The Paleozoic Ere spread an end with a major extinction event, when perhaps as many as 90 percent of all plant and animal species died out. The reason is not known for sure, but many scientists believe that huge volcanic outpourings of lavas in central Siberia, coupled with an asteroid impact, resulting among the fragmented contributive factors.
The Mesozoic Era, sprang into formation and are approximately 248 million years ago, is often characterized as the Age of Reptiles because reptiles were the dominant life forms during this era. Reptiles dominated not only on land, as dinosaurs, but also in the sea, as the plesiosaurs and ichthyosaurs, and in the air, as pterosaurs, which were flying reptiles.
The Mesozoic Era is divided into three geological periods: the Triassic, which lasted from 248 million to 206 million years ago; the Jurassic, from 206 million to 144 million years ago; and the Cretaceous, from 144 million to 65 million years ago. The dinosaurs emerged during the Triassic Period and was one of the most successful animals in Earth's history, lasting for about 180 million years before going extinct at the end of the Cretaceous Period. The first and mammals and the first flowering plants also appeared during the Mesozoic Era. Before flowering plants emerged, plants with seed-bearing cones known as conifers were the dominant form of plants. Flowering plants soon replaced conifers as the dominant form of vegetation during the Mesozoic Era.
Mesozoic was an eventful era geologically with many changes to Earth's surface. Pangaea continued to exist for another 50 million years during the early Mesozoic Era. By the early Jurassic Period, Pangaea began to break up. What is now South America begun splitting from what is now Africa, and in the process the South Atlantic Ocean formed? As the landmass that became North America drifted away from Pangaea and moved westward, a long Subduction zone extended along North America's western margin. This Subduction zone and the accompanying arc of volcanoes extended from what is now Alaska to the southern tip of South America. Abounding of this focus, called the American Cordillera, exists today as the eastern margin of the Pacific Ring of Fire.
During the Cretaceous Period, heat continued to be released from the margins of the drifting continents, and as they slowly sank, vast inland seas formed in much of the continental interiors. The fossilized remains of fishes and marine mollusks called ammonites can be found today in the middle of the North American continent because these areas were once underwater. Large continental masses broke off the northern part of southern Gondwanaland during this period and began to narrow the Tethys Ocean. The largest of these continental masses, present-day India, moved northward toward its collision with southern Asia. As both the North Atlantic Ocean and South Atlantic Ocean continued to open, North and South America became isolated continents for the first time in 450 million years. Their westward journey resulted in mountains along their western margins, including the Andes of South America.
Birds are members of a group of animals called vertebrates, which possess a spinal column or backbone. Other vertebrates are fish, amphibians, reptiles, and mammals. Many characteristics and behaviours of birds are distinct from all other animals, yet they have noticeable similarities. Like mammals, birds have four-chambered hearts and are warm-blooded-having a proportionally constant body temperature that enables them to live in a variety of environments. Like reptiles, birds develop from embryos in eggs outside the mother's body.
Birds are found worldwide in many habitats. They can fly over some of the highest mountains on earth plus both of the earth's poles, dive through water to depths of more than 250 m.'s (850 ft.), and occupy habitats with the most extreme climates on the planet, including arctic tundra and the Sahara Desert. Certain kinds of seabirds are commonly seen over the open ocean thousands of kilometres from the nearest land, but all birds must come ashore to raise their young.
Highly-developed animals, birds are sensitive and responsive, colourful and graceful, with habits that excite interest and inquiry. People have long been fascinated by birds, in part because birds are found in great abundance and variety in the same habitats in which humans thrive. Like people, most species of birds are active during daylight hours. Humans find inspiration in birds' capacity for flight and in their musical calls. Humans also find birds useful-their flesh and eggs for food, their feathers for warmth, and their companionship. Perhaps a key basis for our rapport with birds is the similarity of our sensory worlds: Both birds and humans rely more heavily on hearing and colour vision than on smell. Birds are useful indicators of the quality of the environment, because the health of bird populations mirrors the health of our environment. The rapid declination of bird populations and the accelerating extinction rates of birds in the world's forests, grasslands, wetlands, and islands are therefore reasons for great concern.
Birds vary in size from the tiny bee hummingbird, which measures about 57 mm. (about 2.25 in.) from a beak tip to tail tips and weigh 1.6 g. (0.06 oz.), to the ostrich, which stands 2.7 m. (9 ft.) tall and weighs up to 156 kg. (345 lb.). The heaviest flying bird is the great bustard, which can weigh up to 18 kg. (40 lb.).
All birds are covered with feathers, collectively called plumage, which are specialized structures of the epidermis, or outer layer of skin. The main component of feathers is keratin, a flexible protein that also forms the hair and fingernails of mammals. Feathers provide the strong yet lightweight surface area needed for powered, aerodynamic flight. They also serve as insulation, trapping pockets of air to help birds conserve their body heat. The varied patterns, colours, textures, and shapes of feathers help birds to signal their age, sex, social status, and species that identity of one another. Some birds have plumage that blends in with their surroundings to provide camouflage, helping these birds escape notice by their predators. Birds use their beaks to preen their feathers, often using oil from a gland at the base of their tails. Preening removes dirt and parasites and keeps feathers waterproof and supple. Because feathers are nonliving structures that cannot repair themselves when bigeneric or broken, and they must be renewed periodically. Most adult birds molt-lose and replace their feathers-at least once a year.
Bird wings are highly modified forelimbs with a skeletal structure resembling that of arms. Wings may be long or short, round or pointed. The shape of a bird's wings influences its style of flight, which may consist of gliding, soaring, or flapping. Wings are powered by flight muscles, which are the largest muscles in birds that fly. Flight muscles are found in the chest and are attached to the wings by large tendons. The breastbone, a large bone shaped like the keel of a boat, supports the flight muscles.
Nearly all birds have a tail, which helps them control the direction in which they fly and play a role in landing. The paired flight feathers of the tail, called retrices, extend from the margins of a bird's tail. Smaller feathers called coverts' lie on top of the retrices. Tails may be square, rounded, pointed, or forked, depending on the lengths of the retrices and the way they end. The shapes of bird tails vary more than the shapes of wings, possibly because tail shape is less critical to flight than wing shape. Many male birds, such as pheasants, have ornamental tails that they use to attract mating partners.
Birds have two legs; the lower part of each leg is called the tarsus. Most birds had four toes on each foot, and in many birds, including all songbirds, the first toe, called a hallux, points backwards. Bird toes are adapted in various species for grasping perches, climbing, swimming, capturing prey, and carrying and manipulating food.
Instead of heavy jaws with teeth, modern birds have toothless, lightweight jaws, called beaks or bills. Unlike humans or other mammals, birds can move their upper jaws independently of the rest of their heads. This helps them to open their mouths extremely wide. Beaks occur in a wide range of shapes and sizes, depending on the type of food a bird eats.
The eyes of birds are large and provide excellent vision. They are protected by three eyelids: An upper lid resembling that of humans, a lower lid that closes when a bird sleeps, and a third lid, called a nictitating membrane, that sweeps across the eye sideways, starting from the side near the beak. This lid is a thin, translucent fold of skin that moistens and cleans the eye and protects it from wind and bright light.
The ears of birds are completely internal, with openings placed just behind and below the eyes. In most birds, textured feathers called auriculars form a protective screen that prevents objects from entering the ear. Birds rely on their ears for hearing and for balance, which is especially critical during flight. Two groups of birds, cave swiftlets and oilbirds, find their way in dark places by echolocation-making clicks or rattle calls and interpreting the returning echoes to obtain clues about their environment.
The throats of nearly all birds contain a syrinx (plural, syringes), an organ that is comparable to the voice box of mammals. The syrinx has two membranes that produce sound when they vibrate. Birds classified as songbirds have a peculiarly greater extent-developed syringe. Some songbirds, such as the wood thrush, can control each membrane independently; in this way they can sing two songs simultaneously.
Birds have well-developed brains, which provide acute sensory perception, keen balance and coordination, and instinctive behaviours, along with a surprising degree of intelligence. Parts of the bird brain that are especially developed are the optic lobes, where nerve impulses from the eyes are processed, and the cerebellum, which coordinates muscle actions. The cerebral cortex, the part of the brain responsible for thought in humans, is primitive in birds. However, birds have a hyperstriatum -a forebrain component that mammals lack. This part of the brain helps songbirds to learn their songs, and scientists believe that it may also be the source of bird intelligence.
The internal body parts of all birds, including flightless ones, reflects the evolution of birds as flying creatures. Birds have lightweight skeletons in which many major bones are hollow. A unique feature of birds is the furculum, or wishbone, which is comparable to the collarbones of humans, although in birds the left and right portions are fused. The furculum absorbs the shock of wing motion and acts as a spring to help birds breathe while they fly. Several anical adaptations help to reduce weight and concentrate it near the centre of gravity. For example, modern birds are toothless, which helps reduce the weight of their beaks, and food grinding is carried out in the muscular gizzard, a part of the stomach near the body's core. The egg-laying habit of birds enables their young to develop outside the body of the female, significantly lightening her load. For further weight reduction, the reproductive organs of birds atrophy, or become greatly reduced in size, except the breeding season.
Flight, especially taking off and landing, requires a huge amount of energy-more than humans need even for running. Taking flight is less demanding for small birds than it is for large ones, but small birds need more energy to stay warm. In keeping with their enormous energy needs, birds have an extremely fast metabolism, which includes the chemical reactions involved in releasing stored energy from food. The high body temperature of birds-40° to 42° C.'s (104° to about 108° F.'s)-provides an environment that supports rapid chemical reactions.
To sustain this high-speed metabolism, birds need an abundant supply of oxygen, which combines with food molecules within cells to release energy. The respiratory, or breathing, system of birds is adapted to meet their special needs. Unlike humans, birds have lungs with an opening at each end. New air entered the lungs from one end, and used air goes out the other end. The lungs are connected to a series of air sacs, which simplify the movement of air. Birds breathe faster than any other animal. For example, a flying pigeon breathes 450 times each minute, whereas a human, when running, might breathe only about 30 times each minute.
The circulatory system of birds also functions at high speed. Blood vessels pick up oxygen in the lungs and carry it, along with nutrients and other substances essential to life, to all of a bird's body tissues. In contrast to the human heart, which beats about 160 times per minute when a person runs, a small bird's heart beats between 400 and 1,000 times per minute. The hearts of birds are proportionately larger than the hearts of other animals. Birds that migrate and those that live at high altitudes have larger hearts, compared with their body size, than other birds.
The characteristic means locomotion in birds is flight. However, birds are also variously adapted for movement on land, and some are excellent swimmers and divers.
Like aeroplanes, birds rely on lift-an upward force that counters gravity-to fly. Birds generate lift by pushing down on the air with their wings. This action causes the air, in return, to push the wings up. The shape of wings, which have an upper surface that is convex and a lower surface that is concave, contributes to this effect. To turn, birds often tilt so that one wing is higher than the other.
Different wing shapes adapt birds for different styles of flight. The short, rounded wings and strong breast muscles of quail are ideal for short bursts of powered flight. Conversely, the albatross's long narrow wings enable these birds to soar effortlessly over windswept ocean surfaces. The long, broad wings of storks, vultures, and eagles provide excellent lift on rising air currents.
Feathers play a crucial role in flight. The wings and tails of birds have detailed Flight feathers-the largest and strongest type of feathers-that contribute to lift. Because each of the flight feathers is connected to a muscle, birds can adjust their position individually. As a bird pushes down on the air with its wings, its flight feathers overlap to prevent air from passing through. The same feathers twist open on the upstroke, so that air flows between them and less effort is needed to lift the wings.
Feathers also help to reduce drag, a force of resistance that acts on solid bodies moving through air. Contour feathers, which are the most abundant type of feather, fill and cover angular parts of a bird's body, giving birds a smooth, aerodynamic form.
Bird tails are also important to flight. Birds tip their tail feathers in different directions to achieve stability and to help change direction while flying. When soaring, birds spread their tail feathers to obtain more lift. When landing, birds turn their tails downward, so that their tails act like brakes.
Most birds can move their legs alternately to walk and run, and some birds are adept at climbing trees. Birds' agility on land varies widely among different species. The American robin both hops and walks, while the starling usually walks. The ostrich can run as fast as
64 km./h. (40 mph.). Swifts, however, can neither hop nor run; their weak feet are useful only for clinging to vertical surfaces, such as the walls of caves and houses.
Birds that walk in shallow water, such as herons and stilts, have long legs that simplify wading. Jacanas, which walk on lily pads and mud, have long toes and nails that disperse their weight to help prevent them from sinking. Penguins have stubby legs placed far back from their centre of gravity. So, they can walk only with an upright posture and a short-stepping gait. When penguins need to move quickly, they ‘toboggan' on their bellies, propelling themselves across ice with their wings and feet.
Many birds are excellent swimmers and divers, including such distantly related types of birds as grebes, loons, ducks, auks, cormorants, penguins, and diving petrels. Most of these birds have webbed or lobed toes that act as paddles, which they use to propel themselves underwater. Others, including auks and penguins, use their wings to propel themselves through the water. Swimming birds have broad, raft-like bodies that provide stability. They have dense feather coverings that hold pockets of air for warmth, but they can compress the air out of these pockets to reduce buoyancy when diving.
Many fish-catching birds can dive to great depths, either from the air or from the water's surface. The emperor penguin can plunge into depths of more than 250 m. (850 ft.) and remain submerged for about 12 minutes. Some ducks, swans, and geese perform an action called dabbling, in which they tip their tails up and reach down with their beaks to forage on the mud beneath shallow water.
Like other animals, birds must eat, rest, and defend themselves against predators to survive. They must also reproduce and raise their young to contribute to the survival of their species. For many bird species, migration is an essential part of survival. Birds have acquired remarkably diverse and effective strategies for achieving these ends.
Birds spend much of their time feeding and searching for food. Most birds cannot store large reserves of food internally, because the extra weight would prevent them from flying. Small birds need to eat even more frequently than large ones, because they have a greater surface area in proportion to their weight and therefore lose their body heat more quickly. Some extremely small birds, such as hummingbirds, have so little food in reserve that they enter a state resembling hibernation during the night and rely on the warmth of the sun to energize them in the morning.
Depending on the species, birds eat insects, fish, meat, seeds, nectar, and fruit. Most birds are either carnivorous, meaning they eat other animals, or herbivorous, meaning they eat plant material. Many birds, including crows and gulls, are omnivorous, eating almost anything. Many herbivorous birds feed protein-rich animal material to their undergoing maturation. Some bird species have highly abstractive diets, such as the Everglade kite, which feeds exclusively on snails.
Two unusual internal organs help birds to process food. The gizzard, which is part of a bird's stomach, has thick muscular walls with hard inner ridges. It can crush large seeds and even shellfish. Some seed-eating birds swallow small stones so that the gizzard will grind food more efficiently. Birds that feed on nectar and soft fruits have poorly developed gizzards.
Most birds have a crop-a sac-like extension of the esophagus, the tubular organ through which food passes after leaving the mouth. Some birds store food in their crops and transport it to the place where they sleep. Others use the crop to carry food that they will later regurgitate to their offspring.
The bills of birds are modified in ways that help birds obtain and handle food. Nectar-feeders, such as hummingbirds, have long thin bills, which they insert into flowers, and particularized expansible or brushlike tongues, through which they draw up nectar. Meat-eating birds, including hawks, owls, and shrikes, have strong, hooked bills that can tear flesh. Many fish-eating birds, such as merganser ducks, have tooth-like ridges on their bills that help them to hold their slippery prey. The thick bills and strong jaw muscle of various finches and sparrows are ideal for crushing seeds. Woodpeckers use their bills as chisels, working into dead or living wood to find insect larvae and excavate nest cavities.
At least two species of birds use tools in obtaining food. One is the woodpecker finch, which uses twigs or leaf stalks to extract insects from narrow crevices in trees. The other is the Egyptian vulture, which picks up large stones in its bill and throws them at ostrich eggs to crack them open.
Birds need far less sleep than humans do. Birds probably sleep to relax their muscles and conserve energy but not to refresh their brains. Many seabirds, in particular, sleep very little. For example, the sooty tern, which rarely plummet into settling on water, may fly for several years with only brief periods of sleep lasting a few seconds each. Flying is so effortless for the sooty tern and other seabirds that it takes virtually no energy at all.
Most birds are active during the day and sleep at night. Exceptions are birds that hunt at night, such as owls and night jars. Birds use nests for sleeping only during the breeding season. The rest of the year, birds sleep in shrubs, on tree branches, in holes in trees, and on the bare ground. Most ducks sleep on the water. Many birds stand while they sleep, and some birds sleep while perched on some branch-sometimes using only one foot. These birds can avoid falling over because of a muscle arrangement that causes their claws to tighten when they bend their legs to relax.
To reproduce, birds must find a suitable mate, or mates, and the necessary resources-food, water, and nesting materials-for caring for their eggs and raising the hatched young to independence. Most birds mate during a specific season in a particular habitat, although some birds may reproduce in varied places and seasons, provided environmental conditions are suitable.
Most of all birds have monogamous mating patterns, meaning that one male and one female mate exclusively with each other for at least one season. However, some bird species is either polygynous, that is, the males mate with more than one female, or polyandrous, in which case the females mate with more than one male. Among many types of birds, including some jays, several adults, rather than a single breeding pair, often help to raise the young within an individual nest.
Birds rely heavily on their two main senses, vision and hearing, in courtship and breeding. Among most songbirds, including the nightingale and the sky lark, males use song to establish breeding territories and attract mates. In many species, female songbirds may be attracted to males that sing the loudest, longest, or most varied songs. Many birds, including starlings, mimic the sounds of other birds. This may help males to achieve sufficiently varied songs to attract females.
Many birds rely on visual displays of their feathers to obtain a mating partner. For example, the blue bird of paradise hangs upside down from a tree branch to show off the dazzling feathers of its body and tail. A remarkable courtship strategy is exhibited by male bowerbirds of Australia and New Guinea. These birds attract females by building bowers for shelter, which they decorate with colourful objects such as flower petals, feathers, fruit, and even human-made items such as ribbons and tinfoil.
Among some grouse, cotingas, the small wading birds called shorebirds, hummingbirds, and other groups, males gather in areas called leks to attract mates through vocal and visual displays. Females visiting the leks select particularly impressive males, and often only one or a very few males effectively mate. Among western grebes, both males and females participate in a dramatic courtship ritual called rushing, in which mating partners lift their upper bodies far above the water and paddle rapidly to race side by side over the water's surface. Although male birds usually court females, there are some types of birds, including the phalaropes, in which females court males.
Many birds establish breeding territories, which they defend from rivals of the same species. In areas where suitably nesting habitats is limited, birds may nest in large colonies. An example is the crab plover, which sometimes congregates by the thousands in areas of only about 0.6 hectares (about 1.5 acres).
For breeding, most birds build nests, which help them to incubate, or warm, the developing eggs. Nests sometimes offer camouflage from predators and physical protection from the elements. Nests may be elaborate constructions or some mere scrapes on the ground. Some birds, including many shorebirds, incubate their eggs without any type of nest at all. The male emperor penguin of icy Antarctica incubates the single egg on top of its feet under a fold of skin.
Bird nests range in size from the tiny cups of hummingbirds to the huge stick nests of eagles, which may weigh a ton or more. Some birds, such as the mallee-fowl of southern Australia, use external heat sources, such as decaying plant material, to incubate their eggs. Many birds, including woodpeckers, use tree cavities for nests. Others, such as cowbirds and cuckoos, are brood parasites; they neither build nests nor care for their young. Instead, females of these species lay their eggs in the nests of birds of other species, so that the eggs are incubated-and hatchling duration's bobbed up to raised birds, in so, that they and other hatchling chicks are than the hatchlings' unfeigned by the same rearing nest.
Incubation by one or both parents works with the nest structure to provide an ideal environment for the eggs. The attending parent may warm the eggs with a part of its belly called the brood patch. Bird parents may also wet or shade the eggs to prevent them from overheating.
The size, shape, colour, and texture of a bird egg are specific to each species. Eggs provide an ideal environment for the developing embryo. The shells of eggs are made from calcium carbonates. They contain thousands of pores through which water can evaporate and air can seep in, enabling the developing embryo to breathe. The number of eggs in a clutch (the egg or eggs laid by a female bird in one nesting effort) may be 15 or more for some birds, including pheasants. In contrast, some large birds, such as condors and albatross, may lay only a single egg every two years. The eggs of many songbirds hatch after developing for as few as ten days, whereas those of an albatross and kiwis may require 80 days or more.
Among some birds, including songbirds and pelicans, newly hatched younkers that are without feathers, blind, and incapable of regulating their body temperature. Many other birds, such as ducks, are born covered with down and can feed themselves within hours after hatching. Depending on the species, young birds may remain in the nest for as little as part of a day or as long as several months. Grown older from their young (those that has left the nest) may still rely on parental care for many days or weeks. Only about 10 percent of birds survive their first year of life; the rest die of starvation, disease, predators, or inexperience with the behaviours necessary for survival. The age at which birds begin to breed varies from less than a year in many songbirds and some quail to ten years or more in some albatross. The life spans of birds in the wild are poorly known. Many small songbirds live only three to five years, whereas some albatrosses are known to have survived more than 60 years in the wild.
The keen eyesight and acute hearing of birds help them react quickly to predators, which may be other birds, such as falcons and hawks, or other types of animals, such as snakes and weasels. Many small birds feed in flocks, where they can benefit from the observing power of multiple pairs of eyes. The first bird in a flock to spot a predator usually warns the others with an alarm call.
Birds that feed alone commonly rely on camouflage and rapid flight as means of evading predators. Many birds have highly specific and unusual defence strategies. The burrowing owl in North America, which lives in the burrows of ground squirrels, frightens away predators by making a call that sounds much like a rattlesnake. The snipe, a wading bird, flees from its enemies with a zigzag flight pattern that is hard for other birds to follow.
Many bird species undergo annual migrations, travelling between seasonally productive habitats. Migration helps birds to have continuous sources of food and water, and to avoid environments that are too hot or too cold. Most spectacular of bird migrations are made by seabirds, in which they fly across oceans and along coastlines, sometimes travelling 32,000 km. (20,000 mi.) or more in a single year. Migrating birds use a variety of cues to find their way. These include the positions of the sun during the day and the stars at night; the earth's magnetic field; and visual, olfactory, and auditory landmarks. The strict formations in which many birds fly help them on the journey, for example, migrating geese travel in a V-shaped formation, which enables all of the geese except the leader to take advantage of the updrafts generated by the flapping wings of the goose in front. Young birds of many species undertake their first autumn migration with no guidance from experienced adults. These inexperienced birds do not necessarily reach their destinations; many birds stray in the wrong direction and are sometimes observed thousands of kilometres away from their normal route.
There are nearly 10,000 known species of modern or recently extinct birds. Traditionally, taxonomists (those who classify living things based on evolutionary relationships) have looked at bird characteristics such as skeletal structure, plumage, and bill shape to determine which birds have a shared evolutionary history. More recently, scientists have turned to deoxyribonucleic acid (DNA)-the genetic information found in the cells of all living organisms-for clues about relationships among birds. DNA is useful to volaille bird taxonomists because closely related birds have more similar DNA than do groups of birds that are distantly related. DNA comparisons have challenged some of scientists' previous ideas about relationships among birds. For example, these studies have revealed that vultures of the Americas are more closely related to storks than to the vultures of Europe, Asia, or Africa.
Another method of categorizing birds focuses on adaptive types, or lifestyles. This system groups together birds that live in similar environments or have similar methods for obtaining food. Even among a given adaptive types, birds show tremendous diversity.
Aquatic birds obtain most or all of their food from the water. All aquatic birds that live in saltwater environments have salt glands, which enable them to drink seawater and excrete the excess salt. Albatross, shearwaters, storm petrels, and diving petrels are considered the most exclusively aquatic of all birds. These birds spend much of their time over the open ocean, well away from land.
Many other birds have aquatic lifestyles but live closer to land. Among these are penguins, which live in the southernmost oceans near the Antarctic. Some species of penguins spend most of their lives in the water, coming on land only to reproduce and molt. Grebes and divers, or loons, are found on or near lakes. Grebes are unusual among birds because they make their nests on the water, using floating plant materials that they hide among reeds. Pelicans, known for their long bills and huge throat pouches, often switch between salt water and fresh water habitats during the year. Gulls are generalists among the aquatic birds, feeding largely by scavenging over open water, along shores, or even inland areas. Waterfowls, a group that includes ducks, geese, and swans, often breed on freshwater lakes and marshes, although they sometimes make their homes in marine habitats.
Many long-legged, long-billed birds are adapted to live at the junction of land and water. Large wading birds, including herons, storks, ibises, spoonbills, and flamingoes, are found throughout the world, except near the poles. These birds wade in shallow water or across mudflats, wet fields, or similar environments to find food. Depending on the species, large wading birds may eat fish, frogs, shrimp, or microscopic marine life. Many large wading birds gather in enormous groups to feed, sleep, or nest. Shorebirds often inhabit puddles or other shallow bodies of water. The diversity of shorebirds is reflected in their varied bill shapes and leg lengths. The smallest North American shorebirds, called stints or peeps, have short, thin bills that enable them to pick at surface prey, whereas curlews probe with their long bills for burrowing shellfish and marine worms that are beyond the reach of most other shore feeders. Avocets and stilts have long legs and long bills, both of which help them to feed in deeper water.
Among the best-known birds are the birds of prey. Some, including hawks, eagles, and falcons, are active during the daytime. Others, notably owls, are nocturnal, or active at night. Birds of prey have hooked beaks, strong talons or claws on their feet, and keen eyesight and hearing. The larger hawks and eagles prey on small mammals, such as rodents and other vertebrates. Some birds of prey, such as the osprey and many eagles, eat fish. Falcons eat mainly insects, and owls, depending on the species, have diets ranging from insects to fish and mammalians. Scavengers that feed on dead animals are also considered birds of prey. These include relatives of eagles called Old World vultures, which live in Eurasia and Africa, and the condors and vultures of North and South America.
Some birds, including the largest of all living birds, have lost the ability to fly. The ostriches and their relatives-rheas, emus, cassowaries, and kiwis-are flightless birds settling in Africa, South America, and Australia, including New Guinea and New Zealand. The tinamous of Central and South America are related to the ostrich group, but they have a limited ability to fly. Other birds that feed primarily on the ground and exceed as excellent runners include the bustards (relatives of the cranes) and megapodes, members of a group of chicken-like birds that includes quail, turkeys, pheasants, and grouse. Vegetation is an important part of the diets of running birds.
More than half of all living species of birds are perching birds. Perching birds have been successful in all terrestrial habitats. Typically small birds, perching birds have a distinctive arrangement of toes and leg tendons that enables them to perch acrobatically on small twigs. They have the most satisfactorially-developed and complex vocalizations of all birds. They are divided into two main groups: the sub-oscines, which are mainly tropical and include tyrant flycatchers, antbirds, and oven-birds, and the oscines or songbirds, which make up about 80 percent of all perching bird species, among them the familiar sparrows, finches, warblers, crows, blackbirds, thrushes, and swallow. Some birds of this group catch and feed upon flying insects. An example is the swallow, which opens its mouth in a large trap-like gape to gather food. One apparent group, the dippers, is aquatic; its members obtain their food during short dives in streams and rivers.
Many other groups of birds thrive in terrestrial habitats. Parrots, known for their brilliantly coloured plumage, form a distinctive group of tropical and southern temperate birds that inhabit woodlands and grasslands. Doves and pigeons, like parrots, are seed and fruit eaters but are more widespread and typically more subdued in colour. The cuckoos-including the tree-dwelling species such as the European cuckoo, whose call is mimicked by the cuckoo clock, and ground-inhabiting species, such as roadrunners - are land birds. Hummingbirds are a group of nectars and insect-feeding land birds whose range extends from Alaska to the tip of South America. Woodpeckers and their relatives thrive in forests. Kingfishers are considered land birds despite their habit of eating fish.
Although birds collectively occupy most of the earth's surfaces, most individual species are found only in particular regions and habitats. Some species are quite restricted, occurring only on a single oceanic island or an isolated mountaintop, whereas others are cosmopolitan, living in suitable habitats on most continents. The greatest species diversity occurs in the tropics in North and South America, extending from Mexico to South America. This part of the world is especially rich in tyrant flycatchers, oven-birds, antbirds, tanagers, and hummingbirds. The Australia and New Guinea region have possibly the most distinguishing groups of birds, because its birds have long been isolated from those of the rest of the world. Emus, cassowaries, and several songbird groups, including birds-of-paradise, are found nowhere else. Africa is the unique home to many bird families, including turacos, secretary birds, and helmet-shrikes. Areas that are further from the equator have fewer diverse birds. For example, about 225 bird species breed in the British Isles-approximately half the number of breeding strains that inhabit a single reserve in Ecuador or Peru. Despite the abundance of seabirds at its fringes, Antarctica is the poorest bird continent, with only about 20 species.
The habitats occupied by birds are also diverse. Tropical rain forests have high species diversity, as do savannas and wetlands. Fewer species generally occupy extremely arid habitats and very high elevations. A given species might be a habitat specialist, such as the marsh wren, which lives only in marshes of cattails or tules, or a generalist, such as the house sparrow, which can thrive in a variety of environments.
Many habitats are only seasonally productive for birds. The arctic tundra, for example, teams with birds during the short summer season, when food and water are plentiful. In the winter, however, this habitat is too cold and dry for all but a few species. Many bird species respond to such seasonal changes by undergoing annual migrations. Many bird species that breed in the United States and Canada move south to winter in Central or northern South America. Similar migrations from temperate regions to tropical ones exist between Europe and Africa, northeastern Asia and southeast Asia and India and, to a lesser degree, from southern Africa and southern South America to the equatorial parts of those continents.
Scientists disagree about many aspects of the evolution of birds. Many paleontologists (scientists who study fossils to learn about prehistoric life) believe that birds evolved from small, pillaging dinosaurs called theropods. These scientists say that many skeletal features of birds, such as light, hollow bones and a furculum, were present in theropod dinosaurs before the evolution of birds. Others, however, think that birds evolved from an earlier type of reptile called the codonts - a group that ultimately produced dinosaurs, crocodiles, and the flying reptiles known as pterosaurs. These scientists assert that similarities between birds and theropod dinosaurs are due to a phenomenon called convergent evolution-the evolution of similar traits among groups of organisms that are not necessarily related.
Scientists also disagree about how flight evolved. Some scientists believe that flight first occurred when the ancestors of birds climbed trees and glided down from branches. Others theorize that bird flight began from the ground up, when dinosaurs or reptiles ran along the ground and leaped into the air to catch insects or to avoid predators. Continued discovery and analysis of fossils will help clarify the origins of birds.
Despite uncertainties about bird evolution, scientists do know that many types of birds lived during the Cretaceous Period, which dates to about 138 million to 65 million years ago. Among these birds was Ichthyornis Victor, which resembled a gull and had vertebrae similar to those of a fish, and Hesperonis regalis, which was nearly wingless and had vertebrae like those of today's birds. Most birds of the Cretaceous Period are thought to have died out in the mass extinctions-deaths of many animal species, which took place at the end of the Cretaceous Period.
The remains of prehistoric plants and animals buried and preserved in sedimentary rock or trapped in amber or other deposits of ancient organic matter, provided a record of the history of life on Earth. Scientists who subject in the field of fossil records are called paleontologists, for which having learnt those extinguishing natural archeological remains, are an ongoing phenomenons. In fact, the hundreds of millions of species that have lived on Earth over the past 3.8 billion years, more than 99 percent are already extinct. Some of this happens as the natural result of competition between species and is known as natural selection. According to natural selection, living things must compete for food and space. They must evade the ravages of predators and disease while dealing with unpredictable shifts in their environment. Those species incapable of adapting are faced with imminent extinction. This constant rate of extinction, sometimes called background extinction, is like a slowly ticking clock. First one species, then another becomes extinct, and new species appear almost at random as geological time goes by. Normal rates of background extinction are usually about five families of organisms lost per million years.
More recently, paleontologists have discovered that not all extinction is slow and gradual. At various times in the fossil record, many different, unrelated species became extinct at nearly the same time. The cause of these large-scale extinctions is always dramatic environmental change that produces conditions too severe for organisms to endure. Environmental changes of this caliber result from extreme climatic change, such as the global cooling observed during the ice ages, or from catastrophic events, such as meteorite impacts or widespread volcanic activity. Whatever their causes, these events dramatically alter the composition of life on Earth, as entire groups of organisms disappear and entirely new groups rise to take their place.
In its most general sense, the term mass extinction refers to any episode of multiple loss of species. Nonetheless, the term is generally reserved for truly global extinction events-events in which extensive species loss occurs in all ecosystems on land and in the sea, affecting every part of the Earth's surface. Scientists recognize five such mass extinctions in the past 500 million years. The first occurred around 438 million years ago in the Ordovician Period. At this time, more than 85 percent of the species on Earth became extinct. The second took place 367 million years ago, near the end of the Devonian Period, when 82 percent of all species were lost. The third and greatest mass extinction to date occurred 245 million years ago at the end of the Permian Period. In this mass extinction, as many as 96 percent of all species on Earth were lost. The devastation was so great that paleontologists use this event to mark the end of the ancient, or Paleozoic Era, and the beginning of the middle, or Mesozoic Era, when many new groups of animals evolved.
About 208 million years ago near the end of the Triassic Period, the fourth in mass extinction claimed 76 percent of the species alive at the time, including many species of amphibians and reptiles. The fifth and most recent mass extinction occurred about 65 million years ago at the end of the Cretaceous Period and resulted in the loss of 76 percent of all species, most notably the dinosaurs.
Many geologists and paleontologists speculate that this fifth mass extinction occurred when one or more meteorites struck the Earth. They believe the impact created a dust cloud that blocked much of the sunlight-seriously altering global temperatures and disrupting photosynthesis, the process by which plants derive energy. As plants died, organisms that relied on them for food also disappeared. Supporting evidence for this theory comes from a buried impact crater in the Yucatán Peninsula of Mexico. Measured at 200 km. (124 mi.) in diameter, this huge crater is thought to be the result of a large meteorite striking the Earth. A layer of the element iridium in the geologic sediment from this time provides additional evidence. Unusual in such quantities on Earth, iridium is common in extraterrestrial bodies, and theory supporters suggest iridium travelled to Earth on a meteorite.
Other scientists suspect that widespread volcanic activity in what is now India and the Indian Ocean may have been the source of the atmospheric gases and dust that blocked sunlight. Ancient volcanoes could have been the source of the unusually high levels of iridium, and advocates of this theory point out that iridium is still being released today by at least one volcano in the Indian Ocean. No matter what the cause, the extinction at the end of the Cretaceous Period was so great that scientists use this point in time to divide the Mesozoic Era (also called the Age of Reptiles) from the Cenozoic Era (otherwise known as the Age of Mammals).
Historically biologists-most famous among them British naturalist Charles Darwin-assumed that extinction is the natural outcome of competition between newly evolved, adaptively superior species and their older, more primitive ancestors. These scientists believed that newer, and more peremptorily evolved species simply drove less well-adapted species to extinction. That is, historically, extinction was thought to result from evolution. It was also thought that this process happens in a slow and regular manner and occurs at different times in different groups of organisms.
In the case of background extinction, this holds true. An average of three species becomes extinct every million years, usually because of the forces of natural selection. When this happens, appraisive species, characteristically differs only slightly from the organisms that disappeared-rise to take their places, creating evolutionary lineages of related species. The modern horse, for example, comes from a long evolutionary lineage of related, but now extinct, species. The earliest known horse had four toes on its front feet, three toes on its rear feet, and weighed just 36 kg. (80 lb.). About 45 million years ago, this horse became extinct. It was succeeded by other types of horses with different characteristics, such as teeth better shaped for eating different plants, which made them better in agreement or accorded with their owing environments. This pattern of extinction and the ensuing rise of related species continued for more than 55 million years, ultimately resulting in the modern horse and its relatives the zebras and asses.
In mass extinctions, entire groups of species-such as families, orders, and classes-die out, creating opportunities for the survivors to exploit new habitats. In their new niches, the survivors evolve new characteristics and habits and, consequently, develop into entirely new species. What this course of events means is that mass extinctions are not the result of the evolution of new species, but actually a cause of evolution. Fossils from periods of mass extinction suggest that most new species evolve after waves of extinction. Mass extinctions cause periodic spurts of evolutionary change that shake up the dynamics of life on Earth.
This is perhaps best shown in the development of our own ancestors, the early mammals. Before the fall of the dinosaurs, which had dominated Earth for more than 150 million years, mammals were small, nocturnal, and secretive. They devoted much of their time and energy to evading meat-eating dinosaurs. With the extinction of dinosaurs, the remaining mammals moved into habitats and ecological niches previously dominated by the dinosaurs. Over the next 200 million years, those early mammals evolved into a variety of species, assuming many ecological roles and rising to dominate the Earth as the dinosaurs had before them.
Most scientists agree that life on Earth is now faced with the most severe extinction episode since the event that drove the dinosaurs extinct. No one knows exactly how many species are being lost because no one knows exactly how many species exist on Earth. Estimates vary, but the most widely accepted figure lies between 10 and 13 million species. Of these, biologists estimate that as many as 27,000 species are becoming extinct each year. This translates into an astounding three varieties every hour.
Instead of global climate change, humans are the cause of this latest mass extinction. With the invention of agriculture some 10,000 years ago, humans began destroying the world's terrestrial ecosystems to produce farmland. Today pollution destroys ecosystems even in remote deserts and in the world's deepest oceans. In addition, we have cleared forests for lumber, pulp, and firewood. We have harvested the fish and shellfish of the world's largest lakes and oceans in volumes that make it impossible for populations to recover fast enough to meet our harvesting needs. Everywhere we go, whether on purpose or by accident, we have brought along species that disrupt local ecosystems and, in many cases, drive native species extinct. For instance, Nile perch were intentionally introduced to Lake Victoria for commercial fishing in 1959. This fish proved to be an efficient predator, driving 200 rare species of cichlid fishes to extinction.
This sixth extinction, as it has become known, poses a great threat to our continued existence on the planet. As the sum of all species living in the world's ecosystems, knew as biodiversity, least of mention, losing substance under which a great deal is excessively generically set to one side by much of the resourcefulness from which we depend. Humans use at least 40,000 different plants, animals, fungi, bacteria, and virus species for food, clothing, shelter, and medicines. In addition, the fresh airs we breathe, the water we drink, cook, and wash with, as many chemicals cycles-including the nitrogen cycle and the carbon cycle, so vital to sustain life-depend on the continued health of ecosystems and the species within them.
The list of victims of the sixth extinction grows by the year. Forever lost are the penguin-like great auk, the passengers' pigeon, the zebra-like quagga, the thylacine, the Balinese tiger, the ostrich-like moa, and the tarpan, a small species of wild horse, to name but a few. More than 1,000 plants and animals are threatened by extinction. Each of these organisms has unique attributes-some of which may hold the secrets to increasing world food supplies, eradicating water pollution, or curing disease. A subspecies of the endangered chimpanzee, for example, has recently been identified as the probable origin of the human immunodeficiency virus, the virus that causes acquired immunodeficiency syndrome (AIDS). All the same, these animals are widely hunted in their west African habitat, and just as researchers learn of their significance to the AIDS epidemic, the animals face extinction. If they become extinct, they will take with them many secrets surrounding this devastating disease.
In the United States, legislation to protect endangered species from impending extinction includes the Endangered Species Act of 1973. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), established in 1975, enforces the prohibition of trading of threatened plants and animals between countries. The Convention on Biological Diversity, an international treaty developed in 1992 at the United Nations Conference on the Environment and Development, obligates more than 160 countries to take action to protect plant and animal species.
Scientists meanwhile are intensifying their efforts to describe the species of the world. So far biologists have identified and named around 1.75 million species-a mere fraction of the species believed to exist today. Of those identified, special attention is given to species at or near the brink of extinction. The World Conservation Union (IUCN) maintains an active list of endangered plants and animals called the Red List. In addition, captive breeding programs at zoos and private laboratories are dedicated to the preservation of endangered species. Participants in these programs breed members of different populations of endangered species to increase their genetic diversity, thus enabling the species to cope with further threats to their numbers better.
All these programs together have had some notable successes. The peregrine falcon, nearly extinct in the United States due to the widespread use of the pesticide DDT, rebounded strongly after DDT was banned in 1973. The brown pelican and the bald eagle offer similar success stories. The California condor, a victim of habitat destruction, was bred in captivity, and small numbers of them are now being released back into the wild.
Growing numbers of legislators and conservation biologists, scientists who specialize in preserving and nurturing biodiversity, are realizing that the primary cause of the current wave of extinction is habitat destruction. Efforts have accelerated to identify ecosystems at greatest risk, including those with high numbers of critically endangered species. Programs to set aside large tracts of habitats, often interconnected by narrow zones or corridors, offer the best hope yet of sustaining ecosystems, and by that most of the world's species.
Nevertheless, the Tertiary Period directly following the Cretaceous witnessed an explosive evolution of birds. One bird that lived during the Tertiary Period was Diatryma, which stood 1.8 to 2.4 m. (about six to 8 ft.) tall and had massive legs, a huge bill, and very small, underdeveloped wings. Most modern families of birds can be traced back in the fossil record to the early or mid-Eocene Epoch-a stage within the Tertiary Period that occurred about 50 million years ago. Perching birds, called passerines, experienced a tremendous growth in species diversity in the latter part of the Tertiary; today this group is the most diverse order of birds.
During the Pleistocene Epoch, from 1.6 million to 10,000 years ago, also known as the Ice Age, glacier ice spread over more than one-fourth of the land surfaces of the earth. These glaciers isolated many groups of birds from other groups with which they had previously interbred. Scientists have long assumed that the resulting isolated breeding groups evolved into the species of birds that exist today. This assumption has been modified from studies involving bird DNA within cellular components called mitochondria. Pairs of species that only recently diverged from a shared ancestry are expected to have more similar mitochondrial DNA than are pairs that diverged in the more distant past. Because mutations in mitochondrial DNA are thought to occur at a fixed rate, some scientists believe that this DNA can be interpreted as a molecular clock that reveals the approximate amount of time that has elapsed since two species diverged from one another. Studies of North American songbirds based on this approach suggest that only the earliest glaciers of the Pleistocene are likely to have played a role in shaping bird species.
The evolution of birds has not ended with the birds that we know today. Some bird species are dying out. In addition, the process of Speciation, evolutionary changes that resultant products in some newer species-continues always.
Birds have been of ecological and economic importance to humans for thousands of years. Archaeological sites reveal that prehistoric people used many kinds of birds for food, ornamentation, and other cultural purposes. The earliest domesticated bird was probably the domestic fowl or chicken, derived from jungle fowls of Southeast Asia. Domesticated chickens existed even before 3000 Bc. Other long-domesticated birds are ducks, geese, turkeys, guineas-fowl, and pigeons.
Today the adults, young, and eggs of both wild and domesticated birds provide humans with food. People in many parts of Asia even eat nests that certain swiftlets in southeastern Asia construct out of saliva. Birds give us companionship as pets, assume religious significance in many cultures, and, with hawks and falcons, perform work for us as hunters. People in maritime cultures have learned to monitor seabird flocks to find fish, sometimes even using cormorants to do the fishing.
Birds are good indicators of the quality of our environment. In the 19th century, coal miners brought caged canaries with them into the mines, knowing that if the birds stopped singing, dangerous mine gases had escaped into the air and poisoned them. Birds provided a comparable warning to humans in the early 1960s, when the numbers of peregrine falcons in the United Kingdom and raptors in the United States suddenly declined. This decline was caused by organochlorine pesticides, such as DDT, which were accumulating in the birds and causing them to produce eggs with overly fragile shells. This decline in the bird populations alerted humans to the possibility that pesticides can harm people as well. Today certain species of birds are considered indicators of the environmental health of their habitats. An example of an indicator bird is the northern spotted owl, which can only reproduce within old growth forests in the Pacific Northwest.
Many people enjoy bird-watching. Equipped with binoculars and field guides, they identify birds and their songs, often keeping lists of the various species they have witnessed. Scientists who study birds are known as ornithologists. These experts investigate many behaviours, and, evolutionary histories, ecology, set-classification, and species distributed of both domesticated and rashly or wild birds.
Overall, birds pose little direct danger to humans. A few birds, such as the cassowaries of New Guinea and northeastern Australia, can kill humans with their strong legs and bladelike claws, but actual attacks are extremely rare. Many birds become quite aggressive when defending a nest site; humans are routinely attacked, and occasionally killed, by hawks engaging in such defence. Birds pose a greater threat to human health as carriers of diseases. Diseases carried by birds that can affect humans include influenza and psittacosis.
Negative impacts by birds on humans are primarily economic. Blackbirds, starlings, sparrows, weavers, crows, parrots, and other birds may seriously deplete crops of fruit and grain. Similarly, fish-eating birds, such as cormorants and herons, may adversely influence aquacultural production. However, the economic benefits of wild birds to humans are well documented. Many birds help humans, especially farmers, by eating insects, weeds, slugs, and rodents.
Although birds, with some exceptions, are tremendously beneficial to humans, humans have a long history of causing harm to birds. Studies of bone deposits on some Pacific islands, including New Zealand and Polynesia, suggest that early humans hunted many hundreds of bird species to extinction. Island birds have always been particularly susceptible to predation by humans. Because these birds have largely evolved without land-based predators, they are tame and in many descriptions are flightless. They are therefore easy prey for humans and the animals that accompany them, such as rats. The dodo, a flightless pigeon-like bird on the island of Mauritius in the Indian Ocean, was hunted to extinction by humans in the 1600s.
With colonial expansion and the technological advances of the 18th and 19th centuries, humans hunted birds on an unprecedented scale. This time period witnessed the extinction of the great auk, a large flightless seabird of the North Atlantic Ocean that was easily killed by sailors for food and oil. The Carolina parakeet also became extinct in this intermittent interval of time, although the last one of these birds survived in the Cincinnati Zoo until 1918.
In the 20th century, a time of explosive growth in human populations, the major threats to birds have been the destruction and modification of their habitats. The relentless clearing of hardwood forests outweighed even relentless hunting as the cause of the extinction of the famous passengers' pigeon, whose eastern North American populations may have once numbered in the billions. The fragmentation of habitats into small parcels is also harmful to birds, because it increases their vulnerability to predators and parasites.
Habitat fragmentation and reduction particularly affect songbirds that breed in North America in the summer and migrate to Mexico, the Caribbean, Central America, and Colombia for the winter. In North America, these birds suffer from forest fragmentation caused by the construction of roads, housing developments, and shopping malls. In the southern part of their range, songbirds are losing traditional nesting sites as tropical forests are destroyed and shade trees are removed from coffee plantations.
Pesticides, pollution, and other poisons also threaten today's birds. These substances may kill birds outright, limit their ability to reproduce, or diminish their food supplies. Oil spills have killed thousands of aquatic birds, because birds with oil-drenched feathers cannot fly, float, or stay warm. Acid rain, caused by chemical reactions between airborne pollutants and water and an oxygen providence in the atmosphere, has decreased the food supply of many birds that feed on fish or other aquatic life in polluted lakes. Many birds are thought to be harmed by selenium, mercury, and other toxic elements present in agricultural runoff and in drainage from mines and power plants. For example, loons in the state of Maine may be in danger due to mercury that drifts into the state from unregulated coal-fired power plants in the Midwest and other sources. Global warming, an increase in the earth's temperature due to a buildup of greenhouse gases, is another potential threat to birds.
Sanctuaries for birds exist all over the world-two examples are the Bharatpur Bird Sanctuaries in India's Keoladeo National Park, which protects painted storks, gray herons, and many other bird species, and the National Wildlife Refuge system of the United States. In North America, some endangered birds are bred in settings such as zoos and specialized animal clinics and later released into the wild. Such breeding programs have added significantly to the numbers of whooping cranes, peregrine falcons, and California condors. Many countries, including Costa Rica, are finding they can reap economic benefits, including the promotion of tourism, by protecting the habitats of birds and other wildlife.
The protection of the earth's birds will require more than a single strategy. Many endangered birds need a combination of legal protections, habitat management, and control of predators and competitors. Ultimately, humans must decide that the bird's world is worth preserving along with our own.
Most people did not understand the true nature of fossils until the beginning of the 19th century, when the basic principles of modern geology were established. Since about 1500 AD., scholars had engaged in a bitter controversy over the origin of fossils. One group held that fossils are the remains of prehistoric plants and animals. This group was opposed by another, which declared that fossils were either freaks of nature or creations of the devil. During the 18th century, many people believed that all fossils were relics of the great flood recorded in the Bible.
Paleontologists gain most of their information by studying deposits of sedimentary rocks that formed in strata over millions of years. Most fossils are found in sedimentary rock. Paleontologists use fossils and other qualities of the rock to compare strata around the world. By comparing, they can determine whether strata developed during the same time or in the same type of environment. This helps them assemble a general picture of how the earth evolved. The study and comparison of different strata are called stratigraphy.
Fossils sustain most of the data on which strata are compared. Some fossils, called index fossils, are especially useful because they have a broad geographic range but a narrow temporal one-that is, they represent a species that was widespread but existed for a brief period of time. The best index fossils tend to be marine creatures. These animals evolved rapidly and spread over large areas of the world. Paleontologists divide the last 570 million years of the earth's history into eras, periods, and epochs. The part of the earth's history before about 570 million years ago is called Precambrian time, which began with the earth's birth, probably more than four billion years ago.
The earliest evidence of life consists of microscopic fossils of bacteria that lived as early as 3.6 billion years ago. Most Precambrian fossils are very tiny. Most species of larger animals that lived in later Precambrian time had soft bodies, without shells or other hard body parts that would create lasting fossils, the first abundant fossils of larger animals had been dated from around 600 million years ago. The Paleozoic era lasted to about 330 million years. It includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. Index fossils of the first half of the Paleozoic era are those of the invertebrates, such as trilobites, graptolites, and crinoids. Remains of plants and such vertebrates as fish and reptiles make up the index fossils of the second half of this era.
At the beginning of the Cambrian period (570 million to 500 million years ago) animal life was entirely confined to the seas. By the end of the period, all the phyla of the animal kingdom existed, except vertebrates. The characteristic animals of the Cambrian period were the trilobites, a primitive form of arthropod, which reached their fullest development in this period and became extinct by the end of the Paleozoic era. The earliest snails appeared in this period, as did the cephalopod mollusks. Other groups represented in the Cambrian period were brachiopods, bryozoans, and foraminifers. Plants of the Cambrian period included seaweeds in the oceans and lichens on land.
The most characteristic animals of the Ordovician period (500 million to 435 million years ago) were the graptolites, which were small, colonial hemichordates (animals possessing an anical structure suggesting part of a spinal cord). The first vertebrates-primitive fish-and the earliest corals emerged during the Ordovician period. The largest animal of this period was a cephalopod mollusk that had a shell about three m.'s (about 10 ft.) in length. Plants of this period resembled those of the Cambrian periods.
The most important evolutionary development of the Silurian period (435 million to 410 million years ago) was that of the first air-breathing animal, a scorpion. Fossils of this creature have been found in Scandinavia and Great Britain. The first fossil records of vascular plants-that are, land plants with tissue that carries food-appeared in the Silurian period. They were simple plants that had not developed separate stems and leaves.
The dominant forms of animal life in the Devonian period (410 million to 360 million years ago) were fish of various types, including sharks, lungfish, armoured fish, and primitive forms of ganoid (hard-scaled) fish that were probably the evolutionary ancestors of amphibians. Fossil remains found in Pennsylvania and Greenland suggest that early forms of amphibia may already have existed during the Devonian period. Early animal forms included corals, starfish, sponges, and trilobites. The earliest known insect was found in Devonian rock.
The Devonian is the first period from which any considerable number of fossilized plants have been preserved. During this period, the first woody plants developed, and by the end of the period, land-growing forms included seed ferns, ferns, scouring rushes, and scale trees, the modern relative of club moss. Although the present-day equivalents of these groups are mostly small plants, they developed into treelike forms in the Devonian period. Fossil evidence shows that forests existed in Devonian times, and petrified stumps of some larger plants from the period measure about 60 cm. (about 24 in.) in diameter.
The Carboniferous period lasted from 360 million to 290 million years ago. During the first part of this period, sometimes called the Mississippian period (360 million to 330 million years ago), the seas contained a variety of echinoderms and foraminifers, and most forms of animal life that appeared in the Devonian. A group of sharks, the Cestraciontes, or shell-crushers were dominant among the larger marine animals. The predominant group of land animals was the Stegocephalia, an order of primitive, lizard-like amphibians that developed from the lungfish. The various forms of land plants became diversified and grew larger, particularly those that grew in low-laying swampy areas.
The second part of the Carboniferous, sometimes called the Pennsylvanian period (330 million to 290 million years ago), saw the evolution of the first reptiles, a group that developed from the amphibians and lived entirely on land. Other land animals included spiders, snails, scorpions, more than 800 species of cockroaches, and the largest insect ever evolved, a species resembling the dragonfly, with a wingspread of about 74 cm. (about 29 in.). The largest plants were the scale trees, which had tapered trunks that measured as much as 1.8 m.'s (6 ft.) in diameter at the base and 30 m.'s (100 ft.) in height. Primitive gymnosperms known as cordaites, which had pithy stems surrounded by a woody shell, were more slender but even taller. The first true conifers, forms of advanced gymnosperms, also developed during the Pennsylvanian period.
The chief events of the Permian period (290 million to 240 million years ago) were the disappearance of many forms of marine animals and the rapid spread and evolution of the reptiles. Usually, Permian reptiles were of two types: lizard-like reptiles that lived entirely on land, and sluggish, semiaquatic types. A comparatively small group of reptiles that evolved in this period, the Theriodontia, were the ancestors of mammals. Most vegetation of the Permian period was composed of ferns and conifers.
The Mesozoic era is often called the Age of Reptiles, because the reptile class was dominant on land throughout the age The Mesozoic era lasted to about 175 million years, and includes the Triassic, Jurassic, and Cretaceous periods. Index fossils from this era include a group of extinct cephalopods called ammonites, and extinct forms of sand dollars and sea urchins.
The most notable of the Mesozoic reptiles, the dinosaur, first evolved in the Triassic period (240 million to 205 million years ago). The Triassic dinosaurs were not as large as their descendants in later Mesozoic times. They were comparatively slender animals that ran on their hind feet, balancing their bodies with heavy, fleshy tails, and seldom exceeded 4.5 m. (15 ft.) in length. Other reptiles of the Triassic period included such aquatic creatures as the ichthyosaurs, and a group of flying reptiles, the pterosaurs.
The first mammals also appeared during this period. The fossil remains of these animals are fragmentary, but the animals were apparently small and reptilian in appearance. In the sea, Teleostei, the first ancestors of the modern bony fishes, made their appearance. The plant life of the Triassic seas included a large variety of marine algae. On land, the dominant vegetation included various evergreens, such as ginkgos, conifers, and palms. Small scouring rushes and ferns still existed, but the larger members of these groups had become extinct.
During the Jurassic period (205 million to 138 million years ago), dinosaurs continued to evolve in a wide range of size and diversity. Types included heavy four-footed sauropods, such as Apatosaurus (formerly Brontosaurus); two-footed carnivorous dinosaurs, such as Allosaurus; two-footed vegetarian dinosaurs, such as Camptosaurus; and four-footed armoured dinosaurs, such as Stegosaurus. Winged reptiles included the pterodactyl, which, during this period, ranged in size from extremely small species to those with wingspreads of 1.2 m.'s (4 ft.). Marine reptiles included plesiosaurs, a group that had broad, flat bodies like those of turtles, with long necks and large flippers for swimming, Ichthyosauria, which resembled dolphins, or at times they appear like primitive crocodiles.
The mammals of the Jurassic period consisted of four orders, all of which were smaller than small modern dogs. Many insects of the modern orders, including moths, flies, beetles, grasshoppers, and termites appeared during the Jurassic period. Shellfish included lobsters, shrimp, and ammonites, and the extinct group of belemnites, which resembled squid and had cigar-shaped internal shells. Plant life of the Jurassic period was dominated by the cycads, which resembled thick-stemmed palms. Fossils of most species of Jurassic plants are widely distributed in temperate zones and polar regions, suggesting that the climate was uniformly mild.
The reptiles were still the dominant form of animal life in the Cretaceous period (138 million to 65 million years ago). The four types of dinosaurs found in the Jurassic also lived during this period, and a fifth type, the horned dinosaurs, also appeared. By the end of the Cretaceous, about 65 million years ago, all these creatures had become extinct. The largest of the pterodactyls lived during this period. Pterodactyl fossils discovered in Texas have wingspreads of up to 15.5 m's (50 ft). Other reptiles of the period include the first snakes and lizards. Several types of Cretaceous birds have been discovered, including Hesperornis, a diving bird about 1.8 m's (about 6 ft) in length, which had only vestigial wings and was unable to fly. Mammals of the period included the first marsupials, which strongly resembled the modern opossum, and the first placental mammals, which belonged to the group of insectivores. The first crabs developed during this period, and several modern varieties of fish also evolved.
The most important evolutionary advance in the plant kingdom during the Cretaceous period was the development of deciduous plants, the earliest fossils of which appear in early Cretaceous rock formations. By the end of the period, many modern varieties of trees and shrubs had made their appearance. They represented more than 90 percent of the known plants of the period. Mid-Cretaceous fossils include remains of beech, holly, laurel, maple, oak, plane tree, and walnut. Some paleontologists believe that these deciduous woody plants first evolved in Jurassic times but grew only in upland areas, where conditions were unfavourable for fossil preservation.
The Cenozoic era (65 million years ago to the present time) is divided into the Tertiary period (65 million to 1.6 million years ago) and the Quaternary period (1.6 million years ago to the present). However, because scientists have so much more information about this era, they tend to focus on the epochs that make up each period. During the first part of the Cenozoic era, an abrupt transition from the Age of Reptiles to the Age of Mammals occurred, when the large dinosaurs and other reptiles that had dominated the life of the Mesozoic era disappeared.
The Paleocene epoch (65 million to 55 million years ago) marks the beginning of the Cenozoic era. Seven groups of Paleocene mammals are known. All of them appear to have developed in northern Asia and to have migrated to other parts of the world. These primitive mammals had many features in common. They were small, with no species exceeding the size of a small modern bear. They were four-footed, with five toes on each foot, and they walked on the soles of their feet. Most of them had slim heads with narrow muzzles and small brain cavities. The predominant mammals of the period were members of three groups that are now extinct. They were the creodonts, which were the ancestors of modern carnivores; the amblypods, which were small, heavy-bodied animals; and the condylarths, which were light-bodied herbivorous animals with small brains. The Paleocene groups that have survived are the marsupials, the insectivores, the primates, and the rodents.
During the Eocene epoch (55 million to 38 million years ago), several ancestors direct evolutionary modern animals appeared. Among these animals - all of which were small in stature-were the horse, rhinoceros, camel, rodent, and monkey. The creodonts and amblypods continued to develop during the epoch, but the condylarths became extinct before it ended. The first aquatic mammals, ancestors of modern whales, also appeared in Eocene times, as did such modern birds as eagles, pelicans, quail, and vultures. Changes in vegetation during the Eocene epoch were limited chiefly to the migration of types of plants in response to climate changes.
During the Oligocene epoch (38 million to 24 million years ago), most of the archaic mammals from earlier epochs of the Cenozoic era disappeared. In their place appeared representatives of several modern mammalian groups. The creodonts became extinct, and the first true carnivores, resembling dogs and cats, evolved. The first anthropoid apes also lived during this time, but they became extinct in North America by the end of the epoch. Two groups of animals that are now extinct flourished during the Oligocene epoch: the titanotheres, which are related to the rhinoceros and the horse; and the oreodonts, which were small, dog-like, grazing animals.
The development of mammals during the Miocene epoch (24 million to five million years ago) was influenced by an important evolutionary development in the plant kingdom: the first appearance of grasses. These plants, which were ideally suited for forage, encouraged the growth and development of grazing animals such as horses, camels, and rhinoceroses, which were abundant during the epoch. During the Miocene epoch, the mastodon evolved, and in Europe and Asia a gorilla-like ape, Dryopithecus, was common. Various types of carnivores, including cats and wolflike dogs, ranged over many parts of the world.
The paleontology of the Pliocene epoch (five million to 1.6 million years ago) does not differ much from that of the Miocene, although the period is regarded by many zoologists as the climax of the Age of Mammals. The Pleistocene Epoch (1.6 million to 10,000 years ago) in both Europe and North America was marked by an abundance of large mammals, most of which were fundamentally forward-moving in type. Among them were buffalo, elephants, mammoths, and mastodons. Mammoths and mastodons became extinct before the end of the epoch. In Europe, antelope, lions, and hippopotamuses also appeared. Carnivores included badgers, foxes, lynx, otters, pumas, and skunks, and now-extinct species such as the giant saber-toothed tigers. In North America, the first bears made their appearance as migrants from Asia. The armadillo and ground sloth migrated from South America to North America, and the musk-ox ranged southward from the Arctic regions. Modern human beings also emerged during this epoch.
Earth is one of nine planets in the solar system, the only planet known to harbor life, and the ‘home' of human beings. From space Earth resembles a big blue marble with swirling white clouds floating above blue oceans. About 71 percent of Earth's surface is covered by water, which is essential to life. The rest is land, mostly as continents that rise above the oceans.
Earth's surface is surrounded by a layer of gases known as the atmosphere, which extends upward from the surface, slowly thinning out into space. Below the surface is a hot interior of rocky material and two core layers composed of the metals nickel and iron in solid and liquid form.
Unlike the other planets, Earth has a unique set of characteristics ideally suited to supporting life as we know it. It is neither too hot, like Mercury, the closest planet to the Sun, nor too cold, like distant Mars and the even more distant outer planets-Jupiter, Saturn, Uranus, Neptune, and tiny Pluto. Earth's atmosphere includes just the right degree of gases that trap heat from the Sun, resulting in a moderate climate suitable for water to exist in liquid form. The atmosphere also helps block radiation from the Sun that would be harmful to life. Earth's atmosphere distinguishes it from the planet Venus, which is otherwise much like Earth. Venus is about the same size and mass as Earth, not either too far or nearer from the Sun. Nevertheless, because Venus has too much heat-trapping carbon dioxide in its atmosphere, its surface is extremely hot-462°C's (864°F)-hot enough to melt lead and too hot for life to exist.
Although Earth is the only planet known to have life, scientists do not rule out the possibility that life may once have existed on other planets or their moons, or may exist today in primitive form. Mars, for example, has many features that resemble river channels, suggesting that liquid water once flowed on its surface. If so, life may also have evolved there, and evidence for it may one day be found in fossil form. Water still exists on Mars, but it is frozen in polar ice caps, in permafrost, and possibly in rocks below the surface.
For thousands of years, human beings could only wonder about Earth and the other observable planets in the solar system. Many early ideas, for example, that the Earth was a sphere and that it travelled around the Sun were based on brilliant reasoning. However, it was only with the development of the scientific method and scientific instruments, especially in the 18th and 19th centuries, that humans began to gather data that could be used to verify theories about Earth and the rest of the solar system. By studying fossils found in rock layers, for example, scientists realized that the Earth was much older than previously believed. With the use of telescopes, new planets such as Uranus, Neptune, and Pluto were discovered.
In the second half of the 20th century, more advances in the study of Earth and the solar system occurred due to the development of rockets that could send spacecraft beyond Earth. Human beings can study and observe Earth from space with satellites equipped with scientific instruments. Astronauts landed on the Moon and gathered ancient rocks that revealed much about the early solar system. During this remarkable advancement in human history, humans also sent unmanned spacecraft to the other planets and their moons. Spacecraft have now visited all of the planets except Pluto. The study of other planets and moons has provided new insights about Earth, just as the study of the Sun and other stars like it has helped shape new theories about how Earth and the rest of the solar system formed.
From this recent space exploration, we now know that Earth is one of the most geological activities unbound of all the planets and moons in the solar system. Earth is constantly changing. Over long periods land is built up and worn away, oceans are formed and re-formed. Continents move around, break up, and merge.
Life itself contributes to changes on Earth, especially in, and the way living things can alter Earth's atmosphere. For example, Earth at once had the same amount of carbon dioxide in its atmosphere as Venus now has, but early forms of life helped remove this carbon dioxide over millions of years. These life forms also added oxygen to Earth's atmosphere and made it possible for animal life to evolve on land.
A variety of scientific fields have broadened our knowledge about Earth, including biogeography, climatology, geology, geophysics, hydrology, meteorology, oceanography, and zoogeography. Collectively, these fields are known as Earth science. By studying Earth's atmosphere, its surface, and its interior and by studying the Sun and the rest of the solar system, scientists have learned much about how Earth came into existence, how it changed, and why it continues to change.
Earth is the third planet from the Sun, after Mercury and Venus. The average distance between Earth and the Sun is 150 million km. (93 million mi). Earth and all the other planets in the solar system revolve, or orbit, around the Sun due to the force of gravitation. The Earth travels at a velocity of about 107,000 km./h. (about 67,000 mph.) as it orbits the Sun. All but one planet orbit the Sun in the same plane-that is, if an imaginary line were extended from the centre of the Sun to the outer regions of the solar system, the orbital paths of the planets would intersect that line. The exception is Pluto, which has an eccentric (unusual) orbit.
Earth's orbital path is not quite a perfect circle but instead is elliptical (oval-shaped). For example, at maximum distance Earth is about 152 million km. (about 95 million mi.) from the Sun; at minimum distance Earth is about 147 million km (about 91 million mi.) from the Sun. If Earth orbited the Sun in a perfect circle, it would always be the same distance from the Sun.
The solar system, in turn, is part of the Milky Way Galaxy, a collection of billions of stars bound together by gravity. The Milky Way has arm-like discs of stars that spiral out from its centre. The solar system is found in one of these spiral arms, known as the Orion arm, which is about two-thirds of the way from the centre of the Galaxy. In most parts of the Northern Hemisphere, this disc of stars is visible on a summer night as a dense band of light known as the Milky Way.
Earth is the fifth largest planet in the solar system. Its diameter, measured around the equator, is 12,756 km (7,926 mi). Earth is not a perfect sphere but is slightly flattened at the poles. Its polar diameter, measured from the North Pole to the South Pole, is in a measure less than the equatorial diameter because of this flattening. Although Earth is the largest of the four planets-Mercury, Venus, Earth, and Mars-that makes up the inner solar system (the planets closest to the Sun), it is small compared with the giant planets of the outer solar system-Jupiter, Saturn, Uranus, and Neptune. For example, the largest planet, Jupiter, has a diameter at its equator of 143,000 km (89,000 mi), 11 times greater than that of Earth. A famous atmospheric feature on Jupiter, the Great Red Spot, is so large that three Earths would fit inside it.
Earth has one natural satellite, the Moon. The Moon orbits the Earth, undivided and compelling of one revolution in an elliptical path in 27 days 7 hr 43 min 11.5-sec. The Moon orbits the Earth because of the force of Earth's gravity. However, the Moon also exerts a gravitational force on the Earth. Evidence for the Moon's gravitational influence can be seen in the ocean tides. A popular theory suggests that the Moon split off from Earth more than four billion years ago when a large meteorite or small planet struck the Earth.
As Earth revolves around the Sun, it rotates, or spins, on its axis, an imaginary line that runs between the North and South poles. The period of one complete rotation is defined as a day and takes 23 hr 56 min's 4.1-sec. The period of one revolution around the Sun is defined as a year, or 365.2422 solar days, or 365 days 5 hr. 48 min.'s 46-sec. Earth also moves along with the Milky Way Galaxy as the Galaxy rotates and moves through space. It indirectly takes by more than 200 million years for the stars in the Milky Way to complete one revolution around the Galaxy's centre.
Earth's axis of rotation is inclined (tilted) 23.5° on its plane of revolution around the Sun. This inclination of the axis creates the seasons and causes the height of the Sun in the sky at noon to increase and decrease as the seasons change. The Northern Hemisphere receives the most energy from the Sun when it is tilted toward the Sun. This orientation corresponds to summer in the Northern Hemisphere and winter in the Southern Hemisphere. The Southern Hemisphere receives maximum energy when it is tilted toward the Sun, corresponding to summer in the Southern Hemisphere and winter in the Northern Hemisphere. Fall and spring occur between these orientations.
The atmosphere is a layer of different gases that extends from Earth's surface to the exosphere, the outer limit of the atmosphere, about 9,600 km. (6,000 mi.) above the surface. Near Earth's surface, the atmosphere consists almost entirely of nitrogen (78 percent) and oxygen (21 percent). The remaining 1 percent of atmospheric gases consist of argon (0.9 percent); carbon dioxide (0.03 percent); varying amounts of water vapour; and trace amounts of hydrogen, nitrous oxide, ozone, methane, carbon monoxide, helium, neon, krypton, and xenon.
The layers of the atmosphere are the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. The troposphere is the layer in which weather occurs and extends from the surface to about 16 km (about 10 mi.) above sea level at the equator. Above the troposphere is the stratosphere, which has an upper boundary of about 50 km (about 30 mi) above sea level. The layer from 50 to 90 km (30 to 60 mi.) is called the mesosphere. At an altitude of about 90 km, temperatures begin to rise. The layer that begins at this altitude is called the thermosphere because of the high temperatures that can be reached in this layer (about 1200°C's, or about 2200°F). The region beyond the thermosphere is called the exosphere. The thermosphere and the exosphere overlap with another region of the atmosphere known as the ionosphere, a layer or layers of ionized air extending from almost 60 km (about 50 mi) above Earth's surface to altitudes of 1,000 km (600 mi) and more.
Earth's atmosphere and the way it interacts with the oceans and radiation from the Sun are responsible for the planet's climate and weather. The atmosphere plays a key role in supporting life. Most life on Earth uses atmospheric oxygen for energy in a process known as cellular respiration, which is essential to life. The atmosphere also helps moderate Earth's climate by trapping radiation from the Sun that is reflected from Earth's surface. Water vapour, carbon dioxide, methane, and nitrous oxide in the atmosphere act as ‘greenhouse gases'. Like the glass in a greenhouse, they trap infrared, or heat, radiation from the Sun in the lower atmosphere and by that help warm Earth's surface. Without this greenhouse effect, heat radiation would escape into space, and Earth would be too cold to support most forms of life.
Other gases in the atmosphere are also essential to life. The trace amount of ozone based in Earth's stratosphere blocks harmful ultraviolet radiation from the Sun. Without the ozone layer, life as we know it could not survive on land. Earth's atmosphere is also an important part of a phenomenon known as the water cycle or the hydrologic cycle.
The water cycle simply means that Earth's water is continually recycled between the oceans, the atmosphere, and the land. All of the water that exists on Earth today has been used and reused for billions of years. Very little water has been created or lost during this period of time. Water is always shifting on the Earth's surface and changing back and forth between ice, liquid water, and water vapour.
The water cycle begins when the Sun heats the water in the oceans and causes it to evaporate and enter the atmosphere as water vapour. Some of this water vapour falls as precipitation directly back into the oceans, completing a short cycle. Some water vapour, however, reaches land, where it may fall as snow or rain. Melted snow or rain enters rivers or lakes on the land. Due to the force of gravity, the water in the rivers eventually empties back into the oceans. Melted snow or rain also may enter the ground. Groundwater may be stored for hundreds or thousands of years, but it will eventually reach the surface as springs or small pools known as seeps. Even snow that forms glacial ice or becomes part of the polar caps and is kept out of the cycle for thousands of years eventual melts or is warmed by the Sun and turned into water vapour, entering the atmosphere and falling again as precipitation. All water that falls on land eventually return to the ocean, completing the water cycle.
The hydrosphere consists of the bodies of water that cover 71 percent of Earth's surface. The largest of these are the oceans, which hold more than 97 percent of all water on Earth. Glaciers and the polar ice caps encircle just more than 2 percent of Earth's water as solid ice. Only about 0.6 percent is under the surface as groundwater. Nevertheless, groundwater is 36 times more plentiful than water found in lakes, inland seas, rivers, and in the atmosphere as water vapour. Only 0.017 percent of all the water on Earth is found in lakes and rivers. A mere 0.001 percent is found in the atmosphere as water vapour. Most of the water in glaciers, lakes, inland seas, rivers, and groundwater is fresh and can be used for drinking and agriculture. Dissolved salts compose about 3.5 percent of the water in the oceans, however, making it unsuitable for drinking or agriculture unless it is treated to remove the salts.
The crust consists of the continents, other land areas, and the basins, or floors, of the oceans. The dry land of Earth's surface is called the continental crust. It is about 15 to 75 km (nine to 47 mi) thick. The oceanic crust is thinner than the continental crust. Its average thickness is five to 10 km (three to 6 mi). The crust has a definite boundary called the Mohorovicic discontinuity, or simply the Moho. The boundary separates the crust from the underlying mantle, which is much thicker and is part of Earth's interior.
Oceanic crust and continental crust differ in the type of rocks they contain. There are three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks form when molten rock, called magma, cools and solidifies. Sedimentary rocks are usually created by the breakdown of igneous rocks. They have a tendency to form in layers as small particles of other rocks or as the mineralized remains of dead animals and plants that have fused over time. The remains of dead animals and plants occasionally become mineralized in sedimentary rock and are recognizable as fossils. Metamorphic rocks form when sedimentary or igneous rocks are altered by heat and pressure deep underground.
Oceanic crust consists of dark, dense igneous rocks, such as basalt and gabbro. Continental crust consists of lighter coloured, less dense igneous rock, such as granite and diorite. Continental crust also includes metamorphic rocks and sedimentary rocks.
The biosphere can support life. The biosphere ranges from about 10 km (about 6 mi) into the atmosphere to the deepest ocean floor. For a long time, scientists believed that all life depended on energy from the Sun and consequently could only exist where sunlight penetrated. In the 1970s, however, scientists discovered various forms of life around hydrothermal vents on the floor of the Pacific Ocean where no sunlight penetrated. They learned that primitive bacteria formed the basis of this living community and that the bacteria derived their energy from a process called chemosynthesis that did not depend on sunlight. Some scientists believe that the biosphere may extend deeply into the Earth's crust. They have recovered what they believe are primitive bacteria from deeply drilled holes below the surface.
Earth's surface has been constantly changing ever since the planet formed. Most of these changes have been gradual, taking place over millions of years. Nevertheless, these gradual changes have resulted in radical modifications, involving the formation, erosion, and re-formation of mountain ranges, the movement of continents, the creation of huge super-continents, and the breakup of super-continents into smaller continents.
The weathering and erosion that result from the water cycle are among the principal factors responsible for changes to Earth's surface. Another principal factor is the movement of Earth's continents and sea-floors and the buildup of mountain ranges due to a phenomenon known as plate tectonics. Heat is the basis for all these changes. Heat in Earth's interior is believed to be responsible for continental movement, mountain building, and the creation of new sea-floor in ocean basins. Heat from the Sun is responsible for the evaporation of ocean water and the resulting precipitation that causes weathering and erosion. In effect, heat in Earth's interior helps build up Earth's surface while heat from the Sun helps wear down the surface.
Weathering is the breakdown of rock at and near the surface of Earth. Most rocks originally formed in a hot, high-pressure environment below the surface where there was little exposure to water. Once the rocks reached Earth's surface, however, they were subjected to temperature changes and exposed to water. When rocks are subjected to these kinds of surface conditions, the minerals they contain tend to change. These changes make up the process of weathering. There are two types of weathering: physical weathering and chemical weathering.
Physical weathering involves a decrease in the size of rock material. Freezing and thawing of water in rock cavities, for example, splits rock into small pieces because water expands when it freezes.
Chemical weathering involves a chemical change in the composition of rock. For example, feldspar, a common mineral in granite and other rocks, reacts with water to form clay minerals, resulting in a new substance with totally different properties than the parent feldspar. Chemical weathering is of significance to humans because it creates the clay minerals that are important components of soil, the basis of agriculture. Chemical feed weathering also causes the exit of dissolved forms of sodium, calcium, potassium, magnesium, and other chemical elements into surface and groundwater water. These elements are carried by surface water and groundwater to the sea and are the sources of dissolved salts in the sea.
Erosion is the process that removes lose and weathered rock and carries it to a new site. Water, wind, and glacial ice combined with the force of gravity can cause erosion.
Erosion by running water is by far the most common process of erosion. It takes place over a longer period of time than other forms of erosion. When water from rain or melted snow moves downhill, it can lend support to lose rock or soil with it. Erosion by running water forms the familiar gullies and V-shaped valleys that cut into most landscapes. The forces of the running water removes lose particles formed by weathering. In the process, gullies and valleys are lengthened, widened, and deepened. Often, water overflows the banks of the gullies or river channels, resulting in floods. Each new flood carries more material away to increase the size of the valley. Meanwhile, weathering loosens ever more material so the process continues.
Erosion by glacial ice is less common, but it can cause the greatest landscape changes in the shortest amount of time. Glacial ice forms in a region where snow fails to melt in the spring and summer and instead builds of a functional dynamic spread of ice. For major glaciers to form, this lack of snowmelt has to occur for many years in areas with high precipitation. As ice accumulates and thickens, it flows as a solid mass. As it flows, it has a tremendous capacity to erode soil and even solid rock. Ice is a major factor in shaping some landscapes, especially mountainous regions. Glacial ice provides much of the spectacular scenery in these regions. Features such as horns (sharp mountain peaks), Arêtes (sharp ridges), glacially formed lakes, and U-shaped valleys are all the results of glacial erosion. Wind is an important cause of erosion only in arid (dry) regions. Wind carries sand and dust, which can scour even solid rock. Many factors determine the rate and kind of erosion that occurs in a given area. The climate of an area determines the distribution, amount, and kind of precipitation that the area receives and thus the type and rate of weathering. An area with an arid climate erodes differently than an area with a humid climate. The elevation of an area also plays a role by determining the potential energy of running water. The higher the elevation the more energetic water will flow due to the force of gravity. The type of bedrock in an area (sandstone, granite, or shale) can determine the shapes of valleys and slopes, and the depth of streams.
A landscape's geologic age-that is, how long current conditions of weathering and erosion have affected the area-determines its overall appearance. Younger landscapes tend to be more rugged and angular in appearance. Older landscapes have a tendency to have more rounded slopes and hills. The oldest landscapes tend to be low-lying with broad, open river valleys and low, rounded hills. The overall effect of the wearing down of an area is to level the land; the tendency is toward the reduction of all land surfaces to sea level.
Opposing this tendency toward a levelling is a force responsible for raising mountains and plateaus and for creating new landmasses. These changes to Earth's surface occur in the outermost solid portion of Earth, known as the lithosphere. The lithosphere consists of the crust and another region known as the upper mantle and is approximately 65 to 100 km. (40 to 60 mi.) thick. Compared with the interior of the Earth, however, this region is moderately thin. The lithosphere is thinner in proportion to the whole Earth than the skin of an apple is to the whole apple.
Scientists believe that the lithosphere is broken into a series of plates, or segments. According to the theory of plate tectonics, these plates move around on Earth's surface over long periods. Tectonics comes from the Greek word, tektonikos, which means ‘builder'.
According to the theory, the lithosphere is divided into large and small plates. The largest plates include the Pacific plate, the North American plate, the Eurasian plate, the Antarctic plate, the Indo-Australian plate, and the African plate. Smaller plates include the Cocos plate, the Nazca plate, the Philippine plate, and the Caribbean plate. Plate sizes vary a great deal. The Coco's plate is 2,000 km (1,000 mi) wide, while the Pacific plate is nearly 14,000 km (nearly 9,000 mi) wide.
These plates move in three different ways in relation to each other. They pull apart or move away from each other, they collide or move against each other, or they slide past each other as they move sideways. The movement of these plates helps explain many geological events, such as earthquakes and volcanic eruptions and mountain building and the formation of the oceans and continents.
When the plates pull apart, two types of phenomena come about, depending on whether the movement takes place in the oceans or on land. When plates pull apart on land, deep valleys known as rift valleys form. An example of a rift valley is the Great Rift Valley that extends from Syria in the Middle East to Mozambique in Africa. When plates pull apart in the oceans, long, sinuous chains of volcanic mountains called mid-ocean ridges form, and new sea-floor is created at the site of these ridges. Rift valleys are also present along the crests of the mid-ocean ridges.
Most scientists believe that gravity and heat from the interior of the Earth cause the plates to move apart and to create new sea-floor. According to this explanation, molten rock known as magma rises from Earth's interior to form hot spots beneath the ocean floor. As two oceanic plates pull apart from each other in the middle of the oceans, a crack, or rupture, appear and forms the mid-ocean ridges. These ridges exist in all the worlds' ocean basins and resemble the seams of a baseball. The molten rock rises through these cracks and creates new sea-floor.
When plates collide or push against each other, regions called convergent plate margins form. Along these margins, one plate is usually forced to dive below the other. As that plate dives, it triggers the melting of the surrounding lithosphere and a region just below is known as the asthenosphere. These pockets of molten crust rise behind the margin through the overlying plate, creating curved chains of volcanoes known as arcs. This process is called Subduction.
If one plate consists of oceanic crust and the other consists of continental crust, the denser oceanic crust will dive below the continental crust. If both plates are oceanic crust, then either may be subducted. If both are continental crust, Subduction can continue for a brief while but will eventually ends because continental crust is not dense enough to be forced very far into the upper mantle.
The results of this Subduction process are readily visible on a map showing that 80 percent of the world's volcanoes rim the Pacific Ocean where plates are colliding against each other. The Subduction zone created by the collision of two oceanic plates-the Pacific plate and the Philippine plate-can also create a trench. Such a trench resulted in the formation of the deepest point on Earth, the Mariana Trench, which is estimated to be 11,033 m's (36,198 ft) below sea level.
On the other hand, when two continental plates collide, mountain building occurs. The collision of the Indo-Australian plate with the Eurasian plate has produced the Himalayan Mountains. This collision resulted in the highest point of Earth, Mount Everest, which is 8,850 m's (29,035 ft) above sea level.
Finally, some of Earth's plates neither collide nor pull apart yet slips past each other. These regions are convened by the transforming margins. Few volcanoes occur in these areas because neither plate is forced down into Earth's interior and little melting occurs. Earthquakes, however, are abundant as the two rigid plates slide past each other. The San Andreas Fault in California is a well-known example of a transformed margin.
The movement of plates occurs at a slow pace, at an average rate of only 2.5 cm (one in) per year. Still, over millions of years this gradual movement results in radical changes. Current plate movement is making the Pacific Ocean and Mediterranean Sea smaller, the Atlantic Ocean larger, and the Himalayan Mountains higher.
The interior of Earth plays an important role in plate tectonics. Scientists believe it is also responsible for Earth's magnetic field. This field is vital to life because it shields the planet's surface from harmful cosmic rays and from a steady stream of energetic particles from the Sun known as the solar wind.
Earth's interior consists of the mantle and the core. The mantle and core make up by far the largest part of Earth's mass. The distance from the base of the crust to the centre of the core is about 6,400 km (about 4,000 mi).
Scientists have learned about Earth's interior by studying rocks that formed in the interior and rose to the surface. The study of meteorites, which are believed to be made of the same material that formed the Earth and its interior, has also offered clues about Earth's interior. Finally, seismic waves generated by earthquakes send geophysicists information about the composition of the interior. The sudden movement of rocks during an earthquake causes vibrations that transmit energy through the Earth as waves. The way these waves proceed through the interior of Earth reveals the nature of materials inside the planet.
The mantle consists of three parts: the lower part of the lithosphere, the region below it known as the asthenosphere, and the region below the asthenosphere called the lower mantle. The entire mantle extends from the base of the crust to a depth of about 2,900 km (about 1,800 mi). Scientists believe the asthenosphere is made up of mushy plastic-like rock with pockets of molten rock. The term asthenosphere is derived from Greek and means ‘a weak layer'. The asthenosphere's soft, plastic quality allows plates in the lithosphere above it to shift and slide on top of the asthenosphere. This shifting of the lithosphere's plates is the source of most tectonic activity. The asthenosphere is also the source of the basaltic magma that makes up much of the oceanic crust and rises through volcanic vents on the ocean floor.
The mantle consists of mostly solid iron-magnesium silicate rock mixed with many other minor components including radioactive elements. However, even this solid rock can flow like a ‘sticky' liquid when it is subjected to enough heat and pressure.
The core is divided into two parts, the outer core and the inner core. The outer core is about 2,260 km (about 1,404 mi) thick. The outer core is a liquid region composed mostly of iron, with smaller amounts of nickel and sulfur in liquid form. The inner core is about 1,220 km (about 758 mi) thick. The inner core is solid and is composed of iron, nickel, and sulfur in solid form. The inner core and the outer core also contain a small percentage of radioactive material. The existence of radioactive material is one source of heat in Earth's interior because as radioactive material decays, it gives off heat. Temperatures in the inner core may be as high as 6650°C's (12,000°F).
Scientists believe that Earth's liquid iron core aids to make over a magnetic field that surrounds Earth and shields the planet from harmful cosmic rays and the Sun's solar wind. The idea that Earth is like a giant magnet was first proposed in 1600 by English physician and natural philosopher William Gilbert. Gilbert proposed the idea to explain why the magnetized needle in a compass point north. According to Gilbert, Earth's magnetic field creates a magnetic north pole and a magnetic south pole. The magnetic poles do not correspond to the geographic North and South poles, however. Moreover, the magnetic poles wander and are not always in the same place. The north magnetic pole is currently close to Ellef Ringnes Island in the Queen Elizabeth Islands near the boundary of Canada's Northwest Territories with Nunavut. The magnetic south poles lies just off the coast of Wilkes Land, Antarctica.
Not only do the magnetic poles wander, but they also reverse their polarity-that is, the north magnetic pole becomes the south magnetic pole and vice versa. Magnetic reversals have occurred at least 170 times over the past 100 million years. The reversals occur on average about every 200,000 years and take place gradually over a period of several thousand years. Scientists still do not understand why these magnetic reversals occur but think they may be related to Earth's rotation and changes in the flow of liquid iron in the outer core.
Some scientists theorize that the flow of liquid iron in the outer core sets up electrical currents that produce Earth's magnetic field. Known as the dynamo theory, this theory may be the best explanation yet for the origin of the magnetic field. Earth's magnetic field operates in a region above Earth's surface known as the magnetosphere. The magnetosphere is shaped in some respects like a teardrop with a long tail that trails away from the Earth due to the force of the solar wind.
Inside the magnetosphere are the Van Allen's radiation belts, named for the American physicist James A. Van Allen who discovered them in 1958. The Van Allen belts are regions where charged particles from the Sun and from cosmic rays are trapped and sent into spiral paths resembling Earth's magnetic field. The radiation belts by that shield Earth's surface from these highly energetic particles. Occasionally, however, due to extremely strong magnetic fields on the Sun's surface, which are visible as sunspots, a brief burst of highly energetic particles streams along with the solar wind. Because Earth's magnetic field lines converge and are closest to the surface at the poles, some of these energetic particles sneak through and interact with Earth's atmosphere, creating the phenomenon known. Most scientists believe that the Earth, Sun, and all of the other planets and moons in the solar system took form of about 4.6 billion years. Originating endurably in some lengthily endurance from dust and giant gaseous particles-wave substances known as the solar nebula. The gas and dust in this solar nebula originated in a star that ended its life in an explosion known as a supernova. The solar nebula consisted principally of hydrogen, the lightest element, but the nebula was also seeded with a smaller percentage of heavier elements, such as carbon and oxygen. All of the chemical elements we know were originally made in the star that became a supernova. Our bodies are made of these same chemical elements. Therefore, all of the elements in our solar system, including all of the elements in our bodies, originally came from this star-seeded solar nebula.
Due to the force of gravity tiny clumps of gas and dust began to form in the early solar nebula. As these clumps came together and grew larger, they caused the solar nebula to contract in on itself. The contraction caused the cloud of gas and dust to flatten in the shape of a disc. As the clumps continued to contract, they became very dense and hot. Eventually the s of hydrogen became so dense that they began to fuse in the innermost part of the cloud, and these nuclear reactions gave birth to the Sun. The fusion of hydrogen s in the Sun is the source of its energy.
Many scientists favour the planetesimal theory for how the Earth and other planets formed out of this solar nebula. This theory helps explain why the inner planets became rocky while the outer planets, except Pluto, are made up mostly of gases. The theory also explains why all of the planets orbit the Sun in the same plane.
According to this theory, temperatures decreased with increasing distance from the centre of the solar nebula. In the inner region, where Mercury, Venus, Earth, and Mars formed, temperatures were low enough that certain heavier elements, such as iron and the other heavy compounds that make up rock, could condense of departing - that is, could change from a gas to a solid or liquid. Due to the force of gravity, small clumps of this rocky material eventually came with the dust in the original solar nebula to form protoplanets or planetesimals (small rocky bodies). These planetesimals collided, broke apart, and re-formed until they became the four inner rocky planets. The inner region, however, was still too hot for other light elements, such as hydrogen and helium, to be retained. These elements could only exist in the outermost part of the disc, where temperatures were lower. As a result two of the outer planets-Jupiter and Saturn - are by and large made of hydrogen and helium, which are also the dominant elements in the atmospheres of Uranus and Neptune.
Within the planetesimal Earth, heavier matter sank to the centre and lighter matter rose toward the surface. Most scientists believe that Earth was never truly molten and that this transfer of matter took place in the solid state. Much of the matter that went toward the centre contained radioactive material, an important source of Earth's internal heat. As heavier material moved inward, lighter material moved outward, the planet became layered, and the layers of the core and mantle were formed. This process is called differentiation.
Not long after they formed, more than four billion years ago, the Earth and the Moon underwent a period when they were bombarded by meteorites, the rocky debris left over from the formation of the solar system. The impact craters created during this period of heavy bombardment are still visible on the Moon's surface, which is unchanged. Earth's craters, however, were long ago erased by weathering, erosion, and mountain building. Because the Moon has no atmosphere, its surface has not been subjected to weathering or erosion. Thus, the evidence of meteorite bombardment remains.
Energy released from the meteorite impacts created extremely high temperatures on Earth that melted the outer part of the planet and created the crust. By four billion years ago, both the oceanic and continental crust had formed, and the oldest rocks were created. These rocks are known as the Acasta Gneiss and are found in the Canadian territory of Nunavut. Due to the meteorite bombardment, the early Earth was too hot for liquid water to exist and so existing was impossible for life.
Geologists divide the history of the Earth into three eons: the Archaean Eon, which lasted from around four billion to 2.5 billion years ago; the Proterozoic Eon, which lasted from 2.5 billion to 543 million years ago; and the Phanerozoic Eon, which lasted from 543 million years ago to the present. Each eon is subdivided into different eras. For example, the Phanerozoic Eon includes the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. In turn, eras are further divided into periods. For example, the Paleozoic Era includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods.
The Archaean Eon is subdivided into four eras, the Eoarchean, the Paleoarchean, the Mesoarchean, and the Neoarchean. The beginning of the Archaean is generally dated as the age of the oldest terrestrial rocks, which are about four billion years old. The Archaean Eon came to an end 2.5 billion years ago when the Proterozoic Eon began. The Proterozoic Eon is subdivided into three eras: the Paleoproterozoic Era, the Mesoproterozoic Era, and the Neoproterozoic Era. The Proterozoic Eon lasted from 2.5 billion years ago to 543 million years ago when the Phanerozoic Eon began. The Phanerozoic Eon is subdivided into three eras: the Paleozoic Era from 543 million to 248 million years ago, the Mesozoic Era from 248 million to 65 million years ago, and the Cenozoic Era from 65 million years ago to the present.
Geologists base these divisions on the study and dating of rock layers or strata, including the fossilized remains of plants and animals found in those layers. Residing until the late 1800s scientists could only determine the relative ages of rock strata. They knew that overall the top layers of rock were the youngest and formed most recently, while deeper layers of rock were older. The field of stratigraphy shed much light on the relative ages of rock layers.
The study of fossils also enabled geologists to set the relative ages of different rock layers. The fossil record helped scientists determine how organisms evolved or when they became extinct. By studying rock layers around the world, geologists and paleontologists saw that the remains of certain animal and plant species occurred in the same layers, but were absent or altered in other layers. They soon developed a fossil index that also helped determine the relative ages of rock layers.
Beginning in the 1890s, scientists learned that radioactive elements in rock decay at a known rate. By studying this radioactive decay, they could detect an absolute age for rock layers. This type of dating, known as radiometric dating, confirmed the relative ages determined through stratigraphy and the fossil index and assigned absolute ages to the various strata. As a result scientists can assemble Earth's geologic time scale from the Archaean Eon to the present.
The Precambrian is a time span that includes the Archaean and Proterozoic eons began roughly four billion years ago. The Precambrian marks the first formation of continents, the oceans, the atmosphere, and life. The Precambrian represents the oldest chapter in Earth's history that can still be studied. Very little remains of Earth from the period of 4.6 billion to about four billion years ago due to the melting of rock caused by the early period of meteorite bombardment. Rocks dating from the Precambrian, however, have been found in Africa, Antarctica, Australia, Brazil, Canada, and Scandinavia. Some zircon mineral grains deposited in Australian rock layers have been dated to 4.2 billion years.
The Precambrian is also the longest chapter in Earth's history, spanning a period of about 3.5 billion years. During this time frame, the atmosphere and the oceans formed from gases that escaped from the hot interior of the planet because of widespread volcanic eruptions. The early atmosphere consisted primarily of nitrogen, carbon dioxide, and water vapour. As Earth continued to cool, the water vapour condensed out and fell as precipitation to form the oceans. Some scientists believe that much of Earth's water vapour originally came from comets containing frozen water that struck Earth during meteorite bombardment.
By studying 2-billion-year-old rocks found in northwestern Canada, as well as 2.5-billion-year-old rocks in China, scientists have found evidence that plate tectonics began shaping Earth's surface as early as the middle Precambrian. About a billion years ago, the Earth's plates were entered around the South Pole and formed a super-continent called Rodinia. Slowly, pieces of this super-continent broke away from the central continent and travelled north, forming smaller continents.
Life originated during the Precambrian. The earliest fossil evidence of life consists of Prokaryotes, one-celled organisms that lacked a nucleus and reproduced by dividing, a process known as asexual reproduction. Asexual division meant that a prokaryote's hereditary material was copied unchanged. The first Prokaryotes were bacteria known as archaebacteria. Scientists believe they came into existence perhaps as early as 3.8 billion years ago, by 3.5 billion years ago, and where anaerobic—that is, they did not require oxygen to produce energy. Free oxygen barely existed in the atmosphere of the early Earth.
Archaebacteria were followed about 3.46 billion years ago by another type of prokaryote known as Cyanobacteria or blue
-green algae. These Cyanobacteria gradually introduced oxygen in the atmosphere because of photosynthesis. In shallow tropical waters, Cyanobacteria formed mats that grew into humps called stromatolites. Fossilized stromatolites have been found in rocks in the Pilbara region of western Australia that are more than 3.4 billion years old and in rocks of the Gunflint Chert region of northwest Lake Superior that are about 2.1 billion years old.
For billions of years, life existed only in the simple form of Prokaryotes. Prokaryotes were followed by the relatively more advanced eukaryotes, organisms that have a nucleus in their cells and that reproduces by combining or sharing their heredity makeup rather than by simply dividing. Sexual reproduction marked a milestone in life on Earth because it created the possibility of hereditary variation and enabled organisms to adapt more easily to a changing environment. The inordinate branch of Precambrian time occurred some 560 million to 545 million years ago and seeing an appearance of an intriguing group of fossil organisms known as the Ediacaran fauna. First discovered in the northern Flinders Range region of Australia in the mid-1940s and subsequently found in many locations throughout the world, these strange fossils may be the precursors of many fossil groups that were to explode in Earth's oceans in the Paleozoic Era.
At the start of the Paleozoic Era about 543 million years ago, an enormous expansion in the diversity and complexity of life occurred. This event took place in the Cambrian Period and is called the Cambrian explosion. Nothing like it has happened since. Most of the major groups of animals we know today made their first appearance during the Cambrian explosion. Most of the different ‘body plans' found in animals today-that is, the way of an animal's body is designed, with heads, legs, rear ends, claws, tentacles, or antennae-also originated during this period.
Fishes first appeared during the Paleozoic Era, and multicellular plants began growing on the land. Other land animals, such as scorpions, insects, and amphibians, also originated during this time. Just as new forms of life were being created, however, other forms of life were going out of existence. Natural selection meant that some species can flourish, while others failed. In fact, mass extinctions of animal and plant species were commonplace.
Most of the early complex life forms of the Cambrian explosion lived in the sea. The creation of warm, shallow seas, along with the buildup of oxygen in the atmosphere, may have aided this explosion of life forms. The shallow seas were created by the breakup of the super-continent Rodinia. During the Ordovician, Silurian, and Devonian periods, which followed the Cambrian Period and lasted from 490 million to 354 million years ago, some continental pieces that had broken off Rodinia collided. These collisions resulted in larger continental masses in equatorial regions and in the Northern Hemisphere. The collisions built several mountain ranges, including parts of the Appalachian Mountains in North America and the Caledonian Mountains of northern Europe.
Toward the close of the Paleozoic Era, two large continental masses, Gondwanaland to the south and Laurasia to the north, faced each other across the equator. Their slow but eventful collision during the Permian Period of the Paleozoic Era, which lasted from 290 million to 248 million years ago, assembled the super-continent Pangaea and resulted in some grandest mountains in the history of Earth. These mountains included other parts of the Appalachians and the Ural Mountains of Asia. At the close of the Paleozoic Era, Pangaea represented more than 90 percent of all the continental landmasses. Pangaea straddled the equator with a huge mouth-like opening that faced east. This opening was the Tethys Ocean, which closed as India moved northward creating the Himalayas. The last remnants of the Tethys Ocean can be seen in today's Mediterranean Sea.
The Paleozoic ended with a major extinction event, when perhaps as many as 90 percent of all plant and animal species died out. The reason is not known for sure, but many scientists believe that huge volcanic outpourings of lavas in central Siberia, coupled with an asteroid impact, were joint contributing factors.
The Mesozoic Era, beginning 248 million years ago, is often characterized as the Age of Reptiles because reptiles were the dominant life forms during this era. Reptiles dominated not only on land, as dinosaurs, but also in the sea, as the plesiosaurs and ichthyosaurs, and in the air, as pterosaurs, which were flying reptiles.
The Mesozoic Era is divided into three geological periods: the Triassic, which lasted from 248 million to 206 million years ago; the Jurassic, from 206 million to 144 million years ago; and the Cretaceous, from 144 million to 65 million years ago. The dinosaurs emerged during the Triassic Period and was one of the most successful animals in Earth's history, lasting for about 180 million years before going extinct at the end of the Cretaceous Period. The first birds and mammals and the first flowering plants also appeared during the Mesozoic Era. Before flowering plants emerged, plants with seed-bearing cones known as conifers were the dominant form of plants. Flowering plants soon replaced conifers as the dominant form of vegetation during the Mesozoic Era.
The Mesozoic was an eventful era geologically with many changes to Earth's surface. Pangaea continued to exist for another 50 million years during the early Mesozoic Era. By the early Jurassic Period, Pangaea began to break up. What is now South America begun splitting from what is now Africa, and in the process the South Atlantic Ocean formed? As the landmass that became North America drifted away from Pangaea and moved westward, a long Subduction zone extended along North America's western margin. This Subduction zone and the accompanying arc of volcanoes extended from what is now Alaska to the southern tip of South America. A great deal of this featured characteristic is called the American Cordillera, and exists today as the eastern margin of the Pacific Ring of Fire.
During the Cretaceous Period, heat continued to be released from the margins of the drifting continents, and as they slowly sank, vast inland seas formed in much of the continental interiors. The fossilized remains of fishes and marine mollusks called ammonites can be found today in the middle of the North American continent because these areas were once underwater. Large continental masses broke off the northern part of southern Gondwanaland during this period and began to narrow the Tethys Ocean. The largest of these continental masses, present-day India, moved northward toward its collision with southern Asia. As both the North Atlantic Ocean and South Atlantic Ocean continued to open, North and South America became isolated continents for the first time in 450 million years. Their westward journey resulted in mountains along their western margins, including the Andes of South America.
The Cenozoic Era, beginning about 65 million years ago, is the period when mammals became the dominant form of life on land. Human beings first appeared in the later stages of the Cenozoic Era. In short, the modern world as we know it, with its characteristic geographical features and its animals and plants, came into being. All of the continents that we know today took shape during this era.
A single catastrophic event may have been responsible for this relatively abrupt change from the Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid or comet struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic eras, causing the extinction of many forms of life, including the dinosaurs. Evidence of this collision came with the discovery of a large impact crater off the coast of Mexico's Yucatán Peninsula and the worldwide finding of iridium, a metallic element rare on Earth but abundant in meteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of the dinosaurs opened the way for mammals to become the dominant land animals.
The Cenozoic Era is divided into the Tertiary and the Quaternary periods. The Tertiary Period lasted from about 65 million to about 1.8 million years ago. The Quaternary Period began about 1.8 million years ago and continued to the present day. These periods are further subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and the Holocene, from 10,000 years ago to the present.
Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continents were all clearly outlined. India and other continental masses began colliding with southern Asia to form the Himalayas. Africa and a series of smaller micro-continents began colliding with southern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resemble today's Mediterranean Sea. As the Tethys continued to narrow, the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared as a new island in later Tertiary time, and its active volcanism today shows that sea-floor spreading is still causing the country to grow.
Late in the Tertiary Period, about six million years ago, humans began to evolve in Africa. These early humans began to migrate to other parts of the world between two million and 1.7 million years ago.
The Quaternary Period marks the onset of the great ice ages. Many times, perhaps at least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and parts of Asia. The glaciers eroded considerable amounts of material that stood in their paths, gouging out U-shaped valleys. Anically modern human beings, known as Homo sapiens, became the dominant form of life in the Quaternary Period. Most anthropologists (scientists who study human life and culture) believe that Anically modern humans originated only recently in Earth's 4.6-billion-year history, within the past 200,000 years.
With the rise of human civilization about 8,000 years ago and especially since the Industrial Revolution in the mid-1700s, human beings began to alter the surface, water, and atmosphere of Earth. In doing so, they have become active geological agents, not unlike other forces of change that influence the planet. As a result, Earth's immediate future depends largely on the behaviour of humans. For example, the widespread use of fossil fuels is releasing carbon dioxide and other greenhouse gases into the atmosphere and threatens to warm the planet's surface. This global warming could melt glaciers and the polar ice caps, which could flood coastlines around the world and many island nations. In effect, the carbon dioxide removed from Earth's early atmosphere by the oceans and by primitive plant and animal life, and subsequently buried as fossilized remains in sedimentary rock, is being released back into the atmosphere and is threatening the existence of living things.
Even without human intervention, Earth will continue to change because it is geologically active. Many scientists believe that some of these changes can be predicted. For example, based on studies of the rate that the sea-floor is spreading in the Red Sea, some geologists predict that in 200 million years the Red Sea will be the same size as the Atlantic Ocean is today. Other scientists predict that the continent of Asia will break apart millions of years from now, and as it does, Lake Baikal in Siberia will become a vast ocean, separating two landmasses that once made up the Asian continent.
In the far, far distant future, however, scientists believe that Earth will become an uninhabitable planet, scorched by the Sun. Knowing the rate at which nuclear fusion occurs in the Sun and knowing the Sun's mass, astrophysicists (scientists who study stars) have calculated that the Sun will become brighter and hotter about three billion years from now, when it will be hot enough to boil Earth's oceans away. Based on studies of how other Sun-like stars have evolved, scientists predict that the Sun will become a red giant, a star with a very large, hot atmosphere, about seven billion years from now. As a red giant the Sun's outer atmosphere will expand until it engulfs the planet Mercury. The Sun will then be 2,000 times brighter than it is now and so hot it will melt Earth's rocks. Earth will end its existence as a burnt cinder.
Three billion years is the life span of millions of human generations, however. Perhaps by then, humans will have learned how to journey through and beyond the solar system and begin to colonize other planets in our galaxy, and find yet of another place to call ‘home'.
The Cenozoic era (65 million years ago to the present time) is divided into the Tertiary period (65 million to 1.6 million years ago) and the Quaternary period (1.6 million years ago to the present). However, because scientists have so much more information about this era, they tend to focus on the epochs that make up each period. During the first part of the Cenozoic era, an abrupt transition from the Age of Reptiles to the Age of Mammals occurred, when the large dinosaurs and other reptiles that had dominated the life of the Mesozoic era disappeared
Index fossils of the Cenozoic tend to be microscopic, such as the tiny shells of foraminifera. They are commonly used, along with varieties of pollen fossils, to date the different rock strata of the Cenozoic era.
The Paleocene epoch (65 million to 55 million years ago) marks the beginning of the Cenozoic era. Seven groups of Paleocene mammals are known. All of them appear to have developed in northern Asia and to have migrated to other parts of the world. These primitive mammals had many features in common. They were small, with no species exceeding the size of a small modern bear. They were four-footed, with five toes on each foot, and they walked on the soles of their feet. Most of them had slim heads with narrow muzzles and small brain cavities. The predominant mammals of the period were members of three groups that are now extinct. They were the creodonts, which were the ancestors of modern carnivores; the amblypods, which were small, heavy-bodied animals; and the condylarths, which were light-bodied herbivorous animals with small brains. The Paleocene groups that have survived are the marsupials, the insectivores, the primates, and the rodents
During the Eocene epoch (55 million to 38 million years ago), most direct evolutionary ancestors of modern animals appeared. Among these animals - all of which were small in stature-were the horse, rhinoceros, camel, rodent, and monkey. The creodonts and amblypods continued to develop during the epoch, but the condylarths became extinct before it ended. The first aquatic mammals, ancestors of modern whales, also appeared in Eocene times, as did such modern birds as eagles, pelicans, quail, and vultures. Changes in vegetation during the Eocene epoch were limited chiefly to the migration of types of plants in response to climate changes.
During the Oligocene epoch (38 million to 24 million years ago), most of the archaic mammals from earlier epochs of the Cenozoic era disappeared. In their place appeared representatives of many of modern mammalian groups. The creodonts became extinct, and the first true carnivores, resembling dogs and cats, evolved. The first anthropoid apes also lived during this time, but they became extinct in North America by the end of the epoch. Two groups of animals that are now extinct flourished during the Oligocene epoch: the titanotheres, which are related to the rhinoceros and the horse; and the oreodonts, which were small, dog-like, grazing animals.
The development of mammals during the Miocene epoch (24 million to five million years ago) was influenced by an important evolutionary development in the plant kingdom: the first appearance of grasses. These plants, which were ideally suited for forage, encouraged the growth and development of grazing animals such as horses, camels, and rhinoceroses, which were abundant during the epoch. During the Miocene epoch, the mastodon evolved, and in Europe and Asia a gorilla-like ape, Dryopithecus, was common. Various types of carnivores, including cats and wolflike dogs, ranged over many parts of the world.
The paleontology of the Pliocene epoch (five million to 1.6 million years ago) does not differ much from that of the Miocene, although the period is regarded by many zoologists as the climax of the Age of Mammals. The Pleistocene Epoch (1.6 million to 10,000 years ago) in both Europe and North America was marked by an abundance of large mammals, most of which were basically modern in type. Among them were buffalo, elephants, mammoths, and mastodons. Mammoths and mastodons became extinct before the end of the epoch. In Europe, antelope, lions, and hippopotamuses also appeared. Carnivores included badgers, foxes, lynx, otters, pumas, and skunks, as well as now-extinct species such as the giant saber-toothed tiger. In North America, the first bears made their appearance as migrants from Asia. The armadillo and ground sloth migrated from South America to North America, and the musk-ox ranged southward from the Arctic regions. Modern human beings also emerged during this epoch.
The Cenozoic Era, beginning about 65 million years ago, is the period when mammals became the dominant form of life on land. Human beings first appeared in the later stages of the Cenozoic Era. In short, the modern world as we know it, with its characteristic geographical features and its animals and plants, came into being. All of the continents that we know today took shape during this era.
A single catastrophic event may have been responsible for this relatively abrupt change from the Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid or comet struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic eras, causing the extinction of many forms of life, including the dinosaurs. Evidence of this collision came with the discovery of a large impact crater off the coast of Mexico's Yucatán Peninsula and the worldwide finding of iridium, a metallic element rare on Earth but abundant in meteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of the dinosaurs opened the way for mammals to become the dominant land animals.
The Cenozoic Era is divided into the Tertiary and the Quaternary periods. The Tertiary Period lasted from about 65 million to about 1.8 million years ago. The Quaternary Period began about 1.8 million years ago and continued to the present day. These periods are further subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and the Holocene, from 10,000 years ago to the present.
Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continents were all clearly outlined. India and other continental masses began colliding with southern Asia to form the Himalayas. Africa and a series of smaller micro-continents began colliding with southern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resemble today's Mediterranean Sea. As the Tethys continued to narrow, the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared as a new island in later Tertiary time, and its active volcanism today suggests that sea-floor spreading be still causing the country to grow.
Late in the Tertiary Period, about six million years ago, humans began to evolve in Africa. These early humans began to migrate to other parts of the world between two or 1.7 million years ago.
The Quaternary Period marks the onset of the great ice ages. Many times, perhaps at least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and parts of Asia. The glaciers eroded considerable amounts of material that stood in their paths, gouging out U-shaped valleys. Anically modern human beings, known as Homo sapiens, became the dominant form of life in the Quaternary Period. Most anthropologists (scientists who study human life and culture) believe that Anically modern humans originated only recently in Earth's 4.6-billion-year history, within the past 200,000 years.
Most biologists agree that animals evolved from simpler single-celled organisms. Exactly how this happened is unclear, because few fossils have been left to record the sequence of events. Faced with this lack of fossil evidence, researchers have attempted to piece together animal origins by examining the single-celled organisms alive today.
Modern single-celled organisms are classified into two kingdoms: the Prokaryotes and protists. Prokaryotes, which include bacteria, are very simple organisms, and lack many features seen in animal cells. Protists, on the other hand, are more complex, and their cells contain all the specialized structures, or organelles, found in the cells of animals. One protist group, the choanoflagellates or collar flagellates, contains organisms that bear a striking resemblance to cells that are found in sponges. Most choanoflagellates live on their own, but significantly, some form permanent groups or colonies.
This tendency to form colonies are widely believed to have been an important stepping stone on the path to animal life. The next step in evolution would have involved a transition from colonies of independent cells to colonies containing specialized cells that were dependent on each other for survival. Once this development had occurred, such colonies would have effectively become single organisms. Increasing specialization among groups of cells could then have created tissues, triggering the long and complex evolution of animal bodies.
This conjectural sequence of events probably occurred along several parallel paths. One path led to the sponges, which retain a collection of primitive features that set them apart from all animals. Another path led to two major subdivisions of the animal kingdom: the Protostomes, which include arthropods, annelid worms, mollusks, and cnidarians; and the deuterostomes, which include echinoderms and chordates. Protostomes and deuterostomes differ fundamentally in the way they develop as embryos, strongly suggesting that they split from each other a long time ago.
Animal life first appeared perhaps a billion years ago, but for a long time after this, the fossil record remains almost blank. Fossils exist that seem to show burrows and other indirect evidence for animal life, but the first direct evidence of animals themselves appears about 650 million years ago, toward the end of the Precambrian period. At this time, the animal kingdom stood on the threshold of a great explosion in diversity. By the end of the Cambrian Period, 150 million years later, all of the main types of animal life existing today had become established.
When the first animals evolved, dry land was probably without any kind of life, except possibly bacteria. Without terrestrial plants, land-based animals would have had nothing to eat. Nevertheless, when plants took up life on land more than 400 million years ago, that situation changed, and animals evolved that could use this new source of food. The first land animals included primitive wingless insects and probably a range of soft-bodied invertebrates that have not left fossil remains. The first vertebrates to move onto land were the amphibians, which appeared about 370 million years ago.
For all animals, life on land involved meeting some major challenges. Foremost among these was the need to conserve water and the need to extract oxygen from the air. Another problem concerned the effects of gravity. Water buoys of living things, but air, which is 750 times less dense than water, generates almost no buoyancy at all. To function effectively on land, animals needed support.
In soft-bodied land animals such as earthworms, this support is provided by a hydrostatic skeleton, which works by internal pressure. The animal's body fluids press out against its skin, giving the animal its shape. In insects and other arthropods, support is provided by the exoskeleton (external skeletons), while in vertebrates it is provided by bones. Exoskeletons can play a double role by helping animals to conserve water, but they have one important disadvantage: unlike an internal bony skeleton, their weight increases very rapidly as they get bigger, eventually making them too heavy to move. This explains why insects have all remained relatively small, while some vertebrates have reached very large sizes.
Like other living things, animals evolve by adapting to and exploiting their surroundings. In the billion-year history of animal life, this process could use resources in a different way. Some of these species are surviving today, but these are a minority; an even greater number are extinct, having lost the struggle for survival
Speciation, the birth of new species, usually occurs when a group of living things becomes isolated from others of their kind. Once this has occurred, the members of the group follow their own evolutionary path and adapt in ways that make them increasingly distinct. After a long period-typically thousand of the years-unique features were to mean that they can no longer breed within the former circle of relative relations. At this point, a new species comes into being.
In animals, this isolation can come about in several different ways. The simplest form, geographical isolation, occurs when members of an original species become separated by a physical barrier. One example of such a barrier is the open sea, which isolates animals that have been accidentally stranded on remote islands. As the new arrivals adapt to their adopted home, they become ever more distinct from their mainland relatives. Sometimes the result is a burst of adaptive radiation, which produces several different species. In the Hawaiian Islands, for example, 22 species of honey-creepers have evolved from a single pioneering species of a finch-like bird.
Another type of isolation is thought to occur where there is no physical separation. Here, differences in behaviour, such as mate selection, may sometimes help to split a single species into distinct groups. If the differences persist for a some duration, in that they live long enough new species are created.
The fate of a new species depends very much on the environment in which it evolved. If the environment is stable and no new competitors appear on the scene, an animal species may change very little in hundreds of thousands of years. Nevertheless, if the environment changes rapidly and competitors arrive from outside, the struggle for survival is much more intense. In these conditions, either a species change, or it eventually becomes extinct.
During the history of animal life, on at least five occasions, sudden environmental change has triggered simultaneous extinction on a massive scale. One of these mass extinctions occurred at the end of the Cretaceous Period, about 65 million years ago, killing all dinosaurs and perhaps two-thirds of marine species. An even greater mass extinction took place at the end of the Permian Period, about 200 million years ago. Many biologists believe that we are at present living in a sixth period of mass extinction, this time triggered by human beings.
Compared with plants, animals make up only a small part of the total mass of living matter on earth. Despite this, they play an important part in shaping and maintaining natural environments.
Many habitats are directly influenced by the way animals live. Grasslands, for example, exist partly because grasses and grazing animals have evolved a close partnership, which prevents other plants from taking hold. Tropical forests also owe their existence to animals, because most of their trees rely on animals to distribute their pollen and seeds. Soil is partly the result of animal activity, because earthworms and other invertebrates help to break down dead remains and recycle the nutrients that they contain. Without its animal life, the soil would soon become compacted and infertile.
By preying on each other, animals also help to keep their own numbers in check. This prevents abrupt population peaks and crashes and helps to give living systems a built-in stability. On a global scale, animals also influence some of the nutrient cycles on which almost all life depends. They distribute essential mineral elements in their waste, and they help to replenish the atmosphere's carbon dioxide when they breathe. This carbon dioxide is then used by plants as they grow.
Until relatively recently in human history, people existed as nomadic hunter-gatherers. They used animals primarily as a source of food and for raw materials that could be used for making tools and clothes. By today's standards, hunter-gatherers were equipped with rudimentary weapons, but they still had a major impact on the numbers of some species. Many scientists believe, for example, that humans were involved in a cluster of extinctions that occurred about 12,000 years ago in North America. In less than a millennium, two-thirds of the continent's large mammal species disappeared.
This simple relationship between people and animals changed with domestication, which also began about 12,000 years ago. Instead of being actively hunted, domesticated animals were slowly brought under human control. Some were kept for food or for clothing, others for muscle power, and some simply for companionship.
The first animal to be domesticated was almost certainly the dog, which was bred from wolves. It was followed by species such as the cat, horse, camel, llama, and aurochs (a species of wild cattle), and by the Asian jungle fowl, which is the ancestor of today's chickens. Through selective breeding, each of these animals has been turned into forms that are particularly suitable for human use. Today, many domesticated animals, including chickens, vastly outnumber their wild counterparts. Sometimes, such as the horse, the original wild species has died out together.
Over the centuries, many domesticated animals have been introduced into different parts of the world only to escape and establish themselves in the wild. With stowaway pests such as rats, these ‘feral' animals have often affected native wildlife. Cats, for example, have inflicted great damage on Australia's smaller marsupials, and feral pigs and goats continue to be serious problems for the native wildlife of the Galápagos Islands.
Despite the growth of domestication, humans continue to hunt some wild animals. Some forms of hunting are carried out mainly for sport, but others provide food or animal products. Until recently, one of the most significant of these forms of hunting was whaling, which reduced many whale stocks to the brink of extinction. Today, highly efficient sea fishing threatens some species of fish with the same fate since the beginning of agriculture. The human population has increased by more than two thousand times. To provide the land needed for growing food and housing people, large areas of the earth's landscapes have been completely transformed. Forests have been cut down, wetlands drained, and deserts irrigated, reducing these natural habitats to a fraction of their former extent.
Some species of animals have managed to adapt to these changes. A few, such as the brown rat, raccoon, and house sparrow, have benefited by exploiting the new opportunities that have opened and have successfully taken up life on farms, or in towns and cities. Nonetheless, most animals have specialized ways of life that make them dependent on a particular kind of habitat. With the destruction of their habitats, their number inevitably declines.
In the 20th century, animals have also had to face additional threats from human activities. Foremost among these are environmental pollution and the increasing demand for resources such as timber and fresh water. For some animals, the combination of these changes has proved so damaging that their numbers are now below the level needed to guarantee survival.
Across the world, efforts are currently underway to address this urgent problem. In the most extreme cases, gravely threatened animals can be helped by taking them into captivity and then releasing them once breeding programs have increased their number. One species saved in this way is the Hawaiian mountain goose or n ? n ? . In 1951, its population had been reduced to just 33. Captive breeding has since increased the population to more than 2500, removing the immediate threat of extinction.
While captive breeding is a useful emergency measure, it cannot assure the long-term survival of a species. Today animal protection focuses primarily on the preservation of entire habitats, an approach that maintains the necessary links between the different species the habitats support. With the continued growth in the world's human population, habitat preservation will require a sustained reduction in our use of the world's resources to minimize our impact on the natural world.
Paleontologists gain most of their information by studying deposits of sedimentary rocks that formed in strata over millions of years. Most fossils are found in sedimentary rock. Paleontologists use fossils and other qualities of the rock to compare strata around the world. By comparing, they can determine whether strata developed during the same time or in the same type of environment. This helps them assemble a general picture of how the earth evolved. The study and comparison of different strata are called stratigraphy.
Fossils provide for most of the data on which strata are compared. Some fossils, called index fossils, are especially useful because they have a broad geographic range but a narrow temporal one-that is, they represent a species that was widespread but existed for a brief period of time. The best index fossils tend to be marine creatures. These animals evolved rapidly and spread over large areas of the world. Paleontologists divide the last 570 million years of the earth's history into eras, periods, and epochs. The part of the earth's history before about 570 million years ago is called Precambrian time, which began with the earth's birth, probably more than four billion years ago.
The earliest evidence of life consists of microscopic fossils of bacteria that lived as early as 3.6 billion years ago. Most Precambrian fossils are very tiny. Most species of larger animals that lived in later Precambrian time had soft bodies, without shells or other hard body parts that would create lasting fossils. The first abundant fossils of larger animals date from about 600 million years ago.
At first glance, the sudden jump from 8000 Bc to 10,000 years ago looks peculiar. On reflection, however, the timeline has clearly not lost 2,000 years. Rather, the timeline has merely shifted from one convention of measuring time to another. To understand the reasons for this shift, it will help to understand some of the different conventions used to measure time.
All human societies have faced the need to measure time. Today, for most practical purposes, we keep track of time with the aid of calendars, which are widely and readily available in printed and computerized forms throughout the world. However, long before humans developed any formal calendar, they measured time based on natural cycles: the seasons of the year, the waxing and waning of the moon, the rising and setting of the sun. Understanding these rhythms of nature was necessary for humans so they could be successful in hunting animals, catching fish, and collecting edible nuts, berries, roots, and vegetable matter. The availability of these animals and plants varied with the seasons, so early humans needed at least a practical working knowledge of the seasons to eat. When humans eventually developed agricultural societies, it became crucial for farmers to know when to plant their seeds and harvest their crops. To ensure that farmers had access to reliable knowledge of the seasons, early agricultural societies in Mesopotamia, Egypt, China, and other lands supported specialists who kept track of the seasons and created the world's first calendars. The earliest surviving calendars date from around 2400 Bc.
As societies became more complex, they required increasingly precise ways to measure and record increments of time. For example, some of the earliest written documents recorded tax payments and sales transactions, and indicating when they took place was important. Otherwise, anyone reviewing the documents later would find it impossible to determine the status of an individual account. Without any general convention for measuring time, scribes (persons who wrote documents) often dated events by the reigns of local rulers. In other words, a scribe might indicate that an individual's tax payment arrived in the third year of the reign (or third regnal years) of the Assyrian ruler Tiglath-Pileser. By consulting and comparing such records, authorities could determine if the individual were up to date in tax payments.
These days, scholars and the public alike refer to time on many different levels, and they consider events and processes that took place at any times, from the big bang to the present. Meaningful discussion of the past depends on some generally observed frames of reference that organize time coherently and allow us to understand the chronological relationships between historical events and processes.
For contemporary events, the most common frame of reference is the Gregorian calendar, which organizes time around the supposed birth date of Jesus of Nazareth. This calendar refers to dates before Jesus' birth as Bc (‘before Christ') and those afterwards as ad (anno Domini, Latin for ‘in the year of the Lord'). Scholars now believe that Jesus was born four to six years before the year recognized as ad one in the Gregorian calendar, so this division of time is probably off its intended mark by a few years. Nonetheless, even overlooking this point, the Gregorian calendar is not meaningful or useful for references to events in the so-called deep past, a period so long ago that to be very precise about dates is impossible. Saying that the big bang took place in the year 15,000,000,000 Bc would be misleading, for example. No one knows exactly when the big bang took place, and even if someone did, there would be little point in dating that moment and everything that followed from it according to an event that took place some 14,999,998,000 years later. For purposes of dating events and processes in the deep past and remote prehistory, then, scientists and historians have adopted different principles of measuring time.
In conventional usage, prehistory refers to the period before humans developed systems for writing, while the historical era refers to the period after written documents became available. This usage became common in the 19th century, when professional historians began to base their studies of the past largely on written documentation. Historians regarded written source materials as more reliable than the artistic and artifactual evidence studied by archaeologists working on prehistoric times. Recently, however, the distinction between prehistory and the historical era has become much more blurred than it was in the 19th century. Archaeologists have unearthed rich collections of artifacts that throw considerable light on so-called prehistoric societies. When, contemporary historians realize much better than did their predecessors that written documentary evidence raises as many questions as it does answers. In any case, written documents illuminate only selected dimensions of experience. Despite these nuances of historical scholarship, for purposes of dating events and processes in times past, the distinction between the term's prehistory and the historical era remains useful. For the deep past and prehistory, establishing precise dates is rarely possible: Only in the cases of a few natural and celestial phenomena, such as eclipses and appearances of comets, are scientists able to infer relatively precise dates. For the historical era, on the other hand, precise dates can be established for many events and processes, although certainly not for all.
Since the Gregorian calendar is not especially useful for dating events in the distant period long before the historical era, many scientists who study the deep past refer not to years ‘Bc' or AD' but to years ‘before the present'. Astronomers and physicists, for example, believe the big bang took place between 10 billion and 20 billion years ago, and that planet Earth came into being about 4.65 billion years ago. When dealing with Earth's physical history and life forms, geologists often dispense with year references together and divide time into alternate spans of time. These time spans are conventionally called eons (the longest span), eras, periods, and epochs (the shortest span). Since obtaining precise dates for distant times is impossible, they simply refer to the Proterozoic Eon (2.5 billion to 570 million years ago), the Mesozoic Era (240 million to 65 million years ago), the Jurassic Period (205 million to 138 million years ago), or the Pleistocene Epoch
(1.6 million to 10,000 years ago).
Because the Pleistocene Epoch is a comparatively recent time span, archaeologists and pre historians are frequently able to assign at least approximate year dates to artifacts from that period. As with all dates in the distant past, however, it would be misleading to follow the principles of the Gregorian calendar and refer to dates' Bc. As a result, archaeologists and pre-historians often call these dates' bp (‘before the present'), with the understanding that all dates bp are approximate. Thus, scholars date the evolution of The Homo sapiens to about 130,000 bp and the famous cave paintings at Lascaux in southern France to about 15,000 Bc.
The Dynamic Timeline, of which all date before 8000 Bc refers to dates before the present, and all dates since 8000 Bc categorizes time according to the Gregorian calendar. Thus, a backward scroll in the timeline will take users from 7700 Bc to 7800 Bc, 7900 Bc, and 8000 Bc to 10,000 years ago. Note that the timeline has not lost 2,000 years! To date events this far back in time, the Dynamic Timeline has simply switched to a different convention of designating the dates of historical events.
Written documentation enables historians to establish relatively precise dates of events in the historical era. However, placing these events in chronological order requires some agreed upon starting points for a frame of reference. For purposes of maintaining proper tax accounts in a Mesopotamian city-state, dating an event in relation to the first year of a king's reign might be sufficient. For purposes of understanding the development of entire peoples or societies or regions, however, a collection of dates according to the regnal years of many different local rulers would quickly become confusing. Within a given region there might be many different local rulers, so efforts to establish the chronological relationship between events may entail an extremely tedious collation of all the rulers' regnal years. Thus, to facilitate the understanding of chronological relationships between events in different jurisdictions, some larger frame of reference is necessary. Most commonly these larger frames of reference take the form of calendars, which not only make it possible to predict changes in the seasons but also enable users to organize their understanding of time and appreciate the relationships between datable events.
Different civilizations have devised thousands of different calendars. Of the 40 or so calendars employed in the world today, the most widely used is the Gregorian calendar, introduced in 1582 by Pope Gregory XIII. The Gregorian calendar revised the Julian calendar, instituted by Julius Caesar in 45 Bc, to bring it closer in line with the seasons. Most Roman Catholic lands accepted the Gregorian calendar upon its promulgation by Gregory in 1582, but other lands adopted it much later: Britain in 1752, Russia in 1918, and Greece in 1923. During the 20th century it became the dominant calendar throughout the world, especially for purposes of international business and diplomacy.
Despite the prominence of the Gregorian calendar in the modern world, millions of people use other calendars as well. The oldest calendar still in use is the Jewish calendar, which dates' time from the creation of the world in the (Gregorian) year 3761 Bc, according to the Hebrew scriptures. The year 2000Bc. in the Gregorian calendar thus corresponding to the year am 5761 in the Jewish calendar (am stands for anno Mundi, Latin for ‘the year of the world'). The Jewish calendar is the official calendar of Israel, and it also serves as a religious calendar for Jews worldwide.
The Chinese use another calendar, which, as tradition holds, takes its point of departure in the year 2697 Bc in honour of a beneficent ruler's work. The year AD 2000 of the Gregorian calendar, and with that it corresponds to the year 4697 in the Chinese calendar. The Maya calendar began even earlier than the Chinese-August 11, 3114 Bc. Maya scribes calculated that this is when the cycle of time began. The Maya actually used two interlocking calendars-one a 365-day calendar based on the cycles of the sun, the other a sacred almanac used to calculate auspicious or unlucky days. Despite the importance of these calendars to the Maya civilization, the calendars passed out of general use after the Spanish conquest of Mexico in the 16th century AD.
The youngest calendar in widespread use today is the Islamic lunar calendar, which begins the day after the Hegira, Muhammad's migration from Mecca to Medina in ad 622. The Islamic calendar is the official calendar in many Muslim lands, and it governs religious observances for Muslims worldwide. Since it reckons time according too lunar rather than solar cycles, the Islamic calendar does not neatly correspond to the Gregorian and other solar calendars. For example, although there were 1,378 solar years between Muhammad's Hegira and AD 2000, that year corresponds to the year 1420 in the Islamic calendar. Like the Gregorian calendar and despite their many differences, the Jewish, Chinese, and Islamic calendars all make it possible to place individual datable events in proper chronological order.
Recently, controversies have arisen concerning the Gregorian calendar's designation of Bc and ad to indicate years before and after the birth of Jesus Christ. This practice originated in the 6th century ad with a Christian monk named Dionysius Exiguus. Like other devout Christians, Dionysius regarded the birth of Jesus as the singular turning point of history. Accordingly, he introduced a system that referred to events in time based on the number of years they occurred before or after Jesus' birth. The system caught on very slowly. Saint Bede the Venerable, a prominent English monk and historian, employed the system in his own works in the 8th century ad, but the system came into general use only about AD 1400. (Until then, Christians generally calculated time according to regnal years of prominent rulers.) When Pope Gregory XIII ordered the preparation of a new calendar in the 16th century, he intended it to serve as a religious calendar as well as a tool for predicting seasonal changes. As leader of the Roman Catholic Church, Pope Gregory considered it proper to continue recognizing Jesus' birth as the turning point of history.
As lands throughout the world adopted the Gregorian calendar, however, the specifically Christian implications of the term's Bc and ad did not seem appropriate for use by non-Christians. Really, they did not even seem appropriate to many Christians when dates referred to events in non-Christian societies. Why should Buddhists, Hindus, Muslims, or others date time according to the birth of Jesus? In saving the Gregorian calendar as a widely observed international standard for reckoning time, while also avoiding the specifically Christian implications of the qualification's Bc and ad, scholars replaced the birth of Jesus with the notion of ‘the common era' and began to qualify dates as BCE (‘before the common era') or Ce ("in the common era"). For the practical purpose of organizing time, BCE is the exact equivalent of Bc, and Ce is the exact equivalent of AD, but the term's BCE and Ce have very different connotations than do Bc and AD.
The qualification's BCE and Ce first came into general use after World War II (1939-1945) among biblical scholars, particularly those who studied Judaism and early Christianity in the period from the 1st century Bc (or BCE) and the 1st century ad (or Ce). From their viewpoint, this "common era" was an age when proponents of Jewish, Christian, and other religious faiths intensively interacted and debated with one another. Using the designations, BCE and Ce enabled them to continue employing a calendar familiar to them all while avoiding the suggestion that all historical time revolved around the birth of Jesus Christ. As the Gregorian calendar became prominent throughout the world in the 20th century, many peoples were eager to find terms more appealing to them than Bc and ad, and accordingly, the BCE and Ce usage became increasingly popular. This usage represents only the most recent of many efforts by the world's peoples to devise meaningful frameworks of time.
Most scientists believe that the Earth, Sun, and all of the other planets and moons in the solar system formed about 4.6 billion years ago from a giant cloud of gas and dust known as the solar nebula. The gas and dust in this solar nebula originated in a star that ended its life in an explosion known as a supernova. The solar nebula consisted principally of hydrogen, the lightest element, but the nebula was also seeded with a smaller percentage of heavier elements, such as carbon and oxygen. All of the chemical elements we know were originally made in the star that became a supernova. Our bodies are made of these same chemical elements. Therefore, all of the elements in our solar system, including all of the elements in our bodies, originally came from this star-seeded solar nebula.
Due to the force of gravity tiny clumps of gas and dust began to form in the early solar nebula. As these clumps came together and grew larger, they caused the solar nebula to contract in on itself. The contraction caused the cloud of gas and dust to flatten in the shape of a disc. As the clumps continued to contract, they became very dense and hot. Eventually the s of hydrogen became so dense that they began to fuse in the innermost part of the cloud, and these nuclear reactions gave birth to the Sun. The fusion of hydrogen s in the Sun is the source of its energy.
Many scientists favour the planetesimal theory for how the Earth and other planets formed out of this solar nebula. This theory helps explain why the inner planets became rocky while the outer planets, except Pluto, are made up mostly of gases. The theory also explains why all of the planets orbit the Sun in the same plane.
According to this theory, temperatures decreased with increasing distance from the centre of the solar nebula. In the inner region, where Mercury, Venus, Earth, and Mars formed, temperatures were low enough that certain heavier elements, such as iron and the other heavy compounds that make up rock, could condense out - that is, could change from a gas to a solid or liquid. Due to the force of gravity, small clumps of this rocky material eventually came with the dust in the original solar nebula to form protoplanets or planetesimals (small rocky bodies). These planetesimals collided, broke apart, and re-formed until they became the four inner rocky planets. The inner region, however, was still too hot for other light elements, such as hydrogen and helium, to be retained. These elements could only exist in the outermost part of the disc, where temperatures were lower. As a result two of the outer planets-Jupiter and Saturn-are mostly made of hydrogen and helium, which are also the dominant elements in the atmospheres of Uranus and Neptune.
Within the planetesimal Earth, heavier matter sank to the centre and lighter matter rose toward the surface. Most scientists believe that Earth was never truly molten and that this transfer of matter took place in the solid state. Much of the matter that went toward the centre contained radioactive material, an important source of Earth's internal heat. As heavier material moved inward, lighter material moved outward, the planet became layered, and the layers of the core and mantle were formed. This process is called differentiation.
Not long after they formed, more than four billion years ago, the Earth and the Moon underwent a period when they were bombarded by meteorites, the rocky debris left over from the formation of the solar system. The impact craters created during this period of heavy bombardment are still visible on the Moon's surface, which is unchanged. Earth's craters, however, were long ago erased by weathering, erosion, and mountain building. Because the Moon has no atmosphere, its surface has not been subjected to weathering or erosion. Thus, the evidence of meteorite bombardment remains.
Energy released from the meteorite impacts created extremely high temperatures on Earth that melted the outer part of the planet and created the crust. By four billion years ago, both the oceanic and continental crust had formed, and the oldest rocks were created. These rocks are known as the Acasta Gneiss and are found in the Canadian territory of Nunavut. Due to the meteorite bombardment, the early Earth was too hot for liquid water to exist and so existing was impossible for life.
Geologists divide the history of the Earth into three eons: the Archaean Eon, which lasted from around four billion to 2.5 billion years ago; the Proterozoic Eon, which lasted from 2.5 billion to 543 million years ago; and the Phanerozoic Eon, which lasted from 543 million years ago to the present. Each eon is subdivided into different eras. For example, the Phanerozoic Eon includes the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. In turn, eras are further divided into periods. For example, the Paleozoic Era includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods.
The Archaean Eon is subdivided into four eras, the Eoarchean, the Paleoarchean, the Mesoarchean, and the Neoarchean. The beginning of the Archaean is generally dated as the age of the oldest terrestrial rocks, which are about four billion years old. The Archaean Eon ended 2.5 billion years ago when the Proterozoic Eon began. The Proterozoic Eon is subdivided into three eras: the Paleoproterozoic Era, the Mesoproterozoic Era, and the Neoproterozoic Era. The Proterozoic Eon lasted from 2.5 billion years ago to 543 million years ago when the Phanerozoic Eon began. The Phanerozoic Eon is subdivided into three eras: the Paleozoic Era from 543 million to 248 million years ago, the Mesozoic Era from 248 million to 65 million years ago, and the Cenozoic Era from 65 million years ago to the present.
Geologists base these divisions on the study and dating of rock layers or strata, including the fossilized remains of plants and animals found in those layers. Until the late 1800s scientists could only determine the relative age of rock strata, or layering. They knew that overall the top layers of rock were the youngest and formed most recently, while deeper layers of rock were older. The field of stratigraphy shed much light on the relative ages of rock layers.
The study of fossils also enabled geologists to determine the relative ages of different rock layers. The fossil record helped scientists determine how organisms evolved or when they became extinct. By studying rock layers around the world, geologists and paleontologists saw that the remains of certain animal and plant species occurred in the same layers, but were absent or altered in other layers. They soon developed a fossil index that also helped determine the relative ages of rock layers.
Beginning in the 1890s, scientists learned that radioactive elements in rock decay at a known rate. By studying this radioactive decay, they could determine an absolute age for rock layers. This type of dating, known as radiometric dating, confirmed the relative ages determined through stratigraphy and the fossil index and assigned absolute ages to the various strata. As a result scientists were able to assemble Earth's geologic time scale from the Archaean Eon to the present.
The Precambrian is a time span that includes the Archaean and Proterozoic eons and began about four billion years ago. The Precambrian marks the first formation of continents, the oceans, the atmosphere, and life. The Precambrian represents the oldest chapter in Earth's history that can still be studied. Very little remains of Earth from the period of 4.6 billion to about four billion years ago due to the melting of rock caused by the early period of meteorite bombardment. Rocks dating from the Precambrian, however, have been found in Africa, Antarctica, Australia, Brazil, Canada, and Scandinavia. Some zircon mineral grains deposited in Australian rock layers have been dated to
4.2 billion years.
The Precambrian is also the longest chapter in Earth's history, spanning a period of about 3.5 billion years. During this time frame, the atmosphere and the oceans formed from gases that escaped from the hot interior of the planet because of widespread volcanic eruptions. The early atmosphere consisted primarily of nitrogen, carbon dioxide, and water vapour. As Earth continued to cool, the water vapour condensed out and fell as precipitation to form the oceans. Some scientists believe that much of Earth's water vapour originally came from comets containing frozen water that struck Earth during meteorite bombardment.
By studying 2-billion-year-old rocks found in northwestern Canada, as well as 2.5-billion-year-old rocks in China, scientists have found evidence that plate tectonics began shaping Earth's surface as early as the middle Precambrian. About a billion years ago, the Earth's plates were entered around the South Pole and formed a super-continent called Rodinia. Slowly, pieces of this super-continent broke away from the central continent and travelled north, forming smaller continents.
Life originated during the Precambrian. The earliest fossil evidence of life consists of Prokaryotes, one-celled organisms that lacked a nucleus and reproduced by dividing, a process known as asexual reproduction. Asexual division meant that a prokaryote's hereditary material was copied unchanged. The first Prokaryotes were bacteria known as archaebacteria. Scientists believe they came into existence perhaps as early as 3.8 billion years ago, but certainly by 3.5 billion years ago, and where anaerobic-that is, they did not require oxygen to produce energy. Free oxygen barely existed in the atmosphere of the early Earth.
Archaebacteria were followed about 3.46 billion years ago by another type of prokaryote known as Cyanobacteria or blue-green algae. These Cyanobacteria gradually introduced oxygen in the atmosphere because of photosynthesis. In shallow tropical waters, Cyanobacteria formed mats that grew into humps called stromatolites. Fossilized stromatolites have been found in rocks in the Pilbara region of western Australia that are more than 3.4 billion years old and in rocks of the Gunflint Chert region of northwest Lake Superior that are about 2.1 billion years old
The colonization of Australia/New Guinea was not achieved until the time to which took off around 50,000 years ago. Another extension of human range that soon followed as the one into th coldest parts of Eurasia. While Neanderthals lived in glacial times and were adapted to the cold, they penetrated no farther north than Germany and Kiev. That's not surprising, since Neanderthals apparently lacked needles, sewn clothing, warm houses, and other technological essentials of survival in the coldest climates. Anatomically modern peoples who possess such technology had expanded into Siberia by around 20,000 years ago (there are the usual much olde disputed claims). That expansion may have been responsible for the extinctions of Eurasia's wooly mammoth and wooly rhinoceroses.
With the settlement of Austral/New Guinea, humans now occupied three of the five habitable continents. However, Antartica because it was not reached by humans until the 19th century and has two continents, North America and South America. That left only two continents, North America and South America. They were surely the last ones settled, for the obvious reason tat reaching the Americas fro the Old World required either boats (for which there is no evidence even in Indonesia until 40,000 years ago and nine in Europe until much later) in order to cross by sea, or else it required the occupation of Siberia (unoccupied until about 20,000 years ago) ib order to cross the Bering land bridge.
However, it is uncertain when, between about 14,000 and 35,000 years ago, the Americas were first colonized. The oldest unquestionable human remains in the Americas are at sites in Alaska dated around 12,000 Bc., followed by a profusion of sites in the United States south of the Canadian border and in Mexico in the centuries just before 11,000 Bc. The latter sites are called Clovis sites, named just after the type site near the town of Clovis, New Mexico, where there characteristic large stone spearpoints were first recognized. Hundreds of Clovis sites are now known, blanketing all 48 of the lower U.S. states south into Mexico. Unquestioned and in Patagonia. These facts suggest the interpretation that Clovis sites document the America's first colonized by people, who quickly multiplied, expanded, and filled the two continents.
Nevertheless, it may be all the same, that differences between the long-term histories of peoples of the different continents have been due not to innate differences in the people themselves but to differences in their environments. That is to say, that if the populations of Aboriginal Australia and Eurasia could have been interchanged during the Late Pleistocene, the original Aboriginal Australia would no be the ones occupying most of the Americas and Australia, we well as Eurasia, while the original Aboriginal and Australia, as well as Eurasia, while the original Aboriginal Eurasians would be the ones now reduced to a downtrodden population fragment in Australia. One might at first be inclined to dismiss this assertion as meaningless, because the excrement is imaginary and claims itself its outcome that cannot be verified, but historians are nonetheless able to evaluate related hypotheses by retrospective tests. For instance, one can examine what did happen when European farmers were transplanted to Greenland or the U.S. Great Plains, and when farmers stemming ultimately from China emigrated to the Chatha Islands, the rain forests of Borneo, or the volcanic soil o Java or Hawaii. These tests confirm that the same ancestral peoples either ended up extinct, or returned to living as hunter-gatherers, or went on to build complex states, depending on their environments., similarly, Aboriginal Australian hunter-gatherers, variously transplanted to Finders Island, Tasmania, or southeastern Australia, ended up extinct, or as canal builders intensively managing a productive fishery, depending on their continents.
Of course, the continents differ in innumerable environmental features affecting trajectories of human societies. But merely a laundry list of ever possible difference does not constitute any one answer. Just four sets of differences appear as considered being the mst important ones.
The fist set consists of continental difference in the wild plant and anal species available as starting materials for domestication. That's because food production was critical for the accumulation of food surpluses that could feed non-food producing specialists, and for the buildup of large populations enjoying a military advantage though mere numbers even before they had developed any technological or political advantage.
On each continent, animal and plant domestication was concentrated in a few especially favourable homelands' accounting for only a small fraction of the continent's total area. In the case of technical innovations and political institutions as well, most societies acquire much more from other societies than they invent themselves. Thus diffusion and migration within a continent contribute importantly the development of its societies, which tend in the log run to share each other's development (insofar as environments permit) because of the processes illustrated in much more form by Maori New Zealand's Musket Wars. That is, societies initially lacking an advantage ether acquire it from societies possessing it or (if they fail to do so) are replaced by those other society.
Even so, for billions of years, life existed only in the simple form of Prokaryotes. Prokaryotes were followed by the relatively more advanced eukaryotes, organisms that have a nucleus in their cells and that reproduces by combining or sharing their heredity makeup rather than by simply dividing. Sexual reproduction marked a milestone in life on Earth because it created the possibility of hereditary variation and enabled organisms to adapt more easily to a changing environment. The latest part of Precambrian time some 560 million to 545 million years ago saw the appearance of an intriguing group of fossil organisms known as the Ediacaran fauna. First discovered in the northern Flinders Range region of Australia in the mid-1940s and subsequently found in many locations throughout the world, these strange fossils are the precursors of many fossil groups that were to explode in Earth's oceans in the Paleozoic Era.
At the start of the Paleozoic Era about 543 million years ago, an enormous expansion in the diversity and complexity of life occurred. This event took place in the Cambrian Period and is called the Cambrian explosion. Nothing like it has happened since. Almost all of the major groups of animals we know today made their first appearance during the Cambrian explosion. Almost all of the different ‘body plans' found in animals today-that is, the way and animal's body is designed, with heads, legs, rear ends, claws, tentacles, or antennae-also originated during this period.
Fishes first appeared during the Paleozoic Era, and multicellular plants began growing on the land. Other land animals, such as scorpions, insects, and amphibians, also originated during this time. Just as new forms of life were being created, however, other forms of life were going out of existence. Natural selection meant that some species were able to flourish, while others failed. In fact, mass extinctions of animal and plant species were commonplace.
Most of the early complex life forms of the Cambrian explosion lived in the sea. The creation of warm, shallow seas, along with the buildup of oxygen in the atmosphere, may have aided this explosion of life forms. The shallow seas were created by the breakup of the super-continent Rodinia. During the Ordovician, Silurian, and Devonian periods, which followed the Cambrian Period and lasted from 490 million to 354 million years ago, some of the continental pieces that had broken off Rodinia collided. These collisions resulted in larger continental masses in equatorial regions and in the Northern Hemisphere. The collisions built many mountain ranges, including parts of the Appalachian Mountains in North America and the Caledonian Mountains of northern Europe.
Toward the close of the Paleozoic Era, two large continental masses, Gondwanaland to the south and Laurasia to the north, faced each other across the equator. They're slow but eventful collision during the Permian Period of the Paleozoic Era, which lasted from 290 million to 248 million years ago, assembled the super-continent Pangaea and resulted in some of the grandest mountains in the history of Earth. These mountains included other parts of the Appalachians and the Ural Mountains of Asia. At the close of the Paleozoic Era, Pangaea represented more than 90 percent of all the continental landmasses. Pangaea straddled the equator with a huge mouth like opening that faced east. This opening was the Tethys Ocean, which closed as India moved northward creating the Himalayas. The last remnants of the Tethys Ocean can be seen in today's Mediterranean Sea.
The Paleozoic ended with a major extinction event, when perhaps as many as 90 percent of all plant and animal species died out. The reason is not known for sure, but many scientists believe that huge volcanic outpourings of lavas in central Siberia, coupled with an asteroid impact, were joint contributing factors.
The most notable of the Mesozoic reptiles, the dinosaur, first evolved in the Triassic period (240 million to 205 million years ago). The Triassic dinosaurs were not as large as their descendants in later Mesozoic times. They were comparatively slender animals that ran on their hind feet, balancing their bodies with heavy, fleshy tails, and seldom exceeded 4.5 m's (15 ft) in length. Other reptiles of the Triassic period included such aquatic creatures as the ichthyosaurs, and a group of flying reptiles, the pterosaurs.
The first mammals also appeared during this period. The fossil remains of these animals are fragmentary, but the animals were apparently small in size and reptilian in appearance. In the sea, Teleostei, the first ancestors of the modern bony fishes, made their appearance. The plant life of the Triassic seas included a large variety of marine algae. On land, the dominant vegetation included various evergreens, such as ginkgos, conifers, and palms. Small scouring rushes and ferns still existed, but the larger members of these groups had become extinct.
The Mesozoic Era is divided into three geological periods: the Triassic, which lasted from 248 million to 206 million years ago; the Jurassic, from 206 million to 144 million years ago; and the Cretaceous, from 144 million to 65 million years ago. The dinosaurs emerged during the Triassic Period and was one of the most successful animals in Earth's history, lasting for about 180 million years before going extinct at the end of the Cretaceous Period. The first and mammals and the first flowering plants also appeared during the Mesozoic Era. Before flowering plants emerged, plants with seed-bearing cones known as conifers were the dominant form of plants. Flowering plants soon replaced conifers as the dominant form of vegetation during the Mesozoic Era.
The Mesozoic was an eventful era geologically with many changes to Earth's surface. Pangaea continued to exist for another 50 million years during the early Mesozoic Era. By the early Jurassic Period, Pangaea began to break up. What is now South America begun splitting from what is now Africa, and in the process the South Atlantic Ocean formed? As the landmass that became North America drifted away from Pangaea and moved westward, a long Subduction zone extended along North America's western margin. This Subduction zone and the accompanying arc of volcanoes extended from what is now Alaska to the southern tip of South America. A great deal of this feature, called the American Cordillera, exists today as the eastern margin of the Pacific Ring of Fire.
During the Cretaceous Period, heat continued to be released from the margins of the drifting continents, and as they slowly sank, vast inland seas formed in much of the continental interiors. The fossilized remains of fishes and marine mollusks called ammonites can be found today in the middle of the North American continent because these areas were once underwater. Large continental masses broke off the northern part of southern Gondwanaland during this period and began to narrow the Tethys Ocean. The largest of these continental masses, present-day India, moved northward toward its collision with southern Asia. As both the North Atlantic Ocean and South Atlantic Ocean continued to open, North and South America became isolated continents for the first time in 450 million years. Their westward journey resulted in mountains along their western margins, including the Andes of South America.
The Cenozoic Era, beginning about 65 million years ago, is the period when mammals became the dominant form of life on land. Human beings first appeared in the later stages of the Cenozoic Era. In short, the modern world as we know it, with its characteristic geographical features and its animals and plants, came into being. All of the continents that we know today took shape during this era.
A single catastrophic event may have been responsible for this relatively abrupt change from the Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid or comet struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic eras, causing the extinction of many forms of life, including the dinosaurs. Evidence of this collision came with the discovery of a large impact crater off the coast of Mexico's Yucatán Peninsula and the worldwide finding of iridium, a metallic element rare on Earth but abundant in meteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of the dinosaurs opened the way for mammals to become the dominant land animals.
The Cenozoic Era is divided into the Tertiary and the Quaternary periods. The Tertiary Period lasted from about 65 million to about 1.8 million years ago. The Quaternary Period began about 1.8 million years ago and continued to the present day. These periods are further subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and the Holocene, from 10,000 years ago to the present.
Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continents were all clearly outlined. India and other continental masses began colliding with southern Asia to form the Himalayas. Africa and a series of smaller micro-continents began colliding with southern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resemble today's Mediterranean Sea. As the Tethys continued to narrow, the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared as a new island in later Tertiary time, and its active volcanism today indicates that sea-floor spreading is still causing the country to grow.
Late in the Tertiary Period, about six million years ago, humans began to evolve in Africa. These early humans began to migrate to other parts of the world between two million and 1.7 million years ago.
The Quaternary Period marks the onset of the great ice ages. Many times, perhaps at least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and parts of Asia. The glaciers eroded considerable amounts of material that stood in their paths, gouging out U-shaped valleys. Anically modern human beings, known as Homo sapiens, became the dominant form of life in the Quaternary Period. Most anthropologists (scientists who study human life and culture) believe that Anically modern humans originated only recently in Earth's 4.6-billion-year history, within the past 200,000 years.
With the rise of human civilization about 8,000 years ago and especially since the Industrial Revolution in the mid 1700s, human beings began to alter the surface, water, and atmosphere of Earth. In doing so, they have become active geological agents, not unlike other forces of change that influence the planet. As a result, Earth's immediate future depends largely on the behaviour of humans. For example, the widespread use of fossil fuels is releasing carbon dioxide and other greenhouse gases into the atmosphere and threatens to warm the planet's surface. This global warming could melt glaciers and the polar ice caps, which could flood coastlines around the world and many island nations. In effect, the carbon dioxide removed from Earth's early atmosphere by the oceans and by primitive plant and animal life, and subsequently buried as fossilized remains in sedimentary rock, is being released back into the atmosphere and is threatening the existence of living things.
Even without human intervention, Earth will continue to change because it is geologically active. Many scientists believe that some of these changes can be predicted. For example, based on studies of the rate that the sea-floor is spreading in the Red Sea, some geologists predict that in 200 million years the Red Sea will be the same size as the Atlantic Ocean is today. Other scientists predict that the continent of Asia will break apart millions of years from now, and as it does, Lake Baikal in Siberia will become a vast ocean, separating two landmasses that once made up the Asian continent.
In the far, far distant future, however, scientists believe that Earth will become an uninhabitable planet, scorched by the Sun. Knowing the rate at which nuclear fusion occurs in the Sun and knowing the Sun's mass, astrophysicists (scientists who study stars) have calculated that the Sun will become brighter and hotter about three billion years from now, when it will be hot enough to boil Earth's oceans away. Based on studies of how other Sun-like stars have evolved, scientists predict that the Sun will become a red giant, a star with a very large, hot atmosphere, about seven billion years from now. As a red giant the Sun's outer atmosphere will expand until it engulfs the planet Mercury. The Sun will then be 2,000 times brighter than it is now and so hot it will melt Earth's rocks. Earth will end its existence as a burnt cinder.
Or, perhaps, that a single catastrophic event had been responsible for this relatively abrupt change from the Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid or comet struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic eras, causing the extinction of many forms of life, including the dinosaurs. Evidence of this collision came with the discovery of a large impact crater off the coast of Mexico's Yucatán Peninsula and the worldwide finding of iridium, a metallic element rare on Earth but abundant in meteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of the dinosaurs opened the way for mammals to become the dominant land animals.
The Cenozoic Era is divided into the Tertiary and the Quaternary periods. The Tertiary Period lasted from about 65 million to about 1.8 million years ago. The Quaternary Period began about 1.8 million years ago and continued to the present day. These periods are further subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and the Holocene, from 10,000 years ago to the present.
Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continents were all clearly outlined. India and other continental masses began colliding with southern Asia to form the Himalayas. Africa and a series of smaller micro-continents began colliding with southern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resemble today's Mediterranean Sea. As the Tethys continued to narrow, the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared as a new island in later Tertiary time, and its active volcanism today indicates that sea-floor spreading is still causing the country to grow.
Late in the Tertiary Period, about six million years ago, humans began to evolve in Africa. These early humans began to migrate to other parts of the world between two million and 1.7 million years ago.
The Quaternary Period marks the onset of the great ice ages. Many times, perhaps at least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and parts of Asia. The glaciers eroded considerable amounts of material that stood in their paths, gouging out U-shaped valleys. Anically modern human beings, known as Homo sapiens, became the dominant form of life in the Quaternary Period. Most anthropologists (scientists who study human life and culture) believe that Anically modern humans originated only recently in Earth's 4.6-billion-year history, within the past 200,000 years.
With the rise of human civilization about 8,000 years ago and especially since the Industrial Revolution in the mid 1700s, human beings began to alter the surface, water, and atmosphere of Earth. In doing so, they have become active geological agents, not unlike other forces of change that influence the planet. As a result, Earth's immediate future depends mainly on the behaviour of humans. For example, the widespread use of fossil fuels is releasing carbon dioxide and other greenhouse gases into the atmosphere and threatens to warm the planet's surface. This global warming could melt glaciers and the polar ice caps, which could flood coastlines around the world and many island nations. In effect, the carbon dioxide removed from Earth's early atmosphere by the oceans and by primitive plant and animal life, and subsequently buried as fossilized remains in sedimentary rock, is being released back into the atmosphere and is threatening the existence of living things.
Even without human intervention, Earth will continue to change because it is geologically active. Many scientists believe that some of these changes can be predicted. For example, based on studies of the rate that the sea-floor is spreading in the Red Sea, some geologists predict that in 200 million years the Red Sea will be the same size as the Atlantic Ocean is today. Other scientists predict that the continent of Asia will break apart millions of years from now, and as it does, Lake Baikal in Siberia will become a vast ocean, separating two landmasses that once made up the Asian continent.
In the far, far distant future, however, scientists believe that Earth will become an uninhabitable planet, scorched by the Sun. Knowing the rate at which nuclear fusion occurs in the Sun and knowing the Sun's mass, astrophysicists (scientists who study stars) have calculated that the Sun will become brighter and hotter about three billion years from now, when it will be hot enough to boil Earth's oceans away. Based on studies of how other Sun-like stars have evolved, scientists predict that the Sun will become a red giant, a star with a very large, hot atmosphere, about seven billion years from now. As a red giant the Sun's outer atmosphere will expand until it engulfs the planet Mercury. The Sun will then be 2,000 times brighter than it is now and so hot it will melt Earth's rocks. Earth will end its existence as a burnt cinder.
Three billion years is the life span of millions of human generations, however. Perhaps by then, humans will have learned how to journey beyond the solar system to colonize other planets in the Milky Way Galaxy and find among other different places to call ‘home'.
The dinosaurs were one of a group of extinct reptiles that lived from about 230 million to about sixty-five million years ago. British anist Sir Richard Owen coined the word dinosaur in 1842, derived from the Greek words' deinos, meaning ‘marvellous' or ‘terrible', and sauros, meaning ‘lizard'. For more than 140 million years, dinosaurs reigned as the dominant on land.
Owen distinguished dinosaurs from other prehistoric reptiles by their upright rather than sprawling legs and by the presence of three or more vertebrae supporting the pelvis, or hipbone. They classify dinosaurs into two orders according to differences in pelvic structure: Saurischia, or lizard-hipped dinosaurs, and Ornithischia, or bird-hipped dinosaurs. Dinosaur bones occur in sediments deposited during the Mesozoic Era, the so-called era of middle animals, also known as the age of reptiles. This era is divided into three periods: the Triassic (240 million to 205 million years ago), the Jurassic (205 million to 138 million years ago), and the Cretaceous (138 million to sixty-five million years ago).
Historical references to dinosaur bones may extend as far back as the 5th century Bc. Some scholars think that Greek historian Herodotus was referring to fossilized dinosaur skeletons and eggs when he described griffins—legendary beasts that were part eagle and part lions-guarding nests in central Asia. ‘Dragon bones' mentioned in a 3rd century ad text from China are thought to refer to bones of dinosaurs.
The first dinosaurs studied by paleontologists (scientists who study prehistoric life) were Megalosaurus and Iguanodon, whose partial bones were discovered early in the 19th century in England. The shape of their bones shows that these animals resembled large, land-dwelling reptiles. The teeth of Megalosaurus, which are pointed and have serrated edges, suggest that this animal was a flesh eater, while the flattened, grinding surfaces of Iguanodon teeth suggest that it was a plant eater. Megalosaurus lived during the Jurassic Period, and Iguanodon lived during the early part of the Cretaceous Period. Later in the 19th century, paleontologists collected and studied more comprehensive skeletons of related dinosaurs found in New Jersey. From these finds they learned that Megalosaurus and Iguanodon walked on two legs, not four, as had been thought.
Some ornithischians quickly became quadrupedal (four-legged) and relied on body armour and other physical defences rather than fleetness for protection. Plated dinosaurs, such as the massive Stegosaurus of the late Jurassic Period, bore a double row of triangular bony plates along their backs. These narrow plates contained tunnels through which blood vessels passed, allowing the animals to radiate excess body heat or to warm themselves in the sun. Many also bore a large spined plate over each shoulder. Stegosaurs resembled gigantic porcupines, and they probably defended themselves by turning their spined tails toward aggressors.
During the Cretaceous Period, stegosaurs were supplanted by armoured dinosaurs such as Ankylosaurus. These animals were similar in size to stegosaurs but otherwise resembled giant horned toads. Some even possessed a bony plate in each eyelid and large tail clubs. Their necks were protected by heavy, bony rings and spines, showing that these areas needed protection from the attacks of carnivorous dinosaurs.
The reptiles were still the dominant form of animal life in the Cretaceous period (138 million to 65 million years ago). The four types of dinosaurs found in the Jurassic also lived during this period, and a fifth type, the horned dinosaurs, also appeared. By the end of the Cretaceous, about 65 million years ago, all these creatures had become extinct. The largest of the pterodactyls lived during this period. Pterodactyl fossils discovered in Texas have wingspreads of up to 15.5 m's (50 ft). Other reptiles of the period include the first snakes and lizards. Several types of Cretaceous have been discovered, including Hesperornis, a diving bird about 1.8 m's (about 6 ft) in length, which had only vestigial wings and was unable to fly. Mammals of the period included the first marsupials, which strongly resembled the modern opossum, and the first placental mammals, which belonged to the group of insectivores. The first crabs developed during this period, and several modern varieties of fish also evolved. The most important evolutionary advance in the plant kingdom during the Cretaceous period was the development of deciduous plants, the earliest fossils of which appear in early Cretaceous rock formations. By the end of the period, many modern varieties of trees and shrubs had made their appearance. They represented more than 90 percent of the known plants of the period. Mid-Cretaceous fossils include remains of beech, holly, laurel, maple, oak, plane tree, and walnut. Some paleontologists believe that these deciduous woody plants first evolved in Jurassic times but grew only in upland areas, where conditions were unfavourable for fossil preservation. Becoming the most abundant plant-eating dinosaurs. They ranged in size from small runners that were two m's (6 ft) long and weighed 15 kg (33 lb), such as Hypsilophodon, to elephantine cows that were (32 ft) long and weighed 4 metric tons, such as Edmontosaurus. These animals had flexible jaws and grinding teeth, which eventually surpassed those of the modern cows in their suitability for chewing fibrous plants. The beaks of ornithopods became broader, earning them the name duck-billed dinosaur. Their tooth batteries became larger, their backs became stronger, and their forelimbs lengthened until their arms became elongated walking sticks, although ornithopods remained bipedal. The nose supported cartilaginous sacks or bony tubes, suggesting that these dinosaurs may have communicated by trumpeting. Fossil evidence from the late Cretaceous Period includes extensive accumulations of bones from ornithopods drowned in floods, indicating that duck-billed dinosaurs often migrated in herds of thousands. A few superbly preserved Edmontosaurus skeletons encased within impressions of skin have been discovered in southeastern Wyoming.
Pachycephalosaurs were small bipedal ornithischians with thickened skulls, flattened bodies, and tails surrounded by a latticework of bony rods. In many of these dinosaurs, such as the Pachycephalosaurus, -a large specimen up to eight m's (26 ft) a long-the skull was capped by a rounded dome of solid bone. Some paleontologists suggest that males may have borne the thickest domes and butted heads during mating contests. Eroded pachycephalosaur domes are often found in stream deposits from late in the Cretaceous Period.
The quadrupedal ceratopsians, or horned dinosaurs, typically bore horns over the nose and eyes, and had a saddle-shaped bony frill that extended from the skull over the neck. These bony frills were well developed in the late Cretaceous Triceratops, of which is a dinosaur that could reach lengths of up to eight m's (26 ft) and weighing more than 12 metric tons. The frill served two purposes: It protected the vulnerable neck, and it contained a network of blood vessels on its undersurface to radiate excess heat. Large accumulations of fossil bones suggest that ceratopsians lived in herds.
Controversy surrounds the extinction of the dinosaurs. According to one theory, dinosaurs were slowly driven to extinction by environmental changes linked to the gradual withdrawal of shallow seas from the continents at the end of the dinosaurian era. Proponents of this theory postulate that dinosaurs dwindled in number and variety over several million years.
An opposing theory proposes that the impact of an asteroid or comet caused catastrophic destruction of the environment, leading to the extinction of the dinosaurs. Evidence to support this theory includes the discovery of a buried impact crater (thought to be the result of a large comet striking the earth) that is 200 km (124 mi) in diameter in the Yucatán Peninsula of Mexico. A spray of debris, called an ejecta sheet, which was blown from the edge of the crater, has been found over vast regions of North America. Comet-enriched material from the impact's fiery explosion was distributed all over the world. With radiometric dating (Radiometric Dating), scientists have used the decay rates of certain s to date the crater, ejecta sheet, and fireball layer. Using similar techniques to date the dramatic changes in the record of microscopic fossils, they have found that the impact and the dinosaur extinction occurred nearly simultaneously.
Although large amounts of ash suggest that most of North and South America was devastated by fire from the impact, the longer-term planetwide environmental effects of the impact were ultimately more lethal to life than the fire. Dust blocked sunlight from the earth's surface for many months. Scorched sulfur from the impact site, water vapour and chlorine from the oceans, and nitrogen from the air combined to produce a worldwide fallout of intensely acidic rain. Scientists postulate that darkness and acid rain caused plant growth to cease. As a result, both the herbivorous dinosaurs, which were dependent on plants for food, and the carnivorous dinosaurs, which fed on the herbivores, were exterminated. On the other hand, animals such as frogs, lizards, and small insect-eating turtles and mammals, which were dependent on organisms that fed on decaying plant material, were more likely to survive. Their survival indicates that, in most areas, the surface of Earth did not freeze.
Fossilized dinosaur remains are usually buried in sediments deposited on land. These remains are likely to be found in regions where the silt and sands spread by rivers of the Mesozoic Era are exposed. Fossils are easier to find in arid badlands-rugged, rocky areas with little vegetation, where the sediments are not covered by soil. The excavation of large skeletal fossils involves painstaking procedures to protect the fossils from damage. Fewer than 3,000 dinosaur specimens have been collected to date, and only fifty skeletons of the 350 known varieties of dinosaurs are completely known. Probably less than 10 percent of the varieties of dinosaurs that once lived have been identified.
The shape of dinosaur bones provides clues to how these animals interacted with each other. These bones also reveal information about body form, weight, and posture. Surface ridges and hollows on bones indicate the strength and orientation of muscles, and rings within the bones indicate growth rates. Diseased, broken, and bitten bones bear witnesses to the hazards of life during the dinosaurian age. Cavities in bones reflect the shape of the brain, spinal cord, and blood vessels. Delicate ossicles, or small bony structures in the skull, reveal the shape of the eyeball and its pupil. The structure of the skull and fossilized contents of the abdominal region provide clues to diet.
Organic molecules are also preserved within bones in trace quantities. By studying isotopes of s within these molecules, scientists can gather evidence about body-heat flow and about the food and water consumed by dinosaurs. Impressions in sediment depict skin texture and foot shape, and trackways provide evidence about speed and walking habits.
A 113-million-year-old fossil called Scipionyx samniticus, discovered in southern Italy in the late 1980s, is the first fossil identified that clearly shows the structure and placement of internal organs, including the intestines, colon, liver, and muscles. The fossilized internal organs of Scipionyx samniticus give paleontologists information about how dinosaurs metabolized their food, and other general information about dinosaurs.
Beginning in the late 19th century, the field of paleontology grew as scientific expeditions to find fossil remains became more frequent. American paleontologist Othniel Charles Marsh and his collectors explored the western United States for dinosaurian remains. They identified many genera that have since become household names, including Stegosaurus and Triceratops. In the early part of the 20th century, American paleontologist's Barnum Brown and Charles Sternberg demonstrated that the area now known as Dinosaur Provincial Park in Alberta, Canada, is the richest site for dinosaur remains in the world. Philanthropist Andrew Carnegie-sponsored excavations in the great Jurassic quarry - pits in Utah, which subsequently turned into Dinosaur National Monument. Beginning in 1922, explorer Roy Chapman Andrews led expeditions to Mongolia that resulted in the discovery of dinosaur eggs. More recently, Luis Alvarez, a particle physicist and Nobel laureate, and his son, geologist Walter Alvarez, discovered evidence of the impact of an asteroid or comet debris that coincided with the extinction of the dinosaurs. Among foreign scholars, German paleontologist Werner Janensch, beginning in 1909, led well-organized dinosaur collecting expeditions to German East Africa (modern Tanzania), where the complete skeletal anatomy of the gigantic Brachiosaurus was documented.
One of the most important fossil-rich sites is located in China. In a small town about 400 km (about 200 mi) northeast of Beijing, there is a fossil formation, called the Yixian formation. Of which have yielded many fossilized specimens of primitive and bird-like dinosaurs, and soft parts such as feathers and fur. Some scientists believe these fossils provide evidence that may have evolved from dinosaurs. Among the recent finds in the Yixian formation is an eagle-sized animal with barracuda-like teeth and very long claws named Sinornithosaurus millenii. Although this dinosaur could not fly, it did have a shoulder blade structure in which allowed a wide range of arm motion similar to flapping. Featherlike structures covered most of the animal's body.
Another important dinosaur discovery made in 1993 strengthens the evolutionary relationship between dinosaurs and, A 14-year-old boy who was hunting for fossils near Glacier National Park in northern Montana found a fossil of a nearly complete skeleton of a small dinosaur, later named Bambiraptor feinbergi. The fossil is of a juvenile dinosaur only one m (3 ft) long with a body that resembles that of a roadrunner. It has several physical features similar to those of early, including long, winglike arms, bird-like shoulders, and a wishbone. Some scientists propose that Bambiraptor feinbergi may be a type of dinosaur similar to those from which evolved. Other scientists believe that the animal lived too late in time to be ancestral to, while still other scientists hypothesize that dinosaurs may have led to flying ancestral dinosaurs, from which more than once in evolutionary time.
Argentina is another area rich in fossils. In 1995 a local auto mechanic in Nequén, a province on the eastern slopes of the Andes in Argentina, found the fossils of Giganotosaurus, a meat-eating dinosaur that may have reached a length of more than 13 m's (43 ft). Five years later, in a nearby location, a team of researchers unearthed the bones of what could be the largest meat-eating dinosaur. The newly discovered species is related to the Giganotosaurus, but it was larger, reaching a length of 14 m's (45 ft). This dinosaur was heavier and had shorter legs than the Tyrannosaurus rex. The fossilized bones indicate that the dinosaur's jaw was shaped like scissors, suggesting it used its teeth to dissect prey.
In early 2000 AD., scientists used X-rays to view the chest cavity of a dinosaur fossil found in South Dakota. Computerized three-dimensional imaging revealed the remains of what is thought to be the first example of a dinosaur heart ever discovered. The heart appears to contain four chambers with a single aorta, a structure that more closely resembles the heart of a bird or mammal than the heart of any living reptile. The structure of the heart suggests that the dinosaur may have had a high metabolic rate that is more like that of an active warm-blooded animal than that of a cold-blooded reptile.
Many unusual dinosaur fossils found in the Sahara in northern Africa might be related to dinosaur fossils discovered in South America, indicating that the two continents were connected through most of the dinosaurian period. These findings, along with other studies of the environments of dinosaurs and the plants and animals in their habitats, help scientists learn how the world of dinosaurs resembled and differed from the modern world.
The ancestors of dinosaurs were crocodile-like-creatures called archosaurs. They appeared early in the Triassic Period and diversified into a variety of forms that are popularly known as the thecodont group of reptiles. Many of these creatures resembled later Cretaceous dinosaurs. Some archosaurs led to true crocodiles. Others produced pterosaurs, flying reptiles that possessed slender wings supported by a single spar-like finger. Still other archosaurs adopted a bipedal (two-legged) posture and developed S-shaped necks, and it was certain species of these reptiles that eventually evolved into dinosaurs.
Fossil evidence of the earliest dinosaurs dates from about 230 million years ago. This evidence, found in Madagascar in 1999, consists of bones of an animal about the size of a kangaroo. This dinosaur was a type of saurischian and was a member of the plant-eating prosauropods, which were related to ancestors of the giant, long-necked sauropods that included the Apatosaurus. Before this discovery, the earliest known dinosaur on record was the Eoraptor, which lived 227 million years ago. Discovered in Argentina in 1992, the Eoraptor was an early saurischian, one m (3 ft) long, with a primitive skull.
Scientists have identified the isolated bones and teeth of a few tiny dinosaurs representing ornithischians dating from the beginning of the Jurassic Period, around 205 million years ago. By the middle of the Jurassic Period, around 180 million years ago, most of the basic varieties of saurischian and ornithischian dinosaurs had appeared, including some that far surpassed modern elephants in size. Dinosaurs had evolved into the most abundant large animals on land, and the dinosaurian age had begun.
Earth's environment during the dinosaurian era was far different from it is today. The days were several proceeding moments shorter than they are today because the gravitational pull of the sun and the moon have over time had a braking influence on Earth's rotation. Radiation from the Sun was not as strong as it is today because the Sun has been slowly brightening over time.
Other changes in the environment may be linked to the atmosphere. Carbon dioxide, a gas that traps heat from the Sun in Earth's atmosphere-the so-called greenhouse effect-was several times more abundant in the air during the dinosaurian age. As a result, surface temperatures were warmer and no polar ice caps could form.
The pattern of continents and oceans was also very different during the age of dinosaurs. At the beginning of the dinosaurian era, the continents were united into a gigantic super-continent called Pangaea (all lands), and the oceans formed a vast world ocean called Panthalassa (all seas). About 200 million years ago, movements of Earth's crust caused the super-continent to begin slowly separating into northern and southern continental blocks, which broke apart further into the modern continents by the end of the dinosaurian era.
Because of these movements of Earth's crust, there was less land in equatorial regions than there is at present. Deserts, possibly produced by the warm, greenhouse atmosphere, were widespread across equatorial land, and the tropics were not as rich an environment for life forms as they are today. Plants and animals may have flourished instead in the temperate zones north and south of the equator.
The most obvious differences between dinosaurian and modern environments are the types of life forms present. There were fewer than half as many species of plants and animals on land during the Mesozoic Era than there are today. Bushes and trees appear to have provided the most abundant sources of food for dinosaurs, rather than the rich grasslands that feed most animals today. Although flowering plants appeared during the dinosaurian era, few of them bore nuts or fruit.
The animals of the period had slower metabolisms and smaller brains, suggesting that the pace of life was relatively languid and the behaviour were simple. The more active animals-such as ants, wasps, and mammals-first made their appearance during the dinosaurian era but was not as abundant as they are now.
The behaviour of dinosaurs was governed by their metabolism and by their central nervous system. The dinosaurs' metabolism-the internal activities that supply the body's energy needs-affected their activity level. It is unclear whether dinosaurs were purely endothermic (warm-blooded), like modern mammals, or ectothermic (cold-blooded), like modern reptiles. Endotherms regulate their body temperature internally by means of their metabolism, rather than by using the temperature of their surroundings. As a result, they have higher activity levels and higher energy needs than ectotherms. Ectotherms have a slower metabolism and regulate their body temperature by means of their behaviour, taking advantage of external temperature variations by sunning themselves to stay warm and resting in the shade to cool down. By determining whether dinosaurs were warm or cold-blooded, paleontologists could discover whether dinosaurs behaved more like modern mammals or more like modern reptiles.
Gradual changes in dinosaur anatomy suggest that the metabolic rates and activity levels of dinosaurs increased as they evolved, and some scientists believe this indicates that dinosaurs became progressively more endothermic. Overall, dinosaur body size decreased throughout the latter half the dinosaurian era, increasing the dinosaurs' need for activity and a higher metabolism to maintain warmth. Smaller animals have more surface area in proportion to their volume, which causes them to lose more heat as it radiates from their skin. Well-preserved fossils show that many small dinosaurs were probably covered with hair or feather-like fibres. Dinosaurs' tooth batteries (many small teeth packed together) became larger, enabling them to chew their food more efficiently, their breathing passages became separated from their mouth cavity, allowing them to chew and breathe while, and their nostrils became larger, making their breathing more efficient. These changes may have helped the dinosaurs digest their food and change it into energy more quickly and efficiently, thereby helping them maintain a higher metabolism.
The central nervous system of dinosaurs affected their behavioural flexibility-how much they could adapt their behaviours to deal with changing situations. Scientists believe that the ratio of dinosaurs' brain size to their body weight increased as the animals evolved. As a result, their behavioural flexibility increased from a comparable level to that of modern crocodiles, in the primitive dinosaurs, to a level that is comparable to that of modern chickens and opossums, in some small Cretaceous dinosaurs.
Imprints of the skin of large dinosaurs show that the skin had a textured surface without hair or feathers. The eyes of dinosaurs were about twice the diameter of those of modern mammals. The skeleton of one small dinosaur was found preserved in windblown sand. Its head was tucked next to its forelimbs, resembling the posture of a modern bird, and its tail was wrapped around its body, resembling the posture of a cat.
Many, if not all, dinosaurs laid eggs, and extensive deposits of whole and fragmented shells have been found in China, India, and Argentina, suggesting that large nesting colonies were common. A very few eggs have been identified from the skeletons of embryos contained within them. In proportion to the body weight of the mother, dinosaurs laid smaller eggs in greater numbers than do. Scientists have found what they believe is a typical nest dug into Cretaceous streamside clays in Montana. The nest is a craterlike structure about two m's (6.6 ft) in diameter-thought to be about the diameter of the mother's body.
The large number of bones of small dinosaurs that have been found in nesting colonies indicates that the mortality rate of juveniles was very high. The growth rings preserved in dinosaur bones suggest that primitive dinosaurs grew more slowly than later dinosaurs. The growth rings in some giant dinosaurs suggest that these dinosaurs may have grown to adulthood rapidly and had shorter life spans than some large modern turtles, such as the giant tortoise, which can live 200 years in captivity.
Saurischian dinosaurs were characterized by a primitive pelvis, with a single bone projecting down and back from each side of the hips. This pelvis construction was similar to that of other ancient reptiles but, unlike other reptiles, saurischians had stronger backbones, no claws on their outer front digits, and forelimbs that were usually much shorter than the hind limbs. There were three basic kinds of saurischians: theropods, prosauropods, and sauropods.
Nearly all theropods were bipedal flesh eaters. Some theropods, such as Tyrannosaurus of the late part of the Cretaceous Period, reached lengths of twelve m's (39 ft) and weights of 5 metric tons. In large theropods the huge jaws and teeth were adapted to tearing prey apart. Fossil trackways reveal that these large theropods walked more swiftly than large plant-eating dinosaurs and were more direct and purposeful in their movements. Other theropods, such as The Compsognathus, were small and gracefully built, resembling modernly running such as the roadrunner. Their heads were slender and often beaked, suggesting that these theropods fed on small animals such as lizards and infant dinosaurs. Some of them possessed brains as large as those of modern chickens and opossums.
Other theropods, called raptors, bore powerful claws, like those of an eagle, on their hands and feet and used their flexible tails as balancing devices to increase their agility when turning. These animals appear to have hunted in packs. Many paleontologists believe that may have arisen from small, primitive theropods that were also ancestors of the raptors. Evidence for this theory has been augmented by the discovery of an Oviraptor nest in the Gobi Desert. The nest contains the fossil bones of an Oviraptor sitting on its brood of about fifteen eggs, exhibiting behaviours remarkably similar to that of modern.
Unlike the primitive theropods, the prosauropods had relatively small skulls and spoon-shaped, rather than blade-shaped, teeth. Their necks were long and slender and, because they were bipedal, the prosauropods could browse easily on the foliage of bushes and trees that were well beyond the reach of other herbivores. A large clawed, a hook-like thumb was probably used to grasp limbs while feeding. The feet were broad and heavily clawed. When prosauropods appeared in the fossil record along with the earliest known theropods, they had already reached lengths of three m's (10 ft). By the end of the Triassic Period, the well-known Plateosaurus had attained a length of nine m's (30 ft) and a weight of 1.8 metric tons. During the late Triassic and early Jurassic periods, prosauropods were the largest plant-eating dinosaurs.
Sauropods, which include giants such as Apatosaurus (formerly known as Brontosaurus) and Diplodocus, descended from prosauropods. By the middle of the Jurassic Period they had far surpassed all other dinosaurs in size and weight. Some sauropods probably reached lengths of more than twenty-five m's (82 ft) and weighed about 90 metric tons. These dinosaurs walked on four pillar-like legs. Their feet usually bore claws on the inner toes, although they otherwise resembled the feet of an elephant. The sauropod backbone was hollow and filled with air sacks similar to those in a bird's vertebrae, and the skull was small in proportion to the animals' size. The food they ate was ground by stones in their gizzard, a part of their digestive tract. Indeed, sauropods may be compared with gigantic elephants, with the sauropods' long necks performing the function of an elephant's trunk, and their gizzard stones acting as the strong teeth of an elephant. Some sauropods, such as the late Jurassic Apatosaurus, used their long, thin tails as a whip for defence, while others used their tails as clubs.
In ancestral ornithischians the bony structure projecting down and back from each side of the hips was composed of two bones, so that their hips superficially resembled the hips of. Early ornithischians were small bipedal plant eaters, about one m (3 ft) in length. These animals led to five kinds of descendants: stegosaurs, ankylosaurs, ornithopods, pachycephalosaurs, and ceratopsians.
Paleontology helps to study the prehistoric animal and plant life through the analysis of fossil remains. The study of these remains enables scientists to trace the evolutionary history of extinct and living organisms. Paleontologists also play a major role in unravelling the mysteries of the earth's rock strata (layers). Using detailed information on how fossils are distributed in these layers of rock, paleontologists help prepare accurate geologic maps, which are essential in the search for oil, water, and minerals.
Most people did not understand the true nature of fossils until the beginning of the 19th century, when the basic principles of modern geology were established. Since about 1500, scholars had engaged in a bitter controversy over the origin of fossils. One group held that fossils are the remains of prehistoric plants and animals. This group was opposed by another, which declared that fossils were either freaks of nature or creations of the devil. During the 18th century, many people believed that all fossils were relics of the great flood recorded in the Bible.
Paleontologists gain most of their information by studying deposits of sedimentary rocks that formed in strata over millions of years. Most fossils are found in sedimentary rock. Paleontologists use fossils and other qualities of the rock to compare strata around the world. By comparing, they can determine whether strata developed during the same time or in the same type of environment. This helps them assemble a general picture of how the earth evolved. The study and comparison of different strata are called stratigraphy.
Fossils provide most of the data on which strata are compared. Some fossils, called index fossils, are especially useful because they have a broad geographic range but a narrow temporal one-that is, they represent a species that was widespread but existed for a brief period. The best index fossils have a tendency leaning toward being marine creatures. These animals evolved rapidly and spread over large areas of the world. Paleontologists divide the last 570 million years of the earth's history into eras, periods, and epochs. The part of the earth's history before about 570 million years ago is called Precambrian time, which began with the earth's birth, probably more than four billion years ago.
The earliest evidence of life consists of microscopic fossils of bacteria that lived as early as 3.6 billion years ago. Most Precambrian fossils are very tiny. Most species of larger animals that lived in later Precambrian time had soft bodies, without shells or other hard body parts that would create lasting fossils. The first abundant fossils of larger animals date from about 600 million years ago.
Coming to be, is the Paleozoic era, of which it lasted for about 330 million years. It includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. Index fossils of the first half of the Paleozoic era are those of the invertebrates, such as trilobites, graptolites, and crinoids. Remains of plants and such vertebrates as fish and reptiles make up the index fossils of the second half of this era.
At the beginning of the Cambrian period (570 million to 500 million years ago) animal life was entirely confined to the seas. By the end of the period, all the phyla of the animal kingdom existed, except vertebrates. The characteristic animals of the Cambrian period were the trilobites, a primitive form of arthropod, which reached their fullest development in this period and became extinct by the end of the Paleozoic era. The earliest snails appeared in this period, as did the cephalopod mollusks. Other groups represented in the Cambrian period were brachiopods, bryozoans, and Foraminifera. Plants of the Cambrian period included seaweeds in the oceans and lichens on land.
The most characteristic animals of the Ordovician period (500 million to 435 million years ago) were the graptolites, which were small, colonial hemichordates (animals possessing an anatomical structure suggesting part of a spinal cord). The first vertebrates-primitive fish-and the earliest corals emerged during the Ordovician period. The largest animal of this period was a cephalopod mollusk that had a shell about three m's (about 10 ft) in length. Plants of this period resembled those of the Cambrian periods.
The most important evolutionary development of the Silurian period (435 million to 410 million years ago) was that of the first air-breathing animal, a scorpion. Fossils of this creature have been found in Scandinavia and Great Britain. The first fossil records of vascular plants-that are, land plants with tissue that carries food-appeared in the Silurian period. They were simple plants that had not developed separate stems and leaves.
The dominant forms of animal life in the Devonian period (410 million to 360 million years ago) were fish of various types, including sharks, lungfish, armoured fish, and primitive forms of ganoid (hard-scaled) fish that were probably the evolutionary ancestors of amphibians. Fossil remains found in Pennsylvania and Greenland indicate that early forms of amphibia may already have existed during the Devonian period. Early animal forms included corals, starfish, sponges, and trilobites. The earliest known insect was found in Devonian rock.
The Devonian is the first period from which any considerable number of fossilized plants have been preserved. During this period, the first woody plants developed, and by the end of the period, land-growing forms included seed ferns, ferns, scouring rushes, and scale trees, the modern relative of club moss. Although the present-day equivalents of these groups are mostly small plants, they developed into treelike forms in the Devonian period. Fossil evidence indicates that forests existed in Devonian times, and petrified stumps of some larger plants from the period measure about 60 cm (about twenty-four in) in diameter.
The Carboniferous period lasted from 360 million to 290 million years ago. During the first part of this period, sometimes called the Mississippian period (360 million to 330 million years ago), the seas contained a variety of echinoderms and foraminifer, with most forms of animal life that appeared in the Devonian. A group of sharks, the Cestraciontes-or shell-crushers-were dominant among the larger marine animals. The predominant group of land animals was the Stegocephalia, an order of primitive, lizard-like amphibians that developed from the lungfish. The various forms of land plants became diversified and grew larger, particularly those that grew in low-laying swampy areas.
The second part of the Carboniferous, sometimes called the Pennsylvanian period (330 million to 290 million years ago), saw the evolution of the first reptiles, a group that developed from the amphibians and lived entirely on land. Other land animals included spiders, snails, scorpions, more than 800 species of cockroaches, and the largest insect ever evolved, a species resembling the dragonfly, with a wingspread of about 74 cm (about twenty-nine in.). The largest plants were the scale trees, which had tapered trunks that measured as much as 1.8 m's (6 ft) in diameter at the base and thirty m's (100 ft) in height. Primitive gymnosperms known as cordaites, which had pithy stems surrounded by a woody shell, were more slender but even taller. The first true conifers, forms of advanced gymnosperms, also developed during the Pennsylvanian period.
The chief events of the Permian period (290 million to 240 million years ago) were the disappearance of many forms of marine animals and the rapid spread and evolution of the reptiles. Usually, Permian reptiles were of two types: lizard-like reptiles that lived entirely on land, and sluggish, semiaquatic types. A comparatively small group of reptiles that evolved in this period, the Theriodontia, were the ancestors of mammals. Most vegetation of the Permian period was composed of ferns and conifers.
The Mesozoic era is often called the Age of Reptiles, because the reptile class was dominant on land throughout the age. The Mesozoic era lasted about 175 million years, and included the Triassic, Jurassic, and Cretaceous periods. Index fossils from this era include a group of extinct cephalopods called ammonites, and extinct forms of sand dollars and sea urchins
The most notable of the Mesozoic reptiles, the dinosaur, first evolved in the Triassic period (240 million to 205 million years ago). The Triassic dinosaurs were not as large as their descendants in later Mesozoic times. They were comparatively slender animals that ran on their hind feet, balancing their bodies with heavy, fleshy tails, and seldom exceeded 4.5 m's (15 ft) in length. Other reptiles of the Triassic period included such aquatic creatures as the ichthyosaurs, and a group of flying reptiles, the pterosaurs.
The first mammals also appeared during this period. The fossil remains of these animals are fragmentary, but the animals were apparently small and reptilian in appearance. In the sea, Teleostei, the first ancestors of the modern bony fishes, made their appearance. The plant life of the Triassic seas included a large variety of marine algae. On land, the dominant vegetation included various evergreens, such as ginkgos, conifers, and palms. Small scouring rushes and ferns still existed, but the larger members of these groups had become extinct.
During the Jurassic period (205 million to 138 million years ago), dinosaurs continued to evolve in a wide range of size and diversity. Types included heavy four-footed sauropods, such as Apatosaurus (formerly Brontosaurus); two-footed carnivorous dinosaurs, such as Allosaurus; Two-footed vegetarian dinosaurs, such as Camptosaurus, and four-footed armoured dinosaurs, such as Stegosaurus. Winged reptiles included the pterodactyl, which, during this period, ranged in size from extremely small species to those with wingspreads of 1.2 m's (4 ft). Marine reptiles included plesiosaurs, a group that had broad, flat bodies like those of turtles, with long necks and large flippers for swimming; Ichthyosauria, which resembled dolphins, and primitive crocodiles, as the mammals of the Jurassic period consisted of four orders, all of which were smaller than small modern dogs. Many insects of the modern orders, including moths, flies, beetles, grasshoppers, and termites appeared during the Jurassic period. Shellfish included lobsters, shrimp, and ammonites, and the extinct group of belemnites, which resembled squid and had cigar-shaped internal shells. Plant life of the Jurassic period was dominated by the cycads, which resembled thick-stemmed palms. Fossils of most species of Jurassic plants are widely distributed in temperate zones and polar regions, indicating that the climate was uniformly mild.
The reptiles were still the dominant form of animal life in the Cretaceous period (138 million to sixty-five million years ago). The four types of dinosaurs found in the Jurassic also lived during this period, and a fifth type, the horned dinosaurs, also appeared. By the end of the Cretaceous, about sixty-five million years ago, all these creatures had become extinct. The largest of the pterodactyls lived during this period. Pterodactyl fossils discovered in Texas have wingspreads of up to 15.5 m's (50 ft). Other reptiles of the period include the first snakes and lizards. Several types of Cretaceous have been discovered, including Hesperornis, a diving bird about 1.8 m's (about 6 ft) in length, which had only vestigial wings and was unable to fly. Mammals of the period included the first marsupials, which strongly resembled the modern opossum, and the first placental mammals, which belonged to the group of insectivores. The first crabs developed during this period, and several modern varieties of fish also evolved.
The most important evolutionary advance in the plant kingdom during the Cretaceous period was the development of deciduous plants, the earliest fossils of which appear in early Cretaceous rock formations. By the end of the period, many modern varieties of trees and shrubs had made their appearance. They represented more than 90 percent of the known plants of the period. Mid-Cretaceous fossils include remains of beech, holly, laurel, maple, oak, plane tree, and walnut. Some paleontologists believe that these deciduous woody plants first evolved in Jurassic times but grew only in upland areas, where conditions were unfavourable for fossil preservation.
The Cenozoic era (sixty-five million years ago to the present time) is divided into the Tertiary period (sixty-five million to 1.6 million years ago) and the Quaternary period (1.6 million years ago to the present). However, because scientists have so much more information about this era, they have an aptitude to focus on the epochs that make up each period. During the first part of the Cenozoic era, an abrupt transition from the Age of Reptiles to the Age of Mammals occurred, when the large dinosaurs and other reptiles that had dominated the life of the Mesozoic era disappeared.
Index fossils of the Cenozoic tend to be microscopic, such as the tiny shells of Foraminifera. They are commonly used, along with varieties of pollen fossils, to date the different rock strata of the Cenozoic era.
The Paleocene epoch (sixty-five million to fifty-five million years ago) marks the beginning of the Cenozoic era. Seven groups of Paleocene mammals are known. All of them appear to have developed in northern Asia and to have migrated to other parts of the world. These primitive mammals had many features in common. They were small, with no species exceeding the size of a small modern bear. They were four-footed, with five toes on each foot, and they walked on the soles of their feet. Most of them had slim heads with narrow muzzles and small brain cavities. The predominant mammals of the period were members of three groups that are now extinct. They were the creodonts, which were the ancestors of modern carnivores; the amblypods, which were small, heavy-bodied animals; and the condylarths, which were light-bodied herbivorous animals with small brains. The Paleocene groups that have survived are the marsupials, the insectivores, the primates, and the rodents.
During the Eocene epoch (fifty-five million to thirty-eight million years ago), several direct evolutionary ancestors of modern animals appeared. Among these animals - all of which were small in stature-were the horse, rhinoceros, camel, rodent, and monkey. The creodonts and amblypods continued to develop during the epoch, but the condylarths became extinct before it ended. The first aquatic mammals, ancestors of modern whales, also appeared in Eocene times, as did such modern as eagles, pelicans, quail, and vultures. Changes in vegetation during the Eocene epoch were limited chiefly to the migration of types of plants in response to climate changes.
During the Oligocene epoch (thirty-eight million to twenty-four million years ago), most of the archaic mammals from earlier epochs of the Cenozoic era disappeared. In their place appeared representatives of many modern mammalian groups. The creodonts became extinct, and the first true carnivores, resembling dogs and cats, evolved. The first anthropoid apes also lived during this time, but they became extinct in North America by the end of the epoch. Two groups of animals that are now extinct flourished during the Oligocene epoch: the titanotheres, which are related to the rhinoceros and the horse; and the oreodonts, which were small, dog-like, grazing animals.
The development of mammals during the Miocene epoch (twenty-four million to five million years ago) was influenced by an important evolutionary development in the plant kingdom: the first appearance of grasses. These plants, which were ideally suited for forage, encouraged the growth and development of grazing animals such as horses, camels, and rhinoceroses, which were abundant during the epoch. During the Miocene epoch, the mastodon evolved, and in Europe and Asia a gorilla-like ape, Dryopithecus, was common. Various types of carnivores, including cats and wolflike dogs, ranged over many parts of the world.
The Paleontology of the Pliocene epoch (five million to 1.6 million years ago) does not differ much from that of the Miocene, although the period is regarded by many zoologists as the climax of the Age of Mammals. The Pleistocene Epoch (1.6 million to 10,000 years ago) in both Europe and North America was marked by an abundance of large mammals, most of which were practically modern in type. Among them were buffalo, elephants, mammoths, and mastodons. Mammoths and mastodons became extinct before the end of the epoch. In Europe, antelope, lions, and hippopotamuses also appeared. Carnivores included badgers, foxes, lynx, otters, pumas, and skunks, and now-extinct species such as the giant saber-toothed tiger. In North America, the first bears made their appearance as migrants from Asia. The armadillo and ground sloth migrated from South America to North America, and the muskox ranged southward from the Arctic regions. Modern human beings also emerged during this epoch.
Cave Paint in at Lascaux France, are expressive portions of the cave painting in Lascaux, and was carried out by Palaeolithic artists in or around 13,000 Bc. At the end of the Pleistocene Epoch, the cow and other groups of small horses were painted with red and yellow. Whereas, ochre colours were either blown through reeds onto the wall or mixed with animal fat to apply in the squirted by reeds or thistles. It is believed that prehistoric hunters painted these to gain magical powers that would ensure a successful hunt.
The remains of simple animals provide additional information about climate and climatic change. Because different beetle species are especially well suited for either warm or cool climates, the presence of fossils of a particular type of beetle can give scientists clues to the climate of the region. Fossil algae reveal much about water acidity or alkalinity, water temperature, and the speed of water movement. The Ocean Drilling Program, which collects samples from the sea-floor, has collected enough data to show that the distribution of marine organisms changed significantly during the Pleistocene Epoch.
Invertebrates-animals without backbones, such as shellfish and insects-and plant communities survived the glacial cycles of the Pleistocene Epoch as moderately unscathed. Some animal and plant groups, such as the beetles, moved vast distances but underwent little evolution. Pleistocene mammals, on the other hand, underwent important changes, probably because climate changes affected mammals more than they did invertebrates. Many mammals have evolved significantly since the Pleistocene. Some changes in familiar animals include greater numbers of species of mice and rats and the appearance of modern species of the dog family.
Many mammalian species have become extinct since the Pleistocene. A few of the spectacular mammals that disappeared during the last 20,000 years include the woolly rhinoceros, the giant ground sloth, the saber-toothed tigers, the giant cave bear, the mastodon, and the woolly mammoth. These animals existed just when early humans, and drawings of them exist on cave walls in Europe. Recent theories suggest that these huge mammals could not reproduce quickly enough to replace the number of animals that humans killed for food, and were therefore driven to extinction by human hunting.
Humans continued to evolve during the Pleistocene Epoch. Two genera, Australopithecus and Homo, existed during the early Pleistocene. The last Australopithecines disappeared about one million years before present. Several species of the genus Homo existed during the Pleistocene. Modern humans (Homo sapiens sapient) probably arose from The Homo erectus, which are thought to have evolved from Homo habilis. Paleontologists have found fossils that support the transition between Homo erectus and Homo sapiens dating from about 500,000 years before present to about 200,000 years before present. Anatomically modern humans (Homo sapiens) arose from an earlier human species that lived in Africa. A likely ancestor, known as Homo ergaster, evolved from around 1.9 million years ago. This ancestor arose from an earlier Pleistocene species in Africa, perhaps one known as Homo rudolfensis. Anatomically a modern Homo sapiens appears to have evolved by 130,000 years ago, if not earlier. For a time our species also coexisted in parts of Eurasia with another species of The Homo, Homo neanderthalensis, until between 35,000 and 30,000 years ago. Since then only, our species has survived.
Evidence from both lands and sea environments shows that, at least before the human-induced global warming of the last two centuries, the worldwide climate has been cooling naturally for several thousand years. Ten thousand years have already passed since the end of the last glaciation, and 18,000 years have passed since the last maximum. This may suggest that Earth have entered the beginning of the next worldwide glaciation.
Several possible causes of ice ages exist. Scientists have proposed many theories to explain their occurrence. In the 1920s Yugoslav scientist Milutin Milankovitch proposed the Milankovitch Astronomical Theory, variations in Earth's position can cause which state's climatic fluctuations and the onset of glaciation compared with the Sun. Milankovitch calculated that this deviation of Earth's orbit from its almost circular path occurs every 93,408 years. We have also linked the movement of Earth's crustal plates, called plate tectonics, to the occurrence of ice ages. The positions of the plates in polar regions may contribute to ice ages. Changes in global sea level may affect the average temperature of the planet and lead to cooling that may cause ice ages. Separate theories explaining the causes of ice ages such as the substantial variations of heat output of the Sun, or the bearing of interplanetary dust cloud, that absorb the sun's heat for reaching the Earth, and, perhaps from a meteorite affect-have not yet been supported by any solid evidence.
The Milankovitch Astronomical Theory best explains regular climatic fluctuations. The theory is based on three variations in the position of Earth compared with the Sun: the eccentricity (elongation or circularity of the shape) of Earth's orbit, the tilt of Earth's axis toward or away from the Sun, and the degree of wobble of Earth's axis of rotation. The total effect of these changes causes one region of Earth-latitude 60° to seventy north, near the Arctic Circle-to receive low amounts of summer radiation about once every 100,000 years. These cool summer periods last several hundred to several thousand years and thus provide sufficient time to allow snowfields to expand and merge into glaciers in this area, signalling the beginning of glaciation.
When glaciers expand during an ice age, the sea level drops because the water that forms glaciers ultimately comes from the oceans. Global sea level affects the overall temperature of the planet because solar radiation, or heat, is better absorbed by water than by land. When sea levels are low, more land surfaces becomes exposed. Since the land is not able to absorb as much solar radiation as the water can, the overall average temperature of the planet decreases, or cools, and may contribute to the onset of an ice age.
A map showing Earth during an ice age would look very different from a map of contemporaneousness resulting to our world divergence. During the Wisconsin glaciation of 115,000 to 10,000 years ago, two ice sheets, the Laurentides and the Cordilleran, covered the northern two-thirds of North America, including most of Canada, with ice. Other parts of the world, including Eurasia and parts of the North Atlantic Ocean, were also blanketed in sheets of ice
The Laurentides continental ice sheet extended from the eastern edge of the Rocky Mountains to Greenland. The separate Cordilleran Ice Sheet was composed of mountains and ice cap valley glaciers. Flowing onto the surrounding lowlands, in as much as these partial reservoirs were equally part of northern Alaska, and of the Sierra Nevada. The Cascade Range and the Rocky Mountains and as far south as New Mexico contains the whole from which begins their feeding waters from the Cordilleran Ice sheet. Where the continental shelf between Alaska and Siberia was uncovered, the Bering land bridge formed. In northern Eurasia, continental ice extended from Great Britain eastward to Scandinavia and Siberia. Separate mountains, and glacial systems covered the Alps, the Himalayas, and the Andes. The extensive ice sheets on Antarctica and Greenland did not expand very much during to each of glaciation. Sea ice grew worldwide, particularly in the North Atlantic Ocean.
Years of investigation and research, coupled with resolution and courage to follow wherever truth might lead, have established the certainty of a future world cataclysm during which most of the earth's population will be destroyed in the same manner as the mammoths of prehistoric times were destroyed. Such an event has occurred each time that one or two polar ice caps grew to maturity; a recurrent event in global history is clearly written in the rocks of a very old earth.
The earth is approximately four ½ billion years old. Human beings have been living on it for at least 500,000 years and perhaps even one million years. To appreciate the immensity of these figures, one might imagine the age of the earth represented by the period of about one week; the duration of our own epoch, 7,000 years, is then but one second! By a similar analogy, men have lived on an earth that is one week old for just two minutes. Evidently, our own epoch is but a very short and insignificant period in the life of our planet and our species.
In past epochs there have been ice caps at one or both of the geographical poles. The heat of the sun caused these ice caps to grow larger. As the sun heats the air of the hemisphere, the heated air expands, becomes lighter, and rises. The updrafts are greatest in the tropics. As the earth is virtually spherical, the currents of warm air converge at the poles. Meeting head-on from every direction, they create areas of air pressure, become colder and heavier, turn downward, reversing the direction of their flow, and pour back toward the Equator from the polar centres with high velocities. Thus, there is a continuous circulation of raising humid warm air journeying poll ward and a down draft of cold dehumidified air returning from the poles at low or ground altitudes. Air acts like a sponge. When warm, it absorbs water, when cold it cannot hold much water, and in cooling releases any surplus moisture to fall as rain or snow.
Most of the snow that falls in the polar regions does not melt; the air temperature is too low. Instead, the snow is stored, changing to glacial ice. As this process continues through time, the ice masses at the poles constantly grow in volume.
As the prehistoric ice caps grew larger, they're lent of an ectomorphic drift turning the rotating planet off balance because of the wobble of the earth, causing the earth to roll around sideways to its direction of rotation.
Another analogy will make this clear. When you place a weight at the end of a string and then rotate the string in a circle, the weighted end of the string rises to a horizontal plane. Now, imagine yourself and the string as the earth, the weight at the end of the string as the weight of a growing ice cap, and imagine that, instead of intentionally swinging the weighted string, the rotational motion encompasses you, the string and the weight, as though you were standing on a rotating platform. In this depiction, then, your body represents both the present Axis of Spin and Axis of Figure of the earth. Your body does not move; the Axis of Spin remains the same. However, your arm and the weighted extend like a string, here representing a radius of the earth, rise from the vertical (directed toward the pole) to the horizontal (directed toward the Equator). The sphere of which your arm and the weighted string are some radiuses is rolled sideways; the weight, representing the imbalance of an ice cap, rotates from a polar position to an equatorial position. The Axis of Figure, previously represented by your vertical arm, is now changed; the old Axis of Figure is now perpendicular to the Axis of Spin.
The rotating equilibrium thrown off balance by the weight of the growing ice caps, causes the spinning globe to roll over on its side. Yet such an event does not occur lightly. The oceans, like water in a bowl that is suddenly moved, are cast from their basins to flood the land. The winds, previously settled into patterns dependent upon a stable globe, are whipped asunder by the sudden shifting of the globe. The sudden meeting of warm and cold air creates great pressure zones that spawn new rains and hurricanes to sweep across the earth. The forces of nature, loosed from their equilibrium, range wildly in search of new equilibrium. Bringing us to lay upon the Stone Age, of which this period of human technological development characterized by using stone as the principles raw material for tools. In a given geographic region, the Stone Age normally predated the invention or spread of metalworking technology. Human groups in different parts of the world began using stone tools at different times and abandoned stone for metal tools at different times. Broadly speaking, however, the Stone Age generated by times generations estimate the concurrent evidence around 2.5 million years ago. Ending of a partial differentiation that globally extends by 5,000 years ago, moreover, the world's regional intervals became intermittently more recent. Today only a few isolated human populations rely largely on stone for their technologies, and that reliance is rapidly vanishing with the introduction of tools from the modern industrialized world.
Human ancestors living before the Stone Age likely used objects as tools, a behaviour that scientists find today among chimpanzees. Wild chimpanzees in Africa exhibit a range of tool-using behaviours. For example, they used bent twigs to fish for termites, chewed wads of leaves to soak up liquid, and branches and stones as hammers, anvils, missiles, or clubs. However, when prehistoric humans began to make stone tools they became dramatically distinct from the rest of the animal world. Although other animals may use stone objects as simple tools, the intentional modification of stone into tools, and using tools to make other tools, is behaviourally unique to humans. Although those Africans of 100,000 years ago had more modern skeletons than did their Neanderthal contemporaries, they made essentially the same crude stone tools as Neanderthals, still lacking standardized shapes. This stone Toolmaking and tool-uses were a haling behaviour that became indispensable to the way early humans adapted to their environment and partially affected human evolution
Human technology developed from the first stone tools, in use by two and a half million years ago, to the 1996 laser printer that replaced the outdated 1992 laser printer and used to print out manuscripts depicted of these pages. The rate of development was undetectably slow at the beginning, when hundreds of thousands of years passed with no discernible change in our stone tools and with no surviving evidence for artifacts made of other materials. Today, technology advances so rapidly that it is reported in the daily newspaper.
Archaeologists believe the Stone Age began in the vicinity to 2.5 million years ago because that marks the age of the earliest stone tool remnants ever discovered. The earliest recognizable stone artifacts mark the beginnings of the archaeological record-that is, material remnants of ancient human activities. As recently as 5,000 years ago all human societies on the face of the earth were essentially still living in the Stone Age. Therefore, more than 99.8 percent of humans' time as Toolmaker-from 2.5 million years ago to 5,000 years ago-took places during the Stone Age. During the Stone Age our ancestors went through many different stages of biological and cultural evolution. It was long after our lineage became anatomically modern that we began to experiment with innovations such as metallurgy, heralding the end of the Stone Age.
The term Stone Age has been used since the early 1800s as a designation for an earlier, prehistoric stage of human culture, one in which stone rather than metal tools were used. By the early 1800s different archaeological sites had equalled uncovered those European involvements embedded by some mysterious components from apparently foregoing of prehistoric intervals. Christian Thomsen, curator of the National Museum in Copenhagen, Denmark, developed a classification scheme to organize the museum's growing collections into three successive technological stages in the human past: Stone Age, Bronze Age, and Iron Age. The three set -value-class were quickly adopted and spread throughout the museums in Europe. In addition, among excavators, who were to their finding results of remnant categories, that each set class was of a constant basis as justified freely by these three set -stages. The fact that Stone Age remnants were found at the bottom layers showed that they were the oldest
The study of the Stone Age falls under the fields of anthropology, which is the study of human life and cultural origins of human life up to the present, and Archaeology, which is the study of the material remains of humans and human ancestors. Archaeologists seek out, explore, and study archaeological sites, locations around the world where historic or prehistoric people left behind traces of their activities. Archaeologists use the data collected to make theories about how human ancestors lived.
Archaeologists normally use the term artifact to refer to objects modified by human action, either intentionally or unintentionally. The term tool is used to refer to something used by a human or a human ancestor for some purpose and may be modified or not. For instance, a thrown rock is a tool, even if it were not modified. Giving a demonstration of a particular stone artifact is usually difficult as once used as a prehistorical tool, so in practice, archaeologists prefer to use the term artifact instead. In relation to the earlier stages of the Stone Age the unused debris or waste from the manufacture of stone tools is also considered artifactual.
Stone artifacts are important to archaeologists who study prehistoric humans, because they can yield a wide range of information about ancient peoples and their activities. Stone artifacts are, in fact, often the principle archaeological remnants that persist after the passage of time and as such can give important clues as to the presence or absence of ancient human populations in any given region or environment. Careful analysis of Stone Age sites can yield crucial information regarding the technology of prehistoric Toolmaker. Yet leaving to no doubt that we are dealing with biologically and behaviourally modern humans, and, in turn are we to give anthropologists insight into the levels of cognitive (thinking) ability at different stages of human evolution.
Cro-Magnon garbage heaps yield not only stone tools but also tools of bone, whose suitability for shaping, for instance, into fish hooks has apparently gone unrecognized by previous humans. Tools were produced for their adaptivity and distinctive adjustive measures of shapes, their modernity for functions as needles, awls, engraving tools, and so on obviously tell their own story. Instead of only single-piece tools such as hand-held scrapers, Multi-piece tools made their appearance. Recognizable Multi-piece weapons at Cro-Magnon suites include harpoons, spear-throwers, and eventually the bow and arrows, the precursors of rifles and other Multi-piece modern weapons. Those efficient means of killing at a safe distance permitted the hunting of such dangerous prey as rhinos and elephants, while the invention of rope for nets, lines, and snares allowed the addition of fish and to our dirt. Remains of houses and sewn clothing testify to a greater improved ability to survive in cold climates.
During the Stone Age, Earth experienced the most recent in a succession of ice ages, in which glaciers and sea ice covered a large portion of Earth's surface. The most recent ice age period lasted from 1.6 million to 10,000 years ago, a period of glacial and warmer interglacial stages known as the Pleistocene Epoch. The Holocene Epoch began at the end of the ice age 10,000 years ago and continued to the present time.
Early hominids made stone artifacts either by smashing rocks between a hammer and anvil (known as the bipolar technique) to produce usable pieces or by acceding to a greater extent the controlled process termed flaking, in which stone chips were fractured away from a larger rock by striking it with a hammer of stone or other hard material. Subsequently, throughout the last 10,000 years, additional techniques of producing stone artifacts were to include pecking, grinding, sawing, and boring, in so that it turns into other traditional standards. The most excellent rock for flaking lean of a hard, fine-grained, or amorphous (having no crystal structure) rocks, including lava, obsidian, ignimbrites, flint, chert, quartz, silicified limestone, quartzite, and indurated shale. Ground stone tools could be made on a wider range of raw material types, including coarser grained rock such as granite.
Flaking produces several different types of stone artifacts, which archaeologists look forward to at prehistoric sites. The parent pieces of rock from which chips have been detached are called cores, and the chips removed from cores are called flakes. A flake that has had yet smaller flakes removed from one or more edges to become sharper or form the contours of the known of a retouched piece. The stone used to knock flakes from cores is called a hammerstone or a precursor. Other flaking artifacts include fragments and chunks, most of which are broken cores and flakes.
The terms culture and industries both refer to a system of technology (Toolmaking technique, for example) shared by different Stone Age sites of the same broad time. Experts now prefer to use the term industry instead of culture to refer to these shared Stone Age systems.
Archaeologists have divided the Stone Age into different stages, each characterized by different types of tools or tool-manufacturing techniques. The stages also imply broad time frames and are perceived as stages of human cultural development. The most widely used designations for the successive stages are Palaeolithic (Old Stone Age), Mesolithic (Middle Stone Age), and Neolithic (New Stone Age). British naturalist Sir John Lubbock in 1865 defined the Palaeolithic stage as the period in which stone tools were chipped or flaked. He defined the Neolithic as the stage in which ground and polished stone axes became prevalent. These two stages also were associated with different economic and subsistence strategies: Palaeolithic peoples were hunters-gatherers while Neolithic peoples were farmers. Archaeologists subsequently identified a separate stage of stone tool working in Eurasia and Africa between the Palaeolithic and the Neolithic, called the Mesolithic. This period is characterized by the creation of microliths, small, geometric-shaped stone artifacts attached to wood, antler, or bone to form tools such as arrows, spears, or scythes. Microliths began appearing between 15,000 and 10,000 years ago at the end of the Pleistocene Ice Age.
The Palaeolithic/Mesolithic/Neolithic division system was first applied only to sites in Europe, but is now widely used (with some modification) to refer to prehistoric human development in much of Asia, Africa, and Australasia. Different terminology is often used to describe the cultural-historical chronology of the Americas, which humans did not reach until some point between 20,000 and 12,000 years ago. However, there is a general similarity, the transitional form of flaked stone tools are associated with prehistoric hunters-gatherers to both flaked and ground stone tools associated with the rise of early farming communities. The period in the Americas up to the end of the Pleistocene Ice Age about 10,000 years ago, when most humans were hunters-gatherers, is convened as Paleo-Indian and the subsequent, post-glacial period is known as Archaic.
Archaeologists subdivide the Palaeolithic into the Lower Palaeolithic (the earliest phase), Middle Palaeolithic, and Upper Palaeolithic (the later phase), based upon the presence or absence of certain classes of stone artifacts.
The Lower Palaeolithic dates from approximately 2.5 million years ago until about 200,000 years ago and include the earliest record of human Toolmaking and documents much of the evolutionary history of the genus Homo from its origins in Africa to its spread into Eurasia. Two successive Toolmaking industries characterize the Lower Palaeolithic: the Oldowan and the Acheulean.
The Oldowan industry was named by British Kenyan anthropologists Louis Leakey and Mary Leakey for early archaeological sites found at Olduvai Gorge in northern Tanzania. It is also sometimes called the chopper-core or pebble tool industry. Simple stone artifacts made from small stones or blocks of stone characterize the Oldowan industry. Mary Leakey classified Oldowan artifacts as either heavy-duty tools or light-duty tools, as both their classifications deemed to be heavy-duty tools, which include core types such as choppers, discoids, polyhedrons, and heavy-duty scrapers. Many of these cores may have been produced to generate sharp-edged flakes, but some could have been used for chopping or scraping activities as well. Light-duty tools include retouched forms such as smaller scrapers, awls (sharp, pointed tools for punching holes in animal hides or wood), and burins (chisel like flint tools used for engraving and cutting). Oldowan techniques of manufacturing included hard hammer percussion, or detaching flakes from cores with a stone hammer; the anvil technique, striking a core on a stationary anvil to detach flakes; and bipolar technique, detaching flakes by placing the core between an anvil and the hammerstone.
Early humans probably also made tools from a wide range of materials other than stone. For example, they probably used wood for simple digging sticks, spears, clubs, or probes, and they probably used shell, hide, bark, or horn to fashion containers. Unfortunately, organic materials such as these do not normally survive from earlier Stone Age times, so archaeologists can only speculate about whether such tools were used.
Two of the antiquated Oldowan settings are in Ethiopia, overcoming (as, arrested nearly 2.5 million years ago) and formerly (2.3 million years ago), as this differential studies of the Oldowan localities include Lokalalei (2.3 million years ago), Koobi Fora (1.9 million to 1.4 million years ago) in Kenya, Olduvai Gorge (1.9 million to 1.2 million years ago) in Tanzania, and Ain Hanech (possibly about 1.7 million years ago) in Algeria. The cave deposits at Sterkfontein and Swartkrans (estimated to be from 2.0 million to 1.5 million years ago), in South Africa.
Theories about the intelligence and culture of prehistoric man are beginning to be drastically revised. Accumulated evidence now depicts European men living between 100,000 and 10,000 years ago as communal men who were skilled hunters and Toolmaker, who had developed formal burial rites for members of their tribes and perhaps the orienting initiation among those of whom gainfully employed them as the first religiousities to express a newer beginning of beliefs with the ritual burials for an animals and fellow tribesman alike. Who had some belief in an afterlife, who took excellent care of they're sick and elderly, and who, in their heyday, carried around pocket sized calendars of their own making.
A ten-member international expedition, led by Ralph S. Solecki of Columbia University, found the bones of a dismembered deer ritually buried by Neanderthal men about 50,000 years ago. The bones of the deer's foot, jaw, and back, its shoulder blades, and the top of its skull were found buried 5 feet deep in the Nahr Ibrahim Cave, north of Beirut, Lebanon. The presence of the skull, the bed of stones on which the bones were placed, and the red earth colouring of the bones, which was not native to the cave, said that a ritual known as hunters' magic was involved in the burial. Solecki interpreted the burial as an attempt "to ensure a successful hunt by the ceremonial treatment of an animal." Although evidence existed show that bears were ritually treated by Neanderthal men, this was the first discovery of a lone deer buried in this manner.
An American expedition, also led by Solecki, excavated a mountain cave near Shanidar in Iraqui Kurdistan and discovered evidence that Neanderthals practiced a form of religious burial suggesting a belief in an afterlife: at least one actualized in totality from nine skeletons uncovered in the cave was buried with flowers. Also found in the cave was the skeleton of a man of about forty, comparable to a modern age of eighty, who had been born with a deformed right arm. A Neanderthal doctor had skilfully amputated the arm above the elbow, and judging by his death at a ripe old age, the man was carefully cared for from his boyhood until he died because of a rock fall inside the cave, a common peril then.
Recent pale ontological examinations of skeletons suggest that the Neanderthals' stooped posture was the result of a vitamin D deficiency. Lack of sunlight during the Ice Age might have caused their upright posture to become deformed by rickets.
In January it was revealed that a sophisticated system of notation charting the phases of the moon was used throughout most of Europe during the last Ice Age, beginning around 34,000 years ago. Convincingly between such as scribbles and gouges on pieces of bones, antlers, and stone, may have previously seemed regarded as decorations, but were manifested to be representations of the lunar calender. Alexander Marshack, a research associate at the Peabody Museum of Archaeology and Ethnology at Harvard University, began investigating the markings in 1964 and published the results of his study this year in France. The inscribed objects he studied represented all cultural levels from 34,000 to 10,000 years ago. All were pocket sized, and as many as twenty-four tools were used to cut a single sequence, some covering a year or more. This system of notation seems to anticipate the development of a calendar, the idea of number, and the use of abstract symbols. It had been thought that such cognitive abilities developed only after the start of an agricultural society, less than 10,000 years ago.
A tribe of about twenty-four people living a Stone Age way of life was found in the Tasaday Forest on the southern Philippines' Mindanao Island in July. Anthropologists speculate that the tribe has been cut off from the rest of the world for at least 400 years and maybe as much as 2,000 years.
The tribe was first discovered five years ago by an official conducting a census survey. He described the finding a tribe of "jungle people so mysterious that they were known only as the bird who walks the forest like the wind." A long search led to the Tasaday. Interpreters at first had trouble understanding the tribe's language, which is related to Manubo, a native Filipino tongue in the Malayo-Polynesian family.
As communication became easier, it was found that the tribe calls itself the Tasaday because "the man who owns the forest in which they live told their ancestors in a dream to call themselves Tasadays, after a mountain." When asked whether they had ever been off the island, the Tasadays replied that they did not know leaving was possible; in fact, it was found that they had never even seen the ocean. The Tasadays are monogamous in mating but communal in all other ways, have no leader, know no other tribe, have known no unfriendly people, and have never heard of fighting.
The Tasadays committing not to encourage the famished foods but endeavouring to venture as afar from their clearing, consisting of the food, which subsists easily, and founded in the flourishing vegetation of the forest, through which they dwell. The staple of their diet is the pith of the wild palm. To supplement this, they catch tadpoles and small fish with their hands from the nearby streams. Monkey meat is considered a delicacy. After the monkey's hair is singed in a fire and cut away with bamboo blades sharpened by small stones, the meat is roasted.
The group includes six families with thirteen children, nine of whom are boys. All matters of mutual concern, such as food gathering, are decided in an open meeting.
New information about the Mayan civilization, the most highly developed civilization in the New World before the arrival of the white man, was gained from the discovery of an 11 -page codex fragment of a Mayan calendar book. (A codex is a manuscript copy of an ancient text.) The fragment is said to be part of a larger book about twenty pages long. The three other known codices were brought to Europe during the Spanish conquest but did not emerge as important historical material until the 1900's. The newly discovered codex is the first to be found in over a century.
Composed of bark cloth, like the other three, the 11- page codex is expected to reveal "pictorial information on the Venus calendar and its influence on Mayan religion and astrology," according to Michael D. Co., professor of anthropology at Yale University. The fragment dates to the late Mayan period, between 1400 AD. and 1500 AD. The new fragment reveals that the Mayans viewed all four phases of the Venus cycle as threatening. Previously, only the first phase was thought to have been considered sinister.
All four cycles of Venus as seen from the earth were measured by Mayan priests, who calculated that each cycle took 584 days to be completed. Modern astronomers calculate 583.92 days for each complete cycle. The complete 20- page codex would have covered sixty-five Venus cycles.
The co. believes the fragment to be authentic "because it is on bark cloth, [because of] the condition of the fragment. In addition, none of the applicative material duplicates or imitate anything we know about, being identical to the Venus calendar. Lastly, because no forger could be unscrupulous enough to invent material displaying so much knowledge of Mayan life."
Early Slavic tribes formed an organized state in the fourth to sixth centuries, about 500 years earlier than was believed, according to evidence reported in Tass, the Soviet press agency. Arkady Bugai, the Ukrainian archaeologist accredited with the discovery, predicated his determination on radiocarbon dating of charred wood detected in the remains of so named Serpentine Wall, a 500-mile network of defensive earthen works that once encircled the present site of Kiev, the Ukrainian capital. The charred wood used in the radiocarbon tests was from what is believed to be the remains of trees burned to clear ground for the wall. Bugai reasoned that a highly organized state was required to move the seven billion cubic feet of earth that made up the wall, which rises to a height of thirty to 35 feet and is 50 feet wide at its base.
The Serpentine Wall, which enclosed a roughly triangular area, was assumed to have been built to defend the Kiev area from hostile tribes. Ukrainian scholars now believe that the area must have had a population of approximately one million people of whom all was constructed. It was formerly believed that the first consolidation of Russian tribes occurred around the tenth century, during the rise of Kievan Russia.
An expedition bent on disproving the theory that the American man came to North America by crossing a land bridge over what is now the Bering Strait began in September. Gene Savoy, the American explorer who is known for his 1964 discovery of the ruined Inca city of Vilcabamba in Peru, believes that American man originated in the jungles east of the Andes Mountains in South America, where he thinks advanced civilizations flourished as long ago as 1500 Bc. The discovery of a new species of human ancestors and of fossils of the oldest human beings yet to be found in Europe dominated the news in anthropology in 1995.
The discovery of fossils of a new species of a human ancestor-Australopithecus Anamensis-at sites near Lake Turkana in Kenya was announced in August. Anamensis, a small-scale brained upright walker resembling the famous Lucy skeleton (identified with the species' Australopithecus afarensis), weighed about 110 pounds. The complete upper and lower jaws, a set of lower teeth, a skull fragment, the teeth of several individuals, and a shinbone were dated to between 4.1 million and 3.9 million years ago, according to Meave Leakey (wife of Richard Leakey), one of the accorded archeological researchers.
Anamensis: investigations point to what may seem directly ancestral too consequential afarensis (dated at 3.6 million years old). The shinbone abides of the oldest overseer was evidentially discovered for uprightness: a bipedal locomotion with the ability to walk on two legs: a defining trait of humans. The earliest known evidence before this was the track (3.7 million years old) of three humanlike individuals, probably australopithecines, who strolled across a bed of fresh volcanic ash in what is now Laetoli, Tanzania.
The relationship between Anamensis (from anam, a native Kenyan term for ‘lake') and an even older species whose discovery was announced in 1994 was unclear. The older species, found in the Middle Awash region of Ethiopia, was first named Australopithecus ramidus. The genus name was later changed to Ardipithecus (‘ground apes'). The teeth and scanty bone fragments of Ardipithecus ramidus were dated at 4.4 million years old.
Fragmentary fossil remains of at least four humans thought to be intermediate between Homo erectus and archaic forms of The Homo sapiens, the later species to which all modern humans belong, were found in caves in Atapuerca in northern Spain, according to a report published in August. Dated as at least 780,000 years old by means of a Paleomagnetic dating technique, the stone tools and skeletal fragments -including some from an adolescent and some from a child-of skulls, hands, and feet represent the oldest humans yet discovered in Europe. The researchers who found the fossils said they could possibly be distant ancestors of the Neanderthals who appeared in Europe hundreds of thousands of years later.
The Spanish fossils partly fill a gap in the history of human evolution and expansion around the world. Previously, the oldest human fossils found in Europe, dating back 500,000 years, belonged to Heidelberg man, a likely ancestor of the Neanderthals, found at the Mauer site in Germany near the French border. It is known, however, that descendants of the earliest humans had spread from Africa to Asia well more than a million years ago. Among reasons given by anthropologists for the late occupation of Europe by Homo is the harshness of Europe's Ice Age climate.
Finds of Neanderthaloid skulls and skeletons continue to be reported from widely separated areas. Digging in a cave at Mount Circeo on the Tyrrhenian sea, 50 miles south of Rome, Italy, Alberto Carlo Blanc uncovered an almost perfectly preserved Neanderthal skull, perfect except a fracture in the right temporal region. It is the third of this type found in Italy. The two skulls previously reported were found in 1929 and 1935 in the Sacopastore region, near Rome, but in not nearly so well preserved a condition as the present find. No other human bones were found here, but the skull was accompanied by fossilized bones of elephants, rhinoceri, and giant horses, all fractured, thus giving some evidence of the mode of life of Neanderthal man. Professor Sergio Sergi, of the Institute of Anthropology at the Royal University of Rome, who has studied this skull in detail believes it to be 70,000 to 80,000 years old. He concludes also that Neanderthal man walked comparably as modern man and not with head thrust forward as had previously been assumed.
Another Neanderthal skeleton is reported to have been found in a cave in Middle Asia by A. P. Okladnikoff of the Anthropological Institute of Moscow University and the Leningrad Institute of Anthropology. The bones of the skeleton were badly shattered, but the jaw and teeth of the skull, it crushed at the back, were almost complete
Hominids that were contemporary with Oldowan sites included two major lineages. Its first functional lines are the robust australopithecines (so called because their cheek teeth were larger than those of other australopithecines). This robust australopithecines - such as Australopithecus aethiopicus and Australopithecus boisei in East Africa, and Australopithecus robustus in South Africa-were bipedal and had small brains, large jaws, and large molars. The other lineage is made up of bipedal, larger brain size, and smaller toothed early members of the genus Homo, such as the Homo habilis, Homo's rudolfensis, and early Homo erectus. The oldest fossils of The Homo erectus (sometimes called Homo ergaster) found in Africa dates back to about 1.85 million years ago. This species is characterized by an even larger brain and smaller teeth than earlier hominids and by a larger body size. (In 1984 anthropologists in Kenya found a nearly complete skeleton of an adolescent Homo erectus who would have been 1.8 m.'s
(6 ft.) tall as an adult.)
Experts are intuitively certain that these species were responsible for individual Oldowan sites. These species may have made and used Oldowan style stone tools to varying degrees. However, anthropologists have long suspected that the larger-brained and smaller-toothed Homo was probably a more habitual toolmaker. It is likely that Homo erectus was responsible for many Oldowan sites more recent than 1.85 million years ago. In any case, by one million years ago, all these species but The Homo erectus had gone extinct, so researchers can be certain that at least the Homo's lineage was involved in using and making stone tools. The Homo erectus appears to have moved out of Africa and into Eurasia sometime before one million years ago, although some anthropologists think this geographic spread of hominids may have occurred nearly two million years ago.
The everyday life of Oldowan hominids is largely a matter of archaeological conjecture. Most sites in East Africa are found near lakes or along streams, suggesting that they preferred to live near water sources. Studies of rock sources call to mind that Oldowan hominids sometimes transported stone several kilometres to the sites where stone artifacts are found. Well-preserved sites often have collections of stone artifacts and fragmented fossil animal bones associated together, often in dense concentrations of several thousand specimens. Scholars disagree regarding the nature of these sites. Some archaeologists interpret them as camps, home bases, or central foraging places, similar to those formed by modern hunter-gatherers during their daily activities. Others think that such sites represent scavenging stations where hominids were primarily involved in processing and consuming animal carcasses. Still others view these accumulations as stone caches where hominids collected stone in areas where such raw materials did not occur naturally.
Fossil remains from some Oldowan sites suggest that Oldowan hominids used stone tools to process meat and marrow from animal carcasses, some weighing several hundred pounds. Although some archaeologists have argued that large game hunting may have occurred in the Oldowan, many Oldowan specialists believe these early Stone Age hominids likely obtained most of their meat from large animals primarily through scavenging. The early hominids may have hunted smaller animals opportunistically, however. Modern experiments have shown that sharp Oldowan flakes are especially useful for the processing of animal carcasses -for example, skinning, dismembering, and non-anaesthetized defleshing. The bulk of early hominid diet likely consisted of a variety of plant foods, such as berries, fruits, nuts, leaves, flowers, roots, and tubers, but there are little archaeological records of such perishable foodstuffs.
The term Acheulean was first used by 19th- century French archaeologist Gabriel de Mortillet to refer to remnants of a prehistoric industry found near the town of Saint Acheul in northern France. The distinguishing feature of this site is an abundance of stone hand axes, tools more sophisticated than those found at Oldowan sites. The term Acheulean is now used to refer to hand axe industries in Africa, the Near East, Europe, and Asia dating near 1.5 million years ago to 200,000 years ago and spanning human evolution from A Homo erectus to early archaic Homo sapiens.
The characteristic Acheulean hand axe is a large, pointed or oval shaped form. These hand axes were often made by striking a blank (a rough chunk of rock) from a larger stone and then shaping the blank by carefully removing flakes around its perimeter. Usually, both sides, or faces, of the blank were flaking, a process called bifacial flaking. Later Acheulean hand axes may have been produced by the soft hammer technique, in which a softer hammer of stone, bone, or antler produced thinner, more carefully shaped forms. Other associated forms include cleavers, bifacial artifacts with a sharp, guillotine like bit at one end, its profound thickness and directed artifacts known as picks. Simpler, typical Oldowan artifacts are usually also found at Acheulean sites, and a range of retouched flake tools such as scrapers. Experiments have implicated that Acheulean hand axe and cleavers are excellent tools for heavy-duty slaughtering activities, such as severing animal limbs. Some archaeologists, however, believe they may have served other functions, or perhaps were general, all purpose tools.
Acheulean tools did not entirely replace Oldowan tools. Archaeologists have discovered many sites where Oldowan tools were used throughout the Acheulean time, sometimes in the same geographic region as Acheulean industries. Interestingly, the Acheulean might be especially restricted to Africa, Europe, and western Asia, with few sites in East Asia of stone industries with typical Acheulean hand axes and cleavers during the Lower Palaeolithic. Most of the industries found in East Asia have tendencies toward being simpler, Oldowan like technologies that can be seen at sites at Nihewan and the cave of Zhoukoudian in northern China.
Well-studied Acheulean sites include those at Olduvai Gorge and Isimila, in Tanzania, Olorgesailie, in Kenya, Konso Gardula and Melka Kunture, in Ethiopia: Kalambo Falls, in Zambia, Montagu Cave in South Africa, and Tabun and Gesher Benot Ya'aqov in Israel, Abbeville and Saint Acheul in France, and Swanscombe and Boxgrove, in England, Torralba and Ambrona, of Spain. Most anthropologists think that Acheulean populations of The Homo erectus and early Homo sapiens were probably more efficient hunters than Oldowan hominids. Recently discovered wooden spears from about 400,000 years ago at Schöningen, Germany, and a 300,000-year old wooden spear tips from Clacton, England, suggest that the hominids who made applicable may have hunted game extensively.
Experts disagree about whether Acheulean hominids and their contemporaries harnessed the use of fire. Archaeologists have found evidence such as apparent burnt bone and stone, discoloured sediment, and the presence of charcoal or ash at most sites, including Cave of Hearths, in South Africa, Zhoukoudian, in China, and Terra Amata in France. Discrete fireplaces (hearths), however, may be quite rare. Similarly, there is only questionable evidence of huts or other architectural features.
The Middle Palaeolithic Epoch extends from around 200,000 years ago until about 30,000 years ago. It is also called the Mousterian Industry in Europe, the Near East, and North Africa and called the Middle Stone Age in sub-Saharan Africa.
Toolmaker in the Middle Palaeolithic used a range of retouched flake tools, especially sided scrapers, serrated scrapers, backed knives (blade tools with the non-blade and very much as to be dull sided in the apparatuses to fit comfortably in the hand), and points. Experts believe these tools were used to work animal hides, to shape wood tools, and as projectile points. This period is also characterized by specially prepared cores. Using the disc core method, a circular core could produce many flakes to serve as blanks for retouched tools. With the Levallois method (named after a suburb of Paris, France, where the first such artifacts were discovered), flakes of a predetermined shape were removed from specially prepared cores. This process resulted in ovally-shaped flakes or large, triangular points, depending on the type of Levallois core. Levallois core and flakes are first seen at some late Acheulean sites but become much more common in the Middle Palaeolithic/Middle Stone Age.
Some regional variation can be seen among Middle Palaeolithic industries. A North African variant known as African produced tools and point characterized by tangs (stems projecting from the base of the tool or point, to allow the tool to be attached to a handle or shaft). In Eastern Europe, a variant called Szeletian produced two-sided, leaf-shaped points, a style not usually seen elsewhere until the Upper Palaeolithic Age. In Central Africa, a variant called the Sangoan produced a range of heavy-duty picks and axes.
Middle Palaeolithic/Middle Stone Age archaeological sites are often found in the deposits of caves and rock shelters. Well-studied caves include Pech de l'Aze, Combe Grenal, La Ferrassie, La Quina, and Combe Capelle, in France. Tabun, Kebara, Qafzeh, and Skhul, in Israel, Shanidar, in Iraq, Haua Fteah, in Libya and Klasies River Mouth, in South Africa, also in East Asia, site that are contemporary with the Middle Palaeolithic Ages, are often too, exhibit a simpler Toolmaking technology, without as much standardization of the flake tool forms as in much of the rest of Eurasia and Africa.
Hominids associated with the Middle Palaeolithic Epoch is to include Neanderthals and other archaic Homo sapiens (Homo sapiens predating anatomically modern humans, who lived from about 200,000 to 35,000 years ago). In Europe, the Middle Palaeolithic Age is associated with Homo sapiens neanderthalensis, or Neanderthals, who lived from about 200,000 to 35,000 years ago. Neanderthals were short, robust humans with fully modern cranial capacity. They had more jutting faces, more prominent brow ridges, thicker cranial bones, and larger nose cavities than modern humans. Skeletal remains show that Neanderthals were very robust and muscular. Healed injuries to some skeletons suggest that Neanderthals led stressful, rigorous lives. Famous Neanderthal discoveries include Neander Valley, in Germany, La Chapelle-aux-Saints and La Ferrassie, in France, Krapina, in Croatia, Monte Circeo and Saccopastore, in Italy, Shanidar, in Iraq and Tabun and Amud, in Israel. Fossils of an archaic Homo sapiens from this time have been found at sites such as Dali and Maba, in China and at Florisbad, in South Africa, and Ngaloba, in Tanzania. In addition, fossils interpreted as early anatomically modern humans have been found at some Middle Palaeolithic/Middle Stone Age sites in parts of Africa and the Near East, such as at Qafzeh and Skhul, in Israel, and Klasies River Mouth, in South Africa.
Middle Palaeolithic hominids appear to have been more successful hunters than their predecessors. Abundant animal remains suggest that these hominids ate many kinds of large mammals. It is unknown, however, how much of the meat consumed was obtained through hunting, as opposed to scavenging. Accumulations of remains at some sites show that some animals were of a common species and were adults in their prime, which some researchers suggest is an indication of efficient hunting behaviour. Several sites in Europe that contain the carcass of one or more large animals are believed to be butchery sites, where early humans processed the spoils of kills. Some archaeologists have also argued that some Middle Palaeolithic stone points were probably attached to spears, a development in hunting technology. At Klasies River Mouth Cave in South Africa, archaeologists discovered a buffalo vertebra with a broken tip of what was probably a spearhead embedded in it, which could be evidence that the large mammal was hunted or trapped by hominids.
Middle Palaeolithic hominids have tendencies given to evidence of more behavioural complexity than their predecessors. For example, although most of the stone found at most Middle Palaeolithic sites are local-its sources within a few kilometres of a site-an increasing percentage is exotic stone, transported from its sources tens of kilometres away. Simple hearths at many Middle Palaeolithic sites suggest habitual fire use and possible firemaking as well. Evidence of housing is still quite uncommon, but is present at some sites. For example, at Molodovo, Ukraine, a circle of mammoth bones has been interpreted as a hut structure. Microscopic studies of residues on Middle Palaeolithic scraper tools suggest that they may have been used for woodworking and to work animal hides for use as clothing or in shelters.
Over the Middle Palaeolithic Epoch, hominids spread across much of Eurasia. The use of fire and clothing and the ability to build more substantial shelters may have helped them survive in cold regions, such as the central Asian steppe. By 40,000 years ago, near the end of the Middle Palaeolithic Age, when humans entered Australia, which apparently would have required traversing some distance of open ocean, probably in some form of craft. Some Middle Palaeolithic sites have skeletal remains interpreted as simple burials. Non-representational art is known from this period, although occasional ornaments such as beads have been found at late Middle Palaeolithic/Middle Stone Age sites.
Opinion is divided among anthropologists about whether Neanderthals and other archaic Homo sapiens had fully modern cognitive abilities, particularly the ability to recognize and are known through symbolic gesture, a skill required to form modern languages. On one hand, the large cranial capacities of these populations might suggest modern human cognitive and behavioural capabilities. On the other hand, their technological development was very slow, and they left behind no trace of the use of symbols, such as representational cave paintings. Archaeologists have found much greater evidence of symbolism and cultural complexity during the Upper Palaeolithic.
The Upper Palaeolithic Age extends from approximately 40,000 years ago until the end of the last ice age, about 10,000 years ago. This era is known as the Paleo-Indian period in the Americas, and as the Later Stone Age in a sub-Saharan Africa, where it extended much longer, even to historical times in parts of the continent. In the Upper Palaeolithic, standardized blade industries appear and become much more widespread than in previous times. The first of these industries to appear in the Near East and Europe is known as Aurignacian. Later Upper Palaeolithic industries include the Perigordian, Solutrean, and Magdalenian. The Upper Palaeolithic Epoch is usually characterized by specially prepared cores from which blades (flakes at least twice since they are wide) were struck off with a bone or antler punch. Upper Palaeolithic humans also developed new forms of scrapers, backed knives, burins, and points. Beautifully made, two-sided, leaf-shaped points are also common in some Upper Palaeolithic industries. Toward the end of the Upper Palaeolithic, microliths (small, geometric shaped blade segments) became increasingly common in many areas.
By the end of the Upper Palaeolithic period and the end of the last ice age about 10,000 years ago, human populations had spread to every continent except Antarctica. Humans had effectively adapted to the northern latitudes of Eurasia and had dispersed into the American continents. The earliest documented occupation of the Americas appears to have been during the late ice age, about 12,000 to 10,000 years ago. The first recognized Paleo-Indian industry is known as Clovis, which was followed by Folsom. These industries produced delicately crafted, bifacial points fluted, meaning that the base of the point is thinned by removing a large flake from one or both sides. Fluted Clovis points have been found at mammoth kill sites, while Folsom points are associated with bison kills, mammoths being extinct by that time.
Famous Upper Palaeolithic occupation sites include Laugerie Haute, La Madeleine, Abri Pataud, and Pincevent, in France; Castillo, Altamira, and El Juyo, in Spain; Dolní Vestonice, in the Czech Republic; Mezhirich, in Ukraine; Sungir and Kostenki, in Russia; Ksar Akil, in Lebanon; Kebara, in Israel; Zhoukoudian Upper Cave, in China, Haua Fteah, in Libya and Taforalt, in Morocco, and the well-known Later Stone Age sites in sub-Saharan Africa includes Lukenya Hill, in Kenya, Kalemba, in Zambia, Rose Cottage Cave, Wilton Cave, Nelson Bay Cave, and Boomplaas in South Africa. The most famous Paleo-Indian sites are those moved to the United States near the eastern New Mexico towns of Clovis and Folsom, which gave the industries their names.
Human fossil identifiers are characterized with the Upper Palaeolithic, Paleo-Indian, and Later Stone Ages are usually anatomically modern human, or The Homo sapiens, when in the 19th century, Homo sapiens skeletal remains were grounded within the associated generation with early Upper Palaeolithic artifacts at the unrefined refuge of Cro-Magnons in southern France. The term Cro-Magnon Man has occasionally been manipulated by the frame reference that is anatomically contemporaneous of the humans existing out of the Upper Palaeolithic Era. Not all humans were anatomically modern in this period, however. In the initial depictions of the Upper Palaeolithic Epoch, the localities that make up the Chatelperronian industry remain associated with late Neanderthals, conceivably by it is unduly influenced persuaded by modern humans participating with Aurignacian technology.
During the Upper Palaeolithic Age, tools of bone, antler, and ivory become common for the first time. These tools include points, barbed harpoons, spear throwers, awls, needles, and tools interpreted as spears shaft straighteners. The presence of eyed needles depicts the use of sewn clothing (presumably of hide and possibly early textiles) or hide coverings for tents or shelters. In some carvings from this period, human figures are depicted wearing hooded parkas or other vestments. Other technological innovations include lamps (in hollowed out stones filled with flammable substances such as oil or animal fat) and probably the bow and arrow (small projectile points have been interpreted as arrowheads). Many Upper Palaeolithic artifacts might be evidence of composite technology, in which multiple components were combined to form one tool or process. For example, spear tips were attached with binding material to spear shafts, which were flung using spear throwers (sometimes called atlatls). A spear thrower usually took the form of a length of wood or bone with a handle on one end and a peg or socket at the other to hold the butt of a spear or dart. When swung overhand together, the spear thrower provided greater thrust on the spear.
Upper Palaeolithic populations appear to have been competent-hunter-gatherers. The use of mechanical devices such as spear throwers and, probably, arm bows and an arrowed weapon allowed them to increase the velocity, penetrating force, and distance of projectiles. Many Upper Palaeolithic sites contain large quantities of mammal bones, often with one species predominating, such as red deer, reindeer, or horse. It is believed that many of these Upper Palaeolithic hunter-gatherers could effectively predict the timing and location of seasonal resources, such as reindeer migrations or salmon runs.
Many Upper Palaeolithic sites feature elements interpreted as evidence of housing. These are commonly patterns of bone or stone concentrations that seem to delineate hut or tent structures. At the sites of Étiolles and Pincevent, in France, the distribution of stone artifacts, animal bones, hearths, and postholes has been interpreted as evidence of clearly defined huts. At Mezhirich, in the Ukraine, and Kostenki, in Russia, hut structures were found made of stacked or aligned mammoth bones. Distinctive hearths, often lined or ringed with rocks, is much more common in the Upper Palaeolithic Period than in earlier times.
Stone for tools was often obtained from more distant sources, sometimes in larger quantities than seen previously in the Stone Age. Occasionally, stone was traded or carried over several hundred kilometres. It seems likely, therefore, that trade and transport routes were more formalized than they had been in earlier times. The Upper Palaeolithic Period, without exception unites the trade of exotic materials, through proper documentation such oceanic marine life and exotic semiprecious stones, for personal ornamentation clears itself through the parchments and leaves such ornaments as beads or necklaces, or attire in needs of immediate purchase.
In the Upper Palaeolithic Period, evidential rituals are held in respective set prioritized classification for those categorical to a burial, least of mention, there is a strong possibility that by some sorted and less than are today's religious ceremonials that might have had to some beginning during its time. For example, at Sungir, in Russia, three individuals were buried with ivory spears, pendants and necklaces of shells and animal teeth, and thousands of ivory beads that had apparently been sewn into their clothing.
The prehistoric psychological subject of the humanities, in favouring the arts, would paint, sculpture, and engrave. Comparably or by contrast this subject is inclined by such are the interests concerning the mind's eye. Sites in Europe are famous for their artwork, but prehistoric Stone Age art has also been richly documented in Africa, Australia, and other parts of the world. Animals are common subjects of Upper Palaeolithic art, and human figures and abstract elements such as lines, dots, chevrons, and other geometric designs are also found.
Early humans around the world used natural materials such as red and yellow ochre, manganese, and charcoal to create cave art. Among the hundreds of European sites with Upper Palaeolithic cave paintings, the setting classifications confirming Altamira, in Spain, and Lascaux and the more recently discovered (and archaeologically oldest) Chauvet, in France, gives to a better understanding of imaginative Creationism. Animals such as bison, wild cattle, horses, deer, mammoths, and woolly rhinoceroses are represented in European Upper Palaeolithic cave arts, along with human figures that are uncommon. Later Stone Age paintings of animals have been found at sites such as in Apollo 11 Cave, in Namibia, and stylized engravings and paintings of circles, animal tracks, and meandering patterns have been found in Australia's Koonalda Cave and Early Man Shelter.
Many small sculptures of human female forms (often called Venus figurines) have been found in many sites in Europe and Asia. Small, stylized ivory animal figures made more than 30,000 years ago were discovered in Vogelherd, Germany, and clay sculptures of bison were found in Le Tuc d Audoubert, in the French Pyrenees. In addition, many utilitarian objects-such as spear throwers and batons-were superbly decorated with engravings, sculptures of animals, and other motifs.
The earliest known musical instruments also come from the Upper Palaeolithic. Flutes made from long bones and whistles made from deer foot bones have been found at a number of sites. Some experts believe that Upper Palaeolithic people may have used large bones or drums with skin heads as percussion instruments.
The archaeological record of the Upper Palaeolithic shows a creative explosion of new technological, artistic, and symbolic innovations. There is little doubt that these populations were essentially modern in their biology and cognitive abilities and had fully developed language capabilities. There is a much greater degree of stylistic variation geographically (Some archaeologists have suggested that this be evidence of the emergence of ethnicity) and a more rapid developmental pace during the Upper Palaeolithic Period than in any previous archaeological period. Anthropologists debate whether these new Upper Palaeolithic patterns are due to biological transition or whether they are simply the products of accumulated cultural knowledge and complexity through time.
The Mesolithic (also known as the Epipaleolithic) extends from the end of the Pleistocene Ice Age, about 10,000 years ago, until the period when farming became central to peoples' livelihood, which occurred at different times around the world. The term Mesolithic is generally applied to the period of post-Pleistocene hunting and gathering in Europe and, sometimes, parts of Africa and Asia. In the Americas the post-glacial hunters-a gatherer stage that predates the dominance of agriculture is usually called the Archaic. In the rest of the world, Mesolithic sites are usually characterized by microliths. Microlithic blade segments were commonly retouched into a range of shapes, including crescents, triangles, rectangles, trapezoids, and rhomboids, and thus the tools are often called geometric microliths. These forms often have multiple sharp edges. Many of these microliths probably served as elements of composite tools, such as barbed or a knife edge-tipped spears or arrows, or wooden-handled knives. The microliths were likely inserted into shafts or handles of wood or antler and reinforced with some type of adhesive.
The end of the ice age brought rapid environmental change in much of the world. With the warmer, post-glacial conditions of the Holocene Epoch, ice sheets retreated and sea levels rose, inundating coastal areas worldwide. Temperate forests spread in many parts of Europe and Asia. As these climatic and vegetative changes occurred, large herds of mammals, such as reindeer, were replaced by more solitary animals, such as a red deer, roe deer, and wild pig. Cold-adapted animals, such as the reindeer, elk, and bison, retreated to the north, while others, such as the mammoth, giant deer, and woolly rhinoceros, went extinct. The rich artistic traditions of Upper Palaeolithic Western Europe declined markedly after the end of the ice age. This may in part be because the changing environment made the availability of food and other resources less predictable, requiring population to spend more time searching for resources, leaving less time to maintain the artistic traditions.
Well-studied Mesolithic/Archaic sites include Star Carr, in England; Mount Sandel, in Ireland; Skara Brae, in Britain's Orkney Islands; Vedbæk, in Denmark, Lepenski Vir, in Serbia, Jericho, in the West Bank, Nittano, in Japan, Carrier Mills, in Illinois, and Gatecliff Rockshelter, in Nevada. In the sub-Saharan Africa, many Later Stone Age sites of the Holocene Epoch could broadly be termed Mesolithic, due to their geometric microliths and bows and arrow technology.
During the Mesolithic, human populations in many areas began to exploit a much wider range of foodstuffs, a pattern of exploitation known as broad spectrum economy. Intensifying benefits from unidentified foods made up of wild cereals, seeds and nuts, fruits, small game, fish, shellfish, aquatic mammals and, tortoises, and invertebrates such as a snail. Seemed, nonetheless, that domesticated dogs or other smaller mammals were crucial to those societies possessing them, whereas, wolves were domesticated in Eurasia and North America to become, our dogs used as hunting companions, sentinels, and, in some societies, food.
Initially, one of the most puzzling features of animal domestication is the seeming arbitrariness with which some species have been domesticated while their close relatives have not. It turns out that all but a few candidates for domestication have been eliminated by the Anna Karenina principle. Humans and most animal species make for an unhappy marriage, for one or more of many possible reasons: the animal's diet, growth rates, mating habits, disposition, tendencies to panic, and several distinct features of social organization. Only a few defiant mammal interchanges had ended in happy marriages with humans, by virtue of compatibility on all those separate counts.
To appreciate the changes that have developed under domestication compare the evolution of wolves, the wild ancestors of domestic dogs, particularly placed on parallel with the many breeds of dogs. Some dogs are much bigger than wolves (Great Danes), while others are much smaller (Pekingese). Some are slimmer and built for racing (greyhounds), while others are short-legged and useless for racing (dachshunds). They vary enormously in hair form and colour, sand some are even hairless. Polynesians and Aztecs developed dog breeds specifically raised for food. Comparing a dachshund with a wolf, you would not even suspect that the former had been derived from the latter if you did not already know it. Wolves were independently domesticated to become dogs in the Americas and probably in several different parts of Eurasia, including China and Southwest Asia. Modern pigs are derived from independent sequences of domestication in China, western Eurasia, and possibly other areas well. These examples reemphasize that the same few suitable wild species attracted the attention of many different human societies.
That is, domestication involves wild animals' being transformed into something in addition as applicatory to humans. Truly domesticated animals differ in two ways from their wild ancestors. These differences result from two processes: human selection of those individual animals more useful to humans than other individuals of the same species, and automatic evolutionary responses of animals to the alternate forces of natural selection operating in human environments as compared with wild environments.
The wild ancestors of the Ancient Species of Big Domestic Herbivorous Mammals were spread unevenly over the globe. South America had only one such ancestor, which gave rise to the Ilama and alpaca. North America, Australia and sub-Saharan Africa had none at all. The lack of domestic mammals indigenous to sub-Saharan Africa is especially astonishing, since a main reason why tourist s visit Africa today is to see its abundant and diverse wild mammals. In contrast, the wild ancestors of the Big Herbivorous Mammals were confined to Eurasia.
We are reminded to the many ways in which big domestic mammals were crucial to those human societies that possessing them. Most notably, they provided meat, milked products, fertilizer. Land transport, leather, military assault vehicles. Plow traction, and wool, as well as germs that killed previously unexposed people s.
In addition, of course, small domestic mammals and domestic birds and insects have been useful to humans. Many birds were domesticated for meat, eggs, and feathers: the chicken in China, various duck and goose species in parts of Eurasia, turkey in Mesoamerica, guinea fowl in Africa, and the Muscovy duck in South America. Wolves were domesticated in Eurasia and North America to become dogs used as hunting companions, sentinel, pets, and in some societies, food. Rodent s and other small mammals domesticated for food include the rabbit in Europe, the guinea pig in the Andes, a giant rat in West Arica, and possibly a rodent called the hutia on Caribbean islands. Ferrets were domesticated in Europe to hunt rabbits, and cats were domesticated in North Africa and Southwest Asia t o hunt rodent pests. Small mammals domesticated as recently as the 19th and 20th centuries include foxes, mink, and chinchillas grown for fur and hamsters kept as pets. Even some insects have been domesticated, notably Eurasia's honeybee and China's silkworm moth, kept fo r honey and silk, respectfully.
Many of these small animals thus yielded food, clothing, or warmth. But none of them pulled plows or wagons, none bore riders, none except dogs pulled sleds or became war machines, and none of them have been as important food as have big domestic mammals.
That is, domestication involves wild animals' being transformed into something more useful to humans. Truly domesticated animals differ in processes: human selection of those individual animals more useful to humans than other individuals of th same species, and automatic evolutionary responses of animals to the altered forces of natural selection operating in human environment ‘s' as compared with wild environment ‘s'.
The ways in which domesticate animals have diverged from their wild ancestors include many species changing in size: cows, pigs and sheep became smaller under domestication, while guinea pigs became larger. Sheep and alpacas were selected for retention of wool and reduction of loss of hair, while cows have been selected for high milk yields. Several species of domestic animals have smaller brains and less developed sense organs than their wild ancestors, because they no longer need the bigger brains and more developed sense organs on which the r ancestors depended to escape from wild predators.
Dates of domestication provide a line of evidence confirming Galton's view that early herding-peoples quickly domesticated all big mammal species suitable for being domesticated. All species for whose dates of domestication have archaeological evidence were domesticated between about 8000 and 2500 Bc. - that is, within the first few thousand years of th sedentary farming-herding societies that arose after the end of the last Ice Age. Although the era of big mammal domestication began with the sheep, goat, and pig and ended with camels. Since 2500 Bc., there have been no significant additions.
It's true, of course, that some small mammals' wee first domesticated long after 2500 Bc. For example, rabbits were not domesticated for food until the Middle Ages, ice and rats for laboratory research not until the 20th century, and hamsters for pets not until the 1930s. The continuing developments of domesticated small mammals aren't surprising, because there are literally thousand's of wild species as candidates, and because they were of too little value to traditional societies to warrant the effort of raising them. But big mammal domestication virtually ended 4,500 years ago. By then, all of the world's 148 candidate big species must have been tested innumerable times, with the result that only a few passed the test and no other suitable ones remained.
Almost all of domesticated large mammals that live in herds, they maintain a well-developed dominance hierarchy between herd members and the herds occupy overlapping home ranges than mutually exclusive territories. When the herd is on the move, its members maintain a stereotype order: in the rear, the stallion. In the front, the top-ranking female, followed by her foals in order of age, with the youngest first, an behind her, the other mares in order of rank, each followed by her foals in order of age. In this way, many adults coexist in the herd without constant fighting and wth each knowing in its rank.
That social structure is ideal for domestication, because of a pack line follow the human leaders, they would normally follow the top-ranking female. Herds o packs of sheep, goat, cow and ancestral dogs (wolves) have a similar hierarch, as young animals grow up in such a herd, they imprint onto the animals that they regularly see nearby. Under wild conditions these are members of their own species, but captive young animals also see humans nearby and imprint on humans as well.
Such social animals lend themselves to herding. Since they are tolerant of each other, they can be bunched up. Since they instinctively follow a dominant leader a will imprint on humans as that leader, they can readily be driven by a shepherd or sheepdog. Herd animals do well when penned in crowed conditions, because they are accustomed to ling on densely packed groups in the wild.
In contrast, members of most solitary territorial animal species cannot be herded. They do not tolerate each other, they do not imprint on humans, and the are not very submissive. Whoever saw a line of cats (solitary and territorial in the wild) following a human or allowing themselves to be herded by a human? Every cat lover knows that cats are not submissive too human in the way dogs instinctively are. Cats and ferrets are the sole territorial mammal species that were domesticated, because our motive for doing so was not to herd them in large roup raised for food but to keep them as solitary hunters or pets.
While most solitary territorial species haven't been domesticated, it's not conversely thus haven't been domesticated. It's not conversely the case that most herd species can be domesticated Most can't for one of several additional reasons.
Most herd species don't have overlapping home ranges but instead domesticated. Mst can't for one several additional reason.
Herds of many species don't have overlapping home ranges but instead maintain exclusive territories against other herds. It's n more possible to pen two such herds together than to pen two males in solitary species. Again, many species that live in herds for par of the yer are territorial in the breeding season, when they fight and do not tolerate each other's presence.
Once, agin, many species that living in herds for part of the year are territorial in the breading season, when they fight and do not tolerate each other's presence. That's true of most deer and antelope species (again with the exception of reindeer), and it's one of the main factors that has disqualified all the social antelope species for which Africa is famous from being domesticated. While one's first association to African antelope is "vast dense herd spreading across the horizons." In fact, the males of those herds space themselves into territories and fight fiercely with each other when breeding. Hence those antelope cannot be maintained in crowded encloses in captive, as can sheep or goats or cattle. territorial behaviour similarly y combines with a fierce disposition and a slow growth rate to banish rhinos from the farmyard.
Finally, many herd species, including again most deer and antelope, do not have a well-defined dominance hierarchy and are not instinctively prepared to become imprinted on a dominant leader (hence to become minim-printer on humans). As a result, though many deer and antelope species have been tamed (think of all those true Bambi stories), one never sees such tame deer and antelope driven in herds like sheep. That problem also derailed domestication of North American bighorn sheep, which belong to the same genus as Asiatic mouflon sheep, ancestors of our domestic sheep. Bighorn sheep are suitable to us similar to mouflons in most respects except a crucial one: they lack the mouflon's stereotypical behaviour whereby some individuals behave submissively toward other individual whose dominance e they acknowledge.
The Fertile Crescent's biological diversity over small distances contributed to an advantage-its wealth in ancestors not only on valuable crops but also of domesticated big mammals, least of mention, there were few or no wild mammal species suitable for domestication in the other Metatherian zones of California, Chile, southwestern Australia, and South Africa. In contrast, four species of big mammals-the goat, sheep, pig and cowwere domesticated very early in the Fertile Crescent, possibly earlier than any other animal except the dog anywhere else in the world. Those species remain today four of the world's five most important domesticated mammals. Yet, their wild ancestors were commonest in different parts of the Fertile Crescent, so that the four species were domesticated in different places: sheep possibly in the central part, goats either in the eastern part at higher elevation (the Zagros Mountains of Iran) or in the southwestern part (the Levant), pigs in the northern-central part, and cows in the western part, including Anatolia. Nevertheless, although the areas of abundance of these four wild progenitors thus differed, all four lived in sufficiently equivalent proximity that they were readily transferred after domestication from one part of the Fertile Crescent to another, and the whole region ended with all four species.
Agriculture was launched in the Fertile Crescent by the early domestication of eight crops, termed "founder crops" (because they founded agriculture in the region and possibly in the world). Those eight founders were the cereal's emmer wheat, einkorn wheat, and barley, the pulse's lentil, pea, chickpea, and bitter vetch, and the fibre crop flax. Of these eight, only two, flax and barley, range in the wild at all widely outside the Fertile Crescent and Anatolia. Two of the founders had very small ranges in the wild, chickpea being confined to southeastern Turkey and emmer wheat to the Fertile Crescent itself. Thus, agriculture could arise in the Fertile Crescent from domestication of locally available wild plants, without having to wait for the arrival of crops derived from plants domesticated elsewhere. Conversely, two of the eight founder crops could not have been domesticated anywhere in the world except in the Fertile Crescent, since they did not occur wild elsewhere.
Early food production in the Fertile Crescent is that it may have faced less competition from the hunter-gatherer lifestyle than that in another area, including the western Mediterranean. Southwest Asia has a few large rivers and only a short coastline, providing merger aquatic resources (in the form or river and coastal fish and shellfish). One of the important mammal species hunted for meat, the gazelle, originally lived in huge herds but was over exploited by the growing human population and reduced to low numbers. Thus, the food production package quickly became superior to the hunter-gatherer package. Sedentary villages based on cereals were already in existence before the rise of food production and predisposed those hunter-gatherers to agriculture and herding. In the Fertile Crescent the transition from hunter-gatherer to food production took place fast: as late as 9000 Bc. People still had no crops and domestic animals and were entirely dependent on wild foods, however by 6000 Bc. Some societies were almost dependent on crops and domestic animals.
The situation in Mesoamerica contrasts strongly: that area provided only two domesticable animals (the turkey and the dog), whose meat yield was far lower than that of cows, sheep, goats, and pigs, and corns, Mesoamerica's staple grain, was, difficult to domesticate and perhaps slow to develop. As a result, domestication may have begun in Mesoamerica until around 3500 Bc. (The date remains very uncertain): those first developments were undertaken by people who were still nomadic-hunter-gatherers, and settled villages did not arise there until around 1500 Bc.
Our comparison begins with food production, a major determinant of local population size and society complexity-so, an ultimate factor behind the conquest. The most glaring difference between American and Eurasian food production involved big domestic mammal species. The encountering evolutionary principles can be extended to understanding much else about life besides marriage. We have by tendencies to seek easy, single-factor explanations of success. For most important things, though, success actually required avoiding many separate possible causes of failure. The Anna Karenina principle explains a feature o animal domestications that had heavy consequences for human history-namely that so many seemingly suitable big wild mammal species, such as zebra and peccaries, have never been domesticated and that the successful domestications were almost exclusively Eurasian. Having had of happenings, some active events were readily marked by some untold story that so many wild plant species are seemingly suitable for domestication, least of mention, that they were never domesticated.
That has an enormous set of differences between Eurasian and Native American societies due largely to the Late Pleistocene extinction (extermination?) Of most North and South America's former big wild mammal species, was, that if it had been for those extinctions, modern history might have taken a different course. When Cortés and his bedraggled adventurers landed on the Mexican coast in 1519, they might have been driven into the sea by thousands of Aztec cavalry mounted on domesticated native American horses. Instead of the Aztec's dying of smallpox, the Spaniards might have been wiped out by American germs transmitted by disease-resistant Aztecs. American civilizations resting on animal power might have been sending their own conquistadores t ravage Europe. However, it is, nonetheless, that these hypothetical outcomes were foreclosed by mammal extinction's thousands of years earlier.
Apparently standing, it was nonetheless, parts of the explanation for Eurasia's having been the main site of big mammal domestication is that it was the continent with the most candidates' species of wild mammals to start with, and lost the fewest candidates to extinction in the last 40,000 years. It is also true that the percentage of candidates was domesticated is highest in Eurasia (18 percent), and is especially low in sub-Saharan Africa (no species domesticated out of fifty-one candidates.) Particularly surprising is the large number of species of African mammals that were never domesticated, despite there having Erasmian findings, under which are some counterparts that were zebras? Why Eurasia's pigs, but not American peccaries or Africa's three species of true wild pigs? Why Eurasia's five species of wild cattle (aurochs, water buffalo, yaks, gaur, the banteng), but not the African buffalo or America bison? Why the Asian mouflon sheep (ancestor of our domestic sheep), bu not North American bighorn sheep.
Nevertheless, no one would seriously describe this evolutionary process as domestication, because and bats and other animal consumers do not fulfill the other part of the definition: they do not, consciously grow plants. In the same way, the early unconscious stages of crop evolution from wild plants consist of plants evolving in ways that attracted humans to eat and disperse their fruit without yet intentionally growing them. Human latrines, like those of aardvarks, may have been a testing ground of the first unconscious crop breeders.
Latrines are merely one many places where we accidentally sow the seeds of wild plants that we eat. When combining edible wild plants, then venturing home with our bounty, some seedlings spill en route or at our house. Some fruit rots while still containing perfectly good seeds, and gets thrown out uneaten into the garbage. As parts of the fruit that we take into our mouths, strawberry seeds are tiny and inevitably swallowed and defecated, but other seeds are large enough to be spat out. Thus, our spittoon and garbage dumps joined our latrines to form the fist agricultural research laboratories.
From your berry-picking days, you know that you select particular berries or berry bushed. Eventually, when the first farmers began to sow seeds deliberately, they would inevitably sow those from the plants they had chosen to gather, though they didn't understand the genetic principle that big barriers have seeds likely to grow into bushed yielding more big berries. So, then, amid the mosquitoes on a hot humid day, you do not do it for just any strawberry bush. Even if unconsciously, you decide which bush looks most promising, and whether its worth it at all. What are your unconscious criteria?
One criterion, of course, is size. You prefer large berries, because it is not worth your while to getting sunburnt and mosquitos bitten for some lousy little berries. That provides part of the explanation why many crop plants have much bigger fruits than their wild ancestors do. It is especially familiar that modern supermarket strawberries and blaeberries are gigantic compared with wild ones, and those differences arose in recent centuries.
Still another obvious difference between seeds that we grow and many of their wild ancestors are in bitterness. Many wild seeds evolved to be bitter, bad-tasting and some poisonous, to deter animals from eating them. Thus, natural selection acts oppositely on seeds and on fruits. Certain plants, and their maturing fruit and seeds bore of a secured ripening, in that animals and the seeds themselves held within the fruit stayed of a bad-taste. Otherwise, the animal would also chew up the seed, and it could not sprout.
While size and tastiness are the most obvious criteria that human hunter-gatherers select wid plants, other criteria include fleshy or seedless fruits, oily seeds, and long and fibrous wild squashes and pumpkins have almost no fruit around their seeds, however, the preferences of early farmers selected for squashes and pumpkins consisting of far more flesh than seeds. Cultivated bananas were selected long ago to be all flesh and no seed, by that inspiring modern agricultural scientist to develop seedless oranges, grapes, and watermelon as well. Seedlessness provides a good example of how human selection can completely reverse the original evolved function of a wild fruit, which in nature serves as a vehicle for dispersing seeds.
In ancient times many plants were similarly selected for oily fruits or seeds. Among the earliest fruit trees domesticated in the Mediterranean world were olives, cultivated since around 4000 Bc. and used for their oil. Olive crops are not only bigger but also bursting oilers than wild ones. Ancient farmers selected sesame, mustard, poppies, and flax as well for oily seeds, while modern plant scientists have done the same for sunflower, safflower, and cotton.
It seems, least of mention, that's why Darwin, in his On the Origin of Species, didn't start with an account of natural selection, but instead smithed a lengthy account of how our domesticated plants and animals arose through artificial selection by humans. Rather than discussing the Galápagos Islands that were usually associated with him, Darwin began by discussing-how farmers develop varieties of gooseberries. He Wrote, "I have seen great surprise expressed in horticultural works at the wonderful skill of gardeners, in having produced such splendid results from such poor materials, but the art g=has been simple and regarding the result, has been followed almost unconsciously. It has consisted in always cultivating the best-known variety, sowing its seeds, and, when a better variety chanced to appear, selecting it, and so onward." Those principles of crop development by artificial selection still serve as our most understandable model of the origin of species by natural selection.
Nonetheless, it is to say, that conflicting and the uncertainties of environmental characterology are ordinarily subjected to the instability but founded similarities that entice those propensities that these animals revisit their domesticated areas, in that, it is, nevertheless, that they lived independently in the surrounding of several different sites. Such cases can often be detected by analysing the resulting morphological, genetic, or chromosomal differences between specimens of the same crop or doleritic animal in different areas. For instance, India's zebu breeds of domestic cattle possess humps lacking in western Eurasian cattle's breeds, and genetic analyses show that the ancestor of modern Indian and western Eurasian cattle breeds diverged from each other hundreds of thousands of years go, long before animals were domesticated anywhere. That is, cattle were domesticated independently in Indian western Eurasia, within the last 10,000 years, starting with wild Indian and western Eurasian cattle subspecies that had diverged hundreds of thousands of years earlier.
Did all those peoples of Africa, the Americas, and Australia, despite their enormous diversity, nonetheless share some cultural obstacles to domestication not shared with Eurasian peoples? For example, did Africa's abundance of big mammals, available to kill by hunting, make it superfluous for Africans to go to the trouble of tending domestic stock?
The answer to that question is unequivocal: No. The interpretation is refuted by differing types of evidence: rapid acceptance of Eurasian domesticates by non-Eurasian peoples, the universal human penchant for keeping pets, the rapid domestication of the Ancient Species of Big Domestic Mammals, the repeated independent domestications of some of them, and the limited successes of modern efforts art further domestications.
When Eurasia's domestic mammal reached sub-Saharan Africa, they were adopted by the most diverse African people whenever conditions permitted. Those African herders thereby achieved ca huge advantage over African hunter-gatherers and quickly y displaced them. In particular, Bantgu farmers who acquired cows and sheep spread out of their homeland in Wes t Africa and within a short time overran the former hunter-gatherers in most of the rest of sub-Saharan Africa. Even without acquiring crops, Khoisan peoples who acquired cows and sheep around 2,000 years ago, displaced Khoisan hunter-gatherers over much of southern Africa. The arrival of the domestic horse in West Africa transformed warfare there and turned the are a into a set of kingdoms dependent on cavalry. The only factor that prevented horse s from spreading beyond West Africa was trypanosomic diseases borne by tsetse flies.
The same pattern repeated itself elsewhere in the word, whenever peoples lacking native wild mammal species suitable for domestication finally had the opportunity y to acquire Eurasian domestic animals. European horses were eagerly adopted by Native American in both North and South America, within a generation of the escape of horses from European settlements. For example, by the 19th century y North America's Great Plain Indians wee famous as expert horse-mounted warriors and bison hunters, bu t they did not eve n obtain horses until the late 17th century, sheep acquired from Spaniards similarly transformed Navajo Indian societies and led to, among other things, th e weaving of beautiful woolen blankets fo r that the Navajo have become renowned. Within a decade of Tasmania's settlement by Europeans with dogs, Aboriginal Tasmanians, used in hunting. Thus, among the thousands of culturally diverse native peoples of Austral, the Americas, and Africa, no universal cultural taboo stood in the way of animal domestication.
Surely, in some local wild mammal species of those continents had been domesticable, some Australian, American, and African peoples would have domesticated them and gained advantage from them, just as they benefited from the Eurasian domestic animals that they immediately adopted when those became available. For instance, consider all the peoples of sub-Saharan Africa living within the range of wild zebras and buffalo, why wasn't there at least one African hunter-gatherer tribe that domesticated those zebras and buffalo and that thereby gained sway over other Africans, without having to await the arrival of Eurasian horses and cattle? All these facts indicate that th explanation for the lack of native mammal domestication outside Eurasia lay with the locally available wid mammals themselves, not with the local peoples.
Nonetheless, strong domesticate evidence for the same interpretation comes from pets. Keeping wild animals as pets, and taming them, constitute an initial stage in domestication. But pets have been reported from virtually all traditional human societies on all continents. The variety of wild animals thus tamed is far greater than the variety eventually domesticated, and includes some species that we would scarcely have imagined as pets.
For example, in the New Guinea villages it is often seen in the accompaniment to people with pet kangaroos, possums, and birds ranging from flycatchers to ospreys. Most of these captives are eventually eaten, though some are kept just as pets. New Guineans even regularly capture chicks of wild cassowaries (an ostrich-like large, flightless bid) and raise them to et s a delicacy - even though captive adults' cassowaries are extremely dangerous and now and then disembowel village people. Some Asian people tame eagles for purposes of hunting, although those powerful pets have also been known o n occasion to kill their human handlers. Ancient Egyptians and Assyrians, and modern Indians, tamed cheetahs for use in hunting. Painting made by ancien Egyptians show that they further tamed the hoofed mammals such as gazelles and harthebeests, birds such as cranes, surprisingly the giraffes (which can be dangerous), and also hyenas. African elephants were tamed in Roman times despite the obvious danger, an Asian elephants are still being tamed today. Perhaps the most unlikely pet is the European brown bear, which the Ainu people of Japan regularly capture as young animals, tamed, and reared to kill and eat in a ritual ceremony.
Thus, many wild animal species reached the sequential sessions of animal-human relations leading to domestication, but only a few emerged at the other end of that sequence as domestic animals. Over a century ago, the British scientist Francis Galton summarized this discrepancy succinctly: "It would appear that every wild animal has had its chance of being domesticated, which [a] few . . . were domesticated long ago, but that the large remainder, who failed sometimes in only one small particular, are destined to perpetual wildness."
Still. A line of evidence shows that so mammal species are much more suitable than others are provided by the repeated independent domestications of the same species. Genetic evidence based on the portions of our genetic material known as mitochondrial DNA recently confirmed, as had long been suspected, that humped cattle of India and humpless European cattle were derived from two separate populations of wild ancestral cattle that had diverged hundreds of thousands of years ago. This is, Indian peoples domesticated the local Indian subspecies of wild aurochs, Southwest Asians independently domesticated their own Southwest Asian subspecies of aurochs, and North Africa may have independently domesticated the North African aurochs.
Similarly, wolves were independently y domesticated to become dogs in the Americas and probably in several different parts of Eurasia, including China and Southwest Asia. Modern pigs are derived from independent sequences of domestication in China, western Eurasia, and possibly other areas as well. These examples reemphasize that the same few suitable wild species attracted the attention of many different human societies.
The failure of modern efforts provides a final type of evincing that ast failure to domesticate the large residue of wild candidate spacies arose from shortcomings o those species, than from shortcomings of ancient humans. Europeans today are heirs to one of the longest traditions of animal domestication on Earth - that which began in Southwest Asia around 10,000 years ago. Since the fifteenth century, Europeans have spread around the globe and encountered wild mammal species not found in Europe. European settlers, such as those that have encountered New Guinea with pet kangaroos and possums, have tamed or made pts of many local mammals, just as have indigenous peoples. European herders and farmers emigrating to other continents have also made serious efforts to domesticate some local species.
In the 19th and 20th centuries at least six large mammals - the eland, elk, moose, musk ox, zebra, and American bison - have been the subjects of especially well-organized projects aimed at domestication, carried out by modern scientific animal breeders and geneticists. For example, eland, the largest African antelope, have been undergoing selection fo meat quality and milk quantifies in the Askaniya-Nova Zoological Park in the Ukraine, as well as in England, Kenya, Zimbabwe and South Africa; an experimental farm for elk (red deer, in British terminology) has been operated by the Rowett Research Institute at Aberdeen Scotland, and an experimental farm for moose has operated in the Pechero-Ilych National Park in Russia. Yet these modern efforts have achieved only very limited successes. While bison meat occasionally appears in some U.S. supermarkets, and while moose has been ridden, milked and used to pull sleds in Sweden and Russia, none of these efforts has yielded a result of sufficient economic value to attract many ranchers. It is especially striking that recent attempts to domesticate elands within Africa itself, where its disease resistance and climate tolerance would give it a big advantage over introduced Eurasian wild stock susceptible to African disease, have not caught on.
Thus, neither indigenous herders with access to candidate species over thousands of years, nor modern geneticists, have succeeded in making useful domesticates of large mammals beyond the Ancient Species of Big Herbivorous Domestic Mammals, which were domesticated by at least 4,500 years ago. Yet scientists today could undoubtedly, if they wished, fulfill of many species in that part of the definition of domestication, in that specifies the control of breeding and food supply. for example, the San Diego and Los Angeles zoos are now subjecting the last surviving California condors to a more draconian control of bleeding than that imposed upon any domesticated species. All individual condors have been genetically identified, and a computer programs determine which male will mate with which female in order to achieve human goals. Zoos are conducting similar breeding program for man y other threatened species, including gorillas and rhinos. But the zoos' rigorous selection of Californian condors shows no prospect of yielding an economically useful product. Nor do zoos' efforts with rhinos, although rhinos offer up to more than three tons of meat on the hoof. As we will now see and for the most of other big mammals, presents insuperable obstacles to domestication.
Meanwhile, are areas in which food production arose altogether independently, with the domestication of many indigenous crops (and, sometimes, animals) before the arrival to any crops or animals from other areas. There are only five such areas for which the evidence is now detailed and compelling: Southwest Asia, also known as the Near East or Fertile Crescent, China: Mesoamerica (the term applied to central and southern Mexico and adjacent areas of Central America: , the Andes of South America, and possibly the adjacent Amazon Basin as well: and the eastern United States. Some or all these centres that most comprise several nearby centres where to production often or lest were independent, such as North China's Yellow River valley and South China's Yangtze River valley. Besides these five areas where food production definitely arose de novo, four others-Africa's Sahel zone, tropical West Africa, Ethiopia and New Guinea-are candidates for that distinction. However, there is some uncertainty in each case. Although indigenous wild plants were undoubtedly domesticated in Africa's Sahel zone just south of the Sahara, cattle herding may have preceded agriculture there, and it is not yet certain whether those were independently domesticated Sahel cattle or, instead, domestic cattle of Fertile Crescent origin whose arrival triggered local plant domestication. It remains similarly uncertain whether the arrival of those Sahel crops then triggered the undoubted local domestication of indigenous wild plants in tropical West Africa, and whether the arrival of Southwest Asian crops is what triggered the local domestication of indigenous wild plants in Ethiopia. As for New Guinea, archaeological studies there have provided evidence of early agriculture well before food production in any adjacent areas, but the crops grown have not been definitely identified
Food production in the Fertile Crescent is that it may have faced less competition from the hunter-gatherer lifestyle that in another area, including the western Mediterranean. Southwest Asia has few late rivers and only a short coastline, providing relatively meagre aquatic resources (as river and coastal fish and small shellfish). One of the important mammal species hunted for meat, the gazelle, originally lived in huge herds but was over exploited by the growing human population and reduced to low numbers. Thus, the food production package quickly became superior to the hunter-gatherer package. Sedentary villagers based on cereals were already in existence before the rise of food production and predisposed those hunter-gatherers for balancing equations of integral separations, included by an absence to no agriculture nor to any herding. In the Fertile Crescent the transition from hunter-gatherer to food production took place as perhaps as late as 9000 Bc. people still had no crops and domestic animals and were entirely dependent on wild foods, however by 6000 Bc. some societies were almost completely dependent on crops and domestic animals.
Evidently, most of the Fertile Crescent's founder crops were never domesticated again elsewhere after the initial domestication in the Fertile Crescent. Had they been repeatedly domesticated independently, they would exhibit legacies of those multiple origins as varied chromosomal arrangements or varied mutations. Therefore, these are typical examples of the phenomenon of preemptive domestication that have quickly spread of the Fertile Crescent or elsewhere, to domesticate the same wild ancestors. Once the crop had become available, there was no further need to gather it from the wild and by that set it in the path to domestication again.
In addition, of course, small domestic mammals and domestic and insects have also been useful to humans. Many were domesticated for meat, eggs, and feathers: the chicken in China, various duck and goos e species in parts of Eurasia, turkeys in Mesoamerica, guinea fowl in Africa, and the Muscovy duck in South America. Wolves were domesticated in Eurasia and North America to become our dogs used as hunting companions, sentinels, pets, and in some societies, food. Rodents and other small mammals domesticated for food included the rabbit in Europe, the guineapig in the Andes, a giant rat in West Africa, and possibly a rident called the hutia on Caribbean islands. Ferrets were domesticated in Europe to hunt rabbits, and cats were domesticated in North Africa and Southwest Asia to hunt rodent pests. Small mammals domesticated as recently as the 19th and 20th centuries include foxes, mink, and chinchilla grown for fur and hamsters kept as pets. Even some insects have been domesticated, notably Eurasia's honeybee and China's silkworm moths, kept for honey and slk, respectfully.
Many of these small animals thus yielded food, clothing, or warmth. However, none of them pulled plows or wagons, non-bore riders, none except dogs pulled sleds or became war machines, and none of them have been as important for food as have big domestic mammals.
It is true, of course, that some small mammals were first domesticated long after 2500 Bc. For example, Rabbits weren't domesticated for food until the Middle Ages, mice and rats for laboratory research not until the 20th century, and hamsters for pets not until the 1930s. The continuing development of domesticated small mammals is not surprising, because there are literally thousands of wild species as candidates, and because they were of too little value to traditional societies to warrant the effort of raising them. Nonetheless, big mammals domesticated virtually ended 4,500 years ago. By then, all of the world's 148 candidates big species have been tested innumerable tomes, so that only a few passed the test and no other suitable ones remained.
The failure of modern efforts provides a final type of evidence that past failures to domesticate the large residue of wild candidates species arose from shortcomings of those species, than from shortcomings of ancient humans. Europeans today are heirs to one of the longer traditions of animal domestication on Earth, which began in Southwest Asia around 10,000 years ago. Since the fifteenth century, Europeans have spread around the globe and encountered wid mammals species not found in Europe. European settlers, such as those in New Guinea with pet kangaroos and possums, have tamed or made pets of many local mammals, just as have indigenous people. European herders and farmers emigrating to other continents have also attempted to domesticate some local species.
Africa's domesticated animal species can be summarized much more quickly than its plants, because there are so few of them. The sole animal that is know for sure was domesticated in Africa, because its wild ancestors are confined there, are some turkey-like bird's cattle, the guineas fowl. Wild ancestors of domestic cattle, donkeys, pigs, and most domesticate is the dog. The house cat was a native to North Africa but also to Southwest Asia, so we cannot yet be certain where they were first domesticated, although the earliest dates currently known for domestic donkeys and house cats favour Egypt. Recent evidence suggests that cattle may have been domesticated independently in North Africa, Southwest Asia, and India, and that all three of these stocks have contributed to modern African cattle breeds. Otherwise, all the remainders of Africa's domestic mammals must have been domesticated elsewhere and introduced as domesticates to Africa, because the inheritor's wild ancestors fared out only in Eurasia. Africa's sheep and goats were domesticated in Southwest Asia, its chicken in Southeast Asia, its horses in southern Russia, and its camels probably in Arabia.
Like New Guinea, with no domesticable mammals, as the sole foreign domesticated mammal adopted in Australia was the dog, which arrived from Asia (presumably in Austronesian canoes) around 1500 Bc. Achieving the obtainable was to establish itself in the outbacks of primitive Australia of becoming the dingo. Native Australian kept captive dingos as companions, watchdogs, and even as living blankets, leading to the expression ‘five-dogs- night' to mean a very cold night. However, they did not use dingos/dogs for food, as did Polynesian, or for cooperative hunting of wild animals, as did New Guinea.
Some Mesolithic hunter-gatherers, such as the Natufian of the Near East, appear to have lived in small settlements based on an economy involving gazelle hunting and the harvesting of wild cereals using sickles with flint blade segments inset in bone handles. In the Near East and North Africa, Mesolithic populations processed wild plant foods using grinding stones.
Other Mesolithic technological innovations include the adz and axe (woodworking tools consisting of flaked stone blades set in bored antler sleeves and fastened to wooden handles), fishing weirs and traps, fish hooks, the first preserved bows and arrows, baskets, textiles, sickles, dugout canoes and paddles, sledges, and early skis. The Jomon culture of Japan produced pottery by 10,000 years ago, as did the Ertebølle culture of Scandinavia moderately later.
The development of broad spectrum economies in the post-glacial Mesolithic/Archaic period laid the foundations for the plants and animals, which in turn led to the rise of farming communities in some parts of the world. This development marked the beginning of the Neolithic.
Farming originated at different times in different places as early as about 9,000 years ago in some parts of the world. In some regions, farming arose through indigenous developments, and in others it spread from other areas. Most archaeologists believe that the development of farming in the Neolithic was one of the most important and revolutionary innovations in the history of the human species. It allowed more permanent settlements, much larger and denser populations, the accumulation of surpluses and wealth, the development of more profound status and rank differences within populations, and the rise of specialized crafts.
Neolithic Toolmaking generally shows an advanced portion of technological continuity with the Mesolithic, however, Neolithic industries often include blade and bladelet (small blades) technologies, and sometimes in the accompaniment with microliths. A vast horizon widened by a range-over of retouched tools, including endscrapers (narrower scrapers for working hides). Moreover, be in the back blades or bladelets (some of which were set into handles and used as sickles), and a widened range of activated points. In addition, ground and polished axes and adzes-which would have been used for forest clearance to plant crops, and for woodworking activities—are characteristic of the Neolithic. Such tools, although labour-intensive to manufacture, has a propensity to take a long time without requiring resharpening and consequently were highly prized by these early farmers. Large-scale trade networks of axes and stone are documented in the Neolithic, with artifacts sometimes found hundreds of miles from their sources. Other technological developments in the Neolithic include grinding stones, such as mortars and pestles, for the processing of cereal foods, the widespread use of pottery for surplus food storage and cooking, the construction of granaries for storage of grains, the use of domesticated plant fibres for textiles, and weaving technology.
Archaeologists have several theories to explain why humans began farming. The reasons probably differed moderately from one region to another. Some theories maintain that population pressure or changes in environment may have forced humans to find new economic strategies, which led to farming. Another theory maintains that a population of humans may have lived in a region where domesticating wild plants and animals was easily made in the development of agriculture seemed as an indispensably historical accident. Still another theory proposes that the rise of farming may have varied with social change, as individuals began to use agriculture as a means to take on wealth as food surpluses.
Different plant crops were cultivated in different places, depending on what wild plants grew naturally and how well they responded to cultivation. In the Near East, important crops included wheat, barley, rye, legumes, walnuts, pistachios, grapes, and olives. In China, millet and rice predominated. In Africa, millet, sorghum, African rice, and yams were commonly grown. Rice, plantains, bananas, coconuts, and yams were important in Southeast Asia. Finally, in the Americas, corn, squash, beans, potatoes, peppers, sunflowers, amaranths, and goose-foots were commonly grown.
Domesticated animals also varied from one region to another according, again, to availability and their potential to be domesticated. In Eurasia, Neolithic people domesticated dogs, sheep, goats, cattle, pigs, chickens, ducks, and water buffalo. In the Americas, domesticated animals included dogs, turkeys, llamas, alpacas, and guinea pigs. In Africa, the primary domesticated animals-cattle, sheep, and goats-probably spread from the Near East.
Well-studied early farming sites in Eurasia include Jericho, in the West Bank; Ain Ghazal, in Jordan; Ali Kosh, in Iran; Mehrgarh, in Pakistan; Banpocun (Pan-p'o-ts un), in China; and Spirit Cave, in Thailand. Important African sites include Adrar Bous in Niger, Iwo Eleru in Nigeria, and Hyrax Hill and Lukenya Hill in Kenya. In the Americas, sites showing early plant include Guila Naquitz, in Mexico, and Guitarrero Cave, in Peru.
Larger Neolithic settlements show a variety of new architectural developments. For instance, in the Near East, conical beehive-shaped houses or rambling, connected apartments-style housing was often constructed with mud bricks. In Eastern Europe, houses were made with wattle and daub (interwoven twigs plastered with clay) walls, and, in later times, long houses were constructed with massive timbers. In China, some settlements contain semisubterranean houses dug into clay, with evidence of walls and roofs made out of thatch or other materials and supported by poles.
The of plants and animals led to profound social change during the Neolithic. Surpluses of food, such as stored grain or herds of livestock, could become commodities of wealth for some individuals, leading to social differentiation within farming communities. Trade of raw materials and manufactured products between different areas increased markedly during the Neolithic, and many foreign or exotic goods appear to have developed special symbolic value or status. Some Neolithic graves contain rich stores of goods or exotic materials, revealing differentiations in terms of wealth, rank, or power
In certain areas, notably parts of the Near East and Western Europe, Neolithic peoples built massive ceremonial complexes, efforts that would have required extensive, dedicated work forces. Large earthworks and megalithic (‘giant stone') monuments from the Neolithic (including the Avebury stone circle and the earliest stages of Stonehenge, in England, and the monuments of Carnac, in France), suggest more highly organized political structures and more complex social organization than among most hunters-gatherer populations. In the Americas, sites such as the mounds of Cahokia, in Illinois, also depict more complex, organized political and social order. The technological innovations and economic basis established and spread by Neolithic communities ultimately set the stage for the development of complex societies and civilizations around the world.
Humans produced metal tools and ornaments from beaten copper as early as 12,000 years ago in some parts of the world. By 6,000 years ago, early experiments in metallurgy, particularly extracting metals from copper ore (smelting), were being conducted in some parts of Eurasia, notably in Eastern Europe and the Near East. By 5,000 years ago, copper and tin ores were being smelted and alloyed in some regions, marking the dawn of the Bronze Age. Sculpting of bronze tools, such as axes, knives, swords, spearheads, and arrowheads became increasingly common over time. At first, copper and bronze tools were rare and stone tools were still inordinate common, but as time went on, metal tools gradually replaced stone as the principal raw material for edged tools and weapons.
In Eurasia and parts of Africa, the rise of metallurgical societies appears to coincide with the rise of the earliest state societies and civilizations, such as ancient Egypt, Sumer, Minoan Culture, Mycenae, and China. In the Americas, parts of sub-Saharan Africa, Australia, and the Pacific Islands, societies continued to use stone and other nonmetal materials as the principal raw materials for tools up to the time of European contact, starting in the 15th century ad. Although, technically, populations in these areas could have been said to be Stone Age groups, many had become agricultural societies and had formed flourishing civilizations.
Stone technology enjoyed a brief resurgence within iron-using societies with the coming of flintlock firearms, beginning in the 17th century. Carefully shaped flints-reminiscent of the geometric microliths of the Mesolithic and early Neolithic-were struck against steel to create a spark to ignite the firearm. By the end of the 20th century few human groups had a traditional stone technology, although a few groups on the island of New Guinea still relied on the use of stone adzes. Tools of metal, plastic, and other materials had replaced stone technologies virtually everywhere.
Cave Dwellers, is the term used to designate an ancient people who occupied caves in various parts of the world. Cave dwellers' date generally from the Stone Age period known as the Palaeolithic, which began as early as 2.5 million years ago. Caves are natural shelters, offering shade and protection from wind, rain, and snow. As archaeological sites, caves are easy to colonize and often provide conditions that encourage the preservation of normally perishable materials, such as bone. As a result, the archaeological exploration of caves has contributed significantly to the reconstruction of the human past.
Cave Painting, of Lascaux, France where some Palaeolithic artists painted scenes in caves more than 15,000 years ago, such as the one here found in Lascaux, France. The leaping cow and group of small horses were painted with red and yellow ochre that were either blown through reeds onto the wall or mixed with animal fat and applied with reeds or thistles. It is believed that prehistoric hunters made these paintings to gain magical powers that would ensure a successful hunt.
Wherever caves were available, prehistoric nomadic hunters and gatherers incorporated them into the yearly cycle of seasonal camps. Most of their activities took place around campfires at the cave mouth, and some caves contain stone walls and pavements providing additional protection from winds and dampness. Hunting, particularly of reindeer, horse, red deer, and bison, was important; many caves are situated on valley slopes providing views of animal migration routes.
Stone Toolmaking Humans first made tools of stone at least 2.5 million years ago, initiating the so-called Stone Age. The Stone Age advanced through three stages over time-the Palaeolithic (which is subdivided into Lower, Middle, and Upper periods), Mesolithic, and Neolithic. Blade Toolmaking, was a development of the Upper Palaeolithic Epoch, which began about 40,000 years ago. This technique produced a far greater variety and higher quality of tools than did earlier methods of Toolmaking.
Artifacts have been found in caves in France, Spain, Belgium, Germany, Italy, and Great Britain. The association of these remains with the bones of extinct animals, such as the cave bear and saber-toothed tiger, makes evidently the great antiquity of many cave deposits. A variety of stone and bone heads were discovered in excavated caves, as by rule their documents the importance of spears until the bow and arrow appeared in the late Palaeolithic era. Other common tools included stone scrapers for working hides and wood, burins for engraving, and knives for butchering and cutting. Throughout the Palaeolithic period such tools became increasingly diverse and well made. Bone needles, barbed harpoons, and spear-throwers were made and decorated with carved designs. Evidence of bone pendants and shell necklaces also exists. Among the caves that have yielded relics of early humans are the Cro-Magnon and Vallonnet in France.
Wall paintings and engravings have been found in more than 200 caves, largely in Spain and France, dating from 25,000 to 10,000 years ago. Frequently found deep inside the caves, and the paintings depict animals, geometric signs, and occasional human figures. In the cave of La Colombière in France, a remarkable series of sketches engraved on bone and smoothed stones was unearthed in 1913. In caves such as Altamira in Spain and Lascaux in France, multicolored animal figures were drawn using mineral pigments mixed with animal fats. Some paintings adorn walls of large chambers suitable for ritual gatherings; others are found in narrow passages accessible only to individuals. Hunting and fertilities seem to have been important artistic themes. The ritual gatherings themselves promoted communication and intermarriage among the normally scattered small groups. Chinese caves contain some earliest evidence of human use of fire
On every continent, prehistoric foragers used caves. In the Zhoukoudian (Chou-k'ou-tien) Cave near Beijing, China, remains of bones and tools of The Homo erectus (Peking Man) have been discovered. Chinese caves contain some earliest evidence of human use of fire, approximately 400,000 years ago. In the Shânîdâr Cave in Iraq, 50,000-year-old Neanderthal skeletons were unearthed in 1957. Ancient pollen buried with them has been interpreted as evidence that these cave dwellers had developed funeral rituals. In the western deserts of North America, caves have been found that contain plant foods, woven sandals, and baskets, representing the desert culture of a belated 9000 years ago. Early inhabitants of Australia, the Middle East, and the Peruvian Andes have also left remains in caves.
Gradually people learned to grow food, rather than forage for it. This was the beginning of the Neolithic age, which, although ending in western Europe some 4500 years ago, continued elsewhere in the world until modern times. Once agriculture became important, people established villages of permanent houses and found new uses for caves, mainly as hunting and herding campsites and for ceremonial activities. In Europe, Asia, and Africa caves continued to be used as shelters by nomadic groups.
Cave Dwellings, as these concave inlets held of themselves the cave dwellings that are found in the Cappadocia region of Central Anatolia Göreme, Turkey. Known as ‘fairy chimneys', they were carved into soft volcanic rock by anchorite (hermitic) Christian monks in the 4th century AD. Many of these dwellings are still occupied by Göreme Turks, who consider them to have the quality of being healthy and make a reduction in-rate as to call by name the place of to live.
In dry caves, preservation is often excellent, due to moistureless air and limited bacterial activity. Organic remains such as charred wood, nutshells, plant fibres, and bones are sometimes found intact. In wet caves, artifacts and other remains are often found encrusted with, or buried beneath, calcareous unloads of drip-stone. The collected evidence of human habitation on the cave floor was often buried under rock falls from the ceilings of caverns. Intentional burials have also been found in several cave sites.
Because of the unusual preservative nature of caves and the great age of many remains found in them, the fallacious belief has arisen that a race of cave people existed. Most cave sites represent small, seasonal camps. Because prehistoric people spend a copious measurement of the year in open-air camps, the caves contain the remains of only part of a group's total activities. Also, the cultural remains outside caves were subject to greater decay. Thus, the archaeological record of remote times is better seen in cave deposits.
Caves have been systematically excavated during the past one hundred years. Since they often contain the remains of repeated occupations, caves can document changing cultures. For example, the economic transition from food collecting to agriculture is manifested to finds in highland Mexico and in Southeast Asia. Some caves in the Old World continued to be inhabited even after the close of the Stone Age; relics from the Bronze and Iron ages have been found in cave deposits. On occasion, material dating from the time of the Roman Empire has been recovered. The famous Dead Sea Scrolls, discovered in 1947, were preserved in caves.
In 1935 Doctor F. Kohl-Larsen discovered fragments of two skulls in the gravel at the northeast end of Lake Eyassi, Tanganyika Territory, Africa, in association with fossilized bones of antelopes, pigs, and hyenas resembling types of animals now living in that area. The two hundred fragments of the skulls have been painstakingly assembled by Doctor Hans Weinert of Kiel, Germany, so that there are now available for study the skull cap of one individual and part of the face of another. Though critical study of these East African finds is still far from completion, their closest resemblance may be to Pithecanthropus erectus, the famous Java ape man. These remains have been tentatively dated as 100,000 years ago.
Doctor Robert Broom of the Transvaal Museum, Pretoria, has continued his study of the human-like ape remains found in South Africa. He believes the Australopithecus skulls to be the most definite apes-like, except their teeth, which show a closer similarity to those of man than of the gorilla or chimpanzee, and therefore that they are not actual ancestors of man, but only, survivors of a possible apelike ancestral stock that existed before Ice Age times.
The distal end of a humerus, the proximal ends of an ulna, and the distal phalanx of a toe of Paranthropus robustus, and the distal ends of a femur of Plesianthropus were excavated in the Pleistocene bone breccia of Kromdrai, near Krugersdorp, South Africa, under the direction of Doctor Broom, thus suggesting that this early type of ape-man had to his graduation a diploma in Bipedalism, or he was ably of walking of an erect posture, making a distinct departure from previous assumptions as to posture of this species.
Professor Raymond Dart of Witwatersrand University (South Africa), the discoverer of the controversial Taungs skull (Australopithecus africanus) states that a high culture existed in the present habitat of the Bantu-speaking peoples of South Africa in the Late Stone Age before their coming in that part of Africa. Skeletons associated with the Mapungobwa finds appear to implicate that the civilization cantering to this place was associated with a race said to be intermediate between, and possibly a hybrid of, Cro-Magnon and Neanderthal types, which as known in Europe, are distinct races'
Finds of Neanderthaloid skulls and skeletons continue to be reported from widely separated areas. Digging in a cave at Mount Circeo on the Tyrrhenian sea, 50 miles south of Rome, Italy, Alberto Carlo Blanc uncovered an almost perfectly preserved Neanderthal skull, perfect except a fracture in the right temporal region. It is the third of this type found in Italy. The two skulls previously reported were found in 1929 and 1935 in the Sacopastore region, near Rome, but in not nearly so well preserved a condition as the present find. No other human bones were found here, but the skull was accompanied by fossilized bones of elephants, rhinoceri, and giant horses, all fractured, thus giving some evidence of the mode of life of Neanderthal man. Professor Sergio Sergi, of the Institute of Anthropology at the Royal University of Rome, who has studied this skull in detail believes it to be 70,000 to 80,000 years old. He concludes also that Neanderthal man walked as moderately of an erect positional stance, as modern man and not with its head thrust forward as had previously misfortunes of others.
Another Neanderthal skeleton is reported to have been found in a cave in Middle Asia by A. P. Okladnikoff of the Anthropological Institute of Moscow University and the Leningrad Institute of Anthropology. The bones of the skeleton were badly shattered, but the jaw and teeth of the skull, such as for themselves were crushed at the back, were almost complete.
The famous Chokoutien site near Peking, China, the home of ancient Peking man (Sinanthropus) previously reported, now proves also to have yielded additional more modern type skeletons studied by Doctor Franz Weidenreich and Doctor W. C. Pei, the leaders in research at this site. In the portion of the site known as the upper cave were found the remains of an advanced culture suggesting a resemblance to the Late or Upper Palaeolithic in Europe, thus implying an age of 100,000 to 200,000 years. These cultural remains were accompanied by skeletons of bear, hyena, and ostrich, long extinct forms, and tiger and leopard that longs since disappeared from this part of Asia. The three human skulls in properly positioned placement seemed of its properties that accorded themselves in a very detailed subjective study, so, to implicate that they probably belong to three different racial groups. Of the two female skulls studied, one bears close resemblance to the skulls of modern Melanesians, with frontal deformation, are the second skull deformations to Eskimo skulls who are the first. The brain case of the masculine skull in some valuing quality is much more than is primitive, almost in the Neanderthaloid stage, but in other features is reminiscent of Upper Palaeolithic Man. The face, is similar to, though not identical with, recent Mongolians. From this evidence it seems that racial mixture is no product of modern times, but has its roots in extreme antiquity. It should be noted also that though Mongolian types resembling the modern population of North China were not found in the upper cave, it does not necessarily mean that they were nonexistent during that period. It has been suggested that the population represented in the upper cave may have been a migrating group. Historic and prehistoric American Indian skulls resembling Melanesian, Eskimo, or more primitive types have been reported from time to time in America, so that it would appear from the present finds at Chokoutien that long before migrations from Asia to America are assumed to have taken place, types similar to those composing the native American populations were living permanently, or at least moving around in Eastern Asia.
In the more recent past, the movement and counter-movement of peoples have led to accelerated mixing of stocks and mutual infusion of physical characteristics. Perhaps more important than the transmission of physical characteristics has been the transmission of cultural characteristics. The diffusion of cultures, including tools, habits, ideas, and forms of social organization, was a prerequisite for the development of modern civilization, which would probably have taken place much more slowly if people had not moved from place to place. For instance, use of the horse was introduced into the Middle East by Asian invaders of ancient Sumer and later spread to Europe and the Americas. Even important historical events can be linked to distant migrations; the downfall of the Roman Empire in the 3rd to the 6th century AD, for example, was probably hastened by migrations following the building of the Great Wall of China, which prevented the eastward expansion of Central Asian tribes, thus turning them in the direction of Europe.
A group of people may migrate in response to the lure of a more favourable region or because of some adverse condition or combination of conditions in the home environment. Most historians believe that non-nomadic peoples are disinclined to leave the places to which they are accustomed, and that most historic and prehistoric migrations were stimulated by a deterioration of home conditions. This belief is supported by records of the events preceding most major migrations.
The specific stimuli for migrations may be either natural or social causes. Among the natural causes are changes in climate, stimulating a search for warmer or colder lands; volcanic eruptions or floods that render sizable areas uninhabitable; and periodic fluctuations in rainfall. Social causes, however, are generally considered to have prompted many more migrations than natural causes. Examples of such social causes are an inadequate food supply caused by population increase; defeat in war, as in the forced migration of Germans from those parts of Germany absorbed by Poland after the end of World War II in 1945; a desire for material gain, as in the 13th-century invasion of the wealthy cities of western Asia by Turkish tribes; and the search for religious or political freedom, as in the migrations of Huguenots, Jews, Puritans, Quakers and other groups to North America.
Throughout history, the choice of migratory routes has been influenced by the tendency of groups to seek a type of environment similar to the one they left, and by the existence of natural barriers, such as large rivers, seas, deserts, and mountain ranges. The belts of steppe, forest, and arctic tundra that stretches from central Europe to the Pacific Ocean have been a constant encouragement to east-west migration of groups situated along their length. On the other hand, migrations from tropical to temperate areas, or from temperate to tropical areas, have been rare. The desert regions of the Sahara in northern Africa separated the African from the Mediterranean peoples and prevented the diffusion southward of Egyptian and other cultures, and the Himalayas' mountain system of South Asia cut off approach to the great subcontinent of India but from its eastern and western borders. Consequently of these and similar barriers, certain mountain passes and land bridges became traditional migratory routes. The Sinai Peninsula in northeastern Egypt, bounded on the east by the Arabian Peninsula, linked Africa and Asia; the Bosporus region of northwestern Turkey connected Europe and the Middle East; the Daryal Gorge in the Caucasus Mountains of Georgia, Armenia, Azerbaijan, and southwestern Russia was used by the successive tribes that poured out of the European steppes into the Middle East; and the broad valley between the Altay Mountains and the Tian Shan mountain system of Central Asia provided the route by which Central Asian peoples swept westward.
Among the distinct effects of migration are the stimulations of further migration through the displacement of other peoples; a reduction in the numbers of the migrating group because of hardship and warfare, changes in physical characteristics through intermarriage with the groups encountered; changes in cultural characteristics by adoption of the cultural patterns of peoples encountered; and linguistic changes, also affected by adoption. Anthropologists and archaeologists have traced the routes of many prehistoric migrations by the current persistence of such effects. Blond physical characteristics among some of the Berbers of North Africa are thought to be evidence of an early Nordic invasion, and the Navajo and Apache of the southwestern United States are believed to be descended from peoples of northwestern Canada, with whom they have a linguistic bond. The effects of migration are particularly evident in North, Central, and South America, where peoples of diverse origins live with common cultures.
Among the most far-reaching series of ancient migrations were those of the peoples who spread the Indo-European family of languages. According to a prevalent hypothesis, a large group of Indo-Europeans migrated from east-central Europe eastward toward the region of the Caspian Sea before 3000 Bc. Beginning shortly after 2000 Bc, the Indo-European people known as the Hittites crossed into Asia Minor from Europe through the Bosporus region, and when the bulk of the Indo-Europeans in the Caspian Sea area turned southward. The ancestors of the Hindus went southeastward into Punjab, in northwestern India, and along the banks of the Indus. and Ganges rivers; the Kassites went south into Babylonia, and the Mitanni of northern Mesopotamia went southwestward into the valleys of the Tigris and Euphrates rivers and other parts of the region between the Persian Gulf and the Mediterranean Sea known as the Fertile Crescent.
A migration of great importance to Western civilization was the invasion of Canaan (later known as Palestine) by the tribes of the Hebrew confederacy, which developed the ideas on which the Jewish, Christian, and Islamic religions are founded. These nomadic Semitic tribes, from the Arabian Peninsula and the deserts southeast of the Jordan River, moved (15th-10th century Bc) into a settled region that was alternately under the control of Egypt and Babylonia.
The civilizations of the ancient world were reached cities and countries situated along the edges of the great European and Asian landmass, around the Mediterranean Sea, in the Middle East, in India, and in China. The huge interior area was crossed and recrossed by nomadic tribes, which periodically overran the coastal settlements. Central Asia was the main reservoir of these nomadic hordes, and from it successive waves of migrations penetrated eastward into China, southward into India, and westward into Europe, driving before them subsidiary waves of displaced tribes and peoples. In the 3rd century Bc, the Xiongnu (Hsiung-nu), who were possibly related to the Huns, advanced eastward from Central Asia toward China and westward toward the Ural Mountains, driving other groups before them.
In another movement the Cimbri, thought to have been a Germanic people, drove southward from the eastern Baltic Sea region and twice entered the Roman Empire in the 2nd century Bc. In the 1st century Bc, Germanic groups from the southwestern Baltic area, possibly as a consequence of Cimbri pressure, also drove down into central Europe, occupying the territory between the Rhine and the Danube rivers. By the 3rd century AD, a newly expanding group, the Mongols, had arisen in Central Asia. Because of their pressure, the Huns invaded China and crossed over the Ural Mountains into the Volga River region. This migration displaced the Goths, who travelled from southwestern Russia toward the European domains of the Roman Empire, and in turn forced the Germanic Vandals into Gaul and Spain at the beginning of the 5th century ad. The Visigoths (western Goths) continued their westward advance through Italy, Gaul, and Spain, driving the Vandals before them into northern Africa and eastward to present-day Tunis. The Ostrogoths (eastern Goths) followed the Visigoths into Italy and settled there. The Huns, who had begun their movement in Central Asia eight centuries earlier, followed the Goths into Europe, after being displaced by the Mongols, and settled in what is now Hungary about the middle of the 5th century. The Mongols also forced many Slavs into eastern Europe. Thus, one of the most momentous and far-reaching events of history, the disintegration of the Roman Empire in the 3rd to the 6th century of the Christian era, was largely caused by migrations.
After the Hun invasions in the 3rd and 5th centuries, a period of equilibrium began. In the East, the Chinese maintained their strength against the nomads. In the West, Europe consolidated its own strength.
The weakness and decay of the Persian and the Byzantine empires encouraged the spread of a new migration out of Semitic Arabia that was far more extensive than that of the Hebrews into Canaan. United under the banner of Islam in the 7th and early 8th centuries, Arab tribes swept eastward through Persia to Eastern Turkistan and into northwest India; westward through Egypt and across northern Africa into Spain and southern France; and northwestwards through Syria into Asia Minor. The Arab penetration into Central Asia stimulated nomadic raids on the frontiers of the Chinese Empire and forced the western Asian Magyar tribes to move in the direction of Europe, crossing the Ural Mountains and southern Russia and finally reaching Hungary, where they settled in the 9th century.
Expansion of Chinese frontiers under the Song (Sung) dynasty in the 11th century forced the Seljuk Turkish tribes out of Central Asia. These tribes moved westward across the Ural Mountains into the Volga River region and thence south into Persia, Armenia, Asia Minor, and Syria, settling among the peoples there. In the 13th century, Mongol tribes under famed conqueror Genghis Khan, in one of the most astounding military migrations of recorded history, swept out of Mongolia and captured China, Turkistan, Afghanistan, Iran, Mesopotamia, Syria, Asia Minor, southern Russia, and even parts of eastern Europe. The Ottoman Turks, forced from their pasturelands in western Asia during the brief period of Mongol supremacy, migrated westward and entered Asia Minor in the 14th century, taking Constantinople (then the capital of the Byzantine Empire, in what is now northwestern Turkey) and advancing as far as Vienna, Austria, in the 15th century.
The maritime region consisting of Scandinavia and other lands bordering the North and the Baltic seas was a subsidiary reservoir of migratory groups. In the 5th and 6th centuries, Angles, Saxons, and Jutes, displaced by the Visigoths, sailed from northwest Germany and overran southern Britain. Norwegian mariners captured the Shetland, Orkney, Faroe, and Hebrides islands in the 7th and 8th centuries. In the 9th century, Swedish fighters poured out of the Baltic region through southern Finland, sweeping down into Russia and through the Ukraine along the Dnieper River. During the 9th century, Norwegians settled in Iceland and in Normandy (Normandie) in France. Icelanders reached Greenland in the late 10th century and established a colony there. Subsequently, they sailed even as far as North America but left no permanent settlers. The growth of the system of nation-states in Europe during the 2nd millennium AD again restored the equilibrium in the West, and no important ethnic invasions occurred thereafter.
More people have moved and resettled during the past 450 years than in any similar period of human history. The migrations preceding this period were collective acts, mostly voluntarily undertaken by the members of a group, but many of the more recent migrations have differed in at least two significant ways: They have been either voluntary individual acts or they have been enforced group movements, entirely against the will of the people who are being moved. The two types of migration began almost simultaneously after Europeans arrived in America in the late 1400s, and they have continued in one form or another up to the present day.
The era of modern migrations that began with the opening of the western hemisphere was continued under the impetus of the Industrial Revolution. Millions of western, and then eastern, Europeans, seeking political or religious freedom or economic opportunity, settled in North and South America, Africa, Australia, New Zealand, and other parts of the globe. As many as 20 million Africans were forcibly carried to the Americas by slave traders and sold into bondage. Millions of Chinese settled in Southeast Asia and moved overseas to work in the Philippine Islands, Hawaii, and the Americas. A large colony of Hindus was established in southern Africa, and many people from Arab lands migrated to North and South America.
The migrations from Europe were principally voluntary, in the sense that the emigrants could have stayed in their respective original homelands if they had accepted certain religions, creeds, political allegiances, or economic privations. The involuntary migrations were primarily those of the Africans captured for slave labour, but slave shipments were halted during the first half of the 19th century. However, a large-scale, essentially forced migration took place from southern Africa to the central and eastern parts of the continent, spurred by the expansionist force of the Zulu. Finally, many of the Chinese, Indian, and other Asian migrations, as well as some of the migrations of eastern and southern Europeans, were not strictly definable as either or free or unfree. The individual migrants signed agreements to travel in consignments of contract labour. Although ultimately many of these labourers settled permanently and with equal rights in the lands to which they went, the terms of their original contracts often severely limited their freedom and, in effect, left them little better than slaves for long periods through time.
It's migration into the Americas, early movement or movements of humans to the Americas. The first people to come to the Americas arrived in the Western Hemisphere during the late Pleistocene Epoch (1.6 million to 10,000 years before present). Most scholars believe that these ancient ancestors of modern Native Americans were hunter-gatherers who migrated to the Americas from northeastern Asia.
For much of the 20th century it was widely believed the first Americans were the Clovis people, known by their distinctive spearpoints and other tools found across North America. The earliest Clovis sites date to 11,500 years ago. However, recent excavations in South America show that people have lived in the Americas at least 12,500 years. A growing body of evidence-from other archaeological sites to studies of the languages and genetic heritage of Native Americans-suggests the first Americans may have arrived even earlier.
Many details concerning the first settlement of the Americas remain shrouded in mystery. Today the search for answers involves researchers from diverse fields, including archaeology, linguistics, skeletal anatomy, and molecular biology. The challenge for researchers is to find evidence that can help determine when the first settlers arrived, how these people made their way into the Americas, and if migrating groups travelled by different routes and in multiple waves. Some archeologists and physical anthropologists have suggested that one or more of these migrations originated from places outside Asia, although this view is not widely accepted.
Whoever they were and whenever they arrived, the first Americans faced extraordinary challenges. These hardy settlers encountered a vast, trackless new world, one rich in animals and plants and yet entirely without other peoples. As they entered new territories, they had to locate essential resources, such as water, food, and materials to make or repair their tools. They had to learn which of the unfamiliar animals and plants would feed or cure them that might hurt or kill them. Their efforts ultimately proved successful. By the time European exploration of the Americas began in the late 15th century, the descendants of these ancient colonizers numbered in the millions.
From their evolutionary origins in Africa, anatomically modern humans, Homo sapiens, steadily spread out across Earth's landmasses by 25,000 to 35,000 years ago humans had reached the far eastern reaches of modern Siberia in northeastern Asia-a region believed to be the most likely point of departure for any early migration to North America. Humans arrived in this remote corner of the world during the last major period of the Pleistocene Epoch, or Ice Age. Great glaciers covered much of the Northern Hemisphere at this time. In North America two immense ice sheets, the Laurentide in the east and the Cordilleran in the west, buried much of modern Canada and Alaska, as well as northern portions of the continental United States.
Pleistocene climates and environments were different from they are today, and so too were the Earth's surfaces. Glaciers had captured a significant amount of the world's water on land. Because that water no longer drained back to the oceans, worldwide sea levels dropped. Average sea levels were as much as 135 m (440 ft) lower than they are today.
As sea levels fell, large expanses of previously submerged continental shelf became dry land, including the area beneath what is now the Bering Sea. This area formed a 1,600-km- (1,000-mi-) wide land bridge that connected the northeastern tip of Asia and the western tip of modern Alaska. Known as Beringia, this natural land bridge existed from about 25,000 to nearly 10,000 years ago. It was a flat, cold, and dry landscape, covered primarily in grassland, with occasional shrubs and small trees. People and animals could use Beringia to walk from Siberia to Alaska and back.
Migrants from northeastern Asia could have trekked to Alaska with relative ease when Beringia was above sea level. Even travelling south from Alaska to what is now the continental United States posed significant challenges for any would-be colonizers. There were two possible routes south for migrating people: down the Pacific coast, or by way of an interior passage along the eastern flank of the Rocky Mountains. When the Laurentide and Cordilleran ice sheets were at their maximum extent, both routes were likely impassable. The Cordilleran reached across to the Pacific shore in the west and its eastern edge abutted the Laurentide, near the present border between British Columbia and Alberta.
Geological evidence suggests the Pacific coast route was open for overland travel before 23,000 years ago and after 14,000 years ago. During the coldest millennia of the last ice age, roughly 23,000 to 19,000 years ago, lobes of glaciers hundreds of kilometres wide flowed down to the sea. Deep crevasses scarred their surfaces, making travel across them dangerous. Even if people travelled by boat-a claim for which there is currently no direct archaeological evidence-the journey would have been difficult. There were almost certainly fleets of icebergs to outmanoeuvre. Rivers of sediment draining Cordilleran glacial fields severely restricted the availability of near-shore marine life, which early colonizers would have relied on for nourishment. By 14,000 to 13,000 years ago, however, the coast was ice-free. By then, too, the climate had warmed, and coastal lands were covered in grass and trees. Hunter-gatherer groups could have readily replenished their food supplies, repaired clothing and tents, and replaced broken or lost tools.
The warming climate gradually opened a second possible migration route through the massive frozen wilderness in the continental interior. Geologic evidence indicates that by 11,500 years ago the Cordilleran and Laurentide ice sheets had retreated far enough to open a habitable ice-free corridor between them. By then, much of the exposed land was probably restored enough to support plants and animals on which migrating hunter-gatherer peoples depended.
Scientific inquiry into the peopling of the Americas began in the 1870s. Then, many scholars wondered if modern humans had lived in the Americas for as long as they had in Europe, where numerous Stone Age sites indicated a Pleistocene-era occupation. Excavations at these sites revealed hand axes and other relatively simple stone tools, human bones, and the remains of several now-extinct animals, including the woolly mammoth. The discovery of Pleistocene-age animals alongside human bones and artifacts helped 19th-century archeologists establish the age of ancient human encampments in Europe.
Yet, search as they might, American archeologists found no comparable evidence of a Pleistocene-era human presence. Nonetheless, several sites revealed stone artifacts that some scholars believed looked similar to the ancient stone tools found in Europe. On the basis of this similarity, these experts claimed the American artifacts must be as old. By the 1890s, however, other scholars had challenged this claim. They argued the American and European artifacts did not really look alike, and they noted the American artifacts were of uncertain antiquity because none were found securely embedded in Pleistocene-age geological deposits. A lengthy debate ensued between those who saw evidence for ancient human settlement in the Americas and those who did not. This debate-often loud and sometimes bitter-remained unresolved for more than three decades.
In 1927 archaeologists finally demonstrated that humans had occupied the Americas during the Pleistocene. This breakthrough occurred at a site discovered by ranch foreman George McJunkin near Folsom in northeastern New Mexico. Excavations at the site uncovered a stone projectile point embedded in the rib bones of a now-extinct bison, an ancestor of the modern North American buffalo. Clearly, a human hunter had killed this Pleistocene-era animal. The Folsom discovery proved beyond doubt that humans had lived in the Americas since the last ice age.
The spearpoints used to bring down the Folsom bison where distinctive, finely made points possess a flute, or channel, on each face. These Folsom points were quite unlike those of the European Stone Ages. American archaeologists coined the term Paleo-Indian to identify the ancient Pleistocene Americans who had produced these well-crafted artifacts.
In the decade after Folsom, more Paleo-Indian sites were discovered. Some held Folsom spearpoints, but others revealed larger, less finely made fluted points. These large points occasionally appeared with the bones of mammoths. The first such find became known in 1933 at a site near Clovis in eastern New Mexico, where archaeologists found spearpoints and fossils in sediments below those that had produced Folsom artifacts. This meant that the Clovis people, as they came to be known, represented an even older Paleo-Indian culture. Just how much older was determined soon after the development of radiocarbon dating in the late 1940s This modern dating technology showed that the people who made Clovis artifacts had inhabited North America by about 11,500 years ago-some 600 years before the Folsom culture appeared.
The age of the earliest Clovis sites coincided neatly with geological evidence that by 11,500 years ago the Laurentide and Cordilleran ice sheets had retreated far enough to open a habitable ice-free corridor-a fact first recognized by University of Arizona archaeologist C. Vance Haynes. It appeared that Clovis groups had moved south from Alaska through the continental interior right after it became possible to do so. That no excavated site older than Clovis were found, at least initially, seemed to confirm that Clovis people were the first colonizers of the Americas.
Once they had travelled south of the ice sheets, Clovis groups spread rapidly. Soon after 11,500 years ago, Clovis and Clovis-like materials appear throughout North America. The oldest sites are in the Great Plains and the southwestern United States; younger sites are found in eastern North America. No subsequent group would achieve such a wide distribution, but Clovis groups did not stop in North America. According to the Clovis-first theory, they must have continued to South America. As these groups pushed south, the traditional thinking went, they developed different tools and other artifacts that were no longer readily recognizable as Clovis. They arrived at Tierra del Fuego on the southern tip of South America within 1,000 years of leaving Alaska.
The rapid dispersal of Clovis peoples throughout the hemisphere was remarkable given the landscape they traversed. Not only did they travel through desert, plains, and forest, they did so during the environmental upheaval that marked the end of the last Ice Age. Climates were growing warmer-drier in some areas and wetter in others-and the distributions of plants and animals were shifting in complex ways in response to the changing climates. As they entered each new habitat, they must have quickly learned to find suitable plant and animal foods. They would need stone to repair their toolkits, freshwater to drink, and the ability to overcome environmental challenges encountered along the way.
A long-favoured explanation for the rapid spread of Clovis people was that they preyed on large animals, such as mammoth and mastodon. These animals were themselves wide-ranging in their distribution. Archaeologists believed a reliance on big-game hunting meant that Clovis groups would have less need to learn about available local resources.
Archaeologists initially found some support for the big-game hunting hypothesis in archaeological excavations, as well as in the Clovis toolkit itself. Along the San Pedro River in Arizona, for example, are four Clovis sites separated by less than 20 km (12 mi). Each site yielded Clovis points embedded in the skeletons of mammoths. So similar are the points at these sites that they may be the handiwork of a single group, which obviously found good hunting in the area. The artifacts at San Pedro and other Clovis sites include a variety of tools handy for hunting, killing, and butchering game animals. There are the distinctive fluted spearpoints, shown experimentally by University of Wyoming archaeologist George Frison to be capable of bringing down elephant-sized animals. In addition, there are stone knives, scrapers, gravers (tools for scoring bone), drill, and a few preserved artifacts of ivory and bone. These tools, which occur in Clovis sites across North America, support the view that Clovis peoples were practicing the same way of life.
Clovis tools were typically made of superior quality fine-grained stone, including chert, jasper, and chalcedony. Such stone is durable and readily flaked by skilled Toolmakers into a desired, sharp-edged form. More important, it is easily resharpened and reused. That would be important to hunters pursuing wide-ranging big game. They could continue to use their stone tools as they tracked game far from the quarries where they acquired their stone. Analysis of these tools suggests that Clovis groups commonly travelled distances of 300 km (185 mi). In one instance, a dozen Clovis points quarried from the Texas Panhandle were left as a cache in northeastern Colorado, 485 km (300 mi) away. These distances indicate a range of movement across the landscape far greater than is observed in later periods of American prehistory.
The idea that Clovis people where big-game hunters could help explain an unsolved puzzle of the Americas in the late Pleistocene: the catastrophic extinction of dozens of species of large animals. Across the Americas millions of large animals known as megafauna disappeared. These animals included the mammoth, mastodon, and the giant ground sloth, as well as the horse, the camel, and many other herbivores. Some very large and formidable carnivores also died out, including the American lion, the saber-toothed tiger, and the giant short-faced bear. These extinctions were thought to coincide with the arrival of Clovis groups, a chronological coincidence that led University of Arizona ecologist Paul Martin to propose the hypothesis of Pleistocene overkill. This hypothesis, first put forward in 1967, contends that Clovis big-game hunters caused the extinctions. Martin suggested that overkill was especially likely-even inevitable-if Clovis group were the first Americans. For if the megafauna had never before faced human hunters, they would have been especially vulnerable prey to this new, dangerous, two-legged predator.
For decades the Clovis-first theory seemed to fit well with the available geologic and archaeological evidence. However, some archaeologists always harbored doubts about the Clovis-first scenario. These doubts intensified toward the end of the 20th century. A reassessment of Clovis subsistence led many to challenge the traditional view of Clovis people as big-game hunting specialists. In addition, the discovery of a pre-Clovis human presence in the Americas has undermined the claim that Clovis people were the first Americans.
Since the 1980s there has been increasing skepticism about the traditional view that Clovis groups were dependent on big-game hunting. Despite many years of searching, few Clovis archaeological sites have yielded evidence to support this view. The San Pedro Valley sites have proved to be the exception, not the rule. There are scarcely a dozen Clovis big-game kill sites known, mostly in western North America, with two possible kill sites in eastern North America. These contain the skeletal remains of just two of the Pleistocene Megafaunal-mammoth and mastodon. Clovis people did kill big game, but apparently not as often as once supposed.
A broader view of Clovis subsistence now suggests that they often targeted slower, smaller, less dangerous prey. The roasted remains of turtles, for example, have been found at many sites, including Aubrey and Lewisville in Texas, Little Salt Spring in Florida, and even at the original Clovis site in New Mexico. Other sites indicate that the diet in Clovis times included small and medium-sized mammals, such as beaver, snowshoe hare, and caribou, as well as fish and a variety of gathered plants.
Over time, the Pleistocene overkill hypothesis was clearly not strongly supported by the archaeological record. Archaeologists have yet to document a single Clovis sloth kill, horse kill, camel kill, or a kill of any of the other several dozen megafaunal species. Whatever caused the extinction of these animals, it was not human hunting. Scientists are currently pursuing alternative hypotheses to explain megafaunal extinctions, such as the possibility they were caused by late Pleistocene climatic and environmental change, or perhaps disease. The puzzle remains unsolved.
A revised view of Clovis subsistence coincides with a reevaluation of the Clovis toolkit. Analysis of Clovis spearpoints shows they were adequate weapons for bringing down big game, but they were not always used that way. Few spearpoints show the kinds of damage that routinely occurs when stone projectiles meet animal bone. Clovis point, like many items in the Clovis toolkit, were most likely used as multipurpose tools; many spearpoints show wear patterns indicating they were used as knives. There is also more variety in the Clovis toolkit than traditionally supposed. Clovis groups in different areas occasionally fashioned tools needed for particular tasks in the environments in which they found themselves. In addition, they probably made tools-perhaps wooden digging sticks or woven plant fibre nets with which to catch fish or small game, hat has not been preserved from that remote time. A varied, multipurpose toolkit is to be expected of groups that hunt and gather a range of foods.
If they were not pursuing wide-ranging big game, why were Clovis groups moving such great distances across the landscape? The answer may be exploration. Hunter-gatherer peoples need to know where to go when resources in one location begin to diminish, as animals are hunted out or flee and as available plants are gathered up. For colonizers in an unfamiliar landscape, that means ranging widely across newly discovered lands to see what resources occur where, when, and in what abundance. Not knowing where they might encounter stone to refurbish their tools on their journeys, it is not surprising that Clovis explorers selected only the highest quality stone for their toolkits, or that they left caches of tools along their way-as the cache in Colorado demonstrates. They could return to the caches to replace diminished supplies without having to walk all the way back to a distant stone quarry.
Claims of a pre-Clovis human occupation in the Americas have been around for decades. By the 1980s, dozens of such sites had been reported, some estimated to be as much as 200,000 years old.
Archaeologists have carefully scrutinized each site to determine if three basic criteria are present. Sites lacking all three criteria cannot be accepted as valid. First, the site must have genuine artifacts produced by humans or human skeletal remains. Second, these artifacts or remains must be found in unmixed geological deposits to ensure that younger objects are not accidentally buried in older layers of sediment. Third, these artifacts or remains must be accompanied by reliable radiocarbon dates that indicate a pre-Clovis occupation. For decades all sites reputed to be of pre-Clovis age failed to meet these criteria. All, that is, except one.
In the mid-1970s University of Kentucky archaeologist Tom Dillehay began excavating at Monte Verde, a site on the banks of Chinchihuapi Creek in southern Chile. Monte Verde is an extraordinary site. Unusual geological conditions quickly buried the remains of an ancient camp beneath wet, swampy sediments. Since the remains left on the surface by the site's inhabitants were not exposed to the air, many organic remains-which normally decays and disappear-were preserved.
Dillehay's team found an astonishing array of organic materials. These included wooden foundation timbers of roughly rectangular huts, finely woven string, and chewed leaves, seeds and other plant parts from nearby species-many with food or medicinal value. In addition, excavations revealed burned bones of mastodon along with pieces of its meat and hide. Some bits of hide still clung to pieces of wooden timbers, the apparent remnants of hide-coverings that once draped over the huts. Also found was the footprint of a child in the once-sticky mud, an assortment of hearths, and hundreds of stone, bone, and wood artifacts. Dillehay's team firmly radiocarbon dated these organic remains to 12,500 years ago-1, 000 years before Clovis times.
The excavations at Monte Verde lasted nearly a decade, and the laboratory research, analysis, and writing about what Dillehay's team had found took another dozen years. Dillehay's findings had to be carefully studied and presented in order to overcome the skepticism of archaeologists who had grown accustomed to seeing pre-Clovis claims fail. When Dillehay's second book on the results of his investigations appeared in 1997, most archaeologists were convinced; the Clovis barrier had fallen at last.
Since Monte Verde, several new candidates for a pre-Clovis settlement in North America have appeared. The Cactus Hill site in Virginia has yielded artifacts below layers in which Clovis-like fluted points were found. Precisely how old those more deeply buried artifacts might be are uncertain, however. The layer in which they were found has produced widely varying radiocarbon ages, from 16,000 years ago to modern times. It therefore remains unclear how old these artifacts might be. Archaeologists have also refocused attention on the Meadowcroft Rockshelter in Pennsylvania. Excavations at Meadowcroft in the 1970s and 1980s produced unmistakable artifacts in deposits perhaps as much as 14,250 years old. Questions remain, however, about whether the artifacts and organic remains are as old as the radiocarbon-dated charcoal. For the time being, neither site, nor any of the other sprinkling of recent pre-Clovis claims, is fully accepted by a still-cautious archaeological community.
The excavations at Monte Verde conclusively demonstrated that people inhabited the Americas in pre-Clovis times. Yet Monte Verde also raised many new questions about the first Americans. Several new theories have been advanced to explain the identity, antiquity, and entryway of the first Americans.
Most archaeologists believe the first Americans-whether travelling in a single migration or multiple migrations-originated in northeastern Asia. This view is based mainly on geological evidence that a land bridge once connected Asia and North America and on genetic similarities between northeastern Asian peoples and Native Americans. It is not, however, founded on any direct archaeological evidence. The kinds of tools typical of the Monte Verde site or Clovis culture are not found in either northeastern Asia or Beringia. All the same, Monte Verde is far from that region and in a very different environmental setting, so it is not surprising its artifacts are different.
Although archaeologists have yet to find a single Clovis spearpoint in northeastern Asia, one artifact comes close: a stone points from the site of Uptar in Siberia that has a flute on one face. Even so, the age of the spearpoint is unknown, and it is not otherwise similar to Clovis fluted points. There are archaeological sites in Alaska, such as the Nenana Complex, that slightly predates Clovis. However, these sites lack the hallmark of Clovis technology: fluted stone projectile points. A few Clovis-like fluted points have been found in Alaska, but these are younger, not older, than those to the south.
The absence of similar artifacts in Siberia or Alaska is not surprising. Finding archaeological traces of a small group, or several groups, that briefly passed through this vast area is a difficult task. In addition, the most recognizable feature of the first Americans' eventfully occurred some 2,500 years before a habitable ice-free corridor opened in the North American interior. A coastal migration could explain how people arrived in Monte Verde 12,500 years ago. By the time the interior route opened, the ancient Monte Verdeans had long departed from the banks of Chinchihuapi Creek.
Finding sites occupied by coastal migrants, however, is no easy task. Much of the late Pleistocene-age shoreline along which migrating groups would have travelled was later submerged when the continental ice sheets melted and their waters returned to the sea. To meet this challenge, researchers are using sonar and taking core samples from the sea floor to explore and probe underwater landscapes and coastlines.
Archaeological excavations have occurred at sites on several islands off the coasts of Alaska and British Columbia. The effort has had some initial success. A cave on Prince of Wales Island in southeastern Alaska has yielded artifacts and human remains' radiocarbon dated to about 10,000 years ago. Bear remains from another part of the same cave are dated to 41,000 years ago. These findings provide tantalizing hope that still older traces of a human presence can be found in this area. Further south, on one of the Channel Islands off the coast of California, and at several coastal Peruvian sites, material as much as 11,000 years old has been found. Still, none of these sites have produced remains old enough to be those of the first Americans.
Some archaeologists believe the first Americans did not come from northeastern Asia, but from Europe, crossing the North Atlantic Ocean by boat. No ancient boats have been found, but proponents note that modern humans travelled by boat to Australia perhaps 30,000 to 40,000 years ago. Archaeological support for this theory is based mainly on similarities observed between Clovis artifacts and those of the Solutrean Period of prehistoric Europe. Some researchers also find support for a North Atlantic route in several ancient human skeletons found in the Americas. These skeletons, proponents argue, appear to have more anatomical similarities with modern Europeans than with modern Native Americans.
Despite the claimed similarities, Solutrean and Clovis artifacts have important differences-in form, method of manufacture, and materials. Most obviously, Solutrean points lack fluting, and Solutrean sites include many stone artifacts and bone tools never found in the Americas. Most archaeologists believe the similarities in artifacts that do exist can be explained as the result of cultural convergence. The concept of cultural convergence suggests that different groups at different times and places might create or use similar materials or tools in similar ways. Solutrean and Clovis cultures are also separated by many thousands of kilometres, most of which is ocean, and by 5,000 years. The Solutrean period ended more than 16,500 years ago, while the earliest Clovis site is only 11,500 years old.
The ancient American skeletons considered by some archaeologists to be anatomically distinct from modern Native American's also fail to support a North Atlantic route. After more detailed anatomical study, those remains, such as the 8,500-year-old skeleton found in Washington State known as Kennewick Man-proved to be far less similar to Europeans than initially believed. Kennewick Man does differ from modern Native Americans. However, many physical anthropologists believe this individual, as with all other ancient skeletal remains found in the Americas, are ancestral Native Americans. The fact that ancient and modern Native Americans do not precisely resemble each other is not surprising: many thousands of years of anatomical and evolutionary change separate them. In addition, for several thousand years after the Americas were first settled, the human population was small, widely scattered, and groups were relatively isolated for long periods of time. Under these circumstances, variability in anatomical features can emerge. Groups of ancient Americans would not necessarily look alike, let alone resemble their descendants many thousands of years later.
If the first Americans migrated from northeast Asia, then the study of modern Native American people-descendants of the first Americans-may hold vital clues about the number and timing of the ancestral migrations to the Americas. Linguists and geneticists have searched for these clues in the languages and genetic heritage of modern Native Americans.
Linguistic studies are based on the assumption that ancient elements, or ‘echoes', of an ancestral language can still be heard in the shared words, grammar, sounds, and meanings of the diverse languages spoken by modern Native Americans. By searching for these elements, researchers hope to learn if all Native American languages evolved from a single ancestral tongue. This common ancestral tongue, if present, may be the language spoken by the earliest Americans. If these elements are not present, however, they could indicate the Americas were peopled at different times by groups speaking distant or unrelated languages.
Linguists are still searching for answers. Most linguists, however, believe the sheer number and variety of Native American languages-of which hundreds are known-bespeaks a long period of language diversification. University of California linguist Johanna Nichols estimates that language diversification in the Americas began as early as 35,000 years ago.
Historical studies of the genetic material of modern Native Americans appear to offer additional clues about the earliest Americans. These studies are based on the knowledge that some types of deoxyribonucleic acid (DNA, the chemical that encodes genetic information) are inherited strictly from one parent or the other, but not both. Mitochondrial DNA (mtDNA) is passed from mothers to their offspring, and Y. Chromosome DNA is passed from fathers to sons. Genetic change in these types of DNA is a result of mutation, not recombination of the parents' DNA. By looking at the genetic difference in mtDNA or Y-chromosome DNA over time, researchers can determine how closely related certain populations are and how much time has elapsed since they were members of the same population.
Genetic studies have shown that virtually all Native Americans share a set of four major mtDNA lineages, and at least two such lineages on their Y chromosome. This indicates these groups are all closely related to one another. The nearest relatives of Native Americans beyond the Americas are the native peoples of northeastern Asia. Native Americans are unrelated genetically to Europeans. Geneticists have variously estimated that peoples of Asia and the Americas were part of the same population from 21,000 to 42,000 years ago.
Geneticists, like linguists, still debate when and how many migratory bands may have trekked from Asia to the Americas. Some scholars believe the evidence indicates a single migration. Others see support for multiple movements of people across Beringia and back. How this is resolved, and how the genetic heritage and languages of modern Native Americans are linked to ancient archaeological data, such as Clovis artifacts, remain important unsolved challenges.
One of the most obvious ways of directly linking ancient and modern Native Americans is by examining the DNA found in prehistoric human skeletal remains. Such remains are extremely rare, however, and recovering DNA from ancient remains can be difficult, if it is even preserved. In the United States the difficulty of linking ancient remains with modern Native Americans might be a strictly scientific concern was it not for legislation that has influenced the progress and conduct of such research.
The Native American Graves Protection and Repatriation Act (NAGPRA), signed into law in 1990, was aimed at righting the wrongs of earlier generations of scientists. In the past, researchers sometimes indiscriminately collected the bones of Native Americans for study and display in museums and universities. Native American peoples were not the only groups to receive such treatment, but their remains and artifacts were gathered in lopsided numbers. To many Native Americans, this was one more instance of mistreatment at the hands of Euro-Americans. In response, NAGPRA required institutions in possession of Native American skeletal remains and artifacts to return them at the request of known lineal descendants.
In the wake of NAGPRA, thousands of skeletons and associated artifacts were returned to Native American peoples. Many of these objects are only a few hundred years old. In such cases, debates over the identity of the descendants have been rare. Other cases, particularly those involving older remains, are more difficult to resolve. Proving lineal descent in cases of greater antiquity is no easy task. This is because descendants of early Americans formed new groups as populations grew, and these groups moved away to settle new lands. A group living 11,000 years ago would almost certainly be ancestral to many modern Native American tribes, not just one. In the future, geneticists may identify sufficiently precise genetic markers to link DNA extracted from ancient human skeletal remains with a group of modern tribes. Nonetheless in most cases, making the link to only one tribe will be difficult.
In one prominent case involving the 8,500-year-old remains of Kennewick Man, the debate over lineal descent ended in a court of law. These remains were found in Washington State in 1996 on property belonging to the federal government. Five Native American tribes living in the area submitted a joint claim under NAGPRA for the return of the remains. A group of archaeologists and physical anthropologists then filed a lawsuit to block the return until detailed scientific studies, including analysis of Kennewick Man's DNA, could be conducted.
The lawsuit sparked several years of legal and scientific wrangling. The Native American groups felt scientific studies were an unnecessary desecration of the remains. They believed they had lived in the area since the beginning of human prehistory in the Americas; therefore, Kennewick Man must be one of their ancestors. The scientists bringing the lawsuit, however, argued that ancestry could not be ascertained without detailed study. This research, they noted, would also add vital information to the meager knowledge about ancient American peoples. Both sides were well intentioned, and under the ambiguous terms of NAGPRA, both were right. NAGPRA allows lineal descendants to be identified not just by DNA, but also by tribal traditions and geographic proximity. The dispute remains unresolved.
Fortunately, few NAGPRA cases have been as contentious as that surrounding Kennewick Man. The human remains from Prince of Wales Island, found about the same time, were excavated and analysed without pitting science against tribal tradition, or archaeologists against Native Americans. Ensuring there is room for both perspectives remains and important challenge under the framework established by NAPGRA.
Studies of the first Americans entered the 21st century on the cusp of change. The traditional view that the first Americans were fast-moving Clovis big-game hunters who migrated into the North American interior on the heels of retreating ice sheets has been undermined. Evidence from Monte Verde demonstrates that humans arrived in the Western Hemisphere in pre-Clovis times, and a reassessment of Clovis subsistence suggests Clovis people were not the big-game hunting specialists imagined in the past. As yet, no widely accepted theory has arisen to replace the older Clovis-first theory. Researchers are proposing many new ideas. Which of these ideas will succeed or fail remains to be seen.
The instruments of archaeological study continue to improve at a rapid pace. Shovels and trowels, the traditional tools of excavation, are now being used alongside ground-penetrating radar, seismic studies of surface features, and other techniques to find now-buried sites. A variety of new studies are providing information about where the materials to make ancient stone artifacts were acquired, how the artifacts were made, and how they were used. These include studies of the geological sources of stone artifacts, experimental work in stone fracture mechanics better to understand how stone tools were made, and analyses of microscopic wear patterns visible on such artifacts. A batteries of techniques are now available to study the chemical composition of bone, plant, shell, and other organic and inorganic remains, providing archaeologists with a clearer picture of the environments to which the first Americans adapted. New dating techniques under development should allow archaeologists reliably to date sites more than 50,000 years old-the current limits of radiocarbon dating. These techniques could prove useful in the event sites of greater antiquity are eventually found in the Americas.
The time-honoured process of acquiring archaeological evidence through careful and meticulous site excavation continues. Where the oldest preserved sites might be is not yet known. There are obvious places to look, however, including eastern Siberia, which is still relatively unknown to archaeologists. Other promising locations for future research include the remnants of Beringia, coastal islands of the Pacific, the Isthmus of Panama-through that any group headed into South America must have passed, and perhaps places not yet imagined. Some of the most interesting discoveries in years to come may even be made in museums, when new techniques for analysis are applied to old collections of artifacts and human remains. Affirmatively done with the interest and cooperation of Native American groups.
Archaeologists may never find evidence of the very first humans to arrive in the Western Hemisphere. It is, after all, a very big place. However, ongoing research is sure to reveal much about how the first Americans colonized a new world.
In America, the search for additional evidence of Folsom Man continues. Near Fort Collins, Colorado, Doctor Frank H. Roberts, Jr., continued excavation at a camp site, uncovering a variety of tools and weapons, and the first known decorated objects from any Folsom site, two decorated beads. This earliest American. Folsom Man, may have lived contemporaneously with Old World Cro-Magnon Man, or some 25,000 years ago. This tentative date was assigned recently by Doctors Kirk Bryan and Louis L. Ray of Harvard University based on studies made at the Folsom camp site, known as the Lindenmeier site, in northeastern Colorado. Many stone points, identified as typically Folsom were found in an earth stratum above the floor of an ancient valley that is traceable to a terrace on a local stream. The terrace has been dated as of the late Ice Age. The dating is based on the assumed correlation between this late Ice Age stage with the Mankato of the Middle West and the Pomeranian of Europe. From this it appears that the culture-bearing layer of the Lindenmeier site was developed at the end of the glacial advance, or 25,000 years ago.
An attempt has been made to adapt the method of dating ruins by analysis of tree rings, so successfully carried out in America, specifically in Southwestern United States, to Viking ruins in Southern Norway. E. de Geer, who has been carrying on this work, reports, that, from a study of the remaining timbers in a wooden burial chamber in a Viking mound, it was constructed in 931 ad. A Swedish fort in Gotland was found by the same method to have been built in five AD.
The Homo habilis, is an extinct primate in the categorical classification set order of the group called the subfamily. A set-classification of group members made up of the humans. Scientists believe this species lived in Africa between two million and 1.5 million years ago. H. habilis are the earliest known members of the genus Homo, the branch of Hominines believed to have evolved into modern humans. The term Homo habilis means handy man, a name selected for the deposits of primitive tools found near H. habilis discovered fossils.
Scientists distinguish H. habilis from australopithecines, the more primitive Hominines from which it evolved, by analysing key physical characteristics. H. habilis had a larger brain than australopithecines. The braincase of H. habilis measured at least 600 cubic centimetres (thirty-seven cu inches) compared with the 500 cu cm (thirty-one cu in) typical of australopithecines. Australopithecines had long arms and short legs, similar to the limbs of apes. The overall body form of australopithecines was also apelike in having large body bulk to its height. Proportionally, H. habilis resembled modern humans with its limbs and small body bulk compared with its height. H. habilis had smaller cheek teeth (molars) and a less protruding face than earlier Hominines. H. habilis were taller than australopithecines, but shorter than A Homo erectus, a later, more humanlike species.
The use of primitive tools implies that H. habilis had developed a different way of gathering food than earlier Hominines, which fed only on vegetation. H. habilis probably ate meat plus fruits and vegetables. Anthropologists disagree on whether H.habilis obtained this meat through hunting, scavenging, or a combination of both techniques
British-Kenyan anthropologist Louis Leakey discovered the first fossil evidence of H. habilis at Olduvai Gorge in northern Tanzania in 1960. Other anthropologists have since discovered specimens in northern Kenya, South Africa, and Malawi. Although all these specimens had a larger brain than australopithecines, some had especially large brains (almost 800 cu cm or forty-nine cu. in.) and more modern skeletons. However, their large and slightly protruding faces seem more primitive than those of other H. habilis specimens. Most scientists now believe that these fossils represent a distinct species named Homo rudolfensis. Scientists debate over which of these two species evolved into the later, even larger-brained H. erectus. Many consider H.rudolfensis the more likely candidate because of its large brain and more modern skeleton. For anthropology, the science of man, 1964 was an eventful and exciting year. Perhaps the most important development of 1964 was the discovery in Africa of a new humanlike, tool-using species, possibly a direct ancestor of man. This was not the only remarkable thing. The new species, named Homo habilis, was very old, probably 1.75 million years old, which makes him nearly twice as old as any previously known tool-using animal. The appearance of Homo habilis on the scene caused great excitement between paleontologists and physical anthropologists and has led many of them to a major reconsideration of much of man's biological history
The new discovery, like so many other important finds of recent years, was made by the top fossil finder of the 20th century, Louis S. B. Leakey, curator of the Coryndon Museum Centre for Prehistory and Palaeontology, Nairobi, Kenya. Professor Leakey's work is invariably done with his wife. Mary, a geologist, and their three sons, who have also recovered important fossil materials. The finds were made in the incredible fossil-rich Olduvai Gorge, an arid chasm in the Serengeti Desert of mainland Tanzania (formerly Tanganyika). The section of Olduvai Gorge excavated by the Leakeys is the most spectacular single prehistoric site in the world. The gorge cuts directly through four main stratigraphic levels, or beds, and in these four beds there are undisturbed paleontological and archaeological deposits covering a time span of nearly two million years. The gorge contains the stratified records of the development of stone tools from the most simple beginnings to elaborately fashioned hand axes; it contains fossil evidence of four major types of men or near-men; and it is rich in fossil remains of ancient fauna, including insects, fish, reptiles, and mammals of the lower and middle Pleistocene periods.
The Homo's habilis excavations were announced by Dr. Leakey at the National Geographic Society in Washington, D.C., and in the April, 4, 1964, issue of Nature. The Olduvai fossil remains are being studied by Professor Phillip Tobias, University of Witwatersrand, Johannesburg, and Dr. John R. Napier, Royal Free Hospital School of Medicine, London.
The Leakeys found bones and teeth representing sixteen hominid individuals in beds' I and II (the two lowest beds) of Olduvai Gorge. One of these was the well-known Zinjanthropus, which is placed roughly in the genus Australopithecus. The australopithecines were a genuses of near-men living about one million years ago, perhaps a little earlier. They were originally considered close to the direct line of man's ancestry, but this is now in doubt. All of the other ascertaining affectualities were considered by Leakey, Tobias, and Napier to represent Homo habilis, a more advanced hominid with a size and shape intermediate between Australopithecus and Homo. Acquainted with the genus that includes modern man and his immediate ancestors that of time were past over by 500,000 years. The specific name habilis are from the Latin and means ‘able, handy, mentally skilful, vigorous'.
Not all of the 206 bones making a complete skeleton of Homo habilis have yet been discovered. The recovered parts, however, are numerous enough to give a good picture of his anatomy and, by inference, of his behaviour. The recovered parts include the remains of two or three skulls, three mandibles (jawbones), about forty teeth, parts of a hand and foot, the bones of a lower leg, a collarbone, and some rib fragments.
Some features distinguishing modern man from his ancestors of earlier epochs include legs and feet adapted for an upright posture and bipedal gait; hands adapted for tool use rather than the locomotion; teeth and jaws adapted for a meat -eater rather than a purely herbivorous diet; a brain adapted for good hand-eyes coordination in tool manufacture and used; and the ability to express with language of the human sort. Except language, the foot, hands, jaws, teeth, and brain case of Homo habilis suggest his close relatives cut an end of the line between the pre-humans and human grades.
The fossil foot is nearly complete, lacking only the back part of the heel and the toes. The foot bones are within the range of variation of The Homo sapiens. The large toe is stout and carried parallels to the other toes; the longitudinal and transverse arch system is like ours. The bones of the foot and leg show the adult Homo habilis had an upright posture and bipedal locomotion, a slender body build, and a stature of about four feet.
The hands are not entirely apelike, nor are they typically human. The hand bones are heavier than ours, and the finger bones are curved inward. The tips of the fingers and thumb are broad, stout, and covered by flat nails, as our modern man's. Probably Homo habilis could not oppose his thumb and fingertips in the precision, pen-holding the grip of modern man, but his hand can make stone tools.
The jaws are smaller than those of Australopithecus; the front of the lower jaw is retreating, with no development of an external bony chin. The incisor teeth are large, the canines are large compared with the premolars, and the premolars and molars are narrow in the tongue-to-cheek dimension. Both the manlike proportions of the teeth and the remains of fish, reptiles, and small mammals found in his living sites show that Homo habilis had an omnivorous diet.
The skull is intermediate in shape between Australopithecus and modern man. The mass of the facial about the cranial part of the skull is reduced and is thus more like the advanced forms. The greatest breadth of the skull is high on the vault. The curvature of the parietal bones is intermediate; that of the occipital bone resembles A Homo sapiens.
The brain case of the Olduvai specimen known as No. 7 has an estimated endocranial volume of 680 cc. The endocranial volume for australopithecines ranges from 435 to 600 cc., that of pithecanthropines from 775 to 1,225 cc., and that of modern man from about 1,000 to 2,000 cc., with an average of about 1,350 cc. Thus, the brain of Homo habilis, although both absolutely and proportionally larger than any of the australopithecines, were not large, either absolutely or relationally by contrast, in co-occurrences among a modern humans. A typical adult Homo habilis had a body weight of about 75 pounds and a brain weight of a little more than one pound, whereas a modern man of 150 pounds has a brain weight of about 3 pounds. In the period following Homo habilis, hominid body weight doubled, but the weight of the brain tripled.
The stone tools found in association with Homo habilis are typical of the Oldowan industry first recognized by Leakey 30 years ago. Similar tools are found elsewhere in East Africa, and in South Africa, Angola, and North Africa. These tools are commonly called pebble tools because most of them are made from waterworn pebbles. Most of the Oldowan choppers are worked on both faces to produce a sharp but irregular cutting edges.
These rough choppers made from potato-sized pieces of stone are the earliest known stone tools; they date from the very beginning of the Pleistocene. There is abundant evidence from Olduvai Gorge showing that the great hand ax or Chelles-Acheul culture evolved directly from the Oldowan stone industry.
Oldowan pebble tools and the skeletal remains of Homo habilis are associated in six sites. At some East and South African sites, pebble tools are also found in association with Australopithecus, but Homo habilis are, according to Professor Tobias, always associated with Oldowan tools, whereas Australopithecus is not. The evidence from the six sites unquestionably shows that early hominids regularly manufactured tools of a set design before they developed hands or brains like those of modern man.
The ages of Homo habilis are as standardly forged and unforeseeable as the fossils themselves. Before these new finds most anthropologists thought the earliest Toolmaker lived less than one million years ago. The potassium-argon process of dating had more than doubled the age of known tool manufacture.
The principle of the potassium-argon technique is simple. The radioactive isotope potassium forty (K40) found in volcanic rock is known to disintegrate into calcium forty and argon forty (A40), an inert gas. The rate of transmutation is constant and very slow; one half the K40 atom changes to A40 atoms, each stand for about 1.3 billion years. The phosphorus-containing mineral anorthoclase is found in the volcanic deposits of Olduvai Gorge. While the lava was in a molten state beneath the earth, no A40 accumulated in the mineral because the gas boiled away. After the lava erupted and cooled, however, nearly all newly formed A40 atoms were imprisoned in the crystalline structure of the anorthoclase. By removing the mineral at a low temperature and then heating it, scientists have succeeded in collecting the released A40 atoms to be counted in a mass spectrometer. Because no A40 was initially present and because the rate of accumulation is also known, this count gives an estimate of the age of the rock. Several samples give age estimates ranging from 1.57 to 1.89 million years, or an average of 1.75 for Bed I in Olduvai, where Homo habilis were found and where the first tools of hominid manufacture appear.
The Homo erectus is an extinct primate classified in the subfamily Homininae and the genus Homo, which include humans. Scientists learn about extinct species, such as The Homo erectus, by studying fossils-petrified bones buried in sedimentary rock. Based on their analysis of these fossils, scientists believe that Homo erectus lived from about 1.8 million to 30,000 years ago. Until recently, The Homo erectus was considered an evolutionary ancestor of modern humans, or Homo sapiens.
The anatomical features of A Homo erectus are more humanlike than those of earlier Hominines, such as australopithecines and Homo habilis. The Homo erectus had a larger brain, measuring up to 1150 cc, and a rounder cranium-the portion of the skull that covers the brain-than earlier Hominines. A Homo erectus was also taller, with a flatter face and smaller teeth. Large differences in body size between males and females, characteristic of earlier hominine species, are less evident in Homo erectus specimens.
This larger brain and more modern body-enabled The Homo erectus to do many things its hominine ancestors had never done. A Homo erectus appears to have been the first hominine to venture beyond Africa. It was the first hominine who effectually engaged of systematic hunting, the first to make anything resemble home bases (campsites), and the first to use fire. Evidence suggests that the childhood of The Homo erectus archeologic remains that periods longer than of earlier Hominines, providing an extended period in which to learn complex skills. These skills are reflected in the proportionally sophisticated stone tools associated with The Homo erectus are being included of their archeological remains. Although still primitive compared with the tools made by early Homo sapiens, the tools made by Homo erectus are much more complex than the simple, small pebble tools of earlier Hominines. The most characteristic of these tools was a teardrop-shaped hand ax, known to archaeologists as an Acheulean ax.
Scientific study of The Homo erectus began in the late 19th century. Excited by Charles Darwin‘s theory of evolution and fossil discoveries in Europe, scientists began to search for the fossilized remains of ‘the unknown factor', the evolutionary ancestor of both human beings and modern apes. In 1891 Dutch anthropologist Eugene Dubois travelled to Java, Indonesia, where he unearthed the top of a skull and a leg bone of an extinct hominine. Measurements of the skull suggested that the creature had possessed a large brain, measuring 850 cc, while the leg-bone anatomy suggested that it had walked upright. In recognition of these characteristics, Dubois named the species Pithecanthropus erectus, or ‘erect ape-man.
Canadian anthropologist Davidson Black found similar fossils in China in the late 1920s. Black named his discovery Sinanthropus pekinensis, or ‘Peking Man'. Later studies by Dutch scientist G. H. von Koenigswald and German scientist Franz Weidenreich showed that the fossils discovered by Dubois and Black came from the same species, which was eventually named The Homo erectus.
Since these earliest discoveries, Homo erectus fossils have been found in East Africa, South Africa, Ethiopia, and various parts of Asia. Kenyan fossil hunter Kamoya Kimeru discovered an almost complete Homo erectus skeleton, known as the Turkana boy, near Lake Turkana in northern Kenya in 1984. The oldest known specimen, dated at almost two million years old, also comes from northern Kenya. Recently developed dating methods have shown that Homo erectus also lived in Java almost two million years ago, but Scientific assumptions about The Homo erectus have changed dramatically since the early 1990s. Anthropologists long assumed that the species spread from Africa to parts of Asia and Europe and that these dispersed populations gradually evolved into The Homo sapiens, or modern humans. Most anthropologists now think it more likely that Homo sapiens originated from a small population in Africa within the past 200,000 years. According to this theory, descendants of this African population of Homo sapiens spread throughout the eastern hemisphere, replacing populations of more ancient Hominines, perhaps with limited interbreeding.
Many anthropologists now believe that some Homo erectus specimens should be classified as a separate species named Homo ergaster. According to this view, Homo ergaster appeared first in East Africa and quickly spread into Asia, where it evolved into The Homo erectus. The Homo sapiens arose in Africa from a population descended from Homo ergaster. Until recently, A Homo erectus was thought to have died out about 300,000 years ago. Recent studies of Homo erectus populations in Java suggest that they may have lived until as recently as 30,000 years ago, long after the evolution of modern humans.
Anthropologists also debate whether Homo erectus used language. Some scientists argue that the brain size of The Homo erectus, the shape of its vocal structures, and the complexity of its behaviour suggest that it had a capacity for spoken language far beyond the rudimentary vocalizations of apes. Other anthropologists reject this conclusion. They point out that the first evidence of artistic expression, a trait closely linked with language, appears only about 40,000 years ago. These skeptics also point to the primitive quality of the tools associated with The Homo erectus. Some anatomical evidence also suggests that Homo erectus overran language abilities. The spinal column of early Homo erectus was much narrower than that of modern humans. This anatomical characteristic implies that Homo erectus had fewer nerves to control the subtle movements of the rib cages required for the production of spoken language. This question may remain unanswered, because, unlike stone tools, spoken words never become part of the archaeological record.
The skulls and teeth of early African populations of the middle Homo differed subtly from those of later H. erectus populations from China and the island of Java in Indonesia. H. ergaster makes a better candidate for an ancestor of the modern human line because Asian H. erectus has some specialized features not seen in some later humans, including our own species. H. heidelbergensis has similarities to both H. erectus and the later species H. neanderthalensis, although it may have been a transitional species evolving between Middle Homo and the line to which modern humans belong.
The Homo's ergaster probably first evolved in Africa around two million years ago. This species had a rounded cranium with a brain size of between 700 and 850 cu. cm. (forty-nine to fifty-two cu. in.), a prominent brow ridge, small teeth, and many other features that it shared with the later H. erectus. Many paleoanthropologists consider H. ergaster a good candidate for an ancestor of modern humans because it had several modern skull features, including proportionally thin cranial bones. Most H. ergaster fossils come from the time range of 1.8 million to 1.5 million years ago.
The most important fossil of this species yet found is nearly a complete skeleton of a young male from West Turkana, Kenya, which dates as early as 1.55 million years ago. Scientists determined the sex of the skeleton from the shape of its pelvis. They also found from patterns of tooth eruption and bone growth that the boy had died when he was between nine and twelve years old. The Turkana boy, as the skeleton is known, had elongated leg bones and arm, leg, and trunk proportion that essentially match those of a modern humans, in sharp contrast with the apelike proportions of H. habilis and Australopithecus afarensis. He appears to have been quite tall and slender. Scientists estimate that, had he grown into adulthood, the boy would have reached a height of 1.8 m (6 ft) and a weight of 68 kg (150 lb). The anatomy of the Turkana boy shows that H. ergaster was particularly well adapted for walking and perhaps for running long distances in a hot environment (a tall and slender body dissipates heat well) but not for any significant amount of tree climbing. The oldest humanlike fossils outside Africa have also been classified as H. ergaster, dated of nearly 1.75 million years' old. These finds, from the Dmanisi site in the southern Caucasus Mountains of Georgia, consist of several crania, jaws, and other fossilized bones. Some of these are strikingly like East African H. ergaster, but others are smaller or larger than H. ergaster, suggesting a high degree of variation within a single population.
H. ergaster, H. rudolfensis, and H. habilis, in summing up to possibly two robust Australopiths, all might have coexisted in Africa around 1.9 million years ago. This finding goes against a traditional paleoanthropological view that human evolution consisted of a single line that evolved progressively over time-an Australopiths species followed by early Homo, then Middle Homo, and finally H. sapiens. It appears that periods of species diversity and extinction have been common during human evolution, and that modern H. sapiens has the rare distinction of being the only living human species today.
Although H. ergaster appears to have coexisted with several other human species, they probably did not interbreed. Mating rarely succeeds between two species with significant skeletal differences, such as H. ergaster and H. habilis. Many paleoanthropologists now believe that H. ergaster descended from an earlier population of Homo-perhaps one of the two known species of early Homo-and that the modern human line descended from H. ergaster.
Sophisticated dating techniques combined with new fossil discoveries suggest that skeletal remains unearthed in Africa in 1995 come from the earliest known human ancestors to walk upright, according to a report published in the journal Nature on May 7, 1998.
Researchers said the new findings suggested that Bipedalism (walking on two legs) emerged 4.07 million to 4.17 million years ago, about 500,000 years earlier than was previously believed. Experts said the new research had important implications for the study of human origins because Bipedalism is widely considered a key, evolutionary adaptation that set the human lineage apart from that of other primates.
The new findings are based on fossils found three years ago in northern Kenya near Lake Turkana. Scientists identified the fossils as belonging to a newly discovered primordial human species, the Australopithecus Anamensis, a creature with apelike teeth and jaws, long arms, and a small brain.
Initial efforts to set the age of the sediments in which the fossils were discovered failed, raising doubts about the fossils' antiquity. In addition, a lower-leg bones provide for critical evidence of Bipedalism was found in a different sedimentary layer, suggesting the bone could be younger or from a different species.
Nevertheless, a new dating effort, led by anthropologist Meave G. Leakey of the National Museums of Kenya, used an argon-dating analysis technique that examined crystals in sedimentary volcanic ash. Researchers said the technique showed the lower -leg bone to be a ‘little' younger than the other fossils dated at 4.07 million to 4.17 million years ago. This finding showed the remains belonged to the same species. The dating analysis was further supported by the subsequent discovery of dozens of new fossils in the area, the researchers said.
Before the discovery of Australopithecus Anamensis, the earliest known bipedal human ancestor was Australopithecus afarensis, the famous "Lucy" skeleton discovered in Ethiopia in 1974 and estimated to be three million to 3.7 million years old. Based on the new findings, some scientists believe that A. Anamensis may be the most ancient species of australopithecine.
One of the earliest defining human traits, Bipedalism-walking on two legs as the primary form of locomotion-evolved more than four million years ago. Fossils show that the evolutionary line leading to us had achieved a substantial upright posture by around four million years ago, then began to increase in body size and in relative brain size around 2.5 million years ago. However, other important human characteristics-such as a large and complex brain, the ability to make and use tools, and the capacity for language-developed more recently. Many advanced traits-including complex symbolic expression, such as art, and elaborative cultural diversity emerged mainly during the past 100,000 years.
Few books have rocked the world the way that. On the Origin of Species did. Influenced in part by British geologist Sir Charles Lyell's theory of a gradually changing earth, British naturalist Charles Darwin spent decades developing his theory of gradual evolution through natural selection before he published his book in 1859. The logical-and intensely controversial-extension of Darwin's theory was that humans, too, evolved through the ages. For people who accepted the biblical view of creation, the idea that human beings shared common roots with lower animals was shocking. In this excerpt form, on the Origin of Species, Darwin carefully sidesteps the issue of human evolution (as he did throughout the book), focussing instead on competition and adaptation in lower animals and plants the Darwinian evolution process by natural selection is fundamentally very simple: natural selection occurs whenever genetically influenced variation among individuals affects their survival and reproduction. If a gene codes for characteristics that result in fewer viable offspring in future generations, that gene is gradually eliminated. For instance, genetic mutations that increase vulnerability to infection, or cause foolish risk taking o lack of interest in sex, will never become common. On the other hand, genes that cause resistance to infection, appropriate risk tasking, and success in choosing fertile mates are likely to spread in the gene pool, even if they have substantial costs.
A classical example is the spread of a gene for dark wing colour in a British moth population living downwind from winds major sources of air pollution. Pale moths were conspicuous on smoke-darkened trees and easily caught by rare mutant forms of the moth, whose colour more closely matched that of the bark escaped the predators' beaks. As the tree trunks became darker, the mutant gene spread rapidly and largely displaced the gene for pale wing colour. That is all there is to it. Natural selection involves no plan, no goal, and no direction-just genes increasing and decreasing in frequency depending on whether individuals with those genes have, relatively to other individuals, greater or lesser reproductive success.
The simplest of natural selection has been obscured by many misconceptions. For instance, Herbert Spencer's nineteenth-century catch phrase ‘survival of the fittest' is widely thought to summarize the process, but it actually promotes several misunderstandings. First, survival is of no consequence by itself. This is why natural selection has created some organisms, such as salmon and annual plants, that reproduces only once, then die. Survival increases fitness only because it increases later reproduction. Genes that increase lifetime reproduction will be selected for even if they result in reduced longevity. Conversely, a gene that decreases total lifetime reproduction will obviously be eliminated by selection even if it increases an individual's survival.
Further confusion arises from the ambiguous meaning of ‘fittest'. The fittest individual, in the biological sense, is not necessarily the healthiest, strongest, or fastest. In today's world, and many of those of the past, individuals of outstanding athletic accomplishment need not be the ones who produce the most grandchildren, a measure that should be roughly correlated with fitness. To someone who understands natural selection, it is no surprise that parents are so concerned about their children's reproduction.
A gene or an individual cannot be called ‘fit' in isolation but only as for a particular species in a particular environment. Even in single environment, every gene involves compromises. Consider a gene that makes rabbits more fearful and by that helps to keep them from the jaws of foxes. Imagine that half the rabbits in a field have this gene. Because they do more hiding and less eating, these timid rabbits might, on average, a bit less fed than their bolder companions. If, hunkered down in the March snow waiting for spring, two thirds of them starve to death while this is the fate of only one third of the rabbits who lack the gene for fearfulness, then, come spring, only a third of the rabbits will have the gene for fearfulness. It ha been selected against. It might be nearly eliminated by a few harsh winters. Milder winters or an increased number of foxes could have the opposite effect. It all depends on the current environment.
While natural selection has been changing us in many small ways in the last ten thousand years, his is but a moment on the scale of evolutionary time. Our ancestors of tn thousand or perhaps fifty thousand years ag looked and acted fully human, if we could magically transport babies from that time a rear the in modern families. We could exec them to grow up into perfectly modern lawyers or farmers of athletes or cocaine addicts.
The point of the rest, is that we are specifically adapted to Stone Age conditions. These conditions ended a few thousand years ago, n=but evolution has not had time since then to adapt us to a world of dens population, modern socioeconomic conditions, low levels of physical activity, and the many other novel aspects of modern environment, we ae not referring merely to the world of offices, classrooms and fast-food restaurants. Life on any primitive farm or in any third-world village may also be thoroughly abnormal for people whose bodies s were designed for the word of the Stone Age hunter-gatherer.
Even more specifically, we seem too adapted to the ecological and socioeconomic condition experienced by tribal societies living in the semiarid habitat characteristic of sub-Saharan Africa. This is most likely where our species originated and lived for tens o thousand of years and where we spent perhaps 90 precent of it history after becoming fully human and recognizable as the species we are today. Prior to that was a far longer period of evolution in Africa in which our ancestor's skeletal features lead scientist to give them names, such as Homo erectus and Homo habilis. Yet even these more remote ancestors walked erect and used their hand for making and using tools. We can only guess at many aspects of their biology speech capabilities and social organization is not apparent in stone artifacts and fossil remains, but there is no reason to doubt that their ways of life were rather similar to those of more recent hunter-gatherers.
Technological advance later allowed our ancestors to invade other habitats and regions, such as deserts, bungles, and forests. Beginning about one hundred thousand years ago, our ancestors began to disperse from Africa to parts of Eurasia, including seasonally frigid regions made habitable advances in clothing, habitation and food acquisition and storage, yet despite the geographical and climatic diversity, people still lived in small tribal groups with hunter-gatherer economies. Grainfield agriculture, with its revolutionary alteration of human dit and socioeconomic systems, was practiced fist in southwestern Asia about eight thousand years ago, and shortly therefore after in India and China. It took another thousand years or more to spread to central and western Europe and tropical Africa and to begin independently in Latin America. Most of our ancestors of a few thousand years still lived in bands of hunter-gatherers. We are, the words of some distinguished anthropologist, "Stone Ages, in the fast lane."
Even so, it is nevertheless, that, all humans are primates. Physical and genetic similarities show that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the so-called great apes (large apes) of Africa-chimpanzees (including bonobos, or so-called pygmy chimpanzees) and gorillas-have the same ancestors that lived sometime between eight million and six million years ago. The earliest humans evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between six million and two million years ago come entirely from Africa. Humans and great apes of Africa have the same ancestor that lived between eight million and five million years ago.
Most scientists distinguish among twelve to nineteen different species of early humans. Scientists do not all agree, however, about how the species are related or which ones simply died out. Many early human species-probably most of them-left no descendants. Scientists also debate over how to identify and classify particular species of early humans, and about what factors influenced the evolution and extinction of each species.
The tree of Human Evolution where fossil evidence suggests that the first humans too evolved was from ape ancestors, at least six million years ago. Many species of humans followed, but only some left descendants on the branch leading to Homo sapiens. In this slide show, white skulls represent species that lived around the time stated to the point; gray skulls represent extinct human species.
Early humans first migrated out of Africa into Asia probably between two million and 1.7 million years ago. They entered Europe in some respects later, generally within the past one million years. Species of modern humans populated many parts of the world much later. For instance, people first came to Australia probably within the past 60,000 years, and to the Americas within the past 35,000 years. The beginnings of agriculture and the rise of the first civilizations occurred within the past 10,000 years.
The scientific study of human evolution is called Paleoanthropology. Paleoanthropology is a sub-field of anthropology, the study of human culture, society, and biology. Paleoanthropologists search for the roots of human physical traits and behaviour. They seek to discover how evolution has shaped the potentials, tendencies, and limitations of all people. For many people, Paleoanthropology is an exciting scientific field because it illuminates the origins of the defining traits of the human species, and the fundamental connections between humans and other living organisms on Earth. Scientists have abundant evidence of human evolution from fossils, artifacts, and genetic studies. However, some people find the idea of human evolution troubling because it can seem to conflict with religious and other traditional beliefs about how people, other living things, and the world developed. Yet many people have come to reconcile such beliefs with the scientific evidence.
Modern and Early Humans have undergone major anatomical changes over evolution. This illustration depicts Australopithecus afarensis (centre), the earliest of the three species; Homo erectus, -an intermediate species, and Homo sapiens and modern human. H. erectus and modern humans are much taller than A. afarensis and have flatter faces with a much larger brain. Modern humans have a larger brain than H. erectus and almost flat face beneath the front of the braincase.
All species of organisms originate through the process of biological evolution. In this process, new species arise from a series of natural changes. In animals that reproduce sexually, including humans, the term species refers to an ordered set-groups of adult members regularly interbreed, resulting in fertile offspring-that is, offspring themselves adequate of reproducing. Scientists classify each species with a unique, two -part scientific names. In this system, modern humans are classified as Homo sapiens.
The mechanism for evolutionary change resides in genes-the basic units of heredity. Genes affect how the body and behaviour of an organism develop during its life. The information contained in genes can be change-a process known as mutation. The way particular genes are expressed how they affect the body or behaviour of an organism-can also change. Over time, genetic change can alter a species's overall way of life, such as what it eats, how it grows, and where it can live.
Genetic changes can improve the ability of organisms to survive, reproduce, and, in animals, raise offspring. This process is called adaptation. Parents pass adaptive genetic changes to their offspring, and ultimately these changes become common throughout a population-a group of organisms of the same species that share a particular local habitat. Many factors can favour new adaptations, but changes in the environment often play a role. Ancestral human species adapted to new environments as their genes changed, altering their anatomy (physical body structure), physiology (bodily functions, such as digestion), and behaviour. Over long periods, evolution dramatically transformed humans and their ways of life.
Geneticists estimate that the human line began to diverge from that of the African apes between eight million and five million years ago (paleontologists have dated the earliest human fossils to at least six million years ago). This figure comes from comparing differences in the genetic makeup of humans and apes, and then calculating how long it probably took for those differences to develop. Using similar techniques and comparing the genetic variations among human populations around the world, scientists have calculated that all people may share common genetic ancestors that lived sometime between 290,000 and 130,000 years ago.
Humans belong to the scientific order named Primates, a group of more than 230 species of mammals that also includes lemurs, lorises, tarsiers, monkeys, and apes. Modern humans, early humans, and other species of primates all have many similarities and some important differences. Knowledge of these similarities and differences helps scientists to understand the roots of many human traits, and the significance of each step in human evolution.
The origin of our own species, Homo sapiens, is one of the most hotly debated topics in Paleoanthropology. This debate centres on whether or not modern humans have a direct relationship to H. erectus or to the Neanderthals, are well-known as to a greater extent a nontraditional set-grouped of humans who evolved within the past 250,000 years. Paleoanthropologists commonly use the term anatomically modern Homo sapiens to distinguish people of today from these similar predecessors.
Traditionally, paleoanthropologists gave to a set-classification as Homo sapiens, any fossil human younger than 500,000 years old with a braincase larger than that of H. erectus. Thus, many scientists who believe that modern humans descend from a single line dating back to H. erectus. The name archaic Homo sapiens to refer to a variety of fossil humans that predate anatomically modern H. sapiens. The defining term ‘archaic', denotes a set of physical features typical of Neanderthals and other species of a late Homo before modern Homo sapiens. These features include a combination of a robust skeleton, a large but low braincases (positioned amply behind, than over, the face), and a lower jaw lacking a prominent chin. In this sense, Neanderthals are sometimes classified as subspecies of archaic H. sapiens-H. Sapient, or categorized as neanderthalensis. Other scientists think that the variation in archaic fossils existently falls into clearly identifiable sets of traits, and that any type of human fossil exhibiting a unique set of traits should have a new species name. According to this view, the Neanderthals belong to their own species, H. neanderthalensis.
The Neanderthals lived in areas ranging from western Europe through central Asia from about 200,000 to about 28,000 years ago. The name Neanderthal comes from fossils found in 1856 in the Feldhofer Cave of the Neander Valley in Germany, which-is the modern form of thal and means "valley" in German. As for the swing of moving forward, did it take place in one geographical area, in one group of humans, who were by that enabled to expand and replace the former human populations of other parts of the world? Or did it occur in parallel in different regions. In each, of which the human populations living today would be descendants of the populations living there before its move forwards. The modern-looking human skulls from Africa around 100,000 years ago have been taken to support the former view with a forwarded occurrence that in Africa. Molecular studies (of so-called mitochondrial DNA) were initially also interpreted as to an American origin of modern humans though the meaning of those molecular findings is currently in doubt. On the other hand, skulls of humans living in China and Indonesia hundreds of thousands of years ago are considered by some physical anthropologists to exhibit features still found ion modern Chinese and in Aboriginal Australians, respectfully. If true, that finding would suggest parallel evolution and multi-regional origins of modern humans, than origins in a single Garden of Eden. The issue remains unresolved.
While Neanderthals lived in glacial times and were adapted to the cold, they penetrated no farther north than northern Germany and Kiev. That is not surprising, since Neanderthals apparently lacked needles, sewn clothing, warm houses, and other technology essential to survive in the coldest climates. Anatomically modern peoples who did possess such technology had expanded into Siberia by around 20,000 years ago (there are the usual much older disputed claims). That expansion may have been responsible for the extinction of Eurasia's wooly mammoth and wooly rhinoceros.
Scientists realized several years later that prior discoveries-at Engis, Belgium, in 1829 and at Forbes Quarry, Gibraltar, in 1848-also represented Neanderthals. These two earlier discoveries were the first early human fossils ever found. In the past, scientists claimed that Neanderthals differed greatly from modern humans. However, the basis for this claim came from a faulty reconstruction of a Neanderthal skeleton that showed it with bent knees and a slouching gait. This reconstruction had given the common yet mistaken impression that Neanderthals were much simpler descents who lived crude lifestyles. On the contrary, Neanderthals, like the species that preceded them, walked fully upright without a slouch or bent knees. In addition, their cranial capacity was quite large at 1,500 cu. cm. (about ninety cu. in.), larger on average than that of modern humans. (The difference probably relates to the greater muscle mass of Neanderthals as compared with modern humans, which usually correlates with a larger brain size.)
Along with many physical similarities, Neanderthals differed from modern humans in several ways. The typical Neanderthal skull had a low forehead, a large nasal area (suggesting a large nose), a cultivated but projecting nasally and cheek region, a prominent brow ridge with a bony arch over each eye, a non-projecting chin and a visible space behind the third molar (in front of the upward turn of the lower jaw).
Neanderthal and Modern Human Skulls the skull of Homo neanderthalensis differs considerably from that of anatomically modern humans, or Homo sapiens. Neanderthals had thick-walled skulls, sloping foreheads, and heavy brow ridges. This contrasts with the thin-walled skulls, high foreheads, and flat faces of modern humans. Neanderthals also had more pronounced and powerful jaws but less of a chin than do modern humans.
Neanderthal also had a much heavily assembled body and larger boned skeletons than do modern humans. Other Neanderthal skeletal features included a bowing of the limb bones in some individuals, broad scapulae (shoulder blades), hip joints turned outward, a long and thin pubic bone, short lower leg and arm bones on the upper bones, and large surfaces on the joints of the toes and limb bones. Together, these traits made a powerful, compact body of short stature-males averaged 1.7 m. (5 ft. 5 in.) tall and 84 kg. (185 lb.), and females averaged
1.5 m. (5 ft.) tall and 80 kg. (176 lb.).
The short, stocky build of Neanderthals conserved heat and helped them withstand extremely cold conditions that prevailed in temperate regions beginning about 70,000 years ago. The last known Neanderthal fossils come from western Europe and date from approximately 36,000 years ago.
What is more, as Neanderthal populations grew in number in Europe and parts of Asia, other populations of nearly modern humans arose in Africa and Asia. Scientists also commonly refer to these fossils, which are distinct from but similar to those of Neanderthals, as archaic. Fossils from the Chinese sites of Dali, Maba, and Xujiayao display the long, low cranium and large face typical of archaic humans, yet palaeanthropologist also has features similar to those of modern people in the region. At the cave site of Jebel Irhoud, Morocco, scientists have found fossils with the long skull typical of archaic humans but also the modern traits modern of measure have higher forehead and flat mid-face. Fossils of humans from East African sites older than 100,000 years-such as Ngaloba in Tanzania and Eliye Springs in Kenya-also seem to show a mixture of archaic and modern traits.
Ancient Human Footprints the oldest known footprints of an anatomically modern human are embedded in rock north of Cape Town, South Africa. Geologist David Roberts and palaeanthropologist Lee Berger announced the discovery of the footprints in August 1997. A human being made the footprints about 117,000 years ago by walking through wet sand, which eventually hardened into rock.
The oldest known fossils that possess skeletal features typical of modern humans assign a date to between 130,000 and 90,000 years ago. Several key features distinguish the skulls of modern humans from those of archaic species. These features include a much smaller brow ridge, if any; a globe-shaped braincase; and a flat or parallelled projecting face of reduced in size and found under the front of the braincase. Among all mammals, only humans have a face positioned directly beneath the frontal lobe (forward-most area) of the brain. As a result, modern humans have a tendency in having a higher forehead than did Neanderthals and other archaic humans. The cranial capacity of modern humans ranges from about 1,000 to 2,000 cu cm (sixty to 120 cu in), with the average being about 1,350 cu cm (eighty cu in).
Scientists have found both fragmentary and nearly complete cranial fossils of early anatomically modern Homo sapiens from the sites of Singha, Sudan; Omo, Ethiopia; Klasies River Mouth, South Africa and Skhûl Cave, Israel. Based on these fossils, many scientists conclude that modern H. sapiens had evolved in Africa by 130,000 years ago and started spreading to diverse parts of the world beginning on a route through the Near East sometime before 90,000 years ago.
The 1994 discovery in Sierra de Atapuerca, Spain, of well -preserved hominid bones pushed back the date for the arrival in Europe of our early human ancestors to 800,000 years ago. Anthropology professor Brian Fagan discusses these and other recent findings about the first members of the human family to live in Europe, and he dispels the widespread myth that Neanderthals were dumb and brutish.
Paleoanthropologists are engaged in an ongoing debate about where modern humans evolved and how they spread around the world. Differences in opinion rest on the question of whether the evolution of modern humans took place in a small region of Africa or over a broad area of Africa and Eurasia. By extension, opinions differ about whether modern human populations from Africa displaced all existing populations of earlier humans, eventually resulting in their extinction. Those in who think of modern humans as originating only in Africa and then spread from place to place the world support of the out of Africa hypotheses. Those who think modern humans evolved over a large region of Eurasia and Africa support the so -called multi-Regional hypothesis.
Researchers have conducted many genetic studies and carefully assessed fossils to detect which of these hypotheses agrees more with scientific evidence. The results of this research do not entirely confirm or reject either one. Therefore, some scientists think a compromise between the two hypotheses is the best explanation. The debate between these views has implications for how scientists understand the concept of race in humans. The question raised is whether the physical differences among modern humans evolved deep in the past or most recently.
Scientists reported in the May 16, 1996, issued of the journal Nature that later Neanderthals were likely interactively relational, and, perhaps even traded goods, with Cro-Magnons, their anatomically modern human neighbours. Researchers in Arcy-sur-Cure, France, 35 km (22 mi) southeast of Auxerre, said they found hominid fossils alongside bone and ivory jewellery nearly identical to artifacts attributed to anatomically modern humans.
The fossils were found in Arcy-sur-Cure long ago, but scientists could not determine to which human species the bones belonged. The shape of the inner ear gave anthropologists a clue that the 34,000-year-old fossil remains found decades ago were from a Neanderthal, not a modern human. The ear morphology may also shed light on the relationship of Neanderthals to humans of today.
The ornaments found at the Arcy site included a bone ring, grooved animal teeth, and animal claws with small holes made at one end, presumably so they could be strung on a cord and hung around the neck. They resemble jewellery found at sites in northern Spain and central and southwestern France where Cro-Magnons lived. Anthropologists Jean-Jacques Hublin of the Musée de l'Homme in Paris, France, and Fred Spoor of University College in London, England, the coauthors of the report, concluded that the presence of jewellery at the Arcy site nearly identical to jewellery at the Cro-Magnon sites showed that Neanderthals probably traded with Cro-Magnons rather than imitated the style of their contemporary neighbours. The resemblance was too close in appearance to nearby Cro-Magnon finds for imitation, they believe. Anatomically modern humans first arrived in Europe about 40,000 years ago.
The relationship of Neanderthals to modern humans has long been a topic of scientific debate. The fossil record suggests Neanderthals disappeared from 30,000 to 40,000 years ago. Neanderthals characteristics differ most obviously from anatomically modern humans in the formation of the skull and face. The Neanderthal had a sloping forehead, no chin, protruding browridges, large teeth, and strong jaw muscles. The brains of Neanderthals were larger than those of modern humans. Apart from the face, the Neanderthals had thicker bones and larger musculature, long bodies and short legs. Some Neanderthal's features, especially body proportion, were cold-weather adaptations similar to those developed by modern people living in arctic conditions, such as the Inuit.
Hublin and Spoor used high-resolution, computerized X rays to scrutinize a temporal (side) bone from the skull of a one -year-old Neanderthal. They found that the ear canal-known as the labyrinth-within the bone was distinctly different in size and location from the same bone in Homo erectus, an early human ancestor, and anatomically modern humans. The labyrinth consists of three hollow rings and is involved in maintaining balance.
Some scientists classify the Neanderthal as a separate species, Homo neanderthalensis. Because the features of the Neanderthal's labyrinth do not exist in modern humans, the scientists believe that the muscular hominid belongs to a separate species, or at least is not an ancestor of modern humans. Some experts believe that Neanderthals evolved from an archaic Homo sapiens into an evolutionary dead end. Other researchers have speculated that later Neanderthals may have interbred with Cro-Magnons, but Hublin argues that his new evidence does not support that theory. In their report to Nature, Hublin and Spoor said their findings did not show any trend toward more modern human characteristics.
Archives consist of articles that originally appeared in Collier's Year Book because they were published shortly after events occurred, they reflect the information available then. Cross references refer to Archive articles of the same year. Archaeology Top stories in archaeology in 1995 included new dates for the Neanderthals and the discovery of the frozen bodies of a Scythian equestrian and an Inca woman. Last Neanderthals. New dates from Zafarraya Caves in southern Spain suggest that Neanderthals had lived in some previous millennia after scholars assumed they had become extinct. The dates also suggest that Neanderthals coexisted with modern humans in Western Europe for 10,000 years or more, rather than being replaced quickly by overwhelmingly superior modern groups, as many archaeologists have argued. Samples of animal bones and teeth found with Neanderthal remains and artifacts were subjected to both carbon and thorium/uranium testing, producing dates of around 30,000 years ago. In northern Spain stone tools of a type generally associated with modern humans appeared between 40,000 and 38,000 years ago. Elsewhere in Europe, Neanderthal and modern human populations mixed, but in southern Spain, Neanderthals survived without strong biological or cultural interaction with the newcomers, probably because they were isolated. The existence of a Neanderthal population in southern Spain long after modern humans arrived in the north makes it unlikely that modern humans reached Western Europe from Africa via the Strait of Gibraltar. Frozen Bodies. A frozen Scythian equestrian, dated to around 500 Bc, was found in Siberia's Altai Mountains. The man, 25-30 years of age, was buried with his horse, bow and arrows, an ax, and a knife. He was wearing a thick wool cap, high leather boots, and a coat of marmot and sheepskin. On his right shoulder is a large tattoo of a stag. The horse's harness was decorated with wood carvings of griffins and animals covered in gold foil. The horseman's body, like the body of a richly attired woman who also was discovered in the same area in 1993, had been buried in a log-lined chamber under more than 2 metres (7 feet) of permafrost. The horseman's mummy was moved to a Moscow lab for preservation. In southern Peru the frozen body of an Inca woman of 12-14 years of age, probably a sacrificial victim, was found near the summit of a 6,300-metre (20,700-foot) peak. The remains, dated around Ad. 1500, were discovered 60 metres (200 feet) below a stone sanctuary. The peak is usually ice-covered, but the recent eruption of a nearby volcano had blanketed it with ash. The dark-coloured ash absorbed the sun's warmth instead of reflecting it as the ice had, and the ice melted. Two more bodies were later found farther down the slope, along with the remains of a camp used by the sanctuary's builders and priests. Several small figurines of gold, silver, gold-copper alloy, and oyster-like shell were found near the girl's body, and two had been wrapped in the layers of wool and cotton cloth in which it was bundled. The body had an elaborate feather headdress. The most important aspect of the find is that the bodies were frozen, providing an opportunity to study Inca diet and health. Early Bone Points From Africa. Archaeologists dated barbed bone points found in eastern Zaire to 90,000 years or older. The ability to make such tools earlier supports an African origin of behaviourally and biologically modern humans, the archaeologists said. Barbed points do not occur before 14,000 to 12,000 years ago at sites in Eurasia. The barbed points, and unbarbed points and a flat dagger-shaped object with rounded edges, came from three sites at Katanda in the Semliki River valley. Dating of the immediately overlying sands and hippo teeth found in them suggests an age of 80,000 to 90,000 years ago for the site. The barbed points were found with mammal and fish remains, of which catfish were most abundant. The catfish were probably caught during the rainy season when they spawned on the inundated flood-plain and were easy to catch. The Katanda sites show that a complex bone industry and seasonal use of aquatic resources had developed by 90,000 years ago, following a specialized subsistence pattern most often associated in Europe with the end of the Ice Age nearly 80,000 years later. Earliest Weaving. Impressions of woven fabric on four fragments of clay from Pavlov I, an Upper Palaeolithic site in the Czech Republic, have been the earliest evidence of weaving ever found. The fragments were carbon dated in 1995 to between 26,980 and 24,870 years ago. The dates are at least 7,000 to 10,000 years earlier than those of any other evidence of weaving. Two of the better-preserved specimens of tightly spaced rows characteristically of a finely woven bag or mat. The fineness and the method of weaving used, known as twining, suggested that the material may have been produced using a loom and that the weavers were accomplished and not experimenting with a new technology. This means that the actual arrival of weaving may be even earlier than the date of the Pavlov specimens. The impressions from Pavlov I show that a wide range of items, such as baskets, nets, and snares, were likely to have been available to the hunter-gatherers of the Upper Palaeolithic Epoch. Chauvet Art. The spectacular decorated Grotte Chauvet in southern France, whose discovery was announced in January, has proved to have the world's oldest known cave paintings, carbon dated to more than 30,000 years ago. The cave also contains human and bear footprints, flints, bones, and hearths. Submarines and Archaeology. The Confederate vessel Hunley, the first submarine ever to sink a warship in combat, was discovered in May off the coast of Charleston, SC. Famous for its attack on the USS Housatonic during the American Civil War, the submarine went down shortly after sinking the ship on February 17, 1864. The Hunley was made from an iron locomotive boiler and carried a copper canister filled with 40 kilograms (90 pounds) of black powder at the end of a long spar. Manned by volunteers who powered its hand-cranked propeller, the Hunley placed its charge alongside the target and then backed up, detonating the explosive with a long cord that triggered the firing mechanism. The US Navy alleged that in 1995 that the NR-1, a formerly classified submarine, would be used to search the Mediterranean sea-floor for ancient shipwrecks. The submarine's windows and extensive light and sonar arrays give to develop the perfect tense for searching for ancient sunken wrecks, and its remote, and controlled arm can retrieve objects. The NR-1, the world's smallest nuclear submarine, will enable archaeologists to study the open-waters trade routes of antiquity, not just the coastal routes. Its first archaeological mission will be to explore the trade route between Carthage, on the North African coast, and Rome. The discovery in 1995 of the Japanese submarine I-52, which was sunk on June 23, 1944, deepened concerns about the growing accessibility of the deep oceans. American and British treasure hunters were in a race to find the sub and its cargo, 2 metric tons of gold. Both groups hired Russian research vessels with sophisticated sonar and photographic capabilities. In May the American group found the submarine 5,000 metres (17,000 feet) down in the mid-Atlantic. The discoverer stated that the gold, valued at $25 million, would be recovered with the least disturbance possible to the vessel, which may still hold the remains of 109 men. The Japanese government may retain title to both vessel and contents. Nonetheless, the implications lay clear: anyone with sufficient financial backing can happen upon, and, if possible, the person would unscrupulously pillage shipwrecks-ancient, medieval, or modern. Egyptian Tombs. Important discoveries were made in 1995 at both well-known and newly found cemeteries in Egypt. At Saqqara, near Cairo, French archaeologists discovered the necropolis of three queens of the Sixth Dynasty Pharaoh Pepi I (2332-2283 Bc). A pyramid 45 metres (150 feet) high found buried in sand at Saqqara is the tomb of Queen Meretites, a descendant of Pepi I. It may provide information on a turbulent period at the end of the dynasty when powerful governors paid only nominal allegiance to the pharaoh. Egyptian and Canadian archaeologists put a vast pre-dynastic cemetery at Tell Hassan Dawoud, 100 kilometres (60 miles) east of Cairo, dating to 3000 Bc or earlier. Many tombs yielded gold, marble, and ceramic artifacts. Not all of the burials had grave offerings, however, suggesting that Egypt's society was strongly stratified 500 years before the pharaohs. The largest tomb ever found in Egypt's Valley of the Kings was partly explored in 1995. The tomb was the burial place of many 100 or more offspring of Rameses II, who reigned around 1279-1212 Bc. Artifacts recovered from the tomb bear the names of at least four of his sons, and the name of the firstborn, Amon-her-khepeshef, are painted on a wall. Awaiting the presence to the discovery as few combinations were known about the preponderance of the pharaoh's descendants. The tomb is unlikely to hold any great treasure, since a papyrus in Turin records its robbery in 1150 Bc. Its chief importance is the information it may yield about family burials and tomb plans of New Kingdom royalty. Syrian Bronze Age Cemetery. Archaeologists working at Telles-Sweyhat on the Euphrates River in northern Syria discovered an intact tomb in what may be an unplundered cemetery containing up to 150 such tombs. Investigation of several tombs could provide a sample of human remains largely enough to set biological relationships and social organization of the people through DNA analysis. A large sample would also allow study of diet and disease in the population. The tomb, dated around 2500-2250 Bc, held the remains of several individuals. More than 100 ceramic vessels were in the tomb, along with incised bone, beads, and shells. Copper and bronze objects included daggers, axes, and a javelin. Bones of many pigs, sheep, goats, and cows in the tomb are the remains of funerary offerings. Bird eggs had been placed in the eye sockets of one animal skull
However, it was the Primates, of whom are an order of mammals that includes humans, apes, which are the closest living relatives to humans, monkeys, and some less familiar mammals, such as tarsiers, lorises, and lemurs. Humans and other primates share a common evolutionary descent. Consequently, primates have always fascinated scientists because their physical features, social organization, behavioural patterns, and fossil remains provide clues about our earliest human ancestors.
Primates evolved from tree-dwelling ancestors. Although some species, such as humans, have since taken to the ground, all primates' share features that are related to their tree-climbing ancestry. These include arms and legs that can move more freely than those of most other mammals, flexible fingers and toes, forward-facing eyes that can judge distances accurately-a vital aid when moving about high above the ground-and large brains.
Primates live in a wide range of habitats but are restricted by their need for warmth. Most primates live in tropical jungles or dry forests, but some live in dry grasslands, and others have settled in cold, mountainous regions of China and Japan. The world's most northerly primate, the Japanese macaque, has learned to bathe in hot springs to survive through the winter snows. In parts of the tropics, monkeys can be seen within a few miles of busy city centres, but despite this adaptability, most of the world's primates retain a close dependence on trees. Apart from humans, baboons are the only primates that have fully made the transition to life out in the open, and even they instinctively climb to safety if danger threatens.
Some primates, especially the smaller species, are active only at night, or nocturnal, while others are diurnal, active during the day. Most primate species-particularly monkeys-are highly sociable animals, sometimes living in troops of more than 100 members. Smaller primates, especially nocturnal ones, have a disposition to favour solitary and secretive.
Primates range in size from quite small to quite large. The world's largest species, the lowland gorilla at 200 kg (400 lb) is more than 6,000 times the weight of the smallest primate, the pygmy mouse lemur from Madagascar. Measuring just 20 cm (eight in) from nose to tail, and weighing about thirty g (1 oz), this tiny animal was first identified about two centuries ago, but was later assumed to be extinct until its rediscovery in 1993.
There are about 235 species of primates. Scientists use more than one way to classify primates, and one system divides the order into two overall groups, or suborders: the prosimians and the anthropoids.
The prosimians, or ‘primitive primates', make up the smaller of these two groups, with about sixty species, and include lemurs, Pottos, galagos, lorises, and, in some classification systems, tarsiers. Lemurs are only found on the islands of Madagascar and Comoros, where they have flourished in isolation for millions of years. Pottos and galagos are found in Africa, while lorises and tarsiers are found in southeast Asia. Typical prosimians are small to medium-sized mammals with long whiskers, pointed muzzles, and well-developed senses of smell and hearing. Most prosimians are nocturnal, although in Madagascar some larger lemurs are active by day.
In the past, tree shrews were often classified as primates, but their place in mammal classification has been the subject of much debate. Today, based on reproductive patterns and on new fossil evidence, most zoologists classify them in an order of their own, the Scandentia.
The remainder of the world's primates makes up the anthropoid, or "humanlike" suborder, which contains about 175 species. This group consists of humans, apes, and monkeys. Most anthropoids, apart from baboons, have flat faces and a poor sense of smell. With a few exceptions, anthropoids are usually active during the day, and they find their food mainly by sight.
Evolution has affected the thumbs and big toes of primates. In most mammals, these digits bend in the same plane as the other fingers and toes. Nevertheless, in many primates, the thumbs or big toes are opposable, meaning that they are set apart in a way that permits them to meet the other digits at the tips to form a circle. This enables primates to grip branches, and equally importantly, pick up and handle small objects. Instead of having claws, most primates have flat nails that cover soft, sensitive fingertips-another adaptation that helps primates to manipulate objects with great dexterity.
Primate skulls show several distinctive features. One of these is the position of the eyes, which in most species is on the front of the skull looking forward, rather than on the side of the skull looking to the side as in many other mammals. The two forward-facing eyes have overlapping fields of view, which give primates stereoscopic vision. Stereoscopic vision permits accurate perception of distance, which is helpful for handling food or swinging from branch to branch high above the ground. Another distinctive feature of primate skulls, in anthropoids particularly, is the large domed cranium that protects the brain. The inside surface of this dome clearly shows the outline of an unusually large brain-one of the most remarkable characteristics of this group. The shapes of anthropoid brains are different from other mammals; the portion of the brain neuronally devoted toward vision is especially large, while the portion involved with smell is comparatively small.
The primate order includes a handful of species that live entirely on meat (carnivores) and a few that are strict vegetarians (herbivores), but it is composed chiefly of animals that have varied diets (omnivores). The carnivorous primates are the four species of tarsiers, which live in Southeast Asia. Using their long back legs, these pocket-sized nocturnal hunters leap on their prey, pinning it down with their hands and then killing it with their needle-sharp teeth. Tarsiers primarily eat insects but will also eat lizards, bats, and snakes.
Other prosimians, such as galagos and mouse lemurs, also hunt for insects, but they supplement their diet with different kinds of food, including lizards, bird eggs, fruit, and plant sap. This opportunistic approach to feeding is seen in most of monkeys and in chimpanzees. Several species of monkeys, and chimpanzees, but not the other apes, have been known to attack and eat other monkeys. Baboons, the most adept hunters on the ground, often eat meat and sometimes manage to kill small antelope.
Primates display a wide range of mating behaviours. Solitary primates, such as aye-ayes and orangutans, have simple reproductive behaviour. Within the territory that each male controls, several females live, each with their own territory. The male mates with any females within his territory that are receptive. Other species, such as gibbons, form small family groups consisting of a monogamous pair and they're young. Gorillas form harems, in which one adult male lives with several adult females and they're young. Among social primates, breeding can be complicated by the presence of many adults. Males may cooperate in defending their troop's territory, but they often fight each other for the chance to mate. In some species, only the dominant male mates with the females in the group. Chimpanzee females mate promiscuously with several adult males, although they usually pair up with one of the high-ranking males during the final few days of estrus, spending all of their time together and mating together exclusively.
Primates have the most highly developed brains in the animal kingdom, rivalled only by those of dolphins, whales, and possibly elephants. Anthropoid primates in particular are intelligent and inquisitive animals that are quick to learn new patterns of behaviour. This resourcefulness enables them to exploit a wide range of foods and may help them to escape attacks by predators.
Many zoologists believe that primates' large brains initially evolved in response to their tree-dwelling habits and their way of feeding. Anthropoid primates, which have the largest brains, live in a visual world, relying on sight to move about and to settle and manipulate food. Unlike smell or hearing, vision generates a large amount of complex sensory information that has to be processed and stored. In primate brains, these operations are carried out by part of the brain called the cerebral cortex, which evolved into such a large structure that the rest of the brain is hidden beneath it. Some unrelated mammals, such as squirrels, also live in trees, but they have less-developed eyesight and much smaller brain.
Increased brainpower has had impressive effects on the way primates live. It has helped them to move about and find food and enabled them to develop special skills. One of the most remarkable of these is Toolmaking, seen in chimpanzees and, to a far greater extent, in humans. Toolmaking, as opposed to simple tool use, involves a preconceived image of what the finished tool should look like-something that is only possible with an advanced brain.
The intelligence of primates is also evident in their social behaviour. For species that live in groups, daily life involves countless interactions with relatives, allies, and rivals. Mutual cleaning and grooming of the fur, which removes parasites, helps to reinforce relationships, while threats-sometimes followed by combat-maintain the hierarchy of dominance that permeates typical primate troops.
Primates use a variety of methods to communicate. In solitary prosimians, when animals are not within sight of each other, communication is often accomplished by using scents. Such animals use urine, faeces, or special scent glands to mark territory or to communicate a readiness to mate. In social anthropoids, visual and vocal signals are much more important. Most monkeys and apes communicate with a complex array of facial expressions, some of which are similar to the facial expressions used by humans. The earliest fossils of primates discovered date from the end of the Cretaceous Period, about sixty-five million years ago. These early fossils include specimens of a species called Notharctus, which resembles today's lemurs and had a long pointed snout. The ancestors of another prosimian group, the tarsiers, are known from fossils that date from the early Eocene Epoch, about fifty million years ago. In 1996 researchers in China recovered fossil bones of a primitive primate no bigger than a human thumb. The animal, named Eosimias, had existed of some forty-five million years ago. Many scientists believe that Eosimias is an example of a transitional animal in the evolution of prosimians to anthropoids. The origin of anthropoids is difficult to pin down. A single anthropoid fossil has been found that may come from the Eocene Epoch, but conclusive fossil evidence of anthropoids does not appear until the Oligocene Epoch, which was introduced some thirty-eight million years ago. These early anthropoids belonged to a lineage that led to the catarrhine primates-the Old World monkeys, apes, and humans. The platyrrhine primates, which include all New World monkeys, are presumed to have diverged from the Old World monkeys during the Eocene Epoch. They evolved in isolation on what was then the island continent of South America. Genetic analysis shows that New World monkeys clearly have the same ancestry with the catarrhines, which means that they must have reached the island continent from the Old World. Exactly how they did this is unclear. One possibility is that they floated across from Africa on logs or rafts of vegetation, journeying across an Atlantic Ocean that was much narrower than it is today
Of all primate groups, the apes and the direct ancestors of humans have been the most intensively studied. One key question that concerns once the two groups diverged. Based on the comparisons of genes and the structure of body parts, scientists think that the line leading to the orangutan diverged from the one leading to humans about twelve million years ago. The ancestral line leading to chimpanzees did not diverge until more recently, probably between five and seven million years ago. This evidence strongly suggests that chimpanzees are our closest living relatives. Apes and monkeys also play an important role in the field of medical research. Because their body systems work very much like our own, new vaccines and new forms of surgery are sometimes tried on apes and monkeys before they are approved for use on humans. Species that are most often used in this way include chimpanzees, baboons, and rhesus monkeys. This kind of animal experimentation has undoubtedly contributed to human welfare, but the medical use of primates is an increasingly controversial area, particularly when it involves animals captured in the wild.
The species most under threats are those affected by deforestation. This has been particularly severe in Madagascar, the only home of the lemurs, and it is also taking place at a rapid rate in Southeast Asia, threatening gibbons and orangutans. The almost total destruction of Brazil's Atlantic rainforest has proved catastrophic for several species, including the lion tamarins, which are found only in this habitat. Primates are also threatened by collection for the pet trade and by hunting. Illegal hunting is the chief threat facing the mountain gorilla, a rare African subspecies that lives in the politically volatile border region straddling Uganda, Rwanda, and the Democratic Republic of the Congo.
In the face of these threats, urgent action is currently underway to protect many of these endangered species. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) currently forbids the export of many primates, although not all countries have chosen to follow this law. More direct methods of species preservation include habitat protection and captive breeding programs. In some case-for example, the lion tamarin-these programs have met with considerable success. However, without the preservation of extensive and suitable natural habitats, many primate species are destined for
Our closest living relative are three surviving species of great apes: the gorilla, the common chimpanzee, And the pygmy chimpanzee (also known as bonoboo). Their confinement to Africa, along with abundant fossil evidence, strongly suggests that they also played the earliest stages of human evolution out in Africa, human history, as something separate from the history of animals, occurring about seven million years ago (estimates range from five to nine million years ago). Around that time, a population of African apes broke off into several populations, of which one preceded to evolve into modern gorillas, a second into the two modern chimps, and the third into humans. The gorilla line apparently split slightly before the split between the chimp and the human lines.
The primate, is the order of mammals that includes humans, apes, which are the closest living relatives to humans, monkeys, and some less familiar mammals, such as tarsiers, lorises, and lemurs. Humans and other primates share a common evolutionary descent. Consequently, primates have always fascinated scientists because their physical features, social organization, behavioural patterns, and fossil remains provide clues about our earliest human ancestors.
Primates evolved from tree-dwelling ancestors. Although some species, such as humans, have since taken to the ground, all primates' share features that are related to their tree-climbing ancestry. These include arms and legs that can move more freely than those of most other mammals, flexible fingers and toes, forward-facing eyes that can judge distances accurately-a vital aid when moving about high above the ground-and large brains.
Primates live in a wide range of habitats but are restricted by their need for warmth. Most primates live in tropical jungles or dry forests, but some live in dry grasslands, and others have settled in cold, mountainous regions of China and Japan. The world's most northerly primate, the Japanese macaque, has learned to bathe in hot springs to survive through the winter snows. In parts of the tropics, monkeys can be seen within a few miles of busy city centres, but despite this adaptability, most of the world's primates retain a close dependence on trees. Apart from humans, baboons are the only primates that have fully made the transition to life out in the open, and even they instinctively climb to safety if danger threatens.
Some primates, especially the smaller species, are active only at night, or nocturnal, while others are diurnal, active during the day. Most primate species-particularly monkeys-are highly sociable animals, sometimes living in troops of more than 100 members. Smaller primates, especially nocturnal ones, have a tendency to solidarity and secretive.
Primates range in size from quite small to quite large. The world's largest species, the lowland gorilla at 200 kg. (400 lb.) is more than 6,000 times the weight of the smallest primate, the pygmy mouse lemur from Madagascar. Measuring only 20 cm. (8 in.) from nose to tail, and weighing about thirty g. (1 oz.), this tiny animal was first identified about two centuries ago, but was later assumed to be extinct until its rediscovery in 1993.
There are about 235 species of primates. Scientists use more than one way to classify primates, and one system divides the order into two overall groups, or suborders: the prosimians and the anthropoids.
The prosimians, or ‘primitive primates', make up the smaller of these two groups, with about sixty species, and include lemurs, Pontos, galagos, lorises, and, in some classification systems, tarsiers. Lemurs are only found on the islands of Madagascar and Comoros, where they have flourished in isolation for millions of years. Pontos and galagos are found in Africa, while lorises and tarsiers are found in southeast Asia. Typical prosimians are small to medium-sized mammals with long whiskers, pointed muzzles, and well-developed senses of smell and hearing. Most prosimians are nocturnal, although in Madagascar some larger lemurs are active by day.
In the past, tree shrews were often classified as primates, but their place in mammal classification has been the subject of much debate. Today, based on reproductive patterns and on new fossil evidence, most zoologists classify them in an order of their own, the Scandentia.
The remainder of the world's primates makes up the anthropoid, or ‘humanlike' suborder, which contains about 175 species. This group consists of humans, apes, and monkeys. Most anthropoids, apart from baboons, have flat faces and a poor sense of smell. With a few exceptions, anthropoids are usually active during the day, and they find their food mainly by sight.
Apes are found only in Africa and Asia. They have no tails, and their arms are longer than their legs. Monkeys from Central and South America, known as New World monkeys, have broad noses and nostrils that open sideways. They are called platyrrhine, which means broadly-nosed. Monkeys from Africa and Asia, known as Old World monkeys, have narrow noses and nostrils that face downward-a characteristic also seen in apes and humans. Old World Monkeys are called catarrhine, which mean downward-nosed.
During evolution, primates have kept several physical features that most other mammals have lost. One of these is the clavicle, or collarbone. In primates, the clavicle forms an important part of the shoulder joint. It helps to stabilize the shoulder, permitting a primate to support its weight by hanging from its arms alone-something that few other mammals can do. Some primates, particularly gibbons and the siamang, use this ability to move through the trees from one branch to another by swinging from arm to arm. This type of locomotion is called the brachiation.
During evolution, many mammals have gradually lost limb bones as they have adapted to different ways of life: horses, for example, have lost all but a single toe on each foot. Nearly all primates, by contrast, have retained a full set of five fingers and toes, and usually these digits have become increasingly flexible as time has gone through. In the aye-aye, a prosimian from Madagascar, the third finger on each hand is long and thin with a special claw at the end. Aye-ayes use these bony fingers to extract insect grubs from bark.
Evolution has affected the thumbs and big toes of primates. In most mammals, these digits bend in the same plane as the other fingers and toes. However, in many primates, the thumbs or big toes are opposable, meaning that they are set apart in a way that permits them to meet the other digits at the tips to form a circle. This enables primates to grip branches, and equally importantly, pick up and handle small objects. Instead of having claws, most primates have flat nails that cover soft, sensitive fingertips-another adaptation that helps primates to manipulate objects with great dexterity.
Tails are absent in humans and apes, but in most monkeys and prosimians, the tail plays a special role in maintaining balance during movement through the treetops. Many New World monkeys have prehensile tails, which can be wrapped around branches, gripping them like an extra hand or foot.
Primate skulls show several distinctive features. One of these is the position of the eyes, which in most species is on the front of the skull looking forward, rather than on the side of the skull looking to the side as in many other mammals. The two forward-facing eyes have overlapping fields of view, which give primates stereoscopic vision. Stereoscopic vision permits accurate perception of distance, which is helpful for handling food or swinging from branch to branch high above the ground. Another distinctive feature of primate skulls, in anthropoids particularly, is the large domed cranium that protects the brain. The inside surface of this dome clearly shows the outline of an unusually large brain-one of the most remarkable characteristics of this group. The shapes of anthropoid brains are different from other mammals: The portion of which the distributive contribution under which the brain is enwrapped to the visual modalities is especially large, while the compensable portion of attribution to smell is comparatively small.
The primate order includes a handful of species that live entirely on meat (carnivores) and a few that are strict vegetarians (herbivores), but it is composed chiefly of animals that have varied diets (omnivores). The carnivorous primates are the four species of tarsiers, which live in Southeast Asia. Using their long back legs, these pocket-sized nocturnal hunters leap on their prey, pinning it down with their hands and then killing it with their needle-sharp teeth. Tarsiers primarily eat insects but will also eat lizards, bats, and snakes.
Other prosimians, such as galagos and mouse lemurs, also hunt for insects, but they supplement their diet with different kinds of food, including lizards, bird eggs, fruit, and plant sap. This opportunistic approach to feeding is seen in most of monkeys and in chimpanzees. Several species of monkeys, and chimpanzees, but not the other apes, have been known to attack and eat other monkeys. Baboons, the most adept hunters on the ground, often eat meat and sometimes manage to kill small antelope.
Most apes and monkeys eat a range of plant-based foods, but a few specialize in eating leaves. South American howler monkeys and African colobus monkeys eat the leaves of many different trees, but the proboscis monkey on the island of Borneo is more selective, surviving largely on the leaves of mangroves. These leaf-eating monkeys have modified digestive systems, similar to cows, which enable them to break down food that few other monkeys can digest. Other apes and monkeys eat mostly fruit, while some marmosets and lemurs depend on tree gum and sap.
Compared with many other mammals, primates have few young, and their offspring take a long time to develop. The gestational period, the time between conception and birth, is remarkably long compared with other mammals of similar size. A tarsier, for example, gives birth to a single young after a gestational period of nearly six months. By contrast, a similarly sized rodent will often give birth to six or more young after the gestational period lasting just three weeks. Most primates usually give birth to a single baby, although some species, such as dwarf lemurs, usually have twins or triplets.
Once the young are born, the period of parental feeding and protection can be even more drawn out. In small prosimians the young are often weaned after about five weeks, but in apes they are often fed on their mother's milk for three or four years, and they may continue to rely on her protection for six or more years. This long childhood-which reaches its extreme in humans-is a crucial feature of a primate's life because it enables complex patterns of behaviour to be passed on by learning.
Some primates have fixed breeding seasons, but many can breed anytime of the year. In many species, females signal that they are in estrus-receptive and ready to mate-by releasing special scents. In other species, females develop conspicuous swelling around their genitals to signal their readiness for mating. Such swelling is especially noticeable in chimpanzees. While most copulation occurs when the females are receptive, in some species, such as humans and pygmy chimpanzees, copulation frequently occurs even if the female is not in estrus.
Primates display a wide range of mating behaviours. Solitary primates, such as aye-ayes and orangutans, have simple reproductive behaviour. Within the territory that each male control, his imperative territorial rights are in assess of several females live, each with their own territory. The male mates with any females within his territory that are receptive. Other species, such as gibbons, form small family groups consisting of a monogamous pair and they're young. Gorillas form harems, in which one adult male life with several adult females and they're young. Among social primates, breeding can be complicated by the presence of many adults. Males may cooperate in defending their troop's territory, but they often fight each other for the chance to mate. In some species, only the dominant male mates with the females in the group. Chimpanzee females mate promiscuously with several adult males, although they usually pair up with one high-ranking male during the final few days of estrus, spending all of their time together and mating together exclusively.
Primates have the most highly developed brains in the animal kingdom, rivalled only by those of dolphins, whales, and possibly elephants. Anthropoid primates in particular are intelligent and inquisitive animals that are quick to learn new patterns of behaviour. This resourcefulness enables them to exploit a wide range of foods and may help them to escape attacks by predators.
Many zoologists believe that primates' large brains initially evolved in response to their tree-dwelling habits and their way of feeding. Anthropoid primates, which have the largest brains, live in a visual world, relying on sight to move about and to find and manipulate food. Unlike smell or hearing, vision generates a large amount of complex sensory information that has to be processed and stored. In primate brains, these operations are carried out by part of the brain called the cerebral cortex, which evolved into such a large structure that the rest of the brain is hidden beneath it. Some unrelated mammals, such as squirrels, also live in trees, but they have less-developed eyesight and much smaller brain.
Increased brainpower has had important effects on the way primates live. It has helped them to move about and find food and enabled them to develop special skills. One of the most remarkable of these is Toolmaking, seen in chimpanzees and, to a far greater extent, in humans. Toolmaking, as opposed to simple tool use, involves a preconceived image of what the finished tool should look like-something that is only possible with an advanced brain.
The intelligence of primates is also evident in their social behaviour. For species that live in groups, daily life involves countless interactions with relatives, allies, and rivals. Mutual cleaning and grooming of the fur, which removes parasites, helps to reinforce relationships, while threats-sometimes followed by combat-maintain the hierarchy of dominance that permeates typical primate troops.
Primates use a variety of methods to communicate. In solitary prosimians, when animals are not within sight of each other, communication is often accomplished by using scents. Such animals use urine, faeces, or special scent glands to mark territory or to show a readiness to mate. In social anthropoids, visual and vocal signals are much more important. Most monkeys and apes speak with a complex array of facial expressions, some of which are similar to the facial expressions used by humans.
Primates also talk with a repertoire of sounds. These range from the soft clicks and grunts of the colobus to the songs of the gibbon and the roaring of the howler monkey, which can sometimes be heard more than 3 km. (2 mi.) away. Far-carrying calls are used in courtship, both to keep group members from getting separated and to mark and maintain feeding territories. Some primate utterances convey more precise messages, often denoting specific kinds of danger. In the wild, researchers have observed that chimpanzees run through as much as thirty-four different calls, and evidence suggests that they can pass on information-such as the location of food-using this form of communication.
Comparatively, little in effect is known about the origins of primates compared with many other groups of mammals, because primates have left few fossil remains. The chief reason for the scarcity of fossils is that forests, the primary home for most early primates, do not create good conditions for fossilization. Instead of being buried by sediment, the bodies of early primates were more likely to have been eaten by scavengers and their bones dispersed.
The earliest fossils of primates discovered dates from the end of the Cretaceous Period, about sixty-five million years ago. These early fossils include specimens of a species called Notharctus, which resembles today's lemurs and had a long pointed snout. The ancestors of another prosimian group, the tarsiers, are known from fossils that date from the early Eocene Epoch, about fifty million years ago. In 1996 researchers in China recovered fossil bones of a primitive primate no bigger than a human thumb. The animal, named Eosimias, existed in as much as forty-five million years ago. Many scientists believe that Eosimias is an example of a transitional animal in the evolution of prosimians to anthropoids.
The origin of anthropoids has been difficult to pin down. A single anthropoid fossil has been found that may come from the Eocene Epoch, but conclusive fossil evidence of anthropoids does not appear until the Oligocene Epoch, which was launched around thirty-eight million years ago. These early anthropoids belonged to a lineage that led to the catarrhine primates-the Old World monkeys, apes, and humans. The platyrrhine primates, which include all New World monkeys, are presumed to have diverged from the Old World monkeys during the Eocene Epoch. They evolved in isolation on what was then the island continent of South America. Genetic analysis shows that New World monkeys clearly have the same ancestry with the catarrhines, which means that they must have reached the island continent from the Old World. Exactly how they did this is unclear. One possibility is that they floated across from Africa on logs or rafts of vegetation, journeying across an Atlantic Ocean that was much narrower than it is today.
Of all primate groups, the apes are the direct ancestors of humans that bring on the most provocative of studies. One distinguishing query that finds of its vexation is that of two groups diverging. Based on the comparisons of genes and the structure of body parts, scientists think that the human line leading to the orangutan branched from leading lines of human developments around twelve million years ago. The ancestral line leading to chimpanzees did not diverge until more recently, probably between five and seven million years ago. This evidence strongly suggests that chimpanzees are our closest living relatives.
The word primate means ‘the first'. When it was originally coined more than two centuries ago, it conveyed the widely held idea that primates were superior to all other mammals. This notion has since been discarded, but nonhuman primates still generate great interest because of their humanlike characteristics.
In scientific research, much of this interest has focussed on primate behaviour and its correspondence with human behaviour. Attempts have been made to train chimpanzees and orangutans to mimic human speech, but differences in anatomy make it very difficult for apes to produce recognizable words. A more revealing series of experiments has involved training chimpanzees, and later gorillas, to understand words and to respond using American Sign Language. In the late 1960s, a chimp named Washoe learned more than 130 signs. In the 1970s and 1980s, a gorilla named Koko learned to use more than 500 signs and to recognize an additional 500 signs. One outcome of these long-running experiments was that the chimps or gorillas occasionally produced new combinations of signs, suggesting that the animals were not simply repeating tricks that they had learned. More recently, chimps have been trained to talk with humans by using coloured shapes or computer keyboards. They too have shown an ability to associate abstract symbols with objects and ideas-the underlying basis of language.
Apes and monkeys also play an important role in the field of medical research. Because their body systems work very much like our own, new vaccines and new forms of surgery are sometimes tried on apes and monkeys before they are approved for use on humans. Species that are most often used in this way include chimpanzees, baboons, and rhesus monkeys. This kind of animal experimentation has undoubtedly contributed to human welfare, but the medical use of primates is an increasingly controversial area, particularly when it involves animals captured in the wild.
According to figures published by the World Conservation Union (IUCD), more than 110 species of primates-nearly half the world's total-are currently under threat of extinction. This makes the primates among the most vulnerable animals on earth.
The species most under threats are those affected by deforestation. This has been particularly severe in Madagascar, the only home of the lemurs, and it is also taking place at a rapid rate in Southeast Asia, threatening gibbons and orangutans. The almost total destruction of Brazil's Atlantic rainforest has proved catastrophic for several species, including the lion tamarins, which are found only in this habitat. Primates are also threatened by collection for the pet trade and by hunting. Illegal hunting is the chief threat facing the mountain gorilla, a rare African subspecies that lives in the politically volatile border region straddling Uganda, Rwanda, and the Democratic Republic of the Congo.
In the face of these threats, urgent action is currently underway to protect many of these endangered species. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) currently forbids the export of many primates, although not all countries have chosen to follow this law. More direct methods of species preservation include habitat protection and captive breeding programs. Sometimes-for example, the lion tamarin-these programs have met with considerable success. However, without the preservation of extensive and suitable natural habitats, many primate species are destined for extinction.
Humans as primates, have themselves of a physical and genetic similarities showing that the modern human species, Homo sapiens, has a the same close relationship to another group of primate species, the apes. Humans and the so-called great apes (large apes) of Africa-chimpanzees (including bonobos, or so-called pygmy chimpanzees) and gorillas-have the same ancestors that lived sometime between eight million and six million years ago. The earliest humans evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between six million and two million years ago come entirely from Africa.
Humans and great apes of Africa have the same ancestor that lived between eight million and five million years ago. Most scientists distinguish among twelve to nineteen different species of early humans. Scientists do not all agree, however, about how the species are related or which ones simply died out. Many early human species-probably most of them-left no descendants. Scientists also debate over how to identify and classify particular species of early humans, and about what factors influenced the evolution and extinction of each species.
The tree of Human Evolution and fossil evidence shows that the first humans evolved from ape-like descentable ancestries, in, at least six million years ago. Many species of humans followed, but only some left descendants on the branch leading to The Homo sapiens. In this slide show, white skulls represent species that lived during the period shown; gray skulls represent extinct human species.
Early humans first migrated out of Africa into Asia probably between two million and 1.7 million years ago. They entered Europe much later, generally within the past one million years. Species of modern humans populated many parts of the world much later. For instance, people first came to Australia probably within the past 60,000 years, and to the Americas within the past 35,000 years. The beginnings of agriculture and the rise of the first civilizations occurred within the past 10,000 years.
The scientific study of human evolution is called Paleoanthropology. Paleoanthropology is a sub-field of anthropology, the study of human culture, society, and biology. Paleoanthropologists search for the roots of human physical traits and behaviour. They seek to discover how evolution has shaped the potentials, tendencies, and limitations of all people. For many people, Paleoanthropology is an exciting scientific field because it illuminates the origins of the defining traits of the human species, and the fundamental connections between humans and other living organisms on Earth. Scientists have abundant evidence of human evolution from fossils, artifacts, and genetic studies. However, some people find the concept of human evolution troubling because it can seem to conflict with religious and other traditional beliefs about how people, other living things, and the world developed. Yet many people have come to reconcile such beliefs with the scientific evidence.
Modern and Early Humans have undergone major anatomical changes during evolution. This illustration depicts Australopithecus afarensis, the earliest of the three species, the Homo erectus, an intermediate species, whereby the Homo sapiens, a modern human, and Homo's ergaster. The modern humans are much taller than A. afarensis and have flatter faces and a considerable brawny brain. Modern humans have a larger brain than H. erectus and almost flat face beneath the front of the braincase.
All species of organisms originate through the process of biological evolution. In this process, new species arise from a series of natural changes. In animals that reproduce sexually, including humans, the term species refers to a conjunctive organization into groups whose adult members regularly interbreed, resulting in fertile offsprings that are, offsprings themselves and adequately of procreating. Scientists classify each species with a unique, but two-part scientific names. In this system, modern humans are classified as Homo sapiens.
The mechanism for evolutionary change resides in genes-the basic units of heredity. Genes affect how the body and behaviour of an organism develop during its life. The information contained in genes can be change-a process known as mutation. The way particular genes are expressed -how, and in its gross affect, the body or behaviours of an organism-can also change. Over time, genetic change can alter a species's overall way of life, such as what it eats, how it grows, and where it can live.
Genetic changes can improve the ability of organisms to survive, reproduce, and, in animals, raise offspring. This process is called adaptation. Parents pass adaptive genetic changes to their offspring, and ultimately these changes become common throughout a population-a group of organisms of the same species that share a particular local habitat. Many factors can favour new adaptations, but changes in the environment often play a role. Ancestral human species adapted to new environments as their genes changed, altering their anatomy (physical body structure), physiology (bodily functions, such as digestion), and behaviour. Over long periods, evolution dramatically transformed humans and their ways of life.
Geneticists estimate that the human line began to diverge from that of the African apes between eight million and five million years ago (paleontologists have dated the earliest human fossils to at least six million years ago). This figure comes by comparing the differences in the genetic makeup of humans and apes, and, then figuring how long it probably took for those differences to develop. Using similar techniques and comparing the genetic variations among human populations around the world, scientists have calculated that all people may share common genetic ancestors that lived sometime between 290,000 and 130,000 years ago.
Humans belong to the scientific order named Primates, a group of more than 230 species of mammals that also includes lemurs, lorises, tarsiers, monkeys, and apes. Modern humans, early humans, and other species of primates all have many similarities plus some important differences. Knowledge of these similarities and differences helps scientists to understand the roots of many human traits, and the significance of each step in human evolution.
All primates, including humans, share at least part of a set of common characteristics that distinguish them from other mammals. Many of these characteristics evolved as adaptations for life in the trees, the environment in which earlier primates evolved. These include more reliance on sight than smell; overlapping fields of vision, allowing stereoscopic (three-dimensional) sight; limbs and hands adapted for clinging on, leaping from, and swinging on tree trunks and branches; the ability to grasp and manipulate small objects (using fingers with nails instead of claws); large brains in relation to body size; and complex social lives.
The scientific classification of primates reflects evolutionary relationships between individual species and groups of species. Strepsirhini (meaning ‘turned-nosed') primates-of that the living representatives include lemurs, lorises, and other groups of species all commonly known as prosimians-evolved earliest and are the most primitive forms of primates. The earliest monkeys and apes undergo an evolution from transmissiblel haplorhine (meaning ‘simply-nosed') primates, of which the most primitive living representative is the tarsier. Humans evolved from ape ancestors.
Tarsiers have traditionally been grouped with prosimians, but many scientists now recognize that tarsiers, monkeys, and apes share some distinct traits, and group the three together. Monkeys, apes, and humans-who share many traits not found in other primates-together make up the suborder Anthropoidea. Apes and humans collectively carry out the superfamily of the Hominoidea, a grouping that emphasizes the close relationship among the species of these two groups.
Paleoanthropologists Donald C. Johanson of the Cleveland Museum of Natural History and Tim D. White of the University of California, Berkeley, announced that in January the discovery of the most ancient Hominid (humanlike) species yet uncovered, which they have named Australopithecus afarensis. The fossils on which they base this claim are about three million to four million years old and were found during the 1970's at two widely separated localities in East Africa. The majority were collected at Hadar, a remote region of the Afar Depression of Ethiopia, by Johanson; the others were uncovered in northern Tanzania at Laetolil, 30 miles south of Olduvai Gorge, by anthropologist Mary Leakey. The Hadar material consists of bones from at least thirty-five individuals and includes the best preserved australopithecine skeleton yet found. Nicknamed ‘Lucy' by Johanson, this skeleton is about 40 percent complete and is evidently of a female who stood about 3.5-4 feet tall and lived some three million years ago. The Laetolil fossils, closer to four million years old, are astonishingly similar in many ways to the Hadar material. Because of the remarkable completeness and good preservation of both fossil collections, we are afforded a previously unavailable glimpse of early human evolution. Analysis suggests that these creatures had rather small brains, no larger than those of the gorilla, but the leg and pelvic bones clearly indicate that A. afarensis walked on two legs like humans. In these respects the newly described fossils do not differ substantially from previously described australopithecine species, which date from about 1.5 million to 2.5 million years ago. All previously recognized hominids, however, show larger cheek teeth (molars) and smaller front teeth (incisors and canines) than the apes. A. afarensis, in contrast, shows very broad incisors and large, projecting canines, often more like those of an apes. The appearance of such primitive dental characteristics in an australopithecine has profound implications for evolutionary history. The most widely held theory states that the evolutionary lines leading to modern humans and apes diverged some twelve million to fifteen million years ago, when apes from which humans were descendably waved to move out of the trees, and so, began to exploit the resources of open grasslands for food. This change in habitats is thought to have produced the characteristic humanlike denotation, which is more efficient at chewing tough food, such as seeds, roots, and tubers, than is the denotation of the apes. Fossil teeth and jaws of a human type character, is the creature called Ramapithecus, which existed about ten million years ago and is generally considered ancestrally human. However, the primitive, more primordial apes of A. afarensis have now cast doubt on the status of Ramapithecus as an ancestral hominid and made unclear the ultimate reason for the differentiation of human ancestors from the apes.
Researchers in South Africa having discovered what they believe are the oldest and best-preserved skulls and skeletons of one of humanity's earliest ancestors, according to a report published in the December 9, 1998, publication of The South African Journal of Science. Paleontologists said the fossilized remains would exceedingly be every bit as two million years older than the oldest previously known complete hominid skeletons. The new finding is expected to reveal much about the anatomy and evolution of early humans, and may rank among the most important breakthroughs in Paleoanthropology (the archaeological study of early human evolution).'It is one of many key elements from ape to man', said Ronald J. Clarke, a paleoanthropologists at the University of Witwatersrand in Johannesburg, South Africa, who led the team that made the discovery. The skeleton was discovered in the fossil-richly vicinitized near the Sterkfontein Caves, near Krugersdorp in northeastern South Africa. The skeleton is of a small, adult hominid who was about 1.2-m. (4-ft.) tall and weighed about thirty-two k. (70 lb.). Clarke's team dated the bones at 3.2 million to 3.6 million years old.
The bones are believed to belong to a species of australopithecine, an early hominid that had human and apelike features. However, most of the bones remain embedded in rock within the cave. Paleontologists may be accredited for being as far as possible and study of anatomical skeletons until it is removed, a process that Clarke said could take a year or longer. Clarke made the discovery after unexpectedly finding four hominid foot bones in a box of unsorted fossils at the university in 1994. Another search of boxes in a university storage room in May 1997 revealed more foot and lower leg bones. To Clarke's astonishment, all of the bones appeared to belong to the same hominid.
Clarke's initial discovery, announced in 1995, added new evidence to a longstanding debate among anthropologists about the path of early hominid development. Clarke and several of his colleagues argued that the bones of the specimen, dubbed Little Foot, reflected a transition from four-legged tree dwellers to two-can walk upright. In particular, Clarke said the specimen's humanlike ankles and grasping, an apelike big toe suggested that the creature was a capable tree climber who could easily walk on two legs. Other anthropologists dismissed the idea, however, asserting that humans evolved from plains-dwelling hominids and did not live in trees.
After finding additional bones in 1997, Clarke believed that the rest of the skeleton might be present in Sterkfontein's Silberberg Grotto, where the bones had been originally excavated. Within days of searching, his assistants discovered a piece of fossilized bone protruding from the cave wall that perfectly matched one of Clarke's fossil fragments. Although the excavation is at a preliminary stage, Clarke said the remainder of the skeleton might be present and intact, laying face downward in limestone.
Before Clarke's find, the most comprehensive australopithecine skeleton was an Australopithecus afarensis specimen known as Lucy, discovered by anthropologist Donald Johanson in Ethiopia in 1974 and dated at 3.2 million years old. Lucy, however, is only about 40 percent complete. The oldest known complete hominid skeleton was a species of Homo erectus which had ben excavated in Kenya and dated at 1.5 million years old.
The new discovery is considered extraordinary because the fossil record of early hominids is so fragmentary. Paleontologists have had to piece together knowledge about ancient human species by using bone fragments derived from many individuals, making generalizations about anatomy difficult. Once the bones have been chipped from the grotto rock, scientists will examine the hips and legs to figure out whether or not the creature could easily climb trees. In addition, they hope skeletal features will give them clues about the specimen's sex and how these early hominids lived, including their likely diet and possible foraging behaviours.
Scientists also believe the skeleton's intact skull could shed light on another key puzzle of early human evolution: the relation between a brain size and upright locomotion. Many experts believe that it was the ability to walk on two legs -rather than brain size or use of tools-that set the human lineage apart from all other primates
Another mystery scientists will explore is whether the fossils represent of either or a good example of Australopithecus afarensis (like Lucy), of a southern African hominid species, known as Australopithecus africanus, or possibly a new species together. If the species is unrelated to Lucy and is older, then it could force anthropologists to reconsider their views about hominid evolution in Africa. Because of Lucy's age, many scientists now believe that
A. afarensis is a common ancestor to all succeeding australopithecine species.
Nevertheless, some paleontologists cautioned that the age of the new find had not yet been conclusively established and could be only about two million year's old. The most accurate forms of dating require the presence of volcanic ash, which contains radioactive elements that decay in a predicable way. No such material was present in the cave.
To date the skeleton, Clarke and his team finds distinctive animal fossils near the hominid remains. The age of these animal fossils had already been determined at other datable sites. This technique is not foolproof, however, because movements in the rock layers could make fossils from animals that did not coexist appear next to each other, experts said.
Yet, Spanish paleoanthropologists recently described the fossil remains of several human ancestors from the last Ice Age that were found at a cave site in northern Spain. Did their findings identify a distinct human ancestral species, as the Spaniards suggested? The debate over the paths of human evolution continued as new findings about Human ancestors came of Spanish paleoanthropologists, and have added to the complexity of theories about early humans in Europe with their recent description of the fossil remains of several Ice Age human ancestors found in northern Spain. The researchers suggested that these early humans, who lived more than 780,000 years ago, may have been a separate species that preceded both modern humans and the now-extinct early humans known as Neanderthals.
The researchers said that among the fossils were the facial bones of a boy showing both primitive and modern features and identifying these human ancestors as a distinct species. They suggested the name Homo's antecessor for the proposed new species. Spanish paleoanthropologists José Bermúdez de Castro of the National Museum of Natural Sciences in Madrid, Spain, and his colleagues described the fossils in the May 30, 1997, issues of the Journal Science.
Although anthropologists agreed that the fossil find was very important, most were not ready to accept the ancient humans as representing a new species. Anthropologists pointed out that not only are the dental and facial bones of a boy scant evidence on which to identify a new species, but also there is a chance that some boy's features were in an intermediate stage that would have changed when the boy reached adulthood. The Spanish researchers' proposed path of human evolution also was controversial, because it pushes groups of early humans off the direct line leading to modern humans, suggesting that there may have been more dead ends in human evolution than previously thought.
The Spanish scientists first reported finding this group of fossils, the oldest remains of pre-humans ever found in Europe, in August 1995. Previously the oldest known Europeans were a group of early humans sometimes classified as a separate species, Homo heidelbergensis. The earliest known specimens from this group date from roughly 500,000 years ago. Using a technique known as Paleomagnetic analysis, the Spanish researchers dated the fossil remains found recently in northern Spain to at least 780,000 years ago, in the Pleistocene Epoch.
Paleomagnetic dating is because the direction of the earth's geomagnetic field has reversed often during the history of the world. The dates of these irregular reversals in geomagnetic polarity have been well documented. Currently the geomagnetic polarity of the earth is facing north, but less than a million years ago it faced south. Internal magnetic traces in the layers of rock that lay the groundwork for nearby fossils shown that the fossils had been buried before the earth's magnetic field last switched direction, from south to north, 780,000 years ago.
The Spanish paleoanthropologists found more than eighty fossils representing at least six individuals, including both juveniles and adults. The fossils were found in a deep pit at a cave site known as Gran Dolina, in the Atapuerca Mountains near the city of Burgos in northern Spain.
The characterization of early human compliance in Europe is yet unclear. Anthropologists agree that hominids-a family of bipedal primates that includes modern human beings and all extinct species of early humans-have their origins in Africa. The Spanish anthropologists speculated that the early humans they called Homo antecessor may have first evolved in Africa from a primitive human classified by some paleoanthropologists as Homo ergaster and by others as early Homo erectus and that Homo sapiens developed from Homo antecessor in Africa. The researchers further proposed that Homo antecessor migrated to Europe, and that Homo heidelbergensis and then Neanderthals (also called Homo sapiens neanderthals) evolved from this line.
Under this proposed set of relationships, Homo heidelbergensis would not be a common ancestor of Homo sapiens and Neanderthals, as currently believed by many anthropologists. The Spanish anthropologists' model also pushes forward the later members of the Homo erectus, perhaps in a direct inclination away from their evolving lineage leading face-to-face to modern human beings. An early Homo erectus appeared about 1.9 million to two million years ago in Africa, and more recent examples of this early human have been found in China and Java, Indonesia. In the Spanish scientists' proposed model, Asian members of
A Homo erectus becomes a side path on human evolution, rather than an intermediate step between Homo ergaster and Homo heidelbergensis.
The human family tree has usually become less linear and more complex over the past decade as anthropologists have made more discoveries.
Archaeological study covers an extremely long span of time and a great variety of subjects. The earliest subjects of archaeological study date from the origins of humanity. These include fossil remains believed to be of human ancestors who lived 3.5 million to 4.5 million years ago. The earliest archaeological sites include those at Hadar, Ethiopia, Olduvai Gorge and Laetoli, Tanzania, East Turkana, Kenya, and in a balanced significance in East Africa. These sites contain evidence of the first appearance of bipedal (upright-walking), apelike early humans. Laetoli even reveals footprints of humans from 3.6 million years ago. Some sites also contain evidence of the earliest use of simple tools. Archaeologists have also recorded how primitive forms of humans spread out of Africa into Asia about 1.8 million years ago, then into Europe about 900,000 years ago.
The first physically modern humans, The Homo sapiens, appeared in tropical Africa between 200,000 and 150,000 years ago-dates determined by molecular biologists and archaeologists working together. Dozens of archaeological sites throughout Asia and Europe show how people migrated from Africa and settled these two continents during the last Ice Age (100,000 to 15,000 years ago). Archaeological studies have also provided much information about the people who first arrived in the Americas more than 12,000 years ago.
In their search for the original cradle of humanity, anthropologists have long been looking for remains of early man in all corners of the world. In this effort they have eliminated the Americas and Australia from the competition and to convey the honour of having seen man's first emergence to either Africa or Asia. Any find of early man made in these areas takes on, therefore, a particular importance. In 1953 fragments of a human skull without the face were discovered in probably late Pleistocene levels near Hopefield, 76 miles north of Capetown, Africa, close to Saldanha Bay. This Saldanha skull is very big and low, has a strongly receding forehead, tremendous bone ridges across the eyebrows, and its bones are extremely thick. All these features are typical of the most primitive types of early man. Probably the most significant fact is that this new specimen resembles very closely the famous Rhodesian Man known, from a skull found in late Pleistocene levels at Broken Hill in 1921. This Rhodesian lowbrow is one of the most puzzling finds of early man ever made, for he combines certain extremely primitive characteristics with some very modern features. He has enormous brow ridges, the heaviest ever found in any type of early man, a very strongly projecting face and unusually broad palate, a strongly receding forehead, and low cranial vaults. Still, his large skull has a brain volume within the range of recent man, and in spite of the various primitive features, he suffered from a truly modern disease: tooth decay. This plague of modern humans appears in the Neolithic, the period in which pottery was discovered and in which man began to boil his food. Every dentist will tell you that soft food is the greatest enemy of teeth, and thus the boiling of food instead of the earlier roasting caused the frequent occurrence of tooth decay in the Neolithic period. Nevertheless, the Rhodesian Man must have been an unfortunate creature as afar apart from the dubious honour of being the first man who needed a dentist, he suffered from mastoiditis and rheumatism, as appears from a careful inspection of his skull and his tibia. To top it all, Sir Arthur Keith, Britain's most distinguished anthropologist, suggests that this truly sick man suffered from acromegaly, a disease of overgrowth of the head, feet, and hands. Although such a diagnosis has previously been doubted on certain grounds, it is the merit of Saldanha man of really absolving his Rhodesian cousin from at least this latter verdict and saving his face-than reason. Since both skulls resemble each other so closely, the large size of the skull and the enormous orbital ridges can no longer be considered as pathological features but must are typical of early man in Africa.
Fossils suggest that he evolutionary line leading to us had achieved an upright posture by around four million years ago, then began to increase in body size and a relative brain size of around 2.5 million years ago. That protohuman generally brought into a different state of awareness the fact that Australopithecus africanus, Homo habilis and Homo erectus, their apparent evolution into each other by some chronological succession. Although the Homo erectus, the stage reached around 1.7 million years ago, was close to us modern humans in body size, its brain size, but still barely half of ours. Stone tools became common around 2.5 million years ago, but they were merely the crudest of flaked or battered stones. In zoological significance and distinctiveness, The Homo erectus was more than an ape, but still much less than a modern human.
Since the biological regularities of living organisms display an active and intimate engagement with their environment that is categorically different from that of inorganic matter, we can conclude that they represent profound oppositions. Since organic and inorganic matter are constructs that cannot be applied simultaneously in the same situation and yet are both required for a complete description of the situation, they must be as be viewed in compliments. In that, given the lawful regularities displayed by organic and inorganic matter are different. A profound complementary relationship exists between the law of physics and that of biology. For example, a complete description in mathematical physics of all the mechanisms of a DNA molecule would not be a complete description of organic matter for an obvious reason. The quality of life associated with the known mechanism of DNA replication exists except an objectivised description. It seems likened to the seamless web of interactions under which the organism holds with its environment, only to suggest that the laws of nature have accorded for biological regularities. Additionally, as it seems agreed to the behaviours we associate with life, which are not merely those of mathematical physics. Even if we could replicate all of the fundamental mechanisms of biological life by manipulating inorganic matter in the laboratory, this problem would remain. To prove that no laws other than those of mathematical physics are involved, that if we would be obliged to create life without any interaction with an environment in which the life form sustains itself or interacts.
Although most physical scientists probably assume that the mechanism of biological life can be completely explained following the law of mathematical physics, many phenomena associated with life cannot be explained in these terms. For example, the apparent compulsion of individual organisms to perpetuate their gene, ‘selfish' or not, is obviously a dynamic of biological regularities that is not apparent in an isolated system. This contributive functional dynamic cannot be described as for the biochemical mechanisms of DNA or any other aspect of isolated organic matter. The specific evolutionary path followed by living organisms is unique and cannot be completely described as based on prior applications of the laws of physics.
The more complex organisms that evolve from a symbiotic union that is sometimes called in biology texts factories or machines, but, nonetheless, a machine, as Darwin's model for the relationship part and whole suggest, is a unity of order and not of substance, and the order that exist in a machine is external to the parts. As the biologist Paul Weiss has pointed out, however, the part-whole relationship that exists within and between cells in complex life forms is not that of a machine.
The whole within the part that sets the boundary conditions of cells is DNA, and a complete strand of the master molecule of life exists in the nucleus of each cell. DNA evolved in an unbroken sequence from the earliest life form, and the evolution of even the most complex life forms cannot be separated from the co-evolution of microbial ancestors. DNA in the average cell codes for the production of about two thousand different enzymes, and each of these enzymes canalizes a particular chemical reaction. The boundary conditions within each cell resonate with the boundary condition of all other cells and maintain the integrity of uniqueness of whole organisms.
Volution, in biology, defines a complex process by which the characteristics of living organisms change over many generations as traits are passed from one generation to the next. The science of evolution seeks to understand the biological forces that caused ancient organisms to develop into the tremendous and ever-changing variety of life seen on Earth today. It addresses to how, over a time that by the various plant and animal species had branched away to become an entirely new species, in that, how these different species were related through the branching attributions of the family trees in those that extended over the span of millions of years.
Evolution provides an essential framework for studying the ongoing history of life on Earth. A central, and historically controversial, component of evolutionary theory is that all living organisms, from microscopic bacteria to plants, insects, and mammals, have the same ancestor. Species that are closely related share a recent common ancestor, while distantly related species have a common ancestor further in the past. The animal most closely related to humans, for example, is the chimpanzee. The common ancestor of humans and chimpanzees is believed to have lived approximately six million to seven million years ago. On the other hand, an ancestor common to humans and reptiles that had existed of some 300 million years ago, as these common ancestors were more distantly related forms that lived even farther in the past. Evolutionary biologists attempt to figure out the history of lineages as they diverge and how differences in characteristics developed over time.
Throughout history, philosophers, religious thinkers, and scientists have attempted to explain the history and variety of life on Earth. During the rise of modern science in western Europe in the 17th and 18th centuries, a predominant view held that God created every organism on Earth almost as it now exists. However, in that time of burgeoning interest in the study of apes and natural history, the beginnings of a modern evolutionary theory began to take shape. Early evolutionary theorists proposed that all of the life on Earth evolved gradually from simple organisms. Their knowledge of science was incomplete, however, and their theories left too many questions unanswered. Most prominent scientists of the day remained convinced that the variety of life on Earth could only result from an act of divine creation.
In the mid-19th century a modern theory of evolution took hold, thanks to British naturalist Charles Darwin. In his book, On the Origin of Species by Means of Natural Selection, Darwin described the evolution of life as a process of natural selection. Life, he suggested, is a competitive struggle to survive, often in the face of limited resources. Living things must compete for food and space. They must evade the ravages of predators and disease while dealing with unpredictable shifts in their environment, such as changes in the climate. Darwin offered that, within a given population in a given environment, certain individuals possess characteristics that make them more likely to survive and reproduce. These individuals will pass these critical characteristics onto their offspring. The number of organisms with these traits increases as each generation passes on the advantageous combination of traits. Out matched, individuals lacking the beneficial traits gradually decrease in number. Slowly, Darwin argued, natural selection tips the balance in a population toward those with the combination of traits, or adaptations, best fitted in with their environment.
While, On the Origin of Species were an instant sensation and best sellers, Darwin's theories faced hostile reception by critics giving further information of those railed against his blasphemous ideas. Other critics pointed to questions left unresolved by Darwin's careful arguments. For instance, Darwin could not explain the mechanism that caused life forms to change from generation to generation.
Hostility gave to a considerable degree the acclaim as scientists vigorously debated, explored, and built on Darwin's theory of natural selection. As the 20th century unfolded, scientific advances revealed the detailed mechanisms missing from Darwin's theory. A the study of the complex chemistry of all organisms unveiled the fundamental foundations that structurally surface the genes and in what manner they were duplicated, and passed from generation to generation. New statistical methods helped explain how genes in specific populations change over generations. These new methods provided insight into how populations remain adaptable to changing environmental circumstances and broadened our understanding of the genetic structure of populations. Advances in techniques used to find out the age of fossils provided clues about when extinct organisms existed and details about the circumstances surrounding their extinction. New molecular biology techniques compare the genetic structures of different species, enabling scientists to find specific undetectable evolutionary relationships between species. Today, evolution is recognized as the cornerstone of modern biology. Uniting such diverse scientific fields as cell biology, genetics, palaeontology, and even geology and statistics, the study of evolution reveals an exquisitely complex interaction of the forces that act upon every life form on Earth.
Natural selection is tied to traits that organisms pass from one generation to the next. In humans, these traits include hundreds of features such as eye colour, blood type, and height. Nature offers countless other examples of traits in living things, such as the pattern on a butterfly's wings, the markings on a snail's shell, the shape of a bird's beak, or the colour of a flower's petals.
Such traits are controlled by specific bits of biochemical instructions known as genes. Genes are composed of individual segments of the long, coiled molecule called deoxyribonucleic acid (DNA). They direct the synthesis of proteins, molecular labourers that serve as the constructive edifices to all building blocks of cells, control chemical reactions, and transport materials to and from cells. Proteins are themselves composed of long chains of amino acids, and the biochemical instructions found in DNA determine the arrangement of amino acids in a chain. The specific sequence of amino acids dictates the structure and resulting function of each protein.
All genetic traits result from different combinations of gene pairs, one gene inherited from the mother and one from the father. Each trait is thus represented by two genes, often in different forms. Different forms of the same gene are called alleles. Traits depend on very precise rules governing how genetic units are expressed through generations. According to the laws governing heredity, when a dominant allele (say, tongue rolling) and a recessive allele (no tongue rolling) combines, the trait will always be dictated by the dominant allele. The no tongue rolling trait, or any other recessive trait, will only occur in an individual introduced by those sustaining of getting hold of the two recessive alleles.
Evolutionary change takes place in populations over many generations. Since individual organisms cannot evolve in a single lifetime, evolutionary science focuses on a population of interbreeding individuals. All populations contain some variations in traits. In humans, for example, some people are tall, some are short, and some are of medium height.
In interbreeding populations, genes are randomly shuffled among members of the population through sexual reproduction, the process that produces genetically unique offspring. Individuals of different sexes develop specialized sex cells called gametes. In humans and other vertebrates (animals with backbones), these gametes are sperm in males and eggs in females. When males and females mate, these sex cells join in fertilization. A series of cell divisions creates individuals with a unique assembly of genes. No individual members of any population (except identical twins, which develop from a single egg) are alike in their genetic makeup. This diversity, called genetic diversity or variation, is essential to evolution. The greater a population's genetic diversity, the more likely it is to evolve specific traits that enable it to adapt to new environmental pressures, such as climate change or disease. Expressing of some time, an expressing eventful place showing of a distinction of contrast of such pressures might drive a population with a low degree of genetic diversity to extinction.
Sexual reproduction ensures that the genes in a population are rearranged in each generation, a process termed recombination. Although the contributive combinations of genes in individuals change with each new generation, the gene frequency, or ratio of different alleles in the entire population, remains constant if no evolutionary forces act on the population. One such force is the introduction of new genes into the genetic material of the population, or gene pool.
When individuals move between one population and another, new genes may be introduced to populations. This phenomenon, known as gene flow, results from chance dispersal and intentional migration. Take, for example, two populations of related wildflowers, one red and one white, separated by a large tract of land. Under normal circumstances, the two groups do not interbreed because the wind does not blow hard enough to carry pollen between the populations so that pollination can occur. If in agreement that it happens of an unusually strong wind that carries pollen from the red wildflower population to the white wildflower population, the gene for red flowers may be introduced to the white population's gene pool.
Genes themselves are constantly being modified through a process called mutation: a change in the structure of the DNA in an individual's cells. Mutations can occur during replication, the process in which a cell splits itself into two identical copies known as daughter cells. Normally each daughter cell receives an exact copy of the DNA from the parent cell. Occasionally, however, errors occur, resulting in a change in the gene. Such a change may affect the protein that the gene produces and, ultimately, change an individual's traits. While some mutations occur spontaneously, others are caused by factors in the environment, known as mutagens. Examples of mutagens that affect human DNA include ultraviolet light, X-rays, and various chemicals.
Whatever their cause, mutations are a rare but slow and continuous sources of new genes in a gene pool, yet mutations are neutral-that is, they have no effect. Other mutations are detrimental to life, causing the immediate death of any organism that inherits them. Once in a great while, however, a mutation gives an organism an advantageous trait. A single organism with an advantageous trait is only half of the equation, however. For evolution to occur, the forces of natural selection must distribute that trait to other members of a population.
Natural selection sorts out the useful changes in the gene pool. When this happens, populations evolve. Beneficial new genes quickly spread through a population because members who carry them have a greater reproductive success, or evolutionary fitness, and consequently pass the beneficial genes to more offspring. Conversely, genes that are not as good for an organism are eliminated from the population, sometimes quickly and sometimes more gradually, depending on the severity of the gene, because the individuals who carry them do not survive and reproduce with individuals without the bad gene. Over several generations, the gene and most of its carriers are eliminated from the population. Severely detrimental genes may persist at very low levels in a population, however, because they can be reintroduced each generation by mutation.
Natural selection only allows organisms to adapt to their current environment. Should environmental conditions change, new traits may prevail. Moreover, natural selection does not always favour a single version of a trait. Occasionally, multiple versions of the same trait may instill their carriers with equal evolutionary benefit. Nor does natural selection always favour change. If environmental conditions so dictate, natural selection remains unchanged by eliminating extreme versions of a particular trait from the population.
Often, shifts in environmental conditions, such as climate change or the presence of a new disease or predator, can push a population toward one extreme for a trait. In periods of prolonged cold temperatures, for example, natural selection may favour larger animals because they are better able to withstand extreme temperatures. This mode of natural selection, known as directional selection, is evident in cheetahs. About four million years ago, cheetahs were more than twice as heavy as modern cheetahs. Still, quicker and lighter members of the population had greater reproductive success than did larger members of the population. Over time, natural selection takes to be smaller and smaller cheetahs.
Sometimes natural selection acts to preserve the status quo by favouring the intermediate version of a characteristic instead of one of two extremes. An example of this selective force, known as stabilizing selection, was evident in a study of the birth weight of human babies published in the middle of the 20th century. It showed that babies of intermediate weight, about 3.5 kg. (8 lb.), was more likely to survive. Babies with a heftier birth weight had lower chances for survival because they were more likely to cause complications during the delivery process, and lightweight babies were often born premature or with other health problems. Babies of intermediate birth weight, then, were more likely to survive to reproductive age.
Occasionally natural selection favours two extremes, causing alleles for intermediate forms of a trait to become less common in the gene pool. The African Mocker swallowtail butterfly has undergone this form of selection, known as disruptive selection. The Mocker swallowtail evades its predators by resembling poisonous butterflies in its ecosystem. Predators have learned to avoid these poisonous butterflies and to steer away from the look alike Mocker swallowtails. The Mocker swallowtail has a large range, and in different regions, the Mocker swallowtail looks very different, depending on which species of poisonous butterfly it mimics. In some areas the butterfly displays black markings on a white background; in others the markings float on an orange background. Since a Mocker swallowtail appears poisonous to predators, it has a greater chance of survival and therefore a higher evolutionary fitness. Mocker swallowtails that do not look poisonous have a much lower evolutionary fitness because predators quickly eat them. Disruptive selection, then, favours the extreme colour patterns of white or orange, and nothing between.
Speciation may occur even when no isolating mechanism is present. Here, a new species may form through the slow modification of a single group of organisms into an entirely new group. The evolving population gradually changes over generations until the organisms at the end of the line appear very different from the first. Foraminifera, a tiny species of marine animals that live in the Indian Ocean, displays this process, known as vertical or phyletic evolution. From about ten million to six million years ago, the species remained unchanged. These organisms then began a slow and gradual change, lasting about 600,000 years, that left them so unlike their ancestors that biologists consider them an entirely new species.
Whatever the cause of their reproductive isolation, independently evolving populations have propensities in following general patterns of evolutionary descent. Most often, environmental factors determine the pattern followed. A gradually cooling climate, for example, may result in a population of foxes developing progressively thicker coats over successive generations. This pattern of gradual evolutionary change occurs in a population of interbreeding organisms evolving together. When two or more populations diverge, they may evolve to be less alike or more alike, depending on the conditions of their divergence.
In the pattern known as divergent evolution, after two segments of a population diverge, each group follows an independent and gradual process of evolutionary change, leading them to grow increasingly different from each other over time. Over many generations, the two segments of the population look less and less like each other and their ancestor species. For example, when the Colorado River formed the Grand Canyon, a geographic barrier developed between two populations of antelope-squirrels. The groups diverged, resulting in two distinct species of antelope squirrel that have different physical characteristics. On the south rim of the canyon is Harris's antelope squirrel, while just across the river on the north rim is the smaller, white-tailed antelope squirrel.
Sometimes divergence occurs simultaneously among several populations of a single species. In this process, known as adaptive radiation, members of the species quickly disperse to take advantage of the many different types of habitat niches, that is the different ways of obtaining food and shelter in their environment. Such specialization ultimately results in many genetically distinctive but similar-looking species. This commonly occurs when a species colonizes a new habitat in which it has little competition. For example, a flock of one species of birds may arrive on some sparsely populated islands. Finding little competition, the birds may evolve rapidly into several species. Each adapted to one available niche. Charles Darwin noted an instance of adaptive radiation on his visit to the Galápagos Islands off the coast of South America. He surmised that one species of the finch colonized the island's thousands of years ago and produced the fourteen species of finch-like birds that exist there now. Darwin observed that the greatest differences in their appearance lay in the shapes of the bills, adapted for their mode of eating. Some species possessed large beaks for cracking seeds. Others had smaller beaks for eating vegetation, and still others featured long, thin beaks for eating insects.
Sometimes distantly related species evolve in ways that make themselves obtainably appears more closely related. This pattern, known as convergent evolution, occurs when members of distantly related species occupy similar ecological niches. Natural selection favours similar adaptations in each population.
Noticeable examples of convergent evolution are the marsupial mammals of Australia and their placental mammal counterparts on other continents. About fifty million years in the past, the Australian continent separated from the rest of the Earth's continents. Biologists speculate that few if any placental mammals had migrated to Australia by the time the continents split. They also surmise that neither marsupial mammals, nor their placental counterparts could cross the ocean after the landmasses drifted apart. As a result, the animals evolved entirely independently. Yet many marsupial mammals in Australia strongly resemble many placental mammals found on other continents.
For example, the marsupial mole of Australia looks very much like the placental moles found on other continents, yet these animals have evolved entirely independent of one another. The explanation for the moles' similar appearances lies in the principles of convergent evolution. Both species evolved to exploit similar ecological niches, and, here, the realm just beneath the surface of the ground. While millions of generations in both marsupial and placental moles, natural selection favoured adaptations suited for a life of burrowing: tube-shaped bodies, broad, shovel-like feet, and short, silky fur that sheds dirt or sand easily. The most striking difference between placental moles and marsupial moles is the colour of their fur. Placental moles are usually dark brown or gray, a colouration that enables them to blend in with the soil in their habitat. Marsupial moles burrow in the golden or reddish sand of Australia, so natural selection produced golden or golden-red fur.
Often two or more organisms in an ecosystem fall into evolutionary steps with one another, each adapting to changes in the other, a pattern known as coevolution. Coevolution is often apparent in flowers and their pollinators. Hummingbirds, for example, have long, narrow beaks and a poor sense of smell, and they are attracted to the colour red. Fuchsias, flowering plants that rely on hummingbirds for pollination, usually have long, slender flowers in various shades of red, and they have almost no fragrance. What at first may be a remarkable coincidence is, in fact, the product of thousands of generations of evolutionary fine-tuning. More likely to attract. hummingbirds than fuchsias with different colouration, red-flowered, individuals had greater reproductive success. Hummingbirds have a tendency to spend more time extracting nectar from the flower of fuchsias with shapes that matched the size of their slender beaks, thus increasing the likelihood of successful pollination. Similarly, those hummingbirds with long, slender beaks were best able to collect nectar from the long-necked flower. Over many generations, long-necked hummingbirds became the rule, rather than the exception, in hummingbird populations.
Species do not change overnight, or even during one lifetime. Evolutionary change usually occurs in tiny, almost imperceptible increments over thousands of generations -periods that range from decades to millions of years. To study the evolutionary relationships among organisms, scientists must take complex measures to exert effort in the detection of deriving indirect clues from the fossil record, patterns of animal distribution, comparative anatomy, molecular biology, and finally, direct observation in laboratories and the natural environment.
One way biologists learn about the evolutionary relationships between species is by examining fossils. These ancient remains of living things are created when a dead plant or animal is buried under layers of mud or sand that gradually turns into stone. Over time, the organism remains themselves may turn to stone, becoming preserved within the rock layer in which they came to rest. By measuring radioactivity in the rock in which a fossil is embedded, paleontologists (scientists who study the fossil record) can determine the age of a fossil.
Fossils present a vivid record of the earliest life on Earth, and of a progression over time from simple to more-complex life forms. The earliest fossils, for example, are those of the primitive bacteria. Some of which are 3.5 billion years' old, and are embedded in more recent layers of rock. The first animal fossils appear as primitive jellyfish that assign of a date from 680 million years into the past. Still more-complex forms, such as the first vertebrates (animals with backbones), are documented by fossils some 570 million year's old. Fossils also show that the first mammals appeared roughly 200 million years in the past.
Although these ancient forms of life have not existed on Earth for millions of years, scientists have been able, typically, to show a clear evolutionary line between extinct animals and their modern descendants. The horse's lineage, for example, can be traced back about fifty million years to a four-toed animal about the size of a dog. Fossils provide evidence of several different transitional forms between this ancient horselike animal and the modern species. In another example, the extinct, winged creature Archaeopteryx lived next to 145 million years ago. Its fossil shows the skeleton of a dinosaur and the feathers of a bird. Many paleontologists consider this creature an intermediate step in the evolution of reptilian dinosaurs into modern birds. Fossils show clear evidence that the earliest human species had many apelike features. These features included large, strong jaws and teeth; short stature, long, curved fingers; and faces that protruded outward from the forehead. Later species evolved progressively more humanlike features.
Scientists, as well, learn about evolutionary principles studying to what degree the different species of plants and animals mate through the geographical distribution by which each in their natural states are the arousing stimulants and by how they relate to their environmental condition between each other. In particular, populations that exist on islands provide living clues of patterns of evolution. The study of these evolutionary relationships, known as island biogeography, has its roots in Darwin's observations of the adaptive radiation of the Galapagos finches. The Hawaiian Islands provide similar examples, particularly in the species of birds known as honey reapers. Like the Galapagos finches, the honey-creepers of Hawaii evolved from a common ancestor and branched into several species, showing a striking variety of beak shapes adapted for obtaining different food sources in their various niches.
Detailed study of the internal and external features of different living things, a discipline known as comparative anatomy, also provides a wealth of information about evolution. The arm of a human, the flipper of a whale, the foreleg of a horse, and the wing of a bird have different forms and are adapted to different functions. Yet they correspond in some way, and this correspondence extends too many details. For the arm, flipper, foreleg, and wing, for example, each appendage shows a similar bone structure. The study of comparative anatomy has revealed many instances of correspondence within various groups of organisms and these bodily structures are said to be homologous. Evolutionary biologists suggest that such homologous structures originated in a common ancestor. The differences arose as each group diverged from the common ancestor and adapted to different ways of life. The more recent the common ancestor, the more similar the species.
The skeletons of humans, for instance, retain evidence of a tail-like structure that is probably a relic from previous mammalian ancestors. This feature, called the coccyx, or more commonly, the tail-bone, has little apparent function in modern humans. Relic features such as the coccyx are called vestigial organs. Another vestigial organ in humans is the appendix, under which a narrow tube attached to the large intestine. In some plant-eating mammals, the appendix is a functioning organ that helps to digest plant material. In humans, however, the organ lacks this purpose and is considerably reduced in size, serving only as a minor source of certain white blood cells that guard against infection.
The field of embryology, the study of how organisms develop from a fertilized egg until they are ready for birth or hatching, also provides evolutionary clues. Scientists have noted that in the earliest stages of development, the embryos of organisms that share a recent common ancestor are very similar in appearance. As the embryos develop, they grow less similar. For example, the embryos of dogs and cats, both members of the mammal order Carnivora, are more similar in the early stages of development than just before birth. The same is true of human and ape embryos, biology in the last few decades, researchers seek evolutionary clues at the smallest level: within the molecules of living organisms. Despite the enormous variety of form and function seen in living things, the underlying genetic code, under which the molecular building material of life displays a striking uniformity. Most living organisms have DNA, and in each case it consists of different pairings of the same building blocks: four nucleotide bases called adenine, thymine, guanine, and cytosine. Using different combinations of these bases, DNA directs the assembly of amino acids into functional proteins. The same uniform code operates within all living things.
These molecules contain more than the master plan for living organisms, but each is a record of an organism's evolutionary history. By examining the makeup of such molecules, scientists gain insights into how different species are related. For example, scientists compare the protein cytochrome from different species. In closely related species, the proteins have amino-acid sequences that are very similar, perhaps varying by one or a few amino acids. More distantly related organisms generally have proteins with fewer similarities. The more distant the relationship, the less alike the proteins.
The idea that species become genetically more different as they diverge from a common ancestor laid the groundwork for the concept of the molecular clock. Scientists know that, statistically, neutral mutations tend to accumulate at a regular rate, like ticks of a clock. Therefore, the number of molecular differences in a shared molecule is proportional to the time that has elapsed since the species had the same ancestor. This calculation has provided new knowledge of the evolutionary relationship between modern apes and modern humans. The ‘molecular clock' concept is controversial, however, and has caused much disagreement between evolutionary scientists who study molecules and those who study fossils. This disagreement arises particularly when the molecular clock time estimates do not agree with the estimates derived from studying the fossil record.
Information about evolutionary processes is also obtained by direct observation of species that undergo rapid modification in only a few generations. One of the most powerful tools in the study of evolutionary mechanisms is also one of the tiniest common fruit flies. These insects have short life spans and, therefore, short generations. This enables researchers to observe and manipulate fruit fly reproduction in
the laboratory and learn about evolutionary change in the process.
Scientists also study organisms in their natural environments to learn about evolutionary processes, for example, how insects develop genetic resistance too human -made pesticides, such as DDT. While pesticides are often initially effective in killing crop-destroying pests, sometimes the insect populations bounce back. In every insect population a few individual insects are not affected by the pesticide. The pesticide wipes out most of the population, leaving only the genetically resistant individuals to multiply and flourish. Gradually, resistant individuals predominate in the population, and the pesticide loses its effectiveness. The same phenomenon has been observed in strains of disease-causing bacteria that have become resistant to even the most powerful antibiotics. Bacterial resistance forces scientists to develop new antibacterial compounds continuously. Scientists have hoped that curbing overuse of antibiotics might cause the drugs to become effective again. Recent research, however, suggests that bacteria may retain their resistance to antibiotics over many generations, even if they have not been exposed to the agent.
How life changes and diversifies over time, some evolutionary biologists are trying to understand how life originated on Earth. This too requires the careful examination and interpretation of many indirect clues. In one well-known series of experiments in 1953, the American chemists' Stanley L. Miller and Harold C. Urey attempted to reproduce the atmosphere of the primitive Earth nearly four billion years ago. They circulated a mixture of gases believed to have been present at the time (hydrogen, methane, ammonia, and water vapour) over water in a sterile glass container. They then subjected the gases to the energy of electrical sparks, simulating the action of lightning on the primitive Earth. After about a week, the fluid turned brown and found to contain amino acids: the constructively stabling blocks of proteins. Subsequent work by these scientists and others also succeeded in producing nucleotides, the all-important constructions to accompany the building blocks of DNA and other nucleic acids. While the artificial generation of these molecules in laboratories did not produce a living organism, this research offers some support that the first building blocks of life could have arisen from raw materials that were present in the environment of the primitive Earth.
Other theories regarding the origin of life on Earth point to outer space. Molecules formerly believed to be produced only by living systems have been found spontaneously to form in great abundance in space. Some scientists speculate that the building blocks of early life might have reached the primitive Earth on meteorites or from the dust of a comet tail.
Once all the raw materials were in place, nucleic acids, proteins, and the other components of simple cells,-it is not clear how the first self-replicating life forms came about. Recent theories centre on the role of a particular nucleic acid -ribonucleic acid (RNA), which, in modern cells, carries out the task of translating the instructions coded in DNA for the assembling of proteins. RNA also acts as a catalyst, that is, to cause other chemical reactions and perhaps most significantly, to make copies of itself. Some scientists believe that the first self-replicating organisms were based on RNA.
According to the fossil record, the first single-celled bacteria appeared some 3.5 billion to 3.9 billion years ago. These microscopic creatures lived in the water, converting the Sun's light into chemical energy. This metabolic process, called photosynthesis, released oxygen gas as a byproduct. Photosyn thesis slowly changed the composition of the early atmosphere, adding more oxygen to what scientists believe was a mixture of sulfur and carbon gases and watered vapour. Perhaps two billion years ago, more-complex cells appeared. These were the first eukaryotic cells, containing a nucleus and other organized internal structures. At around the same time, the oxygen in the Earth's atmosphere increased to nearly what it is today, which was yet another step that was crucial to the development of early life. Around one billion years ago, the first multicellular life forms began to appear. The beginning of the Cambrian Period (around 540 million years in the past), known as the Cambrian explosion, marked an enormous expansion in the diversity and complexity of life. Following this great diversification, plant life found its way to land, while the first fishes evolved, ultimately producing amphibians. Later came reptiles and, later still, mammals. The tumult of evolution was in full swing, as it remains today.
The origins of life on Earth have been a source of speculation among philosophers, religious thinkers, and scientists for thousands of years. Many human civilizations used rich and complex creation stories and myths to explain the presence of living organisms. Ancient Greek philosophers and scientists were among the earliest to apply the principles of modern science to the mysterious complexity and variety of life around them. During early Christian times, ancient Greek ideas gave way to Creationism, the view that a single God created the universe, the world, and all life on Earth. For the next 1,500 years, evolutionary science was at a standstill. The dawn of the Renaissance in the early 14th century brought a renewed interest in science and medicine. Advances in anatomy highlighted physical similarities in the features of widely different organisms. Fossils provided evidence that life on this planet was vastly different millions of years ago. With each development came new ideas and theories about the nature of life.
The Greek philosopher Anaximander, who lived in the 500's Bc, is generally credited as the earliest evolutionist. Anaximander believed that the Earth first existed in a liquid state. Further, he believed that humans evolved from fish-like aquatic beings who left the water once they had developed sufficiently to survive on land. Greek scientist Empedocles speculated in 400 Bc that plant life arose first on Earth, followed by animals. Empedocles proposed that humans and animals arose not as complete individuals but as various body parts that joined randomly to form strange, fantastic creatures. Some of these creatures, being unable to reproduce, became extinct, while others thrived. Outlandish as his ideas seem today, Empedocles' thinking anticipates the fundamental principles of natural selection.
The Greek philosopher and scientist Aristotle, who lived in the 300's Bc, referred to a ‘ladder of nature', a progression of life forms from lower too higher, but his ladder was a static hierarchy of levels of perfection, not an evolutionary concept. Each rung on this ladder was occupied by organisms of higher complexity than the rung before it, with humans occupying the top rung. Aristotle acknowledged that some organisms are incapable of meeting the challenges of nature and so cease to exist. As he saw it, successful creatures possessed a gift, or perfecting principle, that enabled them to rise to meet the demands of their world. Creatures without the perfecting principle died out. In Aristotle's view it was this principle-not evolution, which accounted for the progression of forms in nature.
Many centuries later, the idea of a perfect and unchanging natural world. The product of divine creation was predominant, not only in religion and philosophy but in science. Gradually, however, as knowledge accumulated from seemingly disparate areas, the beginnings of modern evolutionary theory began to take shape. A key figure in this regard was the Swedish naturalist Carolus Linnaeus, who became known as the father of modern taxonomy, the science of classifying organisms.
In his major work Systema Naturae (The System of Nature), first published in 1735, Linnaeus devised a system of classification of organisms that is still in use today. This system places living things within increasingly specific categories based on common attributions from a general grouping (kingdom) down to the specific individual (species). Using this system, Linnaeus named nearly 10,000 plant and animal species in his lifetime. Not an evolutionist by any means, Linnaeus believed that each species was created by God and was incapable of change. Nevertheless, his orderly groupings of living things provided important insights for later theorists. Perhaps the most prominent of those who embraced the idea of progressive change in the living world was the early 19th-century French biologist Jean-Baptiste Lamarck. Whom of which as, Lamarckism theory, now known as Lamarckism and based in part on his study of the fossils of marine invertebrates, was that species do change over time. He believed, furthermore, that animals evolve because unfavourable conditions produce needs that animals try to satisfy. For example, short-necked ancestors of the modern giraffe voluntarily stretched their necks to reach leaves high in trees during times when food was scarce. Proponents of Lamarckism thought that this voluntary employment slightly changed the hereditary characteristics controlling neck growth: The giraffe then transmitted these alterations to its offspring as what Lamarck called acquired characteristics. Modern scientists know that adaptation and natural selection are far more complicated than Lamarck supposed, relate to an animal's voluntary efforts. Nonetheless, the idea of acquired characteristics, with Lamarck as its most famous proponent, persisted for many years.
French naturalist and paleontologist Georges Cuvier feuded with Lamarck. Unearthing the fossils of mastodons and other disappeared or vanquished species. Cuvier produced proof of long-extinct life forms on Earth. Unlike Lamarck, however, Cuvier did not believe in evolution. Instead, Cuvier believed that floods and other cataclysms destroyed such ancient species. He suggested that after each cataclysmic event, God created a new set of organisms.
At around the same time that Cuvier and Lamarck were squabbling, British economist Thomas Robert Malthus proposed ideas extremely influential in evolutionary theory. In his 1798 work, An Essay on the Principle of Population Malthus theorized that the human population would increase at a much greater rate than its food sources. This theory introduced the key idea of competition for limited resources, that is, there is not enough food, water, and living space to go around, and organisms must somehow compete with each other to obtain resources necessary for survival. Another key idea came from Scottish geologist Charles Lyell, who supplied a deeper understanding of Earth's history. In his book Principles of Geology (1830), Lyell set forth his case that the Earth was millions of years old rather than only a few thousand years old, as was maintained by those who accepted the biblical story of divine creation as fact.
In 1831, Charles Darwin, who was intending to become a country minister, had an opportunity to sail as ship's naturalist aboard the HMS Beagle on a five-year, around-the-world mapmaking voyage. During the journey, as the ship anchored off South America and other distant shores, Darwin had the opportunity to travel inland and make observations of the natural world. In the Galápagos Islands, he noted how species on the various islands were similar but distinct from one another. He also observed fossils and other geological evidence of the Earth's great age. The observation's Darwin made on that voyage seemed to suggest the evolution, rather than the creation, of the many local forms of life.
In 1837, shortly after returning to England, Darwin began a notebook of his observations and thoughts on evolution. Although Darwin had developed the major components of his theory of evolution by natural selection in a 1842 unpublished paper circulated among his friends, he was unwilling to publish the results until he could present as complement by which its case of the possibility. He laboured for almost twenty additional years on his theory of evolution and on its primary mechanism, natural selection. In 1858 he received a letter from British naturalist Alfred Russel Wallace, a professional collector of wildlife specimens. Much to Darwin's surprise, Wallace had independently hit upon the idea of natural selection to explain how species are modified by adapting to different conditions. Not wanting Darwin to be unfairly deprived of his share of the credit for the theory. Some of Darwin's scientific colleagues presented extracts of Darwin's work along with Wallace's paper at a meeting of the Linnean Society, a London-based science organization, in June 1858. Wallace's paper stimulated Darwin to finish his work and get it into print. Darwin published, On the Origin of Species by Means of Natural Selection on November 24, 1859. All 1,250 copies of the first printing were sold on that day.
Darwin's book and the theory it popularized evolution through natural selection, which set off a storm of controversy. Some protest came from the clergy and other religious thinkers. Other objections came from scientists. Many scientists continued to believe in Lamarckism, the idea that living things could consciously strive to accumulate modifications during a lifetime and could pass these traits onto their offspring. Other scientists objected to the seemingly random quality of natural selection. If natural selection depended upon random combinations of traits and variations, critics asked, how could it account for such refined and complex structures as the human eye? Perhaps the most serious question, one for which Wallace and Darwin had no answer -concerning the inheritance of traits. How exactly were traits passed along to offspring?
Darwin did not know it, but the answer was nearby -although it would not be acknowledged in his lifetime. In the Augustinian monastery at Brünn (now Brno in the Czech Republic), Austrian monk Gregor Mendel experimented with the breeding of garden peas, observing how their traits were passed down through generations. In crossbreeding pea plants to produce different combinations of traits' - colour, height, smoothness, and other characteristics,-Mendel noted that although a given trait might not appear in every generation, the trait did not disappear. Mendel discovered that the expression of traits hinged on whether the traits were dominant or recessive, and on how these dominant and recessive traits combined. He learned that contrary to what most scientists believed at the time. The mixing of traits in sexual reproduction did not result in a random blending. Traits were passed along in discrete units. These units are now known as genes. Mendel created hundreds of experiments and produced precise statistical models and principles of heredity, now known as Mendel's Laws, showing how dominant and recessive traits are expressed over generations. However, no one appreciated the significance of Mendel's work until after his death. However, his work ultimately gave birth to the modern field of genetics?
In 1900, the Dutch botanist Hugo Marie de Vries and others independently discovered Mendel's laws. The following year, de Vries's book The Mutation Theory challenged Darwin's concept of gradual changes over long periods by proposing that evolution occurred in abrupt, radical steps. Having observed new varieties of the evening primrose plant coming into existence in a single generation, de Vries had subsequently determined that sudden change, or mutation, in the genetic material was responsible. As the debate over evolution continued in the early 20th century, some scientists came to believe that mutation, and not natural selection, was the driving force in evolution. In the face of these mutationists, Darwin's central theory threatened to fall out of favour.
As the science of genetics advanced during the 1920s and 1930s, several key scientists forged a link between Mendel's laws of inheritance and the theory of natural selection prentirely geometric unopposed by Darwin and Wallace. British mathematician Sir Ronald Fisher, British geneticist J.B.S. Haldane, and American geneticist Sewall Wright pioneered the field of population genetics. By mathematically annualizing the genetic variation in entire populations, these scientists showed that natural selection, and not just mutation, could result in evolutionary change.
Further investigation into population genetics and such fields as palaeontology, taxonomy, biogeography, and the biochemistry of genes eventually led to what is called the modern synthesis. This modern view of evolution integrated discoveries and ideas from many different disciplines. In so doing, this view reconciled the many disparate ideas about evolution into the all-encompassing evolutionary science studied today. The modern synthesis was advanced in such books as Genetics and the Origin of Species, published in 1937 by the Russian-born American geneticist Theodosius Dobzhansky; Evolution: The Modern Synthesis (1942) by British biologist Sir Julian Huxley, least of mention, Systematics and the Origin of Species (1942) by German-born American evolutionary biologist Ernst Mayr. In 1942, American paleontologist George Gaylord Simpson showed from the fossil record that rates and modes of evolution are correlated: New kinds of organisms arise when their ancestors invade a new niche, and evolve rapidly to exploit the conditions in the new environment best. In the late 1940's American botanist G. Ledyard Stebbins showed that plants display evolutionary patterns similar to those of animals, and especially that plant evolution has shown diverse adaptive responses to environmental pressures and opportunities.
In addition, biologists reviewed a broad range of genetic, ecological, and anatomical evidence to show that observation and experimental evidence strongly supported the modern synthesis. The theory has formed the basis of evolutionary science since the 1950s. It has also led to an effort to classify organisms according to their evolutionary history, and their physical similarities. Modern scientists use the principles of genetics and molecular biology to study relationships first proposed by Carolus Linnaeus more than 200 years ago.
In 1953, American biochemist James Watson and British biophysicist Francis Crick described the three-dimensional shape of DNA, the molecule that contains hereditary information in nearly all living organisms. In the following decade, geneticists developed techniques to compare DNA and proteins from different organisms rapidly. In one such procedure, electrophoresis, geneticists evaluate different specimens of DNA or proteins by observing how they behave in the presence of a slight electric charge. Such techniques opened entirely new ways to study evolution. For the first time geneticists could quantitatively determine, for example, the genetic change that occurs during the formation of new species.
Electrophoresis and other biochemical techniques also proved to geneticists that populations varied extensively at the molecular level. They learned that much of the population variation at the molecular or biochemical level has no apparent benefit. In 1968 Japanese geneticist Motoo Kimura proposed that much of the variation at the molecular level results not from the forces of natural selection, but from chance mutations that do not affect an organism's fitness. Not all scientists agree with the neutral gene theory.
In recent decades, another branch of evolutionary theory has appeared, as researchers have explored the possibility that not only physical traits, but behaviour itself, might be inherited. Behavioural geneticists have studied how genes influence behaviour, and more recently, the role of biology in social behaviour has been explored. This field of investigation, known as Sociobiology, was inaugurated in 1975 with the publication of the book Sociobiology: The New Synthesis by American evolutionary biologist Edward O. Wilson. In this book, Wilson proposed that genes influence much of the animals and humanizing behaviours, and, least of mention, that these characteristics are also subject to natural selection.
Sociobiologists examine animal behaviours called altruistic, that is, unselfish, or demonstrating concern for the welfare of others. When birds feed on the ground, for example, one individual may notice a predator and sound an alarm. In so doing, the bird also calls the predator's attention to itself. What can account for the behaviour of such a sentry, who seems to derive no evolutionary benefit from its unselfish behaviour and so seem to defy the laws of natural selection?
Darwin was aware of altruistic social behaviour in animals, and of how this phenomenon challenged his theory of natural selection. Among the different types of bees in a colony, for example, worker bees are responsible for collecting food, defending the colony, and caring for the nest and the young, but they are sterile and create no offspring. Only by her, that the beehive area of infactoring takes apart that which only the queen bee has inherently given that which she could reproduce. If natural selection rewards those who have the highest reproductive success, how could sterile worker bees come about by natural selection when worker bees devote themselves to others and do not reproduce?
Scientists now recognize that among social insects, such as bees, wasps, and ants, the sterile workers are more closely related genetically to one another and to their fertile sisters, the queens, than brothers and sisters are among other organisms. By helping to protect or nurture their sisters, the sterile worker's bees preserve their own genes more so than if they reproduced themselves. Thus, the altruistic behaviour evolved by natural selection.
Evolutionary theory has undergone many further refinements in recent years. One such theory challenges the central idea that evolution goes on by gradual change. In 1972 the American paleontologist's Stephen Jay Gould and Niles' Eldredge proposed the theory of punctuated equilibria. According to this theory, trends in the fossil record cannot be attributed to gradual transformation within a lineage, but result from quick bursts of rapid evolutionary change. In Darwinian theory, new species arise by gradual, but not necessarily uniform, accumulation of many small genetic changes over long periods of geologic time. In the fossil record, however, new species generally appear suddenly after long periods of the stasis-that are, no change. Gould and Eldredge recognized that Speciation more likely occurs in small, isolated, peripheral populations than in the main population of the species, and that the unchanging nature of large populations contributes to the stasis of most fossil species over millions of years. Occasionally, when conditions are right, the equilibrium state becomes ‘punctuated' by one or more Speciation events. While these events probably require thousands or tens of thousands of years to establish effective reproductive isolation and distinctive characteristics, this is but an instant in geologic time compared with an average life span of more than ten million years for most fossil species. Proponents of this theory envision a trend in evolutionary development to be more like climbing a flight of stairs (punctuations followed by stasis) than rolling up an inclined plane.
In the last several decades, scientists have questioned the role of extinction in evolution. Of the millions of species that have existed on this planet, more than 99 percent are extinct. Historically, biologists regarded extinction as a natural outcome of competition between newly evolved adaptively superior species and they are older, more primitive ancestors. Recently, however, paleontologists have discovered that many different, unrelated species living in, and large ecosystems tend to become extinct at nearly the same time. The cause is always some sort of climate change or catastrophic event that produces conditions too severe for most organisms to endure. Moreover, new species evolve after the wave of extinction removes many species that previously occupied a region for millions of years. Thus extinction does not result from evolution, but causes it.
Scientists have identified several instances of mass extinction, when species apparently died out on a huge scale. The greatest of these episodes occurred during the end of the Permian Period, by some odd 245 million years ago. Then, according to estimates, more then 95 percent of species, nearly all life on the planet-died out. Another extensively studied, but extinction took place at the boundary of the Cretaceous Period and the Tertiary Period, roughly sixty-five million years ago, when the dinosaurs disappeared. In all, more than twenty global mass extinctions have been identified. Some scientists theorize that such events may even be cyclical, occurring at regular intervals.
In that made or broke into the genetic chain no less the chromosomal cells that carry the DNA and inhibiting functions in the transmission of hereditary information, for which the helical hereditary information is necessary for cell growth.
Other theories have entered on abrupt changes in the levels of the world's oceans, for example, or on the effect of changing salinity on early sea life. Another theory blames catastrophic events for mass extinction. Strong evidence, for example, supports the theory that a meteorite some 10 km. (6 mi.) in diameter struck the Earth 65 million years in the past. The dust cloud from the collision, according to this impact theory, shrouded the Earth for months, blocking the sunlight that plants need to survive. Without plants to eat, the dinosaurs and many other species of land animals were wiped out.
Extinction as a cause of evolution rather than the result of it is perhaps best shown as for our own ancestors,-ancient mammals. During the time of the dinosaurs, mammals made up only several the animals that roamed the planet. The demise of dinosaurs provided an opportunity for mammals to expand their numbers and ultimately to become the dominant land animal. Without the catastrophe that took place sixty-five million years into the past, mammals may have remained in the shadow of the dinosaurs is not exclusively a natural phenomenon. For thousands of years, as the human species has grown in number and technological sophistication, we have shown our power to cause extinction and to upset the world's ecological balance. In North America alone, for example, about forty species of birds and more than thirty-five species of mammals have become extinct in the last few hundred years, mostly from human activity. Humans default upon the plants and animals by their extermination through their relentless hunting or harvesting them. What is more, by destroying and replacing their habitat with farms and other forms of development, they also have allowed to introduce the foreign species that hunt or compete with local species, and by poisoning them with chemicals and other pollutants.
The rain forests of South America and other tropical regions offer a particularly troubling scenario. Upwards of fifty million acres of rain forest disappear every year as humans raze trees to make room for agriculture and livestock. Given that a single acre of rain forest may contain thousands of irreplaceable species of plant and animal life, the threat to bio-diversity is severe. The conservation of wildlife is now an international concern, as evidenced by treaties and agreements enacted at the 1992 Earth Summit in Rio De Janeiro, Brazil. In the United States, federal laws protect endangered species. The problem, nonetheless, of dwindling bio-diversity seems certain to worsen as the human population continues to expand, and no one knows for sure how it will affect evolution.
Advances in medical technology may also affect natural selection. The study from the mid-20th century showing that babies of medium birth weights were more likely to survive than their heavier or lighter counterparts would be difficult to reproduce today. Advances in neonatal medical technology have made it possible for small or premature babies to survive in a great deal higher of numbers.
Recent genetic analysis shows the human population contains harmful mutations in unprecedented levels. Researchers attribute this to genetic drift acting on small human populations throughout history. They also expect that improved medical technology may exacerbate the problem. Better medicine enables more people to survive to reproductive age, even if they carry mutations that in past generations would have caused their early death. The genetic repercussions of this are still unknown, but biologists speculate that many minor problems, such as poor eyesight, headaches, and stomach upsets may be attributable to our collection of harmful mutations.
Humans have also developed the potential to affect evolution at the most basic level,-the genes. The techniques of genetic engineering have become commonplace. Scientists can extract genes from living things, alter them by combining them with another segment of DNA, and then place this recombinant DNA back inside the organism. Genetic engineering has produced pest-resistant crops and larger cows and other livestock. To an increasing extent, genetic engineers fight human disease, such as cancer and heart disease. The investigation of gene therapy, in which scientists substitute functioning copies of a given gene for a defective gene, is an active field of medicine, and that in this way the tinkering with genetic material will affect evolutionary remains, yet to be determined.
The most contentious debates over evolution have involved religion. From Darwin's day to the present, members of some religious faiths have perceived the scientific theory of evolution to be in direct and objectionable conflict with religious doctrine regarding the creation of the world. Most religious denominations, however, see no conflict between the scientific study of evolution and religious teachings about creation. Christian Fundamentalists and others who believe literally in the biblical story of creation choose to reject evolutionary theory because it contradicts the book of Genesis, which describes how God created the world and all its plant and animal life in six days. Many such people maintain that the Earth is comparatively young-perhaps 6,000 to 8,000 years old-and that humans and all the worlds' species have remained unchanged since their recent creation by a divine hand.
Opponents of evolution argue that only a divine intelligence, and not some comparatively random, undirected process, could have created the variety of the world's species, not to mention an organism as complex as a human being. Some people are upset by the oversimplification that humans evolved from monkeys. In the eyes of some, a divine being placed humans apart from the animal world. Proponents of this view find any attempt to place humans within the context of natural history deeply insulting.
For decades, the teaching of evolution in schools has been a flash point in the conflict between religious fundamentalism and science. During the 1920's, Fundamentalists lobbied against the teaching of evolution in public schools. Four states-Arkansas, Mississippi, Oklahoma, and Tennessee-passed laws outlawing public-school instruction in the principles of Darwinian evolution. In 1925 John Scopes, a biology teacher in Dayton, Tennessee, assigned his students readings about Darwinism, in direct violation of state law. Scopes was arrested and placed on trial. In what was the major trial of its time, American defence attorney Clarence Darrow represented Scopes, while American politician William Jennings Bryan argued for the prosecution. Ultimately, Scopes was convicted and customarily received a small fine. However, the ‘Monkey Trial,' as it became called, was seen as a victory for evolution, since Darrow, in cross examining Bryan, succeeded in pointing out several serious inconsistencies in Fundamentalists belief.
Laws against the teaching of evolution were upheld for another forty years, until the Supreme Court of the United States, in a 1968 decision in the case Epperson V. Arkansas, ruled that such laws were an unconstitutional violation of the legally required separation of church and state. Over the next few years, Fundamentalists responded by de-emphasizing the religious content in their doctrine and instead casting their arguments as a scientific alternative to evolution called creation science, now also called intelligent design theory. In response to Fundamentalist pressure, twenty-six states debated laws that would require teachers to spend equal amounts of time teaching creation science and evolution. Only two states, Arkansas and Louisiana, passed such laws. The Arkansas law was struck down in federal district court, while proponents of the Louisiana law appealed all the way to the Supreme Court. In its 1987 decision in Edwards v Aquillard, the Court struck down such equal time laws, ruling that creation science is a religious idea and thus an illegal violation of the church-state separation. Despite these rulings, school board members and other government officials continue to grapple with the long-standing debate between creation and evolution scientists. Even so, efforts to permit the teaching of intelligent design theory in public schools have been unsuccessfully as scientists have sought-and found-evidence for evolution. The fossil record demonstrates that life on this planet was vastly different millions of years ago. Fossils, furthermore, provide evidence of how species change over time. The study of comparative anatomy has highlighted physical similarities in the features of widely different species-proof of common ancestry. Bacteria that mutate and develop resistance to antibiotics, along with other observable instances of adaptation, demonstrate evolutionary principles at work. The study of genes, proteins, and other molecular evidence has added to the understanding of evolutionary descent and the relationship among all living things. Research in all these areas has led to overwhelming support for evolution among scientists.
Nevertheless, evolutionary theory is still, in some cases, the cause of misconception or misunderstanding. People often misconstrue the phrase ‘survival of the fittest'. Some people interpret this to mean that survival is the reward for the strongest, the most vigorous, or the most dominant. In the Darwinian sense, however, fitness does not necessarily mean strength so much as the capacity to adapt successfully. This might mean developing adaptations for more efficiently obtaining food, or escaping predators, or enduring climate change-in short, for thriving in a given set of circumstances.
Yet it bears repeating that organisms do not change their characteristics in direct response to the environment. The key is genetic variation within a population,-and the potential for new combinations of traits. Nature will select those individuals that have developed the ideal characteristics with which to flourish in a given environment or niche. These individuals will have the greatest degree of reproductive success, passing their successful traits onto their descendants.
Another misconception is that evolution always progresses to better creatures. In fact, if species become too narrowly adapted to a given environment, they may ultimately lose the genetic variation necessary to survive sudden changes. Evolution, in such cases, will lead to extinction.
Once upon a time, in Human Evolution, now considered as pensively the process though which a lengthy period of change is admissively given by people who have originated from apelike ancestors. Scientific evidence shows that the physical and behavioural traits shared by all people evolved over a period of at least six million years.
One of the earliest defining human traits, Bipedalism -
walking on two legs as the primary form of locomotion-undergoing an evolution of more than four million years ago. Other important human characteristics'-such as a large and complex brain, the ability to make and use tools, and the capacity for language-developed more recently. Many advanced traits',-including complex symbolic expression, such as art, and elaborate cultural diversity-emerged mainly during the past 100,000 years.
Our closest living relatives are three surviving species of great apes: the gorilla, the common chimpanzee, and the pygmy chimpanzee (also known as bonobo). Their confinement to Africa, along with abundant fossils evidence, suggests that the earliest stages of human evolution were also played out in Africa, human history, as sometimes separate from the history of animals, took the initiative in that location about seven million years ago (estimated range from five to nine million years ago). Around that time, a population of African apes split into several populations, of which one went on to evolve into modern gorillas, a second into the two modern chimps, and the third into humans. The gorilla line apparently split before the split between the chimp and the human lines.
Fossils indicate that the evolutionary line leading to us had achieved an upright posture by around four million years ago, then began to increase in body size and in relative brain size around 2.5 million years ago. That protohuman is generally known as Australopithecus africaanus. Homo habilis, and Homo erectus, which apparently evolved into each other in that sequence. Although the Homo erectus, the stage extends to around 1.7 million years ago, was close to us modern humans in body size, its brain size was still barely half of ours. Stone tools became common around 2.5 million years ago, but they were merely the crudest of flaked or battered stones. In zoological significance and distinction, The Homo erectus was more than an ape, but still much less than a modern human.
All of that human history, for the first five or six million years after our origins about seven million years ago, remained confined to Africa. The first human ancestor to spread beyond Africa was The Homo erectus, as it is attested by fossils discovered on the Southeast Asian island of Java and conventionally known as Java man the oldest Java ‘man': archeological remains-of course, they may have belonged to a Java woman,-have usually been argued that they date from about a million years ago. However, it has recently been argued that they date from 1.8 million years ago. (Strictly speaking, the name Homo erectus belongs to these Javan fossils, and the African fossils classified as Homo erectus may warrant a different name). At present, the earliest unquestioned evidence for humans in Europe stems from around half a million years ago, but there are claims of an earlier presence. One would assume that the colonization of Asia also permitted the simultaneous colonization of Europe, since Eurasia is a single landmass not bisected by major barriers.
Nearly half a million years ago, human fossils had diverged from older Homo erectus skeletons in, they're enlarged, rounder, and fewer angular skulls. African and European skulls of half a million years ago were sufficiently similar to skulls of a modern that they are classified in our species, Homo sapiens, instead of in Homo erectus. This distinction is arbitrary, since The Homo erectus evolved into The Homo sapiens. However, these early Homo sapiens still differed from us in skeletal details, had brains significantly smaller than ours, and were grossly different from us in their artifacts and behaviour. Modern stone-tool-making peoples, such as Yali's great grandparents, would have scorned the stone tools of a half million years ago as very crude. The only significant addition to our ancestor's cultural repertoire that can be documented with confidence around that time was the use of fire.
No art, bone tools, or anything else has come down to us from an early Homo sapiens except their skeletal remains, and those crude stone tools, there were still no humans in Australia, because it would have taken boats to get there from Southern Asia. There were also no humans anywhere in the Americas, because that would have required the occupation of the nearest part of the Eurasian continent (Siberia), and possibly boat-building skills as well. (The present, shallow Bering Strait separating Siberia from Alaska, alternated between a strait and a broad intercontinental bridge of dry land, as sea level repeatedly rose and fell during the Ice Ages). Nevertheless, boat building and survival in cold Siberia were both far beyond the capabilities of an early Homo sapiens. After half a million years ago, the human population of Africa and western Eurasia proceeded to diverge from each other and from East Asia populations in skeletal details. The population of Europe and western Asia between 130,000 and 40,000 years ago is recreated by especially many skeletons' known as Neanderthals and sometimes classified as some separate spacies,
Yet their stone tools were still crude by comparison with modern New Guineans' polished stone axes and were usually not yet made in standardized diverse shapes, each with a clearly recognizable function.
The few preserved African skeletal fragments contemporary with the Neanderthals are more similar to our modern skeletons than do Neanderthal skeletons. Even fewer preserved East Asian skeletal fragments are known, but they appear different again from both Africans and Neanderthals. As for the lifestyle at that time, the best-preserved evidence comes from stone artifacts and animal bones accumulated at southern African sites. Although those Africans of 100,000 years ago had more modern skeletons than did their Neanderthal contemporized, they made especially the same crude stone toots as Neanderthals, still lacking standardized shapes. They had no preserved art. To judge from the bone evidence of animal species under which their targeted prey and hunting skills were unimpressive and mainly directed at easy-to-kill, not-at-all-dangerous animals. They were not yet in the business of slaughtering buffalo, pig, and other dangerous prey. They could not even catch fish: their sites immediately on the seacoast lack fish bones and fishhook. They and their Neanderthal contemporaries still rank as less than fully human.
While Neanderthals lived in glacial times and were adapted to the cold, they penetrated no farther north than northern Germany and Kiev. Nonetheless, Neanderthals apparently lacked needles, sewn clothing, warm houses, and other technology essential to survival in the coldest climates. Anatomically modern peoples who did posses such technology had expanded into Siberia by around 20,000 years ago (there are the usual much older disputed claims). That expansion may have been responsible for the extinction of Eurasia's wooly mammoth and wooly rhinoceroses likewise, to note, while the settlements of Australia/New Guinea, humans now occupied three of the five habitable continents, least that we omit Antartica because it was not reached by humans until the 19th century and has never had any self-supporting human population. That left only two continents, North America and South America. For obvious reason that reaching the Americas from the Old world required boats (for which either there is no evidence even in Indonesia until 40,000 years ago and none in Europe until much later) to cross by sea, or else it required the occupation of Siberia (unoccupied until about 20,000 years ago) to cross the Bering Strait. However, it is uncertain when, between about 14,000 and 35,000 years ago, the Americas were first colonized.
Meanwhile, human history at last took off around 50,000 years ago, while of the easiest definite signs had come from East African sites with standardized stone tools and the first preserved jewellery (ostrich-shell beads). Similar developments soon appear in the Near East and in southeastern Europe, then (some 40,000 years ago) in southwestern Europe, where abundant artefacts are associated with fully modern skeletons of people termed Cro-Magnons. Thereafter, the garbage preserved at archaeological sites rapidly becomes ever more interesting and leaves no doubt that we are dealing with biologically and behaviourally modern human, however.
Cro-Magnons' garbage heaps yield not only stone tools but also tools of bone, whose suitability for shaping (for instance, into fish hooks) had apparently gone unrecognized by previous humans. Tools were produced in diverse. Distinctive shapes do modernly that their function as needles, awls, engraving tools, and so on are obvious to us. Instead of only single-piece tools such as hand-held scrapers, and multi-piece tools made their appearance. Recognizable multi-piece weapons at Cro-Magnon sites include harpoons, spear-throwers, and eventually bow and arrows, the precursors of rifles and other multi-piece modern weapons. Those efficient means of killing at a safe distance permitted the hunting of dangerous prey as rhinos and elephants, while the invention of rope for nets, lines, and snares allowed the addition of fish and bird to our diet. Remains of horses and sewn clothing testify to a greater improved ability to survive in cold climates, and remains of jewellery and carefully buried skeletons indicate revolutionary aesthetic and spiritual development.
Of the Cro-Magnons' products preserved, the best known are their artworks: Their magnificent cave paintings, statues, and musical instruments, which we still appreciate as art today. Anyone who has experienced firsthand the overwhelming power of the life-sized painted bulls and hoses in the Lascaux Cave of southern France will understand, if not imagine, that their creators must have been as modern in their minds as they were in their skeletons.
Obviously, some momentous change took place in our ancestors' capabilities between about 100,000 and 50,000 years ago. Presenting us with two major unresolved questions, regarding its triggering cause and its geographic location. As for its case, it can be argued for the perfection of the voiced box and hence for the anatomical basis of modern language, on which the exercise of human creativity is so dependent. Others have suggested instead that a change in brain organization around that time, without a change in brain size, made modern language possible.
As this occurring leap, and its location, did it take place primarily in one geographic area, in one group of humans, who were thereby enabled to expand and replace the former human populations of other parts of the world? Or did it occur in parallel in different regions, in each of which the human populations living today would be descendants of the populations living there before the connective leap? The conventionally advanced-looking human skull from Africa around 100,000 years ago has been taken to support the former view, within occurring specifically in Africa. Molecular studies (of so-called mitochondrial DNA) were initially also interpreted about an African origin of modern humans, though the meaning of those molecular findings is currently in doubt. On the other hand, skulls of humans living in China and Indonesia hundreds of thousands of years ago are considered by some physical anthropologists to exhibit features still found in modern Chinese and in Aboriginal Australians, respectfully. If true, that in the finding would suggest parallel evolution and multi-regional origins of modern humans, rather than origins in a single Garden of Eden. The issue remains unresolved.
The evidence for a localized origin of modern humans, followed by their spread and then their replacement of other types of humans elsewhere, seems strongly for Europe. Some 40,000 years ago, into Europe came the Cro-Magnons, with their modern skeleton, superior weapons, and other advanced cultural traits. Within a few thousand years there were no more Neanderthals, who had been evolving as the sole occupants of Europe for hundreds of thousands of years. The sequence strongly suggests that the modern Cro-Magnon somehow used their far superior technology, and their language skills or brains, to infect, kill, or displace the Neanderthals, leaving behind no evidence of hybridization between Neanderthals and Cro-Magnons.
Physical and genetic similarities show that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the so -called great apes (large apes) of Africa-chimpanzees (including bonobos, or so-called pygmy chimpanzees). Gorilla's,-share a common ancestor that lived sometime between eight million and six million years ago. The earliest humans evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between six million and two million years ago come entirely from Africa.
We should be reminded of the ways in which big domestic mammals were crucial to those human societies possessing them. Most notably, they provided meat, milk products, fertilizer, land transportation, leather, military assault, plow traction, and wool, and germs that killed previously unexposed peoples.
In addition, of course, small domestic mammals and domestic birds and insects have also been useful to humans. Many birds were domesticated for meat, eggs, and feathers: the chicken in China, various duck and goose species in parts of Eurasia, turkeys in Mesoamerica, guinea fowl in Africa, and the Muscovy duck in South America. Wolves were domesticated in Eurasia and North America to become our dogs used as hunting companions, sentinels, pets, and, in some societies, food. Rodent and other small mammals domesticated for food include the rabbit in Europe, the guinea pig in the Andes, a giant rat in West Africa, and possibly a rodent called the hutia on Caribbean islands. Ferrets were domesticated in Europe to hunt rabbits, and cats were domesticated in North Africa and Southern Asia to hunt rodent pests. Small mammals domesticated as recently as the 19th and 20th century include foxes, mink, and chinchillas grown for fur and hamsters as pets. Even some insects have been domesticated, not ably Europe's honeybee and China's silkworm moth, kept for hone y and silk, respectively.
Many of these small animals thus yielded food, clothing or warmth, but none of them pulled plows or wagons, none bore riders, none except dogs pulled sleds nor became war machines, and nine of them have been as important for food as have big domesticated mammals.
Most scientists distinguish among twelve to nineteen different species of early humans. Scientists do not all agree, however, about how the species are related or which ones simply died out. Many early human species',- probably most of them left no descendants. Scientists also debate over how to identify and classify particular species of early humans, and about what factors influenced the evolution and extinction of each species.
Early humans first migrated out of Africa into Asia probably between two million and 1.7 million years ago. They entered Europe later, generally within the past one million years. Species of modern humans populated many parts of the world much later. For instance, people first came to Australia probably within the past 60,000 years, and to the Americas within the past 35,000 years. The beginnings of agriculture and the rise of the first civilizations occurred within the past 10,000 years.
The scientific study of human evolution is called palaeanthropology. Palaeanthropology is a Studfield of anthropology, the study of human culture, society, and biology. Paleoanthropologists search for the roots of human physical traits and behaviour. They seek to discover how evolution has shaped the potentials, tendencies, and limitations of all people. For many people, palaeanthropology is an exciting scientific field because it illuminates the origins of the defining traits of the human species, and the fundamental connections between humans and other living organisms on Earth. Scientists have abundant evidence of human evolution from fossils, artifacts, and genetic studies. However, some people find the concept of human evolution troubling because it can seem to conflict with religious and other traditional beliefs about how people, other living things, and the world became. Yet many people have come to reconcile such beliefs with the scientific evidence.
All species of organisms originate through the process of biological evolution. In this process, new species arise from a series of natural changes. In animals that reproduce sexually, including humans, the term species refers to a group whose adult members regularly interbreed, resulting in fertile offspring,-that is, offspring themselves capable of reproducing. Scientists classify each species with a unique, and two-party scientific name. In this system, modern humans are classified as Homo sapiens.
The mechanism for evolutionary change resides in genes' - the basic units of heredity. Genes affect how the body and behaviour of an organism develop during its life. The information contained within the genes can reserve the change of a process known as mutation. The way particular genes are expressed,-how they affect the body or behaviour of an organism can also change. Over time, genetic change can alter a species overall way of life, such as what it eats, how it grows, and where it can live.
Genetic changes can improve the ability of organisms to survive, reproduce, and, in animals, raise offspring. This process is called adaptation. Parents pass adaptive genetic changes to their offspring, and ultimately these changes become common throughout a population-a group of organisms of the same species that share a particular local habitat. Many factors can favour new adaptations, but changes in the environment often play a role. Ancestral human species adapted to new environments as their genes changed, altering their anatomy (physical body structure) physiology (bodily functions, such as digestion, and behaviour). Over long periods, evolution dramatically transformed humans and their ways of life.
Geneticists estimate that the human line began to diverge from that of the African apes between eight million and five million years ago (paleontologists have dated the earliest human fossils to at least six million years ago). This figure comes from comparing differences in the genetic makeup of humans and apes, and then calculating how long it probably took for those differences to develop. Using similar techniques and comparing the genetic variations among human populations around the world, scientists have calculated that all people may share common genetic ancestors that lived sometime between 290,000 and 130,000 years ago.
Humans belong to the scientific order named Primates, a group of more than 230 species of mammals that also includes lemurs, lorises, tarsiers, monkeys, and apes. Modern humans, early humans, and other species of primates all have many similarities and some important differences. Knowledge of these similarities and differences helps scientists to understand the roots of many human traits, and the significance of each step in human evolution.
All primates, including humans, share at least part of a set of common characteristics that distinguish them from other mammals. Many of these characteristics evolved as adaptations for life in the trees, the environment in which earlier primates evolved. These include more reliance on sight than smell; overlapping fields of vision, allowing stereoscopic (three-dimensional) appearance; limbs and hands adapted for clinging on, leaping from, and swinging on tree trunks and branches; the ability to grasp and manipulate small objects (using fingers with nails instead of claws); large brains in relation to body size; and complex social lives.
The scientific classification of primates reflects evolutionary relationships between individual species and groups of species. Strepsirhini (meaning ‘turned-nosed') primate's,-of which the living representatives include lemurs, lorises, and other groups of species all commonly known as prosimians evolved earliest and are the most primitive forms of primates. The earliest monkeys and apes evolved from ancestral haplorhine (meaning ‘simple-nosed') primates, of which the most primitive living representative is the tarsier. Humans evolved from ape ancestors.
Tarsiers have traditionally been grouped with prosimians, but many scientists now recognize that tarsiers, monkeys, and apes share some distinct traits, and group the three together. Monkeys, apes, and humans-who share many traits not found in other primates-together make up the suborder Anthropoidea. Apes and humans together make up the super-family as contributive members of Hominoidea, a grouping that emphasizes the close relationship among the species of these two groups.
Strepsirhines are the most primitive types of living primates. The last common ancestors of Strepsirhines and other mammals creatures similar to tree shrews and classified as Plesiadapiformes,-evolved at least sixty-five million years ago. The earliest primates evolved about fifty-five million years ago, and fossil species similar to lemurs evolved during the Eocene Epoch (about fifty-five million to thirty-eight million years ago). Strepsirhines share all of the basic characteristics of primates, although their brains are not particularly large or complex and they have a more elaborate and sensitive olfactory system (sense of smell) than do other primates.
Tarsiers are the only living representatives of a primitive group of primates that ultimately led to monkeys, apes, and humans. Fossil species called Omomyid, with some traits similar to those of tarsiers, evolved near the beginning of the Eocene, followed by early tarsier-like primates. While the Omomyid and tarsiers are separate evolutionary branches (and there is no living Omomyid), they share features concerning a reduction in the olfactory system, a trait shared by all haplorhine primates, including humans.
The anthropoid primates are divided into New World (South America, Central America, and the Caribbean Islands) and Old World (Africa and Asia) groups. New World monkeys,- such as marmosets, capuchins, and spider monkeys,-belong to the infra-order of platyrrhine (broad-nosed) anthropoids. Old World monkeys and apes belong to the infra-order of catarrhine (downward-nosed) anthropoids. Since humans and apes together make up the hominoids, humans are also catarrhine anthropoids.
The first catarrhine primates evolved between fifty million and thirty-three million years ago. Most primate fossils from this period have been found in a region of northern Egypt known as Al Fayy? Um (or the Fayum). A primate group known as Propliopithecus, one lineage of which is sometimes called Aegyptopithecus, had primitive catarrhine features-that is, it had many basic features that Old World monkeys, apes, and humans share today. Scientists believe, therefore, that Propliopithecus resembles the common ancestor of all later Old World monkeys and apes. Thus, Propliopithecus may also be considered an ancestor or a close relative of an ancestor of humans evolved during the Miocene Epoch (twenty-four million to five million years in the past). Among the oldest known hominoids is a group of primates known by its genus name, Proconsul. Species of Proconsul had features that suggest a close link to the common ancestor of apes and humans,-for example, the lack of a tail. The species Proconsul heseloni lived in the trees of dense forests in eastern Africa about twenty million years ago. An agile climber, it had the flexible backbone and narrow chest characteristic of monkeys, but also a wide range of movement in the hip and thumb, traits characteristic of apes and humans.
Large ape species had originated in Africa by twenty-three million or twenty-two million years ago. By fifteen million years ago, some of these species had migrated to Asia and Europe over a land bridge formed between Africa-Arabian and Eurasian continents, which had previously been separated.
Early in their evolution, the large apes underwent several radiations-periods when new and diverse species branched off from common ancestors. Following Proconsul, the ape genus Afropithecus evolved about eighteen million years ago in Arabia and Africa and diversified into several species. Soon afterward, three other ape genera evolved,-Griphopithecus of western Asia about 16.5 million years ago, the earliest ape to have spread from Africa, as did the genus Kenyapithecus of Africa about fifteen million years ago, moreover the Dryopithecus of Europe about twelve million years ago. Scientists have not yet determined which of these groups of apes may have caused the common ancestor of modern African apes and humans.
Scientists do not all agree about the appropriate classification of hominoids. They group the living hominoids into either two or three families: Hylobatidae, Hominidae, and sometimes Pongidae. Hylobatidae consists of the small or so-called lesser apes of Southeast Asia, commonly known as gibbons and siamangs. The Hominidae (hominids) includes humans and, according to some scientists, the great apes. For those who include mere humans associated with the Hominidae, all of the great apes, including the orangutans of Southeast Asia, belong to the family Pongidae.
In the past only humans were considered to belong to the family Hominidae, and the term hominid referred only to species of humans. Today, however, genetic studies support placing all of the great apes and humans together in this family and the placing of African apes-chimpanzees and gorillas-together with humans at an even lower level, or subfamily
According to this reasoning, the evolutionary branch of Asian apes leading to orangutans, which separated from the other hominid branches nearly thirteen million years ago, belongs to the subfamily Ponginae. The ancestral and living representatives of the African ape and human branches together belong to the subfamily Homininae (sometimes called Hominines). Lastly, the line of early and modern humans belongs to the tribe (classificatory level above genus) Hominini, or hominins.
This order of classification corresponds with the genetic relationships between ape and human species. It groups humans and the African apes together at the same level in which scientists group together, for example, all types of foxes, all buffalo, or all flying squirrels. Within each of these groups, the species are very closely related. However, in the classification of apes and humans the similarities among those mention's of hominoid, hominid, hominine, and hominin may admit to contradiction. In this context, the term early human refers to all species of the human family tree since the divergence from a common ancestor with the African apes. Popular writing often still uses the term hominid to mean the same thing.
About 98.5 percent of the genes in people and chimpanzees are identical, making chimps the closest living biological relatives of humans. This does not mean that humans evolved from chimpanzees, but it does indicate that both species evolved from a common ape ancestor. Orangutans, the great apes of Southeast Asia, differ much more from humans genetically, indicating a more distant evolutionary relationship.
Modern humans have several physical characteristics reflective of an ape ancestry. For instance, people have shoulders with a wide range of movement and fingers capable of strong grasping. In apes, these characteristics are highly developed as adaptations for brachiation, -swinging from branch to branch in trees. Although humans do not brachiate, the general anatomy from that earlier adaptation remains. Both people and apes also have larger brains and greater cognitive abilities than do most other mammals.
Human social life, too, shares similarities with that of African apes and other primates,-such as baboons and rhesus monkeys-that live in large and complex social groups. Group behaviour among chimpanzees, in particular, strongly resembles that of humans. For instance, chimps form long-lasting attachments with each other, participate in social bonding activities, such as grooming, feeding, and hunting; and form strategic coalitions with each other in order to increase their status and power. Early humans also probably had this kind of elaborate social life.
In whatever manner, modern humans fundamentally differ from apes in many significant ways. For example, as intelligent as apes are, people's brains are much larger and more complex, and people have a unique intellectual capacity and elaborate forms of culture and communication. In addition, only people habitually walk upright, can precisely manipulate very small objects, and have a throat structure that makes speech possible.
By around six million years ago in Africa, an apelike species had evolved with two important traits that distinguished it from apes: (1) small canine, or eye, teeth (teeth next to the four incisors, or front teeth) and (2) Bipedalism,-that is, walking on two legs as the primary form of locomotion. Scientists refer to these earliest human species as australopithecines, or Australopiths for short. The earliest Australopiths species known today belong to three genera: Sahelanthropus, Orrorin, and Ardipithecus. Other species belong to the genus Australopithecus and, by some classifications, Paranthropus. The name australopithecine translates literally as ‘southern ape', concerning South Africa, where the first known Australopiths fossils were found.
The Great Rift Valley, a region in eastern Africa in which past movements in Earth's crust have exposed ancient deposits of fossils, has become famous for its Australopiths finds. Countries in which scientists have found Australopiths fossils include Ethiopia, Tanzania, Kenya, South Africa, and Chad. Thus, Australopiths ranged widely over the African continent.
Fossils from several different early Australopiths species that lived between four million and two million years ago clearly show a variety of adaptations that marks the transition from ape too human. The very early period of this transition, before four million years ago, remains poorly documented in the fossil record, but those fossils that do exist show the most primitive combinations of ape and human features.
Fossils reveal much about the physical build and activities of early Australopiths, but not everything about outward physical features such as the colour and texture of skin and hair, or about certain behaviours, such as methods of obtaining food or patterns of social interaction. For these reasons, scientists study the living great apes-particularly the African apes to understand better how early Australopiths might have looked and behaved, and how the transition from ape too human might have occurred. For example, Australopiths probably resembled the great apes in characteristics such as the shape of the face and the hair on the body. Australopiths also had brains roughly equal in size to those of the great apes, so they probably had apelike mental abilities. Their social life probably resembled that of chimpanzees.
Most of the distinctly human physical qualities in Australopiths related to their bipedal stance. Before Australopiths, no mammal had ever evolved an anatomy for habitual upright walking. Australopiths also had small canine teeth, as compared with long canines found in most other catarrhine primates.
Other characteristics of Australopiths reflected their ape ancestry. They had a low cranium behind a projecting face, and a brain size of 390 to 550 cu. cm. (24 to thirty-four cu. in.) - between an ape's brain. The body weight of Australopiths, as estimated from their bones, ranged from twenty-seven to 49 kg. (sixty to 108 lb.), and they stood 1.1 to 1.5 m. (3.5 to 5 ft.) tall. Their weight and height compare closely to those of chimpanzees (chimp height measured standing). Some Australopiths species had a large degree of sexual dimorphism-males were much larger than females-a trait also found in gorillas, orangutans, and other primates.
Australopiths also had curved fingers and long thumbs with a wide range of movement. In comparison, the fingers of apes are longer, more powerful, and more curved, making them extremely well adapted for hanging and swinging from branches. Apes also have very short thumbs, which limits their ability to manipulate small objects. Paleoanthropologists speculate about whether the long and dexterous thumbs of Australopiths allowed them to use tools more efficiently than do apes.
The anatomy of Australopiths shows several adaptations for Bipedalism, in both the upper and lower body. Adaptations in the lower body included the following: The australopithilium, or pelvic bone, which rises above the hip joint, was much shorter and broader than it is in apes. This shape enabled the hip muscles to steady the body during each step. The Australopiths pelvis also had a bowl-like shape, which supported the internal organs in an upright stance. The upper legs angled inward from the hip joints, which positioned the knees better to support the body during upright walking. The legs of apes, on the other hand, are positioned almost straight down from the hip, so that when an ape walks upright for a short distance, its body sways from side to side. Australopiths also had short and fewer flexible toes than do apes. The toes worked as rigid levers for pushing off the ground during each bipedal step.
Other adaptations occurred above the pelvis. The Australopiths spine had a S-shaped curve, which shortened the overall length of the torso and gave it rigidity and balance when standing. By contrast, apes have a straight spine. The Australopiths skull also had an important adaptation related to Bipedalism. The opening at the bottom of the skull through which the spinal cord attaches to the brain, called the foramen magnum, was positioned more forward than it is in apes. This position set the head in balance over the upright spine.
Australopiths clearly walked upright on the ground, but paleoanthropologists debate whether the earliest humans also spent a significant amount of time in the trees. Certain physical features indicate that they spent at least some of their time climbing in trees. Such features included they're curved and elongated fingers and elongated arms. However, their fingers, unlike those of apes, may not have been long enough to allow them to brachiate through the treetops. Study of fossil wrist bones suggests that early Australopiths could lock their wrists, preventing backward bending at the wrist when the body weight was placed on the knuckles of the hand. This could mean that the earliest bipeds had an ancestor that walked on its knuckles, as African apes do.
Compared with apes, humans have very small canine teeth. Apes-particularly males-have thick, projecting, sharp canines that they use for displays of aggression and as weapons to defend themselves. The oldest known bipeds, who lived at least six million years ago, still had large canines by human standards, though not as large as in apes. By four million years ago Australopiths had developed the human characteristic of having smaller, flatter canines. Canine reduction might have related to an increase in social cooperation between humans and an accompanying decrease in the need for males to make aggressive displays.
The Australopiths can be divided into an early group of species, known as gracile Australopiths, which arose before three million years ago, and a later group, known as robust Australopiths, which evolved after three million years ago. The gracile Australopiths,-of which several species evolved between 4.5 million and three million years in the past,-generally had smaller teeth and jaws. The later-evolving robust had larger faces with large jaws and molars (cheek teeth). These traits indicate powerful and prolonged chewing of food, and analyses of wear on the chewing surface of robust Australopiths molar teeth's support this idea. Some fossils of early Australopiths have features resembling those of the later species, suggesting that the robustus evolved from one or more gracile ancestors.
Paleoanthropologists recognize at least eight species of early Australopiths. These include the three earliest established species, which belong to the genuses' Sahelanthropus, Orrorin, and Ardipithecus, a species of the genus Kenyanthropus, and four species of the genus Australopithecus.
The oldest known Australopiths species is Sahelanthropus tchadensis. Fossils of this species were first discovered in 2001 in northern Chad, Central Africa, by a research team led by French paleontologist Michel Brunet. The researchers estimated the fossils to be between seven million and six million years old. One of the fossils has a fracture but nearly completes the cranium that shows a combination of apelike and humanlike features. Apelike features include small brain size, an elongated brain case, and areas of bone where strong neck muscles would have attached. Humanlike features are to include small, flat canine teeth, a short middle part of the face, and a massive brow ridge (a bony, protruding ridge above the eyes) similar to that of later human fossils. The opening where the spinal cord attaches to the brain is tucked under the brain case, which suggests that the head was balanced on an upright body. It is not certain that Sahelanthropus walked bipedally, however, because bones from the rest of its skeleton have yet to be discovered. Nonetheless, its age and humanlike characteristics suggest that the human and African ape lineages had divided from one another by at least six million years ago.
In addition to reigniting debate about human origins, the discovery of Sahelanthropus in Chad significantly expanded the known geographic range of the earliest humans. The Great Rift Valley and South Africa, from which most other discoveries of early human fossils came, are apparently not the only regions of the continent that preserve the oldest clues of human evolution.
Orrorin tugenensis lived about six million years ago. This species was discovered in 2000 by a research team led by French paleontologist Brigitte Sent and French geologist Martin Pickford in the Tugen Hills region of central Kenya. The researchers found more than a dozen early human fossils dating between 6.2 million and six million years old. Among the finds were two thighbones that possess a groove indicative of an upright stance and bipedal walking. Although the finds are still being studied, the researchers consider these thighbones to be the oldest evidence of habitual two-legged walking. Fossilized bones from other parts of the skeleton show apelike features, including long, curved finger bones useful for strong grasping and movement through trees, and apelike canine and premolar teeth. Because of this distinctive combination of ape and human traits, the researchers gave a new genus and species name to these fossils, Orrorin tugenensis, which in the local language means ‘original man in the Tugen region. The age of these fossils suggests that the divergence of humans from our common ancestor with chimpanzees occurred before six million years ago.
In 1994 an Ethiopian member of a research team led by American paleoanthropologists Tim White discovered human fossils estimated to be about 4.4 million year's old. White and his colleagues gave their discovery the name Ardipithecus ramidus. Ramid means ‘root' in the Afar language of Ethiopia and refers to the closeness of this new species to the roots of humanity. At the time of discovery, the genus Australopithecus was scientifically well established. White devised the genus name Ardipithecus to distinguish this new species from other Australopiths because its fossils had a very ancient combination of apelike and humanlike traits. More recent finds indicate that this species may have lived as early as 5.8 million to 5.2 million years ago.
The teeth of Ardipithecus ramidus had a thin outer layer of enamel,-a trait also seen in the African apes but not in other Australopiths species or older fossil apes. This trait suggests a close relationship with an ancestor of the African apes. In addition, the skeleton shows strong similarities to that of a chimpanzee but has slightly reduced canine teeth and adaptations for Bipedalism.
In 1965 a research team from Harvard University discovered a single arm bone of an early human at the site of Kanapoi in northern Kenya. The researchers estimated this bone to be four million years old, but could not identify the species to which it belonged or return at the time to look for related fossils. It was not until 1994 that a research team, led by British-born Kenyan paleoanthropologists Meave Leakey, found numerous teeth and fragments of bone at the site that could be linked to the previously discovered fossil. Leakey and her colleagues determined that the fossils were those of the very primitive species of Australopiths, which was given the name Australopithecus Anamensis. Researchers have since found other A. Anamensis fossils at nearby sites, dating between about 4.2 million and 3.9 million years old. The skull of this species appears apelike, while its enlarged tibia (lower leg bone) indicates that it supported its full body weight on one leg at a time, as in regular bipedal walking
Australopithecus Anamensis was quite similar to another, much better-known species, A. afarensis, a gracile Australopiths that thrived in eastern Africa between about 3.9 million and three million years ago. The most celebrated fossil of this species, known as Lucy, is a partial skeleton of a female discovered by American paleoanthropologists Donald Johanson in 1974 at Hadar, Ethiopia. Lucy lived 3.2 million years ago. Scientists have identified several hundred fossils of A. afarensis from Hadar, including a collection representing at least thirteen individuals of both sexes and various ages, all from a single site.
Researchers working in northern Tanzania have also found fossilized bones of A. afarensis at Laetoli. This site, dated at 3.6 million years old, is best known for its spectacular trails of bipedal human footprints. Preserved in hardened volcanic ash, these footprints were discovered in 1978 by a research team led by British paleoanthropologists Mary Leakey. They provide irrefutable evidence that Australopiths regularly walked bipedally.
Paleoanthropologists have debated interpretations of the characteristics of A. afarensis and its place in the human family tree. One controversy centres on the Laetoli footprints, which some scientists believe show that the foot anatomy and gait of A. afarensis did not exactly match those of the modern humans. This observation may indicate that early Australopiths did not live primarily on the ground or at least spent a significant amount of time in the trees. The skeleton of Lucy also indicates that A. afarensis had longer, more powerful arms than most later human species, suggesting that this species was adept at climbing trees. Another controversy relates to the scientific classification of the A. afarensis fossils, compared with Lucy, who stood only 1.1 m. (3.5 ft.) tall, other fossils identified as A. afarensis from Hadar and Laetoli came from individuals who stood up to 1.5 m. (5 ft.) tall. This great difference in size leads some scientists to suggest that the entire set of fossils now classified as A. afarensis represents two species. Most scientists, however, believe the fossils represent one highly dimorphic species,-that is, a species that has two distinct forms (in this case, two sizes). Supporters of this view may note that the two large (presumably male) and small (presumably female) adults occur together in one site at Hadar.
A third controversy arises from the claim that A. afarensis was the common ancestor of both later Australopiths and the modern human genus, Homo. While this idea remains a strong possibility, the similarity between this and another Australopiths species-one from southern Africa, named Australopithecus africanus-makes it difficult to decide which of the two species led to the genus Homo.
Australopithecus africanus thrived in the Transvaal region of what is now South Africa between about 3.3 million and 2.5 million years ago. Australian-born anatomist Raymond Dart discovered this species-the first known Australopiths-in 1924 at Taung, South Africa. The specimen that of a young child, became known as the Taung Child. For decades after this discovery, almost no one in the scientific community believed Dart's claim that the skull came from an ancestral human. In the late 1930's teams led by Scottish-born South African paleontologist Robert Broom unearthed many more A. africanus skulls and other bones from the Transvaal site of Sterkfontein.
A. africanus generally had a more globular braincase and less primitive-looking face and teeth than did A. afarensis. Thus, some scientists consider the southern species of early Australopiths to be a likely ancestor of the genus Homo. According to other scientists, however, certain heavily built facial and cranial features of A. africanus from Sterkfontein identify it as an ancestor of the robust Australopiths that lived later in the same region. In 1998 a research team led by South African paleoanthropologists Ronald Clarke discovered an almost complete early Australopiths skeleton at Sterkfontein. This important find may resolve some of the questions about where A. africanus fits in the story of human evolution.
Working in the Lake Turkana's region of northern Kenya, a research team led by paleontologist in which Meave Leakey uncovered 1999 a cranium and other bone remains of an early human that showed a mixture of features unseen in previous discoveries of early human fossils. The remains were estimated to be 3.5 million years old, and the cranium's small brain and earhole was similar to those of the earliest humans. Its cheekbone, however, joined the rest of the face in a forward position, and the region beneath the nose opening was flat. These are traits found in later human fossils from around two million years ago, typically those classified in the genus Homo. Noting this unusual combination of traits, researchers named a new genus and species, Kenyanthropus platy ops, or ‘flat-faced human from Kenya.' Before this discovery, it seemed that only a single early human species, Australopithecus afarensis, lived in East Africa between four million and three million years ago. Yet Kenyanthropus indicates that a diversity of species, including a more humanlike lineage than A. afarensis, lived in this period, just as in most other eras in human prehistory.
The human fossil record is poorly known between three million and two million years ago, from which carries over recent finds from the site of Bouri, Ethiopia, particularly important. From 1996 to 1998, a research team led by Ethiopian paleontologist Berhane Asfaw and American paleontologist Tim White found the skull and other skeletal remains of an early human specimen about 2.5 million years old. The researchers named it Australopithecus garhi; the word garhi means ‘surprise' in the Afar language. The specimen is unique in having large incisors and molars in combination with an elongated forearm and thighbone. Its powerful arm bones suggest a tree-living ancestry, but its longer legs indicate the ability to walk upright on the ground. Fossils of A. garhi are associated with some of the oldest known stone tools, along with animal bones that were cut and cracked with tools. It is possible, then, that this species was among the first to make the transition to stone Toolmaking and to eating meat and bone marrow from large animals.
By 2.7 million years ago the later, robust Australopiths had evolved. These species had what scientists refer to as megadont cheek teeth-wide molars and premolars coated with thick enamel. Their incisors, by contrast, were small. The robusts also had an expanded, flattened, and more vertical face than did gracile Australopiths. This face shape helped to absorb the stresses of strong chewing. On the top of the head, robust Australopiths had a sagittal crest (ridge of bone along the top of the skull from front to back) to which thick jaw muscles attached. The zygomatic arches (which extend back from the cheek bones to the ears), curved out wide from the side of the face and cranium, forming very large openings for the massive chewing muscles to pass through near their attachment to the lower jaw. Together, these traits indicate that the robust Australopiths chewed their food powerfully and for long periods.
Other ancient animal species that specialized in eating plants, such as some types of wild pigs, had similar adaptations in their facial, dental, and cranial anatomy. Thus, scientists think that the robust Australopiths had a diet consisting partly of tough, fibrous plant foods, such as seed pods and underground tubers. Analyses of microscopic wear on the teeth of some robust Australopiths specimens appear to support the idea of a vegetarian diet, although chemical studies of fossils suggest that the southern robust species may also have eaten meat.
Scientists originally used the word robust to refer to the late Australopiths out of the belief that they had much larger bodies than did the early, gracile Australopiths. However, further research has revealed that the robust Australopiths stood about the same height and weighed roughly the same amount as Australopithecus afarensis and A. africanus.
The earliest known robust species, Australopithecus aethiopicus, lived in eastern Africa by 2.7 million years ago. In 1985 at West Turkana, Kenya, American paleoanthropologists Alan Walker discovered a 2.5-million-year-old fossil skull that helped to define this species. It became known as the ‘black skull' because of the colour it had absorbed from minerals in the ground. The skull had a tall sagittal crest toward the back of its cranium and a face that projected far outward from the forehead. A. aethiopicus shared some primitive features with
A. afarensis,-that is, features that originated in the earlier East African Australopiths. This may indicate that A. aethiopicus evolved from A. afarensis.
Australopithecus boisei, the other well-known East African robust Australopiths, lived over a long period, between about 2.3 million and 1.2 million years ago. In 1959 Mary Leakey discovered the original fossil of this species-a nearly complete skull-at the site of Olduvai Gorge in Tanzania. Kenyan-born paleoanthropologists Louis Leakey, husband of Mary, originally named the new species Zinjanthropus boisei (Zinjanthropus translates as ‘East African man'). This skull-dating from 1.8 million years ago-has the most specialized features of all the robust species. It could withstand extreme chewing forces, and molars four times the size of those in modern humans. Since the discovery of Zinjanthropus, now recognized as an Australopiths, scientists have found many A. boisei fossils in Tanzania, Kenya, and Ethiopia.
The southern robust species, called Australopithecus robustus, lived between about 1.8 million and 1.3 million years ago in Transvaal, the same region that was home to A. africanus. In 1938 Robert Broom, who had found many A. africanus fossils, bought a fossil jaw and molar that looked distinctly different from those in A. africanus. After finding the site of Kromdraai, from which the fossil had come, Broom collected many more bones and teeth that together convinced him to name a new species, which he called Paranthropus robustus (Paranthropus meaning ‘beside man'). Later scientists dated this skull at about 1.5 million years old. In the late 1940's and 1950 Broom discovered many more fossils of this species at the Transvaal site of Swartkrans.
Many scientists believe that robust Australopiths represent a distinct evolutionary group of early humans because these species share features associated with heavy chewing. According to this view, Australopithecus aethiopicus diverged from other Australopiths and later produced A. boisei and A. robustus. Paleoanthropologists who strongly support this view think that the robusts should be classified in the genus Paranthropus, the original name given to the southern species. Thus, these three species are sometimes called, P. aethiopicus, P. boisei, and P. robustus.
Other paleoanthropologists believe that the eastern robust species, A. aethiopicus and A. boisei, may have evolved from an early Australopiths of the same region, perhaps A. afarensis. According to this view, A. africanus gave rise only to the southern species, A. robustus. Scientists refer to such a case -in that two or more independent species evolve similar characteristics in different places or at different times,-as parallel evolution. If parallel evolution occurred in Australopiths, the robust species would make up two separate branches of the human family tree.
The last robust Australopiths died out about 1.2 million years ago. At about this time, climate patterns around the world entered a period of fluctuation, and these changes may have reduced the food supply on which robusts depended. Interaction with larger-brained members of the genus Homo, such as Homo erectus, may also have contributed to the decline of late Australopiths, although no compelling evidence exists of such direct contact. Competition with several other species of plant-eating monkeys and pigs, which thrived in Africa at the time, may have been an even more important factor. Nevertheless, the reasons why the robust Australopiths became extinct after flourishing for such a long time are not yet known for sure.
Scientists have several ideas about why Australopiths first split from the apes, initiating the course of human evolution. Nearly all hypotheses suggest that environmental change was an important factor, specifically in influencing the evolution of Bipedalism. Some well-established ideas about why humans first evolved include (1) the savanna hypothesis, (2) the woodland-mosaic hypothesis, and (3) the variability hypothesis.
The global climate cooled and became drier between eight million and five million years ago, near the end of the Miocene Epoch. According to the savanna hypothesis, this climate change broke up and reduced the area of African forests. As the forests shrunk, an ape population in eastern Africa became separated from other populations of apes in the more heavily forested areas of western Africa. The eastern population had to adapt to its drier environment, which contained larger areas of grassy savanna.
The expansion of dry terrain favoured the evolution of terrestrial living, and made it more difficult to survive by living in trees. Terrestrial apes might have formed large social groups in order to improve their ability to find and collect food and to fend off predators-activities that also may have required the ability to communicate well. The challenges of savanna life might also have promoted the rise of tool use, for purposes such as scavenging meat from the kills of predators. These important evolutionary changes would have depended on increased mental abilities and, therefore, may have correlated with the development of larger brains in early humans.
Critics of the savanna hypothesis argue against it on several grounds, but particularly for two reasons. First, discoveries by a French scientific team of Australopiths fossils in Chad, in Central Africa, suggest that the environments of East Africa may not have been fully separated from those farther west. Second, recent research suggests that open savannas were not prominent in Africa until sometime after two million years ago
Criticism of the savanna hypothesis has spawned alternative ideas about early human evolution. The woodland-mosaic hypothesis proposes that the early Australopiths evolved in patchily wooded areas-a mosaic of woodland and grassland-that offered opportunities for feeding both on the ground and in the trees, and that ground feeding favoured Bipedalism.
The variability hypothesis suggests that early Australopiths experienced many changes in environment and ended up living in a range of habitats, including forests, open-canopy woodlands, and savannas. In response, their populations became adapted to a variety of surroundings. Scientists have found that this range of habitats existed at the time when the early Australopiths evolved. So the development of new anatomical characteristics,-particularly Bipedalism-combined with an ability to climb trees, may have given early humans the versatility to live in a variety of habitats.
Scientists also have many ideas about which benefits of Bipedalism may have influenced its evolution. Ideas about the benefits of regular Bipedalism include that it freed the hands, making it easier to carry food and tools; allowed early humans to see over tall grass to watch for predators; reduced vulnerability of the body and too hot of the sun, provided an increased exposure to cooling winds; improved the ability to hunt or use weapons, which became easier with an upright posture; and made extensive feeding from bushes and low branches easier than it would have been for a quadruped. Scientists do not overwhelmingly support any one of these ideas. Recent studies of chimpanzees suggest, though, that the ability to feed more easily might have particular relevance. Chimps carry through an action on two legs most often when they feed from the ground on the leaves and fruits of bushes and low branches. Chimps cannot, however, walk in this way over long distances.
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