Prehistoric humans may be defined as the prehistoric populations of the living human species, Homo sapiens, together with other, ancestral species of the genus Homo. The origins, evolution, and cultural development of early humankind comprise the subject matter of paleoanthropology. Paleoanthropologists are concerned with reconstructing the evolutionary history and ways of life of prehistoric Homo sapiens and of the extinct human species \THomo erectus\t and \THomo habilis\t. They seek the origin of the genus Homo among the early hominids, or prehumans, and attempt to trace the origin of the hominids among still earlier hominoid, or apelike, \Lprimate\ls. HISTORY, AIMS, AND METHODS OF PALEOANTHROPOLOGY Paleoanthropology originated with the recognition that the oddly shaped stones found in the 18th and 19th centuries in certain ancient river gravels of Europe were in fact artifacts--tools made by humans--rather than natural phenomena. In the early days of paleoanthropology, during the 18th century, a literal interpretation of the Bible was generally accepted. Theologians calculated that the Earth was created about 6,000 years ago, and geological features such as ancient gravels were interpreted as evidence for Noah's flood. Some interpreted the stone artifacts as the work of antediluvian (pre-flood) humans. With the growth of scientific \Tgeology\t in the 19th century, most biologists came to accept a time scale of earth history running into millions of years. Along with this theory grew the notion of a stage in human history, before the invention of metalworking, when stone tools alone were used. The discovery by the Western world of "stone age" peoples still living in places such as Australia reinforced the idea that human society and technology had developed, or evolved, through a series of stages. The idea, however, that the human species had itself evolved physically from a nonhuman species was not generally accepted until the last quarter of the 19th century. Human fossils discovered before this time were usually dismissed as the distorted remains of freaks or cripples. This outlook changed as biologists began to accept the evolutionary theories of Charles Darwin (see \Tevolution\t). Darwin and his colleagues Thomas Henry Huxley and Ernst Haeckel showed that humans share many anatomical features with chimpanzees, gorillas, and orangutans, and they argued that Homo sapiens probably evolved from a more primitive species that resembled these apes in many respects. Naturalists were stimulated by this work to search for the fossil remains of physically intermediate creatures that would demonstrate the reality of human evolution by "linking" humans to apelike ancestors. Despite the scientists' protests that no single intermediate form was likely to have existed, this search became, in the popular imagination, a hunt for "the missing link" that would prove Darwin correct. During the past century numerous human, prehuman, and hominoid fossils were discovered that in a general way linked the modern human species to its apelike ancestors. At the same time, ideas of human cultural evolution also became more refined. Excavations and discoveries enabled archaeologists to recognize additional stages in human \Tprehistory\t. The "stone age" was subdivided into the \TPaleolithic Period\t, or Old Stone Age, preceding the invention of agriculture, and the \TNeolithic Period\t, or New Stone Age, that succeeded it. The Paleolithic was subdivided into lower, middle, and upper divisions, defined by the invention of new techniques of stone working, mainly in Europe. Again using evidence mainly from Europe, the later stages of prehistory were distinguished as the \TBronze Age\t and the \TIron Age\t. As archaeological research has been applied to regions outside Europe, some of these stages have been found to be inappropriate (Africa, for instance, had no Bronze Age), and modern archaeologists are more concerned with local sequences of cultural evolution than with universal stages. Nevertheless, they still emphasize the importance of advances in technology to human cultural evolution. A modern project in paleoanthropology is typically a combined operation, pooling the skills of various specialists. Central to the operation are paleontologists and archaeologists. Paleontologists are concerned with the collection, description, and interpretation of fossils, including those of prehistoric humans (see \Tpaleontology\t). They use their knowledge of comparative anatomy to deduce the evolutionary relationships and ways of life of the species whose fossil remains they discover. Archaeologists use careful excavation to recover the physical evidence of human activities in the past (see \Tarchaeology\t). The evidence includes tools, buildings, and other artifacts and refuse such as cooked bones and even feces. It is used to reconstruct a picture of the way of life of the people who lived on the site, from their eating habits and the way they made their tools, to the size and composition of the group in which they lived. Important additional evidence is provided by specialists in other fields. Geologists help to interpret the rock layers in which fossils are found and to relate them to a time scale of Earth history. Palynologists reconstruct the vegetation of the habitat from fossil pollen. Taphonomists attempt to reconstruct the story of bones from death to fossilization. A significant advance of recent decades has been the development by geophysicists of techniques of absolute dating--assigning an age in years directly to a fossil, artifact, or layer of rock. For the anthropologist the most important of these methods have been carbon-14 and potassium-argon dating (see \Tradiometric age-dating\t), although recently some carbon-14 dates have been called into question. Both methods rely on the fact that radioactive elements decay, or change their chemical nature, at a constant rate. The carbon-14 method can be used to date organic substances, such as wood, bone, and shell, up to about 50,000 years old. The potassium-argon method can be used to tell the approximate age of volcanic rocks more than half a million years old. Many sites are unsuitable for absolute dating, however, and in these cases the anthropologist relies on other methods--such as comparing the animals they contain with those found in absolutely dated sites. THE PLEISTOCENE SETTING OF HUMAN EVOLUTION Fossil evidence indicates that the earliest true humans (members of the genus Homo) appeared close to the end of the Pliocene Epoch, about two to three million years ago. Most of human evolution therefore occurred during the Pleistocene Epoch, which stretches from about 2.5 million years ago to the present. The Pleistocene has been a time of unusually great environmental variation when compared to most of the history of the Earth. Comparatively short warm periods have alternated with periods of glaciation, when the climate cooled and ice sheets spread from the poles (see \Tice ages\t). The fluctuating climate of the Pleistocene is the culmination of a trend that began many millions of years ago. Until about 40 million years ago warm climates extended to the Arctic. By about 14 million years ago Antarctica and Greenland bore ice caps; by the time of the appearance of Homo, glaciers had formed on the higher mountains in mid-latitudes. During the past 2.5 million years, the cooling trend culminated in continental glaciations, in which sheets of ice, hundreds of feet thick, blanketed much of Europe and North America. During glacial episodes plants and animals adapted to warm climates were replaced by arctic plants and animals such as the reindeer. Although the climate was harsh, the plains of glacial Europe and North America supported large herds of game and were therefore rich hunting grounds for any human populations hardy enough to inhabit them. Glacial episodes alternated with interglacials, when warmer conditions returned. Until recently, geologists recognized only four or five glacial advances, separated by long interglacials. However, new evidence from deep-sea cores, in which a very complete record of climatic fluctuation is preserved, documents a more complex picture. Many more than five glaciations occurred in a wildly fluctuating manner, and interglacials were quite short--lasting only about 10,000 years. Climatic variation during the Pleistocene also occurred in the tropics. Evidence exists, especially in Africa, that cool, moist periods, when forests spread, alternated with drier intervals, when forests retreated before dry grasslands and desert. The relationship between rainy periods in the tropics and glaciations in mid-latitudes is not clear. An important side effect of glaciation was to alter the level of the oceans. With much of the Earth's water frozen, the oceans shrank, exposing much of the continental shelves and providing new dry-land corridors, notably between Asia and North America by way of Siberia and Alaska (see \TBering Land Bridge\t). Climatic fluctuation in the Pleistocene was probably an important factor in human evolution. Not only did it open up new territory for colonization, by creating dry-land bridges; the rapid changes and often rigorous climates were also important in providing the environmental challenge that spurred human physical and cultural adaptation. HUMAN ORIGINS The human species is a member of the mammalian order Primates. It is related, in descending order of closeness, to apes, monkeys, tarsiers, and lemurs. The early part of human evolutionary history, therefore, is the story of the emergence of the ancestors of these groups. The earliest traces of fossil primates are found in rocks about 70 million years old, which also contain the remains of the last dinosaurs. Within a few million years the dinosaurs had disappeared, and mammals, including primates, became much more common. The remains of these very early primates, which are quite common in fossil sites of the American West and Western Europe, indicate that they were a diverse group of small, ratlike animals, some already adapted to life in the trees. By about 45 million years ago a side branch of this primitive ancestral group had given rise to more advanced primates that were quite similar to modern lemurs. Among this group can be seen characteristics that are distinctive of modern primates: relatively large brains, a well-developed visual sense, and nails rather than claws. All of these characteristics were evidently adapted for life in the trees. Early Ancestral Forms About 35 million years ago appeared the first evidence of primitive monkeylike primates. The largest and best known of these, Propliopithecus (see \TAegyptopithecus\t), was about the size of a cat and is believed to be in the ancestral line of apes and humans. Although still primitive in some respects, primates of this period constitute a link between the earlier, lemurlike primates and the true monkeys and apes that first appeared about 22 million years ago, at the beginning of the long geological epoch known as the Miocene. From rocks of this age have been found the earliest fossils representing the primates of sub-Saharan Africa, the home of many living monkeys and apes and the probable place of origin of the human family. Early Miocene apes and monkeys probably behaved and looked not unlike their living relatives. Among the apes was the group called \TDryopithecus\t, believed to include the evolutionary ancestor of both hominids and later apes such as the chimpanzee and gorilla. For most of the Miocene, Dryopithecus was widespread and successful. However, as the climate grew drier during the later part of the period, Dryopithecus disappeared from the fossil record. Presumably, it retreated with the forests to the tropical regions where the great apes still survive. For many years anthropologists assumed that the origin of hominids as a group separate from apes must have occurred in Europe between 14 and 10 million years ago. Most anthropologists now believe that the split occurred much later and that it occurred in Africa. \TOreopithecus\t, an apelike primate that lived in Europe about 10 million years ago was proposed as the first hominid, but it is now believed that this primate died out completely without leaving descendants. Likewise, \TRamapithecus\t and its close relative Sivapithecus were once championed as the first hominids. It was argued that Ramapithecus, in common with the hominids, had adapted to life outside the forest and lived off the hard, tough vegetable foods of the grasslands. It is, however, highly unlikely that Ramapithecus walked upright or used tools more than living apes. Moreover, most anthropologists now feel that its early origin rules out the possibility that it was a hominid. Pointing to the remarkable similarity in blood chemistry and genetics between chimpanzees and humans, they maintain that the earliest hominids originated from an apelike stock no more than 8 million years ago. In this case, the earliest true hominid is probably \TAustralopithecus\t. Australopithecus The genus Australopithecus, first described on the basis of a single skull from South Africa, is now represented by many fossils from several areas of the African continent. Important sites include \TOlduvai Gorge\t, Tanzania; Lake Rudolf (Turkana), Kenya; and Hadar, Ethiopia. The genus appears to have been confined to Africa, where it existed during the time range between 5.5 and 1 million years ago. Although primitive in some respects, Australopithecus is classified within the human family, Hominidae, because it shares with humans certain significant advances over earlier forms. In particular, its leg bones show that it walked upright; its brain, although still within the ape range, was relatively larger than that of most apes; and neither sex had the projecting canine teeth (fangs) that are used by apes in fighting. Presumably, Australopithecus used simple clubs or threw stones, instead of biting, to defend itself. Australopithecus varied considerably in size--from less than 1.2 m (4 ft) to about the size of a modern human. Three species are generally distinguished (although some classification schemes list five): Australopithecus africanus, the smallest and earliest, which may have been ancestral both to humans and to later Australopithecus, and two larger, more specialized and robust species (Australopithecus robustus and Australopithecus boisei), which overlap in time with early members of the genus Homo and which evidently became extinct. An australopithecine fossil known as the "black skull," similar to A. bosei is thought by some to represent a new species. All species of Australopithecus lived in open woodland and grassland rather than in forests as had the apes, and the ways in which they differ physically from apes can be seen as adaptations to the new habitat. Whereas their front teeth were quite small, their back, grinding teeth (molars and premolars) were huge--evidently an adaptation to hard chewing. Studies of minute scratches on their dental enamel suggest that their diet consisted largely of hard, chewy seeds and berries. Although Australopithecus probably ate small animals, as do chimpanzees, most scientists consider it unlikely that Australopithecus was a systematic hunter, or "killer ape," as this species used to be depicted. Whether Australopithecus regularly made and used tools is a matter of debate. Some paleoanthropologists have suggested that the broken bones and tusks of antelopes and other animals found at \TMakapansgat\t, South Africa, and at other Australopithecus cave sites may have been used as tools and weapons; other paleoanthropologists consider it more likely that the broken animal bones were the leavings of leopards and hyenas that also frequented the caves. Although some sites with Australopithecus fossils have also yielded well-made stone tools, these tools are more likely to be the handiwork of early, true humans, whose remains are also found at these sites. If, as seems probable at present, Australopithecus was no more of a hunter and tool maker than the modern chimpanzee, why did it, unlike the apes, develop two-legged locomotion, or bipedalism? The reasons for this development, too, are debated. One theory is that Australopithecus babies were more helpless and had to be carried in the arms of their mothers. Another emphasizes the importance of free hands to an animal that had no large fangs to defend itself. Yet another sees the advantage of bipedalism as freeing the hands to gather the small, scattered vegetable foods on which Australopithecus usually fed. These ideas are not mutually exclusive. The brain of Australopithecus, although less than half the size of a modern human's, was relatively larger than that of an ape. Brain expansion may have been favored by a more complex social organization, perhaps one in which "families" of one adult male and one or more females and their young clustered in troops for protection. EARLY HUMANS From deposits dating from about 2 million years ago have emerged the first direct evidence of behavior that decisively separates the species Homo from other animals. This behavior includes the regular use of stone tools and other artifacts and the life-style called hunting and gathering. In contrast to the foraging of nonhuman primates, the hunting and gathering of the first humans involved a division of labor. Some group members (probably males) hunted animals for meat, whereas the rest searched for small game and wild vegetable foods. All shared the food they collected. This cooperative way of life strongly favored the evolution of technology (cutting tools and containers, especially). Additionally, there is evidence that these early humans meticulously planned their hunts. So the human capacity for abstract thought, foresight, and adaptation to local conditions of life also apparently improved. The evidence for these changes is seen in the gradually increasing size of the brain, the development of increasingly complex technology (represented by stone working), and the rapid geographical spread of the human species. Homo Habilis After the extinction of Australopithecus, no more than one hominid species existed at any time. However, the single human species is given different names at different stages of its evolution. The earliest humans are known as Homo habilis. Physically, they were much like Australopithecus, apart from the larger size of their brains. Most Homo habilis fossils have been discovered in East Africa. They are often found with simple \TOldowan\t tools, including stone choppers, cores, and sharp-edged flakes. At some sites evidence exists that animals up to hippopotamus size were butchered and eaten, but whether the meat was obtained by hunting or by scavenging the carcasses of dead animals has not been determined. Homo Erectus As well as favoring the evolution of the brain, the early development of technology and culture also affected the evolution of the teeth and jaws. As tools (and later fire) were used to prepare and soften food, the teeth of early humans became smaller and the jaws less robust. By about 1.6 million years ago, these trends had produced a mentally and physically more advanced population called Homo erectus. At about the same time, the hand ax, a finely chipped, versatile, two-edged stone implement first appeared. Hand axes typify the \TAcheulean\t tool industry, which also included a variety of pounders and flakes. Although first recognized in Asia (see \TJava man\t and \TPeking man\t), Homo erectus populations also lived throughout the warmer parts of the Old World. In Europe the jaw of \THeidelberg man\t and many scattered hand axes attest to their presence (although some researchers consider this fossil to belong to an archaic Homo Sapiens). In Africa both fossil remains and habitation sites have been found throughout the length of the continent, from Algeria to South Africa. On the whole, Homo erectus seems to have preferred open or lightly wooded country, where game would have been most plentiful. Many sites attest to the skill of these people in hunting big game--elephants, antelope, and even giant baboons--as well as gathering small game and vegetable foods. Although the colder regions of the far north were not inhabited, Homo erectus was sufficiently adaptable to survive in a variety of habitats, from tropical Africa to chilly central China. Undoubtedly, the more rigorous climate of the north stimulated technological inventions. One of the most important of these was the use of fire, in cooking, for warmth, and in the hunt. This vital step probably occurred about 500,000 years ago. Archaic Homo Sapiens and the Neanderthalers By about 250,000 years ago humans had become sufficiently advanced to be assigned to Homo sapiens. However, until about 40,000 years ago, they were not identical to modern humans. They retained many ancestral features recalling Homo erectus: a large face with big teeth and a low skull with heavy brow ridges and little or no forehead. In brain size, however, they were within the modern range, which distinguishes them from the small-brained Homo erectus. Fossils of these archaic humans have been found at many sites: among the best-known early specimens are the skulls from Steinheim, Germany (see \TSteinheim man\t); Swanscombe, England (see \TSwanscombe man\t); and Broken Hill, Zambia (see \TBroken Hill man\t). An important sign of technological advance was the invention, about 100,000 years ago, of the Levallois technique of stone working, in which a large thin flake is struck from a core and used as a blank for making more specialized tools such as knives and scrapers (see \TLevalloisian\t). As human populations began to exploit a wider variety of habitats, new, local tool traditions appeared: the Fauresmith on the plains of South Africa, the Sangoan on the fringes of the African forest, and the \TMousterian\t in the frigid plains of glacial Europe. The makers of the Mousterian tools were the \TNeanderthalers\t (Homo sapiens neanderthalensis), who flourished between 100,000 and 40,000 years ago. Far from being the brutish, semierect "apemen" of popular imagination, the Neanderthalers were an advanced human group whose ingenuity enabled them to wrest a living from the most challenging habitat then occupied by humankind. Mousterian tools were adapted to a wide variety of tasks: cutting and preparing meat, scraping hides, working wood, and many others. Evidence of rituals--and careful burial of the dead--suggests the existence of religious beliefs. Some of the burials are of aged or handicapped people who must have been supported by the rest of the group, perhaps in return for the benefit of their wisdom and knowledge. Given the evidence for ritual and complex beliefs, it is likely that the brain of archaic Homo sapiens was sufficiently evolved to permit the use of true language. However, anthropologists are divided over whether the Neanderthalers' vocal chords were sufficiently developed to be capable of human speech. About 40,000 years ago humans of modern type replaced the archaic humans such as the Neanderthalers. Some anthropologists believe that only a very few archaic populations evolved directly into Homo sapiens sapiens (the taxonomic classification of fully modern humans), the rest being displaced as the moderns expanded their range. Others hold that archaic groups everywhere became "modern" by evolutionary change. Both ideas are probably to some extent correct; evolution of archaic populations into the modern human type probably occurred in many regions, but in other regions, such as western Europe, the archaics may have been absorbed and displaced by invading modern populations. Upper Paleolithic and Mesolithic Populations In Europe and elsewhere in the Old World the remains of the earliest physically modern humans occur with tools that attest to the invention of new techniques of manufacture, especially the production of long, narrow flake tools, called blades. These innovations define a new period of prehistory, the Upper Paleolithic. In the Old World this period began about 40,000 years ago, in the middle of the last glaciation. In the Americas the comparable stage, the Paleo-Indian, began at least 20,000 years ago with the migration of people across the Bering Strait from Siberia (see \TNorth American archaeology\t). In both continents culture based on hunting and gathering reached its peak of development during this period. Hunters became more expert, devising sophisticated techniques that involved large numbers of people working in cooperation to kill whole herds of game. In areas such as West and Central Europe, where game was most plentiful, permanent communities sprang up, and the population rose in numbers and density. Besides the stone blade, which could be fashioned into any one of a variety of handy small tools, technical innovations included tools made of bone and ivory, clothing sewn together and decorated with beads, and among some groups a system of reckoning time by the Sun and Moon. Among the finest productions of the Upper Paleolithic are the paintings and engravings (mostly animal representations) executed on stone slabs or ivory or on the walls of caves. The quality of this art is such that for many years critics refused to believe that it could be the work of prehistoric peoples. Even today, when its authenticity is established, its function is still obscure but is thought to have involved hunting magic or a ritual use (see \Tprehistoric art\t). About 10,000 years ago the ice sheets and tundra vegetation in the north gave way rapidly to coniferous and hardwood forest. The great herds of bison, horses, reindeer, and mammoths were replaced by more elusive, hard-to-hunt animals such as moose and elks. Human society and technology evolved in adaptation to the changing conditions. The resulting cultures are called Mesolithic in the Old World and Archaic in North America. New tools included microliths, tiny stone blades that were hafted in wood or antler handles. New weapons such as the bow and arrow enabled hunters to pursue the solitary game animals of the forest. Ingenious traps, snares, and nets enabled people to exploit resources such as wildfowl and fish that abounded in the lakes left by the retreating glaciers. Settlements became smaller, more dispersed, and less permanent (see \TMesolithic Period\t). Food Production and Urbanization In some areas humans adapted in a markedly different way to the end of the glaciation. Rather than diversifying their use of resources, as the Mesolithic people of the northern woodlands were doing, they focused their attention on a few reliable resources. The land to the east of the Mediterranean was one such area with evidence of this new orientation. Here some populations began to concentrate on exploiting wild sheep and goats and a few species of wild grasses that produced edible seeds. Gradually, as revealed in the archaeological record, a mutual dependence developed between the human populations and the animals and plants they exploited and protected. The favored species, breeding under human protection, became modified so that they could not have survived without it. The humans, in turn, living in permanent settlements near the resources they exploited and tended, could not easily revert to nomadic hunting. The people had become farmers, and the animals and plants, domesticates. Western Asia is the best-known, and perhaps the earliest, center of domestication, but several other early centers existed in various parts of the world. In Mexico an agriculture was developed on the basis of maize, beans, and squash. Other less-well-known centers of plant and animal domestication existed in Southeast Asia, China, and probably tropical Africa. In areas adjacent to some of these centers food production spread as hunters were displaced by farmers or acquired domesticates from their neighbors. Contrary to a common notion, prehistoric villagers did not live more easily than hunters. Villagers tended to eat a poorer diet, work harder, and suffer from more diseases. However, they also tended to produce more offspring and thus built up a much denser population. Moreover, primitive agriculture and herding often exhausted the soil in a few seasons, forcing the early farmers to move on and wrest new territory from the hunters. So powerful was this process that within a few thousand years most hunting and gathering peoples had been replaced by cultivators or herders in all continents except Australia. Along with their tendency to expand, food-producing societies are distinguished from hunter-gatherer societies by their emphasis on property. A hunter's principal assets are his weapon kit and his acquired skills. A farmer, by contrast, owns wealth in the form of land, herds, and the right to call on the labor of his friends and kinfolk. If he is lucky in these respects, or skillful in their management, he can both accumulate wealth and use it to buy the labor of others. By passing his wealth to his heirs, he can create a family of hereditary "notables" or headmen. In this way a stratified society emerges. Unmistakable indications of social stratification appeared within a few thousand years of the beginnings of agriculture. Within a few centuries more the process had culminated, in a few favored centers, in the appearance of complex societies in which specialist artisans and merchants plied their trades, a priestly elite presided over religious ceremonials, and a bureaucratic organization commanded the labor of the landless. And so, with the appearance of \Tcivilization\t, ended two million years of human prehistory. Clifford Jolly Bibliography: Bishop, W. W., and Miller, J. A., eds., Calibration of Hominoid Evolution (1972); Bordaz, Jacques, Tools of the Old and New Stone Age (1970); Butzer, K. W., Environment and Archaeology (1971); Butzer, K. W., and Isaac, G. L., eds., After the Australopithecines (1975); Clark, W. E. Le Gros, Antecedents of Man, 2d ed. (1971); DeVore, Irven, and Eimerl, Sarel, The Primates (1969); Eiseley, Loren, Darwin's Century: Evolution and the Men Who Discovered It (1958); Fagan, Brian M., The Great Journey (1987); Isaac, G. 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