• A principal force in the shaping of human evolution has been the nature of human society. After splitting from the apes, those in the human line of descent evolved upright stature and developed dark skin in place of the ape’s body hair. But the most significant change—a steady increase in brain size—probably evolved in response to the most critical aspect of the environment, the society in which an individual lived. Judging whom to trust, forming alliances, keeping score of favors given and received—all were necessities made easier by greater cognitive ability. By 50,000 years ago, the social benefits of more efficient communication had prompted the evolution of a novel ability possessed by no other social species, the faculty of language.
• The human physical form was attained first, followed by continued evolution of human behavior. Anatomically modern humans, people whose physical remains resemble the skeletons of people today, became common 100,000 years ago. But they showed no sign of the advanced behaviors that emerged 50,000 years later, probably made possible by the evolution of language. With this new faculty and the greater social cohesion it provided, the first behaviorally modern humans were able to break out of Africa and displace the archaic humans like the Neanderthals who had left Africa many thousands of years previously.
• Most of human prehistory occurred in, and was shaped by, the last ice age. The first modern humans to leave Africa probably crossed over the Red Sea at its southern end and into Arabia. Reaching India, the population went separate ways. One group traveled along the coasts of southeast Asia, arriving in Australia some 46,000 years ago. Another explored the land route northwest from India, reaching Europe and slowly evicting the Neanderthals from their ancient homeland. The expansion into the cold northern latitudes of Eurasia required technical innovation and probably genetic adaptations too. Then a climatic catastrophe, the return of the glaciers 20,000 years ago, emptied Europe and Siberia of people. Descendants of the survivors spread north again several thousand years later as the Pleistocene ice age drew to a close. Some of these new northerners, the Siberians in the eastern half of Eurasia, contrived the first domestication, that of the dog, and discovered the land bridge that then joined Siberia to Alaska and the Americas.
• The adaptations for three principal social institutions, warfare, religion and trade, had evolved by 50,000 years ago. The ancestral human population, the first to possess the power of fully articulate modern speech, may have numbered only 5,000 people, confined to a homeland in northeast Africa. These ancestral people, though less cognitively advanced than people today, possessed all the distinctive features of human nature and had developed, at least in rudimentary form, the institutions that are found in societies throughout the world. These may have included warfare centered round a defense of territory, religious ceremony as a means of social cohesion, and an instinct for reciprocity that governed social relations within the group and trade with those outside it.
• The ancestral people had a major limitation to overcome: they were too aggressive to live in settled communities. Early human societies lived as small bands of hunter-gatherers, their existence dominated by incessant warfare. For 35,000 years after leaving the ancestral homeland, these nomads were unable to settle down. Only gradually did humans evolve to become less aggressive. The tempo of warfare eased and a more gracile, or delicately boned, human form evolved in populations throughout the world. In the Near East, around 15,000 years ago, people at last accomplished a decisive social transition, the founding of the first settled communities. In place of the hunter-gatherers’ egalitarianism and lack of possessions, people in settled societies developed a new social structure with elites, specialization of roles, and ownership of property. Human groups started for the first time to produce storable surpluses of food and other products, which led to more complex societies and to increased trade between groups.
• Human evolution did not halt in the distant past but has continued to the present day. The ancestral human population of 50,000 years ago differed greatly from the anatomically modern humans of 100,000 years ago, and people today have had just as long to evolve away from the ancestral population. The human genome bears many marks of recent evolution, prompted by adaptations to events such as cultural changes or new diseases.More visible evidence of recent evolution is the existence of human races. After the dispersal of the ancestral population from Africa 50,000 years ago, human evolution continued independently in each continent. The populations of the world’s major geographical regions bred for many thousand years in substantial isolation from each other and started to develop distinctive features, a genetic differentiation which is the basis for today’s races. But these separate evolutionary paths were to some extent parallel as people in different continents responded to the same challenges. Gracilization occurred worldwide. Lactose tolerance, the genetic ability to digest lactose in adulthood, evolved among cattle-herding people in Europe 5,000 years ago but also among pastoral peoples of Africa and the Middle East.
• People probably once spoke a single language from which all contemporary languages are derived. Just as the ancestral population, after its dispersal, diverged into different races and ethnic groups, the ancestral tongue split into a growing family of different languages. Some of these languages expanded under the influence of factors such as warfare or agriculture, so that certain language families, like Indo-European, are now spoken over large areas while others, like many in South America or New Guinea, have a range of a few miles. Because language splits follow population splits, the genealogy of human languages must mirror, to some extent, the tree of descent of human populations; some biologists hope that the genealogy of human languages can be reconstructed far into the past, perhaps even near to its root, the mother tongue of the ancestral human population.
• The human genome contains excellent records of the recent past, providing a parallel history to the written record. The genome evolves so fast that whenever any community starts to breed in isolation, whether for reasons of religion, geography or language, within a few centuries its genetics assume a distinctive signature. DNA sheds a novel light on the history of peoples such as Jews, Icelanders and the inhabitants of the British Isles. It records the genetic impact of male dynasties like those of the Mongols and the Manchus. And for those who know to ask the right questions, it retains the secret family history of individuals such as Thomas Jefferson.
The compilers of the book of Genesis did their best, from available myths and legends, to frame a coherent account of human origins. They sought to address such questions as why people speak so many languages, suffer pain in childbirth and wear clothes to conceal their nakedness. Human origins can now be explained in another way. Given that so little has been preserved of the distant human past, it is remarkable how much is now being retrieved. Many of the findings described here have been made in the last few years. Though the frontiers of science are turbulent, throwing up many claims that require revision in light of further evidence, the flood of new findings described in these pages includes many unmistakable advances. The biological framework of human origins and nature is beginning to emerge with surprising clarity as the record of past evolutionary change now streams forth from the sequence of the human genome. In the long search to understand ourselves, our obscure origins, our strange and contradictory nature, and the fragmentation of the once united human family into different races and warring cultures speaking thousands of different languages, we can begin at last to comprehend the long darkness before the dawn.
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METAMORPHOSIS
In each great region of the world the living mammals are closely related to the extinct species of the same region. It is therefore probable that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; and as these two species are now man’s nearest allies, it is somewhat more probable that our early progenitors lived on the African continent than elsewhere.
CHARLES DARWIN, THE DESCENT OF MAN
F
IFTY THOUSAND YEARS AGO, in the northeastern corner of Africa, a small and beleaguered group of people prepared to leave their homeland. The world then was still in the grip of the Pleistocene ice age. Much of Africa had been depopulated and the ancestral human population had recently dwindled to a mere 5,000.
Those departing, a group of perhaps just 150 people, planned to leave Africa altogether. Forsaking their familiar habitat was a serious risk since, as hunters and gatherers, their survival depended on intimate knowledge of local plants and animals. Nor is long distance travel ever easy for foragers who own no pack animals and must carry every necessity—weapons, infants, food and water.
The emigrants faced another danger in the world beyond. The lands outside Africa were not unoccupied. About 1.8 million years ago, during a warm interlude before the Pleistocene ice age began, early humans had left Africa in one or more migrations. Once separated from the main human population in the African homeland, these archaic people had followed their own evolutionary paths and in the course of time had become the distinct species known as Homo erectus and Homo neanderthalensis. Erectus settled in East Asia. The Neanderthals occupied Europe and intermittently parts of the Near East.
The Neanderthals in particular were formidable adversaries. They had large brains, larger in absolute size even than those of contemporary people, and were heavily muscled. They had developed serious weaponry, including stone-tipped thrusting spears. They surrounded or occupied the main exit point from Africa at the southeastern corner of the Mediterranean, including the area that is now Israel. The human lineages evolving in Africa may have tried many times to escape into the world beyond. But none had succeeded, and the Neanderthals’ encirclement of the exits from northeast Africa seems a likely reason.
Why was the little group that left 50,000 years ago able to succeed when all earlier emigrants had failed? What drove them to take such a chance? What ties bound these people together and gave them the means to prevail?
To address such questions requires stepping back to the point in evolutionary time when the human line of descent split from the chimpanzee line. This is the moment at which our ancestors started to acquire the first of the adaptations that differentiated them from apes. These changes affected not just physical form but also the set of behaviors that together make up human nature. Of particular importance are the social behaviors, because both apes and people survive not as individuals but in social groups. In this sense the essence of human evolution is the transformation of ape society into human society.
Given the acute social intelligence of chimpanzees and bonobos, the emergence of the human society was perhaps not so great a leap. But the human lineage had the fortune to move down evolutionary paths that enlarged the brain and made possible the acquisition of language. The reason the ancestral human population was eventually able to burst out from its homeland seems to have been that 5 million years after having parted company with apes, it had at last perfected this critical component of human sociality.
Transforming Ape Society into Human Society
The ape society from which humans evolved lived some 5 million years ago somewhere in equatorial Africa. No fossil remains of these ancestral apes have yet been identified with certainty. Yet much can be inferred about them through the study of the two other living species descended from them, chimpanzees and their cousins the bonobos.
There are several reasons to suppose that the ancestral apes were very chimplike—an important assumption, if true, because it means that today’s chimpanzees serve as a reasonably close surrogate for them. One is that gorillas, which split off from the ape lineage before the human-chimp split, are themselves rather chimplike, suggesting that so too was the lineage that led to the human-chimp ancestor. Another is that the earliest fossils on the human side of the chimp-human split are quite hard to distinguish from chimpanzees. A third reason is that the chimps of west and east Africa are very similar to each other, in both looks and behavior, despite having split apart 1.5 million years ago. Given that they have changed hardly at all in the last 1.5 million years, they may well have been just as conservative in the previous 3.5 million years.
The likely reason for this lack of change is that chimps still live mostly in forest, as did the joint ancestor, whereas the human lineage at some early stage left its forest home and took its chances in the open woodland, adapting to a quite different set of challenges. Chimps could stay much as they were because they were never under great evolutionary pressure to adapt to new environments.
If no fossils of the joint ancestor have yet been found, how can anyone know when it lived? The answer comes from genetics. By estimating the number of differences between corresponding stretches of DNA in the great apes and people, geneticists can construct a family tree whose branches are proportional in length to the evolutionary distances between the various species. The tree implies that the split between chimps and people occurred just over 5 million years ago (the most recent estimate suggests between 4.6 and 6.2 million years ago).a3
Genetic comparison also indicates why it is that chimps are the closest living species to humans. The chimp branches show four living subdivisions—and a fifth if the bonobo is counted—whereas the human branch is unnaturally straight, as if all competing human lines had fallen extinct, perhaps because they were pruned away by members of the one surviving lineage.4
The estimate of 5 million years for the chimp-human divergence fits quite well with a salient event in the earth’s climatic history. Global climate cooled between 10 and 5 million years ago, with the period from 6.5 to 5 million years being particularly harsh. Water was locked up in massive glaciers, and sea level fell so low that the Mediterranean sea was repeatedly drained, depriving Africa of a source of moisture. In the dry, cold climate, the equatorial forests shrank and in places fragmented into woodland.5 In such a habitat, with open canopy and large spaces between trees, forest tree dwellers would have had to spend more time on the ground, at considerably greater risk from large predators. These cruel years placed such stress on the forest apes that many went extinct.
FIGURE 2.1. FAMILY TREE OF HUMANS AND OTHER GREAT APES.
It is just this kind of environmental stress that has forced evolutionary change since the beginning of life on earth. In response to new conditions, a species must adapt, by drawing on the genetic variation available in the members of its population. The individuals better endowed to meet the new conditions will thrive and leave more descendants. As the descendants continue to adapt to the new conditions, their genetic constitution, over the course of the generations, will differ increasingly from that of their ancestors.
The drought that occurred in Africa 5 million years ago may have been the agent that forced evolutionary change on the apes ancestral to both chimps and humans. Despite the serious lack of fossil evidence, much can be inferred about the joint human-chimp ancestor. Its population numbered somewhere between 50,000 and 100,000 breeding individuals, according to genetic calculations.6 Assuming that its way of life resembled that of chimpanzees, it would have lived in communities about a hundred or so strong, structured around a group of related males. By inference from chimpanzee society, the males would have defended their territory aggressively against the males of neighboring communities, with frequent fatalities. Each community’s survival strategy lay in defending as large as possible an area of fruit trees for its females to feed in.
The male apes, again on the assumption the species resembled chimpanzees, were much larger than the females and paid little attention to them except to mate. There was no particular bond between male and female. Each sex had its own hierarchy, with the females subservient to the males. The males spent their day, when they weren’t fighting, building alliances with other males and trying to work their way up the male hierarchy of their community. The risks of being alpha male were considerable, but the payoff considerable, at least in Darwinian terms: the alpha male and his allies got to father most of the community’s offspring
.
So consider this population of 100,000 chimplike apes somewhere in the eastern side of equatorial Africa 5 million years ago. Times are tough and their forest homeland is shrinking. The trees no longer carry enough fruit. The apes are forced to spend a lot of time on the ground searching for other sources of food. Large cats stalk or ambush the unwary. Each generation is tested by this harsh new environment, and in each generation the better adapted produce more offspring.
There are two kinds of survivor. One, clinging to the remnants of forest, manages to continue in much the same way of life: this is the lineage that leads to chimpanzees, and because it clings to the same habitat it has no great need to change its way of life or physical form.
The other manages to survive by venturing into a new niche—it learns to occupy both the trees and the new spaces that have appeared in between them. Helping it survive on the ground is the emergence of a critical new ability—that of walking on two feet.
Two-footedness—bipedalism in paleoanthropologists’ parlance—is the first great stride toward becoming human. Standing upright is in fact not so big an adjustment for apes, who move in the trees by hanging under branches and swinging from one arm to the other. Monkeys, on the other hand, prefer to run along the tops of branches; so when some of them started to live on the ground—becoming baboons—they preserved their four-footed style of travel.
Chimpanzees get around on the ground by knuckle-walking—using the knuckles of the hand as front feet. So why did those on the human side of the split prefer bipedalism? Many advantages of bipedalism have been cited as decisive—it frees up the hands for carrying things, it allows better surveillance of the surroundings—but the most likely reason for its emergence is simply that upright walking is more efficient than knuckle-walking. For the same expense of energy, a chimp can knuckle-walk 6 miles a day but a man can walk 11.7 Bipedalism probably evolved because it was a better way of getting about, and its other benefits were at first incidental.
Before the Dawn: Recovering the Lost History of Our Ancestors Page 2