The Origin of Humankind

by Richard Leakey

  Anthropologists disliked Darwin’s suggestion intensely, not least because tropical Africa was regarded with colonial disdain: the Dark Continent was not viewed as a fit place for the origin of so noble a creature as Homo sapiens. When additional human fossils began to be discovered in Europe and in Asia at the turn of the century, yet more scorn was heaped on the idea of an African origin. This attitude prevailed for decades. In 1931, when my father told his intellectual mentors at Cambridge University that he planned to search for human origins in East Africa, he came under great pressure to concentrate his attention on Asia instead. Louis Leakey’s conviction was based partly on Darwin’s argument and partly, no doubt, on the fact that he was born and raised in Kenya. He ignored the advice of the Cambridge scholars and went on to establish East Africa as a vital region in the history of our early evolution. The vehemence of anthropologists’ anti-Africa sentiment now seems quaint to us, given the vast numbers of early human fossils that have been recovered in that continent in recent years. The episode is also a reminder that scientists are often guided as much by emotion as by reason.

  Darwin’s second major conclusion in The Descent of Man was that the important distinguishing features of humans—bipedalism, technology, and an enlarged brain—evolved in concert. He wrote:

  If it be an advantage to man to have his hands and arms free and to stand firmly on his feet, ... then I can see no reason why it should not have been more advantageous to the progenitors of man to have become more and more erect or bipedal. The hands and arms could hardly have become perfect enough to have manufactured weapons, or to have hurled stones and spears with true aim, as long as they were habitually used for supporting the whole weight of the body ... or so long as they were especially fitted for climbing trees.

  Here, Darwin was arguing that the evolution of our unusual mode of locomotion was directly linked to the manufacture of stone weapons. He went further and linked these evolutionary changes to the origin of the canine teeth in humans, which are unusually small compared to the dagger-like canines of apes. “The early forebears of man were ... probably furnished with great canine teeth,” he wrote in The Descent of Man; “but as they gradually acquired the habit of using stones, clubs, or other weapons for fighting with their enemies or their rivals, they would use their jaws and teeth less and less. La this case, the jaws, together with the teeth, would become reduced in size.”

  These weapon-wielding, bipedal creatures developed a more intense social interaction, which demanded more intellect, argued Darwin. And the more intelligent our ancestors became, the greater was their technological and social sophistication, which in turn demanded an ever-larger intellect. And so on, as the evolution of each feature fed on the others. This hypothesis of linked evolution was a very clear scenario of human origins, and it became central to the development of the science of anthropology.

  According to this scenario, the original human species was more than merely a bipedal ape: it already possessed some features we value in Homo sapiens. The image was so powerful and plausible that anthropologists were able to weave persuasive hypotheses around it for a very long time. But the scenario went beyond science: If the evolutionary differentiation of humans from apes was both abrupt and ancient, a considerable distance was inserted between us and the rest of nature. For those with a conviction that Homo sapiens is a fundamentally different kind of creature, this viewpoint offered comfort.

  Such a conviction was common among scientists in Darwin’s time, and well into this century, too. For instance, the nineteenth-century English naturalist Alfred Russel Wallace—who also invented the theory of natural selection, independently of Darwin—balked at applying the theory to those aspects of humanity we most value. He considered humans too intelligent, too refined, too sophisticated to have been the product of mere natural selection. Primitive hunter-gatherers would have had no biological need for these qualities, he reasoned, and so they could not have arisen by natural selection. Supernatural intervention, he felt, must have occurred to make humans so special. Wallace’s lack of conviction in the power of natural selection greatly upset Darwin.

  The Scottish paleontologist Robert Broom, whose pioneering work in South Africa in the 1930s and 1940s helped establish Africa as the cradle of mankind, also expressed strong views on human distinctiveness. He believed that Homo sapiens was the ultimate product of evolution and that the rest of nature had been shaped for its comfort. Like Wallace, Broom looked to supernatural forces in the origin of our species.

  Scientists such as Wallace and Broom were struggling with conflicting forces, one intellectual, the other emotional. They accepted the fact that Homo sapiens derived ultimately from nature through the process of evolution, but their belief in the essential spirituality, or transcendent essence, of humanity led them to construct explanations for evolution which maintained human distinctiveness. The evolutionary “package” embodied in Darwin’s 1871 description of human origins offered such a rationalization. Although Darwin did not invoke supernatural intervention, his evolutionary scenario made humans distinct from mere apes right from the beginning.

  Darwin’s argument remained influential until a little more than a decade ago, and was effectively responsible for a major dispute over when humans first appeared. I will describe the incident briefly, because it illustrates the seductiveness of Darwin’s linked-evolution hypothesis. It also marks the end of its sway over anthropological thinking.

  In 1961, Elwyn Simons, then at Yale University, published a landmark scientific paper in which he announced that a small apelike creature named Ramapithecus was the first known hominid species. The only fossil remains of Ramapithecus known at the time were parts of an upper jaw that had been found by a young Yale researcher, G. Edward Lewis, in India in 1932. Simons saw that the cheek teeth (the premolars and molars) were somewhat humanlike, in that they were flat rather than pointed, as ape teeth are. And he saw that the canines were shorter and blunter than those of apes. Simons also asserted that the reconstruction of the incomplete upper jaw would show it to be humanlike in shape—that is, an arch, broadening slightly toward the rear, and not a “U” shape, as in modern apes.

  At this time, David Pilbeam, a British anthropologist from Cambridge University, joined Simons at Yale, and together they described these supposedly humanlike anatomical features of the Ramapithecus jaw. They went further than anatomy, however, and suggested, on the strength of the jaw fragments alone, that Ramapithecus walked upright on two feet, hunted, and lived in a complex social environment. Their reasoning was like Darwin’s: the presence of one putative hominid feature (tooth shape) implied the existence of the rest. Thus, what was thought to be the very first hominid species came to be viewed as a cultural animal—that is, as a primitive version of modern humans rather than as an acultural ape.

  The sediments from which the original Ramapithecus fossils were recovered were ancient, as were those yielding subsequent similiar discoveries in Asia and Africa. Simons and Pilbeam therefore concluded that the first humans appeared at least 15 million years ago, and possibly 30 million years ago, and this view was accepted by the vast majority of anthropologists. Moreover, the belief in so ancient an origin placed a comforting distance between humans and the rest of nature, which many welcomed.

  In the late 1960s, two biochemists at the University of California, Berkeley, Allan Wilson and Vincent Sarich, came to a very different conclusion about when the first human species evolved. Instead of working with fossils, they compared the structure of certain blood proteins from living humans and African apes. Their aim was to determine the degree of structural difference between human and ape proteins—a difference that should increase at a calculable rate with time, as a result of mutation. The longer humans and apes had been separate species, the greater the number of mutations that would have accumulated. Wilson and Sarich calculated the mutation rate and were therefore able to use their blood-protein data as a molecular clock.

  According to the c
lock, the first human species evolved only about 5 million years ago, a finding that was dramatically at variance with the 15 to 30 million years of prevailing anthropological theory. Wilson and Sarich’s data also indicated that the blood proteins of humans, chimpanzees, and gorillas are equally different from each other. In other words, some kind of evolutionary event 5 million years ago caused a common ancestor to split in three directions simultaneously—a split that led to the evolution not only of modern humans but of modern chimpanzees and modern gorillas. This, too, was contrary to what most anthropologists believed. According to conventional wisdom, chimpanzees and gorillas are each other’s closest relatives, with humans standing a great distance apart. If the interpretation of the molecular data was valid, then anthropologists would have to accept a much closer biological relationship between humans and apes than most believed.

  An almighty dispute erupted, with anthropologists and biochemists criticizing each other’s professional techniques in the strongest of language. Wilson and Sarich’s conclusion was criticized on the ground, among others, that their molecular clock was erratic and therefore could not be relied upon to give an accurate time for past evolutionary events. Wilson and Sarich, for their part, argued that anthropologists placed too much interpretive weight on small, fragmentary anatomical features, and were thus led to invalid conclusions. I sided with the anthropological community at the time, believing Wilson and Sarich to be incorrect.

  The debate raged for more than a decade, during which time more and more molecular evidence was produced—by Wilson and Sarich and also independently by other researchers. The great majority of these new data supported Wilson and Sarich’s original contention. The weight of this evidence began to shift anthropological opinion, but the change was slow. Finally, in the early 1980s, discoveries of much more complete specimens of Ramapithecus-like fossils, by Pilbeam and his team in Pakistan and by Peter Andrews, of London’s Natural History Museum, and his colleagues in Turkey, settled the issue (see figure 1.1).

  The original Ramapithecus fossils are indeed humanlike in some ways, but the species was not human. The task of inferring an evolutionary link based on extremely fragmentary evidence is more difficult than most people realize, and there are many traps for the unwary. Simons and Pilbeam had been ensnared in one of those traps: anatomical similarity does not unequivocally imply evolutionary relatedness. The more complete specimens from Pakistan and Turkey revealed that the putative humanlike features were superficial. The jaw of Ramapithecus was V-shaped, not an arch; this and other features indicated that it was a species of primitive ape (the jaw of modern apes is U-shaped). Ramapithecus had lived a life in the trees, like its later relative the orangutan, and was not a bipedal ape, still less a primitive hunter-gatherer. Even diehard Rama-pithecus-dLS-hommid anthropologists were persuaded by the new evidence that they had been wrong and Wilson and Sarich had been right: the first species of bipedal ape, the founding member of the human family, had evolved relatively recently and not in the deep past.

  Although in their original publication Wilson and Sarich had proposed a date of 5 million years ago for this event, the consensus of molecular evidence these days pushes it back to close to 7 million years ago. There has, however, been no retreat from the proposed biological intimacy between humans and African apes. If anything, that relationship may be even more intimate than had been supposed. Although some geneticists believe that the molecular data still implies an equal three-way split between humans, chimpanzees, and gorillas, others see it differently. In their view, humans and chimpanzees are each other’s closest relatives, with gorillas at the greater evolutionary distance.

  FIGURE 1.1

  Molecular evidence. Before 1967, anthropologists interpreted fossil evidence as indicating an ancient evolutionary divergence between humans and apes: at least 15 million years ago. But in that year, molecular evidence was presented that showed the divergence to be much more recent: close to 5 million years ago. Anthropologists were reluctant to accept the new evidence but eventually did so.

  The Ramapithecus affair changed anthropology in two ways. First, it demonstrated the perils of inferring a shared evolutionary relationship from shared anatomical features. Second, it exposed the folly of a slavish adherence to the Darwinian “package.” Simons and Pilbeam had imputed a complete lifestyle to Ramapithecus, based on the shape of the canine teeth: if one hominid feature was there, all such features were assumed to be present. As a result of the undermining of the hominid status of Ramapithecus, anthropologists began to be uncertain about the Darwinian package.

  Before we follow the course of this anthropological revolution, we should look briefly at some of the hypotheses that over the years have been proposed to explain how the first hominid species might have arisen. It is interesting that as each new hypothesis gained popularity, it often reflected something of the social climate of the time. For instance, Darwin saw the elaboration of stone weapons as important in initiating the evolutionary package of technology, bipedalism, and expanded brain size. The hypothesis surely reflected the prevailing notion that life was a battle and progress was won by initiative and effort. This Victorian ethos permeated science, and shaped the way the process of evolution, including human evolution, was viewed.

  In the early decades of this century, the heyday of Edwardian optimism, the brain and its higher thoughts were said to have made us what we are. Within anthropology, this prevailing social worldview was expressed in the notion that human evolution had been propelled initially not by bipedalism but by an expanding brain. By the 1940s, the world was in thrall to the magic and power of technology, and the “Man the Toolmaker” hypothesis became popular. Proposed by Kenneth Oakley of the Natural History Museum, London, this hypothesis held that the making and using of stone tools—not weapons—provided the impulse for our evolution. And when the world was in the shadow of the Second World War, a darker differentiation of humans from apes was emphasized—that of violence against one’s fellows. The notion of “Man the Killer Ape,” first proposed by the Australian anatomist Raymond Dart, gained wide adherence, possibly because it seemed to explain (or even excuse) the horrible events of the war.

  Later, in the 1960s, anthropologists turned to the hunter-gatherer way of life as the key to human origins. Several research teams had been studying modern populations of technologically primitive people, particularly in Africa, most notable among whom were the !Kung San (incorrectly called Bushmen). There emerged an image of people in tune with nature, exploiting it in complex ways while respecting it. This vision of humanity coincided well with the environmentalism of the time, but anthropologists were in any case impressed by the complexity and economic security of the mixed economy of hunting and gathering. Hunting, however, was what was emphasized. In 1966, a major anthropological conference entitled “Man the Hunter” was held at the University of Chicago. The overriding tenor of the gathering was simple: hunting made humans human.

  Hunting is generally a male responsibility in most technologically primitive societies. It is therefore not surprising that the growing awareness of women’s issues in the 1970s threw into question this male-centered explanation of human origins. An alternative hypothesis, known as “Woman the Gatherer,” held that as in all primate species, the core of society is the bond between female and offspring. And it was the initiative of human females in inventing technology and gathering food (principally plants) which could be shared by all that led to the formation of a complex human society. Or so it was argued.

  Although these hypotheses differed in what was claimed as the principal mover in human evolution, all have in common the notion that the Darwinian package of certain valued human characteristics was established right from the beginning: the first hominid species was still thought to possess some degree of bipedalism, technology, and increased brain size. Hominids were therefore cultural creatures—and thus distinct from the rest of nature—right from the start. In recent years, we have come to recognize tha
t this is not the case.

  In fact, concrete evidence of the inadequacy of the Darwinian hypothesis is to be found in the archeological record. If the Darwinian package were correct, then we would expect to see the simultaneous appearance in the archeological and fossil records of evidence for bipedality, technology, and increased brain size. We don’t. Just one aspect of the prehistoric record is sufficient to show that the hypothesis is wrong: the record of stone tools.

  Unlike bones, which only rarely become fossilized, stone tools are virtually indestructible. Much of the prehistoric record is therefore made up of them, and they are the evidence on which the progress of technology from its simplest beginnings is constructed.

  The earliest examples of such tools—crude flakes, scrapers, and choppers made from pebbles with a few flakes removed—appear in the record about 2.5 million years ago. If the molecular evidence is correct and the first human species appeared some 7 million years ago, then almost 5 million years passed between the time our ancestors became bipedal and the time when they started making stone tools. Whatever the evolutionary force that produced a bipedal ape, it was not linked with the ability to make and use tools. However, many anthropologists believe that the advent of technology 2.5 million years ago did coincide with the beginnings of brain expansion.

  The realization that brain expansion and technology are divorced in time from human origins forced anthropologists to rethink their approach. As a result, the latest hypotheses have been framed in biological rather than cultural terms. I consider this a healthy development in the profession—not least because it allows ideas to be tested by comparing them with what we know of the ecology and behavior of other animals. In so doing, we don’t have to deny that Homo sapiens has many special attributes. Instead, we look for the emergence of those attributes from a strictly biological context.