The Origin of Humankind

by Richard Leakey

  The earliest stone-tool assemblages that have been found are 2.5 million years old; they include, besides flakes, larger implements, such as choppers, scrapers, and various polyhedrons. In most cases, these items, too, were produced by the removal of several flakes from a lava cobble. Mary Leakey spent many years at Olduvai Gorge studying this earliest of technologies—which is known as the Oldowan industry, after Olduvai Gorge—and in so doing established early African archeology.

  As a result of his experimental toolmaking, Nicholas Toth suspects that the earliest toolmakers did not have the specific shapes of the individual tools in mind—a mental template, if you like—when they were making them. More likely, the various shapes were determined by the original shape of the raw material. The Oldowan industry—which was the only form of technology practiced until about 1.4 million years ago—was essentially opportunistic in nature.

  An interesting question arises about the cognitive skills implied by the production of these artifacts. Were the earliest toolmakers employing mental abilities comparable to those of apes, but in a different way? Or did it require them to be of higher intelligence? The brain of the toolmakers was some 50 percent bigger than that of apes, so the latter conclusion seems intuitively obvious. Nevertheless, Thomas Wynn, an archeologist at the University of Colorado, and William McGrew, a primatologist at the University of Stirling, Scotland, disagree. They analyzed certain manipulative skills displayed by apes, and in a paper they published in 1989, called “An Ape’s View of the Oldowan,” concluded: “All the spatial concepts for Oldowan tools can be found in the minds of apes. Indeed, the spatial competence described above is probably true of all great apes and does not make Oldowan tool-makers unique.”

  I find this statement surprising, not least because I have seen people try to make “Stone Age” tools by bashing two rocks together, with little success. That’s not how it was done. Nicholas Toth has spent many years perfecting techniques for making stone tools, and he has a good appreciation of the mechanics of flaking stone. To work efficiently, the stone knapper has to choose a rock of the correct shape, bearing the correct angle at which to strike; and the striking motion itself requires great practice in order to deliver the appropriate amount of force in the right place. “It seems clear that early tool-making proto-humans had a good intuitive sense of the fundamentals of working stone,” Toth wrote in a paper in 1985. “There’s no question that the earliest toolmakers possessed a mental capacity beyond that of apes,” he recently told me. “Toolmaking requires a coordination of significant motor and cognitive skills.”

  An experiment under way at the Language Research Center, in Atlanta, Georgia, is putting this question to the test. For more than a decade, Sue Savage-Rumbaugh, a psychologist, has been working with a pygmy chimpanzee on developing communication skills. Toth recently began a collaboration with her, to try to teach the chimp, named Kanzi, how to make stone flakes. Kanzi has undoubtedly displayed innovative thinking to produce sharp flakes, but so far he has not reproduced the systematic flaking technique used by the earliest toolmakers. I suspect this means that Wynn and McGrew are wrong and that the earliest toolmakers were using cognitive skills beyond those present in apes.

  That said, it remains true that the earliest tools, the Oldowan industry, were simple and opportunistic. About 1.4 million years ago in Africa, a new form of assemblage appeared, which archeologists call the Acheulean industry, named after the site of St. Acheul, in northern France, where these tools, in later versions, were first discovered. For the first time in human prehistory, there is evidence that the toolmakers had a mental template of what they wanted to produce—that they were intentionally imposing a shape on the raw material they used. The implement that suggests this is the so-called handaxe, a teardrop-shaped tool that required remarkable skill and patience to make (see figure 2.5). It took Toth and other experimenters several months to acquire the skill to produce handaxes of the quality that are found in the archeological record of the time.

  FIGURE 2.5

  Tool technologies. The bottom two rows are representative of the Oldowan technology, which first appears in the archeological record at about 2.5 million years ago and is comprised of a hammer-stone (the white cobble), simple choppers and scrapers (in the same row as the hammerstone), and small, sharp flakes (the row above). The top two rows are examples of items from the Acheulean industry, which first appeared in the record at about 1.4 million years ago and is characterized by handaxes (the two teardrop-shaped implements), cleavers, and picks, in addition to small tools similar to those found in Oldowan assemblages. (Courtesy of N. Toth.)

  The appearance of the handaxe in the archeological record follows the emergence of Homo erectus, the putative descendent of Homo habilis and ancestor of Homo sapiens. As we will see in the following chapter, it is a reasonable deduction that the makers of the handaxes were Homo erectus individuals, endowed as they were with a significantly larger brain than Homo habilis.

  When our ancestors discovered the trick of consistently producing sharp stone flakes, it constituted a major breakthrough in human prehistory. Suddenly, humans had access to foods that had previously been denied to them. The modest flake, as Toth has often demonstrated, is a highly effective implement for cutting through all but the toughest of hides to expose the meat inside. Whether they were hunters or scavengers, the humans who made and used these simple stone flakes thereby availed themselves of a new energy source—animal protein. Thus they would have been able not just to extend their foraging range but also to increase the chances for successful production of offspring. The reproductive process is an expensive business, and the expansion of the diet to include meat would have made it more secure.

  An age-old question for anthropologists has been, of course, Who made the tools? When tools appear in the archeological record, several australopithecine species existed, and probably several Homo species, too. How can we decide who the toolmaker was? This is extremely difficult. If we found tools in association only with fossils of Homo and never with australopithecines, that might be taken to imply that Homo was the only toolmaker. The prehistoric record is not as clear-cut as this, however. Randall Susman has argued from the anatomy of what he believes are A. robustus hand bones from a site in South Africa that this species had sufficient manipulative ability to make tools. But there is no way of being certain whether it actually did so or not.

  My own position is that we should look for the simplest explanation. We know from the prehistoric record that after 1 million years ago only Homo species existed, and we also know that they made stone tools. Until there is good reason to suppose otherwise, it seems cautiously wise to conclude that only Homo made tools earlier in prehistory. The australopithecine species and Homo clearly had different specific adaptations, and it is likely that meat eating by Homo was an important part of that difference. Stone toolmaking would have been an important part of a meat eater’s abilities; plant eaters could do without these tools.

  In his studies of tools from archeological sites in Kenya, and in his experimental toolmaking exercises, Toth made a fascinating and important discovery. The earliest toolmakers were predominantly right-handed, just as modern humans are. Although individual apes are preferentially right- or left-handed, there is no population preference; modern humans are unique in this respect. Toth’s discovery gives us an important evolutionary insight: some 2 million years ago, the brain of Homo was already becoming truly human, in the way we know ourselves to be.

  CHAPTER 3

  A DIFFERENT KIND OF HUMAN

  Exciting and imaginative research performed only recently has allowed us to use fossils to gain insights into aspects of the biology of our extinct ancestors in a way that no one could have predicted a few years ago. It is now possible, for example, to make reasonable estimates of when individuals of a particular human species were weaned, when they became sexually mature, what their life expectancy was, and so on. Armed with the means of uncovering information of this
type, we have come to see that Homo was a different kind of human right from its first appearance. The discovery of a biological discontinuity between Australopithecus and Homo has fundamentally changed our understanding of human prehistory.

  Until the appearance of Homo, all bipedal apes had small brains, large cheek teeth, and protruding jaws and pursued an apelike subsistence strategy. They ate mainly plant foods, and their social milieu probably resembled that of the modern savanna baboon. These species—the australopithecines—were humanlike in the way that they walked but in nothing more. At some time prior to 2.5 million years ago—we still can’t say exactly when—the first large-brained human species evolved. The teeth changed, too—probably an adaptation produced by a shift in diet from one made up exclusively of plant foods to one that included meat.

  These two aspects of the earliest Homo—the changes in brain size and tooth structure—have been apparent since the first fossils of Homo habilis were uncovered, three decades ago. Perhaps because we modern humans are dazzled by the importance of brain power, anthropologists have focused strongly on the jump in the size of the brain—from some 450 cubic centimeters to more than 600 cubic centimeters—that occurred with the evolution of Homo habilis. No doubt this was an important part of the evolutionary adaptation that took human prehistory in a new direction. But it was only a part. The new research into the biology of our ancestors reveals that many other things changed, too, moving them away from being apelike to being more like humans.

  One of the most significant aspects of human development is that infants are born virtually helpless and experience a prolonged childhood. Moreover, as every parent knows, children go through an adolescent growth spurt, during which they put on inches at an alarming rate. Humans are unique in this respect: most mammalian species, including apes, progress almost directly from infancy to adulthood. A human adolescent about to embark on his or her growth spurt is likely to increase in size by about 25 percent; by contrast, the steady trajectory of growth in chimpanzees means that the adolescent adds only 14 percent to its stature by the time it reaches maturity.

  Barry Bogin, a biologist at the University of Michigan, has an innovative interpretation of the difference in growth trajectories. The body’s growth rate in human children is low compared with that in apes, even though the rate of brain growth is similar. As a result, human children are smaller than they would be if they followed the normal simian growth rate. The benefit, Bogin suggests, has to do with the high degree of learning that young humans must achieve if they are to absorb the rules of culture. Growing children learn better from adults if there is a significant difference in body size, because a student-teacher relationship can be established. If young children were the size they would be on an apelike growth trajectory, physical rivalry rather than a student-teacher relationship might develop. When the learning period is over, the body “catches up,” by means of the adolescent growth spurt.

  Humans become human through intense learning not just of survival skills but of customs and social mores, kinship and social laws—that is, culture. The social milieu in which helpless infants are cared for and older children are educated is much more characteristic of humans than it is of apes. Culture can be said to be the human adaptation, and it is made possible by the unusual pattern of childhood and maturation.

  The helplessness of newborn human infants is, however, less a cultural adaptation than a biological necessity. Human infants come into the world too early, a consequence of our large brain and the engineering constraints of the human pelvis. Biologists have recently come to understand that brain size influences more than just intelligence. It correlates with a number of what are known as life-history factors, such as the age of weaning, the age at which sexual maturity is reached, gestation length, and longevity. In species with big brains, these factors tend to be stretched out: infants are weaned later than those in species with small brains, sexual maturity is reached later, gestation is longer, and individuals live longer. A simple calculation based on comparisons with other primates reveals that gestation length in Homo sapiens, whose average brain capacity is 1350 cubic centimeters, should be twenty-one months, not the nine months it actually is. Human infants therefore have a year’s growth to catch up on when they are born, hence their helplessness.

  Why has this happened? Why has nature exposed human newborns to the dangers of coming into the world too early? The answer is the brain. The brain of a newborn ape, on average about 200 cubic centimeters, is about half that of adult size. The required doubling in size occurs rapidly and early in the ape’s life. By contrast, the brains of human newborns are one-third the adult size, and triple in size in early, rapid growth. Humans resemble apes in that their brains grow to adult size early in life; thus, if, like the apes, they were to double their brain size, human newborns’ brains would have to measure 675 cubic centimeters. As every woman knows, giving birth to babies with normal-size brains is difficult enough, and sometimes life threatening. Indeed, the pelvic opening increased in size during human evolution, to accommodate the increasing size of the brain. But there were limits to how far this expansion could go—limits imposed by the engineering demands of efficient bipedal locomotion. The limit was reached when the newborn’s brain size was its present value—385 cubic centimeters.

  From an evolutionary point of view, we can say that in principle humans departed from the apelike growth pattern when the adult brain exceeded 770 cubic centimeters. Beyond this figure, brain size would have to more than double from birth, thus beginning the pattern of helplessness in infants who came into the world “too early.” Homo habilis, with an adult brain size of about 800 cubic centimeters, appears to be on the cusp between the ape growth pattern and that of the human being, while the brain of early Homo erectus, some 900 cubic centimeters, pushes the species significantly in the human direction (see figure 3.1). This, remember, is an argument “in principle”; it assumes that the birth canal in Homo erectus was the same size as it is in modern humans. In fact, we were able to get a clearer idea of how human Homo erectus had become in this respect from measurements of the pelvis of the Turkana boy, the early Homo erectus skeleton my colleagues and I unearthed in the mid-1980s not far from the western shore of Lake Turkana.

  FIGURE 3.1

  Homo erectus. (a), (b), and (c) show three views of the skull KNMER 3733, found east of Lake Turkana in 1975. This individual, with a brain capacity of 850 cubic centimeters, lived about 1.8 million years ago. For comparison, (d) shows a Homo erectus from China (Peking Man), which lived a million years later than 3733 and had a brain capacity of almost 1000 cubic centimeters. (Courtesy of W. E. Le Gros Clark/Chicago University Press, and A. Walker and R. E. F. Leakey/ Scientific American, 1978, all rights reserved.)

  In humans, the pelvic opening is similar in size in males and females. So, by measuring the size of the Turkana boy’s pelvic opening, we obtained a good estimate of the size of his mother’s birth canal. My friend and colleague Alan Walker, an anatomist at Johns Hopkins University, reconstructed the boy’s pelvis from bones that had been separate when we unearthed them (see figure 3.2). He measured the pelvic opening, found that it was smaller than in Homo sapiens, and calculated that the newborns of Homo erectus had brains of about 275 cubic centimeters, which is considerably smaller than the brain size of modern human newborns.

  The implications are clear. Homo erectus infants were born with brains one-third the adult size, as modern humans are, and, as modern humans do, must have come into the world in a helpless state. We can infer that the intense parental care of infants which is part of the modern human social milieu had already begun to develop in early Homo erectus, some 1.7 million years ago.

  We cannot do similar calculations for Homo habilis, the immediate ancestor of erectus, because we have yet to discover a habilis pelvis. But if habilis babies were born with erectus-size neonate brains, then they, too, would need to be born “too early,” but not by as much; they, too, would have been helples
s at birth, but not for as long; and they, too, would have required a humanlike social milieu, but to a lesser degree. It therefore seems that Homo moved in a human direction from the very beginning. Similarly, the aus-tralo-pithecine species had ape-size brains, and so would have followed an apelike pattern of early development.

  An extended period of helplessness in infancy—a period during which intensive parental care was required—was already characteristic of early Homo: this much we have established. But what of the remainder of childhood? When did this become prolonged, enabling practical and cultural skills to be absorbed, followed by an adolescent growth spurt?

  The prolongation of childhood in modern humans is achieved through a reduced rate of physical growth compared with that in apes. As a result, humans reach various growth milestones, such as tooth eruption, later than apes do. For instance, the first permanent molar appears in human children at about the age of six, compared with three in apes; the second molar erupts between the ages of eleven and twelve in humans and at age seven in apes; and the third molar shows up at eighteen to twenty in humans and nine in apes. In order to answer the question of when childhood became prolonged in human prehistory, we needed a way of looking at fossil jaws and determining when the molars erupted.