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Authors: Richard Leakey
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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 helpless 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.
FIGURE 3.2
Turkana boy. The reconstructed skeleton of this nine-year-old Homo erectus shows how very humanlike this species was in its bodily structure. Alan Walker, who directed the excavation of the skeleton, stands by its side. (Courtesy of A. Walker/National Museum of Kenya.)
For example, the Turkana boy died just as his second molar was beginning to show through. If Homo erectus followed the slower, human pattern of childhood development, this would mean that the boy died when he was about eleven years old. If, however, the species had an apelike growth trajectory, he would have been seven. In the early 1970s, Alan Mann, of the University of Pennsylvania, performed an extensive analysis of fossil human teeth and concluded that all species of Australopithecus and Homo followed the human pattern of slowed childhood growth. His work was extremely influential, and bolstered the conventional wisdom that all hominid species, including the australopithecines, followed the modern human pattern. Indeed, when we found the Turkana boy’s jaw and I saw the second molar erupting, I assumed that he had been eleven when he died, because that is what he would have been were he like Homo sapiens . Likewise, the Taung child, a member of the species Australopithecus africanus , was thought to have died at the age of seven, because his first molar
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 helpless 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.
FIGURE 3.2
Turkana boy. The reconstructed skeleton of this nine-year-old Homo erectus shows how very humanlike this species was in its bodily structure. Alan Walker, who directed the excavation of the skeleton, stands by its side. (Courtesy of A. Walker/National Museum of Kenya.)
For example, the Turkana boy died just as his second molar was beginning to show through. If Homo erectus followed the slower, human pattern of childhood development, this would mean that the boy died when he was about eleven years old. If, however, the species had an apelike growth trajectory, he would have been seven. In the early 1970s, Alan Mann, of the University of Pennsylvania, performed an extensive analysis of fossil human teeth and concluded that all species of Australopithecus and Homo followed the human pattern of slowed childhood growth. His work was extremely influential, and bolstered the conventional wisdom that all hominid species, including the australopithecines, followed the modern human pattern. Indeed, when we found the Turkana boy’s jaw and I saw the second molar erupting, I assumed that he had been eleven when he died, because that is what he would have been were he like Homo sapiens . Likewise, the Taung child, a member of the species Australopithecus africanus , was thought to have died at the age of seven, because his first molar
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