Free The Origin of Humankind by Richard Leakey

thereafter would have been pregnant every three or four years. These patterns tell us that by the time of early Homo erectus , human ancestors had already moved in the direction of modern human biology and away from ape biology, while the australopithecines remained in their ape grade.
    The evolutionary shift by early Homo toward modern human patterns of growth and development occurred in a social context. All primates are social, but modern humans have developed sociability to the highest degree. The change in biology we inferred from the evidence of teeth in early Homo tells us that social interaction in this species had already begun to intensify, creating an environment that fostered culture. It appears that the entire social organization was significantly modified, too. How can we know this? It is evident from a comparison of the body size of males and females, and from what we know of such differences in modern primate species, such as baboons and chimpanzees.
    In savanna baboons, as noted earlier, males are twice the size of females. Primatologists now know that this size difference occurs when there is strong competition among mature males for mating opportunities. As in most primate species, male baboons, when they reach maturity, leave the troop into which they were born. They join another troop, often one nearby, and are from then on in competition with the males already established in the group. Because of this pattern of male migration, the males of most groups are usually unrelated to each other. They therefore have no Darwinian (that is, genetic) reason for cooperating with each other.
    However, in chimpanzees, for reasons that are not fully understood, males remain in their natal group and females transfer. As a consequence, the males in a chimpanzee group have a Darwinian reason for cooperating with each other in acquiring females, because as brothers they have half their genes in common. They cooperate in defending against other chimp groups, and on occasional hunting forays, when they usually try to corner a hapless monkey in a tree. This relative lack of competition and enhanced cooperation are reflected in the size of males compared with females: they are a mere 15 to 20 percent bigger.
    With regard to size, australopithecine males follow the baboon pattern. It is a reasonable assumption, therefore, that social life in australopithecine species was similar to what we see in modern baboons. When we were able to make a comparison of male and female body size in early Homo , it immediately became obvious that a significant shift had occurred: males were no more than 20 percent bigger than females, just as we see in chimpanzees. As the Cambridge anthropologists Robert Foley and Phyllis Lee have argued, this change in body-size differential at the time of the origin of the genus Homo surely represents a change in social organization, too. Very probably, early Homo males remained in their natal groups with their brothers and half brothers, while the females transferred to other groups. Relatedness, as I’ve indicated, enhances cooperation among the males.
    We can’t be certain what prompted this shift in social organization: enhanced cooperation among males must have been strongly beneficial for some reason. Some anthropologists have argued that defense against neighboring troops of Homo became extremely important. Just as likely, and perhaps more so, is a change centered on economic needs. Several lines of evidence point to a shift in diet for Homo —one in which meat became an important energy and protein source. The change in tooth structure in early Homo indicates meat eating, as does the elaboration of a stone-tool technology. Moreover, the increase in brain size that is part of the Homo package may even have demanded that the species supplement its diet with a rich energy source.
    As every biologist knows, brains are metabolically expensive organs. In modern humans, for example, the brain constitutes a