Free The Origin of Humankind by Richard Leakey Page A

mere 2 percent of body weight, yet consumes 20 percent of the energy budget. Primates are the largest-brained group of all mammals, and humans have extended this property enormously: the human brain is three times the size of the brain in an ape of equivalent body size. The anthropologist Robert Martin, of the Institute of Anthropology in Zurich, has pointed out that this increase in brain size could have occurred only with an enhanced energy supply: the early Homo diet, he notes, must have been not only reliable but nutritionally rich. Meat represents a concentrated source of calories, protein, and fat. Only by adding a significant proportion of meat to its diet could early Homo have “afforded” to build a brain beyond australopithecine size.
    For all these reasons, I suggest that the major adaptation in the evolutionary package of early Homo was significant meat eating. Whether early Homo hunted live prey or merely scavenged carcasses, or both, is a highly controversial issue in anthropology, as we will see in the next chapter. But I have no doubt that meat played an important part in our ancestors’ daily lives. Moreover, the new subsistence strategy of obtaining not just plant foods but meat as well probably demanded significant social organization and cooperation.
    Every biologist knows that when a basic change occurs in a species’ pattern of subsistence, other changes usually follow. Most often, such secondary changes involve the species’ anatomy, as it adapts to the new diet. We have seen that the tooth and jaw structure of early Homo is different from that of the australopithecines, presumably as an adaptation to a diet that includes meat.
    Very recently, anthropologists have come to believe that, in addition to dental differences, early Homo differed from the australopithecines in being a much more physically active creature. Two independent lines of research converged on the conclusion that early Homo was an efficient runner, the first human species to be so.
    A few years ago, the anthropologist Peter Schmid, a colleague of Robert Martin’s in Zurich, had an opportunity to study the famous Lucy skeleton. Using fiberglass casts of the fossil bones, Schmid began assembling Lucy’s body, with the full expectation that it would be essentially human in shape. He was surprised with what he saw: Lucy’s rib cage turned out to be conical in shape, like an ape’s, not barrel-shaped, as would be seen in humans. Lucy’s shoulders, trunk, and waist also turned out to have a strong apelike aspect to them.
    At a major international conference in Paris in 1989, Schmid described the implications of what he had found, and they are highly significant. Australopithecus afarensis , he said, “would not have been able to lift its thorax for the kind of deep breathing that we do when we run. The abdomen was potbellied, and there was no waist, so that would have restricted the flexibility that’s essential to human running.” Homo was a runner; Australopithecus was not.
    The second line of evidence that bore on this issue of agility flowed from Leslie Aiello’s work on body weight and stature. She obtained measures of these features in modern humans and apes and compared them with similar data gleaned from human fossils. Present-day apes are heavily built for their stature, being twice the bulk of a human of the same height. The fossil data, too, fell into a clear pattern—one that by now was becoming familiar. The australopithecines were apelike in their body build, while all Homo species were humanlike. Both Aiello’s findings and Schmid’s work are consistent with Fred Spoor’s discovery of the difference in anatomical structure of the inner ear in australopithecines and Homo: a greater commitment to bipedality goes along with the new body stature.
    I hinted in the previous chapter that major changes other than that of brain size occurred with the evolution of the genus Homo . We can see now what it was: