The Origin of Humankind by Richard Leakey Read Free Book Online Page A

pattern seen previously from this time period—that of a great range of body sizes—is confirmed and even extended by the new finds. How is this fact to be interpreted? Is the issue of one species or more at the brink of resolution?
    Unfortunately it is not. Those who considered that the size range of the previously discovered fossils indicated a difference in stature between males and females viewed the new ones as supporting that position. Those of us who suspected that so broad a size range must indicate a difference between species, not a within-species difference, interpreted the new fossils as strengthening that view. The shape of the family tree earlier than 2 million years ago must therefore be regarded as an unresolved question.
    The discovery of the Lucy partial skeleton in 1974 seemed to offer the first glimpse of the degree of anatomical adaptation to bipedal locomotion in an early hominid. By definition, the first hominid species to have evolved, some 7 million years ago, would have been a bipedal ape of sorts. But until the Lucy skeleton came along, anthropologists had no tangible evidence of bipedalism in a human species older than about 2 million years. The bones of the pelvis, legs, and feet in Lucy’s skeleton were vital clues to this question.
    FIGURE 2.4
    Family trees. The existing fossil evidence is interpreted differently by different scholars, although the overall shape of the inferred evolutionary history is similar. Two versions are presented here, somewhat simplified. My preference is for B, in which specimens of the genus Homo are among the earliest known fossils; this would be ancestral to what we know as Homo habilis . The fossil record does not extend back as far as the origin of the human family—some 7 million years ago, as inferred from molecular genetic evidence.
    From the shape of the pelvis and the angle between the thighbone and knee, it is clear that Lucy and her fellows were adapted to some form of upright walking. These anatomical features were much more humanlike than apelike. In fact, Owen Lovejoy, who performed the initial anatomical studies on these bones, concluded that the species’ bipedal locomotion would have been indistinguishable from the way you and I walk. Not everyone agreed, however. For instance, in a major scientific paper in 1983 Jack Stern and Randall Susman, two anatomists at the State University of New York, Stony Brook, offered a different interpretation of Lucy’s anatomy: “It possesses a combination of traits entirely appropriate for an animal that had traveled well down the road toward full-time bipedality, but which retained structural features that enabled it to use the trees efficiently for feeding, sleeping or escape.”
    One of the crucial pieces of evidence that Stern and Susman adduced in favor of their conclusion was the structure of Lucy’s feet: the bones are somewhat curved, as is seen in apes but not in humans—an arrangement that would facilitate tree climbing. Lovejoy discounts this view and suggests that the curved foot bones are a mere evolutionary vestige of Lucy’s apelike past. These two opposing camps enthusiastically maintained their differences of opinion for more than a decade. Then, early in 1994, new evidence, including some from a most unexpected source, seemed to tip the balance.
    First, Johanson and his colleagues reported the discovery of two 3-million-year-old arm bones, an ulna and a humerus, that they attribute to Australopithecus afarensis . The individual had obviously been powerful, and its arm bones had some features similar to those seen in chimpanzees while others were different. Commenting on the discovery, Leslie Aiello, an anthropologist at University College, London, wrote in the journal Nature: “The mosaic morphology of the A. afarensis ulna, together with the heavily muscled and robust humerus, would be ideally suited to a creature which climbed in the trees but also walked on two legs when on the