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to be Homo because of its more humanlike (that is, less robust) cranium. He therefore proposed shifting the cerebral Rubicon to 600 cubic centimeters, thereby admitting the new Olduvai hominid to the genus Homo . This tactic surely raised the emotional level of the vigorous debate that ensued. Ultimately, however, the new definition was accepted. (It later developed that 650 cubic centimeters is rather small for the average adult brain size in Homo habilis; 800 cubic centimeters is a closer figure.)
    Scientific names aside, the important point here is that the pattern of evolution beginning to emerge from these findings was of two basic types of early human. One type had a small brain and large cheek teeth (the various australopithecine species); the second type had an enlarged brain and smaller cheek teeth (Homo) (see figure 2.2 ). Both types were bipedal apes, but something extraordinary had clearly happened in the evolution of Homo . We will explore this “something” more fully in the next chapter. In any case, anthropologists’ understanding of the shape of the family tree at this point in human history—that is, at around 2 million years ago—was rather simple.
    FIGURE 2.2
    Early Homo . This fossil, known by its museum acquisition number of 1470, was found in Kenya in 1972. It lived almost 2 million years ago and is the most complete early specimen of Homo habilis; it shows significant brain expansion and reduction in tooth size, compared with the australopithecines. (Courtesy of A. Walker and R. E. F. Leakey /Scientific American , 1978, all rights reserved.)
    The tree bore two main branches: the australopithecine species, all of which became extinct by 1 million years ago, and Homo , which eventually led to people like us.
    Biologists who have studied the fossil record know that when a new species evolves with a novel adaptation, there is often a burgeoning of descendant species over the next few million years expressing various themes on that initial adaptation—a burgeoning known as adaptive radiation. The Cambridge University anthropologist Robert Foley has calculated that if the evolutionary history of the bipedal apes followed the usual pattern of adaptive radiation, at least sixteen species should have existed between the group’s origin 7 million years ago and today. The shape of the family tree begins with a single trunk (the founding species), spreads out as new branches evolve through time, and then reduces in bushiness as species go extinct, leaving a single surviving branch— Homo sapiens . How does all this match up with what we know from the fossil record?
    For many years after the acceptance of Homo habilis , it was thought that 2 million years ago there were three australopithecine species and one species of Homo . We would expect the family tree to be heavily populated at this point in prehistory, so four coexisting species doesn’t sound like much. And in fact it has recently become apparent—through new discoveries and new thinking—that at least four australopithecines lived at that period, cheek by jowl with two or even three species of Homo . This picture is by no means settled, but if human species were like species of other large mammals (and there is no reason to think that they were not, at that point in our history), then such is what biologists would expect. The question is, What happened earlier than 2 million years ago? How many branches were there on the family tree, and what were they like?
    As noted, the fossil record quickly becomes sparse beyond 2 million years ago and blank further back much more than 4 million years ago. The earliest-known human fossils are all from East Africa. On the east side of Lake Turkana, we have found an arm bone, a wrist bone, jaw fragments, and teeth from around 4 million years ago; the American anthropologist Donald Johanson and his colleagues have recovered a leg bone of similar age from the Awash region of Ethiopia. These are slim pickings