The Origin of Humankind Read Online Free Page B

realization in place, the task of the anthropologist in accounting for human origins refocused on the origin of bipedalism. Even stripped down to this single event, the evolutionary transformation was not trivial, as Owen Lovejoy, an anatomist at Kent State University, has noted. “The move to bipedalism is one of the most striking shifts in anatomy you can see in evolutionary biology,” he wrote in a popular article in 1988. “There are important changes in the bones, the arrangement of the muscles that power them, and the movement of the limbs.” A glance at the pelvises of humans and chimpanzees is sufficient to confirm this observation: In humans, the pelvis is squat and boxlike, while in chimps it is elongated; and there are major differences in the limbs and trunk, too (see figure 1.2 ).
    The advent of bipedalism is not just a major biological transformation but a major adaptive one as well. As I argued in the preface, the origin of bipedal locomotion is so significant an adaptation that we are justified in calling all species of bipedal ape “human.” This is not to say that the first bipedal ape species possessed a degree of technology, increased intellect, or any of the cultural attributes of humanity. It didn’t. My point is that the adoption of bipedalism was so loaded with evolutionary potential—allowing the upper limbs to be free to become manipulative implements one day—that its importance should be recognized in our nomenclature. These humans were not like us, but without the bipedal adaptation they couldn’t have become like us.
    What were the evolutionary factors that promoted the adoption of this novel form of locomotion in an African ape? The popular image of human origins often includes the notion of an apelike creature leaving the forests and striding onto the open savanna. A dramatic image no doubt, but entirely inaccurate, as has recently been demonstrated by researchers at Harvard and Yale Universities who have analyzed soil chemistry in many parts of East Africa. The African savannas, with their great migrating herds, are relatively recent in the environment, developing less than 3 million years ago, long after the first human species evolved.
    FIGURE 1.2
    Different modes of locomotion. The shift from quadrupedal to bipedal locomotion demanded substantial changes in the body’s anatomical structure. For instance, humans have longer hind limbs, shorter fore-limbs, a squatter pelvis, shorter and noncurved digits, and a reduced lumbar region, compared with chimpanzees and gorillas. Australopithecus afarensiSy the earliest-known hominid, undoubtedly was a biped, but retained some anatomical features of tree dwellers. (Courtesy of John Fleagle/Academic Press.)
    If we take our minds back to an Africa of 15 million years ago, we find a carpet of forest from west to east, home to a great diversity of primates, including many species of monkeys and apes. In contrast with the situation today, ape species greatly outnumbered monkey species. Geological forces were stirring, however, which would dramatically alter the terrain and its occupants during the next few million years.
    The earth’s crust was tearing itself apart beneath the eastern part of the continent, in a line from the Red Sea through present-day Ethiopia, Kenya, Tanzania, and into Mozambique. As a result, the land rose blisterlike in Ethiopia and Kenya, forming great highlands more than 9000 feet in altitude. These great domes transformed not only the continent’s topography but its climate. Disrupting the previously uniform west-to-east airflow, the domes threw the lands to the east into rain shadow, depriving the forests of their sustenance. The continuous tree cover began to fragment, leaving a mosaic environment of forest patches, woodland, and shrubland. Open grassland, however, was still rare.
    About 12 million years ago, a continuation of tectonic forces further changed the environment, with the formation of a long, sinuous