A publication of the Archaeological Institute of America
The gift of gab, artistic sensibilities, and an incomparably clever brain are all characteristics we think of when asked what makes us uniquely human. But Craig Stanford has something less cerebral in mind. In his latest book, Upright (New York: Houghton Mifflin, 2003; $23), the University of Southern California anthropologist and codirector of the Jane Goodall Primate Research Center argues eloquently that the fundamental turning point in human evolution occurred when our ancestors shifted from four legs to two. Indeed, it was this change of posture and locomotion, Stanford contends, that laid the groundwork for the cognitive revolution that molded our minds much later.
Paleoanthropologists have long recognized bipedalism as a hallmark of humanity. Some of the earliest putative humans on record (dating back nearly six million years) have been categorized as such strictly because they appear to have walked on two legs--even though they more closely resembled apes in nearly every regard. Yet for all the importance accorded our bizarre mode of locomotion, there is little agreement as to how it came about. Researchers have variously proposed that it enabled better spotting of predators over the tall grass; helped to keep the brutal African sun off the back; freed the hands to carry tools, food, and children--the list goes on and on.
Stanford is unpersuaded by these popular explanations. Surveying a century of research into the question, he concludes that a more plausible theory for the origins of bipedalism can be derived from observations of our closest living relative, the chimpanzee. Drawing on his own fieldwork in Uganda's Bwindi Impenetrable National Park, Stanford describes having seen the animals standing bipedally in trees and on the ground to reach ripe fruits overhead. It's not difficult to envision how a primeval human forebear in a similar situation might have then taken a few tentative steps to reach the next fruit tree, he notes. Over time, natural selection favored those individuals who were able to remain upright longer and thereby obtain food more effciently--initially by reaching for fruit and shuffling between trees, and later by traversing long distances in open country.
Rather than considering the first two-legged steps as a radical departure from the ancestral ape way of life (as a number of the other hypotheses do), Stanford says "it makes more sense to view bipedalism as emerging when an ape that was already doing something began to do it more often."
However it transpired, the switch to upright traveling had far-reaching effects on the human body. For one, the accompanying changes in breathing may have also paved the way for speech to evolve. And as early hominids refined their stride, they were able to leave the forest for the savanna, where calorie-packed meat was more readily available.
Only when our ancient forebears had regular access to large amounts of calories and nutrients could their brains expand, because gray matter requires an enormous amount of energy. Bipedalism, then, probably played a key role in enabling the eventual evolution of our large brains. In other words, that first small step for our great, great grand-ape was a giant leap for humankind.
Kate Wong is a writer and editor at Scientific American.
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