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COPYRIGHT 2003 Natural History Magazine, Inc.
Ask any paleoanthropologist what got humankind started on its unique evolutionary trajectory, and the reflex answer will almost certainly be "the adoption of upright bipedalism." And whatever the exact characteristics of the most ancient hominid may have been, there is no question that the adoption of upright locomotion on the ground was an epoch-making event for our hominid family.
The idea that Homo sapiens might be descended from some ancient apelike animal that walked around on its two hind legs goes back at least as far as Jean-Baptiste Lamarck's great Philosophie Zoologique, which appeared in the opening decade of the nineteenth century. And Darwin famously expressed a similar viewpoint in The Descent of Man, published in 1871. Darwin speculated that the importance of bipedalism was that it freed the hands from the demands of locomotion, thereby opening the way for toolmaking and other manual activities that make us uniquely human. If so, it took some time for our precursors to realize the potential of their upright posture: it is now clear that the origin of stone toolmaking postdated the acquisition of bipedalism by millions of years. Still, it is hard to resist the idea that bipedalism was a necessary condition for all that followed, even if it might not have been a sufficient one.
Since Darwin's day, paleoanthropologists have energetically sought the key to hominid erectness in many different places. Nearly always, though, these scientists have sought the Holy Grail of a single critical function: what exactly was it about being upright that gave early hominids the edge? For, given that teetering along on a single pair of feet is, to all appearances, hardly an optimal solution for a hominid whose ancestors almost certainly got around using four limbs, isn't it intuitively obvious that the particular advantage of walking upright on two limbs must have been an overwhelming one? And, at the very least, it's clear that upright bipedalism is not an automatic primate response to descending from the trees to live on the ground. Even patas monkeys, apart from ourselves the most committed-to-ground-dwelling of all living primates, have accomplished that shift by becoming even more specialized quadrupeds than their more arboreal ancestors had been before them.
So just what was going on when our ancient forebears, in a period of climate change that transformed their ancestral forested habitats in Africa into one of trees, shrubs, and grasses, started opting for upright, two-legged locomotion on the ground? There...
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