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Stand and deliver: why did early hominids begin to walk on two feet?
Natural History, Nov, 2003 by Ian Tattersall
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.
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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 has been no dearth of suggestions, all based on adaptation to some aspect Or another of ground-dwelling life. I have to confess here that I have long been suspicious of the profligate use of "adaptation" to simultaneously explain any and all evolutionary innovations. After all, any individual is made up of a whole host of features that one could describe as adaptations, whereas natural selection can only vote up and down on the whole thing, warts and all. Still, there is no doubt that paleoanthropologists have come up with a whole host of terrific stories on the subject.
The first to describe a truly ancient biped was the Australianborn physical anthropologist and paleontologist Raymond Dart, in 1925. Dart understood that life on the predator-ridden open savannas would have been pretty dangerous for the relatively small, slow, and defenseless early hominids. He suggested that standing up would have enabled the creatures to peer over tall grass and spot dangerous animals at a distance. Other investigators have pointed out that an animal looks bigger when it stands up, which might help discourage predators from attacking it. Corroborating that idea, contemporary studies do seem to show that the predatory interest of big cats is more readily triggered by horizontal silhouettes than by vertical ones.
Those who prefer to look upon even our remotest ancestors as bush-league versions of ourselves have tended to side with Darwin. They see bipedalism as a mechanism for freeing the hands to carry food and other objects back to home base, or as a way of making it easier for mothers to tote babies around. The most recent wrinkle in this hypothesis has been the suggestion (by male paleoanthropologists) that bipedal early hominid males used their free hands to carry food back to hapless females, whose baby-toting activities had dramatically curtailed their food seeking. This social behavior supposedly led in turn to such far-reaching consequences as pair-bonding, concealed ovulation, and the prominence of female breasts. The story has the undeniable attraction of tying bipedalism to a variety of human physical and social peculiarities, but it is no less controversial for that, and it has recently come under attack on a variety of grounds. Feminist anthropologists, for example, perhaps in retaliation for the perceived sexual slight, have directly blamed erect bipedalism on the appalling exhibitionist tendencies of males.
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