why did we evolve to become bipeds? -- 9/25/19

Today's selection -- from The Story of the Human Body by Daniel E. Lieberman. Why did we evolve to become bipeds?

"Plato once defined humans as featherless bipeds, but he didn't know about dinosaurs, kangaroos, and meerkats. In actual fact, we humans are the only striding, featherless, and tailless bipeds. Even so, tottering about on two legs has evolved only a few times, and there are no other bipeds that resemble humans, making it hard to evaluate the comparative advantages and disadvantages of being a habitually upright hominin. If hominin bipedalism is so exceptional, why did it evolve? And how did this strange manner of standing and walking influence subsequent evolutionary changes to the hominin body?

"It is impossible to ever know for sure why natural selection favored adaptations for bipedalism, but I think the evidence most strongly supports the idea that regularly standing and walking upright was initially selected to help the first hominins forage and obtain food more effectively in the face of major climate change that was occur­ring when the human and chimpanzee lineages diverged. ...

"Between 10 and 5 million years ago, the entire earth's climate cooled considerably. Although this cooling happened over millions of years and with endless fluc­tuations between warmer and colder periods, the overall effect in Africa was to cause rain forests to shrink and woodland habitats to expand. ... if you had the misfortune to be living at the margins of the forest, then this change must have been stressful. As the for­est around you shrinks and becomes woodland, the ripe fruits you hunger after become less abundant, more dispersed, and more sea­sonal. These changes would sometimes require you to travel farther to get the same amount of food, and you'd resort more frequently to eating fallback foods, which are more abundant but lower in qual­ity than preferred foods such as ripe fruit. Typical fallback foods for chimpanzees include the fibrous stems and leaves of plants, as well as various herbs, and the evidence for climate change sug­gests that the first hominins would have needed to find and eat such foods more often and more intensely than chimps do. ...

"Just as the tough get going when the going gets tough, natural selection acts most strongly not during times of plenty, but during times of stress and scarcity. ... The first shift is that hominins with bigger, thicker cheek teeth and the ability to chew more forcefully would have been better able to consume more tough, fibrous fallback foods. The second but more extensive shift, bipedalism, is a little harder to appreciate as an adaptation to cli­mate change but was probably even more important in the long run for several reasons, one of which may be surprising.

"One obvious advantage of bipedalism is that standing on two feet can make it easier to forage for certain fruits. Orangutans, for example, sometimes stand nearly upright on branches when feed­ing in trees, reaching for precariously hanging foods by keeping their knees straight and holding on to at least one other branch. ... [and] being able to stand and walk upright more effectively might have helped hominins to carry more fruit, as chimps sometimes do when competition is intense.

"A second, more surprising, and possibly more important advan­tage of bipedalism is that walking on two legs may have helped early hominins save energy when traveling. Recall that the LCA [last common ancestor] was probably a knuckle walker. Knuckle walking is a decidedly peculiar way to walk on all fours, and it is also energetically costly. Laboratory studies that have enticed chimps to walk on treadmills while wearing oxygen masks have found that these apes spend four times more energy to walk (on either two or four limbs) a given distance than humans. Four times! This extraordinary differ­ence occurs because chimps have short legs, they sway from side to side, and they always walk with bent hips and knees. As a result, chimps constantly spend lots of energy contracting their back, hip, and thigh muscles to keep from toppling over and collapsing to the ground. Not surprisingly, chimps walk comparatively little, only about 2 or 3 kilometers a day (about 1 to 2 miles). For the same amount of energy, a human can walk between 8 and 12 kilome­ters (5 to 7.5 miles). Therefore, if early hominins were able to walk bipedally with less lurching and with straighter hips and knees, they would have had a substantial energetic advantage over their knuckle-walking cousins. Being able to walk farther using the same amount of energy would have been a very beneficial adaptation as the rain forests shrank, fragmented, and opened up, causing pre­ferred foods to become rarer and more dispersed."

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Daniel E. Lieberman


The Story of the Human Body




Copyright 2013 by Daniel E. Lieberman


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