Why do humans walk on two legs?

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A study examining chimpanzees who were taught to walk on two legs indicates that humans began walking on two legs to be more energy efficient. See more pictures of monkeys.
Photographer: Norma Cornes | Agency: Dreamstime.com

For years, scientists and armchair philosophers have pondered one of humanity's great evolutionary concerns: Why do humans walk on two legs? We have four limbs, and our ape friends seem to get along well on all fours (by doing what's called knucklewalking), so what makes walking on two legs better? Or, if it's not better, why do we do it at all?

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Scientists know that bipedalism was one of the first features to develop in early hominids -- early human ancestors that had already diverged from apes. And a variety of theories attempt to explain why, yet some of the theories seem to contradict one another. A study published in the July 2007 issue of Proceedings of the National Academy of Sciences attempts to provide a definitive answer. It claims that human bipedalism boils down to one thing: energy.

The study, performed by three researchers from the University of Arizona, the University of California, Davis, and Washington University in St. Louis, examined differences in upright walking between four adult humans and five adult chimpanzees. Chimpanzees were used because they're the closest modern-day relative to humans. (Four to seven million years ago, humans and chimpanzees diverged from a common ancestor. They then developed independently.)

Researchers taught the five adult chimpanzees to walk on treadmills. They walked upright on their hind legs and knucklewalked on all fours. The chimps wore masks that tracked how much oxygen they used. The researchers also measured how much pressure was exerted on the treadmill. This revealed which muscles the animals were using. The same measurements were taken for the four adult humans.

The walking tests showed that the chimps, as a group, averaged the same energy expenditure walking on all fours as they did walking on two legs. As a group, the humans used 75 percent less energy walking upright than the chimps used walking on all fours. Essentially, walking upright seemed to be beneficial because it saved energy.

But what appeared even more interesting was that the amount of energy expended by the chimps varied between them. One chimp used less energy on two legs than on all fours. Another used the same in both walking positions, and the other three used more energy when walking on two legs. The variations were traced to how the chimps walked and differences in body structures. One of the scientists expressed excitement at this variation, saying that it reflected an essential part of evolution [source: Discovery News].

Analysis of videotapes of the walking chimps showed that chimps generally use large hip muscles and take short steps when walking upright. Humans tend to use smaller muscles, like those in their lower legs, and to take longer steps. This leads to improved energy efficiency. Not coincidentally, the chimps that took longer upright strides than their peers consumed less energy.

The experiment's results illustrate how energy expenditure contributed to human evolution. Early humans who adapted to expend less energy walking upright are represented in the fossil record. Fossils show that some ancient humans developed longer legs, different hip structures or thicker leg bones, consistent with modern-day humans. These adaptations made upright walking easier and lessened the amount of energy required to walk upright.

On the next page, we'll look at other theories about why humans walk upright and at more links between humans and primates.

Why Humans Walk on Two Legs: Other Theories

Natasha, a macaque in an Israeli zoo, began walking exclusively on two legs after almost dying from an illness.
Natasha, a macaque in an Israeli zoo, began walking exclusively on two legs after almost dying from an illness.

Scientists claim that walking on two legs was one of the keys to humans' development from ancient ape-like ancestors. Walking on two legs saved energy and allowed the arms to be used for activities like hunting, crafting simple tools and interacting with objects. Charles Darwin proposed long ago that having two limbs free to use tools constituted a key element of advanced intelligence [source: National Geographic].

Prior to the chimp-treadmill study, which was the first study examining upright walking in adult chimpanzees, scientists debated a variety of explanations for why humans walk upright. Many of these explanations invoked the idea of energy conservation in one way or another.

One theory proposed that walking on two legs freed the arms, which could then be used to collect food to bring to the family unit. If a primate walked on all fours, he couldn't carry much food back to the family without embarking on multiple hunting or foraging expeditions. It required less energy to provide for a family if the male could walk upright and return to his mate and young with enough food for all of them. The female could then stay with the young and take care of them, ensuring their health and protection from predators.

A second theory proposed that hominids started walking upright when traveling through water. Chimps do this today, rising up on their hind legs to wade through a pool or creek.

Still another theory proposed that our ancient ancestors rose up on their hind legs in order to cool themselves. By standing upright, they exposed less of their bodies to the sun.

Changing habitats and ecological conditions can have a dramatic effect on animal behavior, sometimes forcing species to adapt, flee or die. Some researchers believe that several million years ago, a warming climate and declining forest habitats meant that our forebears had to undergo longer journeys to find food. Walking on two legs made these journeys less taxing. A related possibility is that the changing climate forced primates to become primarily ground-dwellers, rather than living in trees and forest canopies. Food sources became more plentiful on the ground, where primates would have had an advantage by walking on two legs.

The final theory asserts that our ancestors never had to leave the trees to learn to walk on two legs. Instead, they learned while still living above ground. Orangutans provide a modern-day analog, as they often stand on two legs on tree branches and grab onto other branches with their front limbs in order to stay balanced.

The treadmill study may provide the best case for explaining why humans evolved to walk upright. But a fascinating story from 2004 shows that nature continually surprises and confounds even the most experienced researchers. That year, a five-year old black macaque named Natasha, living in a zoo in Israel, almost died from a bad case of the stomach flu. After regaining her health, Natasha inexplicably began walking upright all the time -- and with remarkably good posture. While monkeys often walk upright for short periods, they never do so consistently and straight-backed like a human. Three other monkeys living with Natasha had the stomach flu, but none of them displayed her post-illness behavior. The veterinarian treating her said that brain damage might be the cause but that he had never heard of a monkey walking only upright before.

For more information on monkeys, apes, human evolution and related topics, please check out the links on the next page.

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  • "Chimps on treadmills offer evolution insights." Reuters. CNN.com. July 17, 2007. http://www.cnn.com/2007/TECH/science/07/17/chimps.on.treadmills.reut/
  • "Introduction to Archaeology: Glossary." Archaeological Institute of America. http://www.archaeological.org/webinfo.php?page=10299#h
  • Connor, Steve. "The truth about why we walk on two legs: it saves energy." The Independent. July 17, 2007. http://news.independent.co.uk/sci_tech/article2776141.ece
  • Knight, Meredith. "Walking Tall: Why Did Humans Switch From Four- to Two-Legged Strides?" Scientific American. July 17, 2007. http://www.sciam.com/article.cfm?articleID=D4EDD3B5-E7F2-99DF-3780F5F065506592&chanID=sa007
  • Mayell, Hillary. "Fossil Pushes Upright Walking Back 2 Million Years, Study Says." National Geographic News. Sept. 2, 2004. http://news.nationalgeographic.com/news/2004/09/0902_040902_upright_hominid.html
  • Schmid, Randolph E. "Why We Walk on Two Legs: It's Easier." Associated Press. Discovery News. July 17, 2007. http://dsc.discovery.com/news/2007/07/17/walking_arc.html?category=archaeology
  • Sockol, Michael D., Raichlen, David A. and Pontzer, Herman. "Chimpanzee locomotor energetics and the origin of human bipedalism." Proceedings of the National Academy of Sciences. July 16, 2007. http://www.pnas.org/cgi/content/abstract/0703267104v1?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=walking+upright&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT
  • Waldman, Dan. "Monkey apes humans by walking on two legs." Associated Press. MSNBC.com. July 21, 2004. http://www.msnbc.msn.com/id/5479501/