Why do humans walk on two legs? To save energy

Four legs are good--but two legs are more energy-efficient, theorizes a researcher at the University of California at Davis.

For some, walking on two legs consumes less energy than walking on all fours, according to a paper from UC Davis. The findings may help explain why human ancestors evolved into bipeds 10 million years ago.

The study--which compared data from humans and specially trained chimps on treadmills--found that humans used about 75 percent less energy and burned 75 percent fewer calories than walking on all fours or two legs for chimpanzees, according to the report.

Interestingly enough, some of the chimps in the experiment--who were taught to walk on two legs and to "knucklewalk"--also did better on two legs.

For three chimps, bipedalism consumed more energy than walking on all fours. One chimp, however, expended as much energy walking on four legs as two legs, and one other chimp consumed less energy walking upright.

"We were prepared to find that all of the chimps used more energy walking on two legs--but that finding wouldn't have been as interesting. What we found was much more telling," Andrew Sockol, a Ph.D. candidate in anthropology at UC Davis, said in a statement. "This isn't the complete answer, but it's a good piece of a puzzle humans have always wondered about: How and why did we become human? And why do we alone walk on two legs?"

The researchers also found that, for some of the chimps, walking on two legs required no more energy than knucklewalking.

These two chimps also had different gaits and anatomy than the others. Their anatomy and skeletal characteristics, in fact, were similar to early hominid fossils that allowed for greater extension of the hind limb.

Sockol studied the biomechanics and oxygen consumption of specially trained chimps on a treadmill. While the chimps worked out, the scientists collected metabolic and kinetic data as well as information on oxygen consumption. The same data was gathered for human subjects.

One of the more difficult parts of the project was getting the chimps to walk on two legs and knucklewalk. It took two years to find a trainer--for the chimps, that is.

Fossil and molecular evidence suggests that climate changes in equatorial Africa some 8 million to 10 million years ago prompted a change in human evolution. The area had been forested, but began to become drier. This may have increased the distance between food patches. This would have forced early hominids to travel longer distances. Those that used less energy had an advantage.

The research appears this week in the online early edition of the Proceedings of the National Academy of Sciences of the United States of America.

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