Four legs are good--but two legs are more energy-efficient, theorizes（推斷）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（黑猩猩）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"（用關節走）--also did better on two legs.

For three chimps, bipedalism（兩足動物） 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（人類學） 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（步法）and anatomy身體結構）than the others. Their anatomy and skeletal（骨骼） characteristics（特點）, in fact, were similar to early hominid（原始人類） fossils（化石） that allowed for greater extension（伸長） of the hind limb（腿）.