Why did humans evolve to walk upright? Perhaps because it's just plain easier. Make that "energetically less costly," in science-speak.

Bipedalism - walking on two feet - is one of the defining characteristics of being human, and scientists have debated for years how it came about. In the latest attempt to find an explanation, researchers trained five chimpanzees to walk on a treadmill¹ while wearing masks that allowed measurement of their oxygen consumption.

The chimps² were measured both while walking upright and while moving on their legs and knuckles³. That measurement of the energy needed to move around was compared with similar tests on humans and the results are published in this week's online edition of Proceedings⁴ of the National Academy of Sciences.

It turns out that humans walking on two legs use only one-quarter of the energy that chimpanzees use while knuckle-walking on four limbs. And the chimps, on average, use as much energy using two legs as they did when they used all four limbs.

However, there was variability among chimpanzees in how much energy they used, and this difference corresponded to their different gaits and anatomy⁵.

One of the chimps used less energy on two legs, one used about the same and the others used more, said David Raichlen, assistant professor of anthropology⁶ at the University of Arizona.

"What we were surprised at was the variation," he said in a telephone interview. "That was pretty exciting, because when you talk about how evolution works, variation is the bottom line, without variation there is no evolution."

Walking on two legs freed our arms, opening the door to manipulating the world, Raichlen said. "We think about the evolution of bipedalism as one of first events that led hominids down the path to being human."

The research was supported by the National Science Foundation and the L. S. B. Leakey Foundation.