Scientists studied chimps walking on treadmills to help understand why our human ancestors might have started towalkon two feet10 million years ago.

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A trained chimp works out to help answer why humans walk on two legs. The face mask measures oxygen consumption while the paint spots on the chimp's joints aid in video-assisted biomechanical analysis. ((Cary Wolinsky/University of California at Davis) )

In an article published this week in the Proceedings of the National Academy of Sciences, researcherscame to the conclusion that a human walking with two feetis about 75 per cent more efficientin terms of burning energy and calories when compared to the way apesget around using two legs or all fours.

For the study, five adult chimps were trained over the course of four months to walk on a treadmill on both four and two limbs. The apes wore masks enabling the scientists to measure how much oxygen they used. Their energy expenditure was then measured andcompared to the chimps' anatomy and the biomechanics of their gait.

"Walking upright on two legs is a defining feature that makes us human," said Herman Pontzer, one of the study's authors, in a release. "It distinguishes our entire lineage from all other apes."

Pontzer,an assistant professor of Anthropology in Arts & Sciences at Washington University in St. Louis conducted the study with Michael Sokol of University of California and David Raichlen of the University of Arizona.

There has been a host of research looking at the evolution of bipedal walking. This lateststudy strengthens the theory that our two-legged gait evolved because it saves energy.

"Our results, therefore, support the hypothesis that energetics played an important role in the evolution of bipedalism," said the article.

The research team also looked at fossil records of early humans, which they found to support the theory that the structure of the human bodyevolved to makeit better atwalking upright.

"However, regardless of the context under which bipedalism evolved, our biomechanical analysis of adult chimpanzee costs, coupled with previous analyses of early hominin pelvic and hindlimb morphology, suggests that improved locomotor economy may have accrued very early within the hominin lineage."

Scientists saidthe study was the first of its kind.Similar research was conducted in 1973, whichevaluatedjuvenile chimpanzees, and not adult chimpanzees as in this week's article.