Standing Up

WASHINGTON (AP) -- Why did humans evolve into stand-up guys?

Scientists say walking upright is easier and uses less energy. They established that by testing chimps on a treadmill, having them walk both upright and on their legs and knuckles. The measurement of the energy needed to move around was compared with similar tests on humans. 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.

Bipedalism, or walking on two feet, is one of the defining characteristics of being human, and scientists have debated for years how it came about.

The results are published in this week's online edition of Proceedings of the National Academy of Sciences.

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