clipped from: dsc.discovery.com   
Why We Walk on Two Legs: It's Easier
Straining for Science
Why did humans evolve to walk upright? Perhaps because it's just plain easier. Make that "energetically less costly," in science-speak, and you have the conclusion of researchers who are proposing a likely reason for our modern gait.

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 treadmill while wearing masks that allowed measurement of their oxygen consumption.

The chimps were measured both while walking upright and while moving on their legs and knuckles. 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 Proceedings of the National Academy of Sciences.