Why walk upright? Efficiency

WASHINGTON - 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.

So, 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 on 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.

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.

Past explanations for walking upright have included the need to use the arms to gather food, the need to use upper limbs to bring food to a mate and offspring and raising the body higher to dissipate heat in the breeze. *