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