Why did humans evolve to walk upright? Perhaps because it's just plain easier. Make that "energetically less costly", scientifically speaking. Bipedalism--walking on two feet, is one of the defining characteristics of being humans, and scientists have debated for years how it came about. In the latest attempt to find an explanation, researchers trained five chimps 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 chimps use while knucklewalking 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 were differences among chimps in how much energy they used, and this difference corresponded to their different manner of walking and anatomy .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. Interview. "That was pretty exciting, because when you talk about how evolution works, variation is the bottom line, without variation there is no evolution." Walking on two legs freed our arms, opening the door to drive the world, said Raichlen. "We think about the evolution of _ as one of first events that led hominids down the path to being humans." The research was supported by the National Science Foundation and the L.S.B.Leakey Foundation. We can infer from the passage that  _  .
scientists have no idea on how humans' walking on two legs came about scientists have had different views on why chimps walk on four legs scientists have had different views on how humans' walking on two legs vame about scientists have had similar views on how humans' walking on two legs came about