A team of anthropologists that studied chimpanzees trained to use treadmills has gathered new evidence suggesting that our earliest apelike ancestors started walking on two legs because it required less energy than getting around on all fours. The researchers found that human walking used about 75 percent less energy and burned 75 percent fewer calories than quadrupedal and bipedal walking in chimpanzees. They also found that for some but not all of the chimps, walking on two legs was no more costly than knucklewalking.