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Why Humans Walk on Two Legs A team of scientists 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. When our earliest ancestors started walking on two legs，they took the first steps toward becoming human, said lead researcher Michael Sockol of UC Davis1.Our findings help an,swer why. Sockol worked for two years to find an animal trainer willing to coax adult chimps to walk on two legs and to knucklewalk on all fours on the sort of treadmill found in most gyms. The five chimps also wore face masks used to help the researchers measure oxygen consumption.While the chimps worked out，the scientists collected metabolic and other data that allowed them to calculate which method of locomotion used less energy and why.The team gathered the same information for four adult humans walking on a treadmill. Tile researchers found that human walking used about 75 percent less energy and burned 75 percent fewer calories than quadrupedal and bipedal2 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. We were prepared to find that all of the chimps used more energy walking on two legs but that finding wouldnt have been as interesting, Sockol said.What we found was much more telling. For three chimps，bipedalism3 was more expensive，but for the other two chimps，this wasnt the case.One spent about the same energy walking on two legs as on four.The other used less energy walking upright. These two chimps had different gaits and anatomy than their knucklewalking peers. | 
