Why Humans Walk on Two Legs

　　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.

　　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.

　　Taken together，the findings provide support for the hypothesis that anatomical differences affecting gait existed among our earliest apelike ancestors，and that these differences provided the genetic variation which natural selection could act on when changes in the environment gave bipeds an advantage over quadrupeds.

　　This isnt the complete answer, Soekol said.But its a good piece of a puzzle humans have always wondered about;How and why did we become human? And why do we alone walk on two legs?

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