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.

　　Fossil and molecular evidence suggests the earliest ancestors of the human family lived in forested areas in equatorial Africa in the late Miocene era some 8 to 10 million years ago，when changes in climate may have increased in distance between food patches.That would have forced our earliest ancestors to travel longer distances on the ground and favored those who could cover more ground using less energy.

　　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?

　　词汇：

　　chimpanzee/5tFimpEn5zi:/n.黑猩猩

　　biped/5baiped/n.二足动物

　　gait/^eit/n.步态，步法

　　anatomic/7AnE5tCmik/adj.解剖 的

　　quadruped/5kwCdrJped/n.四足动物的

　　Miocene/5maiEsi:n/n..adj.中新世

　　Calorie/5kAlEri/n.卡 ,小卡

　　treadmill/5tredmIl/n.踏轮;踏车;单调工作

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