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 wouldn’t 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 wasn’t 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.
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 isn't the complete answer,” Soekol said.“But it’s 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 (又作 chimp )
biped/5baiped/n.二足动物
gait/^eit/n.步态，步法
anatomic/7AnE5tCmik/adj.解剖 (学)的
quadruped/5kwCdrJped/n.四足动物的
Miocene/5maiEsi:n/n.&。adj.中新世 (的)
Calorie/5kAlEri/n.卡 (路里) ,小卡
treadmill/5tredmIl/n.踏轮;踏车;单调工作
coax/kEuks/v.哄，哄劝
knucklewalk v.用膝关节走路
metabolic/7metE5bClik/adj.(新陈) 代谢的
locomotion/lEJkE5mEJF(E)n/n.运动 (力)
注释：
1. UC Davis：加州大学戴维斯分校。UC是University of California(加利福尼亚大学)的缩写，该校有多个分校，UC Davis是其中一个。Davis以前是UC Berkeley分校的农学院，后来独立小来。Davis是一个大学城，很小的城市，Davis校园基本就构成了这个城市。
2. quadrupedal andbipedal：quad-：构词词素，是“四”的意思;bi-：构词词素，是“二”的意思; ped-：构词词素，是“脚”的意思;-al是构成形容词的后缀，表示“……的”。因此，quadrupedal 意为“凹足动物的”，bipedal意为“二足动物的”。
3. bipedalism：系bipedal(二足动物的)的对应名词，这里后缀-ism表示“特征”、“特性”。
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