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 answer 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 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, Sockol 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?　　chimpanzee n. 黑猩猩的　　metabolic adj.代謝的Miocene n. adj. 中新世　　注釋：　　2. quadrupedal and bipedal：quad-：構詞詞素，是四的意思;bi-:構詞詞素，是二的意思;ped-：構詞詞素，是腳的意思;-al是構成形容詞的后綴，表示...的。因此，quadrupedal意為四足動物的，bipedal意為二足動物的。　　練習：　　A) The evidence why chimpanzees can be trained to use treadmills.　　C )The evidence why our apelike ancestors came to walk on two legs.　　2. Which of the following best interprets the meaning ofWhile the chimps worked out, .?　　B) While the chimps exercised in the gym.　　D) While the chimps tried to understand the instructions.

　　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 answer 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 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, Sockol 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?　　chimpanzee n. 黑猩猩的　　metabolic adj.代謝的Miocene n. adj. 中新世　　注釋：　　2. quadrupedal and bipedal：quad-：構詞詞素，是四的意思;bi-:構詞詞素，是二的意思;ped-：構詞詞素，是腳的意思;-al是構成形容詞的后綴，表示...的。因此，quadrupedal意為四足動物的，bipedal意為二足動物的。　　練習：　　A) The evidence why chimpanzees can be trained to use treadmills.　　C )The evidence why our apelike ancestors came to walk on two legs.　　2. Which of the following best interprets the meaning ofWhile the chimps worked out, .?　　B) While the chimps exercised in the gym.　　D) While the chimps tried to understand the instructions.