Why did humans evolve to walk upright? Perhaps because it's just plain easier. Make that "energetically less costly," in science-speak.

    Bipedalism - walking on two feet - is one of the defining characteristics of being human, and scientists have debated for years how it came about. In the latest attempt to find an explanation, researchers trained five chimpanzees to walk on a treadmill while wearing masks that allowed measurement of their oxygen consumption.

    The chimps were measured both while walking upright and while moving on their legs and knuckles. That measurement of the energy needed to move around was compared with similar tests on humans and the results are published in this week's online edition of Proceedings of the National Academy of Sciences.

    It turns out that humans walking on two legs use only one-quarter of the energy that chimpanzees use while knuckle-walking on four limbs. And the chimps, on average, use as much energy using two legs as they did when they used all four limbs.

    However, there was variability among chimpanzees in how much energy they used, and this difference corresponded to their different gaits and anatomy.

    One of the chimps used less energy on two legs, one used about the same and the others used more, said David Raichlen, assistant professor of anthropology at the University of Arizona.
    
    "What we were surprised at was the variation," he said in a telephone interview. "That was pretty exciting, because when you talk about how evolution works, variation is the bottom line, without variation there is no evolution."

    Walking on two legs freed our arms, opening the door to manipulating the world, Raichlen said. "We think about the evolution of bipedalism as one of first events that led hominids down the path to being human."

    人類為何進化到了直立行走的姿態？可能這是因為直立行走要容易得多。用科學術語來說，直立行走能“節省能量”。

    用兩條腿走路是人類的基本特征之一，多年來，科學家們在這一特征的形成問題上一直爭論不休。在這項最新研究中，研究人員讓五只大猩猩戴著可測算氧氣消耗量的面罩在跑步機上行走。

    研究人員對大猩猩在直立行走和四肢著地行走時消耗的氧氣量分別進行了測量，并將結果與人類的進行了對比。這些結果在本周《國家科學院院刊》的網絡版上公布。

    測算結果表明，人類用兩條腿行走時消耗的能量僅為大猩猩四肢著地行走時所消耗能量的四分之一。平均來看，大猩猩兩條腿行走時消耗的能量與四肢著地行走時的差不多。

    但行走時消耗能量的多寡在這些大猩猩中也存在差異。這與它們不同的步法和解剖學特征有關。

    亞利桑那大學的人類學副教授大衛·里奇倫說：“其中一只大猩猩用兩條腿行走時消耗的能量較少，還有一只兩種行走方式消耗的能量相同。其余的兩條腿行走時消耗的能量較多。”

    他在接受一個電話采訪時說：“讓我們感到驚訝的是變異。這十分令人興奮，因為變異在進化過程中起著關鍵性作用。沒有變異，就不會有進化。”

    里奇倫說，雙腿走路解放了我們的雙臂，為我們主宰世界打開了大門。“雙腿直立行走的進化是原始人向人類進化的最初階段中的一步。”