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."

人类为何进化到了直立行走的姿态？可能这是因为直立行走要容易得多。用科学术语来说，直立行走能“节省能量”。

用两条腿走路是人类的基本特征之一，多年来，科学家们在这一特征的形成问题上一直争论不休。在这项最新研究中，研究人员让五只大猩猩戴着可测算氧气消耗量的面罩在跑步机上行走。

研究人员对大猩猩在直立行走和四肢着地行走时消耗的氧气量分别进行了测量，并将结果与人类的进行了对比。这些结果在本周《国家科学院院刊》的网络版上公布。

测算结果表明，人类用两条腿行走时消耗的能量仅为大猩猩四肢着地行走时所消耗能量的四分之一。平均来看，大猩猩两条腿行走时消耗的能量与四肢着地行走时的差不多。

但行走时消耗能量的多寡在这些大猩猩中也存在差异。这与它们不同的步法和解剖学特征有关。

亚利桑那大学的人类学副教授大卫·里奇伦说：“其中一只大猩猩用两条腿行走时消耗的能量较少，还有一只两种行走方式消耗的能量相同。其余的两条腿行走时消耗的能量较多。”

他在接受一个电话采访时说：“让我们感到惊讶的是变异。这十分令人兴奋，因为变异在进化过程中起着关键性作用。没有变异，就不会有进化。”

里奇伦说，双腿走路解放了我们的双臂，为我们主宰世界打开了大门。“双腿直立行走的进化是原始人向人类进化的最初阶段中的一步。”