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D. A Tale of Legs

Narrator: Why do we have long legs? This is Science Today. Anthropologist Adrienne Zihlman of the University of California, Santa Cruz studies the differences between humans and our close relatives the chimps and gorillas. Even though we evolved from a common ancestor, we're proportioned very differently.

Zihlman: So for example, compared to chimps and gorillas we have long upper limbs but they don't weigh much. They're like six or seven percent of body weight compared to chimps and gorillas, where it's 15 to 17 percent. Similarly, our lower limbs are 30 to 35 percent of body weight, whereas for chimps and gorillas it's 20 to 24 percent.

Narrator: The reason, says Zihlman, is that unlike chimps and gorillas, who don't walk much, and spend time in trees, our early human ancestors walked a lot, and walked long distances.

Zihlman: And if you're going to walk long distances you need a system that's very stable as well as powerful.

Narrator: So natural selection favored early humans who had longer, stronger, more powerful legs. Which we still have, even though we don't use them as much. For Science Today, I'm Steve Tokar.