Humans are excellent two-legged walkers. It's one of the things that make us such successful creatures.

And there are some scientists who say we're naturally born runners as well, that our bodies evolved to run. Now, anthropologist Dan Lieberman, one of the proponents of the "human runner" school, concludes that we do it better without shoes.

He says human ancestors needed to run well — both away from big animals and after small, tasty ones, for example. He based that view on fossil bones. But lately he's been studying runners — living ones.