The appearance of these traits in one ape suggests enough variation in the population for natural selection to have taken hold if necessary, Lieberman says. If the environment caused apes to walk a lot farther, the high energetic cost of knuckle-walking could have changed the behavior over time.
"That's how evolution works," Lieberman says. "One [chimp] turned out to be better than the other chimps, because he adapted a more extended posture."
Though the fossil record does not extend back to when scientists believe the human-chimp split occurred, several leg and hip bones from later time periods—in particular a hip bone three million years old—reflect the changes that decrease the cost of two-legged walking.
"At least by three million years ago," Lieberman says, "hominids figured out how to not have this [energy] cost."
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Posted July 16, 2007