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Scientists have spent more than a century debating how and when humans first began to walk upright. The conventional wisdom is that when our ancestors descended from the trees, they used their knuckles to help them walk on the ground. Then they slowly developed a more upright posture. But a few scientists aren't so sure about that. They say humans may have evolved from primates who skipped the knuckle-walking stage - learning to walk upright while they were still living in trees.
NPR's Jon Hamilton has the story.
JON HAMILTON: The evidence that upright walking began in trees comes from orangutans. Brian Richmond is an expert on ape and human locomotion at the George Washington University in D.C. He says orangutans depend on trees to get around.
Professor BRIAN RICHMOND (Anthropology, George Washington University): The orangutans would be traveling through the trees from one tree to the next. And the way they see the trees as a tree highway. They don't walk on the ground, they have to go from tree to tree to tree to get where they're going.
HAMILTON: But a group of scientists in Britain noticed that orangutans in zoos who do spend time on the ground seem to be able to walk in a more human way than other apes. So they spent a year in Sumatra, watching orangutans in the wild. Richmond, who wasn't part of the team, said the scientists discovered something surprising about how orangutans actually get around in trees.
Prof. RICHMOND: When they're on the big, stout branches, they tend to walk on all fours - on top of the branch. But when the branches gets thinner and thinner and thinner out towards the edge, they use their hands to hold on to the top branches and their feet to use to hold on to branches below.
Professor ROBIN CROMPTON (Primate Evolution and Morphology Research Group, University of Liverpool): It's strikingly human-like. That even their trunk upright and the leg totally in line with the trunk - very much like ourselves.
HAMILTON: Robin Crompton is a member of the team that studied the orangutans. He's also a professor the University of Liverpool. Crompton says there's a clear benefit to being able to navigate thin branches by walking upright. It would have saved the orangutan's many dangerous and time-consuming trips to the ground, as they went from tree to tree.
Prof. CROMPTON: What the orangutan is telling us is that our own kinds of movement, passing upright, walking on the hind legs, could actually have evolved in the trees and not from the ground at all.
HAMILTON: In a paper published in the Journal of Science this week, Crompton said this might mean redrawing our family tree to include a direct ancestor that was more like an orangutan.
But Brian Richmond says there are problems with the British hypothesis. For one thing, it makes it hard to explain why genetic studies show that our closest relatives are chimps and gorillas. And they walk on their knuckles.
Richmond says another problem is the construction of the human wrist. It looks like the wrist of a chimp or a gorilla, not an orangutan.
Prof. RICHMOND: We have eight wrist bones. Almost all the primates have nine wrist bones. And guess what? That's mostly an adaptation to walking on your knuckles on the ground. So this is some of the evidence that points to our ancestors being ones that both climbed in the trees and walked on the knuckles on ground.
HAMILTON: Richmond says walking upright in trees would have helped our ancestors adapt to life on the ground, but he thinks they still had to spend a long time on their knuckles before standing on their own two legs.
Richmond says arguments about the origins of upright walking aren't likely to end anytime soon.
Prof. RICHMOND: The only way to resolve the question of what happened is to find fossil evidence of our early ancestors and the common ancestors of chimpanzees and humans that lived somewhere between six million and eight million years ago. That's really the only way to really find out what happened.
HAMILTON: And scientists are still looking for that evidence.
Jon Hamilton, NPR News.
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