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MARY LOUISE KELLY, host:

So, here's a weighty question: what makes us human? What allowed us to become this dominant species? NPR's Science Desk is trying to pin that down in a series we're calling The Human Edge. And I'm here with science correspondent Christopher Joyce who's been asking anthropologists what they think was the most important change that made us so successful. And, Chris, what did you find out?

CHRISTOPHER JOYCE: That if you ask four experts you'll get five opinions -usually whatever it is the scientist is studying at the time is the thing that made the difference. Here's a taste of what I got.

Dr. MATT TACHERRY(ph) (Anthropologist): The enlarged brain.

Dr. CHRISTIAN TRYON (Anthropologist): I think it's sort of group sharing of knowledge. I think that's it.

Dr. WILL HARCOURT-SMITH (Anthropologist): I would put the foot at the very top.

Dr. TACHERRY: You know, bipedality, walking on two legs.

Dr. TRYON: Our ability to work together in these large societies.

Dr. HARTCOURT SMITH: There's no one thing.

JOYCE: Those were anthropologists Matt Tacherry, Christian Tryon and Will Harcourt-Smith.

KELLY: Wow. And really they're all over the map, but I guess you've got to start the story of human evolution somewhere.

JOYCE: Well, why don't we start with some Fred and Ginger.

(Soundbite of tap dancing)

JOYCE: As Fred Astaire and Ginger Rogers prove, you can do some amazing things with two legs. But two-leggedness, bipedalism, is rare among living things. Four legs is easier. It's stable and you can cover ground a lot faster. And that's been the animal architecture of choice for eons, at least for most mammals.

Our earliest primate ancestors were no exception, whether moving in trees or knuckle-walking on the ground. But somewhere along the line, primates discovered how to stand up and walk on two legs. And after millions of years, it got us to where we are now.

(Soundbite of footsteps)

JOYCE: Now, no one's suggesting that natural selection led our species to wear high heels. Natural selection favors new traits that give their owners a survival advantage. And as anthropologist Ian Tattersall point out, it all starts by chance.

Dr. IAN TATTERSALL (Anthropologist, American Museum of Natural History): You know, natural selection can't call things into existence just because they're desirable. Novelties happen randomly.

JOYCE: And, says Tattersall, who works at the American Museum of Natural History in New York, it takes a long, long time for novelties like standing up to walk to take hold and change a population of animals.

Dr. TATTERSALL: The earliest hominids didn't become upright walkers the way we are, right away.

JOYCE: The hominids our ancestors and their close relations were adapted to living in trees. But there may already have been something going on in their bodies that predisposed them to walk on two legs like standing up and walking in the trees - a bit like orangutans do now, reaching their arms up to branches to brace themselves as they move along.

Dr. TATTERSALL: I don't think you come down to the ground and decide it would be a really good idea to stand upright and move around. I think the only reason you would do it is because this is what came naturally to you in the first place.

JOYCE: This tree-walking may have been something even monkeys did. Primitive monkeys started to split off from the lineage that led to apes and humans over 30 million years ago. Some of them also evolved to walk upright. Their walking style is a lot more primitive than ours, but it could hold clues to how our own ancestors learned to walk.

(Soundbite of monkey screeching)

JOYCE: That's not an anthropologist; it's Parker, one of two spider monkeys at a lab at Stony Brook University in New York.

Susan Larson from the anatomy department is coaxing Parker down an enclosed trackway where scientists videotape monkey walking.

Professor SUSAN LARSON (Anatomy, Stony Brook U.N.): He's very good at it. They do it in the wild.

JOYCE: Larson is trying to understand how the muscles and bones in the monkey allow Parker to walk badly, a sort of high-stepping goosestep, but nonetheless walk.

Prof. LARSON: So, he have structures that appear the same, look the same in humans in monkeys, then the reason why they look the same is because they work the same.

JOYCE: Structures like a wide blade at the top of the hip bone that allows for more muscle attachments to help in balancing. Features like these seem to be part of a kind of walking tool kit. If some of the same tools show up in fossil bones of our ancestors, that could explain the early evolutionary changes that allowed us to walk.

When our ancestors first walked on the ground seems to be about six or seven million years ago. But why they did it is harder to agree on. Anthropologist William Jungers at Stony Brook, favors the look, ma - hands hypothesis.

Dr. WILLIAM JUNGERS (Anthropologist, Stony Brook University): When animals walk bipedally other primates it is often in the context of carrying, whether they are carrying food items, stones. I think carrying is the most plausible precursor for bipedality.

JOYCE: Free hands may be the devil's playground, but they gave our ancestors the means to gather and carry food, and eventually make tools. Free hands improved the chances of a long life and many offspring.

There are other ideas, though. One holds that the African climate dried out and forests turned into savanna. Without dense forests, our ancestors had to walk instead of swing on a branch to find the next tree.

Whatever the reason, though, walking was still just a part-time thing for millions of years. Trees were still our home base.

Dr. JUNGERS: When you're going to travel, you come to the ground and you move. Whether it's through the forest, across the savanna, it doesn't matter you're a biped when you're moving. But where do you sleep? You sleep in the trees. Where do you go to escape predators? You move to the trees.

JOYCE: You can see evidence of this split-level lifestyle in the bones of one of our early relatives: Lucy, the pint-sized Australopithecus who lived about three million years ago. She had longish legs and a big toe in line with the other toes, both good for walking. But her hands and arms were still built for climbing. Her strategy was to be mistress of both worlds. But mistresses eventually have to make a choice.

Dr. JUNGERS: The real question is why we abandoned that strategy or chose an alternative strategy?

JOYCE: The alternative being full-time living on terra firma. Again, no consensus on why - maybe the Africa forests thinned out or walking led to running and hunting down bigger meals. One thing for sure, though, the more we walked, the more our bodies changed.

Dr. JUNGERS: We've elongated this lower part of our body.

JOYCE: What did that give us?

Dr. JUNGERS: Locomotion really is priced by the step, so increased stride length has tremendous economical advantages.

JOYCE: The longer leg meant we could go farther with a given amount of effort. Other changes followed: a springy arch in our foot also made us more efficient walkers. We developed a spine that centers our weight over our legs to keep our balance.

As a result, we did better than other primates. First-rate walkers could travel to find food, water or the best stone for tools. Wanderlust was born romantic walks on the beach.

Bipedality made a big difference in a hostile and unforgiving world. Eventually a new genus was born: Homo - humans. Not the noble savage, but the mobile savage. And except for a few throwbacks...

Unidentified Man (Actor): (As Tarzan) Tarzan.

Unidentified Woman (Actress): (As Jane) Jane, Jane.

Unidentified Man: (As Tarzan)

JOYCE: ...we've kept both feet on the ground ever since.

(Soundbite of Tarzan yell)

KELLY: Oh boy. Well, it's going to be hard to top Tarzan. Christopher Joyce, what's next in the series for us next week?

JOYCE: We're going to take a look at those two feet on the ground that I mentioned. We're going to understand the human foot and see what it tells us.

KELLY: All right. We'll look forward to it.

(Soundbite of music)

KELLY: And there's an interactive graphic at NPR.org. That, among other things, demonstrates the differences between Emma Darwin - she's the wife of Charles -and chimps.

(Soundbite of music)

KELLY: This is NPR News.

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