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The tree of life just got a little more complicated. Scientists working in Ethiopia have unveiled a trove of fossil bones that they say come from our oldest known ancestor. Among the bones are those of a female - who's been named Ardi. She apparently weighed about 110 pounds and stood about four feet high. She lived in trees, but could walk on the ground.

As NPR's Christopher Joyce explains, Ardi pushes back human history a long way.

CHRISTOPHER JOYCE: Move over Lucy, that diminutive creature discovered 35 years ago in Ethiopia became the poster child for our earliest out-of-the-trees ancestors. But now comes Ardi. Ardi lived 4.4 million years ago - that's a million years before Lucy. Scientists say that makes Ardi the closest they've come to finding the elusive common ancestor of humans and chimpanzees.

Anthropologists suspect that about six or seven million years ago, some kind of ape-like creature gave rise to two evolutionary lines. One led to chimpanzees and other apes, the other to hominids, who eventually became us. C. Owen Lovejoy of Kent State University, one of Ardi's discoverers says people thought the common ancestor was probably a lot like a chimp. But Lovejoy says he knows chimps and Ardi was no chimp.

Dr. C. OWEN LOVEJOY (Professor of Anthropology, Kent State University): The last common ancestor was more Ardi-like than it was chimpanzee or gorilla-like. So the old attitude that the last common ancestor was chimp-like is completely wrong.

JOYCE: What Lovejoy has to go on are more than 100 bones from 36 individuals, formerly known as Ardipithicus ramidus. Older Ardipithicus specimens are known but consist of just a few teeth and bones. Tim White, an anthropologist at the University of California, Berkeley is the team's field director. He brought a model of the skull to Washington to display. It's about the size of a grapefruit. When White found it, he said it was crushed.

Professor TIM WHITE (Anthropologist, University of California, Berkeley): I'm pulling out a couple of pieces of wrapping here. The height of this cranium was about an inch. The face was completely missing. We had none of it. So we went back to the field.

JOYCE: Over 15 years, they collected a body.

Prof. WHITE: This thing has all the teeth in the lower jaw and the upper jaw and the bones of the face and the entire cranium.

Unidentified Man: And the hands and feet. It was a nightmare to put it together.

JOYCE: White says they also recreated the creatures' environment.

Prof. WHITE: Collecting every pollen grain, every seed, every piece of wood, every shrew, every bat, every elephant, every giraffe, every horse, every monkey, all of this provides the high resolution snapshot of what Africa was like in this place our ancestors lived four and half million years ago.

JOYCE: White and Lovejoy describe Ardi as a mosaic of ape, monkey and yet to evolve descendent Lucy, formerly known as Australopithecus. Ardi lived in a lush wooded environment, not the grassy savanna often associated with human ancestors. And it was no knuckle walker. It could walk on its own two feet, though those feet were still pretty monkey-like. But it probably spent most of its time in the trees. Its hands differed from the chimps as well. And then there are the teeth. The canines are small, unlike the long threatening canines chimps have. Male chimps use the canines to intimidate other males and gain access to females. Lovejoy says a male Ardipithicus probably did not need big teeth to get what he wanted.

Prof. LOVEJOY: So females are picking males that are using some other technique to obtain reproductive success. And that technique probably is exchanging food for copulation.

JOYCE: That, says Lovejoy, paved the way for the kind of social life humans eventually adopted.

Professor ALAN WALKER (Anthropology and Biology, Penn State University) This is much more important than Lucy.

JOYCE: That's Alan Walker, an anthropologist at Penn State University. He says the fossil will kick start plenty of debate among anthropologists.

Prof. WALKER: As soon as everybody in the field goes to read these things and digest them, people will start the usual academic criticism.

JOYCE: One who's already started is Bernard Wood, an anthropologist at George Washington University.

Professor BERNARD WOOD (Anthropologist, George Washington University) What I'm not seeing is anything that really convinces me that it's the ancestor of modern humans.

JOYCE: Wood says sometimes unrelated species evolve very similar traits. Bats and birds both have wings, for example. Wood says Ardipithicus could have been a twig on a very different branch from the human line.

Prof. WOOD: We know if you look at the rest of the tree of life, the minority of branches reach the surface. Most of the branches never make it.

JOYCE: The discoverers have published their findings in the journal Science. And they say they're ready for that debate. They've had 15 years to prepare.

Christopher Joyce, NPR News.

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