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Fossils of human ancestors are rare. You could easily fit all that scientists have found in the back of one pickup truck. But a remarkable site in Georgia, in the former Soviet Union, has produced a cornucopia of bones dating back almost two million years.

As NPR's Christopher Joyce reports, scientists are excited and a bit puzzled by what they've uncovered there.

CHRISTOPHER JOYCE, BYLINE: The fossil hunters found the cache of bones over a decade ago in a place called Dmanisi. The team kept most of it under wraps. Now, they've lifted the veil. What they've got are the fossil remains of five creatures who lived 1.8 million years ago, including a magnificent adult male skull.

William Jungers, a professor of anatomy at Stony Brook University in Long Island, is impressed.

WILLIAM JUNGERS: That skull is incredible. It's got to be one of the most complete skulls ever discovered in the fossil record of human evolution.

JOYCE: And they've got bones from five individuals. Team member Marcia Ponce de Leon, of the Anthropological Institute in Switzerland, says that's unique this far back in time.

MARCIA PONCE DE LEON: For the first time, we can see a population. We only had individuals before.

JOYCE: Having a population means scientists could look for similarities in the bones that would characterize what this entire species was like. What puzzled them were the big differences, though, in the bones. There was a lot of variety, almost a grab bag of features, small brain case, a big protruding jaw, giant teeth. They looked like a mix of species. But Ponce de Leon's colleague, Christoph Zollikofer, notes that all five apparently died within centuries of each other in the same place.

CHRISTOPH ZOLLIKOFER: So we are pretty sure that the variation that we see is that within a species, a single evolving lineage.

JOYCE: So OK, they found a human ancestral species with a lot of physical variety from one individual to the next, but this poses a problem. The conventional wisdom on early human evolution is it that there were several species that arose in Africa: Home habilis, Homo rudolfensis, Homo erectus, maybe even more. But this new discovery suggests that a single species, like the Dmanisi Five, can have a lot of physical variety and that means, says, Zollikofer, maybe there weren't numerous early human species. There was just one.

JUNGERS: You know, I think there are going to be people who won't like this.

JOYCE: Those people, says William Jungers at Stony Brook, have argued that there were lots of sibling species of early humans that popped up independently in Africa, Asia and Europe, only to die out. They were stages, or experiments in human evolution. The new research suggests a different narrative.

JUNGERS: There may have been one very successful species that emerges from Africa, subsequently and rapidly spreads to Southeast Asia. That's the picture of a very successful, cosmopolitan species.

JOYCE: Like most new ideas on human evolution, this one has skeptics. Brian Richmond is an anthropologist at George Washington University. Richmond says the techniques used in this new research gloss over the true amount of variation among those earliest African fossils.

BRIAN RICHMOND: It doesn't get at the more fine-grained aspects of anatomy that actually distinguish species from one another. It's a little bit like using a telescope when in this case they need a magnifying glass.

JOYCE: Richmond says the discovery, published in the journal Science, is nonetheless a treasure of new data for scientists to ponder and to argue about. Christopher Joyce, NPR News.

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