The biggest payoff of the summer was the discovery of more fragments of the three individuals found earlier, which added to the evidence of hybrid traits.
Trinkaus said the Oase fossils show features of modern humans: projecting chin, no brow ridge, a high and rounded brain case. But they also have clear archaic features that place them outside the range of variation for modern humans: a huge face, a large crest of bone behind the ear and enormous teeth that get even larger toward the back.
A researcher shows part of a skull found in the Pestera cu Oase cave in Romania.
(Erik Trinkaus And Ricardo Rodrigo)
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Trinkaus made a CT scan of the face to measure the unerupted teeth. "To find wisdom teeth that big," he said, "you have to go back 500,000 years."
The team considered whether early humans might have interbred with other hominids with Neanderthal-like features, but "in this time period," said Trinkaus, "the only archaic humans those modern humans could have interbred with were Neanderthals." The mosaic of Neanderthal and modern traits remind Trinkaus and Zilhao of similar traits they found in a 25,000-year-old fossil of a child in Portugal.
Researchers pondering why the Neanderthals died out have speculated that early humans might have killed them off, and Zilhao said the signs of interbreeding do not exclude that possibility. "We know that even when people fight, the winner might kill the males and keep the females from the other side," he said.
The signs of interbreeding challenge the standard wisdom that Neanderthals were a distinct, less intelligent species.
"If you look at the archaeological evidence," argued Trinkaus, "which includes things like burials, there is very little difference between what we find associated with Neanderthals and what we find associated with early modern humans -- from the same time period."
Richard Klein of Stanford University thinks this holds true only until about 50,000 years ago, when modern human behavior changed dramatically. "There could have been interbreeding," Klein conceded. "But all the genetic evidence we have suggests that, if it occurred, it was remarkably rare."
Six years ago, Zilhao and Francesco d'Errico of the University of Bordeaux published evidence that Neanderthals independently invented and used personal ornamentation. Zilhao said these finds have changed the view that Neanderthals were an inferior species.
Klein said the picture is changing, but not in that direction. The real question today, he said, is "whether modern humans fully replaced the Neanderthals or simply swamped them" genetically, with greater numbers. "And it may never be possible to say."
