WASHINGTON (Reuters) - A 40,000-year-old skull found in a Romanian cave shows traits of both modern humans and Neanderthals and might prove the two interbred, researchers reported on Monday.
If the findings are confirmed, the skull would represent the oldest modern human remains yet found in Europe.
DNA samples taken from Neanderthal bones suggest there was no mixing, or at least that any Neanderthal genetic contribution did not make it to the modern DNA pool.
But Erik Trinkaus of Washington University in St. Louis has in the past found bones that he believes show both modern human and Neanderthal traits, and now he and colleagues have found a skull.
The skull, probably that of a teenager, has been dated to 40,000 years ago and shows many modern traits. But it also is a little flatter than most modern Homo sapiens, and exceptionally large upper molars more associated with Neanderthals.
"Such differences raise important questions about the evolutionary history of modern humans," said Joao Zilhao of the University of Bristol in Britain, who worked on the study.
It could be "evolutionary reversal" he said -- humans changing back into archaic forms.
"They could also reflect admixture with Neanderthal populations as modern humans spread through western Eurasia," Zilhao said in a statement.
"This mixture would have resulted in both archaic traits retained from the Neanderthals and unique combinations of traits resulting from the blending of previously divergent gene pools."
Modern humans are believed to have spread into Europe around 45,000 to 50,000 years ago, and had completely replaced the older Neanderthals by 30,000 years ago.
But that means at least 10,000 years of living side by side, and artifacts attributed to the more modern humans have been found at Neanderthal sites.
Neanderthals were also once designated Homo sapiens, although are a designated subspecies -- Homan sapiens neanderthalis. But some experts now designate them as a separate species -- Homo neanderthalis.
Zilhao, Trinkaus and colleagues examined the skull from the Pestera cu Oase, or the Cave with Bones, in southwestern Romania. It was mostly full of bones from cave bears but then the researchers found some human skull fragments.
They are the earliest modern human remains found in Europe, although last week researchers reported finding 45,000-year-old human artifacts in Russia south of Moscow.
