They probably also interbred with modern humans, the scientists said.

Previous evidence suggested that Neanderthals died out some 34,000 years ago, and were replaced by modern Homo sapiens. But the international team did more tests on the Neanderthal bones, found in the 1970s, and found they were much younger than that.

"The new radiocarbon dates suggest Neanderthals would have coexisted with early modern humans in central Europe for several millennia," said Fred Smith of the anthropology department at Northern Illinois University, US.

No immediate wipe out

Professor Smith and his colleagues argue that their findings contradict the idea that modern humans were so superior to Neanderthals that on arrival they immediately wiped them out.

"I think what is most likely is that early modern humans dispersed into Europe, in some places competed with and replaced Neanderthals, in other places assimilated them," said Erik Trinkaus, an anthropologist at Washington University in St. Louis, who led the study.

"The differences between the two groups in basic behaviour and abilities must have been small and rather subtle," he said.

Professors Trinkaus and Smith have put forward the controversial idea that Neanderthals and early modern humans did interbreed.

Mixed message

Earlier this year Trinkaus reported finding the 24,500-year-old bones of a child in Portugal that he said showed characteristics of Neanderthals and of modern humans.

Professor Smith said he also thought the Croatian bones had some "modern" characteristics: "For example, they tend to show features that are a bit more like modern humans than other Neanderthals show."

"And most early modern Europeans exhibit some features that are hard to explain if Neanderthals are totally excluded from their ancestry." Professor Smith added.

Professor Trinkaus said the new evidence shows the two populations would have had plenty of time to mix in the thousands of years they lived alongside one another.