A hybrid skeleton showing features of both Neanderthal and early modern humans has been discovered, challenging the theory that our ancestors drove Neanderthals to extinction.

The skeleton of a young boy was found in Portugal.

Scientists say it shows for the first time that Neanderthals, who became extinct tens of thousands of years ago, mated with early members of our own species.

The scientists believe that the offspring of the interbreeding could be ancestors of modern man.

"This skeleton, which has some characteristics of Neanderthals and others of early modern humans, demonstrates that early modern humans and Neanderthals are not all that different. They intermixed, interbred and produced offspring," said Erik Trinkaus of Washington University.

But Joao Zilhao of the Portuguese Archaeological Institute said more research was needed to back up the controversial theory.

And Dr Robert Foley of Cambridge University told the BBC: "The fossil evidence as we currently understand it doesn't show the signs of hybrids between Neanderthals and modern humans, so it would be a novel and unusual find."

Child skeleton found in rabbit hole

The skeleton, thought to be that of a four-year-old boy, was found when an archaeologist explored a rabbit hole near the coast north of Lisbon.

The child had been given a ritual burial, with red ochre and pierced shells.

He had the pronounced chin and teeth of modern humans, but his sturdy limbs were more characteristic of the Neanderthals.

The Neanderthals were a powerfully-built species who evolved to cope with the challenging climate of Ice Age Europe.

While their brains were bigger than our own, Neanderthals never developed the sophisticated culture and technology that became the hallmark of their modern human contemporaries.

'Out of Africa' theory challenged

Most anthropologists believe that modern humans evolved in Africa by about 100,000 years ago. They eventually spread across the world - the so-called "Out of Africa" theory.

By 20,000 years ago, the Neanderthals were extinct. But it was not known whether modern humans destroyed them, or whether their distinctive characteristics disappeared through interbreeding.

Dr Trinkaus says the Portuguese skeleton provides powerful evidence for the interbreeding theory.

"This find refutes strict replacement models of modern human origins - that early modern humans all emerged from Africa and wiped out the Neanderthal population," said Dr Trinkaus.

The scientists believe that raises the possibility that people alive today could have some genes inherited from Neanderthal ancestors.

"A major contribution"

Chris Stringer, an expert on Neanderthal man at the Museum of Natural History in London, said he expected the find to make a "major contribution" to the debate on how the Neanderthals died out.

The hybridisation theory has been difficult to prove because only fragments of skeletons have previously been found, Dr Stringer said.

"The Iberian peninsula is an area where there was a significant overlap in time and space between Neanderthal and modern man. They could have coexisted for as long as 10,000 years," he said.