But our closest ancestors may well have been able to speak as well as us, said Prof Svante Paabo from Germany's Max Planck Institute.

He was speaking in Chicago, US, where he announced the "first draft" of a complete Neanderthal genome.

The genetics information has been gleaned from fossils found in Croatia.

Prof Svante Paabo confirmed that Neanderthals shared the FOXP2 gene associated with speech and language in modern humans.

A total of three billion "letters", covering 60% of the Neanderthal genome, have been sequenced by scientists from Max Planck Institute for Evolutionary Anthropology and 454 Life Sciences Corporation, in Branford, Connecticut.

The majority of the sequence comes from bones from Vindija Cave in Croatia.

The draft genome can give us clues to the genetic regions which make us "uniquely human", Prof Paabo told BBC News.

"It was always a dream to look at the DNA of our closest evolutionary relatives.

"Now that we have the Neanderthal genome, we can look for areas in the human genome where a change seems to have swept rapidly through us since we separated from Neanderthals.

"There, something special may have happened in us. The cool thing is, now that we have the whole genome, we can look for these changes without bias."

Prof Paabo released details about the latest stage of his research at the annual meeting of the American Association for the Advancement of Science (AAAS).

Tongue twister

Neanderthals lived in Europe and parts of Asia until they became extinct about 30,000 years ago.

They were the closest relatives of

currently living humans, sharing between 99.5% to 99.9% of our DNA sequence.

Pinpointing the differences may reveal the crucial evolutionary changes that enabled modern humans to leave Africa and rapidly spread around the world, starting around 100,000 years ago.

Accordingly, Prof Paabo and his team have focused on genes of special interest in recent human evolution, such as FOXP2, which is involved in speech and language.

Humans differ from chimpanzees at two key points in the FOXP2 gene.

But the preliminary results suggest that Neanderthals shared these same variations.

"There is no reason to believe they couldn't speak like us," said Prof Paabo.

"But of course there are many other genes involved in speech and language, so there are many more studies to be done."

Breeds apart

Since Neanderthals lived side by side with modern humans in Europe for many thousands of years, it has been speculated that we may have inherited some Neanderthal DNA in our genome today, thanks to interbreeding.

But Professor Paabo's team have found no evidence for this.

They focused on a gene implicated in brain development - microcephalin-1 - which shows significant variation among present day humans.

It has been suggested that a particular variant of the gene, found commonly in Europeans, was contributed by Neanderthals.

But the Croatian Neanderthal fossils harboured an ancestral form of the microcephalin-1 gene, which today is also found among Africans.

Overall, it seems that Neanderthals have contributed, at most, a "very limited" fraction of

the variation found in contemporary human populations, said Prof Paabo.

"What is exciting now, is that we are beginning to look the other way - from early modern humans into Neanderthals."

Professor Chris Stringer, from the Natural History Museum, London, UK, commented: "If the the Neanderthal genome data show little evidence of potential hybridisation, that would fit with my view from the fossil evidence that, while interbreeding was probably possible, it may have occurred only rarely, with trivial impact on modern humans.

"The populations had been separate for hundreds of thousands of years and I think there would have been significant physical and behavioural differences between them. However, larger samples would be desirable to get a more complete picture, and hopefully those will follow soon."

Dead end

Prof Paabo said the team did not expect to find any clues which might help solve the riddle of the Neanderthals' demise.

"I don't think they became extinct due to something in their genome," he said.

"It was clearly something in their interaction with the environment or with modern humans that caused them to be extinct.

"That will not be something you can see from their DNA sequence."

And Prof Paabo was quick to pour cold water on any suggestion that the genome sequence would facilitate the cloning of a Neanderthal.

"We are talking about a very complex mammal," said the 53-year-old.

"I don't think that technology will improve fast enough to make this possible in my lifetime.

"It is more in the realm of science fiction than science."