Modern humans and Neanderthals had sex across the species barrier, according to a leading geneticist who is overseeing a project to compare their genomes.

Professor Svante Paabo, director of genetics at the renowned Max Planck Institute for Evolutionary Anthropology in Leipzig, will shortly publish his analysis of the entire Neanderthal genome, using DNA retrieved from fossils. He aims to compare it with the genomes of modern humans and chimpanzees to work out the ancestry of all three species.

Modern humans arrived in Europe from Africa about 40,000 years ago to find Neanderthals already living there. The two species then co-existed for 10,000-12,000 years before Neanderthals died out — a fact that has caused endless academic speculation about whether they interbred.

Paabo recently told a conference at the Cold Spring Harbor Laboratory near New York that he was now sure the two species had had sex — but a question remained about how “productive” it had been.

“What I’m really interested in is, did we have children back then and did those children contribute to our variation today?” he said. “I’m sure that they had sex, but did it give offspring that contributed to us? We will be able to answer quite rigorously with the new [Neanderthal genome] sequence.”

Such an answer might ease the controversy over recent contradictory discoveries regarding Neanderthals. Some fossils seem to have both modern human and Neanderthal features, suggesting that the two species interbred. Yet DNA scans have shown that Neanderthal genes were very different from those of modern man.

Last week Professor Chris Stringer, head of human origins at the Natural History Museum, presented a conference at the Royal Society in London with an idea that could accommodate both sets of evidence.

“It’s possible that Neanderthals and humans were genetically incompatible, so they could have interbred but their children would have been less fertile,” said Stringer.

This phenomenon is seen in many other species such as when lions breed with tigers and horses breed with zebras.

“I used to believe Neanderthals were primitive,” said Stringer, “but in the last 10,000-15,000 years before they died out, around 30,000 years ago, Neanderthals were giving their dead complex burials and making tools and jewellery, such as pierced beads, like modern humans.”

Due to the length of time that has elapsed since Neanderthals became extinct, any trace of their DNA in modern humans could have been diluted below detectable levels. Paabo hopes to overcome this by scanning the Neanderthal genome for the genes of modern humans.

Click here to watch an interview with Professor Svante Paabo, who is overseeing the neanderthal genome project, where he talks about whether neanderthals and modern humans had sex.

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