New genetic research appears to confirm that the ancestors of modern non-Africans interbred with Neanderthals.
Some of the human X chromosome originates from Neanderthals and is found exclusively in people outside Africa, say researchers led by Damian Labuda of the University of Montreal and the CHU Sainte-Justine Research Center.
The results support a similar conclusion from University of California geneticists late last year.
"This confirms recent findings suggesting that the two populations interbred," says Labuda. "In addition, because our methods were totally independent of Neanderthal material, we can also conclude that previous results were not influenced by contaminating artifacts."
He believes that this interbreeding took place early on, probably at the crossroads of the Middle East.
Neanderthals, whose ancestors left Africa about 400,000 to 800,000 years ago, evolved in what is now mainly France, Spain, Germany and Russia, and are thought to have lived until about 30,000 years ago. Meanwhile, early modern humans left Africa about 80,000 to 50,000 years ago.
Almost ten years ago, Labuda and his team identified a piece of DNA called a haplotype in the human X chromosome whose origins they questioned. When the Neanderthal genome was sequenced last year, they compared 6,000 chromosomes from all parts of the world to the Neanderthal haplotype.
They found that Neanderthal sequence was present in people across all continents, including Australia, except for sub-Saharan Africa.
"There is little doubt that this haplotype is present because of mating with our ancestors and Neanderthals. This is a very nice result, and further analysis may help determine more details," says Dr Nick Patterson, of the Broad Institute of MIT and Harvard University.