Mating between modern humans and their Neanderthal ancestors, in Europe and Asia tens of thousands of years ago, is now the only plausible explanation for the genetic similarity between the two, say scientists.
A new study, using an original genetic technique, says alternative explanations can now be ruled out.
It has long been recognised that modern-day Europeans and Asians bear closer genetic similarity than Africans to our ancient cousins.
Dr. Konrad Lohse, of the University of Edinburgh, led the study. He said:
Two alternative theories had emerged to explain this phenomenon. One idea was that the modern humans that left Africa had evolved from the same African sub-population that had earlier given rise to the Neanderthals.
The other, more likely, explanation was that modern humans had met and mated with the Neanderthals in Europe and Asia.
We are very confident that we can now rule out the idea that this genetic similarity originated in Africa. We hope our study settles this issue.
Previous studies have sequenced large numbers of modern human genomes, comparing them with Neanderthal samples to reconstruct the likely order of events. But none had been able to provide a definitive answer on the question of interbreeding.
Here, the team split single genomes from an African, a Eurasian, a Neanderthal and a chimpanzee into smaller, more manageable chunks. This allowed them to carry out a statistical analysis of the genetic code that would have been impossible on the full length of the genetic sequence.
The results are published in the journal Genetics. The team now hope to use their new method to see whether similar evolutionary phenomena have occurred in other species. Lohse said:
We want to see how often these evolutionary loops occur throughout the tree of life, where species diverge but then meet again to form hybrids. We should be able to use these techniques on lots of different species.