Humans bred with Neanderthals after leaving Africa

Humans last interbred with Neanderthals well after they spread out of Africa, a genetic analysis has shown.

The reason that Neanderthals are most closely related to people outside Africa, say Harvard and Max Planck Institute scientists, is that they last interbred when modern humans carrying Upper Paleolithic technologies encountered Neanderthals as they expanded out of Africa.

When the Neanderthal genome was sequenced in 2010, scientists discovered that people outside Africa share slightly more genetic variants with Neandertals than Africans do.

One explanation is that modern humans mixed with Neanderthals when they came out of Africa. Another, more complex, theory is that African populations ancestral to both Neanderthals and modern humans remained subdivided over a few hundred thousand years, and that those more related to Neanderthals subsequently left Africa.

To try and answer this question, the team measured the length of DNA pieces in the genomes of Europeans that are similar to Neanderthals. Since recombination between chromosomes when egg and sperm cells are formed reduces the size of such pieces in each generation, the Neanderthal-related pieces will be smaller the longer they have spent in the genomes of present-day humans.

And the results indicate that Neanderthals and modern humans last exchanged genes between 37,000 and 86,000 years ago – well after modern humans appeared outside Africa, but potentially before they started spreading across Eurasia.

This suggests that Neanderthals,or their close relatives, had children with the direct ancestors of present-day people outside Africa.