A team of Harvard and Max Planck Institute researchers has estimated the date when Neanderthals and modern Europeans last shared ancestors to explore why Neanderthals are most closely related to people outside Africa.
The study, published in the journal PLoS Genetics, provides a historical context for the interbreeding. It suggests that it occurred when modern humans carrying Upper Paleolithic technologies encountered Neanderthals as they expanded out of Africa.
Previous genetic studies have revealed that people outside Africa share slightly more genetic variants with Neanderthals than Africans do.
One scenario that could explain this observation is that modern humans mixed with Neanderthals when they came out of Africa.
An alternative, but more complex, scenario 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.
Now the team has 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 people.
The researchers have found 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.
Bibliographic information: Sriram Sankararaman et al. 2012. The Date of Interbreeding between Neandertals and Modern Humans. PLoS Genet 8(10): e1002947; doi: 10.1371/journal.pgen.1002947