Cross-species liaisons occurred as Stone Age wound down
Web edition : 9:41 am
A new study suggests that present-day Europeans share more genes with now-extinct Neandertals than do living Africans, at least partly because of interbreeding that took place between 37,000 and 86,000 years ago.
Cross-species mating occurred when Stone Age humans left Africa and encountered Neandertals, or possibly a close Neandertal relative, upon reaching the Middle East and Europe in the latter part of the Stone Age, says a team led by geneticist Sriram Sankararaman of Harvard Medical School.
The new study, published online October 4 in PLOS Genetics, indicates that at least some interbreeding must have occurred between Homo sapiens and Neandertals, Sankararaman says. But it’s not yet possible to estimate how much of the Neandertal DNA found in modern humans comes from that interbreeding and how much derives from ancient African hominid populations ancestral to both groups.
A separate analysis of gene variants in Neandertals and in people from different parts of the world also found signs of Stone Age interbreeding outside Africa. That study, published online April 18 in Molecular Biology and Evolution, was led by evolutionary geneticist Melinda Yang of the University of California, Berkeley.
Results from Sankararaman and Yang’s groups “convincingly show that the finding of a higher proportion of Neandertal DNA in non-Africans compared to Africans can be best explained by gene flow from Neandertals into modern humans,” says evolutionary geneticist Johannes Krause of the University of Tübingen in Germany.
Other studies have found that ancient interbreeding may not be necessary to explain the presence of Neandertal DNA in modern humans. It may be possible that African populations ancestral to both H. sapiens and Neandertals possessed some genes that became part of both species’ genomes. Evolutionary ecologists Anders Eriksson and Andrea Manica of the University of Cambridge recently demonstrated the plausibility of this scenario using a model based on more than 100 populations of human-Neandertal ancestors spread across Africa, Europe and West Asia.
Sankararaman’s analysis assumes that the common ancestors of humans and Neandertals more than 230,000 years ago consisted of two African populations and one population outside Africa. It’s not clear whether a more complex model that includes 100 or more populations of human-Neandertal ancestors would yield any signs of late Stone Age interbreeding, says Cambridge’s Manica.
Sankararaman and his colleagues measured the lengths of DNA segments shared by Neandertals and present-day Europeans. Since genetic reshuffling via sexual reproduction reduces the size of such segments over time, lengths of Neandertal-related chunks of DNA in people today can be used to calculate the time since those chunks entered the human genome.
The researchers say that mating between European Neandertals and modern humans most likely occurred between 47,000 and 65,000 years ago.
Other evidence finds slightly more Neandertal DNA in present-day East Asians and South Americans than in Europeans (SN: 9/22/12, p. 5). Interbreeding might have occurred separately in Asia and Europe, Sankararaman says. Or large numbers of people migrating into Europe after interbreeding took place could have diluted the Neandertal genetic contribution to populations already living there.