NEW YORK (GenomeWeb News) – The most recent mingling between modern humans outside of Africa and Neandertals appears to have occurred during the late Stone Age, according to a new genetic analysis by American and German researchers.
To assess the time frame for Neandertal gene flow into non-African populations, a team led by Harvard University's David Reich and Svante Pääbo, director of the Max Planck Institute for Evolutionary Anthropology's evolutionary genetics department, did linkage disequilibrium analyses using genome sequences for European-America, East Asian, and West African individuals sequenced for the 1000 Genomes Project pilot.
Their findings, published online last night in PLoS Genetics, pegged the most recent introduction of Neandertal genes into the ancestors of modern Europeans at roughly 37,000 to 86,000 years ago. The team's less stringent estimates put this gene flow in a slightly narrower window: within the last 47,000 to 65,000 years.
The analysis supports the notion that Neandertals and modern humans mixed with one another relatively recently, the study authors explained, and hints that "interbreeding may have occurred when modern humans carrying Upper Paleolithic technologies encountered Neandertals as they expanded out of Africa."
Since the Neandertal genome sequence was reported in 2010, researchers have found that human populations living outside of Africa carry Neandertal sequences in their genomes — sequences that are commonly believed to be genetic remnants of ancient breeding events between modern humans and Neandertals following human migration out of Africa.
Even so, the study authors noted, there is an alternative population substructure model that might also explain these patterns. In that scenario, the Neandertal sequences in present-day non-African populations would be predicted to stem from the existence of multiple hominin populations in Africa prior to the appearance of distinct modern human and Neandertal forms.
"If this substructure persisted until modern humans carrying Upper Paleolithic technologies expanded out of Africa so that the modern human population that migrated was genetically closer to Neandertals," they wrote, "people outside Africa today would share more genetic variants with Neandertals than people in sub-Saharan Africa."
When the researchers looked at specific SNP linkage disequilibrium patterns in genome sequences of 59 West African, 60 East Asian, and 60 European-American individuals sequenced for the 1000 Genomes Project, though, they found evidence indicating that gene flow from Neandertals or a closely related species into some human populations happened much more recently than the modern human-Neandertal split.
Through analyses that included genetic distance mapping for SNP pairs against a genetic reference map developed by Decode Genetics, the team estimated that gene flow from Neandertals occurred as recently as 47,000 to 65,000 years ago.
That argues against a population substructure-based explanation for Neandertal sequences in some human populations, since fossil records for modern humans and Neandertals each go back at least 200,000 years.
While such findings are consistent with the modern human-Neandertal admixture model, the team noted, it doesn't rule out the possibility that population substructure also existed in ancestral human populations. If that were the case, the researchers explained, simulations suggest that the timing of Neandertal-human interbreeding may have happened over a somewhat broader time span, occurring during the past 37,000 to 86,000 years.
"Genetic analyses by themselves offer no indication of where gene flow may have occurred geographically," the researchers noted. "However, the data in conjunction with the archaeological evidence suggests that the two populations likely met somewhere in Western Eurasia."