It is known that there was gene flow from Neanderthals to modern humans around 50,000 years ago; now, analysis of a Neanderthal genome from the Altai Mountains in Siberia reveals evidence of gene flow 100,000 years ago in the other direction—from early modern humans to Neanderthals. Sergi Castellano and colleagues analyse genomic data from Neanderthal and Denisovan modern humans from the Altai Mountains in Siberia and from Neanderthals from Spain and Croatia. Using a Bayesian method for inference of demographic models known as G-PhoCS (Generalized Phylogenetic Coalescent Sampler), the authors obtain preliminary quantitative estimates of previously reported gene flow events between modern and archaic humans. They also report evidence of gene flow from an early modern human population to the ancestors of Neanderthals from the Altai Mountains more than 100,000 years ago, in the opposite direction to the instances of gene flow from Neanderthals to modern humans. It has been shown that Neanderthals contributed genetically to modern humans outside Africa 47,000–65,000 years ago. Here we analyse the genomes of a Neanderthal and a Denisovan from the Altai Mountains in Siberia together with the sequences of chromosome 21 of two Neanderthals from Spain and Croatia. We find that a population that diverged early from other modern humans in Africa contributed genetically to the ancestors of Neanderthals from the Altai Mountains roughly 100,000 years ago. By contrast, we do not detect such a genetic contribution in the Denisovan or the two European Neanderthals. We conclude that in addition to later interbreeding events, the ancestors of Neanderthals from the Altai Mountains and early modern humans met and interbred, possibly in the Near East, many thousands of years earlier than previously thought.

Ancient gene flow from early modern humans into Eastern Neanderthals