Did Neanderthals and modern humans interbreed? While several methods, including the study of mitochondrial DNA of Neanderthals, failed to resolve this longstanding controversy, a comparison of the genomes of Neanderthals and modern humans shows that between 1 and 4 per cent of the DNA of humans living today came from Neanderthals. The results of the study were published recently in Science. Researchers compared DNA sequences of Neanderthals obtained from bones found in different locations with those of five present-day humans — from southern Africa, West Africa, France, China, and Papua New Guinea. Though there is evidence of gene flow from Neanderthals to modern humans, the level of flow is relatively low. According to the researchers, the low level of gene flow indicates that interbreeding may have been “very limited.” However, there is no evidence of a reverse flow. When a small expanding population, in this case modern humans who have moved out of Africa, encounters a resident population (Neanderthals) and interbreeds, there is a greater possibility of gene flow from the resident population to the small expanding population. The gene flow might be low, but individuals from China and Papua New Guinea have been found to carry the Neanderthal genes. Morphologically recognisable Neanderthal fossils have been found only in Europe and West Asia. The presence of Neanderthal genes in people from China and Papua New Guinea could have resulted only if the interbreeding had happened in the Middle East soon after modern humans moved out of Africa and long before they moved to other parts of the world.

While individuals from China and Papua New Guinea carry Neanderthal genes, individuals from West Africa and Southern Africa do not. Neanderthals appear to be genetically closer to individuals in Eurasia than to those in Africa. The presence of the Neanderthal genome in present-day non-Africans challenges the simple ‘Out-of-Africa' model. This model is based on the notion that all present-day humans trace all their ancestry to a small African population. While this is largely true, the small portion of the Neanderthal genome in people outside Africa could have played a minor role in the non-African genetic ancestry. Another achievement of this study is the successful comparison of the genomes of Neanderthals and present-day humans despite the researchers not knowing the fixed differences in the nuclear genome of the two. We now know they shared ancestors — and that Neanderthals resembled present-day humans more closely than was believed.