Speciation by distance and temporal overlap: a new approach to understanding Neanderthal evolution

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Neanderthals are the best-known fossil hominid group, but at the same time many aspects of their evolution are still poorly understood. The variation of numerous characters in Neanderthal populations shows a geographical gradient. From west to east, characters become less and less Neanderthal-like and more and more modern humanlike. Moreover, in Central Europe and the Near East, post-Neanderthal populations still exhibit some Neanderthal features, which is not the case in Western Europe. The spread of the first humans into Europe involved differentiation of this species by distance, whereas consecutive populations were linked by gene flow. Hence, from Western Europe to the Near East, there was a succession of human populations that developed, over time, Neanderthal characters that were more and more marked from east to west. Then, modern humans spread rapidly into Europe at about 40,000 years ago, but at least in the western part of the continent, no convincing evidence of hybridization with Neanderthals has been found. By contrast, interbreeding was still possible in the eastern part of Europe and in the Near East, but became less and less so towards the west. This hypothesis implies that the ancestors of Neanderthals arrived and evolved in Europe at a time when gene flow between Western Europe and Near Eastern populations was very limited. Hence, Near East Neanderthals cannot be interpreted as the result of a migration of a European population toward the east, but as a continuum in space and time of European inhabitants.