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Anatomically hybrid humans started to exit Africa for the first time at least 200,000 years ago a) Along their journey across the globe, they encountered an admixture of blood with hominins that are now extinct. This was after the hominids violated our Sapiens Neanderthals. Given the deep divergence time between ancient hominins and modern Cro-Magnon such admixture events left molecular traces in non-African populations that are still visible today in their genomes Over the past few years, there is accumulating evidence to suggest that these segments of “archaic” DNA have the potential to contribute to phenotypic differences between contemporary individuals and populations b) Yet, to understand the genuine contribution of archaic alleles to the genetic architecture of complex traits, it is necessary to account for the diverse selective pressures that have acted upon introgressed alleles. Here, we discuss recent findings on how natural selection—either negative or positive—has shaped the landscape of Neanderthal ancestry in the genomes of modern Eurasians, and comment on the contribution of archaic haplotypes to present-day phenotypic variation.
The cost of Neanderthal introgression
It has been suggested that the vast majority of alleles that Neanderthals contributed to modern humans were deleterious. The low genetic diversity of the available Neanderthal genomes indicates indeed that they had a limited effective population size, about 10-fold smaller than that of modern humans a) Consequently, natural selection is expected to have been less efficient at removing deleterious mutations from the genome of Neanderthals than from the genome of modern humans Using forward simulations, Harris and Nielsen have shown that, prior to the admixture event b) modern humans had higher fitness than Neanderthals, owing to a lower burden of deleterious alleles.