March 20, 2008 4:24 PM Subscribe
Any admixture would have to be driven by male Neanderthals. Two years ago
posted by wantwit (19 comments total)
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we discussed morphological evidence of nontrivial interbreeding
. Since then Neanderthal DNA has
been examined for
genetic support for this model of human evolution, largely contradicting
the belief in Neanderthal contribution to modern humanity. Indeed any contribution from the Neanderthal gene pool to the evolution of modern humans might be very rare and indeed it appears that the best candidate gene thus (MC1R
) far likely was a result of convergent evolution
EXCERPT: It is now clear that the level of interbreeding between the two populations, if any, was so low that we are unlikely to find any neutrally evolving Neanderthal alleles in modern populations. However, it is possible that low levels of interbreeding could have led to the adaptive transfer of some alleles between species (introgression). Beneficial alleles can persist in interspecific hybrids even when the hybrids are less fit than either parent population as long as the hybrids are fertile . As hybrids back-cross to a parent population, most introduced alleles will be lost to drift or to negative selection; some beneficial alleles, however, may be maintained in subsequent generations. Claims have been made for adaptive introgression from Neanderthals into populations of modern humans at the microcephalin  and the tau  loci. Some proponents of the multiregional model have gone so far as to suggest that adaptive introgression was a primary source of beneficial alleles during the evolution of modern humans . While we regard this latter idea as unsupported by the available Neanderthal and human genome sequences, it is worth considering the possibility that a very limited amount of adaptive introgression has occurred.
MC1R is a good a priori candidate for adaptive introgression. It is thought that light skin is favored in Europe as a compromise between the need for vitamin D synthesis and the need to prevent folate photolysis, both caused by UV radiation . Several genes affecting skin color are known to have been positively selected in European populations [21,22], though studies of MC1R evolution have come to different conclusions [22,42,43]. Jolly has pointed out that the easiest way for early modern humans entering Europe to evolve light skin would be to acquire the necessary genes from Neanderthals rather than to evolve them de novo . If the low-activity MC1R variant is positively selected in Europe, then MC1R presents a good opportunity to test for evidence of adaptive introgression from Neanderthals to modern humans. However, although Neanderthals and modern Europeans share a low-activity MC1R phenotype, the genotype is different (see above), which argues against significant adaptive introgression. The hypothesis could be tested more rigorously using Neanderthal sequence from other loci affecting skin color with a clearer signal of positive selection in Europeans.