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Neandertals Hot To Trot

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The gene microcephalin (MCPH1) regulates brain size during development and has experienced positive selection in the lineage leading to Homo sapiens. Within modern humans, a group of closely related haplotypes at this locus, known as haplogroup D, rose from a single copy ~37,000 years ago and swept to exceptionally high frequency (~70% worldwide today) because of positive selection … we show that haplogroup D likely originated from a lineage separated from modern humans ~1.1 million years ago and introgressed into humans by ~37,000 years ago. This finding supports the possibility of admixture between modern humans and archaic Homo populations (Neanderthals being one possibility).

The features that we recognize as Neandertal features, were defined as Neandertal features by virtue of the fact that they are mostly gone! That means that any alleles correlated with Neandertal morphological features were almost certainly selected against, or were at best neutral. That means that those recognizably Neandertal genes are gone! But here we have a gene that looks to have come from some archaic population. Adaptive introgression occurs when adaptive alleles are selected, and broken apart from their genetic background.

Introgression is the transfer of alleles across species or subspecies boundaries. In other words, it describes gene flow between populations that are partially isolated … Mammal species just don’t establish reproductive barriers very quickly. Comparing mammals, postzygotic isolating mechanisms take between 2 and 10 million years to evolve. No primate species pairs have evolved postzygotic isolation on the timescale represented by the evolution of Homo. When archaic and modern humans were in contact, they certainly interbred.

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