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Dryad

A relatively higher mutation rate in African humans is the dominant mechanism causing Neanderthals to appear closer to non-Africans, not introgression

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May 26, 2022 version files 3.82 GB

Abstract

It is widely thought that humans carry genetic legacies due to inter-breeding with Neanderthals, but all methods used to infer legacies ignore recurrent mutations and assume a constant mutation rate. These assumptions cause automatic rejection of a second hypothesis, where a higher mutation rate in Africans caused increased divergence from Neanderthals. Any fair test should strive to treat both hypotheses equally. Here I use mutation spectra, the relative frequencies of different mutating three-base combinations, to compare contrasting expectations from the two hypotheses. I find that putative introgressed bases are strongly enriched for recurrent mutations and lie in regions with unusually high mutation rates, distorted mutation spectra and unusually large African minus non-African heterozygosity differences. Moreover, putative introgressed bases should be absent from Africa and rare outside, yet almost the entire signal of introgression is carried by sites where putative Neanderthal alleles are fixed in non-Africans and at high frequency in Africans. Together, these observations support a model where signals of introgression are driven mostly or even entirely by mutation rate differences between human populations. This new model helps to explain why introgression is inferred ubiquitously, including in scenarios involving great apes where inter-breeding is biologically implausible.