An ancestral recombination graph of human, Neanderthal, and Denisovan genomes:
We note that our estimated TMRCA to Neanderthal within Neanderthal-introgressed segments in all non-African populations is recent, ~74 ka ago, and implies therefore that little genetic drift separates admixed humans from sequenced Neanderthals in these segments. This recent TMRCA suggests that the majority of Neanderthal ancestry in modern humans originated from Neanderthal gene flow into the ancestors of all non-Africans before populations diversified. It also suggests that at least one of the Neanderthal genomes used here is closely related to the Neanderthal(s) involved in this admixture event. The slightly elevated Neanderthal ancestry that others have described in Central and East Asian populations also appears to have originated in this first pulse, as Central and East Asian Neanderthal haplotypes are mostly shared with other, geographically distant populations. This observation favors the hypothesis that the increased Neanderthal ancestry in these populations relative to others is due to weaker selection against alleles that may be mildly deleterious (32), made possible because of smaller historical population sizes in this part of Eurasia, rather than to additional admixture events (22). Our evidence of many small-scale, population-specific admixture events, however, together with a simulation study that found a single-pulse admixture model followed by drift unable to explain the discrepancies in admixture proportions in European and Asian genomes (49), hints at a complex history of admixture throughout Eurasia not fully captured by either of these two hypotheses.
I don’t think that it’s weaker selection in East Asians. I think it’s complex demography.
This 74,000 year dating of the Neanderthal introgression in Eurasians seems to be older than usually assumed, no? I believe based on the Ust Ishim genome introgression was previously dated to around 55,000-60,000 years ago.
This older date is important, because then we can assume that the proto-Eurasian population was actually in Eurasia at some point before 74,000 years ago, since presumably the admixture with Neanderthals didn’t occur in Africa.
i assume the 74k bp coalescence is the neanderthal population bottleneck, not the introgression date
I’m guessing Neanderthal introgression into both paleo west/east Eurasian populations was originally about the same with basal Eurasian (whom supposedly had little to none) admixture diluting it in the former.