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PSMC estimates of Neanderthal effective population size (Ne) exhibit a roughly 5-fold decline across the most recent 20 ky before the death of each fossil. To explain this pattern, this article develops new theory relating genetic variation to geographic population structure and local extinction. It argues that the observed pattern results from subdivision and gene flow. If two haploid genomes are sampled from the same subpopulation, their recent ancestors are likely to be geographic neighbors and therefore coalesce rapidly. By contrast, remote ancestors are likely to be far apart, and their coalescent rate is lower. Consequently, Ne is larger in the distant past than in the recent past. New theoretical results show that modest rates of extinction cause substantial reductions in heterozygosity, Wright's FST, and Ne. 

Admixture appears increasingly ubiquitous in the evolutionary history of various taxa, including humans. Such gene flow likely also occurred among our closest living relatives: bonobos ( Pan paniscus ) and chimpanzees ( Pan troglodytes ). However, our understanding of their evolutionary history has been limited by studies that do not consider all Pan lineages or do not analyze all lineages simultaneously, resulting in conflicting demographic models. Here, we investigate this gap in knowledge using nucleotide site patterns calculated from whole-genome sequences from the autosomes of 71 bonobos and chimpanzees, representing all five extant Pan lineages. We estimated demographic parameters and compared all previously proposed demographic models for this clade. We further considered sex bias in Pan evolutionary history by analyzing the site patterns from the X chromosome. We show that 1) 21% of autosomal DNA in eastern chimpanzees derives from western chimpanzee introgression and that 2) all four chimpanzee lineages share a common ancestor about 987,000 y ago, much earlier than previous estimates. In addition, we suggest that 3) there was male reproductive skew throughout Pan evolutionary history and find evidence of 4) male-biased dispersal from western to eastern chimpanzees. Collectively, these results offer insight into bonobo and chimpanzee evolutionary history and suggest considerable differences between current and historic chimpanzee biogeography.