Population biology and population genetics of pleistocene hominins

The population genetics of Pleistocene hominins is deduced from three types of data: coalescent processes and haplotype trees estimated from surveys of genetic variation in present-day human populations, ancient DNA extracted from fossils, and overlays of current quantitative genetic variance/covariance matrices upon hominin fossils. The haplotype trees are subjected to nested clade phylogeographic analyses. These analyses show that there were three major expansion events of hominins out of Africa during the last 2 Myr. The first expansion event marked the original dispersal of Homo erectus out of Africa into Eurasia. The quantitative genetic analysis of hominin fossils indicates that there was relaxed selection upon at least some morphological features at this time, perhaps due to an increased use of cultural inheritance in dealing with the environment. Coalescent analyses indicate that the colonization of Eurasia was marked by strong selection at many loci, so although morphological selection may have been relaxed, adaptive processes were still proceeding as humans colonized this new geographical area. A second expansion out of Africa was marked by the spread of the Acheulean culture, implying that the spread of this culture was due to a spread of peoples and not just ideas. The expanding Acheulean populations interbred with existing Eurasian populations, and recurrent gene flow between Eurasian and African populations was established although restricted by isolation by distance after the Acheulean expansion. A third expansion out of Africa marked the spread of many anatomically modern traits that had earlier appeared in Africa. This expansion was also marked by interbreeding, so regional continuity persisted for some traits. Total replacement of Eurasian populations is rejected with a p < 10⁻¹⁷, under nested clade phylogeographic analysis, and this strong conclusion has been confirmed by subsequent phylogeographic analysis using Approximate Bayesian Computation after correcting for some statistical errors. Direct studies on ancient DNA also support limited admixture rather than total replacement. Coalescent studies are inconclusive and contradictory both about the size of hominin populations before this last out-of-Africa expansion and the degree of population growth during the expansion phase. Because of admixture and gene flow, humanity evolved into its modern form as a single evolutionary lineage but with geographical differentiation at any given time due to isolation by distance and local adaptation.