Chapter 7 - Human Population History Over the Last Two Million Years

Abstract

The amount and patterns of genetic variation in the current human gene pool is profoundly affected by past evolutionary history. One method of investigating this past history is to use surveys of genetic variation in present-day populations and look for the genetic signatures of past evolutionary events or processes. Such historical information is primarily contained at the multilocus level, and those areas of the genome with little to no recombination are the clearest windows into the past. An analysis of these genomic areas with nested clade analysis reconstructed a model of human evolutionary history remarkably consistent with fossil, archaeological, and paleontological data, and indicated that admixture events and gene flow have dominated human evolution since the mid-Pleistocene. Another method of using genetic data to make inference about the past is to use parametric models that also allow estimates of past gene flow rates, admixture amounts, etc. Many of these models are based on the assumption of a population tree or treelike structure with only weak or sporadic gene flow. Such treelike structures in humans are strongly falsified when tested with either current and/or ancient DNA surveys. A direct genetic survey of past populations is now feasible due to the ability to study ancient DNA. The ancient DNA studies strongly support the inferences from nested clade analysis that much of human evolutionary history has been dominated by movements of populations and individuals from the mid-Pleistocene on, sometimes across, and between continents. These movements were not followed by replacement, splitting, or isolation, but rather by admixture and gene flow. Genetic interchange has been the hallmark of much human evolution, both past and present.