Human Demography in the Pleistocene: Do Mitochondrial and Nuclear Genes Tell the Same Story?

Traditionally, research on modern human origins has centered on questions of the time and geographical place of origin, with less attention given to the complex population dynamics of our evolutionary history. Recently, however, a focus has emerged within molecular anthropology that concentrates on the demographic aspects of the origin of modern humans.1-3 A popular hypothesis proposes that modern human populations passed through a bottleneck (or episodic reduction in size) in the late Middle or early Late Pleistocene, at which time there existed perhaps only several thousand breeding individuals, and that this was followed by a rapid, large expansion.1,2,4 Supporting evidence comes largely from the pattern of DNA sequence variation observed in mitochondrial genes. However, because the mitochondrial genome is only a very small fraction of the entire genome, its evolutionary history is not necessarily concordant with the history of the bulk of the genome, the nuclear genome. An important distinction can be made between evolutionary forces that affect just one locus and those forces that act on all the genes of a population. Population-level phenomena such as bottlenecks, expansions, population subdivisions, and speciation events are expected to produce similar patterns of genetic variation across many loci. In contrast, natural selection usually affects a small region of tight linkage, such as a single genetic locus. Therefore, hypotheses about population histories should be tested across many loci.5-8