The evolution of human population size: A synthesis of genetic and paleoanthropological data.

Abstract

This thesis is an examination of the size of the human population during the Late Pliocene and Pleistocene. In it, I consider several hypotheses concerning the size of the human population during the period from 2 million years ago to 10,000 years ago, and how it changed over time. For the first time, evidence from archaeological, paleontological, and genetic sources are drawn together in order to generate and test models of population size changes. In order to examine whether the current pattern of data may allow the rejection of any hypotheses of population size changes during this time period, I develop a simulation model that can be used to test the predictions of different models of population size changes with regard to the level and pattern of human genetic variation. To delimit the range of plausible hypotheses about human population size before the Neolithic, a model of population size change is drawn from the fossil and archaeological records. This model includes a speciation event at two million years ago, which may be related to a small population size at that time, followed by a gradual expansion during most of the Pleistocene. This model is shown to be compatible with the level of genetic variation among human nuclear genes, but not the pattern of that variation. A model of sudden population expansions during the late Pleistocene, from an effective population size on the order of 10,000 individuals to an effective population size of several million, has been posited as a likely explanation for the pattern of variation present within and among non-recombining human genes. This model, however, is also shown to be insufficient to account for the pattern of variation present within the human nuclear recombining genes used in this analysis. It is therefore likely that some factor other than changes in population size has influenced the level and pattern of human genetic variation. Several mechanisms, including those involving selection and population structure, may have affected the relationship of the level of genetic variation to the population size of our species.