View Full Version : Population Bottlenecks and Pleistocene Human Evolution

Saturday, January 1st, 2005, 05:24 PM
Population Bottlenecks and Pleistocene Human Evolution

John Hawks, Keith Hunley, Sang-Hee Lee, and Milford Wolpoff

We review the anatomical and archaeological evidence for an early population bottleneck in humans and bracket
the time when it could have occurred. We outline the subsequent demographic changes that the archaeological
evidence of range expansions and contractions address, and we examine how inbreeding effective population size
provides an alternative view of past population size change. This addresses the question of other, more recent,
population size bottlenecks, and we review nonrecombining and recombining genetic systems that may reflect them.

We examine how these genetic data constrain the possibility of significant population size bottlenecks (i.e., of
sufficiently small size and/or long duration to minimize genetic variation in autosomal and haploid systems) at
several different critical times in human history. Different constraints appear in nonrecombining and recombining
systems, and among the autosomal loci most are incompatible with any Pleistocene population size expansions.

Microsatellite data seem to show Pleistocene population size expansions, but in aggregate they are difficult to
interpret because different microsatellite studies do not show the same expansion. The archaeological data are only
compatible with a few of these analyses, most prominently with data from Alu elements, and we use these facts to
question whether the view of the past from analysis of inbreeding effective population size is valid. Finally, we
examine the issue of whether inbreeding effective population size provides any reasonable measure of the actual
past size of the human species. We contend that if the evidence of a population size bottleneck early in the evolution
of our lineage is accepted, most genetic data either lack the resolution to address subsequent changes in the human
population or do not meet the assumptions required to do so validly. It is our conclusion that, at the moment,
genetic data cannot disprove a simple model of exponential population growth following a bottleneck 2 MYA at
the origin of our lineage and extending through the Pleistocene. Archaeological and paleontological data indicate
that this model is too oversimplified to be an accurate reflection of detailed population history, and therefore we
find that genetic data lack the resolution to validly reflect many details of Pleistocene human population change.

However, there is one detail that these data are sufficient to address. Both genetic and anthropological data are
incompatible with the hypothesis of a recent population size bottleneck. Such an event would be expected to leave
a significant mark across numerous genetic loci and observable anatomical traits, but while some subsets of data
are compatible with a recent population size bottleneck, there is no consistently expressed effect that can be found
across the range where it should appear, and this absence disproves the hypothesis.