Conclusions
While it is clear that paleontological, archaeological, and genetic approaches are potentially rich sources of hypotheses about human demographic history, each alone has a considerable disadvantage when applied as a test of paleodemographic questions. The weaknesses of fossil and archeological approaches lie in their incompleteness and poor sampling. While paleoanthropologists have learned much about human phylogeny and adaptation, it has been a challenge to obtain any but the roughest estimates of past population size from site distributions, distributions of technologies or traits, or evidence of morphological evolution. Geographically clustered sampling and taphonomy bias the samples, and their sizes are very small. The weakness of genetic data is inaccuracy, especially in the sense of low levels of resolution, as there are many factors other than population size that affect genetic diversity and do so in ways that are largely not quantifiable. Perhaps the most significant of these is selection, and its main effect is to remove the element of predictability from these relationships because the details of past selection are unknown, and perhaps unknowable.
All the currently available genetic, paleontological, and archaeological data are consistent with a bottleneck in our lineage more or less at about 2 MYA. At the moment, genetic data cannot disprove a simple model of exponential population growth following such a bottleneck 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 conclude that genetic data lack the resolution to validly reflect many details of Pleistocene human population change.
However, there is one detail 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. Genetic and anatomical traits, after all, are the raw data the hypothesized bottleneck is meant to explain. 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. There are better ways to explain the data.
Although significant population size fluctuations and contractions occurred, none has left a singular mark on our genetic heritage. Instead, while isolation by distance across the network of population interactions allowed differences to persist, and with selection, local adaptations were able to develop, evolution through selection, along with gene flow, has promoted the spread of morphological and behavioral changes across the human range. It is this pattern of shared ancestry that has left its signature in the variation that we observe today. We know this from many sources of data and argue that no single source can suffice. If the evidence of a population size bottleneck early in the evolution of our lineage is accepted, most genetic data by themselves either lack the resolution to address subsequent changes in the human population or do not meet the assumptions required to do so validly.
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