The effect of ancient population bottlenecks on human phenotypic variation

The origin and patterns of dispersal of anatomically modern humans are the focus of considerable debate(1-3). Global genetic analyses have argued for one single origin, placed somewhere in Africa(4-7). This scenario implies a rapid expansion, with a series of bottlenecks of small amplitude, which would have led to the observed smooth loss of genetic diversity with increasing distance from Africa. Analyses of cranial data, on the other hand, have given mixed results(8-12), and have been argued to support multiple origins of modern humans(2,9,12). Using a large data set of skull measurements and an analytical framework equivalent to that used for genetic data, we show that the loss in genetic diversity has been mirrored by a loss in phenotypic variability. We find evidence for an African origin, placed somewhere in the central/southern part of the continent, which harbours the highest intra-population diversity in phenotypic measurements. We failed to find evidence for a second origin, and we confirm these results on a large genetic data set. Distance from Africa accounts for an average 19-25% of heritable variation in craniometric measurements - a remarkably strong effect for phenotypic measurements known to be under selection.