The effect of ancient population bottlenecks on human phenotypic variation

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Abstract
The 'out of Africa' debate on human origins has enlivened palaeoanthropology for many years. Genetic analyses tended to support a single origin for modern humans in Africa, but measurements of anatomy have produced mixed results. Now that dichotomy is resolved: a new analysis of a large database of skull measurements, informed by advances in ancient demography of anatomically modern humans made possible by large genetic datasets, unequivocally supports a single African origin. The proposed origin of modern humans has been controversial; whereas genetic analyses mostly support a single African origin, measurements of anatomy give mixed results. A new analysis of a large database of skull measurements by Manica and colleagues shows that 'distance from Africa' accounts for up to a quarter of heritable variation in craniometric traits, strongly indicating a common African heritage. The origin and patterns of dispersal of anatomically modern humans are the focus of considerable debate1,2,3. Global genetic analyses have argued for one single origin, placed somewhere in Africa4,5,6,7. 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 results8,9,10,11,12, and have been argued to support multiple origins of modern humans2,9,12. 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.