Early modern human diversity suggests subdivided population structure and a complex out-of-Africa scenario

Abstract

The interpretation of genetic evidence regarding modern human origins depends, among other things, on assessments of the structure and the variation of ancient populations. Because we lack genetic data from the time when the first anatomically modern humans appeared, between 200,000 and 60,000 years ago, instead we exploit the phenotype of neurocranial geometry to compare the variation in early modern human fossils with that in other groups of fossil Homo and recent modern humans. Variation is assessed as the mean-squared Procrustes distance from the group average shape in a representation based on several hundred neurocranial landmarks and semilandmarks. We find that the early modern group has more shape variation than any other group in our sample, which covers 1.8 million years, and that they are morphologically similar to recent modern humans of diverse geographically dispersed populations but not to archaic groups. Of the currently competing models of modern human origins, some are inconsistent with these findings. Rather than a single out-of-Africa dispersal scenario, we suggest that early modern humans were already divided into different populations in Pleistocene Africa, after which there followed a complex migration pattern. Our conclusions bear implications for the inference of ancient human demography from genetic models and emphasize the importance of focusing research on those early modern humans, in particular, in Africa.

Fig. 1.

Anatomically modern humans and archaic forms of Homo in shape space. (Upper) Two-dimensional projection of the first 3 principal components of the neurocranial shape coordinates and one example (here Mladeč 1) for the full set of landmarks and semilandmarks measured on each neurocranium. Recent humans in light brown; UP fossils in blue; early AMH in red; Neanderthals in green, archaic Homo in orange. The graph provides 2 different kinds of information: (i) Ordination of each specimen in the first 3 principal components (PCs, together 71% of total variation), and (ii) nearest neighborhood relations according to full Procrustes shape distance (which uses all dimensions, not just the first three). Connections between nearest neighbors from the same group are shown in their group color, connections between nearest neighbors from different groups are drawn as black lines. Equal frequency ellipsoids (75%) are plotted in respective color for all groups. Ellipsoids for recent humans are based on their geographic origin: Africa, Asia, Australia, and Europe. (For a more detailed view see Fig. S1a). (Lower) Log Centroid Size (log CS), a measure for size in our analysis, is plotted for all specimens (color coding as above). Early AMH exhibit the narrowest distribution of log CS in comparison with all other groups.

Fig. 2.

Variances in shape space for each group and their bootstrap distributions. Bootstrap distributions for the shape variances of the 5 different groups (20,000 bootstraps each; color coding as in Fig. 1). Because of the small and heterogeneous early AMH sample the distribution of its variance is very wide, i.e., shape variance of early AMH is known with low confidence only. Yet, the shape variances of both early AMH and of recent humans are significantly (P < 0.05) larger than the variances of Neanderthals and of archaic Homo.