Our results thus revealed that shape variability of early AMH was highest among all tested groups, i.e., within a sample of the genus Homo embracing the last 1.8 million years. The shortest connections between early AMH are either with other specimens of this group or recent modern humans, for instance, Omo 2 [recently dated to ~195 ka (1)] and LH 18, two of the earliest east African candidates for the emergence of modern human morphology (18), and the Levantine Qafzeh 6 connect with recent Australian aboriginals (cf. ref. 19). We also find a connection between 3,500-km-distant sites in the Levant and northwest Africa, i.e., between the more archaic looking Jebel Irhoud 1 and Skhul 5, whereas Jebel Irhoud 2 connects to recent Europeans. Qafzeh 9 (Levant) is linked to a European UP specimen. We find, however, no single link between Neanderthals and AMH, including Upper Paleolithic specimens.
Our data on neighbors and variability is unsupportive of the strict forms of a single-origin model but does not conflict with another approach, the model of ‘‘isolation by distance,’’ which predicts that genetic and phenotypic dissimilarity increases with geographic distance (24, 29–31). The metapopulation framework would predict the same because frequency and magnitude of genetic exchange would follow the likelihood of 2 populations to meet, which declines with geographical distance from the early AMH epicenter in Africa. Our fossil AMH data, however, suggest that before there was isolation by distance from Africa, there already existed (at least temporally) isolation by distance within Africa during the Pleistocene.But what about the observed low genetic diversity found in humans?
Genetic diversity among living modern humans is known to be very low when compared with extant apes (32, 33). To reconcile this observation with our proposed metapopulation model within Africa, it is necessary to assume that genetic diversity of early AMH(and maybe even earlier fossil groups of Homo) must have been relatively low as well. The only fossil human group for which such genetic data are available, the Neanderthals, support this contention; their level of genetic variability also is low when compared with living apes (34, 35).UPDATE: John Hawks discusses the paper.
Early modern human diversity suggests subdivided population structure and a complex out-of-Africa scenario
Philipp Gunz et al.
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.