Craniometric data supports demic diffusion model for the spread of agriculture into Europe

Abstract

Background: The spread of agriculture into Europe and the ancestry of the first European farmers have been subjects of debate and controversy among geneticists, archaeologists, linguists and anthropologists. Debates have centred on the extent to which the transition was associated with the active migration of people as opposed to the diffusion of cultural practices. Recent studies have shown that patterns of human cranial shape variation can be employed as a reliable proxy for the neutral genetic relationships of human populations.

Methodology/principal findings: Here, we employ measurements of Mesolithic (hunter-gatherers) and Neolithic (farmers) crania from Southwest Asia and Europe to test several alternative population dispersal and hunter-farmer gene-flow models. We base our alternative hypothetical models on a null evolutionary model of isolation-by-geographic and temporal distance. Partial Mantel tests were used to assess the congruence between craniometric distance and each of the geographic model matrices, while controlling for temporal distance. Our results demonstrate that the craniometric data fit a model of continuous dispersal of people (and their genes) from Southwest Asia to Europe significantly better than a null model of cultural diffusion.

Conclusions/significance: Therefore, this study does not support the assertion that farming in Europe solely involved the adoption of technologies and ideas from Southwest Asia by indigenous Mesolithic hunter-gatherers. Moreover, the results highlight the utility of craniometric data for assessing patterns of past population dispersal and gene flow.

Figure 1. Map showing geographic distribution of all OTUs.

Dispersal models involving the active migration of people from SW Asia take two basic forms. Once-off single dispersals from either Anatolia (brown arrow) or the Levant (orange arrows), or continuous dispersal models whereby active population migration continued from southeastern Europe into central Europe (blue arrows). CD = Continuous dispersal, IBD = Isolation-by-distance (null), LGF = Limited gene flow.

Figure 2. Two-dimensional representation of the null model of isolation-by-geographic and temporal distance.

The horizontal axis represents the distance between all OTUs and OTU1 (Natufian) in km. The vertical axis represents the chronological distribution of the OTUs in average years B.P. A. The null model expects all OTUs to be related to each other according to their distribution in space, once time is corrected for (dashed lines). B. The LGF model (solid lines) is a variant of the null model, which expects decreased gene flow between contemporaneous Mesolithic (OTUs 9, 10 and 12) and Neolithic (OTUs 4, 5, 6 and 7) populations as a result of cultural barriers imposed by the adoption of agricultural practices.

Figure 3. Plot of the first two principal co-ordinates illustrating OTU affinity patterns based on craniometric data.

The major axis of variation (horizontal axis, 34.9% variance) shows a clear distinction between all archaeologically defined Neolithic OTUS (green, brown and black circle symbols) and all Mesolithic OTUS (purple and red symbols) plus the Natufian (black triangle). Second axis = 18.7% variance.