The Yamnaya Steppe Herders from Russia

Discussion

In its classical formulation, the Kurgan hypothesis, i.e. a late Neolithic spread of proto-Indo-European languages from the Pontic steppes, regards the GAC people as largely descended from Late Neolithic ancestors from the East, most likely representing the Yamna culture; these populations then continued their Westward movement, giving rise to the later Corded Ware and Bell Beaker cultures. Gimbutas [23] suggested that the spread of Indo-European languages involved conflict, with eastern populations spreading their languages and customs to previously established European groups, which implies some degree of demographic change in the areas affected by the process. The genomic variation observed in GAC individuals from Kierzkowo, Poland, does not seem to agree with this view. Indeed, at the nuclear level, the GAC people show minor genetic affinities with the other populations related with the Kurgan Hypothesis, including the Yamna. On the contrary, they are similar to Early-Middle Neolithic populations, even geographically distant ones, from Iberia or Sweden. As already found for other Late Neolithic populations [18], in the GAC people's genome there is a component related to those of much earlier hunting-gathering communities, probably a sign of admixture with them. At the nuclear level, there is a recognizable genealogical continuity from Yamna to Corded Ware. However, the view that the GAC people represented an intermediate phase in this large-scale migration finds no support in bi-dimensional representations of genome diversity (PCA and MDS), ADMIXTURE graphs, or in the set of estimated f3-statistics.

For a formal test of these findings, mtDNA data, with its absence of recombination, have valuable properties and allow explicit modelling. Analysis of mitochondrial DNA also benefits from the extensive mathematical methodology that has been developed for studying population history based on such data. Population relationships inferred from mitochondrial data closely resembled those inferred from nuclear data, and so it seemed unlikely that the two datasets may reflect very different demographic events. The models including no migration (NOMIG), or a migration from the Pontic steppes before the onset of the GAC (MIG1) found very limited support in the ABC analysis. The best fit in the first round of simulations was obtained including only a later migration from Yamna, i.e. one not affecting the GAC individuals for which we have information (MIG2), followed by the model in which two subsequent migrations are considered (MIG1,2). Adding, in a second round of simulations, a further episode of gene flow, this time eastwards (MIG2,3), improved the fit, but not dramatically so. The proportion of mtDNA lineages possibly derived from those of the Yamna people is less than 50%.



In short, simulation-based tests on mtDNA variation do not suggest that the GAC people of this study have special links with migrants from the Pontic steppes, but show a direct connection between the Yamna and later Central Europe cultures (Corded Ware and Bell beaker), who derived almost half of their mitochondrial variation from them. The emerging picture is thus one in which migrations from the Pontic steppes into Central Europe left a trace in the genomes of the Corded Ware culture, but not in those of the GAC.

At this stage, it is hard to say whether and to what extent the finding that migrations from the Pontic steppes had little or no demographic impact on the GAC also has implications for the Kurgan hypothesis. There is little doubt that in the Late Neolithic there were indeed migration processes from the Pontic steppes into Central Europe, documented by the archaeological and genetic links between the Yamna and Corded Ware cultures. However, depending on the number of people involved, migrations may or may not leave a recognizable trace in the genetic makeup of a population. The data we provide show that such a trace is not apparent in the genomes of the GAC people. Rather, evolutionary connections are evident between the GAC and other European groups, both at the nuclear and mitochondrial level. In short not all population relationships in the Central European late Neolithic correspond to those proposed in the original Kurgan model. Of course, there is also the possibility that GAC sites other than those investigated in this study might show different genomic features, but at this stage, this is only a matter of speculation. Therefore, either the GAC people pre-existed and were extraneous to the Pontic Steppe migration process envisaged by Gimbutas, or the Pontic steppes migrants' contribution was represented by few individuals, too few indeed to leave a trace in the genetic makeup of the GAC population.

To get a deeper insight into the linguistic changes prompted by Neolithic migration, one should have an idea of the languages spoken in that period, which is currently out of reach. Therefore, our work leaves open the possibility that the GAC was in contact with Yamna, but in this case, the interaction was mostly at the cultural level, entailing very limited migration, if any, contrary to the predictions of Gimbutas' Kurgan hypothesis. One alternative is that that the Pontic steppe migration did not profoundly affect mitochondrial variation, as implied by several studies showing an excess of migrating males in expanding pastoral economies (see Saag et al. unpublished data [40] and Kristiansen et al. [41], and references therein). However, in this case, the substantial contribution of males should at least result in some degree of similarity between GAC and Yamnaya at the nuclear level, which did not emerge in this study. In both cases, a transmission of cultural traits from the Pontic steppes to the GAC, and later further West, is conceivable and not ruled out by our data; further archaeological work, including studies of other GAC sites, may shed additional light on this.

Published 22 November 2017.DOI: 10.1098/rspb.2017.1540