Genetic history of the Balkans

Discussion
Archaeogenetic studies are delivering new evidence that is transforming our understanding of human prehistory and prompting an intense and productive dialog between geneticists and archaeologists. So far, relatively fewer studies focus on the historical period, that of writing, which requires engagement with written sources in addition to the material evidence. Triangulation of information from history, the archaeological study of material culture, and genetics opens new possibilities for understanding the human past, with each line of evidence not only providing unique information but also helping to address biases deriving from other types of analysis. To aid in these goals, Data S1, section 1 to this study includes detailed and standardized historical and archaeological information for over twenty sites, along with contextual data for each grave with a newly reported genome. This deeper synthesis of historical, archaeological, and genetic data informs our interpretations, whereas the detailed site and grave reports allow others to refine our reconstructions or extend them as future results become available.
The genomic transect of 1st millennium individuals from the Balkan Peninsula presented here furnishes new insights into the long-term population dynamics of a region that was both a crucial frontier of the Roman Empire as well as an enduring geographic crossroads between east and west, north and south. The results emphasize the importance of continuing population change in historical times and the long-term shifts in the role of socio-political structures across the 1st millennium. The period of Roman control was dominated by internal migration, with sporadic but increasing long-distance migration from outside the territory of the Roman Empire, and this pattern reversed in later centuries, with a relatively larger contribution from populations originating beyond the Danube corridor.
Broadly, our results suggest three phases in the population history of this region in the 1st millennium. First, the high Roman Empire (ca. 1–250 CE) saw the strong impact of Roman culture on the local Iron Age Balkan population. Although this process was accompanied by little detectible contribution from populations with ancestry from the Italian Peninsula, there was significant migration by individuals of Anatolian/Eastern Mediterranean ancestry, either directly or through Italy, whose admixture would leave a long trail in later local populations. Meanwhile, militarization and/or economic vitality attracted migrants from further afield both within and beyond the Roman Empire. In some cases, at least, the small-scale percolation of individuals preceded large-scale population movements of later centuries.
In the late Roman Imperial period (ca. 250–550), internal migration from within the empire lessened, whereas the presence of individuals with ancestry originating in populations from beyond the Danube frontier is evident. Admixture was pervasive both among groups originating beyond the frontier (notably Northern/Central Europeans and Pontic-Kazakh Steppe groups) and among these groups and the local population. Although claims about the identity of individuals or groups have sometimes been made based on material culture discovered in burial contexts, DNA-based ancestry data can reveal the complex role of processes like migration and admixture behind individual and group histories (see above for the example from Kormadin, where a “Gepid” cemetery certainly included individuals with local Iron Age Balkan ancestry). The presence of North/Central European ancestry disappears in later periods, suggesting that individuals with this ancestry were relatively few or that historical processes (such as further migration or differential mortality) selectively reduced their contribution in later centuries. For generations, scholars of late antique history have debated the extent to which the political transitions accompanying the end of Roman rule were fueled by demographic changes and whether these transitions were driven by ethnogenesis or mass migration. Our findings support a nuanced view in which both ethnogenesis and migration were important.
Today, speakers of Slavic languages represent the largest linguistic group in Europe, mainly inhabiting eastern, central, and southeastern Europe. Several aspects of their initial arrival in the Balkans are not yet well understood, such as their place of origin and timing, the mechanisms ranging from colonization, invasion, and infiltration, their degree of demographic impact in the region, and the underlying reasons with demographic pressures, climate change, and depopulation due to the Justinian Pandemic being postulated.46,47 We document a clear signal of Eastern European-related gene flow in the vast majority of individuals in our dataset after 700 CE (n = 49), likely associated with the arrival of Slavic-speaking populations according to historical and archaeological evidence.46 Due to a gap in our sampling between 500 and 700 CE, we cannot determine the exact timing of the earliest arrivals, but the detection of individuals with full Eastern European ancestral origin during the 8th and 9th centuries points to a long process encompassing many generations rather than a short-lived migration event. Unlike the earlier Central/Northern European gene flow, genomic data are consistent with a major contribution of migrations of both sexes and with a long-lasting strong demographic impact in the region that extends to present-day populations. Nevertheless, our results rule out a complete demographic replacement, as we observe significant proportions of Iron Age Balkan-related and Anatolian-related ancestry across the Medieval period up to the present. These demographic processes of mobility and admixture generated an ancestry cline of present-day Balkan populations with relatively similar ancestry profiles but speaking languages from four different families, i.e., Latin, Slavic, Albanian, and Greek, highlighting different cultural processes across the region despite many commonalities in their demographic history. Together, these processes created a regional ancestry profile by the end of the 1st millennium that largely endures across the region.
Limitations of the study
Like any historical evidence, this new genetic dataset has limitations. The main one concerns the inherent fragmentary nature of the archaeological record, impacting our study in three ways. First, the prevalence of cremation burial in the 1st and 2nd centuries limits the size of the sample in the earliest phase and may bias the results toward a local population more likely to be inhumed. Second, the paucity of sixth-century samples may obscure the presence of populations from Northern/Central Europe who arrived in this later period and the earliest phases of the Slavic migrations. Third, urban populations are overrepresented in our study with respect to rural areas, which could be differentially impacted by the demographic processes described in our work. Additional genetic analyses across other Roman frontiers during and after the height of the Empire will help understand how this ancient phase of globalization shaped the current demographic landscape of three continental regions.