The rise and transformation of Bronze Age pastoralists in the Caucasus

The Caucasus region, situated at the crossroads of Europe and Asia, experienced significant population movements and cultural exchanges throughout the Bronze Age. Its diverse ecologies and rich metal resources fostered the development of early steppe pastoralist societies, which later expanded across Eurasia, profoundly impacting genetic makeup, languages, and cultural trajectories. However, the origins of these societies, their interactions with farming communities, and their eventual decline remain unclear. This study aims to address these gaps by analyzing a large dataset of ancient genomes from the Caucasus and surrounding regions, tracing the genetic history of these populations over 6,000 years.

Previous research has highlighted the Caucasus' role as a bridge between Europe and Asia, noting the movement of people, technologies, and ideas across the Greater Caucasus mountain range. Studies have identified the region as the origin of early steppe pastoralist societies, but understanding their origins and interactions with neighboring groups requires more comprehensive genetic data. Existing genomic data has provided insights into some aspects of this history, but this study significantly expands the available data, allowing for a much more detailed analysis.

This research utilized genome-wide data from 131 individuals from 38 archaeological sites spanning approximately 6,000 years. The samples encompassed various time periods, from the Mesolithic and Neolithic to the Late Bronze Age. 84 new radiocarbon dates were generated to enhance the chronological accuracy. After quality control, 102 unrelated individuals were included in population genetic analyses, alongside published ancient and modern-day genomes. Analyses included principal component analysis (PCA), ADMIXTURE analysis, *f*-statistics, and qpAdm modeling to assess genetic affinities, identify ancestry components, and estimate admixture dates. The study also employed ancIBD to estimate genetic relatedness between individuals and analyzed runs of homozygosity (ROH) to assess population structure and inbreeding.

The study identified two distinct genetic clusters before the Neolithic transition: one in the north with Eastern hunter-gatherer ancestry, and another in the south with distinct Caucasus hunter-gatherer ancestry and increasing East Anatolian farmer admixture. During the Eneolithic period, West Eurasian steppe ancestry emerged from a mixture of Eastern hunter-gatherer and Caucasus hunter-gatherer ancestries. The Early and Middle Bronze Age saw long-term genetic stability within the steppe pastoralist groups. The Late Bronze Age was marked by gene flow from diverse sources, leading to the decline of steppe cultures and the eventual assimilation of steppe ancestry into highland populations. Analysis of burial mounds revealed that these were generally not lineage-based, with mostly unrelated individuals buried together. The study found evidence of distinct genetic groups associated with various archaeological cultures (e.g., Maykop, Yamnaya, Kura-Araxes), along with gene flow and admixture events between these groups and populations from neighboring regions (e.g., Central Asia). Differences in ROH patterns indicated smaller effective population sizes in steppe communities compared to Caucasus communities.

The findings significantly advance our understanding of the complex population dynamics of the Caucasus during the Bronze Age. The study demonstrates the long-term genetic continuity of Caucasus hunter-gatherer ancestry and the subsequent interaction with Neolithic farmers. The emergence of Steppe ancestry highlights the role of admixture and the gradual development of pastoralist societies. The Late Bronze Age findings underscore the impact of cultural and environmental shifts on population structure and the integration of steppe and highland groups. The non-lineage-based nature of burial mounds challenges existing assumptions about social organization in Bronze Age societies. The results contribute to the broader understanding of the spread of pastoralism and the genetic and cultural connections between the Caucasus, the Eurasian steppe, and surrounding regions.

This study provides a comprehensive genetic history of the Caucasus region during the Bronze Age, shedding light on the formation, expansion, and ultimate transformation of steppe pastoralist societies. The findings highlight the importance of admixture events and environmental factors in shaping population structure. Future research might focus on more extensive analyses of animal genomes to trace the origins and spread of pastoral technologies, and further investigating the environmental factors that contributed to the decline of steppe cultures in the Late Bronze Age.

The study acknowledges potential limitations related to sampling biases, which may not fully capture the genetic diversity of all populations in the region. Radiocarbon dating reservoir effects can also affect the precise dating of events. While this study significantly expands upon previous genomic data, additional sampling, especially in under-represented regions and time periods, could provide further insights. Furthermore, the interpretation of cultural interactions is based on archaeological correlations with genetic data, and additional studies integrating other lines of evidence (e.g., linguistics, isotopic analyses) could strengthen interpretations.