Population genomics, demography, and circum-Baltic connectivity of Early Medieval southwestern Finland

Abstract

Genome-wide data from 34 ancient individuals (Late Iron Age and Early Medieval, ca. 800-1250 CE) from coastal and inland southwestern Finland, analysed with modern Finnish biobank genomes. Despite distinct cultural-economic profiles, ancient coastal and inland groups were genetically indistinguishable. The inland zone shows close to 1000 years of continuity up to present-day inhabitants, while the coastal zone underwent a later shift: modern coastal Finns carry markedly more Scandinavian and less Baltic ancestry, consistent with documented medieval migration from Sweden after ca. 1300 CE. IBD-based estimates show effective population size rising from about 2500 to about 13,000 by 1250 CE, likely linked to new farming practices and favourable climate, with no sign of close-kin consanguinity. A circum-Baltic IBD network connects Early Medieval Finland to Gotland, Latvia, Russia and Scandinavia, and to victims of the 1675 Kronan warship sinking.