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

Ancient inland southwestern Finland shows nearly a millennium of population continuity, while the coastal zone experienced a significant genetic shift since the Early Medieval period.

• Why it matters: Understanding population dynamics in Early Medieval Finland is limited by scarce contemporary sources, hindering insights into regional demographic and cultural changes during this period.
• What they did: Genomic data from 34 ancient individuals and modern Finns were analyzed to characterize ancestry, reconstruct demographic patterns, and map genetic connections within a circum-Baltic network, revealing shifts in ancestry and population size.
• The result: Findings show ancient inland populations are similar to modern Finns, while coastal populations now have more Scandinavian and less Baltic ancestry, with a population size reaching approximately 13,000 by 1250 CE, highlighting regional demographic and genetic transformations.