Ancient DNA reveals a millennium of stability and a dramatic shift in medieval Finland

For centuries, the story of Early Medieval Finland has been told almost entirely through pottery shards, sword hilts, and burial customs, because the written record from the region is fragmentary and often composed by outsiders. Now, a team of researchers has recovered the voices of the people themselves, encoded in their DNA. By sequencing the genomes of 34 ancient individuals buried in southwestern Finland between roughly 800 and 1250 CE, and comparing them with thousands of modern Finnish genomes, scientists have reconstructed a demographic history that is far more nuanced, and far more surprising, than anyone expected. The findings, published in Genome Biology, reveal a tale of two landscapes: one astonishingly stable for nearly a thousand years, the other transformed beyond recognition within just a few centuries.

The study focused on the Late Iron Age and Early Medieval period, an era that Finnish archaeologists have traditionally called the “Crusade Period,” reflecting the long-held assumption that Swedish kings launched violent military campaigns to convert the local population to Christianity. Modern scholars largely regard these so-called crusades as later literary constructions, and the researchers instead use the term Early Medieval Period to describe the gradual emergence of Christian institutions and Swedish political influence. During this time, Finland was not a unified state but a patchwork of culturally distinct regions, including a coastal zone with strong Scandinavian ties and an inland zone known as Häme, whose material culture blended western and eastern traditions.

Recovering ancient DNA from Finland is notoriously difficult. The country’s acidic soil rapidly destroys unburnt human remains, and before this study only 26 ancient genomes with sufficient genome-wide coverage had ever been published from the entire country. The team, led by researchers at the University of Turku and the Max Planck Institute for Evolutionary Anthropology, generated new target-capture data from 38 individuals buried at 17 archaeological sites, carefully screening each genome for contamination and excluding four that could not be reliably authenticated. The resulting dataset, analysed alongside biobank genomes from thousands of present-day Finns, provides the most detailed genomic portrait of pre-modern Finland ever assembled.

The first major finding concerns the relationship between the coastal and inland zones. Despite their documented differences in economy, burial practice, and material culture, the ancient populations of the two zones were genetically indistinguishable. Statistical analyses of allele frequencies showed that the coastal and inland groups formed a single genetic clade, excluding both modern and other ancient comparison populations. Proximal ancestry modelling revealed that both groups could be described as a three-way mixture of Scandinavian, Baltic Iron Age, and Sámi-related ancestry, with the inland group showing a slight, though not statistically significant, tilt toward the Siberian-associated component that characterises Uralic-speaking populations further east.

That genetic unity, however, did not last. When the researchers compared the ancient genomes with modern inhabitants of the same regions, a striking asymmetry emerged. People living in the inland zone today are the direct descendants of their Early Medieval predecessors: rare identity-by-descent segments shared between ancient and modern genomes map onto a broad regional pattern of connectedness across the inland zone, demonstrating nearly a millennium of population continuity. This extends earlier work on medieval cemeteries in Pirkanmaa from a local phenomenon to a regional one, showing that the persistence is a feature of the whole inland gene pool rather than an artefact of a few closely related burial sites.

The coastal zone tells a completely different story. Present-day coastal Finns carry roughly 25 percentage points more Scandinavian-derived ancestry than their Early Medieval counterparts, and their Baltic-derived ancestry has all but vanished. Haplotype-based analyses found no discernible pattern of shared DNA segments between the ancient coastal population and modern residents of the same municipalities, indicating a genuine population turnover rather than gradual drift. The researchers attribute this shift primarily to large-scale emigration from Sweden, most likely concentrated in the late thirteenth and early fourteenth centuries, when the expanding Kingdom of Sweden consolidated its rule over what medieval documents called Österland, the eastern lands. Crucially, the genetic data show that this immigration was confined to the coast: Swedish settlement did not penetrate the hinterland, and the consolidation of Swedish authority inland appears to have proceeded through administrative and ecclesiastical structures rather than through mass movement of people.

The study also reconstructed the demographic trajectory of the region using a technique called time-transect effective population size estimation, which exploits shared DNA segments among individuals who lived at different times. The results show a population in rapid expansion: the effective population size of southwestern Finland grew from roughly 2,500 individuals around 800 CE to approximately 13,000 by 1250 CE, a fivefold increase in four and a half centuries. This boom coincides with the introduction of new plough technologies from Scandinavia and the Baltics, which allowed farmers to break the heavy clay soils that dominate the region, as well as with pollen and macrofossil evidence for expanding grasslands from the mid-1100s. Notably, the researchers found no evidence of consanguinity: while many individuals carried short runs of homozygosity consistent with distant parental relatedness in a small population, none showed the long stretches that would indicate mating between close relatives, suggesting that social mechanisms to avoid inbreeding were already in place.

Perhaps the most visually compelling result is a network map of shared DNA segments connecting 103 ancient individuals from around the Baltic Sea, from the Viking Age to the Early Modern period. The network resolves into three clusters, Scandinavian, Slavic, and Finnish, linked by a single Viking Age individual from Gotland whose ancestry was previously modelled as more than 90 percent Finnish-like. When adjusted for sample size, Finland shared its strongest connections with Gotland, Latvia, and Russia, mirroring the archaeological record of trade, diplomacy, and intermarriage that tied coastal Finland into the exchange systems of the medieval North. The Finnish word for money, raha, originally meant fur pelt, a linguistic fossil of the fur trade that likely drove much of this connectivity.

Some of the individual connections are remarkably specific. A genetically female individual buried at Laukskola on the Daugava river in Latvia, dated to 1031 to 1147 CE and likely a speaker of the nearly extinct Liv language, shares DNA segments with two individuals from Early Medieval Finland consistent with a relationship as close as sixth cousins. Because the three were largely contemporaneous, their shared ancestry points to a common forebear who moved between the regions within living memory of the burials. Even more striking, the researchers recovered IBD links between Early Medieval Finns and two crew members of the Kronan, a Swedish warship that sank at the Battle of Öland in 1675. Historical records attest that officers for the Kronan were recruited from Finland, and the genetic evidence corroborates that these sailors were either Finnish themselves or recent descendants of Finnish immigrants to Sweden.

The study does have limits. Because all the sampled burials predate the main wave of Swedish immigration, the analysis can only establish that the coastal shift occurred after roughly 1300 CE, and the researchers note that genomes from the fourteenth to sixteenth centuries, particularly from sites along the axis between the two zones, will be needed to trace the migration in finer detail. The Siberian ancestry cline that runs from northeast to southwest across Finland, likely reflecting ancient admixture with Sámi-related groups, also remains only partially resolved, since no ancient genomes from eastern Finland were available. Nevertheless, the big picture is clear. Early Medieval southwestern Finland was not a remote periphery waiting to be conquered, but a genetically homogeneous and rapidly growing population firmly embedded in the circum-Baltic world. When Swedish power arrived, it reshaped the coast through settlement while leaving the inland gene pool untouched, a dichotomy whose imprint is still visible in the DNA of Finns today, nearly eight centuries later.

Subject of Research: Population genomics and demographic history of Early Medieval southwestern Finland using ancient DNA

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

Article References: O’Sullivan, R. J., Nordfors, U., Saari, N.-J., Majander, K., Taavitsainen, J.-P., Salo, K., Rohrlach, A. B., Salmela, E., Lamnidis, T. C., Traverso, L., Krause, J., Peltola, S., & Onkamo, P. (2026). Population genomics, demography, and circum-Baltic connectivity of Early Medieval southwestern Finland. Genome Biology, 27(1), Article 288. https://doi.org/10.1186/s13059-026-04277-7

Image Credits: AI Generated

DOI: 10.1186/s13059-026-04277-7

Keywords: ancient DNA, Finland, Early Medieval period, population genomics, identity-by-descent, Scandinavian ancestry, Viking Age, Baltic Sea, effective population size, Swedish migration, Sámi ancestry, archaeogenetics