A new ancient DNA study reveals that coastal and inland communities in southwestern Finland were genetically similar during the early Middle Ages, before migration from Sweden transformed the coastal population. The research also shows that medieval Finland was deeply connected to networks stretching across the Baltic Sea.
A new genetic study is offering a clearer picture of how migration, trade and population growth shaped Finland during the Viking Age and Middle Ages. Researchers led by the University of Turku analysed genomes from people who lived in southwestern Finland between the Late Iron Age and early Middle Ages. Their findings suggest that the inhabitants of coastal and inland regions were genetically very similar around a thousand years ago, despite differences in their economies and material cultures.
That situation later changed dramatically along the coast. Modern coastal populations have substantially more Scandinavian ancestry than their medieval predecessors, while inland communities show remarkable genetic continuity stretching back nearly a millennium.
The findings, published in Genome Biology, provide new evidence for migration from Sweden into Finland during the later Middle Ages and show that this movement was concentrated particularly in coastal areas.
“Our findings suggest that migration from Sweden began to have a broader impact on Finland’s population relatively late, during the Middle Ages”, says lead author, Postdoctoral Researcher Ronan James O’Sullivan. “Of course, people were already moving across the Baltic Sea earlier than 1250 CE, but migration from Sweden to Finland on a larger scale seems to have taken place only after the Early Medieval period.”
Coastal change, inland continuity
Figure from O’Sullivan et al., Genome Biology (2026), licensed under CC BY 4.0.
The researchers generated genome-wide data from human remains recovered at 17 archaeological sites in southwestern Finland. Of 38 sampled individuals, 34 produced data suitable for the main analyses. Most dated from the Early Medieval period, 1050–1250, while two belonged to the Late Iron Age.
During this period, southwestern Finland contained culturally and economically distinct zones. The coastal region had particularly strong connections with Scandinavia, while inland Häme displayed influences from both eastern and western traditions. Yet the genetic evidence indicates that these cultural distinctions did not correspond to separate populations: ancient coastal and inland inhabitants were genetically very similar.
The subsequent trajectories of the two populations were very different. In inland southwestern Finland, the researchers found evidence for nearly 1,000 years of population continuity. Modern inhabitants retain particularly strong genetic connections to the people who lived there during the early Middle Ages.
Along the coast, however, researchers detected a substantial change. Modern coastal inhabitants showed an approximately 25-percentage-point increase in Scandinavian-derived ancestry compared with the ancient coastal population, together with reduced Baltic-derived ancestry.
The study argues that the large-scale immigration responsible for this change probably occurred mainly after the thirteenth century. Historical research has previously suggested that Swedish migration intensified during the late 13th and early 14th centuries, as southwestern Finland became increasingly integrated into the Kingdom of Sweden.
The genetic evidence suggests that this migration remained heavily concentrated along the coast. According to the researchers, Swedish settlers may have favoured areas close to coastal administrative and ecclesiastical centres, while maritime connections made it easier to maintain economic, political and family relationships with Sweden.
Finland and the Viking-Age Baltic world
Figure from O’Sullivan et al., Genome Biology (2026), licensed under CC BY 4.0.
The study also provides genetic evidence for the extensive connections linking Finland with other parts of the Baltic world long before large-scale Swedish settlement.
During the Viking Age, archaeological evidence already points to considerable movement and exchange across the Baltic. Finnish-made or Finnish-influenced objects have been discovered at Birka in Sweden, while imported glass beads, weapons, jewellery and Islamic, Anglo-Saxon and Germanic coins found in Finland demonstrate participation in long-distance trade networks. Furs were probably an important Finnish export.
The genetic evidence reinforces this picture. Researchers identified shared DNA segments connecting individuals from Finland with people from Viking-Age Sweden, Gotland, Denmark, Norway and Russia, as well as medieval Latvia.
Gotland stands out in particular. Finland had more average normalised DNA sharing with Gotland than with any other Baltic region examined. Several individuals from Viking-Age Gotland also possessed substantial Finnish-like ancestry, suggesting either migration from Finland or recent Finnish ancestry. The researchers argue that these results reinforce archaeological evidence for Gotland’s importance as a centre of movement and exchange across the Baltic.
Another intriguing connection emerged with Latvia. A woman buried at Laukskola on the Daugava River between 1031 and 1147 shared DNA with two individuals from southwestern Finland at a level consistent with distant kinship. The researchers suggest that their common ancestor could have migrated from Latvia to Finland, or in the opposite direction.
“Finland was never isolated from the wider Baltic world,” O’Sullivan points out. “Our results show that people living here were closely connected to the vibrant, international networks that spanned the Baltic Sea”.
A rapidly growing medieval population
Early medieval fashion from Finland. The costumes are based on grave finds from the Yliskylä cemetery in Perniö, Salo. Photo credit: Ulla Nordors
The genetic data also allowed the researchers to investigate demographic change within Finland itself.
Their estimates suggest that the effective population size increased substantially over the period studied, from roughly 2,500 around the beginning of the Late Iron Age to approximately 13,000 by the end of the Early Medieval period. The researchers caution that the earlier estimate is less certain because it is based on fewer individuals.
This expansion coincides with significant changes in medieval Finnish agriculture. New plough technologies, probably arriving from Scandinavia and the Baltic region, made it easier to cultivate the heavy clay soils of southwestern Finland. Archaeological and environmental evidence independently points towards increasing agricultural output and population growth, including an expansion of open grasslands from the mid-12th century.
The researchers found no evidence for widespread close-relative reproduction despite the relatively small population. Instead, the genetic evidence suggests that communities maintained social practices that helped avoid close inbreeding.
One of the study’s most unexpected connections stretches several centuries beyond the Middle Ages. Two people who died aboard the Swedish warship Kronan, which sank during the Battle of Öland in 1675, shared distant genetic connections with early medieval individuals from Finland. Historical evidence records recruitment from Finland for the ship, and the researchers conclude that these individuals either came from Finland or descended from Finnish migrants to Sweden.
Taken together, the findings portray medieval southwestern Finland not as a peripheral or isolated region, but as a growing population deeply involved in the movement of people and goods around the Baltic Sea. At the same time, they reveal how migration after the early Middle Ages produced a lasting divide between coastal and inland populations that can still be detected genetically today.
The article, “Population genomics, demography, and circum-Baltic connectivity of Early Medieval southwestern Finland,” by Ronan James O’Sullivan, Ulla Nordfors, Nelli-Johanna Saari, Kerttu Majander, Jussi-Pekka Taavitsainen, Kati Salo, Adam B. Rohrlach, Elina Salmela, Thiseas C. Lamnidis, Luca Traverso, Johannes Krause, Sanni Peltola and Päivi Onkamo, is published in Genome Biology. Click here to read it.
Top Image: Finland and the eastern Baltic depicted on Olaus Magnus’s Carta marina, published in 1539.
A new ancient DNA study reveals that coastal and inland communities in southwestern Finland were genetically similar during the early Middle Ages, before migration from Sweden transformed the coastal population. The research also shows that medieval Finland was deeply connected to networks stretching across the Baltic Sea.
A new genetic study is offering a clearer picture of how migration, trade and population growth shaped Finland during the Viking Age and Middle Ages. Researchers led by the University of Turku analysed genomes from people who lived in southwestern Finland between the Late Iron Age and early Middle Ages. Their findings suggest that the inhabitants of coastal and inland regions were genetically very similar around a thousand years ago, despite differences in their economies and material cultures.
That situation later changed dramatically along the coast. Modern coastal populations have substantially more Scandinavian ancestry than their medieval predecessors, while inland communities show remarkable genetic continuity stretching back nearly a millennium.
The findings, published in Genome Biology, provide new evidence for migration from Sweden into Finland during the later Middle Ages and show that this movement was concentrated particularly in coastal areas.
“Our findings suggest that migration from Sweden began to have a broader impact on Finland’s population relatively late, during the Middle Ages”, says lead author, Postdoctoral Researcher Ronan James O’Sullivan. “Of course, people were already moving across the Baltic Sea earlier than 1250 CE, but migration from Sweden to Finland on a larger scale seems to have taken place only after the Early Medieval period.”
Coastal change, inland continuity
The researchers generated genome-wide data from human remains recovered at 17 archaeological sites in southwestern Finland. Of 38 sampled individuals, 34 produced data suitable for the main analyses. Most dated from the Early Medieval period, 1050–1250, while two belonged to the Late Iron Age.
During this period, southwestern Finland contained culturally and economically distinct zones. The coastal region had particularly strong connections with Scandinavia, while inland Häme displayed influences from both eastern and western traditions. Yet the genetic evidence indicates that these cultural distinctions did not correspond to separate populations: ancient coastal and inland inhabitants were genetically very similar.
The subsequent trajectories of the two populations were very different. In inland southwestern Finland, the researchers found evidence for nearly 1,000 years of population continuity. Modern inhabitants retain particularly strong genetic connections to the people who lived there during the early Middle Ages.
Along the coast, however, researchers detected a substantial change. Modern coastal inhabitants showed an approximately 25-percentage-point increase in Scandinavian-derived ancestry compared with the ancient coastal population, together with reduced Baltic-derived ancestry.
The study argues that the large-scale immigration responsible for this change probably occurred mainly after the thirteenth century. Historical research has previously suggested that Swedish migration intensified during the late 13th and early 14th centuries, as southwestern Finland became increasingly integrated into the Kingdom of Sweden.
The genetic evidence suggests that this migration remained heavily concentrated along the coast. According to the researchers, Swedish settlers may have favoured areas close to coastal administrative and ecclesiastical centres, while maritime connections made it easier to maintain economic, political and family relationships with Sweden.
Finland and the Viking-Age Baltic world
The study also provides genetic evidence for the extensive connections linking Finland with other parts of the Baltic world long before large-scale Swedish settlement.
During the Viking Age, archaeological evidence already points to considerable movement and exchange across the Baltic. Finnish-made or Finnish-influenced objects have been discovered at Birka in Sweden, while imported glass beads, weapons, jewellery and Islamic, Anglo-Saxon and Germanic coins found in Finland demonstrate participation in long-distance trade networks. Furs were probably an important Finnish export.
The genetic evidence reinforces this picture. Researchers identified shared DNA segments connecting individuals from Finland with people from Viking-Age Sweden, Gotland, Denmark, Norway and Russia, as well as medieval Latvia.
Gotland stands out in particular. Finland had more average normalised DNA sharing with Gotland than with any other Baltic region examined. Several individuals from Viking-Age Gotland also possessed substantial Finnish-like ancestry, suggesting either migration from Finland or recent Finnish ancestry. The researchers argue that these results reinforce archaeological evidence for Gotland’s importance as a centre of movement and exchange across the Baltic.
Another intriguing connection emerged with Latvia. A woman buried at Laukskola on the Daugava River between 1031 and 1147 shared DNA with two individuals from southwestern Finland at a level consistent with distant kinship. The researchers suggest that their common ancestor could have migrated from Latvia to Finland, or in the opposite direction.
“Finland was never isolated from the wider Baltic world,” O’Sullivan points out. “Our results show that people living here were closely connected to the vibrant, international networks that spanned the Baltic Sea”.
A rapidly growing medieval population
The genetic data also allowed the researchers to investigate demographic change within Finland itself.
Their estimates suggest that the effective population size increased substantially over the period studied, from roughly 2,500 around the beginning of the Late Iron Age to approximately 13,000 by the end of the Early Medieval period. The researchers caution that the earlier estimate is less certain because it is based on fewer individuals.
This expansion coincides with significant changes in medieval Finnish agriculture. New plough technologies, probably arriving from Scandinavia and the Baltic region, made it easier to cultivate the heavy clay soils of southwestern Finland. Archaeological and environmental evidence independently points towards increasing agricultural output and population growth, including an expansion of open grasslands from the mid-12th century.
The researchers found no evidence for widespread close-relative reproduction despite the relatively small population. Instead, the genetic evidence suggests that communities maintained social practices that helped avoid close inbreeding.
One of the study’s most unexpected connections stretches several centuries beyond the Middle Ages. Two people who died aboard the Swedish warship Kronan, which sank during the Battle of Öland in 1675, shared distant genetic connections with early medieval individuals from Finland. Historical evidence records recruitment from Finland for the ship, and the researchers conclude that these individuals either came from Finland or descended from Finnish migrants to Sweden.
Taken together, the findings portray medieval southwestern Finland not as a peripheral or isolated region, but as a growing population deeply involved in the movement of people and goods around the Baltic Sea. At the same time, they reveal how migration after the early Middle Ages produced a lasting divide between coastal and inland populations that can still be detected genetically today.
The article, “Population genomics, demography, and circum-Baltic connectivity of Early Medieval southwestern Finland,” by Ronan James O’Sullivan, Ulla Nordfors, Nelli-Johanna Saari, Kerttu Majander, Jussi-Pekka Taavitsainen, Kati Salo, Adam B. Rohrlach, Elina Salmela, Thiseas C. Lamnidis, Luca Traverso, Johannes Krause, Sanni Peltola and Päivi Onkamo, is published in Genome Biology. Click here to read it.
Top Image: Finland and the eastern Baltic depicted on Olaus Magnus’s Carta marina, published in 1539.
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