New study tells a story of gene flow into Scandinavia

Looking at 2,000 years of genetic history

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Image: Hans Dahl, “Leif Erikson Discovers America” / Wikimedia Commons
Contrary to popular belief, Vikings weren’t simply blonde, seafaring Scandinavians. A new study shows people who lived in Scandinavia exhibited high levels of non-Scandinavian ancestry, pointing to a continuous exchange of genetic information across the broader European continent.

Terje Birkedal
Laguna Woods, Calif.

A recent study has revealed that today’s Scandinavians have different genetic makeups than their pre-Viking, Viking, and even medieval ancestors. The differences are not great, but they are nonetheless significant and tell an interesting story about genetic change in Scandinavian populations over the last 2,000 years.

The study was conducted by a vast team of scientists from Scandinavia, as well as many other countries around the world. The findings of these historical genetic sleuths were published in the scholarly journal Cell on Jan. 5, 2023, in an article entitled “The genetic history of Scandinavia from the Roman Iron Age to the present.” The primary authors and researchers were Ricardo Rodriguez-Varela, Kristjan H.S. Moore, S. Sunna Ebenesersdóttir, Kári Stefánsson, Agnar Helgason, and Anders Götherström.

The goal of the researchers was to document the timing and geographical extent of genetic influences from other populations on Scandinavians from the time of Christ to the present. What they hoped to examine is what geneticists call gene flow. Gene flow can be defined as the movement (by interbreeding) of genetic material from one population to another.

To accomplish this ambitious task, they looked at the genomes that had been recovered from the human remains of 297 people who had lived in Scandinavia between the first century to the 19th century. The word genome refers to the complete set of genetic instructions contained in a cell or all the instructions that guide the making of each unique human.

To ensure they sampled ancient populations from all over Scandinavia, they selected ancient genomes from eight distinct areas of Scandinavia: Denmark was treated as one geographic area; Norway was divided into three areas, north, central, and south; Sweden was also divided into north, central, and south; and the large island of Gotland was also treated as a separate area for purposes of the study.

The researchers also collected the full genetic information on a total of 16,638 people living in Sweden, Norway, and Denmark today for purposes of comparison to the 297 ancient Scandinavian genomes. To understand which populations were bringing genetic change to Scandinavia over the past 2,000 years, they also gathered genetic information on 9,000 non-Scandinavians from all over the wider Eurasian geographic continent.

To track historical changes in gene flow through time, the researchers divided the last 2,000 years into five separate periods: pre-Viking (1-749), Viking (750-949), late Viking (950-1099), medieval (1100-1349), and post-medieval (1350-1850). Sufficient human genomes were available out of the total 297 genomes to represent all of these time periods.

So, you may ask, after all this background on methodology, what did the researchers actually learn about Scandinavia’s genetic past? The short answer is a lot. They learned that pre-Viking Scandinavians were different from their descendants in still having echoes of ancestral DNA (the molecular-level information package in every cell) from their Mesolithic and Neolithic ancestors.

Also, during this early period, outside populations seem to have had only low levels of influence on the genetics of Scandinavian populations. There were some genes flowing in from the eastern Baltic region to Sweden, as well as some minor gene flow from the British-Irish Isles into Scandinavia. Interestingly, the remains of a woman in Denmark dating from about 500 exhibited a large amount of British-Irish ancestry. In addition, there was the hint of a gene flow from north to south from Uralic populations, most likely coming from the Sámi people.

The big changes in the genetic make-up of Scandinavians began in the Viking period and these changes continued in their influence through the late Viking period only to begin to ebb during the medieval period. Aided by their sleek newly-invented Viking ships, this was the era when Scandinavians were to be the first peoples to reach four continents in course of their exploration, raiding, trading, and colonization of other lands.

In the Viking and the late Viking periods, there was a “surge” in gene flow into all of Scandinavia from the British-Irish Isles. The authors describe this surge in DNA as “a lasting and widespread gene flow from the British-Irish Isles into Scandinavia, most likely because of migration during the Viking period.”

Based on the array of genes flowing in, the researchers believe many of the migrants were female, though there are genetic clues that men were also reaching Scandinavian shores. It appears that at least some of these people were of high status. In fact, the genome of a high status woman found in an elaborate boat burial in central Sweden showed that she was fully British-Irish in ancestry. However, the majority of the incoming British-Irish were probably of low status and many were most likely brought to Scandinavia as slaves.

Yet, we should not forget that in the late Viking period, England, Norway, southern Sweden and Denmark were united for nearly 30 years under a series of Scandinavian kings. Thus, in this period it is likely that a good number of the migrants to Scandinavia were free, such as missionaries, clerics, and merchants.

Also beginning in the Viking period, but significantly increasing during the late Viking period, was gene flow from the peoples of the eastern Baltic region. The source, based on the genome research, was clearly “female-based” and the geographical direction of this westerly gene flow was largely toward the island of Gotland and central Sweden. Recall that Swedish Vikings dominated raiding and trading in the eastern Baltic Sea.

However, the authors rightly caution that one should not assume that all or most of this Baltic migration to Sweden was by force. Much of it was probably voluntary and involved people freely choosing to move. There was a great deal of trading and tributary relations going on between eastern Baltic peoples and the Swedes during Viking times.

At this time, genes were also flowing into Denmark and southern Norway and Sweden from people from southwestern Europe. This gene flow appears to have been less than that coming from the British-Irish Isles and the eastern Baltic lands, but people of southwestern Europe were definitely having an effect on the genetic landscape of Scandinavia during this time period. However, the researchers were unable to pinpoint their exact lands of origin.

Though the genetic influence of Uralic populations was thought to begin in the pre-Viking period, gene flow from these people accelerated during the Viking period and became prominent in both northern and central Sweden and Norway. The main source of this influence was probably the Sámi, with whom the Vikings had important trade relations.

However, people from Finland living across the Gulf of Bothnia from Sweden, who were Uralic in origin, could also have been a source of some these Uralic genes. Overall, the Uralic genes formed what is called a genetic cline, a geographic variation from higher to lower incidence in both Sweden and Norway that persists to the present day among people of those countries. The incidence of the Uralic genes today, as back then, is greatest in the north and least toward the south.

In the subsequent medieval period, the influence of British-Irish genes on southern and central Norwegians remained strong but began to quickly drop and disappear in northern Norway, Sweden, and Denmark.  Gene flow from southwest Europe in Scandinavia also fell to low levels. In contrast, genes coming from the Baltic region continued to increase in central and southern Swedish populations. And the north-to-south Uralic genetic cline remained in place in Norway and Sweden.

When they looked at the genomes of modern Scandinavians, the researchers were struck by the low levels of British-Irish ancestry that they found.  Nonetheless, the amount of British-Irish ancestry in today’s populations is much greater than that detected in pre-Viking populations. The overall low levels of British-Irish ancestry found in all regions of  modern Scandinavia surprised the researchers and led them to conclude that the “overall magnitude of British-Irish gene flow into Scandinavia appears to have been small, as witnessed by the relatively clear distinction between modern populations from Scandinavia and the British-Irish Isles.”

The same may be true of the gene flow from the eastern Baltic in the Viking and medieval eras, for the researchers also observed only low levels of that ancestry today in central and southern Sweden. But the Uralic north-to-south genetic cline in Scandinavia that first fully appeared in the early Viking age is still present today and documents higher levels of Uralic ancestry among Scandinavians who live in the northern reaches of Norway and Sweden.

The researchers say there is much more to learn about Scandinavian genetic history. They wish to better understand why the genes of migrants from the British Isles and the eastern Baltic dropped to low levels in modern Scandinavian populations over the last 600 years. As migrants, did they have fewer children over the years than native Scandinavians? Or are there other explanations for this phenomenon? The researchers, in particular, wish to obtain a better sampling of genetic material from Bronze Age and even earlier Scandinavians. Unfortunately, those far-ago people often destroyed their researchable DNA because they favored cremating their dead.

For those who would like to read the referenced article in full, it is available online at cell.com/cell/pdf/S0092-8674(22)01468-4.pdf.

This article originally appeared in the June 2023 issue of The Norwegian American.

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Terje Birkedal

Terje G. Birkedal was born in Stavanger, Norway, in 1946. He immigrated to the U.S. as a child and grew up in Colorado. After earning a Ph.D. in Anthropology he served as an archeologist with the National Park Service for 36 years. He has conducted fieldwork in Alaska, the American South and Southwest, Canada, the Great Plains, Guam, and Norway. He served five years as President of Sons of Norway Bernt Balchen Lodge in Anchorage, Alaska, and he has always been passionate about Norwegian prehistory, history, and culture.