It was one of the most brutal massacres of Scandinavia’s pre–Viking age. Around 450 C.E., on an island off the southern coast of what is now Sweden, the residents of an earthen-walled fort were slaughtered. Archaeologists aren’t sure whether the attackers were rival islanders or raiders from across the sea. What they do know is dozens of men, women, and children were killed in a surprise attack.
The assailants left the bodies where they lay, and the coastal fortress, known as Sandby Borg, was abandoned. “It was never reoccupied,” says Kerstin Lidén, an archaeological scientist at Stockholm University. “It’s a very particular event.”
Now, that tragedy—and another horror that occurred more than 1000 years later—is giving scientists an unprecedented snapshot of Scandinavia before and after the Viking era. The study reveals the Viking Age was a high point of migration to what is now Sweden. “The Viking period is extreme,” says team leader Anders Götherström, a geneticist at Stockholm University. “There’s massive variation and migration coming into Viking society.”
Scientists already know, thanks to ancient DNA, that Scandinavia was a hotbed of immigration from Northern Europe and the British Isles during the Viking age, roughly between 750 C.E. and 1050 C.E. But what immigration looked like before and after this period has been less clear.
A big part of that uncertainty arises from how people of the time disposed of their dead. Think of the blazing boat burials written about in Icelandic sagas, or the pyre of legendary Beowulf. “Swedes in the Iron Age had the annoying habit of practicing cremation,” says Aarhus University archaeologist Søren Sindbæk, who was not part of the research. Burnt bodies mean no DNA.
To find genetic material from the pre–Viking era, Götherström and his colleagues turned to sites featuring people who died under unusual circumstances, like Sandby Borg. Excavations in 2014 on the Swedish island of Öland revealed the bones of victims who were left unburied and unburned. “It wasn’t so good for them, but it was great for me,” Götherström says.
His team managed to decode ancient genomes from six men and one woman killed at Sandby Borg. Their DNA, along with samples from a handful of other early sites, showed Iron Age Scandinavians a few centuries before the Viking Age were genetically similar to each other and related to modern-day Swedes, the researchers report today in Cell.
More genomes from other sites confirm that around 750 C.E., as new sailing techniques enabled Scandinavian merchants and raiders to range far beyond their corner of the Baltic, Scandinavia saw an influx of immigrants. People with genetic ancestry from the British Isles, the eastern Baltic, and “Uralic” ancestry from Siberia appear in different parts of Scandinavia. There’s more western ancestry in Sweden and Denmark, for example, and an eastern influence on the island of Gotland.
“They vindicate the idea that there is an upturn in contact with people overseas in the Viking Age,” Sindbæk says.
Another tragedy provided further evidence of Viking Age immigration. In 1676, a Swedish warship called the Kronan capsized and partially exploded in the Baltic, just 6 kilometers off the coast of Öland, taking at least 600 men down with it. Since 1981, divers have recovered coins, textiles, military equipment, and dozens of skeletons from the cold, low-oxygen depths. “It’s like a fridge,” Götherström says. “They’re so well preserved they’re like modern samples.”
Genes from the Kronan’s sailors capture a cross-section of Swedish society at the time, from upper class officers to lower deck gun crews. Most of the skeletons belong to men in their 20s and 30s. “They didn’t recruit mercenaries in those days,” says Lars Einarsson, an archaeologist at the Kalmar Museum and director of the Kronan underwater excavations. “Most of the crew come from what was Sweden.” If the Viking Age results represented a “before” picture of Scandinavia’s population, the Kronan showed researchers the “after.”
The men who died aboard the Kronan closely resembled modern Swedes and showed strong similarities to the people of Sandby Borg, suggesting Viking Age immigration from the British Isles and eastern Baltic didn’t leave much of a mark on Scandinavians’ genetic profile. “We didn’t expect that previously,” Götherström says. “We’re getting people coming from the west and east in the Viking Age, but for some reason they do not have as many children.”
The results sent the research team searching for possible explanations. One possibility is that the Viking-era immigrants were not the types to settle down and have kids, says Ricardo Rodriguez Varela, a co-author and geneticist at Stockholm University. They might have come as traders or diplomats, for example, or as members of social groups that tended not to have large families: “Christian monks or slaves produce less offspring.”
Researchers hope the genetic results will also help piece together more personal stories, such as which Swedish villages were home to the Kronan’s crew members, or the circumstances behind the Sandby Borg massacre. Roman coins found in the fort suggest its residents weren’t native to the island, according to one theory, perhaps making them targets for violence. “Were they really locals, or did they move to Scandinavia?” Götherström asks. “Now, we have the answers.”