The fact that many people died in a relatively short time in one place suggested they might have perished together in an epidemic, lead study author Nicolás Rascovan, a biologist at Aix-Marseille University in Marseille, France, told Live Science.
The study researchers theorized that the ancient sample diverged from other plague strains about 5,700 years ago.
About 5,000 years ago, humans migrated from the Eurasian steppe down into Europe in major waves, replacing the Neolithic farmers who lived in Europe at that time.
However, if the plague specimen from the Swedish grave diverged from other strains 5,700 years ago, it likely evolved before the steppe migrations began — suggesting it was already there.
Eventually, it would have made its way even to relatively distant sites like Frälsegården in Sweden, where the woman the researchers analyzed died.