One of the deadliest diseases in history, the plague once killed between 30 and 60 percent of Europe’s entire population. Exactly when and how it got to Europe has puzzled scientists for some time, but now researchers think the unwelcome bacterium arrived during the late Stone Age.
Led by the Max Planck Institute for the Science of Human History, a team of scientists studied more than 500 human tooth and bone samples from Russia, Germany, Hungary, Croatia, Lithuania, Estonia, and Latvia, looking for any signs of the plague-causing bacterium known as Yersinia pestis.
They managed to sequence entire genomes of Y. pestis from six individual people that lived 4,800 to 3,700 years ago from the Late Neolithic period to the Bronze Age, and reported their findings in Current Biology.
The researchers discovered that all the genomes were quite closely related, despite having infected individuals in a number of different countries.
"This suggests that the plague either entered Europe multiple times during this period from the same reservoir, or entered once in the Stone Age and remained there," explained co-lead author Aida Andrades Valtueña in a statement.
So, to work out how the plague might have made it to Europe, the team used clues from archaeological and ancient DNA representing human movements during the same time period. They concluded that the plague was likely carried to Europe by the nomadic steppe people, who began migrating to the continent about 4,800 years ago from the Pontic-Caspian Steppe. This expanse of steppeland stretches from the northern shores of the Black Sea eastwards to the Caspian Sea.
The steppe people had distinct genetic markers so they can be traced in modern-day Europeans.
"In our view, the human genetic ancestry and admixture, in combination with the temporal series within the Late Neolithic-Bronze Age Y. pestis lineage, support the view that Y. pestis was possibly introduced to Europe from the steppe around 4,800 years ago, where it established a local reservoir before moving back towards Central Eurasia," said study author Alexander Herbig.
The study also suggests that at this time, the severity of plague infections was also changing, but more research is required to find out more. However, fear of infection might have spurred the movement of people into Europe.
"The threat of Y. pestis infections may have been one of the causes for the increased mobility during the late Neolithic-early Bronze Age period," explained co-lead author Johannes Krause. What’s more: "It's possible that certain European populations, or the steppe people, may have had a different level of immunity."
The team hopes that their research will help to unravel how the plague evolved, and explain why it became more aggressive as time went on. While it may seem like a thing of the past, the plague is currently raging through Madagascar, and has killed 195 people thus far, so finding out more about it is as important as ever.