An ancient strain of the plague found in a woman buried in Sweden may be the fatal signature of a devastating pandemic that swept through stone age farmers and set the stage for a massive migration into Europe from the east.

Evidence for the grim scenario came to light when scientists ran genetic tests on a 20-year-old woman from a rural farming community who was among 78 people buried in a passage grave in Gökhem in western Sweden.

DNA collected from the woman’s teeth revealed that she harboured the oldest strain of the plague yet found, dating to nearly 5,000 years ago. Traces of the same strain were also found in another individual at the burial site.

“This is the earliest strain of the plague that we know about, and it probably played a big role in the decline of the population,” said Simon Rasmussen at the University of Copenhagen. “You suddenly have this big outbreak and a lot of people are going to die.”

All forms of the plague are caused by the bacterium Yersinia pestis, but the strain from Gökhem bore the genetic hallmarks of pneumonic plague. This form of the infection takes hold in the lungs and is more virulent than the bubonic plague, which primarily infects the lymph nodes. An epidemic of bubonic plague claimed the lives of up to 200 million people when the Black Death struck Europe in the 14th century.

Rasmussen believes the plague may originally have emerged as a human disease in the unprecedented mega-settlements that started to be built about 6,000 years ago in what are now Ukraine, Romania and Moldova. The settlements were home to tens of thousands. But with so many people living in dreadful sanitary conditions and in close contact with animals, the sites were perfect breeding grounds for bugs. “This is the classic, textbook example of what is needed for new pathogens to develop,” he said.

Writing in the journal Cell, the scientists propose that the plague first emerged in the Eurasian mega-settlements about 5,700 years ago. The infection then spread far and wide, reaching distant farming communities along trade routes with help from the newly-invented horse-drawn cart. The scenario may explain the so-called neolithic decline, a crash in the European population that took place about 5,500 years ago, and why the mega-settlements, then the largest in Europe by far, are known to have been abandoned and burned to the ground.

“The mega-settlements start to collapse just as the seed strain starts to develop,” said Rasmussen. “The plague could have been part of the reason.”

A pandemic of the sort the scientists describe could explain how foraging populations from the Eurasian steppe were able to move into Europe with such ease at the time.

“We know that there was this decline in the population in neolithic times and that made it possible for people to migrate into Europe,” said Rasmussen. “When that happened, it completely changed the genetic makeup of the early Europeans. It turned Europeans into what they are today.”