‘Root’ of the plague reaches back some 5,000 years
In an ancient grave in Sweden, scientists have unearthed the oldest known strain of a deadly bacteria that has killed millions of people over thousands of years.
They call it Yersinia pestis. You may know it as the plague. The discovery suggests that the microscopic bacteria has been wiping out great swaths of the human population for more than 5,000 years — destroying empires, spurring political uprisings and leaving a permanent mark on regional gene pools.
“What we found in the Swedish grave site is not only the oldest sample of the Y. pestis genome, but also the oldest version of the genome,” said Simon Rasmussen, a metagenomics researcher at the Technical University of Denmark, who led the work. “Think of it as the root of the tree.”
The arguably most notorious plague pandemic, known as the Black Death or the Great Plague, started in China in 1334 and spread along trade routes to Constantinople before reaching Europe in the 1340s. It also claimed the lives of an estimated 25 million people, including about half the population of Europe, according to researchers at the Centers for Disease Control and Prevention.
Rat-associated plague can still be found in populations of ground squirrels and other small mammals in the Americas, Africa and Asia. It is now under control in most urban areas across the globe.
The new study, published in the journal Cell, reveals that the relationship between humans and plague goes back even further than scientists had realized. The bacteria identified by Rasmussen and his colleagues may represent a previously unknown outbreak of plague that struck Europe as much as 5,700 years ago.
The researchers already knew that the population of Europe plummeted 5,000 to 6,000 years ago. This sudden plunge is known as the Neolithic decline.
Rasmussen and his colleagues wondered about a plague pandemic.
“We were thinking, ‘Where have we seen this drop before?’ and that got us thinking about the Black Plague,” he said.
The disease was known to have existed across Eurasia at the dawn of the Bronze Age, which started around 4,500 years ago, but there was no evidence of its existence earlier than that.
To see if the plague was in Europe at the end of the Neolithic period, the group used databases of DNA extracted from ancient human remains — specifically, teeth. Because blood circulates through the centre of our teeth, Rasmussen said, it is possible to detect the DNA of pathogens that were present in a person’s bloodstream at their time of death by examining a tooth sample.
“If you die from it and it’s in your blood,” he said, “then we can find it.”
After scanning for genetic sequences resembling modern-day Y. pestis, the group eventually found a match. It was in DNA extracted from the tooth of a 20-year-old woman who died in western Sweden between 5,040 and 4,867 years ago.
“It was the oldest plague sample ever found,” Rasmussen said.
Although the young Swedish woman did not live in prime plague territory, there were other places in Europe where the disease could have flourished in the Neolithic era, Rasmussen said. These were the mega-settlements of the Trypillia Culture, built between 6,100 and 5,400 years ago and located in present-day Ukraine, Romania and Moldova. The largest of these settlements was home to as many as 20,000 people.
Archeologists have shown that these settlements were abandoned and burned about once every 150 years.
The researchers propose that the plague first evolved in these mega-settlements, morphing from a relatively benign stomach bug to a deadly microscopic killer around 5,700 years ago, around the time when the Swedish strain diverged from all others then in existence. This could explain the periodic burning of the buildings — perhaps they were set aflame to eradicate the disease.
The authors also suggest that the plague made its way from these settlements to the small Swedish farming village thanks to a vast trade network that was made possible by the recent expansion of animal-pulled wagons. As the disease spread along trade routes throughout the continent, it could have caused the Neolithic decline.