(CNN) - The fearsome disease known as the plague might help explain a mystery that has puzzled scientists for years.
Between 5,000 and 6,000 years ago, Neolithic farming communities in Europe dramatically declined, and there is still no definitive answer as to why. Now, however, a team of researchers in Sweden has found a new strain of plague that they say may have been partially responsible.
The research, published in the journal Cell, describes the discovery of the oldest-known sample of plague bacteria in the 5,000-year-old genetic material of a 20-year-old woman found in a rural farming community.
She was buried in a passage grave in Gökhem, southern Sweden, alongside 78 people from around the same time. One other person also showed traces of the same strain of plague, and the researchers suggest that this provides evidence of an epidemic.
Scientists previously thought that the plague arrived in Europe with migrants from the Eurasian Steppe, who effectively moved in and replaced the Neolithic communities. The steppe is a belt of grassland that runs from modern-day Hungary through Ukraine and Central Asia then east to China. Steppe peoples moved across the region for most of recorded history, according to Encyclopedia Britannica.
However, the discovery that the plague reached northern Europe far earlier suggests that the disease arrived before the Eurasian migrants.
The study suggests that the plague started as a human disease in "mega-settlements" of 10,000 to 20,000 people were built starting about 6,000 years ago. Poor sanitation and the concentration of people, animals and food may have allowed the plague to develop into a dangerous disease.
"It's the classic textbook example of what's needed to develop a new pathogen," said lead study author Simon Rasmussen, associate professor with the Disease Systems Biology Program at the University of Copenhagen.
The pathogen then spread throughout Europe along trade routes, made possible thanks to the development of technology such as the wheel, the study said, rather than with mass migration.
"You can think of it as the Stone Age ending with a bang," Rasmussen said. "Society as it is just falls apart."
With Neolithic communities decimated by plague, the theory goes, Eurasian migrants found it relatively easy to fan out across Europe, forever changing the genetic makeup of people on the continent.
"This mass migration creates the genetics of the modern European," Rasmussen said.
In 2016, scientists reconstructed the genome of an ancient plague in order to shed light on how certain diseases can either mysteriously disappear or continue to evolve and spread.
The newly discovered strain of ancient plague died out thousands of years ago, but plague still exists today. In 2017, more than 2,000 people contracted the disease in Madagascar in an outbreak that left 165 people dead.
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