Dec. 6 (UPI) -- Scientists have discovered one of the earliest and most primitive strains of bacteria responsible for pneumonic plague. The early strain may explain the demise of European mega-settlements at the end of the Neolithic period.
Researchers found the ancient plague strain by searching genomic databases for DNA sequences similar to modern forms of the deadly bacteria. They traced a match to the remains of a young woman recovered from an ancient burial site in Sweden.
The plague strain found in the DNA of the Stone Age woman is the most primitive form of the bacteria Y. pestis yet discovered -- the closest to the plague's genetic origins.
"The genetic origin of Y. pestis is Y. pseudotuberculosis, a gut pathogen that is relatively harmless compared to Y. pestis," Simon Rasmussen, a computational biologist at the University of Copenhagen, told UPI in an email. "Therefore it is really interesting to see how it evolved from this relatively harmless form into what is one of the deadliest pathogens in our history."
Because the plague bacteria was found in the woman's 5,000-year-old teeth, it was likely circulating in her blood stream. Researchers think the infection was the woman's cause of death. The same plague strain was found in other human remains excavated from the Swedish burial site, further evidence of a deadly outbreak.
Researchers have long sought to explain the decline of European settlements at the end of the Stone Age. Archeological evidence suggests that by 5,400 years ago, Europe's first mega-settlements had begun to collapse. Around 4,800 years ago, the continent was repopulated by people from the steppe regions of Eurasia.
Some scientists have argued early steppe migrators brought the first strains of plague with them to Europe, but the latest discovery proves an early strain was present before their arrival.
Rasmussen and his colleagues think the newly discovered strain could explain why European settlements were on the decline by the time herders from the central Eurasian steppe arrived.
Europe's Neolithic mega-settlements, made possible by the development of agriculture, featured job specialization, new technologies and long-distance trade. These urban centers, with large numbers of people living in close quarters with animals, would have provided the ideal breeding ground for deadly new diseases.
Trade between settlements would have aided the spread of an early plague strain. The movement of early European traders could also explain how a small, rural settlement in Sweden became infected.
If an early form a plague did spell the downfall of Stone Age mega-settlements in Europe, the newly discovered strain is the best candidate yet discovered. It diverged from Y. pseudotuberculosis several thousand years before other strains.
Still, the story of Neolithic collapse in Europe and the emergence of the plague isn't clear. Scientists have yet to find genetic evidence of the plague among remains recovered from ancient European mega-settlements, and at least three strains were present in Europe by the end of the Stone Age.
"At the end of the Neolithic several lineages of strains were present," Rasmussen said. "The one from Sweden, the one from the Bronze Age and the strains that evolved to become the Black Death and modern strains."
"The two basal strains -- Sweden and Bronze Age -- were only capable of causing pneumonic plague and not optimized for flea transmission. The ancestor of the modern strains evolved that, but we haven't found ancient versions of that," Rasmussen said. "The Swedish and Bronze Age strains later disappeared and only the modern strains -- those responsible for the Justinian and Black Death epidemics -- are here today."
Rasmussen and his colleagues plan to continue looking for the "smoking gun" that proves their early plague strain triggered the collapse of Stone Age mega-settlements.
Even without it, scientists now have a better understanding of Y. pestis's early evolution, thanks to the work of Rasmussen and his colleagues.
The new research was published Thursday in the journal Cell.