Ancient Plague and Population Decline

Ancient Plague and Population Decline

     The oldest known plague victims date back to around 5,000 years ago in Europe. Until now, it wasn't clear if the cases in Latvia and Sweden were isolated or part of a larger outbreak. A new study, using ancient DNA from 108 prehistoric individuals found in nine grave sites across Sweden and Denmark, suggests that an ancient form of the plague was widespread among Europe’s first farmers and could be linked to the population's mysterious decline over 400 years.

     "It’s fairly consistent across all of Northern Europe and France, and in Sweden, despite archaeological differences, we see the same pattern: they just disappear," said Frederik Seersholm, a postdoctoral researcher at the Lundbeck Foundation GeoGenetics Centre, Globe Institute, University of Copenhagen, Denmark. Seersholm is the lead author of the study published in the journal Nature.

     Neolithic farmers migrated from the eastern Mediterranean, replacing small bands of hunter-gatherers and introducing agriculture to northwestern Europe around 6,000 to 7,000 years ago. Their legacy includes many megalithic graves and monuments, such as Stonehenge. Archaeologists have debated the cause of their disappearance between 5,300 and 4,900 years ago, suggesting climate change-induced agricultural crises or disease.

     "Suddenly, there’s no more burial at these monuments, and the people responsible for building them are gone," Seersholm said. Violence was unlikely, as the next wave of newcomers, known as the Yamnaya, arrived from the Eurasian steppe after a gap in the archaeological record. The study found that plague-causing bacteria were present in 1 in 6 ancient samples, suggesting the infection was not rare.

     "These plague cases date to the same time frame as the Neolithic decline, indicating strong circumstantial evidence that the plague might have been involved in this population collapse," Seersholm said.

     Yersinia pestis, the bacterium causing plague, was the most prevalent pathogen identified, present in 18 individuals, or 17% of the samples. The true prevalence could have been higher since ancient DNA extraction requires well-preserved remains. The bacterium was detected in remains from multiple generations at some sites, indicating three distinct infection events and different variants of the plague-causing bacterium.

     "We expected to find plague only in the last generation, indicating a swift outbreak, but that wasn't the case," Seersholm said. The team observed that plague genes had reshuffled over generations, possibly affecting the pathogen's virulence.

     The study suggests that plague was likely spread human-to-human rather than from animals, unlike the later bubonic plague. The ancient plague lacked a crucial gene for survival in fleas, suggesting it wasn't identical to the bubonic plague, which caused the Black Death. The findings imply that the ancient plague was less severe and spread differently.

     Mark Thomas, a professor of evolutionary genetics at University College London, said while the study's evidence supports the plague's involvement, he believes the Neolithic decline resulted from multiple factors, including poor farming practices and general ill health. Neolithic people's compromised health made them vulnerable to various pathogens, with Yersinia pestis being one of the more identifiable ones due to DNA visibility.

     Overall, this study sheds light on the complex factors behind the Neolithic population collapse, highlighting the ancient plague's potential role.