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The oldest known plague victims in Europe date back about 5,000 years, but it was unclear whether two recent cases, one in Latvia and one in Sweden, were isolated sporadic cases or evidence of a more widespread epidemic.
A new study based on ancient DNA recovered from 108 prehistoric people excavated from nine graves in Sweden and Denmark suggests that an ancient form of plague may have been widespread among early European farmers and may explain why this population mysteriously declined over a 400-year period.
“Despite some pretty big archaeological differences, it's pretty consistent across Scandinavia, France and Sweden, and you see the same patterns, and they just fade away,” said Frederik Thiersholm, a postdoctoral researcher at the Lundbeck Foundation Geogenetics Centre, Globe Institute, University of Copenhagen in Denmark, and lead author of the study published Wednesday in the journal Nature.
This group, known as the Neolithic Farmers, migrated from the eastern Mediterranean and replaced small groups of hunter-gatherers, first bringing agriculture and settled life to northwestern Europe around 6,000 to 7,000 years ago. Their legacy lives on in the continent's many megalithic tombs and monuments, the most famous of which is Stonehenge.
Archaeologists are fiercely debating what caused the species to disappear between 5,300 and 4,900 years ago: some believe it was caused by agricultural crises due to climate change, while others suspect disease was to blame.
“Suddenly there's no one left to be buried at (these monuments), and the people who built these megaliths are (gone),” Mr Searsholm said.
Mr. Siersholm said it was unlikely that violence played a role because the next wave of newcomers, known as the Yamnaya, arrived from the Eurasian steppes after a gap in the archaeological record.
The study found that a form of the bacterium that causes the plague was present in one in six ancient samples, suggesting that transmission of the disease was not uncommon.
“These plague cases date back to exactly the same time period as the Neolithic population collapse, providing very strong circumstantial evidence that the plague may have been involved in this collapse,” he said.
Genetic information about pathogens can be stored in human DNA, allowing scientists to time travel and study ancient diseases and their evolution.
Yersinia pestis, the bacterium that causes the plague, was the most common of the six pathogens identified in the new study, being present in 18 of the 108 people sampled, or 17 percent.
But the study suggests that the true prevalence of the plague at the time may have been much higher, given that ancient DNA can only be extracted from well-preserved human remains (it's impossible to know for sure whether the people studied died from the plague, only that they were infected).
Still, the study authors said their findings don't necessarily suggest a rapid and deadly plague epidemic: The bacteria was found in remains from four out of six generations buried in some cemeteries.
“We expected that finding that the plague was only present in the last generation would be evidence that it killed them all, and that's exactly what happened,” said Searsholm, who used ancestral information contained in ancient DNA to piece together family trees from the graves.
“I also expected the plague to be exactly the same, with all of the DNA base pairs exactly the same, because that's what you'd expect if a disease spreads rapidly. But that's not what we found,” he said.
Instead, the team found evidence of three separate instances of infection and different mutants of the bacteria that causes the plague.
“So the big question is, why didn't the plague just kill everyone in the first place? This was puzzling even to us, so we started looking at genetics to see if we could find some explanation,” he said.
The team found instances of plague genetic shuffling — where DNA sequences were lost, added or moved — that could have affected the pathogen's virulence over the course of a single generation.
“This is in a genomic region that we know encodes virulence, and that's why we hypothesized that it became more virulent[over generations],” says Searsholm, “but of course this is very difficult to test, because you can't culture ancient[bacteria].”
Given that the bodies were carefully buried in graves, the genetic data from the study may have captured the beginning of a plague outbreak, said Sirsholm, and it's possible that the disease was milder than the bubonic plague that caused the Black Death, the world's most devastating epidemic, which killed half of Europe's population in seven years during the Middle Ages.
What's more, the mutants detected in the samples were missing a gene that geneticists know is essential for the bacteria to survive in the flea's digestive tract, so the resulting disease is likely not identical to the bubonic plague that is spread by rodent-carried fleas, the study said. Bubonic plague still exists today, and symptoms include painful, swollen lymph nodes (buboes) in the groin, armpits and neck area, as well as fever, chills and cough.
Mark Thomas, professor of evolutionary genetics at University College London, said the study suggests that the plague probably spread from person to person in Scandinavia at the time, rather than as a sporadic infection from animals, but it is impossible to know how deadly or chronic the disease was.
But Thomas, who was not involved in the latest study but was part of the team that first identified the Neolithic population declines, said he wasn't convinced that the plague was the main cause of the population collapses that occurred at different times in Europe, which he said were more likely the result of a combination of factors, including poor agricultural practices that exhausted the soil and widespread health problems.
“Neolithic people were very weak in terms of their overall health. Their bones are in terrible condition,” Thomas said.
“It's possible that there was an overall increase in pathogen load,” he added, but “from a DNA perspective,” Y. pestis is one of the more visible diseases to archaeologists, making it easier to identify and study.
