The first farmers of the Old World were killed by the plague
The oldest known victims of the plague discovered in Europe date back to about 5,000 years ago. But it was never clear whether the two cases, one in Latvia and one in Sweden, were isolated and sporadic or indicative of a wider outbreak.
A new study, based on analysis of ancient DNA from 108 prehistoric individuals found in nine burial sites in Sweden and Denmark, suggests that an ancient form of plague may have been widespread among Europe's early farmers and may explain why this population is mysterious has shrunk over the past 400 years, CNN reports.
“It’s pretty consistent across northern Europe, in France and Sweden, despite some pretty big differences in the archaeology, we still see the same pattern, they just disappear,” says Frederik Sirsholm, a research fellow at the Lundbeck Foundation Geogenetics Centre, the Globe Institute, the University of Copenhagen in Denmark and lead author of the study published in the journal Nature on Wednesday.
This group, known as the Neolithic farmers, migrated from the eastern Mediterranean, displacing small groups of hunter-gatherers and first introducing agriculture and sedentism to northwestern Europe around 6,000 to 7,000 years ago. Their legacy is preserved across the continent in a variety of megalithic burial sites and monuments, the most famous of which is Stonehenge.
Archaeologists are actively discussing the reason for the disappearance of this people between 5300 and 4900 years ago. Some attribute their deaths to an agricultural crisis caused by climate change, while others suspect disease.
“Suddenly people were no longer buried at these monuments. And the people who were responsible for building these megaliths are gone,” Searsholm said.
Violence is unlikely to have played a role, Searsholm said, as the next wave of newcomers, known as the Yamnaya, arrived from the Eurasian steppes after break in archaeological excavations.
The study found that forms of the bacteria that cause plague were present in 1 in 6 ancient samples, suggesting that infection with the disease was not uncommon.
«These plague cases are right around the time when we know the Neolithic collapse occurred, so it's very strong circumstantial evidence that plague may have been involved in this demographic collapse,» he said.
Genetic information about pathogens can be stored in a person's DNA, allowing scientists to travel back in time to learn about ancient diseases and how they evolved.
The bacterium Yersinia pestis, which causes the plague, was the most common of the six pathogens identified in the new study and was present in 18 people, or 17% of the 108 samples collected.
However, according to the study, the true prevalence of the plague at that time could be much higher, given that ancient DNA could only be extracted from well-preserved human remains. (It is also impossible to say with certainty whether the people studied died from the plague—we only know that they were infected.)
However, the study authors said their results do not necessarily indicate a rapid and deadly plague epidemic . The bacterium was found in the remains of representatives of four of the six generations buried in some burial sites.
“I expected to find that the plague was only prevalent in the last generation, which would indicate that the plague was killing them all and that was it,” said Searsholm, who compiled family trees from the graves using ancestor information contained in ancient DNA.
“I also expected the plague to be exactly the same as every single base pair of DNA, because that's what you would expect if you saw a rapid outbreak of disease, but we found very different,” he said.
Instead, the team found evidence of three different infections, as well as different variants of the bacterium that causes the plague.
“The big question then is how come the plague didn’t kill everyone at once? And that puzzled us, too, so we started looking at the genes to see if we could find some explanation,” he said.
The team found cases where plague genes were swapped — lost, added, or moved in DNA sequences, which could possibly affect the virulence of the pathogen during the life of one generation.
“This is a region of the genome where we know virulence is encoded, and (that's) the reason our hypothesis is that it has been more virulent (over generations),” Searsholm said. “But of course it's very, very difficult to test because you can't actually just grow ancient bacteria.”
Given that the remains were carefully buried in the grave, Searsholm said it was possible that the genetic data examined in the study captured the very beginning of the plague. It's also likely that the disease was less severe than the bubonic plague that caused the Black Death, the world's most devastating plague outbreak that is estimated to have killed half of Europe's population over seven years during the Middle Ages, CNN says.
Moreover, because the variants found in the samples lacked a gene that geneticists know is critical for the survival of the bacterium in the digestive tract of fleas, the resulting disease was unlikely to be identical to bubonic plague, which was spread by fleas carried by rodents, it said in the study. Bubonic plague still exists today, and symptoms include painful, enlarged lymph nodes, called buboes, in the groin, armpit, or neck, as well as fever, chills, and cough.
The study suggests that in Scandinavia at the time, the plague was likely transmitted from person to person rather than sporadically from animals, although it is impossible to know exactly how fatal or chronic the disease was, said Mark Thomas, professor of evolutionary genetics at University College London.
However, Thomas, who was not involved in the latest study but was part of the team that first identified the Neolithic decline, said he was less convinced that plague was the main cause of the massive population decline that he said occurred at different times in Europe and was likely the result of a combination of factors including poor agricultural practices that depleted the soil and widespread health problems.
“Neolithic people were very weak in terms of general health. Their bones look bad,” Thomas said.
“There may have been a more general increase in pathogens,” he added. However, “from a DNA perspective,” Yersinia pestis is one of the diseases that is more visible to archaeological scientists and therefore easier to identify and study.
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