Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news about fascinating discoveries, scientific breakthroughs and more.
CNN
—
The oldest known plague victims date back about 5,000 years in Europe. But it has never been determined whether the two cases, one in Latvia and one in Sweden, were isolated and sporadic or part of a larger epidemic.
A new study, based on ancient DNA recovered from 108 prehistoric individuals exhumed from nine burial sites in Sweden and Denmark, suggests that an ancient form of the plague may have been widespread among early European farmers and could explain why that population mysteriously collapsed within 400 years.
“It’s pretty consistent across northern Europe, France and Sweden, even though there are big differences in the archaeology, we still see the same pattern, they just disappear,” said Frederik Seersholm, a postdoctoral researcher at the Lundbeck Foundation Center for Geogenetics, Globe Institute, University of Copenhagen in Denmark and lead author of the study published in the journal Nature on Wednesday.
This group, known as Neolithic farmers, migrated from the eastern Mediterranean, replacing small groups of hunter-gatherers and bringing agriculture and a settled lifestyle to northwestern Europe for the first time about 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 hotly debating the causes of the population’s disappearance, 5,300 to 4,900 years ago. Some attribute the disappearance to an agricultural crisis caused by climate change, while others suspect disease.
“All of a sudden, no one is getting buried (on these monuments). And the people who built these megaliths (are gone),” Seersholm said.
Violence is unlikely to have played a role, Seersholm said, with the next wave of new arrivals, known as the Yamnaya, arriving from the Eurasian steppe after a gap in the archaeological record.
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 dated exactly to the period when we know the Neolithic decline occurred, which is very strong circumstantial evidence that plague may have been involved in this population collapse,” he said.
Genetic information about pathogens can be preserved in human DNA, allowing scientists to travel back in time to discover ancient diseases and their evolution.
Yersinia pestis, the bacteria that causes plague, was the most prevalent of the six pathogens identified in the new research, present in 18 individuals, or 17% of the 108 samples.
However, the actual prevalence of the plague at that time could have been much higher, according to the study, given that ancient DNA can only be extracted from well-preserved human remains. (It is also not possible to know for sure whether the people studied died of the plague, only that they were infected.)
Karl-Göran Sjögren
Archaeologists excavate a grave in Frälsegården, Sweden, in 2001. DNA extracted from some of the bones revealed the presence of the bacteria that causes the plague.
The study authors cautioned, however, that their findings did not necessarily suggest a rapid and deadly plague outbreak. The bacteria was detected in the remains of four out of six generations buried at some of the burial sites.
“I expected to find that the plague was only present in the last generation, which would be proof that the plague kills them all, and that’s it,” said Seersholm, who reconstructed the graves’ family trees using ancestral information contained in ancient DNA.
“I also expected the plague to be exactly the same, like every base pair of DNA would be exactly the same, because that’s what you would expect if you had a rapid outbreak of disease, but that’s not what we found,” he said.
Instead, the team found evidence of three separate infection events, as well as different variants of the bacteria that causes plague.
“So the big question is why didn’t the plague kill everyone early on? That also puzzled us, so we started looking at genes to see if we could find an explanation,” he said.
The team found instances where plague genes had been shuffled – lost, added or moved around in DNA sequences – which could have possibly affected the pathogen’s virulence within a generation.
“It’s in an area of the genome where we know virulence is encoded, and that’s why we hypothesized that it was more virulent (over generations),” Seersholm said. “But of course, that’s very, very difficult to test because you can’t really grow an ancient bacterium.”
Given that the remains were carefully buried in a grave, Seersholm said it’s possible that the genetic data examined in the study captured the very beginning of a plague outbreak. 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, which is estimated to have killed half of Europe’s population within seven years in the Middle Ages.
Additionally, because the variants detected in the samples lacked a gene that geneticists know is essential for the bacteria to survive in the flea digestive tract, the resulting disease was likely not identical to bubonic plague, which is transmitted by fleas carried by rodents, the study said. Bubonic plague still exists today, and symptoms include painful, swollen lymph nodes, called buboes, in the groin, armpit or neck, as well as fever, chills and cough.
The study suggests that at that time in Scandinavia, plague was likely transmitted from human to human rather than through sporadic transmission from animals, although it is not possible to know 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 research but was part of the team that first identified the Neolithic decline, said he was less convinced that the plague was the main reason for the wider population collapse, which he said occurred at different times across Europe and was likely the result of a combination of factors including poor farming practices that depleted the soil and widespread ill health.
“Neolithic people were in very poor general health. Their bones are in poor condition,” Thomas said.
“It could be that there was a more general increase in the pathogen load,” he added. However, “from a DNA perspective,” Yersinia pestis happens to be one of the most visible diseases to archaeologists and therefore the easiest to identify and study.