Ancient DNA reveals possible cause of mysterious population collapse 5,000 years ago

The oldest known victims of plague date back to about 5,000 years ago in Europe. But it was never clear whether two cases, one in Latvia and one in Sweden, were isolated and sporadic or evidence of a wider outbreak.

A new study, based on Ancient DNA recovered from 108 prehistoric individuals unearthed at nine burial sites in Sweden and Denmark, suggests that an ancient form of plague may have been widespread among early European farmers and could explain why that population mysteriously collapsed over 400 years.

“It’s quite consistent across Northern Europe, France and it’s in Sweden, although there are some quite large differences in the archaeology, we still see the same pattern, they just disappear,” said Frederik Seersholm, a postdoctoral researcher at the Lundbeck Foundation GeoGenetics Centre, Globe Institute, University of Copenhagen in Denmark and lead author of the study. published in the journal Nature on Wednesday (10).

This group, known as Neolithic farmers, migrated from the eastern Mediterranean, replacing small groups of hunter-gatherers and bringing agriculture and a sedentary lifestyle to northwestern Europe for the first time around 6,000 to 7,000 years ago. Their legacy lives on in the continent’s many tombs and megalithic monuments, the most famous of which is Stonehenge .

Archaeologists have been intensely debating the cause of the disappearance of this population between 5,300 and 4,900 years ago. Some attribute their disappearance to an agricultural crisis caused by climate change, while others suspect disease.

“Suddenly, there are no more people being buried (in these monuments). And the people responsible for building these megaliths (have disappeared),” Seersholm said.

Violence is unlikely to have played a role, Seersholm said, with the next wave of newcomers, known as the Yamnaya, arriving from the Eurasian steppes after a gap in the archaeological record.

The study found that forms of the bacteria that causes plague were present in 1 in 6 ancient samples, suggesting that infection with the disease was not rare.

“These cases of plague, they are dated exactly to the time period in which we know the Neolithic decline happened, so this is very strong circumstantial evidence that plague may have been involved in this population collapse,” he said.

Genetic time travel

Genetic information about pathogens can be preserved in human DNA, allowing scientists to travel back in time to discover ancient diseases and how they evolved.

Yersinia pestisthe 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 sampled.

However, according to the study, the true prevalence of the plague at that time could have been much higher, 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.

However, the study authors said their findings do not necessarily suggest a rapid and deadly plague epidemic. 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 evidence that the plague was killing everyone, and that’s what it was,” said Seersholm, who assembled family trees from the graves using ancestry information contained in ancient DNA.

“I also expected the plague to be exactly the same, like every DNA base pair would be exactly the same, because that’s what you would expect if you saw a rapid outbreak of disease, but that’s not what we found,” he said.

Instead, the team found evidence of three distinct infection events, as well as different variants of the bacteria that causes plague.

“The big question then is, why didn’t the plague kill everyone right at the beginning? And that puzzled us too, so we started looking at genes to see if we could find some kind of explanation,” he said.

The team found cases where plague genes had been rearranged — lost, added or moved in DNA sequences — which could have 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]our hypothesis is that it became more virulent[over generations],” Seersholm said. “But of course, that’s very, very difficult to test, because you can’t just grow an ancient bacterium.”

Transmission of prehistoric plague

Given that the remains were carefully buried in a tomb, Seersholm said it’s possible that the genetic data examined in the study captures the beginning of a plague epidemic. 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 in seven years during the Middle Ages.

Furthermore, because the variants detected in the samples lacked a gene that geneticists know is crucial for the bacteria’s survival in a flea’s digestive tract, the resulting disease was likely not identical to bubonic plague, which was spread by fleas carried by rodents, according to the study. Bubonic plague still exists today, and symptoms include painfully swollen lymph nodes, called buboes, in the groin, armpit or neck, as well as fever, chills and cough.

The study suggests that in Scandinavia at that time, plague was likely being transmitted from human to human rather than sporadically from animals, although it is not possible to know how lethal or chronic the disease was, said Mark Thomas, a 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 is less convinced that plague was the main reason behind 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 very compromised in terms of their overall health. Their bones look bad,” Thomas said.

“There may have been a more general increase in pathogen load,” he added. However, “from a DNA perspective,” Yersinia pestis It is one of the most visible diseases to archaeological scientists and therefore easiest to identify and study.