Did Plague Cause the Mysterious Collapse of Europe’s Early Farmers 5,000 Years Ago?

Some 7,000 years ago, a group of early farmers from the eastern Mediterranean swept into western Europe and Scandinavia. They displaced hunter-gatherers and instituted sophisticated agricultural practices—raising cattle and pigs and using the manure to help grow wheat and barley. Today, their monumental stone tombs and structures, such as Stonehenge in England, still dot the landscape. But the humans, after thriving for a few thousand years, seemingly disappeared.

“Call it a mystery,” Karl-Göran Sjögren, an archaeologist at the University of Gothenburg tells Science’s Andrew Curry. “We have no real explanation for it.”

Now, in a new study published last week in Nature, Sjögren and other researchers suggest these people could have vanished because of the plague. Archaeologists analyzed more than one hundred skeletons buried in Scandinavian Neolithic monumental tombs and found an ancestral strain of the Black Death in many of the remains—about one in six people appeared to be infected. They propose this could explain the sudden decline of Europe’s population around 5,000 years ago.

“These plague cases, they are dated to exactly the time frame where we know the Neolithic decline happened, so this is very strong circumstantial evidence that the plague might have been involved in this population collapse,” study lead author Frederik Seersholm, a geogeneticist at the University of Copenhagen, tells CNN’s Katie Hunt.

The Dark and Deadly History of the Plague

Previously, scientists had found two cases of the plague from that time—one in Latvia and one in Sweden—which suggested the deadly disease was present in Europe during the population decline. But it wasn’t clear if these were just isolated incidents or indications of a widespread outbreak.

To solve the mystery, the researchers in the new study decided to look for evidence of the plague in more remains. They analyzed DNA in the teeth of 108 people buried in stone tombs in Sweden and Denmark between 4,900 and 5,300 years ago. DNA from the plague pathogen would only show up in teeth if the person was actively sick when they died.

The team found three separate strains of plague DNA from the bacteria Yersinia pestis in 18 of the samples. Two of the strains only appeared in a few teeth from people buried in the older tombs. But the third strain had genetic variations that could make it more deadly, and it was found in many more skeletons. It also seems to occur at the very end of the time when these tombs were active, around 4,900 years ago, and coincides with the collapse of the agricultural people then spread across the area.

“It’s present in a lot of individuals,” Seersholm tells the New Scientist’s Michael Le Page. “And it’s all the same version, which is exactly what you would expect if something spreads very quickly.”

The researchers also say the fact that these people were properly buried in tombs suggests their society was still functioning at the time they died. But since the burials ended shortly after those plague victims died, it might be the case that the remains capture only the start of a devastating epidemic.

“Maybe we’re only seeing the very beginning of the outbreak, and then society collapses and they’re not burying anyone,” Seersholm tells Science.

Still, the findings don’t necessarily prove the plague caused the agricultural society’s downfall. The strain was found in people from several generations, suggesting it did not immediately wipe out society. But the researchers say the plague did have the potential to spread among humans and kill entire families, bolstering the case that it contributed in large part to the Neolithic decline.

Other researchers are more cautious. Mark Thomas, an evolutionary geneticist at University College London who first identified the Neolithic population collapse but was not involved in the new study, tells CNN that the decline was likely due to a larger combination of factors, including exhaustion of the soil. Additionally, poor health was widespread in that era. “Neolithic people were very compromised in terms of general health. Their bones look bad,” he tells the publication.

Seersholm says to New Scientist that while researchers will keep looking for new evidence of the plague in remains across Europe, the only way to know for certain how deadly this strain was would be to bring it back to life. But, he adds, that seems like a bad idea.