DNA Study Suggests Plague Caused Neolithic Population Crash in 3,000 BC

The notorious plague outbreak known as the Black Death was one of the most epic mass casualty events in human history, as it may have killed off up to one-third of the European population in the 14th century. But if a new study just released by a team of researchers from Denmark and Sweden is correct, this was not the first time that the black plague rampaged through the European countryside to cause a catastrophic loss of life.

Based on an analysis of recovered DNA samples from skeletons that were buried approximately 5,000 years ago in Scandinavia, genetic experts from the University of Copenhagen and the University of Gothenburg in Sweden determined that the plague was common in the region at that time. As it so happens there was a Neolithic population crash in northwestern Europe between the years 5,300 and 4,900, and the researchers involved in the new study believe they may have found the explanation.

While other scientists have suggested that disease might have played a role in that rapid and prolonged Neolithic period population decline, this is the first time real evidence has been provided to support this theory.

Excavation of the Frälsegården passage grave in 2001. The bones were labeled and measured individually with a total station. (Karl-Göran Sjögren/Nature)

An Ancient Pestilence Strikes

For the purposes of this new study, the genetic researchers from Denmark and Sweden collected DNA data from 108 individuals who lived around 3,000 BC on Scandinavian territory. The majority of these people were buried in megalithic graves found at different locations in Sweden, but some were recovered from stone cist tomb excavated near Stevns, Denmark.

In their analysis of this genetic material, which was extracted from bones and teeth, the scientists were able to find information about the people’s ancestry, their social structure, and their exposure to certain types of pathogens, including the one associated with the plague.

The bacterium known as Yersinia pestis is the agent responsible for causing the plague in human beings, and it can leave behind long-lasting traces in the people it infects. In this new study the remnants of Y. pestis were discovered in the DNA samples of multiple individuals, demonstrating that outbreaks of the plague had indeed occurred in ancient Scandinavia.

“We find that the Neolithic plaque was widespread, detected in at least 17% of the population and across large geographical distances,” the study authors wrote in a paper appearing in the journal Nature.

“We demonstrate that the disease spread within the Neolithic community in three distinct infection events within a period of around 120 years.”

Plague spread in Neolithic Scandinavia. Each individual in the study is represented by colored shapes. Only Neolithic individuals are show. Squares represent males and circles females; triangles represent a person of unknown sex. Colors indicate genetic ancestry, and black crosses designate individuals with the plague. (Seersholm et al./Nature)

The fact that the plague was not confined to a particular place and time was especially significant, since it shows it was recurrent, traveled freely across the landscape, and was implicated in the deaths of a large number of people covering six generations of Neolithic farmers.

In reality, the researchers wrote, that 17% infection rate figure is likely too low. The skeletal remains that produced the DNA samples used in their study were unusually well-preserved in large, elaborate graves, and it is possible that most of those individuals had actually survived their encounters with the plague. But the ancient plague victims who died from the disease likely would have been buried rapidly and without much care, meaning their bones would have decayed away long ago. If these skeletal remains had been available for analysis, the results would have almost certainly increased estimates of the ancient infection rates of this deadly disease .

The skulls of the two second generation individuals in the right branch, FRA022 and FRA023, placed beside a limestone slab covering the ancestral individual, FRA021. (Karl-Göran Sjögren/Nature)

From Ancient to Medieval Times, Europe was Plagued by the Black Death

The conclusion that the European population crash of 5,000 years ago was linked to plague outbreaks is based strictly on indirect evidence, and there could have been other factors involved in the population crash as well, such as massive crop failures due to droughts or long winters caused by climate change.

“We cannot—yet—prove that this was exactly how it happened,” acknowledged Frederik Seersholm, a genetic researcher from the University of Copenhagen, in a press release issued by that institution. “But the fact that we can now show that it could have happened this way is significant.”

It is significant, Seersholm explained, because there has been no evidence of any type supporting any of the theories about what might have caused the mysterious population crash in the region around 3,000 BC. This development was quite surprising, given the way human populations had been expanding as a result of the Neolithic Revolution, when agriculture replaced the hunting-gathering lifestyle as the primary form of subsistence in Europe and elsewhere.

“In connection with the population decline in the end of the Neolithic, both war and outbreaks of infectious diseases, including plague, have been suggested,” Seersholm stated. “There have been several theories involving the plague, and one of them suggested that the plague could not have caused an epidemic—but that assumption no longer holds.”

This enlightening new discovery can be credited to advancements in DNA analysis that allow genetic researchers to extract detailed data from ancient genetic specimens that are heavily degraded. In addition to identifying the presence of Y. pestis remnants in people who died 5,000 years ago, the researchers involved in this study have also been able to establish family connections between the individuals who buried together in the various ancient tombs. It seems these tombs were reused by specific families over multiple generations, all of whom were apparently exposed to outbreaks of the Neolithic version of the Black Death.

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Top image: One of the complete skeletons found in the Frälsegården passage grave.               Source: Karl-Göran Sjögren/Nature

By Nathan Falde