At the end of the Stone Age, some 5,300 years ago, the populations of Scandinavia and northwest Europe plummeted, and farming communities evaporated. “People stopped building megaliths, like Stonehenge,” said Frederik Seersholm, a geneticist at the University of Copenhagen. “Settlements were abandoned. Everyone vanished.”
The so-called Neolithic decline, which lasted several centuries, is believed to have allowed a nomadic herding culture known as the Yamnaya to migrate west, altering the genetic makeup of early Europeans. The cause of this demographic collapse has been an open question, with the suspects including wars and agricultural crises.
A new genomic study published in July in the journal Nature makes the case for another candidate, which had been found in people living at the time but was never thought to have been widespread: the plague.
Until now, it was unclear how virulent the Neolithic plague was within a human population. “There is a hypothesis that the oldest plague bacterium lacked epidemic potential,” said Dr. Seersholm, the lead author of the paper. “That hypothesis no longer holds.”
The researchers propose that a Stone Age pandemic originated in small farming villages and spread to mega-settlements and far-off lands along with traders who traveled by horse-drawn cart. “We can’t prove that this was exactly how it happened, not yet, anyway,” Dr. Seersholm said. “Still, it’s significant that we can show it could have happened.”
‘Bring out your dead’
Plague is caused by the bacterium Yersinia pestis. The bubonic and septicemic forms — mainly infecting the lymph nodes and blood — are typically transmitted through fleas and rats. The more deadly pneumonic form, which affects the lungs, travels on airborne droplets and is contagious in people and animals.
In her book “Plague,” Wendy Orent, an anthropologist in Atlanta, speculates that the first account of bubonic plague was the Old Testament story of the Philistines who stole the ark of the covenant from the Israelites and were afflicted with “swellings.” Since then, Yersinia pestis has been the driver of three documented scourges that erupted on the world stage.
The first recorded reports of the plague were in the Egyptian port town of Pelusium during the summer of 541 A.D. For the next 200 years, the Plague of Justinian — named for the Byzantine emperor who caught the disease but survived — deepened the social and economic collapse of western Europe and claimed the lives of 30 million to 50 million people across three continents.
The next major outbreak of the plague has been pinned on a variant of Yersinia pestis that most likely jumped from wild marmots to humans in what is now northern Kyrgyzstan. The first recorded emergence occurred in the 14th century during the siege of Kaffa, a Genoese trading port in Crimea, where the Mongols reportedly catapulted disease-ridden corpses over the walls.
From 1346-53, the Black Death — a grim reference to the gangrenous blackening and death of tissue, mostly on the extremities — wiped out perhaps half of Europe’s population. It proved to be the initial wave of a nearly 500-year contagion that included the Great Plague of London in 1665-66, as a witness observed, “The warning cry ‘bring out your dead’ and the rumble of the ‘dead-carts’ disturbed the stillness of the night.”
A descendant of that strain is blamed for the third pandemic, known as the Modern Plague, which originated in 1855 in China and traversed the globe over the next several decades, resulting in the deaths of about 12 million people in India alone. Although it still smolders in small pockets worldwide — the United States about seven cases a year, on average — the Modern Plaque is now treatable with antibiotics if caught early.
Generations of pestilence
In 2001, biologists in England successfully decoded the full DNA sequence of Yersinia pestis by mapping a mutation that had killed a Colorado veterinarian after an infected cat sneezed on him. A decade later, scientists reported the discovery of the microbes in human teeth from Eurasia that dated back 5,000 years.
Three years ago, researchers announced that plague genomes had been recovered from the Stone Age skull of a man who lived in what is now Latvia. And in 2023, the earliest known evidence of the plague in Britain was unearthed in the dental pulp of three 4,000-year-old skeletons. “The ability to detect ancient pathogens from degraded samples from thousands of years ago is incredible,” said Pooja Swali, the geneticist at University College London who identified the bacterium.
The authors of the new paper sequenced the genomes of the skeletal remains from 108 people at nine burial sites over a wide geographic area in Sweden and Denmark. All of the bodies were interred between 3300 B.C. and 2900 B.C.
Of the six pathogens observed in the research, Yersinia pestis was the most prevalent, present in about 17 percent of the bodies. “In other words, one in six people had active infections of plague at the time of death,” Dr. Seersholm said. “But that ratio probably underestimates the true frequency of the bacterium.”
The DNA of a family excavated from one tomb was mapped for six generations, spanning about 120 years. Twelve of the 38 family members were infected with Yersinia pestis, which Dr. Seersholm said was almost certainly pneumonic because it was missing some of the genes that would enable the bubonic form to spread from fleas to rodents to humans. “So no rats were required,” he said.
Dr. Seersholm’s team concluded that the plague arrived in three distinct waves, evolving with each new generation. The first two waves appear to have been relatively mild, but the third seems more potent and may have been culpable in the Neolithic decline.
Dr. Swali lauded the scholarship of the project but remained unconvinced that the plague led to mass death rather than isolated infections. “Yersinia pestis may have played a role in the decline or dealt some final blows,” she said. “But there were many other factors that could have contributed to small and already declining population sizes, such as famine or other diseases that may be trickier to detect.”
Kyle Harper, a historian at the University of Oklahoma and author of “Plagues Upon the Earth,” said that not long ago, before archaeologists and geneticists began collaborating, the plague was not credibly implicated in the Neolithic decline. Witness accounts abound for the Justinian Plague and the Black Death, but the prehistoric strain remains an enigma. “How did it spread?” Dr. Harper said. “How did it affect people? These are still huge unknowns.”
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