Ancient DNA Reveals Plague Was Already Killing Humans 5,500 Years Ago

Say the word plague, and most people picture the same scene. Filthy medieval cities. Rats scurrying through crowded streets. Corpses piled up faster than anyone could bury them. New DNA evidence just pushed that picture back thousands of years, to a setting that couldn’t look more different. Small, mobile hunter-gatherer families in the Siberian wilderness, long before cities existed at all, were already dying from the same disease.

Teeth That Held Onto a 5,500-Year-Old Secret

The evidence comes from four ancient cemeteries along the Angara River in southeast Siberia. That river flows out of Lake Baikal, the deepest freshwater lake on Earth, and has drawn archaeologists studying hunter-gatherer life in the region for decades. The burial sites themselves belong to a much larger, long-running research effort called the Baikal Archaeology Project, which has spent years documenting mortuary sites in the region dating back roughly 8,500 years. An international team of researchers extracted ancient DNA preserved inside the teeth of 46 individuals buried at these sites. They used it to reconstruct bacterial genomes nobody had ever seen before. Their findings appeared in the journal Nature in June 2026.

What they found inside those genomes was Yersinia pestis, the bacterium responsible for plague. Eighteen of the 46 people tested, roughly 39 percent, carried evidence of infection. The outbreaks weren’t a single event either. Researchers identified two distinct phases. The first struck the sites known as Shumilikha and Ust’-Ida I. The second hit two more cemeteries, Bratskii Kamen and Serovo, a few centuries later.

A Decades-Old Mystery Finally Explained

Archaeologists had actually noticed something strange about these burial sites long before anyone thought to test for ancient disease. Between 65 and 75 percent of the people buried there died before age 15. That’s a genuinely unusual pattern compared to other hunter-gatherer cemeteries from the same region and era. Archaeologist Andrzej Weber of the University of Alberta had been trying to explain that pattern since the 1990s. Finding plague as the actual cause, he said, felt extraordinary. It also made complete sense once the evidence was finally in front of him.

Reconstructing family relationships between the buried individuals made the picture even clearer. In one grave, three young girls between the ages of four and nine, likely close maternal relatives, all carried plague DNA together. At another site, researchers found the same infection in an aunt and her nephew, buried side by side. The aunt’s teenage niece, buried elsewhere in the same cemetery, tested positive too. This wasn’t a scattering of unrelated, isolated cases. It moved through families.

A Killer That Didn’t Need Rats or Fleas Yet

Here’s the detail that genuinely surprised researchers. The version of plague responsible for later, more famous outbreaks, including the Black Death, spread primarily through fleas living on rats. That single epidemic killed an estimated 25 million people across the 14th century. But that specific flea-borne transmission mechanism didn’t fully evolve until roughly 3,800 years ago. The strains found in these Siberian graves predate that development by well over a thousand years. That means they had to spread a different way, most likely person to person, directly within families and small communities.

Scientists had genuinely debated this for years. Were these earlier, more primitive strains of plague even capable of causing serious outbreaks? Or did they stay relatively mild until the flea-borne version eventually took over? Eske Willerslev, a senior author on the study and professor at the University of Copenhagen and the University of Cambridge, said the new findings settle that question. These ancient strains, he confirmed, were already highly lethal.

How Fast, and How Young

The pattern of death at these sites points to something acute rather than a slow, lingering illness. Researchers found the infections caused sudden, severe mortality concentrated especially among children between roughly eight and eleven years old. That’s a strikingly narrow age band, the kind of pattern epidemiologists associate with a fast-moving, aggressive pathogen rather than a disease that lingers and picks off the already vulnerable over time. At one of the four sites, researchers also detected a second, entirely separate pathogen: the bacterium responsible for brucellosis, in one individual. It’s a reminder that this small hunter-gatherer population was navigating more than one kind of infectious threat at once, long before anyone had a word for either disease.

Rewriting When Epidemics Actually Became Possible

Researchers had previously identified isolated individuals carrying early plague strains elsewhere. One case in Sweden dated back roughly 4,900 years. Another in Latvia was around 5,000 years old. Those were single infected people, though, not evidence of a disease actually sweeping through an entire community. The Siberian findings are different in a meaningful way. They show plague moving through whole family networks, killing children in disproportionate numbers, well before farming or dense urban living ever entered the picture.

That distinction matters for how historians and epidemiologists think about the deep past. The common assumption has long been that truly devastating infectious outbreaks required the crowded, unsanitary conditions that came later with agriculture and city life. This evidence suggests otherwise. Small, mobile, tightly connected family groups, exactly the kind of society humans lived in for most of our existence, were already vulnerable to exactly this kind of lethal, fast-moving disease.