Starting around 1,500 years ago, bouts of plague swept through Europe and Asia, killing millions of people. But previously unknown variants of the plague germ had begun infecting people in both places much, much earlier. That’s the conclusion of a new study.
Eske Willerslev works at the University of Copenhagen, in Denmark. As an evolutionary geneticist, he studies how the genes in DNA change over very long periods of time. He and his colleagues now report finding DNA from the bacteria that cause plague (PLAYG) in teeth from people who lived during the Bronze Age and early Iron Age. That was between 4,800 and 3,000 years ago!
The plague germ is called Yersinia pestis. Initially, this bacterium would have spread from person to person. This might have happened when a sick person coughed, for instance. If the germ triggered a lung infection, the disease would be called pneumonic (Nu-MON-ik) plague. But if the blood became infected, the symptoms would change and the illness would be called septicemic (Sep-tih-SEEM-ik) plague.
“It’s surprising that the plague was widespread 3,000 years before written records of plagues and well before large-scale urbanization,” Willerslev says. Evidence suggests that Bronze Age herders migrated across Europe and Asia. Those travelers “likely caused the spread of early Y. pestis strains,” he now suspects.
Willerslev’s group described its findings October 22 in Cell.
The new data suggest that different forms of the plague germ “survived in Eurasia for a lot longer than previously expected,” says Hendrik Poinar. An evolutionary geneticist, he works at McMaster University in Hamilton, Canada. Early forms of the germ, however, likely did not set off major epidemics, he notes.
But it certainly did in later times. There was one epidemic in the sixth century A.D. This was during Europe’s Byzantine Empire. But best known is the infamous Black Plague. It wiped out up to half of everyone living in Europe during the mid-1300s. Some 500 years later, a worldwide plague pandemic appeared to get its start in China.
Such mass deaths traced to a form of the disease where a victim’s lymph nodes become infected. Called bubonic plague, this form of the disease likely became possible only sometime within the past 3,700 years, Willerslev’s group now proposes. At that time, a single genetic change, or mutation, suddenly allowed the germ to infect lymph nodes and blood. Once that happened, all it would take for the disease to spread was the bite of an infected flea.
