Plagues killed millions of Europeans and Asians starting around 1,500 years ago. But previously unknown variants of the plague-causing bacterium Yersinia pestis infected people several thousand years earlier, a new study finds.
The infectious microbes’ DNA has been found in the teeth of Bronze Age and early Iron Age people who lived between 4,800 and 3,000 years ago, say evolutionary geneticist Eske Willerslev of the University of Copenhagen and his colleagues. Y. pestis was initially passed from person to person — say, when an infected individual coughed on a healthy person — and most likely caused lung infections known as pneumonic plague or blood infections called septicemic plague, the researchers report October 22 in Cell.
“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 (SN: 7/11/15, p. 11). “Those population movements likely caused the spread of early Y. pestis strains,” Willerslev holds.
The new findings suggest that different forms of Y. pestis “survived in Eurasia for a lot longer than previously expected,” says evolutionary geneticist Hendrik Poinar of McMaster University in Hamilton, Canada. Early forms of the plague probably couldn’t cause major epidemics, he says.
Mass deaths from bubonic plague, an infection of the lymph nodes, became possible sometime after 3,700 years ago, Willerslev’s group proposes. Genetic changes in certain Y. pestis strains enabled the bacterium to spread via fleas and to elude hosts’ immune systems (SN: 8/8/15, p. 16).
Until now, researchers have been unable to extract Y. pestis DNA from bones older than 1,500 years.
The microbe is known to have caused a sixth century plague in Europe’s Byzantine Empire; the bubonic plague, or Black Death, which killed up to half of Europe’s population in the mid-1300s; and a worldwide epidemic that emerged in China in the 1850s.
After screening about 89 billion pieces of DNA from teeth of 101 Bronze Age and Iron Age individuals, Willerslev’s team found Y. pestis DNA in seven of them. Two came from Siberia’s roughly 4,800-year-old Afanasievo culture. One approximately 4,500-year-old individual belonged to Estonia’s Corded Ware culture. An infected person from West Asia’s Sintashta culture lived almost 4,200 years ago. A skeleton from the Unetice culture in Poland dates to more than 4,000 years ago, and one from Siberia’s Andronovo culture is about 3,700 years old. An infected individual from the early Iron Age in Armenia lived close to 3,000 years ago.Based on comparisons of modern and ancient Y. pestis DNA, the researchers calculate that the most recent common ancestor of all known strains of the bacteria existed between 5,021 and 7,022 years ago. A previous estimate ranged from 1,505 to 6,409 years ago. Early Y. pestis strains appear to have evolved and died out too quickly to calculate the age of an ancestral strain with any precision, Poinar says.
Y. pestis genomes from the Bronze Age lacked a gene that enabled later forms of the bacterium to survive inside the flea gut. This gene was present in the Iron Age individual, indicating that the plague’s ability to spread via flea bites evolved between 3,700 and 3,000 years ago.
Another gene variant found in previously known Y. pestis strains prevents production of a protein that triggers attacks from hosts’ immune systems. Plague DNA in one of the oldest Bronze Age individuals and in the most recent Bronze Age person lacked this gene variant, suggesting that these forms of Y. pestis were vulnerable to people’s immune defenses.
An ability to evade hosts’ immune systems evolved at different rates in various Y. pestis strains during the Bronze Age, the researchers suspect.