It turns out the plague was plaguing humans far earlier than previously believed. New research using ancient DNA has revealed that plague has been endemic in human populations for more than twice as long as previously thought and that the ancestral plague would have been predominantly spread by human-to-human contact, until genetic mutations allowed Yersinia pestis (Y. pestis), the bacteria that causes plague, to survive in the gut of fleas.
These mutations, which may have occurred near the turn of the 1st millennium BC, gave rise to the bubonic form of plague that spreads at terrifying speed through flea, and consequently rat, carriers. The bubonic plague caused the pandemics that decimated global populations, including the Black Death, which wiped out half the population of Europe in the 14th century.
Before its flea-borne evolution, however, researchers say that plague was in fact endemic in the human populations of Eurasia at least 3,000 years before the first plague pandemic in historical records (the Plague of Justinian in 541 AD).
They say the new evidence that Y. pestis bacterial infection in humans actually emerged around the beginning of the Bronze Age suggests that plague may have been responsible for major population declines believed to have occurred in the late 4th and early 3rd millennium BC.
Researchers found that the Y. pestis lineage originated and was widespread much earlier than previously thought, and they narrowed the time window as to when and how it developed, said senior author Eske Willerslev from the University of Copenhagen.
The underlying mechanisms that facilitated the evolution of Y. pestis are present even today. Learning from the past may help us understand how future pathogens may arise and evolve, he added.
Willerslev added: “These results show that the ancient DNA has the potential not only to map our history and prehistory, but also discover how disease may have shaped it.”
The study appears in Cell.