New research analyzing 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 guts 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). The scientists say the new evidence that Y. pestis bacterial infection in humans actually emerged around the beginning of the Bronze Age (~4,000 BC) suggests that plague may have been responsible for major population declines believed to have occurred in the late 4th and early 3rd millennium BC. The new findings were published as the open-access cover article of the October 22, 2015 issue of Cell.
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