International researchers found that the bacterium that causes plague or "black death" has been present in humans for more than twice as long as previously believed. The findings of the new study, which focused on a 5,000-year-old teeth fossil, suggest new revelations about human history.
Before the plague-causing bacteria Yersinia pestis (Y. pestis) persist to thrive in flea gut, the authors said that that it has already been endemic among humans in Eurasia. Specifically, it already existed about three years before the first global plague outbreak, known as the Plague of Justinian, was reported in 541 AD. With this, they believe that significant population declines during the latter part of the 4th and the early part of the 3rd millennium BC may be accounted for the possible emergence of the plague at the onset of the Bronze Age.
For the study, the researchers studied fossils of human teeth from Eurasia. After sequencing the genome, they found seven adults to have Y. pestis bacteria in their DNA, the oldest of whom died during the earliest report of plague, which is 5,783 years ago.
A notable finding is that six out of the seven samples lack two genetic materials usually found in modern plague strains. One of the components is called ymt or "virulence gene," which shields the bacteria from being destroyed by toxins found in the gut of fleas. In turn, the bacteria multiplies and freezes the digestive tract, causing the flea to bite on anything and thus spread the plague. The other missing genetic material is the activator gene mutation called pla, which enables the bacteria to spread across different body sites.
The discovery of the missing genes prompted the researchers to think that the early forms of the bacteria could neither have fleas as their host nor could it result in bubonic plague, which spread into the lymphatics.
The researchers then suggest that the plague that struck Bronze Age people may have been pneumonic, which target the respiratory system. The mode of transmission of the bacteria back then may be through inhalation thus, warranting human-to-human transfer.
Dr. Marta Mirazón-Lahr, co-author of the study from Cambridge, said that in their specimens, the altered strain of the bacterium was initially observed in Armenia in 951 BC, but is not noted in the most recent specimen, which dates back from 1686 BC. Such discovery may suggest that bubonic plague evolved and steadily stood toward the end of the 2nd and start of the 1st millennium BC. She added, however, that the specimen they obtained in 1686 BC came from Altai mountains near Mongolia, suggesting a longer history of bubonic plague in Middle East and that the bacteria may have spread elsewhere.
The plague described in the bible that occurred in 1320 BC, which according to the World Health Organization appears to be bubonic, adds to the concept that the highly fatal strain of the bacteria may have originated in the Middle East.
Robert Foley, study co-author, also from Cambridge, said that the fatality of bubonic plague may have necessitated the correct demographics before the bacteria could survive. He said every pathogenic organism has to retain a certain state of equilibrium."if it kills a host before it can spread, it too reaches a 'dead end,'" he explained.
"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," concluded Eske Willerslev, one of the authors from the University of Copenhagen.
The study was published in the journal Cell on Thursday, Oct. 22.
