The origin of the plague, a deadly disease that claimed millions of lives throughout history, has been a long-standing mystery. Now, scientists say that the plague had been infecting humans as early as the Bronze Age - 3,300 years earlier than previously thought - and they offer an explanation as to how the bacterium that causes the disease evolved to become so deadly, Live Science reported.
The research team investigated 101 ancient skulls from the Bronze Age and found traces of the bacterium Yersinia pestis in the teeth of seven skulls. However, the scientists found that the bacterium in its early form was not yet capable of causing a pandemic. It was only capable of causing pneumonic plague or septicemic plague, both very harmful diseases but not enough to cause pandemics.
"So at that time we have a kind of intermediate plague," study author Simon Rasmussen of the Technical University of Denmark told Smithsonian. "These Bronze Age strains couldn't cause bubonic plague, but they caused septicemic plague in the blood and pneumonic plague in the lungs, which you can transmit through the air whenever you sneeze or cough."
The researchers believed that the bacterium went through a mutation. They pointed out two mutations that gave Y. pestis its unique characteristics: getting the ymt gene, which allowed it to survive in the guts of fleas, and developing the pla gene, which helped it infect various tissues, causing it to become the bubonic plague.
"Our findings suggest that the virulent, flea-borne Y. pestis strain that caused the historic bubonic plague pandemics evolved from a less pathogenic Y. pestis lineage infecting human populations long before recorded evidence of plague outbreaks," the researchers wrote.
The researchers also said that the plague could have driven human population migrations that caused Europe to lose about 60 percent of its population as people went to other places to avoid the deadly disease.
"You see these very abrupt population replacements, people moved into northern Europe from central Asia, replacing the existing populations - kinds of very abrupt migrations [that] fit very well with plague playing a major role," Lead study author Eske Willerslev told BBC News.
"The underlying evolutionary mechanisms that facilitated the evolution of plague are still present today," Rasmussen told Live Science. "By knowing which new genes and mutations lead to the development of plague, we may be better at predicting or identifying bacteria that could develop into new infectious diseases."
The study was published Oct. 22 in the journal Cell.
