http://www.newsweek.com/ancestor-bubonic-plague-bacteria-found-3800-year-old-teeth-967993

In the teeth of 3,800 year-old human skeletons, scientists have found a historically important bacteria. The pulp of these teeth was home to the ancestor of the bacteria that would one day cause the bubonic plague.

A study published in the journal Nature Communications describes the genome of Yersinia pestis, a bacterium from the bronze age of Eurasia. According to the study, this bacteria had not yet adapted to life on fleas. Y. pestis would later evolve into a bacteria that could hitch rides on fleas and go rapidly from person to person. (Recent studies suggest that rodents had little or nothing to do with the spread of the disease, despite the popular belief that rats were at fault.)