Ancient DNA research also reveals pathogen evolution. When grinding up human remains, we sometimes encounter DNA from microorganisms that were in an individual’s bloodstream when he or she died and so were the likely cause of death. This approach proved that the bacterium Yersinia pestis was the cause of the fourteenth-to-seventeenth-century CE Black Death,14 the sixth-to-eighth-century CE Justinianic plague of the Roman Empire,15 and an endemic plague that was responsible for at least about 7 percent of deaths in skeletons from burials across the Eurasian steppe after around five thousand
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