Yersinia pestis bacteria viewed with an electron microscopeConnect Images/AlamyAn ancient ancestor of the pathogen that would later cause the Black Death and other major pandemics has been identified in a Bronze Age domestic sheep in Russia making it one of the oldest pathogens ever found in an animal.Its DNA closely matches that of plague bacteria found in European human skeletons from the same period, providing the first evidence that the disease could have spread between humans and their own livestock well before the pathogen evolved to jump from rodents to people via fleas.