Two hundred thousand years ago or thereabouts, an African lion killed someone. Along with a meal, the big cat got a wicked stomachache. Today a record of that unfortunate death still survives, in the bacteria that make big cats sick.
The trail of this strange story starts in the 1980s, when scientists discovered that ulcers are caused by bacteria known as known as Helicobacter pylori. H. pylori is found in people around the world, and scientists learned how to recognize the different strains they carried. Based on the patterns of the strains, a team of scientists concluded in 2003 that Helicobacter pylori must have been present early in the history of our species, and was spread across the world during the migration of humans. (I wrote a long post on H. pylori and human evolution when the scientists who discovered its link to ulcers got the Nobel Prize.)
But there were skeptics.
A compelling story. We hear a lot about diseases jumping from animals to humans. Here is an instance of a disease jumping from humans to carnovores. I guess at one time we were not at the top of the food chain.