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. The skeptics proposed that Helicobacter pylori spread into humans in just the past few thousand years. They searched for a closely related species of Helicobacter that might have been its ancestor and found one: Helicobacter acinonychis. Preliminary studies suggested that it was far more like our own Helicobacter pylori than other species of Helicobacter. Helicobacter acinonychis also causes severe gastritis in its hosts--lions, tigers, and cheetahs. In laboratory experiments, scientists found that it could also infect mice. Some of the scientists who had initially proposed the ancient origin of Helicobacter pylori have tested this alternative, and they've published their results in a paper in the journal PLOS Genetics. Their surprising conclusion is that the big cats actually got their ulcer bugs from us. The scientists got to this conclusion by sequencing the genome of H. acinonychis and then comparing it to H. pylori strains. By comparing the mutations in each strain, the scientists found that the bacteria all descend from a common ancestor that lived about 200,000 years ago. But which way did the parasite jump? The scientists argue that it went from humans to cats. The cat bacteria carry 92 fragments of various genes that can be found in good working order in the human strains. It would be hard to imagine how the bacteria could have moved from cats to humans and then repaired all 92 genes. The bacteria must have shifted from humans to cats, and then genes that were no longer essential in the new environment began to mutate. Many of the genes that Helicobacter acinonychis has lost made proteins that appear on the surface of Helicobacter pylori. Losing those proteins may have helped the bacteria escape the immune systems of cats. The bacteria also have some genes not found in Helicobacter pylori, and these show signs of having been delivered to the bacteria by viruses. Exactly what they do for the microbes isn't clear yet. What makes this hypothesis particularly compelling is that it's the same sort of process that scientists have documented in many other species of bacteria. It just so happens that scientists have recently documented it in the evolution of bacteria in yogurt 5,000 years ago, about which I blogged last week. It's pretty easy to guess how a lion or cheetah could have picked up our bacteria: by having one of us for breakfast. Once the bacteria were transferred to a big cat, they then spread to other species. It's no big surprise that big cats ate some of our ancestors. (Check out the recent book Man the Hunted for the evidence.) What is surprising is how clearly the unfortunate fate of some early Homo sapiens has been preserved in living bacteria. And it's also a striking demonstration that it's not just humans that can pick up diseases from animals (such as influenza and HIV). The parasite highway runs both ways.