Like pretty much all multi-cellular organisms, humans enjoy the benefits of helpful bacteria. (As you may have heard, there are more bacteria in the human body than cells.) These mutualistic microbes live within the body of a larger organism, and, like any good long-term houseguest, help out their hosts, while making a successful life for themselves. It’s a win-win situation for both parties.
Scientists still don’t understand exactly how these relationships began, however. To find out, a team of researchers from the University of California, Riverside, used protein markers to create a detailed phylogenic tree of life for 405 taxa from the Proteobacteria phylum—a diverse group that includes pathogens such as salmonella as well as both mutualistic and free-living species.