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.
Those analyses revealed that mutualism in Proteobacteria independently evolved between 34 to 39 times, the researchers report in the journal Proceedings of the Royal Society B. The team was a bit surprised to find that this happened so frequently, inferring that evolution apparently views this lifestyle quite favorably.
Their results also show that mutualism most often arises in species that were originally parasites and pathogens. In other words, the salmonella of the past may today help us break down food in our gut. Moreover, the team reports, those mutualistic lineages “exhibit a paucity of reversals to parasitism or to free-living status.” Once those pathogens experience the sweetness of cooperating with the hosts they once ravaged, they rarely, if ever, go back to the rough life of a pathogen.
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