You may have heard that the appendix is a relic of our past, like the hind leg bones of a whale. Bill Parker, a professor of surgery at the Duke University School of Medicine, heard that, too; he just disagrees. Parker thinks the appendix serves as a “nature reserve” for beneficial bacteria in our gut. When we get a severe gut infection such as cholera (which happened often during much of our history and is common in many regions even today), the bene­ficial bacteria in our gut are depleted. The appendix allows them to be re­stored. In essence, Parker sees the appendix as a sanctuary for our tiny mutualist friends, a place where there is always room at the inn.

Parker’s hypothesis, which he and collaborators first published in 2007 in the Journal of Theoretical Biology, is a fundamentally new idea about how an organ in our body works. A paper published last December provides new data to back up the theory.

James Grendell, chief of the division of gastroenterology, hepatology and nutrition at Winthrop University-Hospital on Long Island, and his team studied 254 patients with a history of gut infections caused by the bacterium Clostridium difficile. C. difficile, known as C. diff among the medical in-crowd, is a deadly pathogen often encountered in hospitals, particularly when patients must be treated by prolonged courses of antibiotics. C. diff does not appear to compete well with the native biota of patients’ guts, but when the native biota are depleted (as is the case after several courses of antibiotics), C. diff can grow quickly and take over. If Parker’s idea is right, individuals without an appendix should be more likely to have a recurrence of C. diff than those with one.

And that is precisely what Grendell’s group found: patients without an appendix were more than twice as likely to have a recurrence of C. difficile. Recurrence in individuals with their appendix intact occurred in 18 percent of cases. Recurrence in those without their appendix occurred in 45 percent of cases.

Where does this leave us? In your body is an organ that appears to be helping out the bacteria in your life so they can, in turn, help keep you alive. More tests, even true experiments, need to be done before we can be sure. Until then, doctors will keep cutting out infected appendixes. When they do, when they hold them up, they hold up a symbol—a somewhat gross, pinky finger–size symbol—both of our complex relationship with other species and of how little we know.

This article was published in print as "Your Appendix Could Save Your Life."