Everyone is born with one, but no one knows what it’s for. The human appendix is a small dead-end tube connected to the cecum, or ascending colon, one section of the large intestine. Everyone lives happily with it until it becomes painfully inflamed, when the only treatment is to remove it surgically.

Then everyone lives happily without it. So why is it there in the first place?

Some experts have guessed that it is a vestige of the evolutionary development of some other organ, but there is little evidence for an appendix in our evolutionary ancestors. Few mammals have any appendix at all, and the appendices of those that do bears little resemblance to the human one.

Last December, researchers published a novel explanation in The Journal of Theoretical Biology. The appendix, they suggest, is a “safe house” for commensal bacteria, the symbiotic germs that aid digestion and help protect against disease-causing germs.

Structurally, the appendix is isolated from the rest of the gut, with an opening smaller than a pencil lead, protected from the fecal stream that might be carrying pathogens. In times of trouble like a diarrheal infection that flushes the system, these commensal bacteria could hide out there, ready to repopulate the gut when the coast is clear.