For years, the appendix got no respect. Doctors regarded it as nothing but a source of trouble: It didn’t seem to do anything, and it sometimes got infected and required an emergency removal. Plus, nobody ever suffered from not having an appendix. So human biologists assumed that the tiny, worm-shaped organ is vestigial  a shrunken remainder of some organ our ancestors required. In a word: Useless.

Now that old theory has been upended. In a December issue of The Journal of Theoretical Biology, a group of scientists announce they have solved the riddle of the appendix. The organ, they claim, is in reality a “safe house” for healthful bacteria  the stuff that makes our digestive system function. When our gut is ravaged by diseases like diarrhea and dysentery, the appendix quietly goes to work repopulating the gut with beneficial bacteria.

“In essence,” says William Parker, a chemist who co-wrote the paper, “after our system crashes, the appendix reboots it.” The theory may explain the location of the appendix: Positioned at the beginning of the colon, it often escapes being voided when a sick colon violently empties itself out the bottom.

If the appendix is indeed crucial, why don’t people who have their appendixes removed die? Because in the modern world hygiene and medicine can keep our levels of healthy bacteria adequate. The appendix may have evolved its rebooting function back when our ancestors lived a more vulnerable life  and an entire village might suffer catastrophic diarrhea. In that situation, each gut had to rely on its own resources to recover after a collapse, so the appendix was crucial.