Why do we have an appendix if we can live without it?

For a long time, the honest answer was: no reason. Because people seemed fine after having it removed, the appendix, a small worm-shaped tube hanging off the large intestine, was written off as a useless leftover of evolution. Today many scientists think it does have a job: it may be a safe house for the friendly bacteria that live in your gut, ready to restock the intestines after an illness flushes them out.

The old story came from Charles Darwin. He suggested that our distant ancestors used this part of the gut to digest tough plant food. Herbivores like the koala still have a big pouch there, the cecum, full of bacteria that break down cellulose. As our ancestors' diet became easier to digest, the idea goes, the pouch shrank until only the appendix was left.

The new story began in 2007, when researchers at Duke University proposed the bacterial safe-house idea. Some evidence backs it up: a 2012 study found that people without an appendix were twice as likely to have a relapse of a nasty gut infection called C. difficile. And evolution seems to like the appendix. One study estimated it evolved independently in mammals 32 to 38 times, but was lost no more than six times. The debate isn't closed, though: the appendix is still often described as vestigial.