Like the male nipple and coccyx, the appendix is often viewed as extraneous evolutionary baggage that never quite got lost on the long-haul journey to modern humanity. Last year, British surgeons whipped out 44,562 of them, mostly during emergency operations when, as a result of infection, they rapidly became life-threatening. And people carry on perfectly well without them.

So it is late in the day that doctors have reported finally working out what this seemingly useless, worm-like few inches of tubing is for. According to a team at Duke University Medical School in North Carolina, it acts as a nursery for good gut bacteria, all ready to replenish your intestines with microbes should the need arise.

In the modern west, that need should never arise. If you suffer an infection that destroyed all your gut bacteria, they would soon re-establish themselves if you live in a densely populated area, because the bugs are easily picked from other people.

But far back in history, when populations were thinly spread, or in countries where contact with others is still infrequent, the appendix could be a life-saver. A serious bout of dysentery or cholera can strip your guts of harmless bacteria that are vital for digesting food. But the appendix is a safe haven that stockpiles the organisms, which would gradually be released back into the intestine.

Duke surgeon Bill Parker, who published a report on the appendix in the Journal of Theoretical Biology, said the location of the appendix, in a cul de sac just below the normal one-way system of the large intestine, supports the idea.

Cases of appendicitis have been falling in the west for the past half-century, largely because of changes in diet to include more fruit and vegetables. Forty years ago, there were nearly three times as many operations for acute appendicitis. Childhood deaths from appendicitis have fallen more than 85% since the 1960s, when death rates were particularly high among children aged four and under.