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.

William Parker, the senior author of the study and an assistant professor of surgery at Duke, emphasized that this was a hypothesis, not experimental proof. “At this point,” Dr. Parker said, “this is a deduction based on a lot of information that we’ve had for many years and some key pieces of information that have only been uncovered recently by our lab and others. It does make sense.

“But an experiment to prove this theory would be very expensive. And in any case, why would you want to spend money to find out something that is not likely to help cure a disease?”

Rebecca E. Fisher, an assistant professor of anatomy at the University of Arizona College of Medicine, said that although the appendix was “likely to be a derived feature, selected for a purpose, the enigma is that we didn’t know what that purpose might be.”

“I think Parker’s study offers an interesting solution,” Professor Fisher said.

Recent studies have found that biofilms, colonies of beneficial microbes that live outside cells, form in the large intestine, where they are dependent on the mucus that lines the bowel. There, they aid digestion and protect against infection, while enjoying the protection and nutrition of the human host.

The researchers, examining tissue from uninfected human appendices removed in kidney-pancreas transplants, found biofilms on the epithelial lining of the appendix, as well. Under their theory, it is in these biofilms in the appendix, well positioned to avoid pathogens in the rest of the gut, that commensal bacteria take refuge.

If that is true, why is it that removing the appendix apparently does not have negative side effects? The scientists contend that in industrialized countries with modern medical care and sanitation, maintaining a reserve of helpful bacteria has become unnecessary. Widespread outbreaks of intestinal disease are so rare that the commensal bacteria face little danger of extermination.

Dr. Parker wants to know whether biofilms function in the same way in other animals. “We already have an idea that the cecum in some animals works the same way,” he said. “But how, for example, do biofilms work in frogs and other species with no cecum and no appendix?”

Whether the human appendix has the function Dr. Parker thinks it has or whether it has no function at all, it is clear, he said, that “if your appendix gets inflamed, forget about the fact that it might have some function.”

“You have to get it out,” he added. “Appendicitis can be life threatening, and the earlier you treat it, the less likely it is that you will die from it.”