The appendix is often only noticed when it necessitates troublesome removal. Wouldn't we be better off never having had an appendix at all? Darwin himself speculated that the appendix never served an independent function, and that it had once been part of a much larger cecum that shriveled as it shrank into a separate, useless fold.

But Darwin didn't have the whole picture. Recent research by William Parker from Duke and other doctors suggests that not only was the appendix once very important, it still is in some countries. The appendix, it turns out, is not vestigial at all.

In 2007, Dr. Parker and a team of immunologists at Duke published a study asserting that the appendix serves as a "safe house" for beneficial gut bacteria in the event of a serious gastrointestinal disease. Once the body's immune system has defeated the disease, the healthy microbes that had been safely ensconced in the appendix quickly repopulate the gut.

"[The appendix] looks like it’s situated perfectly to reboot the system if you get an infectious disease—say cholera or typhoid—some kind of infectious disease of the gut that would cause a diarrheal response," Dr. Parker tells mental_floss.

In 2013, Dr. Parker worked with an international team of researchers on mapping the evolution of the appendix. Their study compiled information on the diets of 361 living mammals and found that the 50 species now considered to have an appendix are so diversely scattered across the mammalian evolutionary tree that the appendix must have evolved independently at least 32 times, and perhaps as many as 38 times.

"It’s what we call a recurrent trait, it comes and goes but only in certain mammal groups, in other mammal groups you just don’t see it at all," Parker explains. Establishing the appendix as a recurrent trait helps to back Dr. Parker's safe house theory. In most cases, when the appendix first begins to appear in a species there is no sign of a dietary shift, as Darwin speculated, that would accompany a shrinking cecum. Evolution was selecting for the development of an appendix over and over again, likely because of the role it played in backing up the immune system.

But does its ability to protect beneficial bacteria still work? "We think so, that’s hypothetically the case. Of course, no one is willing to do that experiment," Dr. Parker said.

In developed countries, "[the appendix] was probably very useful around 1850, maybe even say 1890. But it’s become much less useful since 1950 when toilets were essentially universal in the United States." And it likely still plays an important function in developing nations where infectious gut diseases are more prevalent.

The appendix's apparent ability to contribute to the immune system means that, contrary to popular belief and widespread categorization, it may not be vestigial after all. Instead, as Dr. Parker says, "We’ve had some social changes that have made a lot of parts of our immune system inactive."