Why Is the Appendix Still a Thing?

I first learned what the appendix was from an episode of The Simpsons. There's a joke about Doctor Hibbert performing an emergency appendectomy on the street thanks to his "trusty pocket knife," as mopey junior camper Bart laments that he has not yet earned the right to a knife of his own:

A few seasons later there's a whole episode about Bart having his appendix removed after swallowing a jagged metal Krusty-O. Like many of us, all of my early exposure to this mysterious organ was in context to it being swiftly and requisitely extracted from the body. It seemed to me this was an organ that performed no function other than to become inflamed and require surgery.

In reality, less than 10 percent of the population ever gets appendicitis. But even if it's not dangerous, we've been taught that the appendix is basically a gratuitous organ. People who do have it removed function completely normally. Even the name calls to mind something that is added, extra, unnecessary. So why hasn't our species evolved to no longer have it?

Turns out, the appendix isn't as superfluous as we've been led to believe, and may play an important role in helping out the good bacteria that live in our bellies.

The appendix is a fleshy, worm-shaped tube about four inches long that hangs off of the cecum—a pouch that joins the small and large intestines. Leonardo da Vinci was one of the first to document the appendix in his anatomy drawings, but it was Charles Darwin who started the nasty rumor that it was useless. Darwin theorized that the appendix was a "vestige of evolution," a fold of flesh that used to be part of a large cecum and was now functionless.

"By the time World War I came around, people had found appendices in other species and there was a good understanding that Darwin wasn't right," said William Parker, a professor of surgical science at Duke University who studies how the appendix functions. "But nobody really knew what it did, still. So in the textbooks then, it was still listed as a classic example of a vestige of evolution, and a I think a lot of textbooks are still that way."

Parker told me that by the early 20th century, scientists had identified that the appendix was full of lymphatic tissue and that it served some role in the immune system. But it wasn't until fairly recently that we finally figured out what that role was. In the late 90s, medical science made an important discovery (that these days seems like a no-brainer): the immune system supports the growth of "good bacteria" in the gut. Prior to that point, it was thought that the immune system only ever attacked bacteria with impunity, but this discovery made it clear that the immune system is much more nuanced.

That opened the door to understanding the job of the appendix: research by Parker and his peers suggest it works as a kind of "safe house" for good bacteria, a place where these bacteria can hide out when the body is fighting off an infection. The exact intricacies of how the appendix function are still under debate, but it's become more and more clear that it's not just a "vestige of evolution."

The fact that we haven't evolved to no longer have appendices is a clue that it serves a function, too, according to Heather Smith, an evolutionary biologist at Midwestern University who has studied the appendix in multiple species.

"Any trait or organ or character that has even a small amount of advantage can become adaptive: in other words, it can increase in frequency in the population," Smith told me over the phone. "Even though we can live okay without the appendix, it doesn't mean that it's not better to have one."

Smith's research has shown that the appendix evolved independently in 50 different species more than 30 times, which suggests it provided some kind of advantage. The weird thing is that it's inconsistent: it's in mammals, but not all mammals. Some rodents, some primates, but not all, which only adds to the mystery. When you add in the fact that you can live fine without it and that it's more likely to become inflamed than other organs, it's understandable why we've remained a bit suspicious of this mysterious organ for so long.

But now that we've got some establishing research on the table, and as our understanding of the importance of the bacterial balance in our gut grows, we may start to wonder how we could ever live without the appendix. Y'know, except for when it explodes.

Why Is This Still a Thing is a column exploring the anachronistic, seemingly-outdated technology (or, in some cases, biology) that surrounds us. New columns appear every Friday.