Most of us are familiar with the appendix, that worm-shaped tube at the beginning of the large intestine, even if we can’t remember which side of the abdomen it resides. (Hint: It’s not the left side.)

It’s easy to forget the four-inch-long organ is there at all — until it becomes inflamed or bursts, that is. More than 1 in 20 Americans will develop a case of appendicitis at some point, according to the National Institutes of Health, which can sometimes lead to serious and even fatal complications.

If you don’t like the sound of those odds, you may also be wondering why we’re burdened with the ticking time-bombs in the first place. Is there really an evolutionary benefit to their hiding out in our bellies? Read on to find out.

Good Gut Bacteria

You may be surprised to learn the appendix isn’t relegated only to the realm of humans. A handful of other animals — including great apes and other primates but also opossums, wombats and rabbits — have similar structures. 

In fact, according to one landmark study published in 2013, the appendix evolved in different animals at least 32 times throughout history and de-evolved only seven times. Clearly, it holds some sort of value. Scientists just can’t agree on what that value is.

One theory, as put forth by the same researchers behind the 2013 study, is that the human appendix serves as a reservoir for beneficial gut bacteria. Species with an appendix, they say, also seem to have high concentrations of lymphatic tissue in the beginning of the large intestine — called the cecum — that’s right next door to the appendix. 

This tissue, also called immune tissue, may play a role in the immune system. But we know that it also encourages the growth of healthy gut bacteria; housing an extra supply of this bacteria in the adjoining appendix could come in handy when, say, resetting the entire digestive system after intestinal illness.

Read More: Gut Bacteria’s Role in Anxiety and Depression

Original Appendix Function

Of course, other researchers (including Charles Darwin, the “father of evolution” himself) argue the appendix is merely a thing of the past. They call it a vestigial organ: a useless remnant cast aside during evolution, similar to the human tailbone or wisdom teeth.

Modern-day herbivorous vertebrates, like horses, have fairly large cecums; it’s thought that these aid in the digestion of tough plant material like tree bark, and that the appendix acts as an extension of this organ.

As our ancient ancestors’ diets evolved and began to include more easily digestible foods, however, they no longer needed the extra assistance that the appendix, in theory, provided.

Nowadays, an estimated 1 in 100,000 people are born without an appendix at all — and some scientists believe that, given enough time, the organ will disappear from the human body for good. If that seems a bit difficult to imagine, consider that we’ve been removing appendixes for longer than a century without any discernible health problems.

Read More: How Are Humans Still Evolving?

What Is Appendicitis?

You’ve likely heard the horror stories about what appendicitis feels like: intense abdominal pain, vomiting, diarrhea, fever and bloating. But what causes an incognito appendix to go rogue?

Most often, an abdominal infection or some sort of blockage in the opening of the organ is to blame. Either of these things can cause the bacteria inside to rapidly multiply — eventually, the appendix becomes swollen with pus. 

Left untreated, the pressure inside will continue to build and build and build until (you guessed it) it has no other choice but to rupture. This can happen as soon as 48 to 72 hours after onset of symptoms.

At this point, even with medical treatment, myriad complications can arise. When the appendix bursts, its contents spill throughout the abdomen — bringing nasty infection with it. This condition is called peritonitis and can be life-threatening, particularly if it progresses to sepsis.

What Is an Appendectomy?

Regardless of what stage you’re at, though, an appendectomy is the best and only effective treatment for appendicitis. According to the Cleveland Clinic, 300,000 appendectomies are performed each year in the U.S.

In this procedure, a surgeon either removes the misbehaving organ through one large cut in the abdomen (the standard method) or through a smaller cut with the help of a tiny video camera. Either way, the risk of complications is extremely small — and the benefits may apply to more than just gut health.

Read More: Having Your Appendix Removed Significantly Lowers the Risk of Parkinson’s Disease