The heart pumps blood, the liver removes toxins from the blood, and the kidneys remove waste products and excess fluid from the body. Unsurprisingly, it seems like every organ of the body has an important specialized function, but what about the appendix? This narrow pouch protruding off the cecum in the digestive system has a reputation for being utterly useless unless you count its tendency to become inflamed and cause appendicitis.

But although for many years scientists were unable to find any particular purpose for the human appendix, recent research suggests that it isn’t all that useless after all.

Is the appendix a vestigial organ?

Human vestigiality refers to traits occurring in humans — including both organs and behaviors — that have lost most if not all of their original function through evolution. In his books, On the Origin of Species and The Descent of Man, Charles Darwin defines vestigial organs as “evolutionary remnants” whose function “was critical for survival in the past, but became non-existent over time.”

Some examples of anatomical vestigiality include the human tailbone, wisdom teeth, and the inside corner of the eye.

Likewise, the human appendix was also considered to be a vestigial organ by most biologists, a remnant of the mammalian caecum (also known as cecum). Originally, this part of the intestine had a digestive function, primarily facilitating the digestion of cellulose with the aid of residential microorganisms. But this cellulose digestive trait is lost in the human caecum, although in the human appendix a relative abundance of microorganisms present in biofilms still exists alongside the presence of lymphoid tissue.

While plant-eating vertebrates seem to make good use of the appendix, being an integral part of their digestive systems, the human appendix no longer seems to aid in digestion, perhaps because humans started to include more easily digestible foods in their diet as they evolved. Supporting this assertion is the fact that none of the 1 in 20 people who have had their appendix removed seem to miss it.

However, that doesn’t mean that the organ is completely useless. It just shows that the appendix isn’t critical to our survival or optimal performance. Case in point, modern research suggests that the worm-shaped tube attached to the large intestine may actually serve a minor role in protecting the body from infection.

The appendix may support the immune and digestive systems

According to a 2016 study, the appendix is covered in lymphocytes, white blood cells with an important role in the immune system. It is now believed that the appendix is responsible for some T lymphocyte- and B lymphocyte-mediated immune responses, thereby mounting early defenses that may help prevent serious infections in humans.

Species that have an appendix also had higher average levels of lymphoid tissue in their cecum, according to Heather F. Smith, Ph.D. and associate professor at Midwestern University Arizona’s College of Osteopathic Medicine, whose team of researchers combed through data on the presence or absence of the appendix in 533 mammal species, and its relation to other gastrointestinal and environmental traits.

So, it seems like one of the functions of the appendix is to expose white blood cells to a wide variety of antigens, or foreign substances, present in the gastrointestinal tract, supporting local immunity.

The researchers also claim that the human appendix fosters several strains of gut flora, acting as a sort of ‘safe house’ from which bacteria can repopulate the digestive tract following a calamity that may have destroyed the gut microbiota, such as in the aftermath of taking strong oral antibiotics or following a nasty case of diarrhea.

The fact that the appendix is located right at the lower part of the large intestine through which food and microbes pass, supports the idea that the organ serves a function related to the microbial flora of the digestive system.

Previously, research performed by researchers at Winthrop–University Hospital showed that individuals without an appendix were four times as likely to have a recurrence of Clostridium difficile colitis.

In other words, if the theory that the appendix is a safe house of beneficial bacteria is correct, individuals with an appendix should be more likely to recover from severe gut infections than those without.

The appendix is no fluke of evolution

Researchers led by  William Parker, an immunologist at Duke University Medical Center in Durham, N.C, charted the evolution of the appendix, finding that it is at least 80 million years old. They also found that the appendix appears in nature much more often than previously thought.

According to Parker and colleagues, the appendix evolved at least twice, once in Australian marsupials and another time in rats, lemmings, meadow voles, and other rodents. More than 70 percent of all primate and rodent groups contain species with an appendix, the analysis revealed. In contrast, Darwin thought appendices appear in only a handful of animals.

But if the appendix is so useful, how can evolution account for the huge incidence of potentially deadly inflammation of the appendix, known as appendicitis? It’s important to realize that appendicitis isn’t caused by some faulty organ, but rather by recent cultural changes associated with industrialized society. Oddly enough, due to improved sanitation our immune systems are now less equipped and perform much less work than in ancient times, leaving us vulnerable to infections of the appendix.

The human appendix: not so useless after all

Many biology textbooks still refer to the appendix as a ‘vestigial organ’, but modern research suggests this is incorrect. Even if you can live without it as if nothing happened, this doesn’t mean the appendix doesn’t serve any purpose. The appendix seems to be a vital safehouse where beneficial bacteria lurk until the moment they are needed to repopulate the gut.