Appendix - Friend or Foe?

The human appendix, a useless organ, who’s seemingly only function is to cause us pain and send us into surgery. This is the common view held by most of the public, and masses of doctors and scientists. But in a classic scientific twist, this view is changing.

The appendix is an organ located at the beginning of our large intestine; it has the general size and shape of a worm, and much like worms, the appendix is all too commonly ignored. Historically, we believed that the appendix was a reduced or vestigial part of the large intestine. A different approach to medical science shines new light on our forgotten appendix.

The appendix, with its tight convoluted tube-like structure seems all too perfect for incubating bacterial infections, and in fact quite often does, this is known as appendicitis. The typical treatment for appendicitis is a relatively simple surgery called an appendectomy, resulting in the removal of the appendix. After an appendectomy, patients typically lead normal, healthy, unaffected lives. How could the appendix be useful if it’s always causing us problems, and shouldn’t people be better off without an unimportant organ? These are the features that have made it difficult to pinpoint the real function of the human appendix.

Conventional medicine has a heavy focus on understanding symptoms and their treatments, and oftentimes research does not delve into root causes. In comes evolutionary medicine! Evolutionary medicine’s underlying idea is that both causes and symptoms alike need to be fully understood before we can say we know anything about any given condition. For example, researchers in this area like to ask questions such as: how did the evolution of our species leave us vulnerable to appendicitis? Additionally, given our best evolutionary knowledge of the appendix, what is the best way to treat appendicitis?

So, what do our best insights to date have to say about the appendix? A study led by Michel Laurin, a proponent of evolutionary medicine, proposes that it may serve as a sort of ‘safe house’ for our good gut bacteria (those that help digest and keep our gut healthy). Their idea comes from three key insights: (1) it is well known that biofilms (like plaque buildup on teeth) provide protection to bacteria, (2) the immune system makes efforts to maintain biofilms of good gut bacteria, and (3) the appendix of several species contain immune tissues. These lines of evidence make the appendix out to be an organ with immune functions. But what is it protecting us from?

Think back to a time where you ate something that's gone bad, or were eating in a foreign country for the first time. Diarrhea is one of the body’s responses to infection by a perceived pathogen, such as bacteria. Essentially the body tries to rid itself of the pathogen as fast as possible, however awful it may feel. But at the same time, we lose our established good gut bacteria. We would be better off having these microbial ‘good guys’ around. So a sort of ‘good guy’ hang out space could help keep our good gut bacteria with us in these troublesome times. The ‘safe house’ model of appendix function proposes that the appendix is precisely that place! The appendix’s size may help promote the development of good gut bacteria biofilms, and its location subsequently aids in a timely recolonization of the gut after periods of diarrhea.

As plant-eating animals, or at least animals who should eat plants if they know what's good for them, we come across cellulose from time to time. However, our digestive enzymes do not work with this compound very well, that's where our good gut bacteria would come in. Knowing this ecological feature of our intestines, Heather Smith and her colleagues studied the appendix presence, length, and diameter with diet types across many species of mammals. They found that appendices are present in animals whose diets are rich in plants, and found none in the meat eating class of mammals called carnivora (wolves and dogs, bears, cats, etc.). Additionally, they found that the appendix evolved independently at least 32 times! Clearly, the appendix has a great advantage in plant eating animals, such as our ancestors and ourselves.

But what about all those people who get appendicitis and need surgery? Surely they would have died in times without modern surgery if their appendix ruptured. How could evolution give us an organ that could so easily kill us? There is an evolutionary term for this kind of situation: environmental mismatch. This is where a trait gives an advantage in a certain environment, but after an environmental change, with no time to evolve, a good trait could become a bad trait. In the case of our appendix, it evolved under conditions where infections were more common. The appendix would have been exposed to low grade infection by harmful bacteria somewhat regularly, and as a result, it would be less sensitive or reactive to an infection. Today, in the hyper sterile societies of developed countries, our appendices almost never get infected, but when they do, they can be prone to overreact. This reaction could lead to inflammation so severe the appendix could rupture.

All that said, we should not think negatively about the doctors and researchers that came before our time. We would likely be inclined to come to the same conclusions as they did. After all, the first appendectomy was at least 100 years before Darwin Published the groundwork for evolutionary theory in the Origin of Species. May this be a lesson on the security of our scientific knowledge, and remind us to keep asking questions even if we think we already know the answer.

Further readings

Michel Laurin, Mary Lou Everett, William Parker. 2011. The Cecal Appendix: One More Immune Component With a Function Disturbed By Post-Industrial Culture. The Anatomical Record 294:567–579

Heather F. Smith, William Parker, Sanet H. Kotzé, Michel Laurin. 2013. Multiple independent appearances of the cecal appendix in mammalian evolution and an investigation of related ecological and anatomical factors. Comptes Rendus Palevol 12:339-354.