The Appendix: Don’t close the book on it yet!

The Appendix: Don’t close the book on it yet!

● The vermiform appendix is the poster child of vestigial organs leading to the joke that, “Its major importance would appear to be financial support of the surgical profession.” Like the wings of an ostrich or the eyes of the blind cave-dwelling catfish, the appendix no longer supports the function that it was designed to do: digest tough cell walls of plants. In herbivores, this function resides in the caecum, an off shoot of the large intestine, that houses symbiotic bacteria, producing enzymes (cellulases) by fermentation. Did you know that in the koala, the caecum is longer than the animal itself?! But, as Darwin noted, in hominids -apes and humans, the switch from a leafy to predominantly fruit diet made the caecum redundant and eventually, it degenerated into the finger-like appendix. Although we still eat plants, our vestigial organ does not house enough cellulase-secreting bacteria to digest more than a few grams of cellulose per day. 

So why do we still have an appendix? It is notoriously prone to infection, commonly in children 8-13 years old. Before modern surgical methods, acute appendicitis was often fatal. What a poor design! But there is evidence that the appendix has useful functions. Like the tonsils, the appendix houses lymphoid tissue, or white cells, important for immunity. It has been compared to a “safe house”, lodging beneficial bacteria that can repopulate our gut after an infection wipes out existing microbial flora. 

● A new study by a group at Duke University has concluded that the appendix has arisen independently more than 30 times in the evolution of mammals. By plotting diet on the evolutionary tree of mammals, researchers found that the appearance of the appendix did not correlate with a change away from herbivorous diets. Species with an appendix were scattered so widely on the evolutionary tree that they concluded that the appendix evolved separately along distinct branches. Also, they found that the larger the caecum, the larger the appendix: opposite to what one would expect for a vestigial remnant of the caecum. But naysayers argue, if it is so useful, why don’t all mammals have an appendix? We’ll have to wait until Science adds another Chapter to the Appendix! 

Ref: Multiple independent appearances of the cecal appendix in mammalian evolution and an investigation of related ecological and anatomical factors. Smith et al. (2013) http://goo.gl/zJyviw

Counterpoint: The vestigiality of the human vermiform appendix. A modern reappraisal. http://goo.gl/v9Qvm0

#ScienceEveryday  

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62 Responses to The Appendix: Don’t close the book on it yet!


  1. “…why don’t all mammals have an appendix?”


    Because they can survive long enough to procreate without one?

  2. Rajini Rao says:


    Selective advantage could have kept the individual free from illness or increased fitness so they would mate more successfully. 


  3. Wasn’t it stablished it is a reservorium of intestinal flora?


    Sorry, I’ve read it again more slowly.

  4. Chad Haney says:


    It’s interesting that as we learn more about our microbiome some long held theories are changing. For example, medicine 50 yrs ago would never dream of fecal transplants. To them all bacteria were bad. I wonder how the study of our microbiome will add more to the appendix story.

  5. Rajini Rao says:


    Víktor Bautista i Roca  Well, its dead end construction, ,like a cul-de-sac, is a convenient stash for bacteria, but there is a persuasive argument that it is still a vestigial organ that has acquired a secondary function. The Duke investigators suggest that it arose separately, and is not a vestige.  

  6. Rajini Rao says:


    Chad Haney , indeed, I agree. I almost wrote “repoopulate” in my sentence up there 😉

  7. Jim Carver says:


    I’m sure you guys know this, but what is usually found inside a diseased appendix?


  8. Rajini Rao Cul-de-sac. Never better used! 🙂


    In Catalan and French it literally means “arse-of-sack”. And, at least in Catalan, “sac” in some profanous contexts may mean arse, too. 

  9. Rajini Rao says:


    Oh, the things that I learn on Google+! 😉


  10. If all you eat is raw food, then you might want a lymph node for your intestines, but it still risks getting overwhelmed. Cooking became very popular 50-10,000 years ago

  11. Rajini Rao says:


    James Salsman , good point. Cooking breaks down macromolecules and increases the efficiency of extracting energy from food. I recall reading that the leap in human intelligence (advancement) correlated with cooking food. 

  12. Jim Carver says:


    Yeah, cooking is good. That thing about natural enzymes is bs…but what is usually found in the appendix when they remove it?

  13. Jim Nichols says:


    I understood that the reason we had an appendix was to store bacteria to aid in digestion incase you had cholera or diarrhea and had a die off of beneficial bacteria… Your appendix also makes and trains white blood cells.. 

  14. Rajini Rao says:


    Why don’t you just tell us, Jim Carver ? 🙂


    Jim Nichols , yes..those are the current functions of the appendix. It’s been pressed into secondary use, since the main purpose of breaking down cellulose is no longer essential to humans. My post explains the pros and cons of the arguments for the function. 

  15. Jim Carver says:


    Rajini Rao K, I was just giving some time for people to answer. Unfortunately, it’s fingernails.


  16. Acchi bat hai rajini

  17. Rajini Rao says:


    Hope your innards didn’t twinge too much, Catherine Maguire 🙂

  18. Jim Nichols says:


    From somewhere in the dusty cobwebs of my memory I seem to recall about the appendix was thought to allow us to move from clan to clan without becoming sick during the Neolithic era because it stored bacteria that we were used to and dumped it in our gut to counter unknown bacteria (aka from another clan)  that would make our digestive system upset… 

  19. Rajini Rao says:


    Sure that’s not an old wives tale, Jim Carver ?  Sounds like something an exasperated parent would say to their nail biting kid. Fortunately, I was never a nail biter!


    Jim Nichols , interesting theory! It seems reasonable that our intestinal flora can be regional and give us protection against local pathogens. 

  20. Rajini Rao says:


    John DeLaughter , the article does not mention fingernails in the appendix, which is what Jim Carver mentioned? 


  21. * George was 29 and still single. All of his friends were married, but George just dated and dated. * One day Bill asked him why he wasn’t married. “Don’t you want to settle down? Are you holding out for the perfect woman? Are you having trouble meeting someone compatible?” * “Actually,” George replied, “I’ve found many women I would have been happy to marry. Things always start off fine, but when I bring them home to meet my parents, my mother never approves of them.” * Bill thinks for a moment. “I’ve got the perfect solution, just find a girl who’s just like your mom!” * A few months later Bill ran into George again. George looked a little depressed so Bill asked how things were going. “Did you find the perfect girl? Did your mother approve?” * George shrugged his shoulders. “Yes, I found the perfect girl. Yes she was just like my mom. Yes, you were right, not only did my mom approve, but they became good friends.” * “What’s the problem?” asked Bill. * “My father can’t stand her.”

  22. Jim Carver says:


    It’s in the Encyclopedia Britannica also. Taught in health class when I was a young sprout. It’s true, and you can imagine with all that bacteria and sharp edges.

  23. Rajini Rao says:


    deepak paneru your joke is missing an appendix. 

  24. Peter Harris says:


    fairly funny…


    wondering if you want to hear the one about those port holes… 🙂

  25. Jim Carver says:


    So what’s for dinner? ;D


  26. Great summary and citations Rajini Rao!  I enjoyed reading that quite a bit.  I find myself somewhat skeptical of an adaptationist approach when it comes to the appendix (and a lot of morphology, in fact).  The myth is that functionality implies strong selection, but there is no reason to expect this is the case, even if it appears de novo in some cases, and not as a structural reduction.


    In any case, one note on anatomical terminology: humans do still have a caecum, aside from the appendix.  It is an out-pocketing of the ascending colon (forming the beginning of the colon), and the appendix then arises from there.  So essentially, the caecum is reduced to a relatively indistinct origin of the colon and a particularly pinched portion, which we call the appendix.  The connection of the small intestine to this point is called the ilio-caecal junction.

  27. Rajini Rao says:


    I’m off kitchen duty this evening since I am loitering around here, Jim Carver 🙂


    Peter Harris , appendix jokes, anecdotes and puns welcome (that’s a challenge). 

  28. Rajini Rao says:


    Michael Habib , thanks for that clarification..yes we do have a caecum, a short one -I never did pay attention in my anatomy classes 😉 What did you think of the argument on caecum and appendix length in the Duke paper? Their assumption is that a large appendix is not a vestige. 


    I guessed as much, John DeLaughter . Thanks for the link, it was a good read. 

  29. Jim Carver says:


    Rajini Rao I had a Prof. one time and he said I should have put an Appendix in my paper. I said ‘why?’ , the whole thing was just shit anyway. 🙂

  30. Peter Harris says:


    a passenger on a liner asked the captain why do ships have have round windows…


    the captain replys ..so the water does not hit you square in the eye!


    sorry it’s a wee bit off topic?

  31. Rajini Rao says:


    Tsk, Jim Carver  😀

  32. Jim Carver says:


    Rajini Rao He laughed and gave me the ‘A’ anyway. That was good because it was near the end of the semester and I was out of bribe money.


  33. Rajini Rao: I just read through the paper very quickly, and aside from some concerns about their phylogenetic approach (none major), I thought that the non-vestigal argument was strong. 


    That said, I do not necessarily agree with their conclusion regarding the adaptive nature of the appendix: “The appendix has evolved minimally 32 times, but was lost fewer than seven times, indicating that it either has a positive fitness value or is closely associated with another character that does.”


    They looked at a lot of potential trait co-evolution options and didn’t find much, so their own data don’t support the above conclusion.  Sure, if a given bit of morphology evolves 32 times and is lost seven times, it might be tied to fitness.  So, in that sense, their conclusion is plausible. The idea of a pleiotropic effect is particularly reasonable.


    But the first thing that comes to my mind is a random walk with unequal acquisition and reversal rates because of developmental constraints.  Those constraints could be ties to a trait with positive fitness value, but that’s not required.  They’re assuming that “random” looks like Brownian motion with equal transition rates, but random can easily look like other things.


    I like Jeff Schwartz’s take on biological traits: “If it doesn’t kill you, then you have it”.  I would say that essentially summarizes their results.  Sure, the appendix may also take on microbial safe house functions when it appears, but the fitness value there could very well be small (too small, apparently, to maintain the structure in 7 lineages).

  34. Jim Carver says:


    Rajini Rao  You see jokes about the appendix have never been that funny and don’t go over through it that well. I can usually make a joke about damn near anything, but I don’t do anal jokes very well and that’s what this would have to be.


    Damned interesting topic though Rajini, and out of your normal posting time too. That’s good, change it up is good sometimes.


    I personally have to take breaks from it and work because I get too wound up. I guess you probably noticed that. Well, I’m aware of it. Maybe I expect a little too much. And maybe I expect the standard to be higher also. I think that’s the problem.

  35. Rajini Rao says:


    Hmm, that’s a lot to digest , Michael Habib . I do think that the microbial safe house function could be a secondary utility following the loss of the cellulose-digestive function. 


  36. Argh!  The puns 🙂  But yes, Rajini Rao, I agree: a secondary utility model would seem to fit their data well.

  37. Jim Carver says:


    Rajini Rao I edited my prior post (why?) but I wanted you to read it. K? 🙂 It kinda says it in the edit.

  38. Rajini Rao says:


    Which post, Jim Carver ? 


  39. For small-sample empirical evidence, I went from never getting sick to catching everything after my appendix was repurposed.

  40. Rajini Rao says:


    That’s too bad, Laurel Lawson . I wonder if that is a common complaint after appendicitis. 

  41. Kawthar A says:


    Great read as always Rajini, the appendix has always fascinated me, and I remember one of my co-worker’s 3 years old daughter had to get her appendix removed, and the doctor told them the appendix got infected because the girl used to eat sunflower seeds..yep..that sounds weird! 

  42. Rajini Rao says:


    Thanks, Kawthar AL ABDALLA . It must have been a frightening experience for the family and the 3 year old to have an appendix removed (although, I would suspect that the sunflower seeds are more of a myth!). This reminds me of the French children’s story Madeline : it used to be my favorite reading to my daughter. Madeline lives in an orphanage with a harried and anxious nun, Miss Clavell, in charge. She gets into all sorts of scrapes, and the young Embassy boy next door, Pepito, is a bad hat (he sounds fun!). Anyway, the point of my comment is that in the first story, Madeline is rushed out in the middle of the night to have her appendix removed. When she returns, she is so proud of her scar that all the other little girls set up a big to do, crying, “Boo hoo, we want our appendix out too!”. https://en.wikipedia.org/wiki/Madeline


    William Carter , has Ms. Z been introduced to the delightful Madeline stories? 

  43. Kawthar A says:


    It took the girl so much time to recover. Thanks for the story Rajini 😀

  44. Marta Rauch says:


    Thanks as always for your interesting posts, Rajini Rao 🙂

  45. Rajini Rao says:


    Glad you liked it, Marta Rauch 🙂


  46. Came for the pun stayed for the science 🙂


  47. Haha thats f*** funny ANT


  48. Rajini Rao A very useful post! At school my students ask for many  informations about  vermiform  appendix  when we treat the anatomy and physiology of intestine. 


    For expression “cul-de-sac” there are meaning, use, history, etimology etc here: 


    http://en.wikipedia.org/wiki/Cul-de-sac 

  49. Rajini Rao says:


    annarita ruberto , thank you for that very interesting link on the term cul-de-sac. I use it all the time, since we live in one, but did not know the history! Tolkien fans will be interested to know that he used the literal translation “Bag End” as a joke to make fun of what he considered a pretentious term 🙂


  50. I learned a lot about the appendix! I can’t wait until Science adds another chapter to it!

  51. Rajini Rao says:


    Meanwhile, the Bibliography on the Book of Science is getting longer, Systems Biology 🙂


  52. This George need happy ending a new lady friend.


  53. …and so it goes 🙂


  54. Nice.. It’s a true fact

  55. colin pyper says:


    This is a vestigial comment as I had an emergency appendectomy 40 years ago!

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