Is the appendix a vestigial organ?

One of the main mistakes creationists make is arguing that if a vestigial trait is actually used for something, then it is neither vestigial nor gives us evidence for evolution. (Such features testify to common ancestry.)  Both creationist claims are wrong. They rest on the false argument that if the appendix, for instance, actually has some useful function, then it can no longer be claimed as evidence for evolution—as a now-useless remnant of a much larger part of the intestine that was useful in our ancestors.

Why is that argument false? Because if a feature is an evolutionary modification of an obviously ancestral feature, like the flippers of penguins (which clearly evolved from wings), then it can be both useful and vestigial, and therefore testimony of evolution. It’s important that readers remember this, because creationists conveniently forget it.

One feature that can be both vestigial and useful is the human appendix. Once thought to be not only useless, but positively detrimental (our ancestors died from its inflammation), we’re now finding that it has some use, as it contains immune-system cells that may serve as a refuge for useful bacteria, bacteria that can repopulate our gut if it’s wiped clean by diseases like cholera. A February article on the science site Cosmos—unfortunately called “The Appendix—Darwin’s Mistake“—points out increasing evidence that the appendix has a function.

Author and physician Norman Swan writes this in Cosmos:

. . . over the last few years the thinking has changed. The appendix turns out not to be an evolutionary curiosity but a handy little organ with the potential to resuscitate the bowel. Back in 2007, researchers at Duke University in North Carolina proposed that the appendix was actually a “safe house” for normal gut bacteria that could be put to use when the bowel had been devastated by, say, an infection such as cholera and needed to be repopulated by healthy bacteria.

The Duke group had found colonies of protective microbes known as biofilms were disproportionately produced by the appendix. Ironically, the immune cells found in the gooey mucous lining of the appendix and bowel actually help these biofilms to form.

If this theory were true then people without an appendix might be more vulnerable to dangerous gut infections. A study a few years later found evidence for this. People who’d had their appendix removed were significantly more likely to suffer recurrently from the serious and potentially life-threatening recurrent Clostridium difficile infection.

. . . the lining of the appendix contains a newly discovered class of immune cells known as innate lymphoid cells. Other lymphoid cells must be specifically tuned to attack the latest strains of bacteria or viruses, but these cells come ready wired to respond to the wide range of biological insults that flow down the intestines from our daily diet.

Experimenting in mice, the researchers found that these innate lymphoid cells were critical to maintaining the tissue around the caecum. If the cells were removed, the caecum shrank, suggesting they played a vital role for the integrity of the tissue. They also found that mice without these innate lymphoid cells were more vulnerable to a pathological
gut infection. This supports the study I mentioned earlier where patients without their appendix were more likely to suffer from recurrent C. difficile infections.

Darwin mentioned in The Descent of Man that the appendix is “useless” and “a rudiment”, as well as being variable, so that humans have really different sizes of their appendixes, and some have none at all.

Was he wrong about the appendix being evidence for evolution? No. Yes, he was wrong about its being “useless”, but not about its variability or its status as a rudiment of the larger appendixes in our herbivorous relatives.

Granted, the data above might be true, but that doesn’t detract from the appendix’s use as evidence for evolution. Yet creationists love finding that rudimentary organs may still be useful for something, as they think (not very clearly, as usual) that if something has a use, it can’t possibly be vestigial.

In fact one  creationist website takes this quote from Why Evolution is True to show that the evidence for evolution is weak:

We humans have many vestigial features proving that we evolved. The most popular is the appendix… our appendix is simply the remnant of an organ that was critically important to our leaf-eating ancestors, but is of no real value to us.

But the whole section, appearing on pages 60 and 61, notes that the appendix may have a function:

We humans have many vestigial features showing that we evolved. The most famous is the appendix.  Let’s look at it closely. Known medically as the vermiform (“worm shaped”) appendix, it’s a thin, pencil-sized cylinder of tissue that forms the end of the pouch, or caecum, that sits at the junction of our large and small intestines.  Like many vestigial features, its size and degree of development are highly variable: in humans, its length varies from about an inch to over a foot. A few people are even born without one.

In herbivorous animals like koalas, rabbits, and kangaroos, the caecum and its appendix tip are much larger than ours. This is also true of leaf-eating primates like lemurs, lorises, and spider monkeys. The enlarged pouch serves as a fermenting vessel (like the “extra stomachs” of cows), containing bacteria that help the animal break down cellulose into usable sugars. In primates whose diet includes fewer leaves, like orangutans and macaques, the caecum and appendix are reduced.  In humans, who don’t eat leaves and can’t digest cellulose, the appendix is nearly gone. Obviously the less herbivorous the animal, the smaller the caecum and appendix.  In other words, our appendix is simply the remnant of an organ that was critically important to our leaf-eating ancestors, but of no real value to us.

Does an appendix do us any good at all?  If so, it’s certainly not obvious.  Removing it doesn’t produce any bad side effects or increase mortality (in fact, removal seems to reduce the incidence of colitis).  Discussing the appendix in his famous textbook  The Vertebrate Body,  the paleontologist Alfred Romer remarked dryly, “Its major importance would appear to be financial support of the surgical profession.”  But to be fair, it may be of some small use.  The appendix contains patches of tissue that may function as part of the immune system.  It has also been suggested that it provides a refuge for useful gut bacteria when an infection removes them from the rest of our digestive system.

But these minor benefits are surely outweighed by the severe problems that come with the human appendix.  Its narrowness makes it easily clogged, which can lead to its infection and inflammation, otherwise known as appendicitis.  If not treated, a ruptured appendix can kill you. You have about one chance in 15 of getting appendicitis in your lifetime. Fortunately, thanks to the evolutionarily recent practice of surgery, the chance of dying when you get appendicitis is only 1%.  But before doctors began to remove inflamed appendixes in the late 18th century, mortality probably exceeded 20%. In other words, before the days of surgical removal, more than one person in a hundred died of appendicitis. That’s pretty strong natural selection.

So yes, the appendix may have a function, but it’s still a vestigial organ, and evidence for evolution. The only remaining question is this: is it a detrimental  feature? Well, because of doctors it isn’t now, but it may well have been over the bulk of human evolution, as I note above. And that may be the reason it’s not only small but variable among people. Features that are crucial for our survival and reproduction don’t vary nearly that much. Perhaps its marginal use as a refuge for bacteria wasn’t useful enough to overcome the disadvantage of its being prone to infection.

Saying that there is a “function” to the appendix isn’t enough. To show that its presence is (or was) adaptive compared to its non-presence, you have to show that the benefits of having a bacterial refuge (in terms of future reproduction) outweighed the problems of having an infection-prone organ.  And nobody has showed that. So, it’s still possible that the appendix, while vestigial and rudimentary (and highly variable: the sign of a feature, like wisdom teeth, that’s disappearing over time), may have been detrimental in our ancestors and is detrimental now.

Nevertheless, creationists continue to harp on a functionality of the appendix as disproving evolution. If there is a lesson from this post, just remember: THAT IS NOT TRUE.

But let us for the moment grant the creationists their argument: that the appendix is not a remnant of a useful feature, but a feature that evolved, or is maintained, by a net reproductive benefit to its carrier. Does that disprove evolution? Hardly, given the massive evidence for evolution from a gazillion other areas.

And there are features that don’t seem, even under scrutiny, to have any positive effect on your reproduction. If you want a feature that is almost certainly does not enhance fitness, try our vestigial ear muscles (also variable among people), or, better yet, the hundreds of “dead genes” that we harbor in our genome: genes that had a function in our ancestors but have been silenced. (Olfactory-receptor genes and yolk-protein genes in humans are two examples.) Let the creationists explain why the creator put nonfunctional “dead” genes in our DNA, and just those genes that are active and adaptive in our ancestors.

And seriously, Dr. Swan: “Darwin’s mistake?” What are you implying by that? As I said, it may well be true that, over the bulk of human evolution, having an appendix was, on net, detrimental. “Detrimental” is “worse than useless,” so Darwin might not have been so wrong after all.

h/t: Barry


  1. Posted May 15, 2016 at 1:14 pm | Permalink

    Someone told me the appendix was once used to help us digest things like grass and bark and things we don’t really eat anymore……..anything in that?

    • Mark Sturtevant
      Posted May 15, 2016 at 6:44 pm | Permalink

      We see that monkeys today have a plant based diet,and a big appendix to support that diet as an accessory fermenting organ. Since we are descended from monkeys, it is likely that our ancestors of that sort also had a big appendix that was used similarly. Moving on to ape ancestors, the appendix probably shrunk, and lost that function.

      • gravelinspector-Aidan
        Posted May 16, 2016 at 8:08 am | Permalink

        It occurs to me that a better set of evidence to compare the human appendix would be the various appendices of primates, instead of the organs of rabbits, mice and other more convenient laboratory animals. Whether the data is compiled into a convenient location is another question.
        This review paper from 1980 sounds useful. As does this one from 2000. I’d expect more recent work to cite one or other of those (if they’re so easy to find that I can find them).

        • Posted May 17, 2016 at 12:05 pm | Permalink

          I agree that the Scott (1980) paper is important; he published an anatomical comparative study of primate appendices. His conclusion was that it was not vestigial because there was evidence that it began to develop in Old World monkeys and actually progressively developed in primates. I think this is even more important than the recent studies (2007 and 2009) that show the human appendix seems to serve current functions. Scott’s analysis appears to show the appendix had positive selection pressures in the past.

          However, if one reads the original 2007 Bollinger et. al’s paper they propose that the appendix is currently becoming vestigial, which is even better evidence for evolution since it’s happening during our time:

          “However, to the extent that the primary function of the appendix is the one proposed herein, it might be argued that the human appendix is not important in industrialized countries with modern medical care and sanitation practices. Indeed, maintenance of a reserve supply of commensal bacteria in the event of infection by pathogens may be unnecessary in areas where outbreaks of enteric pathogens do not affect the vast majority of the population at any one point.”

          And another co-author of the study during an interview stated: “Parker and his colleagues, including R. Randal Bollinger, propose something a little bit different: Yes, the organ doesn’t really serve a purpose to modern Americans — but the organ probably became obsolete within the past century or two, not over hundreds of thousands of years.”

          Acute appendicitis is common in humans and is said to be a well recognized cause of death in gibbons who are in captivity. Primate issues with appendicitis is probably a result of modern diets and the great reduction of diarrheal infections in developed societies. It’s great evidence for evolution because it’s transformation to vestigial status is happening before us.

          • gravelinspector-Aidan
            Posted May 17, 2016 at 9:38 pm | Permalink

            Good points. I’m sure PCC(E) will take notes for any intended revised editions.

  2. Posted May 15, 2016 at 1:34 pm | Permalink

    In fairness to the author, titles are often made up by click-hungry editors, sometimes without the author’s input or knowledge.

    • EvolvedDutchie
      Posted May 15, 2016 at 1:41 pm | Permalink

      If that’s the case, then it could have been worse: ‘Scientists Looked At The Appendix: You Won’t Believe What Happened Next!’ 😛

      • Mark Joseph
        Posted May 15, 2016 at 2:42 pm | Permalink

        Or, “17 Functions for the Appendix (#11 will shock you!)”

  3. DiscoveredJoys
    Posted May 15, 2016 at 1:37 pm | Permalink

    Ah! The UK monarchy, vestigial but useful.

  4. EvolvedDutchie
    Posted May 15, 2016 at 1:38 pm | Permalink

    “it can be both useful and vestigial, and therefore testimony of evolution.”

    I think this is the best response to the creationist argument. Whether the appendix is useful or useless doesn’t really matter to the question whether it has evolved or not. Only how it has evolved to the organ it is today.

  5. Randy Schenck
    Posted May 15, 2016 at 2:45 pm | Permalink

    It is amazing how the creationist can dig and misinterpret evidence of evolution and make up conclusions that also make no sense. Without evidence of their own beliefs the desperation is pretty sad.

    It’s not unlike their upside down reasoning and failure to understand something as simple as freedom of religion. Instead of simply accepting it and being glad they live in a place that has it, they make up reasons why they don’t, when all they are doing is forcing their religion on others and becoming irritated when they are told to stop.

  6. Posted May 15, 2016 at 3:07 pm | Permalink

    “both useful and vestigial”

  7. Posted May 15, 2016 at 3:48 pm | Permalink

    Appendicitis has two causes. In children the lymphoid (immune) tissue at the base of the appendix enlarges in response to infection and obstructs the lumen of the appendix causing appendicitis. In adults it is the clogging of the lumen by stool that typically causes appendicitis. Either way, trends toward eliminating the appendix would likely lead to smaller and smaller lumens which would be more easily obstructed and thus detrimental. Those born without appendices usually have some chromosomal or developmental anomaly, such that it is rare to be perfectly healthy and sans appendix. It persists because we can’t get rid of it and it may gave some nominal function, like the fifth toe.

  8. Posted May 15, 2016 at 4:43 pm | Permalink

    Here’s a similar argument from a creationist that the recurrent laryngeal nerve isn’t evidence for evolution. It’s so tortuous and full of impressive-sounding medical terms that I can picture a lot of readers being thoroughly convinced on that basis alone.

    • Posted May 15, 2016 at 8:29 pm | Permalink

      Bergman forgot to mention that the sky fairy cleverly designed the recurrent laryngeal nerve to yield signs of metastasized carcinomas of the lung by producing a very distinctive horse voice without any throat soreness. Clever sky fairy to provide a warning that you’re about to die from something he/she/it won’t bother to fix.

  9. Michael Finfer, MD
    Posted May 15, 2016 at 6:51 pm | Permalink

    I think the point here is that classifying a structure as a vestigial organ is a diagnosis of exclusion. It is very difficult to prove that a structure has no function because it is very difficult to prove a negative. We provisionally assume that a structure is vestigial if we cannot come up with a function for it. That does not exclude the possibility that we might be able to assign a function to it in the future.

    To say that when you find a function for an organ that was originally thought to be vestigial you cannot use that organ as evidence for evolution is nonsense. You might as well say that anything with a function cannot be used as evidence for evolution. It’s just a way to rig the discussion to get the result that you want.

    That’s the entire point of creationism, is it not?

  10. Mark Sturtevant
    Posted May 15, 2016 at 6:53 pm | Permalink

    This is a good summary of the issue. I am of a mind to not hold up our appendix as a very ‘good’ example of a vestigial organ, although it is technically vestigial in that our earlier monkey ancestors probably used a larger appendix as a fermenting organ, on top of the uses that we share with monkeys today. So yes, it has been demoted a little in function, and that makes it vestigial. But just barely.
    Better examples of vestigial organs are those ear muscles that are mentioned, and our arrector pili muscles, and a few other things. We got plenty of ammo against the creationists.

    • Posted May 15, 2016 at 7:49 pm | Permalink

      “Our monkey ancestors”? Excuse me?

      • Mark Sturtevant
        Posted May 15, 2016 at 9:48 pm | Permalink

        ‘Scuse. Monkey Overlords.

  11. Posted May 15, 2016 at 7:53 pm | Permalink

    Outrageous that a 19th Century scientist lacked omniscient knowledge of the natural world. Sheesh. Don’t know why we pay attention to ANYTHING that Darwin guy wrote if he can’t even foresee a function for a specific organ, involving things of which there was no knowledge at the time. Loser.

  12. Willard Bolinger
    Posted May 15, 2016 at 9:38 pm | Permalink

    In my years of reading scientific books and articles I have noticed claims that some organ did not serve any function or some gene was dead and it always seemed that a better statement would have been that a qualifier such have been use “as far as we know” or something similar. Creations do not publish any scientific papers or as far as I known do actual scientific work..No peer reviewed scientific papers! Instead they cull articles and look for statements that they can say-“oh look what so-and -so said” and then play upon the statement.

    • DiscoveredJoys
      Posted May 16, 2016 at 2:42 am | Permalink

      That’s because taking a snippet of ‘sacred’ text out of context and then elaborating it into something they wish had been said originally is what they do. They merely extend that process to the real world.

    • gravelinspector-Aidan
      Posted May 16, 2016 at 8:13 am | Permalink

      It’s called “quote mining”. Contemptible enough to actually get a name.

  13. Richard Bond
    Posted May 16, 2016 at 4:59 am | Permalink

    I recently came across what will henceforth be my favourite example of a vestigial feature the next time that Jehovah’s [why can they not even get the name of their god correct?] Witnesses call: the palmaris longus. It is a muscle-tendon combination in the wrist that can be used for repairing other tendons, (obviously without the usual rejection problems) because its removal has no functional effect. In fact a substantial minority of people do not even have one. The punch line that will upset creationists is the question of the animals in which it is still functional: monkeys!

    • Posted May 16, 2016 at 1:32 pm | Permalink

      Wow, God gave us a spare! What a gracious Creator. Think of those poor souls who have lost their spare tendon thanks to mankind’s wickedness letting sin into this world and corrupting our nature. Satan! Give them back their spare tendons!!

  14. Posted May 16, 2016 at 5:19 am | Permalink

    Here is a great clip by the Irish comedian Dara O’Briain on the evidence for evolution, including the appendix;

    • gravelinspector-Aidan
      Posted May 16, 2016 at 8:18 am | Permalink

      The argument from biting one’s own cheek.

  15. Gregory Kusnick
    Posted May 16, 2016 at 12:08 pm | Permalink

    In humans, who don’t eat leaves

    May I suggest that in future editions of WEIT you change this to “don’t subsist on leaves” or “don’t eat leaves exclusively”.

    Counting by volume, salads and leafy vegetables make up a substantial fraction of our diet. Counting by number of species consumed, I’m guessing the spice shelf of a large grocery store puts us far ahead of any other primate, and perhaps of any herbivore.

  16. Posted May 16, 2016 at 1:08 pm | Permalink

    Since so many elements of the human body have been repurposed over eons and, many of them in tortuous ways (eyes, for example, or the vas deferens), an argument claiming design by God for these oddities while accusing Darwin of error because of something not yet known by anyone seems weird.

    • Posted May 16, 2016 at 3:21 pm | Permalink

      Indeed. A favorite of mine is the fact that the air and food pipes cross in the throat, ensuring that some of the sky fairy’s dearly beloved choke to death on steak each year. It is easy to understand if you know a little about vertebrate evolution. If you think the sky fairy designed it, you’d have to conclude he/she/it isn’t as clever as the average first year engineering major at Lehigh.

  17. Xiaohui
    Posted May 22, 2016 at 2:37 pm | Permalink

    How about wisdom teeth? Are they vestigial?

    • Posted May 22, 2016 at 3:34 pm | Permalink

      Probably. Although one could make the case that they are very important to oral surgeons successfully putting their children through college.

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