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 18^th 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