Field of Science

Darwin was wrong about the human appendix being vestigial

ResearchBlogging.org"Ever since Darwin"™ [1] it has been thought that the human vermiform appendix doesn't serve a function any more, but that the function was lost, and the remnant lingers on - something that is easily explained by evolution, but not so easily explained by creationism.

Then a research team from Duke University came along and changed all that.

First they demonstrated that the appendix does have a function [2]:
the human appendix is well suited as a “safe house” for commensal bacteria, providing support for bacterial growth and potentially facilitating re-inoculation of the colon in the event that the contents of the intestinal tract are purged following exposure to a pathogen.

Proposed function of appendix as providing support for growth of bacteria essential to the function of the intestine.

Then they show [3] that
Although the appendix has apparently been lost by numerous species, it has also been maintained for more than 80 million years in at least one clade, supporting the idea that the structure has a vital biological function.

Part of the phylogenetic tree showing that the appendix most likely evolved more than once (red, appendix present; gray, appendix absent). Note that this part of the tree doesn't show that the monotremes (platypus, echidna) and some marsupials also have something that looks like an appendix.

Both of these papers separately point to evidence that the human vermiform appendix is not a useless structure. The interpretation is not in question - no biologist do (should) not have any problem understanding the implication of this, namely that the status of the appendix has changed to one of use, but that while Darwin was wrong about this particular point, this conclusion in no way takes anything away from evolutionary theory as a whole. Creationists are guaranteed to jump out and imply that this means there is a problem with evolution, but it cannot be said with enough emphasis that it in no way means that. Again, evolution can explain such organs of no use, but creationism can't. That one structure is no longer considered neutral with respect to fitness does not mean that such neutral organs don't exist. Examples:
  • leg bones in whales
  • eyes of blind cavefish
  • pelvis remnants in snakes
  • the human tailbone
  • human goose bumps
  • human muscles to move the ears (I'm very skilled at this myself)
There are many more. Some of these may one day, like the human appendix, be demonstrated to have a function (i.e. increase fitness), but it is highly doubtful (it is unthinkable) that the status will change for all of them.

A question of semantics

What do we call such organs/structures/behaviors that no longer serve any function? If we distinguish between those that used to have one function but now has another (penguin wings, for example) and those that used to have a function, but now doesn't at all (see list above), then I personally find it most useful to call only the latter vestigial. However, some people use the term in both cases (Jerry Coyne is said to do so in Why Evolution Is True, for example), but I find it more informative to call the former structure exapted, preadapted, or co-opted (I use co-opted, because the first to are usually used a nouns, exaptation and preadaptation - and that reminds me that Todd Oakley still owes me to explain what the difference between these three are).

So, I'll say that the human appendix is not a vestigial organ.

A question of politics

All of the above is really fairly straightforward (but exciting) science discourse. However, our own PZ Myers has reacted rather strongly in a post, Darwin and the vermiform appendix.

Before I go on, let me make a few things clear before anyone says otherwise: PZ is a great blogger, when blogging about science as well as about creationism and all things related to either one. I happen to be fairly aligned with him in terms of politics as well as on religion (we are both atheists), and I agree with him on most issues that he brings up on his blog. Notably, I side with him on the question of "Crackergate," "Expelled from Expelled," and share his disdain for the faitheists, just to name a few.

The post from today about the human appendix paper is the first example where I strongly disagree with PZ. He doesn't so much talk about the scientific paper itself, but instead about an article in ScienceDaily, Evolution Of The Human Appendix: A Biological 'Remnant' No More, in which we are introduced to the evolutionary significance of the study:
Now, some of those same researchers are back, reporting on the first-ever study of the appendix through the ages. Writing in the Journal of Evolutionary Biology, Duke scientists and collaborators from the University of Arizona and Arizona State University conclude that Charles Darwin was wrong: The appendix is a whole lot more than an evolutionary remnant. Not only does it appear in nature much more frequently than previously acknowledged, but it has been around much longer than anyone had suspected.

"Maybe it's time to correct the textbooks," says William Parker, Ph.D., assistant professor of surgical sciences at Duke and the senior author of the study. "Many biology texts today still refer to the appendix as a 'vestigial organ.'"

[Emphases added.]
Parker then said
Darwin simply didn't have access to the information we have. If Darwin had been aware of the species that have an appendix attached to a large cecum, and if he had known about the widespread nature of the appendix, he probably would not have thought of the appendix as a vestige of evolution.

He also was not aware that appendicitis, or inflammation of the appendix, is not due to a faulty appendix, but rather due to cultural changes associated with industrialized society and improved sanitation. Those changes left our immune systems with too little work and too much time their hands – a recipe for trouble.

That notion wasn't proposed until the early 1900's, and we didn't really have a good understanding of that principle until the mid 1980's. Even more importantly, Darwin had no way of knowing that the function of the appendix could be rendered obsolete by cultural changes that included widespread use of sewer systems and clean drinking water.
Here is, then, the relevant passage from Darwin [1]:
Not only is [the vermiform appendage of the caecum] useless, but it is sometimes the cause of death

[Emphasis added.]
He doesn't use the term vestigial, but instead is very careful (I gather) to say that it no longer has any use (in fact, it isn't even just neutral, per Darwin's observation, but detrimental).

Here are a few quotes from PZ's post about the ScienceDaily article, and my answers to each of them:
It's rather unseemly to collect a lot of data that Darwin did not have, run it through PAUP 4.0 on a fast computer, map the data onto a molecular consensus phylogeny, and cackle gleefully over discovering something Darwin did not know. Really, it doesn't make you a better scientist than Darwin.
"Cackle gleefully" ? Who, Parker? "Make you a better scientist" ? Where does this come from? Parker isn't implying any such thing.
To make it even worse, people who do this can't even make the corpse-fight a fair fight — they have to stuff the pathetic dead body with straw. In this case, they're padding Darwin's investment in the appendix a fair amount. They cite one work by Darwin, The Descent of Man, which mentions this issue. He wrote one whole paragraph on the topic, and here it is, in its entirety;
Go to PZ's post to see the quote, which includes the sentence I quote from Darwin above, where he labels the appendix as 'useless.' People criticize the work of dead scientists on scientific grounds as a matter of routine. There is absolutely nothing wrong with it. Indeed, this is the way things must be, in order for science to progress. PZ would agree, I know, and I am puzzled why he would scold Parker in this manner.
Note why Darwin classed this appendage as vestigial: because it is greatly reduced compared to the homologous organs in non-human relatives, and because it currently exhibits a great range of variation, which is apparently non-functional. These are criteria which the paper in question does not refute at all. Darwin does say that the appendix is "useless", and the paper will show some evidence that that is wrong. It's also irrelevant.

The reason why it is irrelevant is that the presence of some function is not part of the definition of a vestigial or rudimentary organ
It is not relevant that the appendix is useless? The hell?! PZ harps about the semantics - vestigial vs. useless - but then ignores the meaning behind Parkers's words. Parker clearly means 'useless' when he says 'vestige'. Of that there can be no doubt. How can it be irrelevant whether the appendix has a use, when that is what both papers are all about? (It can't, and it isn't.)
If a portion of the gut, a digestive organ, is diminished in size such that it no longer contributes to the primary function of the organ, but does retain a secondary function, such as assisting in immunity, or as the authors of the recent paper will argue, in acting as a reservoir of bacteria for recolonizing the gut, then it is still a vestigial organ. It has lost much of its ancestral function.
PZ is using one definition of the word, and not the other (see semantics above).
I mentioned that I'd point out errors in Darwin's understanding. They're there, but note that seeing them now 150 years after he wrote his big book does not make me smarter than Darwin, nor does it invalidate the overall picture of his theory.
And Parker neither said nor implied anything to that effect. In fact, he goes to great length to emphasize that "Darwin simply didn't have access to the information we have," and "If Darwin had been aware of the species that have an appendix attached to a large cecum, and if he had known about the widespread nature of the appendix, he probably would not have thought of the appendix as a vestige of evolution." That, and the rest of the Parker's words above from the ScienceDaily article.
Most of my complaints here are with the abysmal presentation of the ideas in it by the popular press, aided and abetted by the scientists themselves. Just keep in mind that whenever these press releases that declare "Darwin was wrong" appear, it's usually an example of grandstanding and the regrettable tendency of competitive scientists to think the way to impress people with the importance of their work is to get into a penis-measuring contest with poor dead Chuck.
I was very careful to put "Darwin was wrong" in the title of this post. This is the "abysmal presentation" that PZ is referring to. PZ co-authored a letter to the New Scientist, when they printed those exact words in big letters on the cover of the January 2009 issue. (That cover story was about the uprooting the tree of life, and how parts of it isn't at all like a tree, but a network. I agree it was foolish to choose that headline for the cover, but, luckily, as things have turned out, it didn't really have much of an effect as far as I have seen.) I also think it is impressive when scientists can finally prove Darwin wrong on something. If no one else have been able to do it for almost 150 years, then let us indeed celebrate it with much fanfare whenever someone can. Darwin would have been proud.

I think PZ has got the heebie-jeebies. He is at the forefront of defending evolution against creationist imbeciles, so it is understandable that he dislikes it when scientists gives them ammo for their cannons (note to self: find a better idiom). However, when communicating science, we should not be hesitant to say things the way they are, and in this case "Darwin was wrong: The appendix is a whole lot more than an evolutionary remnant." Trying to boost your scientific work in the popular media by such statements is fair game. It's very hard to get people interested, and I personally applaud Parker for the way he did it, and for standing up for "poor dead Chuck" at the same time. Yeah!

Lastly, let me briefly say how disagreeable I find the discourse in the comments to many of PZ's posts, this one in particular. People call each other names for voicing opinions not shared by the majority of the commenters. Accusing them of trolling is standard practice, akin to shouting 'witch' back in the middle ages, and one is a 'fucktard' when they don't like you. But worse than that, ScienceBlogs is a place for science bloggers, and the number one trait of a good scientist is skepticism. Pharyngulites as a group don't have too much of it; commenting how much they are in agreement with PZ on whatever he writes about, spiced with sublime derogatory terms, seems to be a hobby of hundreds of regular visitors. A few quotes from comments to the post in question:

"some idjits think that makes the whole work of the innovator wrong."
"As Prof. Myers points out, Darwin was wrong about a lot of things. So what?"
"Picking a fight with Darwin over it is just stupid."
"So, basically, they didn't understand what Darwin meant by "vestigial" because they didn't do all their homework."

It's not all bad, though. Here's one dissenting voice: "If we are to refer to all homologous structures with different functions as vestigial, it seems to have become a useless term."

Exactly my point.

References:

[1] Darwin, C. 1871. The Descent of Man and Selection in Relation to Sex.

[2] Randal Bollinger, R., Barbas, A., Bush, E., Lin, S., & Parker, W. (2007). Biofilms in the large bowel suggest an apparent function of the human vermiform appendix Journal of Theoretical Biology, 249 (4), 826-831 DOI: 10.1016/j.jtbi.2007.08.032

[3] Smith HF, Fisher RE, Everett ML, Thomas AD, Randal Bollinger R, & Parker W (2009). Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix. Journal of evolutionary biology PMID:19678866


Update 11/2:
Bill Parker sent me an email in reply, and I have elected to make it a separate post: More on the appendix.

10 comments:

  1. Hey, that was a cool post. Somebody should comment on it. 22 followers and no adulation. Really?

    ReplyDelete
  2. About 300 visitors spent an average of over 3 minutes reading this post, and no one had any comments. The stats don't tell me why not.

    Disagreeing with the establishment is not good tone, I suppose.

    ReplyDelete
  3. I disagree with the wording that Bollinger et al. "demonstrated" a function for the appendix. The JTB paper proposes, but provides no direct evidence in support of, biofilm generation and maintenance as a function of the appendix. This most recent paper also does not appear to provide any direct evidence of linkage, either, beyond the assumption that any preserved trait must be essential. The anatomical analysis in frogs (page 11 & Fig. 4) revealed that they also possess biofilms in roughly the same distribution as mammals, which is inconsistent with the idea that an appendix is in some way essential for the maintenance of these films (at least under normal circumstances). The distribution of biofilm was initially considered a point in favor of the appendix-biofilm hypothesis, so this frog study must also count against it. Hence, I feel the case for function remains rather weak, and as a result the data presented are as consistent with the possibility that the appendix is an incidental or neutral feature as they are with the proposition that it plays a significant role in mediating host interactions with commensal bacteria. The proof would come from a direct inspection of the rate at which commensal bacteria recolonize the bowel following diarrhea in people (or animals) with/without an appendix. Hopefully, this experiment (which is substantially more difficult to perform than the present analysis) is forthcoming.

    I note that Myers subsequently wrote an analysis of the paper itself (rather than the "news" coverage) that was much less snarky, although he expressed some significant reservations about the paper's conclusions (which I obviously share).

    ReplyDelete
  4. I would have posted, but we were talking about the issue already... sorry,
    Cheers Arend

    ReplyDelete
  5. I think here is an example of PZ reading the science, thinking of how a creationist would react *and then replying to that*. (* for italics).

    Hense the "Cackle gleefully" bit. He's going at this through the mind of a creationist reading it. I got the feeling he wasn't accusing Parkers work...but the way a creationist would read it.

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  6. I got the feeling he wasn't accusing Parkers work

    No, he wasn't. He explicitly stated that he was only at odds with both the science-reporting and Parker "trying to pick a fight with a dead man," which I think is a totally ludicrous way of interpreting his statements.

    ReplyDelete
  7. Genial brief and this post helped me alot in my college assignement. Say thank you you for your information.

    ReplyDelete
  8. We can probably prevent most appendicitis through a naturalistic diet and lifestyle. It's getting blocked, parasites are growing in there, our bowel movements are too slow and infrequent etc. It doesn't just blow up on it's own.


    PZ had another post on the appendix months earlier ridiculing creationists for saying it had a greater function than what he'd accepted at the time. It also mentions natural healers trying to cure it's onset with high enemas, juice diet, and herbs etc. I don't think that's implausible. Scary, primitive and unethical maybe, especially on children, but I think the organ is worth trying to heal save. Just hacking it out is just as arrogant as saying it does nothing. Maybe there is a way to save it using similar methods or better ones.

    C) It's removal has been reliably linked to Crohn's and who knows what else. Even if it's just a reduced immune system or an increased burden on the body somehow when combined with the risks in surgery and the psychological effects of surgery on children, I say it's worth trying to avoid. Apparently about 10% of the time it's done unnecessarily; the organ was healthy.

    I agree that people get up to weird stuff in the natural healing world, and I'm not advocating for that. I am advocating for a more open-minded and less rigid view of these types of things.

    I mean, look at neuroplasticity. They were firm in their idea that it was impossible up until the last few decades, now we know they had it backwards and it's one of the most exciting fields of research. I could write an entire article about how that assumption held back society and science.

    The kind of stuff that goes on at PZ's is pretty brutal, I don't read the comments anymore. He had a post about tone recently and basically defended his right to be a dick to everyone in the name of truth. Hey, that's not why I got into science. It's gotta be fun. We're in this to raise the quality of life for every living being on the planet.

    It's my opinion that the drug rush people get from being part of a group where you're allowed to spout off in those ways has certain benefits I'm just not interested in, at least not anymore.

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  9. Very Interesting!
    Thank You!

    ReplyDelete

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