What's that appendix for, anyway?

Well, it keeps coming up, doesn't it? Famous cdesign proponentsist Dembski brought it up again recently in his list of ID "predictions" (click for epic fail). While his point was nicely deconstructed by Afarensis, I think it's worth examining the paper that attributed a function to the appendix. Just what did Bollinger et al. say about its function? On what basis did they draw their conclusions? And, I suppose most exasperatingly, what does the paper mean for evolution? Is the appendix vestigial or not, and if not, does that vindicate ID, evolution, or both?

The paper in question is an elaborately stated hypothesis, premised on the idea that

The occurrence of the appendix sporadically throughout phylogeny might suggest that the structure is evolutionarily derived for a specific function rather than merely a vestige of a once important digestive organ.

The emphasis in the above is mine, and included primarily to demonstrate that regardless of where the paper takes us it cannot be a successful unique prediction of ID, because the rationale behind the search for a function was evolutionary in the first place. But what exactly is it that the authors think the appendix is doing?

Well, they believe that the appendix is a site at which bowel biofilms are created, and a reservoir for commensal bacteria. What does this mean? Well, as we all ought be aware, we share our bodies with a significant number of bacteria who live on what we fail to digest and occasionally assist us by breaking down what we cannot and feeding bits of it to us. In recent years, as we have generally developed a greater appreciation for the role of the extracellular matrix in various aspects of biology, it has been demonstrated that our intestinal flora are to some extent supported by a rich layer of polysaccharides coating our intestinal surfaces. Bollinger et al. premise a role for the appendix in the support of this biofilm on the basis of three main pieces of evidence.

1. The appendix is located at the proximal end of the colon, where the greatest density of biofilm can be found.
2. Bowel biofilm is known to be associated with mucin and IgA, and the lymphoid tissue of the appendix could abundantly produce these molecules.
3. The position of the appendix protects it from the fecal stream.

Given that biofilms benefit the host organism (us), the idea that the appendix promotes the formation of the biofilm and serves as a reservoir of friendly commensal bacteria in the event that the colon was flushed out in response to a pathogenic event (e.g. diarrhea) is an attractive attribution of function.

There are several reasons one might not find this convincing. The first of these is that biofilms are likely to be a ubiquitous feature of mammalian digestive systems. While this does not disprove the idea that the appendix promotes the formation of biofilms, it does suggest that the observed relationship between biofilm presence and the appendix might be coincidental. Similarly, success of bowel biofilms in such organisms would suggest that the abundance of lymphoid tissue in the appendix is not necessary to populate the film with the requisite immune molecules. Given the propensity for the appendix to suffer a blockage (with resulting appendicitis), one might also find proposition 3 to be tenuous, though I would welcome a rebuttal on this point from a professional gastroenterologist or anyone who studies peristalsis rigorously.

In general this hypothesis is difficult to test due to the fact that direct analogues to the human appendix in other organisms are rather rare. However, there are some experiments that can be performed, the most obvious of which is to take some animal that lacks an appendix (a carnivore of some kind?) and examine the distribution of biofilm in its colon. This would likely address whether point (1) represents a causative relationship or coincidence, and an examination of the molecular composition of the biofilms of these animals would give us good information about whether (2) is a valid reason to attribute this function to the appendix. A comparison of biofilm distribution and composition between human subjects who possess an appendix and those who lack one (at least 6% of the population, it would seem) would also indicate whether removal of the appendix deranges biofilm formation. Comparative studies of biofilm regeneration and intestinal recolonization by commensal species following diarrhea in normal and appendix-free individuals would also be of value in assessing the function of the appendix. So there are further experiments to be performed which can speak to this hypothesis.

But, let us assume for Mr. Dembski's sake that all of these experiments have taken place and yielded results which cast the best possible light on this hypothesis. Would it establish that the appendix was not vestigial? Well, no, not if you genuinely understand what vestigial means. Again, Afarensis makes some good points here, and there is also an interesting discussion of vestigiality on the appendix page at TalkOrigins. The fundamental point that you must appreciate is that vestigial does not mean useless. The word "vestigial" describes a particular kind of evolutionary history of a feature, specifically that a given physiological construct has lost the purpose it possessed in ancestral species. Vestigiality implies nothing about present function. Thus, even if it has a function in humans, the appendix would still be vestigial unless we could use it to ferment all those cellulose-laden tree leaves we eat. So it simply isn't true that the discovery of a function for the appendix means it's not vestigial—indeed, the presence or absence of a modern function doesn't speak to the vestigiality of the appendix at all.

Supposing that the hypothesis proves to be valid, it doesn't pose a real problem for evolution. Remember that the reason Bollinger et al. claim to have developed this hypothesis is to find an evolutionary rationale for the preservation of the appendix. If it turns out to provide a benefit, even in the form of a marginal improvement in biofilm robustness, then this might provide an explanation why the organ has not been lost completely. On the other hand, such an explanation is not strictly necessary; our anatomists may just be catching the appendix on the way out.

And actually, given the ubiquity of biofilms, validation of this hypothesis would represent a(nother) philosophical disaster for the intelligent design conjecture. If the appendix truly provides a significant benefit to biofilm formation, then it seems logical to ask why the designer included it in so few mammals, of such disparate kinds. Why not put it in all of them? If it's not important for that, then why put it in any? The apparently random distribution (and dissimilarity) of appendices in mammals is consistent neither with a purposeful insertion of elements known to be of benefit, nor with a purposeful trimming of elements known to be useless. Evolution, on the other hand, is perfectly at home with this kind of convergent funny business. In this sense, it doesn't matter whether a function is ever found for the appendix or not. Vestigiality aside, the existence of the appendix cannot be explained by the design conjecture if it is useless, nor can it be explained by design conjecture if it possesses a function, as this hypothesis suggests, that would be of benefit to all animals, including those that lack one.

Bollinger, R.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