As my last post may have suggested, I’ve recently been taking a deeper look at the large intestine – specifically the appendix. The appendix is a small, intestinal, diverticulum (basically a little pouch) that protrudes off the cecum (the first part of the large intestine, itself a little pouch- though much bigger than the appendix). You may have heard (and indeed, at the time of writing, Wikipedia has it written) that the appendix is a vestigial structure- a now useless remnant of something that was useful to our ancestors. Darwin actually helped propagate this belief, theorizing that the appendix was a shrunken remnant of a larger cecum. Furthermore, the relatively common and apparently benign surgical removal of the appendix, the procedure known as appendectomy, seems to support the idea that the appendix is of no particular use to humans today.
But is it?
There is an increasing body of information supporting the idea that the appendix is not a vestigial structure and that it has a specific role in human health. This might get a bit lengthy, so I will approach this topic in stages- probably culminating in a few posts.
First things first- is the appendix really vestigial? As I mentioned above, Darwin believed that the appendix was vestigial. He came to this idea because of the (erroneous) belief that hominids were the only primates to possess an appendix. Other primates that eat vast quantities of leaves and fibrous material that needs to be fermented by gut microflora, have large cecums where fermentation can occur. Humans, who don’t rely on copious vegetation for nutrition, only have a small cecum. It was thus hypothesized that the appendix was the shrunken remains of our forbearers’ large cecum. What Darwin was missing, however, was the fact that a number of species, including many primates, have large cecums and ALSO have an appendix. Hmm…
Another clue that the appendix is not simply the excess baggage of our herbivorous forbearers is that according to phylogenetic analysis, the appendix has actually arisen at least twice, independently, in evolutionary history. Such research also suggests that the appendix has been maintained in mammalian evolution for 80 million years [1]. To have evolved twice, independently, and to have been maintained for 80+ million years, suggests the appendix is not a useless remnant.
If the appendix is not vestigial, what is its function?
The dual evolution of the appendix, and the occurrence of an appendix in species with large cecums suggests that the organ plays an important role in normal physiology. Anatomically, the appendix is found at the end of the cecum, in a rather secluded corner of the intestines (if you can imagine such a thing). While the length of the appendix varies greatly from human to human, the diameter remains relatively constant. Another constant is the appendix’s association with a large amount of immune tissue known as GALT (gut-associated lymphoid tissue). While most people tend to think of immune tissue as ‘bacteria-fighting’ stuff, it turns out that some immune tissue produces substances (such as secretory IgA and mucin) that actually support bacterial growth, specifically the growth of biofilms.
Biofilms have been the focus of quite a bit of research recently, and usually not in a good way. Because people tend to think of biofilms (literally aggregates of bacteria embedded in self-produced slime) as pathogenic and problematic conglomerates, the focus of most research has been how best to disrupt and destroy them. It’s not entirely unwarranted either, Biofilms tend to be associated with unpleasant conditions, such as infections of medical implants and dental plaque. However, biofilms of commenselate bacteria (the ones we evolved with, on our skin and in our gut) are a way of safeguarding good bacteria.
When this is all put together, it appears that the appendix, with its relatively constant diameter and with the secreted products of GALT, is well adapted to facilitate and maintain communities of mutualistic intestinal flora [2]. It has thus been theorized that the appendix can act as a source of normal microbiota that can inoculate the gut when needed.
Why would your gut need to be inoculated with normal microbiota? Isn’t that what’s already in your gut?
It has been suggested [2, 3] that the appendix acts as a ‘safe house’ for resident microbiota when a GI infection occurs. When disease-causing bacteria are flushed from the intestines by diarrhea, the normal bacteria are eliminated as well. The appendix safe-guards a population of the normal bacteria that can then repopulate the large intestine after the diarrhea has passed. This function may not seem too important today in the developed world, where we enjoy relatively good hygiene and relatively low levels of epidemic diarrhea, but in the not too distant past and in populations that still suffer from diseases such as cholera, the appendix likely plays an important role in recovering from diarrheal diseases.
While the appendix offers benefits if you live in a developing country, it is less important (though not entirely so- I’ll get to that later) in developed countries with modern hygiene practices such as water treatment and sewage systems. In fact, in the developed world, the appendix has become a bit of a liability, with a surprisingly large portion of the population developing appendicitis at some point during their life. In my next post I’ll discuss the appendix in disease and health, and probably wax poetic about how we should consider this interesting little organ in our modern environment.
1. Smith, H.F., R.E. Fisher, M.L. Everett, A.D. Thomas, R.R. Bollinger, and W. Parker, Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix. J Evol Biol, 2009. 22(10): p. 1984-99.
2. Bollinger, R.R., A.S. Barbas, E.L. Bush, S.S. Lin, and W. Parker, Biofilms in the normal human large bowel: fact rather than fiction. Gut, 2007. 56(10): p. 1481-2.
3. Laurin, M., M.L. Everett, and W. Parker, The cecal appendix: one more immune component with a function disturbed by post-industrial culture. Anat Rec (Hoboken), 2011. 294(4): p. 567-79.
principleintopractice 
Fascinating! I eagerly await your next installment. If the appendix is a safe for our endogenous eubiotics, it must have quite the password protection (microbiota engage in constant cell-cell communication, after all). I wonder if something about the ancestrally inappropriateness of our modern diets (grains? Sugar? industrial oils?) is leading to the bad bugs breaking the password protection, infiltrating the appendix, causing the appendix to mount a huge inflammatory war only to get destroyed by the fallout in the end and bursting. Do we have any descent estimates of appendicitis rates in ancestrally appropriate populations, such as contemporary or recent historical hunter gather and forager groups? I doubt it’s the type of disease to leave a mark in the human skeletal record, so the data sets from natural populations will be severely limited. I’m sure you’re going to address these issues in your next installments.
Thanks Aaron! And yes- part 2 will definitely address these issues! Interestingly, in that 1912 paper I referenced in my last post, there was a brief mention that chimpanzees have a cecum and appendix that is very similar to ours and that in captivity, when fed a human diet and “exposed to human contagion”, appendicitis becomes very common. Much like with “humans in the wild” it’s hard to estimate the rate of appendicitis of chimpanzees in the wild, but it seems unlikely that they would be subject to great rates of the condition in their natural environment.
Victoria,
Ok, that made it!
I’m enjoying your blog and look forward to following it.
As I see you are interested in horses here is great site for equine diet and nutrition guidelines with an emphasis on reducing sugar and insulin response – (sounds familiar?). These articles would be useful for those interested in human digestion as well.
http://www.safergrass.org/articles.html There are article links within the articles – here is an example: http://www.ivis.org/advances/Ralston/hoffman/IVIS.pdf
Seems that horses have the same problems humans do – modern diets. Grains and genetically modified grasses can have simple sugars and starches that can spike equine sugar levels and insulin levels resulting in “hot” behavior episodes and in the worst case laminitis. Colic and obesity are other potential problems.
(Oddly, there is no link to the articles page within the main safergrass homepage.)
Best Regards,
Phil
Hi Phil
So sorry your comments got caught up in the spam filter. I only just realized I should go and check that folder…still figuring out this blogging thing! Thanks for the links, I’ll take a closer look later, but that’s definitely a subject I’m interested in. I try and keep my horses as ‘grass fed’ as possible, but even that can be problematic for horses that are already metabolically deranged (knock on wood mine all handle it well). I’ve heard that one of the issues is that pasture management has pushed towards grasses that are higher in sugar as they are better for fattening cattle and other livestock… Of course for horses the high sugar is problematic and potentially leads to laminitis as you mentioned.
Again, sorry your comments got caught up… I’ll make sure to keep an eye on at folder in the future! Thanks for your thought!
[…] blog for the first time, I’d recommend you go back and read the last two posts where I talk about the fallacy of the appendix as a vestigial organ and how and why this organ sometimes goes off the tracks in our modern […]
This is so interesting! I am behind on reading blogs, so I still need to read the rest of the posts, but I had a question I didn’t want to forget to ask- if the appendix can repopulate gut flora after a bout of diarrhea, do you think it could also repopulate the ‘good’ bacteria that are killed during a round of antibiotics? (I could be over-simplifying a lot here; not a expert in this stuff!) If so, it would be remarkably valuable in our antibiotic-dependent world!
Ah, yes- I should have read the rest of the posts before asking! Thank you for answering my question before you knew I asked it and for such a fantastic series. I learned a lot and will be thinking about this appendix stuff for a while! 🙂
Excellent! Glad you enjoyed the series, and also glad I answered your question, though I’ll give a bit more detail here (I’ve realized that if I start getting into detail in my posts, they rapidly become unmanageably long!).
Short answer (and as I mentioned in my final post), yes- the appendix is probably likely to keep some bacteria safe even during a course of antibiotics, but there are likely limitations. Biofilms offer bacteria protection (actually, most of the research I see on this stuff is how to DESTROY biofilms, so we can kill the bacteria (but these tend to be people studying dental plaques)), so if commensal bacteria are stashed away in our appendix in biofilms, they’re probably somewhat safe from antibiotic treatment, but they’re not immune. The degree to which bacteria are safe probably depends on the type of antibiotic used and the length of treatment. I doubt there’s much left after a 6 month course of Clarithromycin (for example). This, of course, would be something very interesting (and in my opinion very worth while!) to study.
Thanks for commenting!
That is fascinating. I really appreciate the detailed answer. I hope someone studies this further. Again, keep up the amazing work!
[…] my previous post, I discussed a paper that showed that having an appendix (and thus having a safe house for normal commensal bacteria that can repopulate your gut after infection or antibiotic treatment), is protective against a […]
[…] on to talk about some of the evo-med examples I have written about here before. First I discussed the likely role of the appendix (and why we should care) and then I talked about an alternative perspective on the etiology of […]
[…] presentation later this week (I’ve opted to talk briefly about pathology of the appendix because I am oh so fond of it!), but there is no exam at the end of the elective and the final grade is pass/fail. When I’m not […]