Field of Science

A function for the appendix? (sloppy science, slick spin)

A paper proposing a function for the human appendix is all over the news. The paper itself is in Journal of Theoretical Biology (Articles in Press). I'd post a copy of the pdf but the journal is published by Elsevier and they'd probably put me in jail, so instead here's the Abstract
The human vermiform (“worm-like”) appendix is a 5 to 10 cm long and 0.5 to 1 cm wide pouch that extends from the cecum of the large bowel. The architecture of the human appendix is unique among mammals, and few mammals other than humans have an appendix at all. The function of the human appendix has long been a matter of debate, with the structure often considered to be a vestige of evolutionary development despite evidence to the contrary based on comparative primate anatomy. The appendix is thought to have some immune function based on its association with substantial lymphatic tissue, although the specific nature of that putative function is unknown. Based (a) on a recently acquired understanding of immune-mediated biofilm formation by commensal bacteria in the mammalian gut, (b) on biofilm distribution in the large bowel, (c) the association of lymphoid tissue with the appendix, (d) the potential for biofilms to protect and support colonization by commensal bacteria, and (e) on the architecture of the human bowel, we propose that 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.
Their points (a) to (e) are fine, but the conclusion depends on the totally unjustified assumption that a severe bout of diarrhoea eliminates commensal bacteria from the colon. Of course diarrhoea can reduce the number of bacteria in the colon, and cholera is likely to disrupt its epithelial biofilms. But even the most severe diarrhoeal infections are very unlikely to sterilize it, largely because there will be so many microenvironments within the colon that retain at least part of their biofilm. Biofilms are notoriously difficult to remove and to sterilize.

Other issues:
  • What about animals that don't have an appendix? Only humans and anthropoid apes have appendices, but they are certainly not the only animals that get diarrhoeal infections. The authors argue that humans in Western cultures don't need their appendices because they're so rarely at risk of contracting the severe diarrhoeal diseases that the authors postulate necessitate reinoculation from the appendix. But animals are much more like humans in primitive cultures than like affluent humans.
  • Some of the news reports (but not the paper itself) suggest that humans in primitive cultures need their appendix for reinoculation because populations are sparse and reinoculation from other people would not be reliable. But humans always live in social groups, and one thing severe diarrhoea does is spread intestinal bacteria around the environment, making reinoculation even more likely that it would otherwise be.


  1. I would have thought that the bacteria that are distributed in the local environment during an outbreak of any serious bacterial infection of the intestine would be the bad bacteria of the disease itself that the body is obviously removing by passing diarrhea rather than normal faeces. These infections of bad bacteria are exactly what the postulated function of the appendix would help address.
    So the body removes, on mass, via diarrhea, as much of the infection as possible, and repopulates the intestines with good bacteria from the appendix. Sounds perfectly reasonable to me, and fits perfectly with several of my patient histories.
    Two brothers, for example. Both with appendixes removed at ages ten and nine; both with various unexplained (by conventional medicine) intestinal issues; no family history of such issues.
    What we need is much more research to back up or disprove the theory.

  2. Coul you tell me what is the immunological function of the appendix??

  3. Comments such as " they are not trained in evolutionary biology" (found at a wbsite containing a long article on why the appendix most certainly IS NOT reqired in this age of antibiotics!), and "I run a microbiology laborary" that I found here, seem more likely to have been written by people more interested in protecting their turf rather than people seeking truth.

  4. We did not assume that diarrheal illness wipes out all of the bacteria...only that the bacterial count is diminished. A diminished bacterial count would create room for opportunistic pathogens, such as C. diff. In addition, we have managed to deal with the concerns regarding the phylogenetic distribution is a widely read paper in the Journal of Evolutionary Biology (vol 22, pages 1984-1999). Finally, I think it is probably important to keep in mind the fact that epidemiologic studies in developing countries tell us that infection of the GI tract is most serious in young children. With this in mind, we should probably not dismiss the apparent function of the appendix without considering the condition of children in developing countries.


Markup Key:
- <b>bold</b> = bold
- <i>italic</i> = italic
- <a href="">FoS</a> = FoS