What is the appendix for?

One of the most interesting fields of research for me right now is the microbiome. New research in the area comes out every day and the more we learn, the more scientists realise we don’t know.

Although not all bacteria and other microbes have yet been identified – let alone everything they do – we do now know that diversity is key to good health: the more different species there are, the healthier the individual. Hunter-gatherers have a much, much more microbiome than we do in the Western world, and we get many diseases that are unheard of in primitive cultures, e.g. Crohn’s disease.

appendix, appendicitis, microbiome, gut flora, immune system

Appendicitis - infection and inflammation of the appendix - is so common that everyone knows someone who has had it and has had an appendectomy as a result, an operation that probably saved their life.

For as long as we’ve known that this little dead end in our gut exists, doctors and researchers have been wondering what it is for. It was thought to be vestigial, like the coccyx (tailbone) or erector pili (the muscles that make hair stand on end) and body hair. In fact, if you google ‘vestigial organs’ you’ll find the appendix at the top of the list. It is thought not to do anything much, because no purpose has been found and, more than anything, people without one live on without suffering any negative consequences at all.

For other vestigial organs we can find out what there purpose might have been if we look back at our ancestors or closely related species, such as the great apes or monkeys. Our ear muscles, for example, don’t do much anymore, but monkeys use them to twist their ears in different directions, to pick up the sound of approaching danger sooner. Vestigial organs usually have shrunk and/or atrophied: they’re there, but unable to do anything anymore. When it comes to the appendix, however, it appears that apes and humans have a larger and better developed one than monkeys – which suggests that the appendix must be doing something useful.

Most people never get appendicitis, but 6% do, and that’s a lot of people. Of those who do get it, 50% survive it without an operation. Because it’s so common, appendectomy is now routine surgery. So without any medical help, 1 in every 32 people would be wiped out by appendicitis. If it was that dangerous an organ that doesn’t appear to serve any particular purpose, you would expect that over millennia evolution would have made us lose it, not promote it.

It wasn’t until 2005 that it crossed one American scientist’s mind, that perhaps the appendix was a reservoir for bacteria! In order to come up with this idea, there first had to have been knowledge of the gut flora and immunity. The appendix is filled with bacteria, antibodies and lymphatic tissue. Why?

We have more of the same all along the gut, but if we a struck down with infection and disease that causes severe diarrhoea, e. g. cholera, we will lose our gut flora and antibody protection in a very short space of time. Add to that these days enema before colonoscopy or antibiotic treatments. We’ll end up with a very clean gut indeed! But as we now know that is not a very good way to be.

The appendix appears to provide a save haven for bacteria during any such attack, and it is from here that bacteria emerge to recolonise the gut once the threat has passed. So, while we can live without an appendix, there is great benefit in having one. And by the way: appendicitis is another disease that is extremely rare in developing countries. Could the fact that we are so prone to it also have something to do with our Western lifestyle?

For further reading: Rob Dunn - The Wild Life of our Bodies, Harper Perennial, 2011.