Why bother with the appendix?

05 May 2020

GUT MICROBES

GUT MICROBES

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Question

Zaid got in touch on the forum to ask: Why does a useless organ such as the appendix that can get infected and be potentially dangerous exist in the human body at all? How does is fit in the evolution of our species? And is it possible that humans in the future will evolve to not have an appendix?

Answer

Physiologist Sam Virtue pondered this question for us...

The idea that the appendix is something we no longer need, really comes from arguably the greatest evolutionary biologist of all time, Charles Darwin, and dates back about 150 years.

And so what Charles Darwin suggested was as we evolved as organisms from ones eating predominantly grass or leaves, to being able to eat more palatable food, we ceased to need a large caecum.

Now the caecum is a part of the gut, which is really quite big in organisms like koalas, which eat eucalyptus leaves, or horses that eat grass. And it's a location where the grass or the leaves can be broken down by bacteria, sort of a fermentation like process, to turn it into nutrients that our body can absorb.

As we evolved as primates, into apes and humans, we no longer ate things like grass. So we didn't need this capacity to essentially ferment them into usable food and our caecum shrank and just left behind a little pouch of caecum and this vestigial appendix.

And really, up until about 20 years ago, that was the view. That essentially the appendix was something that was an evolutionary relic.

However, more recently, a lot of people have got interested in our gut microbiota. So the gut microbiota are all the bacteria that live with us. There are 10 trillion of our own cells containing our own DNA in our body.

But there are actually a hundred trillion bacteria that live inside us, inside our guts, and come along with us. And these bacteria are good for us on the whole, if they're healthy, they do things like help us to make vitamins that we need like biotin. And they also enable us to absorb minerals, and they protect us against bad bacteria. And the idea started to be formed, that the appendix might actually act as a sort of a safe house for these bacteria at times when they're under attack.

So if you were to get a really nasty disease like cholera, which gives you very, very bad diarrhoea, all of your gut microbiota could be washed away. And this would leave your gut free for really nasty bacteria like Clostridium difficile.

This is a bacterium that causes colitis and can even be fatal in people, to move in. But at times when we have diseases like cholera, where a large amount of our gut's contents have been washed away, and with them, our bacteria, the appendix acts as a place where our own resident bacteria can hide and stay safe, until after all the cholera outbreak has passed. And then they can re-enter our guts and repopulate them and then give us back our nice healthy bacteria, that we like to live with.

In support of the importance of the appendix for having a function, recent research has suggested far from it being evolved away.

The appendix in animals may have evolved as many as 30 different times independently. And if something's being evolved 30 different times independently, it suggests it's pretty important. So I think it's probably unlikely humans will evolve to not have an appendix, but you pretty much can't rule anything out.

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