How did the appendix evolve?

So, what exactly is the appendix; where is it; how did it evolve, and when, and what does it do? To find out more, we put in a call to Heather Smith, professor of anatomy at Midwestern University in Glendale and the editor-in-chief of The Anatomical Record…

Heather - My name's Heather Smith. I'm a professor of anatomy at Midwestern University. I'm the editor-in-chief of The Anatomical Record, which is an official journal of the American Association for Anatomy. The appendix is essentially a little blind pouch. It's what anatomists call a diverticulum, and it projects off part of the large intestine, the colon, called the caecum. And in humans it's referred to as vermiform in shape, which just means worm-like. So it's a little cylindrical tube about a centimetre in diameter, and it's found in the lower right hand quadrant, lower right hand part, of the abdomen. And then you can actually locate the position of your appendix roughly at a position that's called McBurney's point. So if you draw a line between your navel, your belly button, and the point of your hip that kind of sticks out on the right side, anatomists call it your anterior superior iliac spine. About two thirds of the way along that line is about where your appendix will lie. So in cases of suspected appendicitis, it's common that the doctor will apply pressure at that point. And if the patient responds with sensitivity or pain, it's suggestive that they might have appendicitis. And in the old days before laparoscopic surgery, this was the location that surgeons would use to kind of guide their surgical incision.

Chris - And do all animals have one or is it uniquely a human trait?

Heather - Interestingly, not all animals have an appendix, but it's also not unique to humans. So we see appendices in the clade that's referred to as euarchontoglires, which just means primates, rodents, rabbits. But we don't see it in species like carnivores or ungulates, which are just hoofed mammals. So things like deer and horses. But interestingly, the appendix can take different shapes, so it doesn't always look like our vermiform type. So in primates we tend to see this, this vermiform little worm-like type like ours, a little cylinder. But in things like rabbits, we tend to see this very elongated caecum, which is part of the large intestine, that just kind of tapers into an appendix. And in things like marsupials and monotremes, it's straight off the large intestine in the absence of a caecum. And there are even some unusual species like the Florida manatee that have two appendices.

Chris - And do any humans have two appendices and why do manatees need more than one?

Heather - That is a very good question, and I don't know that we know the answer to that. So manatees have two little strange projections, but whether they function like our appendix is unclear. Humans can do okay without it. I mean, it's definitely more beneficial to have an appendix, but those of us who have had ours removed know that we can function okay without it.

Chris - And what do we think it does? Because if it's evolved multiple times, has it evolved each time to do the same job? And what is that job or does it do different jobs in different animals?

Heather - So my research team compiled data from the published literature on the distribution of appendices across the mammal tree of life. And we found that the appendix has evolved independently about 30 different times throughout the course of mammalian evolution. And this is in different groups that have different shaped appendices. So it seems unlikely that the appendix is doing the same thing in all of those different species because the appendix takes a different shape and a different size. So it's probably doing something different in different species. What we know in humans is that the appendix has a healthy layer of bacteria called a biofilm. And so it seems likely that the purpose of the appendix in humans is partly as a reservoir of this good bacteria. So during times of gastrointestinal distress, when all the good healthy gut bacteria gets kind of flushed out of the body, the appendix is a little safe house that kind of preserves the good bacteria of the gut and allows it to kind of repopulate the gut. After the episode, it also contains what we call lymphoid tissue, which is just a type of immune tissue. So it helps the body to kind of mount its immune response against invading pathogens. But whether it does that same function in all mammals is not really well understood.

Chris - And in people who end up with a problem with their appendix, do we know why that happens? Is it just bad luck or are there any kind of strong pointers towards what's happening in people that end up with this, and by and large end up having to have it removed?

Heather - There are several different causes of appendicitis, but they all relate back to the appendix becoming inflamed. And it typically happens because the lumen, the diameter, the hole inside the appendix, is relatively narrow. And so it's very easy for things like bits of food, what we call digesta, to get stuck in the appendix. And that causes a lack of blood flow to the area and therefore inflammation.