Short Wave - Behold! The Anus: An Evolutionary Marvel

Episode Date: June 21, 2021

The anus is an evolutionary marvel. But how and when did this organ evolve into what it is today? Today on Short Wave, Maddie gets to the bottom of these questions with The Atlantic's science writer Katherine Wu. For more of Katherine's reporting, check out 'The Body's Most Embarrassing Organ Is an Evolutionary Marvel' from The Atlantic. If you have stories ideas or comments — email us at [e-mail] pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. To peer into the soul of a sea cucumber, don't look to its face. It doesn't have one. Gently turn that blobby body around and gaze deep into its marvelous, multifunctional anus. This is Catherine Wu, a science writer for the Atlantic. The sea cucumber's posterior is so much more than an exit hole for digestive waste. It is also a makeshift mouth that's a science writer. gobbles up bits of algae, a faux lung latticed with tubes that exchange gas with the surrounding

water, and a weapon that in the presence of danger can launch a sticky, stringy web of internal organs to entangled predators. And if you haven't guessed already, this episode, yes, is about the anus, because the anus is an evolutionary marvel. An anus of a sea cucumber is a spectacular example. It can even, on occasion, be a home for shimmering pearlfish, which wriggle inside the bum when it billows open to breathe. It would not be inaccurate to describe a sea cucumber as an extraordinary anus that just so happens to have a body around it. Now, the anus doesn't get as much respect as it deserves, partially because of our human taboos around them. Or, as Katie puts it, we've turned anuses across the tree of life into cultural underdogs, and scientific ones. too. I imagine it's a little weird for some of these people to like, you know, go into a poster

session and just be like, hey, do you want to look at pictures of all these anuses I've been looking at in my lab? There's this huge taboo around it just in like general pop culture. And I think that definitely does bleed into science. I hope one day, like 50 years from now, maybe sooner, we will have just a ton of butt cons or anus cons around the world. But we're really not there yet. So today on the show, we give anuses the attention they deserve. We'll explore the fiery debate over how and when exactly the first anus arose in evolutionary time, and we'll talk about some of the most remarkable anuses on the planet. I'm Maddie Safaya, and this is Shortwave, the Daily Science podcast from NPR. Okay, so we are here with Katie Wu, a writer for

the Atlantic who wrote a magnificent piece on the evolution of buttholes. All right, Katie. So let's start at the start. So not all of our evolutionary ancestors had anuses, right? Can you tell me about some of those ancestors that came before us that didn't have buttholes and how they dealt with life? Yeah. I think what is really beautiful about this story is that we still see relics of those one whole ancestors around today. Not all animals have anuses, which also means by the transitive property that not everyone poops, despite what you might have read. So, you know, a long, long, long time ago, there was really just one hole. Our ancestors were these floating sacks just musying about through the ocean. And they had what you might call a manus, a mouth anus combo that was just one hole.

You know, food went in. It got digested. Nutrients were distributed. throughout this blobby body, and then it had to exit through the same structure. It was kind of like a parking garage, you know, that one exit entrance. Right, right. But that really, like, slowed things down. It meant, you know, you couldn't eat lunch until breakfast was finished being digested. Right. And then the evolution of anus's allowed animals to do all new kinds of stuff, right?

It wasn't a parking garage anymore. I mean, my favorite thing, anuses allow you to, you know, suddenly eat multiple meals without having to get rid of waste. What else does an anus allow you to do? I mean, the anus is kind of amazing. You basically turn this cul-de-sac into a highway, which means that a bunch of stuff can transit through at the same time. So exactly as you pointed out, you no longer have to stagger out your meal so that, you know, you vacate the digestive tract before you fill it up again. It also means that like everything just gets speeded up and more sophisticated. If you turn, you know, a bowl into a tube. You can start specializing along that tube and have like a complex stomach and

put some, you know, weird microbes in different spots and have different parts of the digestive tract doing different things. You get better at extracting nutrients, which means you can grow bigger and maybe move around a little bit more. It just turns you into a better functioning digestive machine. Right, right. So that's why anuses are great for business. We know that, Katie. But we don't know that much about how they came about, like how we went from one all-purpose hole into two holes, a mouth and an anus, right? Yeah, so I think there are a couple different theories worth mentioning here. I think the idea is, do you preserve that initial hole and just make a second one, or do you

somehow warp that first hole into two? So in the first scenario, you know, picture you're digging a hole and you keep tunneling and you keep tunneling and you keep tunneling until finally you break through to the other side. that's kind of the anus being the second hole to appear idea. And I think it's the one that most scientists are behind right now. The other is that, you know, you preserve that initial hole. There's not really a whole ton of tunneling, but the center of that hole caves in in the middle.

And all of a sudden, you have two holes side by side that eventually kind of migrate further apart. Okay, okay. So we're not entirely sure if it's a splitting in two or a tunneling scenario. but we also aren't sure of when it happened, like in evolutionary time, right? I mean, why is it so hard to know that, Katie? Yeah, I think there are a couple things that are tricky here. One is that, you know, anuses are these soft tissues.

They don't fossilize really well. And so it's kind of hard to do a survey from hundreds of millions of years ago and say, oh, we definitely know the anus appeared here and it stuck around. The other problem is anus has probably appeared and disappeared multiple times, which means, you know, we could find a super ancient anus, but we can't guarantee that, you know, it persisted in all the descendants of that ancient anus. It could just be like something random that fizzled out after a while. It could be, you know, something that just looks entirely unlike what we have today. I think we're probably going to find that there

are multiple origin stories for the anus, and the trick will be figuring out which one is most closely connected to us. Okay, okay, so regardless of how or when, I think you and I can both agree that there are some marvelous anuses in the animal kingdom. And you wrote about a couple of them. So if it's okay with you, I'd like to spend the rest of our time talking about some of our favorite examples. I can start us off. Does this sound good? Absolutely.

Okay. So you wrote about the dragonfly anus and that how young dragonflies suck water into their anus and then spew it out to propel themselves forward. And I don't know why, but I just, I love an anus with some locomotion. You know what I mean? I really do too. And this is just like a really green way to get around. You know, it's hydropowered. Like, I mean, they can refuel like anywhere.

And can you imagine what these insects could do with a bidet? Like, I just. I get it. I couldn't. But now that's the only thing I'll imagine when I look at a dragonfly. Thank you, Katie. All right. Give you some of yours.

Give you some yours. Okay. So we have to talk about. turtles, which are amazing because they can breathe through their buttholes, which just makes a lot of sense. They are covered in shells. They basically have armor all over their body. It gets stuffy in there. And speaking of like really gassy stuff, there are lacewing larvae that can incapacitate termites by shooting death farts out of their anuses. And I mean, it's just like weaponizing the anus is a really beautiful thing to do. What do you mean death farts?

Say more, please. So basically, these larvae are kind of like terrifying little predators. They love to eat termites. And so they will invade their nests. And, you know, they don't have a ton of weapons. Like, they don't bring guns or swords in there. So what are they going to use? But like the gas that comes out of their butts.

So they have this toxic flagellants that just bursts out their ends. And the termites just are completely incapacitated. They're like knocked out. They can't move. And they are ripe for control. assumption after that point. Wow. Okay, Catherine, that was magical. And for real, though, it is amazing what you can learn when you put aside these taboos and just marvel at our bodies and in this case, the anus. You know what I mean? Yeah, absolutely. I mean, I think in writing this piece,

I myself had to get more comfortable with saying the word anus. And it was unfortunate because my partner was working at home during this time, and I hadn't warned him what I was going to be interviewing people about. But I think we're just both very comfortable with a lot of things now. We've reached like a new level of cultural intimacy with this part of our bodies. And it's wonderful. Absolutely. Absolutely. Okay, Catherine Wu, this was amazing. I really appreciate you. And that you decided like, yeah, the people need a piece about anuses because we did. We are the people. Thank you. Thank you so much for helping to spread the good word on anuses. This episode was produced by Thomas Liu, edited by Viet Le and fact-checked by Indy Kara.

The audio engineer for this episode was Marcia Caldwell. I'm Maddie Safaya. This is Shortwave.