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In the beginning, there was nothing.
The back ends of our animal ancestors that swam the seas hundreds of millions of years ago were blank, relegating the entry and exit of all foodstuffs to a single, multipurpose hole. Animals were serially monogamous with their meals, taking food in one glob at a time, then expelling the scraps through the same hole. The appearance of the anus was momentous in animal evolution, turning a one-hole digestive sac into an open-ended tunnel.
Suddenly, animals had the luxury of downing multiple meals without needing to fuss with disposal in between. Digestive tracts lengthened and regionalized, partitioning into chambers that could extract different nutrients and host their own communities of microbes. Many creatures grew into longer and larger body forms, and started to move in new ways.
The benefits of bottoming out the gut are clear, but how the bacteria was excavated isn't because anuses are shrouded in scientific intrigue and a fair bit of squabbling. This is Unexplainable. I'm Noam Hassenfeld, proud owner of an anus.
As is Atlantic writer Katie Wu. I mean, it's kind of funny because the anus is like the ending to the digestive tract, but it's also a lot about the beginning of our story as the animals we know and love today. That was actually an excerpt of a recent article of Katie's that she read at the top, all about the extremely long history of the anus.
So pre-anus, I think you can sort of picture the primordial animal ancestor as this big blobby sack that was just moseying through the oceans, kind of like a balloon with a really, really big out hole. It's like having a parking garage. You know, there's only so many vacancies. You can't eat lunch while your breakfast is still digesting. And so that really limits your potential to extract a certain number of nutrients in one go or even in one span of time.
Enter the anus. This is an organ that basically empowered us to get more out of our food. It helped us get bigger. It probably helped us start moving in more creative ways and burn more energy because we suddenly had nutrients to spare. So we owe a lot to the anus. On this week's show, the story of the anus, one of the most important developments in the history of life on Earth. ♪♪
I think it's safe to say that we would not see the diversity of animals we see in today's world without the appearance of the anus.
What kind of diversity are we talking about here? How many anuses are out there? Oh my God, I could not even tell you. I don't think there is a hard and fast number here. I think we've had so many millions of years to kind of riff on the anus, and the anus has probably evolved multiple times independently. I've seen papers from this past year where people discovered anus
new anuses or new numbers of anuses on animals, which is just very, very cool. Do you have a favorite? I think my number one has to be the sea cucumber because this is probably the animal that has the anus that is closest to like a Swiss army knife anus. It can do everything. Like there is just nothing this anus cannot do. Okay. Little Inspector.
Like if the mouth is busy, it's a second mouth. It's also a breathing apparatus because there are all these little respiratory tubes that circle the anus and allow the sea cucumber anus to exchange gas with the water.
It can serve as a home for a parasite called a pearlfish that will swim in when the anus tries to breathe. Oh, and probably my favorite bit about the sea cucumber anus is that when you disturb a sea cucumber, you know, it can't like beat you with a club or like punch you out, but it can shoot a bunch of internal organs out at you through its anus and defend itself that way. Go, go, anus organ shooter.
So you just, you really don't want to mess with this anus. Okay, so anuses are clearly awesome. No argument there. Where does the scientific intrigue come from that you mentioned at the top? What's the big mystery here? I think probably the biggest anus mystery that is still hanging over all of us is the true origin of the anus. Part of the problem is definitely that this is like an understudied topic.
But it's also so hard to study something that must be millions and millions of years old and doesn't fossilize. Like the anus is not this bony structure that we're going to see in the fossil record. It's soft tissue that degrades super fast. There are probably all these animals that came up with anuses and then went extinct.
So to be able to pinpoint something as like the first anus, that's super tough. And I think this is something that scientists are going to be grappling with for a really long time. Okay, so what's our best guess for where the anus came from? The short answer is we have no idea, but there's at least a couple competing theories.
One of the oldest ones is mouth and anus kind of appeared simultaneously. So remember that we're starting with a single hole that functions as both a mouth and an anus. Some researchers jokingly call it a manus, which I really enjoy. A manus? One idea is that you've got this dual function oval that suddenly pinches in in the middle. And now you have a mouth and an anus that have formed side by side out of the same starting hole. So one hole becomes two holes? Yeah, one hole becomes two holes.
And then those two holes maybe kind of migrate to different ends of the body and voila, you have a mouth and an anus. Okay, besides the obvious one, does this theory have any major holes? Well, if the mouth and the anus grew out of the same original hole, you might expect the same genes to be expressed around them, like in the rim that surrounds each of them.
That's not the case in a lot of animals that seem to have like a pretty old body plan. Instead, they express different genes around each of their holes, suggesting that they might have appeared at different points in evolutionary time. Okay, that seems like enough reason for another theory.
What's another way anuses could have evolved? The other theory is the one that seems to have more support behind it, or at least more of the people that I talked to seem to like this idea better. But you're again starting with a sac-like organism with a single hole at one end. And it looks a bit like a cave. And basically that cave tunnels through until it breaks out the other end and you have a fully fledged tunnel. It's a tube now. So...
It's almost like you're starting with something that has a mouth that blows an anus off the other side. And you said this is the theory that seems to have more support, right? Is there any evidence in the animal kingdom that creatures can, like, generate their own anuses? Yeah, yeah, yeah. It's actually not that hard to form a random hole in your body. Oh, okay. There are actually multiple animals. A lot of them are just random worms that have formed, like,
dozens of anuses over evolutionary time. They have such branched digestive tracts, they look almost like root systems, and each of them terminates an anus, which is just absolutely bonkers. So they can do this almost at will. Like, let's make another anus. That's totally chill.
You did mention that, you know, the evolution of the anus has probably happened multiple times. Could both of these theories be true? Yes and no. I think they probably are both true in the sense that both have likely happened at some point in evolution writ large. But I think one of the biggest questions is, what was the first one that happened? And what was the one that persisted over millions of years and sort of begat the lineage that led to us?
How did the first and most enduring anuses form? Clearly, our ancestors had something going for them that really worked long term. And that's what we stuck with. Yeah. And is there a way to figure out which one of these is more likely as the origin of our anus? Kind of. I mean, I think we will keep converging on an answer. But you
you know, there's no bone that sort of structures the anus and makes it possible to say like, aha, this was clearly like a cave to tunnel situation, or this was a whole splitting situation. But I think one way to do this is just to keep doing comparative studies between species that we think have looked the same pretty much since they evolved. So like certain types of worms or maybe certain types of reptiles or things that, you know, we often consider as more
quote unquote, primitive just because they haven't had a reason to change their body plan since. So a lot of people actually call animals like these, you know, living fossils because they represent what their ancestral forms might have looked like. And maybe it's the closest thing we can get to jumping in a time machine and looking at what the sort of OG anus looked like. Is it possible that we'll just never know the true origin of the human anus?
I am going to be not an anus cynic and say that I'm optimistic that we're going to find our lineage's OG anus, or at least evidence for it, and at least clarify the timeline. I think when we're operating on the scale of hundreds of millions of years, it's not going to be like November 18th, like 400 million years ago BC. Right. But we can at least bracket it, I think, within a few million years, and that's already pretty amazing. Yeah.
Across the animal kingdom, anuses have this fascinating history. But when it comes to specifically human anuses, the story doesn't really measure up. I will say, like, the human anus, super boring, doesn't do much. It poops. That's pretty much it. But unlike so many other animals, humans do have butts. Our butts look like no other butts out there, and I think that is very, very cool. Coming up next, why our butts?
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This is Kara Swisher, host of the podcast On with Kara Swisher from New York Magazine and the Vox Media Podcast Network. It has been a week in Chicago at the Democratic National Convention. I've been zipping around the convention hall, including getting my PivotPod co-host, Scott Galloway, out of Secret Service prison. But I also talked to a bunch of very sharp folks. And of course, I wrangled some of the smartest ones for both podcasts while I was at it.
And who better than David Axelrod, one of the most famous political strategists and also a seasoned convention veteran to help us digest it. I'm convention rich in experience here. I sat down with Ax to talk about the energy, the messaging and the strategy behind it all. The same people who a few weeks ago were mad at me because I was suggesting the president, Biden should get out of the race, are now mad at me because...
they think it's a terrible thing to say, don't be irrationally exuberant. David is always a great conversation and you can tune in in full wherever you get your podcasts. Be sure to follow on with Kara Swisher for more insightful election coverage.
My anus is bleeding. I like big butts, butts, butts, butts, butts. Shake that nasty butt. You touched the butt. You shut your butt. No, you shut your butt. My butt is shut. Unexplainable, we're back, and we're talking about butts. I'm Heather Radke, and I'm writing a book about butts. Do you have a working title for your book, by the way? Yeah, the book is called Butts.
Okay, okay. So what's it like to be writing a book on butts? It's endlessly fascinating. You get science, you get culture, you get histories of race and gender, you get all of it. And is it true that butts are like this uniquely human thing? Yeah. Only humans have butts. Seriously? I mean, you know, of course there's like a joint in a monkey or a chimpanzee or a dog or whatever. Like in a sense, those animals have a butt, but they don't have like
A big gluteus maximus. I mean, that's like a uniquely human thing. And the amount of fat also is a uniquely human thing. Okay, so when we're talking about a butt, we're talking about this uniquely human, muscly, fatty thing we've all got on our backsides. So is the evolution of butts any more settled than the evolution of the anus that we were talking about in the first half? Like, do we know why butts are shaped the way they are?
Well, there's kind of two different parts of the but. One is relatively settled and the other part is quite up for debate.
Okay, what's the part of the butt that we understand pretty well? The muscle and bone part is pretty well studied, and those muscles are big. They're the biggest muscles in your body. And a man named Dr. Daniel Lieberman, he did some really important studies in the 90s and 2000s. They were really studies about running. His theory was that humans developed a set of adaptations that made them excellent long-distance runners. And one of the anatomical adaptations he looked at was
the muscles and bones of the butt. So a human can run slow and long, an antelope can run fast and short, and over a long period of time, slow and long will win because the fast and short animal will get tired out and eventually they'll have to stop. So that allowed them to basically like run down an animal.
So our big butt muscle was sort of essential for this kind of long-distance running? Well, I mean, to be fair to the rest of our bodies, it's not just our butts. But the butt is one adaptation amongst many that helped us many millions of years ago to run and hunt and get enough calories to support another evolutionary adaptation that was critically important, which was like the adaptation of the bigger brain. So we've got a pretty solid idea of why the butt muscle is so big.
What's the other part that's more up for debate? The other half is the fat, and it's much more complicated. Okay. Or at least it's much less known, I should say. Is that because of the same thing that Katie was talking about? Like soft tissue stuff doesn't really leave a fossil record? Yeah. So a bone leaves a fossil, a muscle connects to a bone, and we can learn what those muscles look like based on the marks they leave on the bone. But there's no fossil record of fat, just like there's no fossil record of hair or...
skin or whatever. So what do we know about how much fat we all have? Well, so first of all, humans have one of the highest percentages of body fat on the planet. The brain requires so many calories. So humans need more fat than other animals because we have bigger brains. But why does the fat end up getting stored in the butt? So this is a really good question. I mean, one scientist said to me, like,
It doesn't have to be. Like, you could just have, like, giant globules of fat on your elbows or something. Okay. Which I always thought was a funny image. It's like, oh, right. Yeah. You know, I mean, I think the most straightforward answer to that question is, like, your center of gravity is around your waist. And so it's a sort of convenient place to store fat because...
It doesn't mess up other kinds of locomotion. Like you could imagine if you really did have big fat globules on your knees or something, like it might make it so the running gets a little harder again. Okay. Simple enough. Right. But there's a lot of other theories about why our butts look the way they do that are much more complicated and have a lot more to do with kind of like some difficult and problematic science. Yeah.
Is there an evolutionary explanation as to why we love the booty? Females with a curvier figure are better able to compete to attract high quality mates. Men should have a psychological adaptation to prefer these women as mating partners.
There's a lot of people that think that it's a sort of sexual selection pressure that makes butts look the way they do. So I guess the idea here is that humans find big butts attractive. So more big butt people are having sex and having kids. And that's why humans have so much fat in their butts. Are there studies to back this up? Well, okay. There have been several evolutionary psychology studies where they do things like...
show pictures of women with different size butts to men and ask, like, which of these do you find more attractive? The pictures are kind of wild, actually. They're like these cartoon images of blonde white women in, like, tight black dresses, and they're angling their butts at different angles. And this is a theory that is basically, like,
A bigger butt looks like a woman who's sort of holding her body at a certain angle and that angle would be more efficient for pregnancy while simultaneously hunting and gathering on the savannah, which would be adaptive in ancient times.
I assume this is some of the problematic sides, right? Oh, yeah. Because, I mean, doesn't all of this have to be significantly influenced by culture? Exactly. And like what constitutes a good butt changes profoundly over time. It just sort of feels like the fact that the butt or at least the fat part of the butt is so inherently hard to figure out is...
It feels like this convenient excuse to take a certain element of culture and say, like, this is the way it has to be. It was always supposed to be this way. Yeah. One of the lessons of the butt is how much we kind of project onto it. And...
I think the history of science, particularly in the 19th century, really shows us how much questions of the butt became questions of race. Scientists were looking for ways to categorize humans racially, and one of the categories they used was race.
size of butt and they equated big buttedness with African women and hypersexuality and there was one woman in particular Sarah Bartman who was brought up from South Africa and displayed on a stage in 1810 in London and people would poke her in the butt and
She had died and a famous scientist in the 19th century conducted an autopsy on her body that was very much against her will. And the results of that autopsy were used for, you know, the next hundred and some years as evidence.
Evidence of how Black people and Black women were different and less than white people and white women. Yeah, it just reminds me of all these other ways throughout history that scientists have really misused science to try to say who gets to count as human when ultimately it's really just about their own prejudices. I think one thing I've come to about science of bodies is
It's such an understandable question, but I think now when somebody says to me, why do butts look the way they do? There's a part of me that wants to say, why do you want to know? What is so important about having that question answered? It's both extremely understandable, but it's so caught up with questions of race and gender that you'll never untie them. They cannot be untied. Hmm.
So overall, we've got a pretty clear idea about why we have this big butt muscle. We understand pretty well why a lot of fat is necessary for humans. And we even have some plausible reasons for why that fat would end up being stored in the butt. And then it's like that last 1%, the social, sexual, historical part that's maybe the most difficult to figure out and also the part that should probably make us tread most carefully.
I think that's right. And I really think for the science part of it, there's a real lesson in the kind of social reality of scientific study. Like, there's a racial way that most people think about the butt. There's a gendered way most people think about the butt. And so in order to do any science around the way a butt looks, you'd have to really take in fully those complicated things.
cultural ideas because you would carry them with you into any study you did. But that last 1%, it's a pretty interesting lens we can use to view ourselves, right? Yeah, it's a super interesting... I think it's such an interesting question because it's not actually a question about science. It's a question about the person asking the question. It's a question about
social realities, historical constructions, and what bodies mean. And what bodies mean in our world are questions about race and gender more than they're questions about evolutionary biology.
I mean, it makes a lot of sense what you said at the top, that butts make us unique as humans. I mean, in a number of ways, they make us unique as humans. They allow us to do our ancient man running down the wildebeest stuff. And all of the metaphors and symbolism we've put onto the butt. I don't know if it makes us human, but I do think it's a very human thing that we do to the butt. Heather Radke's book on butts will be out next year, so keep an eye out for it.
Heather's also a contributing editor at Radiolab. And if you're interested in the power of the butt muscle, in particular, she reported a great episode for them called Man Against Horse. You can also find Katie Wu's article on anuses over at The Atlantic. It's called The Body's Most Embarrassing Organ is an Evolutionary Marvel.
This episode was produced and edited by Noam Hassenfeld and me, Meredith Hodnaught, with help from Daniel Gross, Brian Resnick, and Jillian Weinberger. Music from Noam and the Music Library APM. Mixing and sound design by Christian Ayala and Afim Shapiro, with fact-checking from Mandy Nguyen. Bird Pinkerton recently got super into algae and salamanders, so, uh, yeah, stay tuned on that one. And Lauren Katz heads up our newsletter,
Liz Kelly Nelson is the VP of Vox Audio. You can sign up for our newsletter, read our articles, and find show notes at vox.com slash unexplainable. Unexplainable is part of the Vox Media Podcast Network, and we'll be back in your feeds next Wednesday. Have a good week. Where was I?
Bye.