Savannah: After you watch this SciShow compilation, you can find more butt facts on the SciShow Tangents podcast. It's available on all podcast platforms.

You've had a butt your whole life, but you still might not be using it right. I know your butt might *seem* intuitive because, I mean, it comes free and ready to use, no assembly required. But there are multiple ways to do all of the things that a butt does, and some are better than others. So, I'm bringing you the user guide for your butt.

[intro]

If you're watching this video, you probably have a butt. But why? If the only reason were to empty your bowels, there wouldn't be animals that can do that without butts, and there are. So, Olivia's going to talk about those guys and all the weird and wonderful butts there are in the world, and then Michael will fill in why human evolution *didn't* bypass the butt.

[slide: "The True Origin Story of Butts and Poop"]

Olivia: As weird and wacky as our fellow animals get, there's one thing that we pretty much all have in common - we have to eat. And if you have to eat, you also need to release waste. So many animals share a very useful, very familiar feature: a butt. Technically an "anus," but the word "butt" is hilarious. And while the function of that rear exit is pretty straightforward, the evolution of butts is surprisingly complicated and confusing.

When you get right down to it, many of us animals are basically tubes with an opening at either end: a mouth for taking in food and an anus for releasing the unwanted byproducts of digestion. This two-ended tube arrangement is called an "alimentary canal" or a "through-gut," and it's common among animals that belong to the vast group of bilateria. This includes most animals that are symmetrical down the middle, such as worms, spiders, fish, and humans.

But not all animals work this way. Sponges, for example, have no true mouth, gut, or anus at all; they absorb food particles from the water directly into the cells of their bodies. Jellyfish and anemones, on the other hand, have a functioning mouth and anus, but they're the same opening - a mouth-butt. In these animals, the mouth leads directly into a sack-like gut, and then waste is spit back out when -

- digestion is done. 

But there are some serious benefits to having a through-gut like we do. For one thing, the alimentary canal can develop into specialized regions to serve different purposes. Your alimentary canal, for example, includes several organs with specific functions: your stomach's job is to start breaking down your food down into a liquidy mess, and your large intestine reabsorbs fluids and prepares waste for expulsion, and the whole process is helped along by other organs, like the liver and the pancreas. The through-gut can also be expanded and adjusted to fit bodies of all shapes and sizes, from worms to whales. Just imagine a blue whale with a mouth-butt; its meal would have to make a ridiculous round-trip to drop off nutrients and then come back out. 

And very importantly, animals with a through-gut can carry more than one meal at a time. An anemone with a sac has to finish digesting and relieving itself before it can take in new food, but you can happily eat your dinner while lunch is still working its way along. Scientists suspect that the familiar through-gut and anus first evolved in the earliest bilaterian animals and were later inherited by their descendants - like us.

But trying to pin down exactly when and how true anuses evolved is tricky, since the evolution of butts has been a bit wacky. For example, you'll find a separate mouth and anus in starfish and sea urchins, but their close cousins - the brittle stars - have lost their anuses somewhere along the way. They've independently evolved a sac-like gut like an anemone, using their mouth for both input and output. Most flatworms have similarly lost their anuses, but one group in particular - the tapeworms - have gone even further and lost their *entire* digestive tract. No gut, no butt. These parasites don't digest their own food; instead, they sit inside their host's alimentary canal and absorb all the predigested food around them.

And then, some animals have evolved entirely new butts. The flatworm genus Thysanozoon has uniquely evolved numerous waste holes across the top of its body. It's like a back covered in butts. Examples like these suggest that butts have come and gone -

- come and gone many times in animal evolution. This leaves scientists wondering exactly how an animal goes from being buttless to boasting a a functional behind.

To find out, they investigated some of our strangest relatives, such as acoel worms; these microscopic worms can be found in oceans all over the world. They're thought to be extremely primitive members of the bilaterian group, but unlike us, they don't have a true respiratory or circulatory system, and they also don't have a proper gut or anus. What they *do* have on their rear end is a gonopore, an opening which males use to release sperm.

Research into developmental genes - that is, the genes that control the growth and organization of the body - has found that the expression of these genes in the acoel gonopore is similar to what they see in other animals' anuses. This makes scientists wonder if somewhere in our early, worm-like ancestors the gut became linked up with the reproductive opening and - voila - a butt was born. More specifically, a cloaca: an opening that serves dual purposes releasing waste and reproductive material. Cloacas are standard among many modern animals, including fish, amphibians, birds, and reptiles. But that's not the only possible path to a butt; another intriguing option is seen in different groups of marine life - comb jellies.

These are another very ancient branch of the animal family tree, and despite their similar name and lifestyle, comb jellies aren't very closely related to jellyfish. These gelatinous creatures paddle their way around the oceans with oar-like structures known as "combs." For a long time, scientists though comb jellies had a mouth-butt like jellyfish do, but in 2016, researchers caught the jellies on a video pooping out of tiny pores far from their mouths. This was big news at the time because it demonstrated how comb jellies use the full compliment of mouth, gut, and butt - a through-gut.

But a 2019 study revealed that the situation is a bit more complicated. Researchers looked at a species called the "warty comb jelly," placing individuals under a microscope to watch them eat, digest, and poop.

- digest and poop. And they found that as much as 90% of the time warty comb jellies don't have an anus. But as they eat and their guts fill with waste, a tiny hole eventually opens through the body wall, spits out the poop, and then closes back up. The scientists described this as a "transient anus," a butt that only appears when it's needed. 

Not only is this amazing and bizarre, it's another potential clue to the origin of butts. Perhaps our distant ancestors, before evolving true anuses like we have now, went through a transition period with a sometimes-butt, like the warty comb jellies. Or this might be something unique to that species. The researchers have investigated other comb jelly species, and so far, they've only seen permanent anuses. 

In the end, there might not be a simple answer to the question of where our butts came from. Scientists are hopeful that more detailed genetic and anatomical studies will help them sort out the relationships between the diversity of guts and the anuses we see in living species. What's certain is that somewhere in your distant ancestry, some simple creature evolved a simple through-gut, and it's thanks to them that you don't have to poop out of your mouth.

[slide: "Why Do Humans Have Butts?"]

Michael: Why...do we have butts? It might seem like a cheeky question, but if you look around the animal kingdom, even at our closest relatives, big butts are pretty uniquely human. So why would evolution grant us such an asset? We have a few hypotheses.

So, our butts are made primarily of two things - fat and muscle - and each likely has its own evolutionary role to play. Like, in general, fat is a resource that's useful for our brains as they develop. A 2016 study proposed that development for both our large brains and our fat stores coincided with the time our ancestors came down from the trees. See, it takes less energy to transport all that fat when you're walking versus when you're climbing. So by walking on two legs, we could carry more fat, allowing us to develop and power a bigger brain, or so the hypothesis goes. 

There's still plenty we don't know about the evolution of fat and bum fat -

- bum fat in particular. It's tough to track because it isn't preserved very well in fossils. However, we're in slightly firmer territory with the muscle part of all this - the gluteus maximus. It's the biggest muscle in our bodies, which is only the case for humans, not our ape relatives. We're also the only primates that habitually walk on two legs, so for nearly 200 years, scientists have thought that these two observations are probably related.

Now, your gluteus maximus isn't actually used very much when you walk on flat ground, but it sure is used when you run; as your heel hits the ground with each stride, your body angles forward, and it's that big gluteus maximum that's partly responsible for stabilizing your body and pulling it back. Like fat, muscles aren't usually preserved in fossils, but the bones they attach to are. And the functions of bones and muscles are closely related, so we can make inferences about the evolution of one from the other.

We know that of our various ancestors, Homo erectus had a pretty human-looking pelvis around 2 million years ago, which could mean a human-looking butt. This came along with a bunch of other features that make us good runners: things like long Achilles tendons, short toes, and long legs. Humans are the only primates that can run long distances, and scientists think that distance running helped us evolve new strategies for obtaining food - by chasing prey to exhaustion or by quickly getting to carcasses to scavenge them. So, butts would've been a key advantage for hunting.

In support of this idea, the evolution of the butt roughly corresponds to a time anthropologists think our diet increased in protein, possibly because we were better at running after our food. And in turn, all that protein was, once again, helpful for developing those big brains of ours. 

So the next time you're out for a run or even thinking through a tough problem, remember - thank your butt. It's a big part of being human.

[slide]

Savannah: Your butt does so much more than poop, but I mean, it's also incredibly valuable for that purpose. Since you spend a good chunk of your life pooping, here is how to do it better.

[slide: "What's the Best Position for Pooping?"]

Michael: Okay, Internet. You asked, so now, we're answering.

Pooping. It's a thing people do all the time, normally somewhere between three times a day and once every three days. And you may have heard -

- may have heard that you've been doing it wrong; "You're supposed to squat, not sit," or so they say. It's probably true that squatting can help make bowel movements slightly easier, but despite what some people claim, changing your position won't solve all your digestive problems.

Humans spent thousands of years squatting to defecate; it's only in the last few hundred years that Western countries have adopted the raised toilet and started sitting instead. So this whole poop-position controversy is based on the idea that squatting must arrange your anatomy in a helpful way because that's what our bodies evolved to do. 

On its way out of you, solid waste has to pass through your rectum and then your anal canal. There's a muscle called the "puborectalis muscle" that loops around your digestive tract between those two sections, and it helps you keep poop inside you when you aren't trying to eliminate it; basically, it makes an angle between your rectum and anal canal, which puts upward pressure on your rectum and holds everything in. And when the muscle relaxes, your rectum and anal canal straighten out a bit, which - along with other muscles that relax and contract - lets stuff slide out. 

It's probably not surprising that there hasn't been too much research on the anatomy involved in pooping, but a few small studies say that squatting helps align your rectum and anal canal better than sitting does. A couple others have shown that when people squat in stead of sit, they have to strain less, and then the elimination process takes less time. Again, these studies were small; one used 28 volunteers and another used only six. So it's enough to suggest that there might be something to these ideas, but bigger, more carefully controlled studies need to be done before this can be considered scientific fact. 

But. If squatting does make things easier, it's possible that it also helps with hemorrhoids. Hemorrhoids are swollen veins near the anus and rectum, which can be painful and lead to itching and bleeding. They're often caused by too much straining when you poop. So if squatting does help with straining, it could make hemorrhoids less likely, but that's basically all we know about the potential benefits. 

There are companies out there that sells stools to help you pass your stool, and sometimes, they make really exaggerated claims. For example, "squatting might fix irritable bowel syndrome" or even "prevent colorectal cancer." There's just no evidence to support those claims. Gastrointestinal experts seem to agree that if your bowel movements -

- if your bowel movements are normal, there's no real reason to squat when you go. There's probably no harm in it either, but if you're happy with the way you've been pooping until now, there's no need to change up your method.

But. You know, thanks for asking.

[slide]

Savannah: Okay, so maybe it's not necessarily better to squat, but we really just need more butt studies before we make any strong statements about it. And when you're looking for scientific information about butts, one of the greatest resources on the internet is the SciShow Tangents podcast. Every single episode ends with a custom butt-fact related to that week's topic. Like, one week, we learned about the Costa Rican ants that were thought to have two butts until researchers realized that they really have a butt-colored beetle *in* their butts. 

There are literally hundreds more butt-facts to hear on SciShow Tangents, wherever you get your podcasts. Tangents proves that butts and poop can be related to pretty much anything. They've been putting out a solid stream for years, so it's only fitting that one of the hosts of the illustrious podcast, Hank, is here to tell you what happens if you *stop* pooping.

[slide: "What Happens If You Stop Pooping?"]

Hank: This episode's gonna get...real. So. Be prepared for that.

Let's start here: Nobody likes to be constipated. Sometimes because of things like diet or stress, you don't poop for a little while. It happens to me whenever I go on vacation, and I'm like, "Why? I'm supposed to be having a good time. This is not fun." Thankfully, that's usually fixable with coffee or like a bunch of fiber, have a couple bran muffins. But if that "little while" turns into a long time, and you just stop pooping entirely, things can get pretty ugly.

Poop, of course, if your body's way of getting rid of undigested or undigestible food and other wastes that your body produces. After the stuff travels through your small intestine and large intestine, it ends up at the rectum, which stretches and sends a signal to your brain: "It's time to go!" 

Sometimes, though, things can stop chugging along as nature intended, that's where constipation comes in, and it can be caused by anything from holding it in too long to a lack of exercise. Some diseases can also make your intestines struggle to propel the poop along. Parkinson's or inflammatory bowel diseases -

- inflammatory bowel diseases can prevent the muscles and nerves in the colon from working properly, so it can't pass along the feces or gas. In severe cases, that can cause a megacolon - and that's the actual name for this thing - it's an abnormally large or swollen colon, which can create a fecal backup.

Regardless of what causes it though, nothing good happens when your poop gets stuck in your body. When it hangs out in the colon, more and more water is sucked out of it, and that makes it harder, which in turn makes it difficult for your body to push out. And when you do try, you have to push *harder*. That strain can cause hemorrhoids, which are inflamed veins in your rectum, or anal fissures, which are small tears in the lining of the anus. [sarcastic tone] All this sounds so fun.

And if you don't go for a long, long, long time, your poop gets too hard and dry. It can also cause something called a "fecal impaction," a hardened mass of stool that can't move at all. Those masses can cause nausea, ulcers, and press on the bladder, causing urinary incontinence. They can also cause the colon to perforate or rupture, which is life-threateningly dangerous and requires surgery.

And in really, really, really rare cases...things can get... oh, goodness. A severe fecal impaction can actually make the intestines contract in reverse. In an effort to get things moving, they'll contract and relax, which, unfortunately, can send their contents in the wrong direction - they're just trying to save your life. Liquid from the small intestine backs up into the stomach and causes the unfortunately-named condition, fecal vomiting. Now, it's not quite as nasty as it sounds, like, it's not fully formed stools or anything, but also...definitely not pleasant.

Thankfully, fecal impactions can be broken up with laxatives or can be broken up manually by a doctor. If a person is chronically constipated or develops fecal impactions, their intestinal muscles might weaken over time, making it more difficult to regulate the pooping on their own.

So. Make sure to eat plenty of fiber, don't hold it for too long, and if you're not pooping regularly, go talk to a -

- go talk to a doctor, because the alternatives... No!

[slide]

Savannah: That all sounds horrific. Luckily, if all goes according to plan, you won't have to do that because your sphincter never gets tired. So it's not like you'd have to stop pooping and give it a rest. Here's what makes that little squeezer different from your other muscles.

[slide: "Why Doesn't Your Spincter Get Tired?"]

Hank: I don't know about you, but I consider any day I don't poop my pants to be a good day. When you think about it though, it's kind of weird that we're able...to...do that. The muscle that keeps things from leaking out of your body - your anal sphincter - is always clenching. So why doesn't it get tired like any other muscle if you always clenched it?

Turns out, your sphincter isn't super-ultra strong or anything, but it *is* more energy efficient. You have dozens of sphincters in your body, including the one in your anus, which are responsible for maintaining a lot of those bodily functions that run in the background, like digestion. And to help them do that, they're made out of what's known as "smooth muscle." When any type of muscle contracts, two main bundles of proteins - actin and myosin - slide past each other and shorten the muscle. But with regular skeletal muscle, like in your biceps and quads, that process takes more energy.

It all starts when calcium ions flow into the muscle cell and reshape the protein troponin, exposing a stop on the actin strand that mysin really wants to grab onto. With a little bit of added energy, the myosin strand cocks back, grabs onto that actin's sweet spot, and swings forward, shortening the muscle. Repeat this process for a few other muscle fibers, and you got yourself a flexed bicep. But in order to keep the muscles contracting, you need energy in the form of adenosine triphosphate, or ATP.

Skeletal muscle uses readily-available ATP pretty quickly, so after a bit of exercise, your muscles have to pull from other energy sources, and the byproduct of some of those energy sources - whether it's lactic acid, hydrogen ions, or something else - are thought to contribute to that feeling of tiredness.

But smooth muscle has a few built-in safeguards against fatigue. It has much more actin -

- much more actin than myosin when compared to a skeletal muscle, and it has no troponin, so the myosin strand always has a spot to hook onto in the actin strand. That means when smooth muscle contracts, it can stay contracted for a long time without using much ATP, so it won't run out of fuel. Since the muscle is so energy efficient, it doesn't need to rely on those alternative sources of energy, which means it won't produce those metabolic byproducts or get fatigued.

This ultra ATP efficiency might sounds like a fantastic evolutionary advantage, but there's a trade off; it comes with a slower contraction speed, which is not something that you would want in, say, your leg muscles. You'd have a much harder time running away from a potential predator. Because sometimes you need your leg muscles to kick a shark in the face. But when it comes to the inside of your butt or your other sphincters, the slower contraction speed is worth being able to clench for a good long while.

Now, most smooth muscles act involuntarily, but hopefully, you have say in when you go to the bathroom. That's because you actually have multiple layers of anal sphincters, both internal and external. The internal layer contracts involuntarily, but it relaxes when it get the "gotta go" signal from your body. The external sphincter is technically skeletal muscle, so it can contract voluntarily, which you're familiar with if you've ever heard nature's call a little too loudly.

This also mean that it *can* get tired, but luckily, it only really plays a backup role to your internal sphincters. And to that end, we owe our sphincters a big thank you. It's also just a fun word to say.

[slide]

Savannah: You really are indebted to them for holding in your poop. But they're not as good at holding in farts, and here's why that's unfortunate.

[slide: "Why Do Some Farts Smell So Bad?"]

Michael: Don't fart naked near food. That was a tongue-in-cheek conclusion from a couple of Australian scientists in 2001 who conducted an experiment on farts. A nurse had asked them if farting in a sterile environment - like the surgery room she worked in - could cause infections. It's kind of a weird question, but maybe not entirely ridiculous. So these researchers looked into whether farts were potentially harmful or just stinky clouds.

- or just stinky clouds. And their experiment was just one of many that analyzed what farts are made of.

"Flatus," as medical journals call it, is partially produced by swallowing air as you go about your life. But farts are mostly a byproduct by the trillions of tiny microbes that live in your gut digesting carbohydrates and spewing out gases. Humans fart up to 20 times per day - or more - and release mostly odorless gases like nitrogen, oxygen, carbon dioxide, and hydrogen. But there's a tiny percentage of the stuff that smells. Really bad.

It may surprise you that methane isn't the culprit. It's flammable and a greenhouse gas, but it doesn't smell. Not to mention, not all of all of us have gut bacteria that produce methane in the first place. Fart smells are actually due to a handful of compounds that contain sulfur: hydrogen sulfide, methanethiol, and dimethyl sulfide.

Hydrogen sulfide [H2S] is a colorless gas that smells like rotten eggs. It's flammable and in large quantities, also highly poisonous. Like some other toxins, hydrogen sulfide messes with enzymes that let the mitochondria in your cells make energy. Fortunately for us, farts contain tiny amounts of it, so we smell it, but we're not in any real danger.

Methanethiol [CH4S] is another sulfurous chemical, and it smells like rotting vegetables. It's produced by bacteria that are also involved in things like terrible breath and making some cheeses. At super high concentrations, it can be toxic like hydrogen sulfide, but again - the amount in farts is just stinky, not dangerous.

And then there's dimethyl sulfide [C2H6S]; this gas has been described as having a "sweet" odor, but definitely not in a good way - in a "cabbage-y" way.

The amounts of all three of these chemicals in your farts depend on what you eat. If you snack on foods that have a lot of sulfur-containing compounds - like cabbage, broccoli, or eggs - then your gut bacteria will produce more sulfurous gases, and your farts might drive more people from the room.

Fortunately for doctors operating on patients though, that 2001 experiment testing the infectiousness of farts found that bacteria aren't spread if you fart through clothing. So the smell might cause some problems for your coworkers, but it's all stink and no real danger.

[slide: "Why Do Your Farts Smell Worse In the Shower?"]

Hank: Even if you're not a huge fan of talking about it, and I - to be clear, I don't understand that impulse - you, me, and everybody else farts. And the average person does it up to 20 times -

- does it up to 20 times a day. Most farts don't smell that great, even on a good day. But if you've ever let one rip in the shower, you might know that they can smell...worse in there? What? Why? Nothing wakes you up in the morning like a nice, hot nose-full of your own butt gas.

We've actually talked...quite a lot about farting before on SciShow, and if you want to know about what makes them smell the way they do, you can watch our episode about it. Mostly, you can thank bacteria living in your gut, which produce a bunch of lovely, gross-smelling chemicals.

And the smell just seems to get worse when you take a shower. Some of that is because you're stuck in a closed space, and there's nowhere for you or the gas to go, unlike if you farted in a nice, open field, or while you're traveling down an escalator. The main reason it smells especially bad is because of the added humidity.

Your sense of smell is super important: it helps you figure out if your food's gone bad, if there are gas leaks, or if there's a fire nearby. So researchers have been trying to better understand it. In one experiment, they put 75 volunteers in a room, and they changed the room's humidity and air pressure to learn how different conditions affect how we smell things. They did not make the subjects fart, although that would've been amazing, and I hope somebody does that study someday. But they did introduce other odors, and they found that when the room was more humid, the volunteers could smell much lower concentrations of the odors. Essentially, their sense of smell got better when there was more water vapor in the air.

Not many studies have look at that before, so scientists weren't really sure why this happens; one possibility is that the water vapor helps the molecules that make up the smell interact with receptors in your nose. Another idea is that the gas particles bind to the water vapor, so the air acts like a sponge and can hold more smell than regular, dry air.

When you take a hot shower, your bathroom fills up with steamy water vapor. So one way or another, that gross smell ends up being a lot stronger. It also probably doesn't help that you aren't wearing clothes, so there isn't any fabric to absorb the smell or lock it in. But as you probably would guess, researchers have not done many studies on that yet.

Either way, the next time you feel the need to let one go, you might want to wait until you're out of the shower; you probably will thank yourself. Though, at least it's just you in there. No one else has to know.

[slide]

Savannah: So. Here's a recap on the best ways to use your butt:

One, you can use your butt for all sorts of stuff from running to farting. Two, if you gotta go, you gotta go. Don't put off that dump for too long. Three, farting in an open field with pants on will be way more enjoyable than farting in a shower.

And another excellent use of that butt is to sit down and watch another SciShow video.

[outro]