- Hey, smart people.

Joe here.

Here's a word that no one would be embarrassed to say.

However, there are other words that are a bit more taboo, words that nobody likes to talk about, like butt or anus, which is too bad because the end point of your digestive tract is a lot more than just a doody door, excrement exit, or poo-poo portal.

It might be the most important hole in your body.

Okay, Joe, well, what about my eyes or nose or ears or mouth?

Those are fine, I guess, but the bum has gotten a bum rap.

It's a hole so important that it's the first body part you grew.

And the story of the anus is actually the story of how every complex animal on Earth from mole-rats to the mantis shrimp came to be.

An orifice to which we owe our very existence?

Maybe.

It's time that we shine a little light where the sun don't shine.

(jaunty music) - [Narrator] This video contains language such as butt, poop, and anus used in a mostly scientific context.

For our younger viewers and other viewers who still live their parents, discretion may be advised, even though literally everyone poops so it should be that big of a deal, I'm just not trying to get you grounded or anything so don't say I didn't warn you.

- Inside, you are enormous.

The lazy river of digestion, nutrient extraction, and waste excretion connecting your mouth to your rear stretches something like 12 meters.

And laid open, the accordion-like surface area of the alimentary canal would cover a small apartment.

You know, in grammar class, we're taught to never end a sentence with a conjunction, though apparently no one told evolution because all of this ends in a butt.

So what is a butt?

You know one when you see it, but can you define it?

This is a butt, and this and this.

You can even be the butt of a joke.

Butt, as we use it today to refer to the fleshy junk at the base of our collective trunks, likely derives from the Middle English bott, the hindquarters of an animal, and didn't become a slang word for people's posteriors until the 1800s.

But the butt is really defined by its most important feature.

In Latin, the word anus means ring or circle, the same root that gives us words like annular eclipse, named for the halo of light visible during some solar eclipses.

Life comes in all shapes and sizes, but pretty much every animal shares the same few basic needs.

We have to eat, absorb nutrients, and get rid of waste.

Some things do it like this, no mouth, no anus.

I mean, just imagine living a life like a coffee filter, digesting whatever happens to pass by.

Or you can level up and get yourself a mouth.

Now you've turned your body into a sac, you can store a bit of food inside, secrete some digestive enzymes, absorb as many nutrients as you can, and then you eject what's left out the same hole that it went in.

You take a mouth poo.

I mean, imagine pouring some Cheetos into your mouth, you just let them sit there for a few hours and then blech.

Well, that's what you'd do if you were a jellyfish or an anemnemenheeh, or a jellyfish.

That's all fine if your ultimate position in life is to sit on the ocean floor or drift along like some squishy blob of stingy spaghetti.

Plenty of creatures like that exist so it's obviously good enough in the eyes of evolution.

But if you add an anus, things get really interesting.

Don't meme that.

The origin of the anus was a monumental upgrade to the animal body plan and how they could eat, digest, and get rid of waste.

There's a reason most animals alive today are built basically like a tube surrounded by meat and skeleton.

Having a through gut, the anatomical term for a digestive tube with both a front and back door, means that you can feed again before your last meal exits, letting you extract more energy and nutrients from more food, allowing you to grow a larger and more complex body.

Animals that poo out of their mouth sac, they can't do that.

And many animals took their through gut and added accessories, like different chambers to allow digestion of a wider array of stuff.

And many, from termites to wildebeests to humans, added mobile homes for billions of microbes.

These microscopic digestive deputies do the hard work of breaking down wood or grass, or even synthesizing critical vitamins and micronutrients.

The through gut even allows you to store up your waste until you have a nice, quiet place to drop it off.

The longer your digestive tube, the more nutrients you can potentially pull out of your food.

Some animals have built longer guts by making longer bodies.

Others have kept their bodies small and circled their guts back.

Yes, that means pooping right next to your mouth.

A little messy, but whatever floats your digestive boat.

Or you could solve the problem like us, by winding up a long, multi-chambered gut and tossing it into your tummy like last year's Christmas lights.

None of these specialized animal forms would be possible without the anus because it takes two holes to make a tunnel.

Otherwise it's just a cave.

Thankfully, unlike a tunnel, the anus isn't open all the time.

It's one of many sphincters on your body.

These rings of muscle have important jobs, keeping our digestive chambers separated and guarding them from the outside.

While there are six sphincters in your digestive system, the anus is the shining star.

Most muscles in your body spend the majority of their time relaxed.

But your anal sphincter is rare among muscles, spending basically every moment of your life fully flexed, and thankfully for that.

The anus is also among the most densely nerve-packed parts of your body, and for good reason.

It needs to be able to tell difference by feel between solid, liquid, and gas, and be able to selectively release one, two, or maybe all of those.

That is not a decision you wanna mess up.

You're totally unaware of this, but six to eight times every day, a muscle contraction occurs in your body called a mass movement, squeezing the contents of the colon along just like a half-used tube of toothpaste.

Now, this contraction can be triggered by eating, and the more you eat, the bigger the squeeze.

Eventually some of that stuff gets pushed into the rectum, your body's exit hatch.

And here, these special nerve receptors measure pressure.

When enough waste pushes against the rectum walls, it triggers the defecation reflex.

When you poop, you technically don't push it out.

You increase the pressure in your rectum, the walls squeeze in, the sphincter muscles relax, and (can explosion crackles).

Potato chips, anybody?

Across the animal kingdom, butts come in many different shapes, sizes, and even numbers.

So what's the best butt?

Some anuses do double duty.

When they're not defecating, sea cucumbers use their butts to absorb oxygen.

One species also uses its anus as a second mouth.

(chuckles) True bottom feeders.

Reptiles, amphibians, most birds, and monotremes like the platypus have a cloaca, a multi-purpose Swiss Army knife of holes linking urinary, waste, and reproductive systems.

I guess you could say, "One ring to rule them all."

But nature's full of exceptions and, strangely, some animals have lost their anuses.

Brittle stars and many parasites had ancestors with anuses but no longer have them.

Even the microscopic mites that live on our faces burrowed within our pores and our eyelashes, they don't have an anus either.

When they die, all of the poop they've accumulated throughout their life bursts out and this microscopic fecal facial has even been linked to skin inflammation.

Microscopic fecal facial has- (man groans) Microscopic fecal facial has even been linked to.

Everybody poops?

I don't think so.

Lies!

(book clatters) I'm fine.

I'm calm now.

Lies!

Lies!

(book clatters) Some animals even lose their anus within their lifetimes.

A rare group of scorpions will sometimes break off their tails to escape predators, and they make a quick getaway but they say adios to their anus in the process and never poop again.

There are even anuses that come and go.

The warty comb jelly looks like a jellyfish but isn't a jellyfish, and it has an anus that only shows up when it's time to poop, only to disappear like some ghost butt.

This ocean-dwelling flatworm went anus crazy, sprouting multiple anuses across its entire back.

And although they don't use toilets, I cannot help but wonder what theirs would look like.

As varied as the animal kingdom's anuses are, none of them have bones so they don't fossilize well.

That makes it hard to trace the beginning of the end, and scientists still aren't sure when and how the anus originated.

But the origin of your anus is a bit more certain.

It was the first body part you built.

We belong to a class of organisms called deuterostomes.

That's Greek for second mouth, or really, mouth second.

When you were really young, no, younger, basically just a little ball of cells, you looked more like a hollow raspberry than a person.

Then your cells started to fold in on themselves from right here, and that fold burrowed through until it reached the other side, creating your donut-like through gut in the process.

I can see you.

And the end of the tunnel that formed first, that became your anus, while the second opening became your mouth.

Now you know why they call us mouth second.

In other classes of animals, this process is flipped, and the part that folds in first becomes the mouth.

They're called protostomes, Greek for first mouth.

But you and me, we came in the world rear end first, baby.

During life's 3-billion-plus-year journey on Earth, anal evolution happened independently many times on many different branches of life.

And they're so old that butts are more than the holes they hold.

And they do a lot more than just get rid of waste these days.

Wombats have buns of steel that they use for defense, capable of crushing the skulls of their attackers.

And manatees, they fart to maintain the right buoyancy in water.

And dragon fly larva, they use their butts like aquatic jet packs.

But our butts, they're pretty special too.

The human buttock is the most voluminous in the animal kingdom.

Your built-in tush cushion, it's made of fat on top of some pretty hefty gluteal muscles, and that's all thanks to our unique and very useful habit of walking on two legs.

I mean, compared to our chimp cousins and prehistoric human relatives, we have a shorter, wider pelvis with muscles that are oriented and enlarged to handle the weight of our entire upper body when we're walking and running.

Your supersized glutes are the reason that you can get up from a chair or stand on one leg.

I mean, seriously, ask a chimp to do that sometime.

I bet they don't even listen to you.

The fat stores that bulk up our booties are a little more mysterious.

They may have helped our ancestors store energy when food was scarce, but many researchers think it's pretty obvious that they serve as a sexual signal too, with larger, fuller buns unconsciously indicating to potential mates that you're healthy and have plenty of assets.

And one look at our culture's musical, artistic, and fashion tastes makes it clear that bottoms are something everyone thinks about.

But considering how many euphemisms we've invented to avoid actually talking about butts and anuses, well, we're pretty uncomfortable discussing them, which is too bad, because now I hope that you realize, from black holes to back holes, the universe is full of wonder.

And finally, remember to subscribe and don't forget to click the buttfor.

"What's a buttfor?"

You ask.

For pooping, silly.

Stay curious.