Hey there, curious minds! Let’s talk about something that might seem a little awkward but is actually super important: the end of our digestive system, often called the “rear” or “bottom.” While it might not be the most glamorous topic, it’s a fascinating part of our bodies that plays a crucial role in how we function.
Did you know that the rear is one of the first parts of your body to develop? It’s true! This opening is essential for all complex animals, from tiny shrimp to big mammals like us. It’s more than just an exit for waste; it’s a key player in how we eat, digest, and grow.
Inside your body, there’s a long tube that connects your mouth to your rear, stretching about 12 meters! This tube is where digestion happens. It breaks down food, absorbs nutrients, and gets rid of waste. Imagine it like a long, winding river that helps keep you healthy and energized.
Animals have evolved in amazing ways to handle digestion. Some creatures, like jellyfish, have a simple system where food goes in and waste comes out the same opening. But most animals, including us, have a more advanced system with a separate exit for waste. This allows us to eat more efficiently and grow bigger and stronger.
The anus is a special muscle that stays tightly closed most of the time. It’s packed with nerves that help it decide when to release waste. This process happens several times a day without you even noticing, thanks to a muscle movement that pushes waste along like squeezing a tube of toothpaste.
Across the animal world, rears come in all shapes and sizes. Some animals, like sea cucumbers, use their rears for breathing, while others, like certain birds, have a single opening for waste and reproduction. Interestingly, some creatures have even lost their anuses over time!
When you were just a tiny ball of cells, your body started forming a tube that would become your digestive system. The first opening that formed became your anus, and the second became your mouth. This process is part of what makes us unique among animals.
Humans have some of the most developed rears in the animal kingdom. Our strong muscles help us walk upright and perform many activities. Plus, the fat in our rears might have helped our ancestors survive when food was scarce.
Even though we might not talk about it much, our rears are a vital part of who we are. They help us stay healthy and active, and they’re a testament to the amazing complexity of life on Earth. So next time you think about your digestive system, remember how incredible it truly is!
Stay curious and keep exploring the wonders of the world around you!
Using household items like tubes, balloons, and clay, build a model of the human digestive system. Label each part and explain its function. This hands-on activity will help you visualize how food travels through your body.
Organize a relay race where each station represents a different part of the digestive system. At each station, perform a task that mimics the function of that part, such as breaking down “food” (paper) at the stomach station. This will reinforce your understanding of the digestive process.
Research and create a poster comparing the digestive systems of different animals. Highlight unique features and adaptations. This will help you appreciate the diversity of digestive systems in the animal kingdom.
Write a creative story from the perspective of a piece of food traveling through the digestive system. Describe each stage of digestion and what happens to the food. This will enhance your understanding of the digestive process in a fun way.
Create a quiz game with questions about the digestive system. Play with your classmates to test your knowledge and learn from each other. This interactive activity will reinforce key concepts and facts.
Here’s a sanitized version of the provided YouTube transcript:
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Thank you to Morning Brew for supporting PBS. 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 no one likes to talk about. Like “rear” or “bottom.” Which is too bad. Because the end point of your digestive tract is a lot more than just an exit for waste. It might be the most important opening in your body.
Okay Joe, what about my eyes, nose, ears, or mouth? Well, those are fine, I guess? But the rear has gotten a bad reputation! It is an opening so important that it’s the first body part you developed. The story of the anus is actually the story of how every complex animal on Earth, from mole rats to mantis shrimp, came to be. An orifice to which we owe our very existence? Maybe. It’s time we shine a little light where the sun doesn’t shine.
[MUSIC]
“This video contains language such as rear, waste, and anus used in a mostly scientific context. For our younger viewers and other viewers who still live with their parents, discretion may be advised, even though literally everyone has to deal with waste, so it shouldn’t 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 that alimentary canal would cover a small apartment.
In grammar class, we’re taught never to end a sentence with a conjunction. Though, apparently no one told evolution, because all of this ends in a “but.” So what is a rear? You know one when you see it, but can you define it? This is a rear, and this, and this. You can even be the subject of a joke.
“Rear” as we use it today, to refer to the fleshy part at the base of our bodies, 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 rear 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. Imagine living life like a coffee filter, digesting whatever happens to pass by. Or you can level up and get yourself a mouth. Now that 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 eject what’s left out the same hole that it went in.
Imagine pouring some snacks in your mouth, letting them sit there for a few hours, and then… This is what you’d do if you were a jellyfish. That’s all fine if your ultimate position in life is to sit on the ocean floor or drift along like a squishy blob. 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.
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 you can eat 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 eliminate waste from 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 substances. And many, from termites to wildebeest to humans, added mobile homes for billions of microbes. These microscopic digestive helpers do the hard work of breaking down food or 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. And 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 eliminating waste right next to your mouth. Sounds messy, but whatever works for your digestion.
Or you could solve the problem like us, by winding up a long, multi-chambered gut, and storing it in our bodies. None of these specialized animal forms would be possible without the anus. Because it takes two openings 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 in 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. 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 the difference, by feel, between solid, liquid, and gas, and be able to selectively release one, two, or maybe all of those. That’s not a decision you want to mess up. You’re totally unaware of this, but six to eight times a 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.
This contraction can be triggered by eating. And the more you eat, the bigger the squeeze. And eventually, some of that material gets pushed into the rectum, your body’s exit hatch. Here, special nerve receptors measure pressure, and when enough waste pushes against the rectum walls, it triggers the reflex to eliminate waste.
Across the animal kingdom, rears come in many different shapes, sizes, and even numbers. So what’s the best rear? Some openings do double duty. When they’re not eliminating waste, sea cucumbers use their rears to absorb oxygen. One species also uses its rear as a second mouth. True bottom feeders. Reptiles, amphibians, most birds, and monotremes like the platypus have a cloaca, a multipurpose opening linking the urinary, waste, and reproductive systems.
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 pores and eyelashes, don’t have an anus either. When they die, all of the waste they’ve accumulated throughout their life bursts out, and this microscopic waste has even been linked to skin inflammation.
[NARRATOR VOICE] Everybody eliminates waste? I don’t think so. Lies. I’m fine, I’ll calm down. Lies! Some animals even lose their anus within their lifetimes. A rare group of scorpions will sometimes break off their tails to escape predators. They get to make a quick getaway, but they lose their anus in the process and never eliminate waste again.
There are even anuses that come and go. The warty comb jelly, which looks like a jellyfish but isn’t one, has an anus that only shows up when it’s time to eliminate waste, only to disappear again. This ocean-dwelling flatworm has multiple openings across its entire back. And although they don’t use toilets, I can’t help but wonder what theirs would look like.
As varied as the animal kingdom’s openings 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. That fold burrowed through until it reached the other side, creating your donut-like through gut in the process. 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 I came into the world rear-end first.
During life’s three billion-plus year journey on Earth, anal evolution happened independently many times in different branches of life. They’re so old that rears are more than the openings they hold, and they do a lot more than get rid of waste these days. Wombats have strong rears that they use for defense, capable of crushing the skulls of their attackers. Manatees use gas to maintain the right buoyancy in water. And dragonfly larvae use their rears like aquatic jetpacks.
But our rears are pretty special too. The human buttock is the most voluminous in the animal kingdom. Your built-in cushion is made of fat on top of some pretty hefty muscles. And that’s thanks to our unique (and very useful) habit of walking on two legs. Compared to our chimp cousins and prehistoric human relatives, we have a shorter, wider pelvis, with muscles oriented and enlarged to handle the weight of our entire upper body when walking and running.
Your supersized glutes are the reason you can get up from a chair or stand on one leg. Seriously, ask a chimp to do that sometime. I bet they won’t even listen to you. The fat stores that bulk up our rears 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 signal too, with larger, fuller rears indicating to potential mates that you’re healthy and have plenty of… resources.
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 rears and anuses, we’re pretty uncomfortable discussing them. Which is too bad. Because now, I hope you realize, from black holes to back holes, the universe is full of wonder.
Finally, remember to subscribe and don’t forget to click the button. “What’s a button?” you ask? For eliminating waste, silly. Stay curious.
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This version removes explicit language while maintaining the overall message and structure of the original transcript.
Digestion – The process by which the body breaks down food into small molecules that can be absorbed and used for energy and nutrients. – The digestion of food begins in the mouth where enzymes start breaking down carbohydrates.
Anus – The opening at the end of the digestive tract where solid waste leaves the body. – After digestion, waste is expelled from the body through the anus.
Nutrients – Substances in food that provide energy and materials for cell development, growth, and repair. – Fruits and vegetables are rich in essential nutrients that help maintain a healthy body.
Waste – Materials that are not needed by the body and are expelled after digestion and metabolism. – The kidneys filter waste from the blood, which is then excreted as urine.
Animals – Living organisms that feed on organic matter, typically having specialized sense organs and nervous systems. – Animals have evolved various adaptations to survive in their environments.
Evolution – The process by which different kinds of living organisms develop and diversify from earlier forms during the history of the earth. – The theory of evolution explains how species change over time through natural selection.
System – A group of organs that work together to perform a specific function in the body. – The digestive system includes the stomach and intestines, which help break down food.
Mouth – The opening in the face that is used for eating and speaking, where digestion begins. – The mouth contains teeth and saliva that help start the digestion of food.
Rear – The back part of something, often referring to the hind part of an animal’s body. – The tail is located at the rear of many animals and can be used for balance or communication.
Cells – The basic structural and functional units of all living organisms, often called the building blocks of life. – Cells in the human body perform various functions, such as carrying oxygen and fighting infections.
