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（(宗教)禁忌、避讳）, words that nobody likes to talk about, like "but" or "anus." Which is too bad, because the endpoint（端点） of your digestive（消化的） tract（束） is a lot more than just a duty door, excrement（粪便） exit, or poo poo portal. It might be the most important hole in your body. Okay, Joe, 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. This video contains language such as butt（屁股）, poop, and anus, used in a mostly scientific context.

  For our younger viewers and other viewers who still live with their parents, discretion（判断力） may be advised. Even though literally everyone pooped, 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 it in the morning. Inside, you are enormous. The lazy river of digestion（消化）, nutrient（营养的） extraction（取出）, and waste excretion, connecting your mouth to your rear（后方的）, it stretches something like 12 meters. And laid open, the accordion（手风琴）-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 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. But, as we use it today, to refer to the fleshy（肉的） junk at the base of our collective（集体的） trunks, likely derives from the Middle English "but," 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（食）," 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. Just imagine living a life like a coffee filter（滤波器）, 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 sack, you can store a bit of food inside, secrete（隐秘） some digestive（消化的） enzymes（[生化]酶）, absorb as many nutrients as you can, and then you eject（逐出） what's left out the same hole that it went in. You take a mouth poo. I mean imagine pouring some Cheetos in your mouth, you just let them sit there for a few hours, and then... Well, that's what you'd do if you were a jellyfish（水母）. Or a nemenem.

  Or a jellyfish. That's all fine if your ultimate position in life is to sit on the ocean floor, or drift（漂流） 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 mean that. The origin of the anus was a monumental（不朽的） upgrade（升级） 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（内脏）, the anatomical term for a digestive（消化的） tube with both a front and back door, means that you can eat again before your last meal exits. Letting you extract（提取） more energy and nutrients from more food, allowing you to grow a larger and more complex body. Animals that poo out of their mouth sack? They can't do that. And many animals took their through-gut and added accessories, like different chambers（会议室） to allow digestion（消化） of a wider array（数组） of stuff.

  And many, from termites（[昆]白蚁） to wildebeests（羚羊(其中一种)） to humans, added mobile homes for billions of microbes. These microscopic digestive deputies（代理人） do the hard work of breaking down wood, or grass, or even synthesizing（合成） critical vitamins and micronutrients（[生]微量营养素）.