字幕表 動画を再生する 英語字幕をプリント Thanks to Brilliant for supporting this whole week of SciShow. Go to Brilliant.org/SciShow to learn more. When you hear the word “scurvy,” you probably think of jokes about pirates, but this disease can actually be really serious. Its symptoms include bleeding gums; bumpy, rashy skin; and joint pain, and if it's not treated, it can be deadly. It's not just a thing of the past, either. Scurvy still shows up in developing nations, and dozens of cases were reported in the United States in the last decade. Thankfully, it is relatively easy to treat — all you need is a high dose of Vitamin C. But here's the thing: If it weren't for a quirk of evolution, nobody would have ever gotten scurvy in the first place. The reason Vitamin C cures scurvy is because the disease is actually caused by a Vitamin C deficiency. Without this vitamin, the body can't properly form collagen. Collagen is the protein that makes up the vast majority of our connective tissues, so without enough of it, things can go seriously wrong. For example, scurvy patients can develop skin that's thin and fragile, where any wounds they get stay open and can't heal. That can make them more likely to contract a life-threatening infection. Scurvy can also cause the collagen supporting blood vessels to fall apart, which can lead to hemorrhages. So this protein is a pretty big deal and by extension, so is Vitamin C. To be clear, this vitamin isn't the only thing we need to make collagen. But it is the only component of the collagen synthesis process our bodies can't naturally produce. We need to consume it somehow. That's why a lack of it leads to scurvy, and why a high dose of it can cure the disease. But what's interesting is that most of nature doesn't have to deal with this. Lots of animals can make their own Vitamin C without consuming it in their diets. And here's the kicker: Our very distant primate ancestors could do this, too. More than 60 million years ago, the branch of primates that now includes humans did have the ability to make Vitamin C. But then we lost it. More specifically, we lost the ability to make an enzyme called GULO. Just like with collagen, the process of making Vitamin C is... a process. But for this story, all you need to know is that the last step depends on GULO. When you look at different animals' genomes, you'll see that most animals have the gene that codes for this enzyme. And those that do can tweak how much or how little Vitamin C they produce. Like, rabbits pump out way more GULO enzymes during the winter months when Vitamin C-rich foods are in short supply, and goats can really crank it out when they're sick or stressed. Even animals toward the bottom of the evolutionary tree, like sponges and jellyfish, have the gene responsible for GULO, which means it's existed for a very long time. The only members of the no-GULO club are guinea pigs, some bats and birds, most fish... and many primates, including humans. So in our evolutionary past, our very distant ancestors had the ability to make GULO and therefore Vitamin C. But somewhere in primate evolution, a random mutation broke it. This might seem more than a little unfair, but you have to remember, mutations are random. Most of the time, they're harmless and unnoticeable, but sometimes they affect an important gene. And when they do… well, it just kind of stinks. We can tell this happened to GULO because its gene is actually still in our genetic code — although these days, it's just something called a pseudogene. That's a copy of a DNA sequence that accumulated enough mutations that it can't perform its original job — or in the case of the GULO gene, accumulated enough that it's totally nonfunctional. Pseudogenes actually crop up all the time, and the GULO gene is one of about 20,000 others in our genome. For reference, we have about 27,000 genes that actually make what they're supposed to. So, you know… our genomes are kind of a mess. But that's a little beside the point here. The real question is, why did the nonfunctional GULO gene get passed down in the first place? After all, we can literally die without Vitamin C. So it seems like the first primates to get this broken gene should have died before they could pass it along to their offspring. Well, scientists have looked into it, and they've come up with a hypothesis about what happened. A part of it could be that there is some benefit to ditching the GULO gene. GULO can create hydrogen peroxide as a byproduct of its reaction, which can form particles called free radicals when it breaks down. These are little free-floating electrons that have the potential to harm cells and make certain diseases more likely. So it seems helpful to produce as few of them as possible. But then again… that's a marginal benefit at best, considering what happens without Vitamin C. So the better explanation seems to be that the GULO pseudogene wasn't eliminated because there just wasn't enough selective pressure. Not making GULO is easy to compensate for if you're an animal that consumes plenty of Vitamin C in your diet. And at the point in our ancestry where primates lost the GULO gene, they lived mostly in tropical environments, so their diets contained plenty of fruits and vegetables. So we probably never noticed the ability to synthesize this vitamin was gone because, well, we got it pretty easily in our diets. Today, that seems pretty rough, considering that scurvy is still a problem. But unfortunately, that's just how evolution plays out sometimes. The best we can do now is try and make sure everyone gets what they need. Scurvy isn't anything to joke about, but there are plenty of other great jokes you can make about storybook pirates. Like, anything about booty is a good time. Pirates are always collecting it, counting it, and divvying it up among their crew. And if you want to experience some of that for yourself, you can check out the Logic course from Brilliant. It features a quiz that's all about pirates, where you use your new logic skills to figure out who's cheating whom out of the most booty. It's a fun time, and Brilliant also has a bunch of other courses that can help you cultivate your math and scientific thinking skills. They're also available offline using Brilliants iOS and Android app. So whether you're stuck on the subway or sailing the seven seas, you'll be able to keep learning. If you're one of the first 200 people to sign up at Brilliant.org/SciShow, you'll also get 20% off an annual Premium subscription. And you'll be helping SciShow make more content, too!