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  • Thanks to Brilliant for supporting this whole week of SciShow.

  • Go to Brilliant.org/SciShow to learn more.

  • When you hear the wordscurvy,” 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 dowell, 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 jobor 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 knowour 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 againthat'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!

Thanks to Brilliant for supporting this whole week of SciShow.

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壊血病が存在してはいけない理由 (Why Scurvy Shouldn't Exist)

  • 237 10
    Jerry Liu に公開 2021 年 01 月 14 日
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