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  • {♫Intro♫}

  • When somebody's talking about viruses in

  • biology, they're usually talking about things that make you sick

  • stuff like the flu virus.

  • And based on that, you might get the impression

  • that all viruses are terrible, awful, no-good

  • things that just wreak havoc on humanity.

  • But, surprise: That's not actually true! And the truth is way more interesting!

  • In reality, not only have some viruses helped us: A specific clade of them has given us

  • some great adaptations and influenced our evolution for the better.

  • Much of this research focuses on the human genome.

  • Your genome is the complete collection of your genetic material, including all of your

  • DNA and the genes that comprise it. It contains everything you need to know to make a human.

  • A lot of this material has been passed down from species to species for thousands or millions

  • of years, which is why generally, our genomes are pretty similar to those of other animals.

  • But! There are also places in the human genome that are totally uniquemaybe because

  • some sequences got rearranged, or because some were inserted or deleted.

  • Those kinds of changes translate to things like the proteins our bodies make or the ways

  • we develop. They're part of what makes us human.

  • So it's not surprising that researchers have spent a lot of time trying to figure

  • out where these changes came from.

  • Over the years, they've learned that genomes can change thanks to a series of random DNA

  • mutationswhich might sound familiar from biology class.

  • But what most bio classes don't talk about is how viruses also have a huge role to play

  • here. Especially a group called retroviruses.

  • Retroviruses are kind of freeloaders.

  • Like other viruses, they don't have much complex machinery, so they can't reproduce

  • themselves. Instead, they depend on living things to do that for them.

  • Retroviruses take their genetic material, insert it into a host cell's genome, and

  • then rely on the host to replicate that material.

  • Over time, the host helps crank out more and more copies of the virus, and the virus goes

  • on to infect other cells and repeat this process all over again.

  • Normally, when we talk about these things, we tend to focus on the viruses themselves,

  • since those end products can cause illness.

  • But there's a whole different piece of the puzzle here. Because you have to remember:

  • Those retroviruses don't just show up, replicate, and leave.

  • They embed instructions for making new viral products in our DNA.

  • Sometimes, that's not a big deal, because those instructions can get eliminated by our

  • immune systems.

  • Or if the virus only infected something like a skin cell, its genetic code won't get

  • passed on to the host's offspring.

  • But at multiple points in our past, our ancestors picked up viruses that happened to infect

  • germ cells, like egg or sperm cells.

  • That means those viruses' DNA was passed to their host's offspringand their

  • offspring, and their offspring, for thousands or millions of years. Until virtually all

  • modern humans had instructions for making certain types of viruses.

  • These instructionsones that have been passed down through germ cells for generations

  • are called human endogenous retroviruses, or HERVs.

  • They make up almost eight percent of our entire genetic codeeight percent! — and they've

  • had a significant influence on how our bodies work.

  • That influence starts early, too.

  • Like, take one family of retrovirus called HERV-H.

  • It's a family that only primates have, and it started influencing your genetic code just

  • a few days after you were conceived.

  • For a short amount of time, your embryonic cells were pluripotent, meaning they could

  • become any type of cell, from branching neurons to dense muscles.

  • Of course, your body needed specific cells in specific places, so that your brain and

  • heart and muscles would end up in the best spots. So eventually, instructions in your

  • genome told the cells what kind of tissue to permanently become.

  • But before then, HERV-H stepped in.

  • This retrovirus marked your cells as pluripotent and kept them in that statepreventing

  • them from turning into muscle or skin cells for just

  • a little bit longer than they would otherwise.

  • This likely benefits the virus, since the longer cells stay pluripotent, the more time

  • a virus has to be replicated and infect every cell.

  • But this process might also affect human development.

  • In 2015, researchers were comparing the genomes of humans and macaque monkeys, and they found

  • a few identical genes in both species.

  • But they also noticed that those genes were only expressed in humans while the embryo

  • was pluripotent. In other words, the body was only reading those genes and following

  • their instructions during that stage.

  • It's not exactly clear what the implications are there, but if HERV-H is keeping cells

  • pluripotent for longer, it means those genes are expressed for longer, too.

  • The fact that a random virus could hold an embryo like this is kind of a big deal. That's

  • not a small impact!

  • And HERV-H might have other consequences, too. Like, it might play a role in disease.

  • It's still preliminary work, but in 2018, one paper found evidence that this retrovirus

  • could have a role in shaping heart cell developmentmaybe even a role in heart disease.

  • So one way or another, your body might not be quite the same if it weren't for HERV-H.

  • Now, even once your cells were on their way to specialization, they still had months of

  • growing to do, which carried some risk.

  • Like, an infection could have easily come along and wiped out your little clump of cells.

  • But it didn't — maybe thanks to another endogenous retrovirus, called HERV-K.

  • Among other things, HERV-K helps embryos develop a built-in immune system that keeps them safe

  • even before they develop antibodies to pathogens in the outside world.

  • It codes for a small protein called Rec. And Rec helps HERV-K make viral copies and proteins

  • so it can infect other cells.

  • That might sound like bad news for us, because it seems like the last thing an embryo probably

  • needs is a bunch of virus particles. But in reality,

  • this is actually really useful for embryos.

  • These particles trigger embryonic cells to start making antiviral proteins, and that

  • builds one of its first defenses against other viruses.

  • This kind of thing is far from a complete immune system, but for an embryo, it might

  • make the difference between getting an infection or not.

  • And who knows? Maybe this even made a big difference for you, once upon a time.

  • So, endogenous retroviruses affected your pluripotency, and they helped with your immunity

  • as an embryo. But they also played a major role

  • in helping you develop into a fully-grown fetus.

  • At about a week after fertilization, a human embryo is a hollow ball of cells that starts

  • attaching to the uterus.

  • At this time in development, humans, just like all other mammals, will develop a placenta,

  • a temporary organ that forms during pregnancy.

  • This structure is crucial for human pregnancyit provides nutrition to the developing

  • fetus and helps it get rid of waste.

  • But for it to work, it needs to be connected to the fetus.

  • I know, that sounds obvious. But like, somehow, a mechanism had to evolve to do that job.

  • And as it turns out, an endogenous retrovirus might have played a big role there.

  • Researchers have found that the placenta secretes a protein that binds it to the embryo, keeping

  • the two attached for the next few months of development.

  • And the DNA that makes up that protein is nearly identical to a region of a retrovirus

  • that allows the virus to attach to the host cell.

  • I'm definitely not saying that fetuses are basically like viruses attached to a host

  • But this does suggest that without this retrovirus,

  • placentas might not workor at least,

  • might not work the same way.

  • So! HERVs shape how your body develops and

  • how it stays protected and cared for during

  • development. But these things don't stop being important as soon as you're born;

  • they have a real impact on your adult body, too.

  • For example, endogenous retroviruses didn't

  • just help your embryonic immune systems;

  • they help defend your adult body from pathogens as well.

  • In fact, some research suggests that all of our existing immune system pathways depend

  • on enzymes created by retroviruses and things like them.

  • If you think about it, that's kind of amazing. The remnants of ancient viruses are protecting

  • our bodies againstmicrobes and other viruses.

  • HERVs have a role beyond the immune system, though. They're also responsible for small,

  • but significant changes in how the human body works in general.

  • Take this enzyme called amylase.

  • All mammals have it in their pancreas, and it's used to break complex sugars into simple

  • sugars. But a few mammalsincluding humanshave it in their saliva.

  • This could be thanks to a retrovirus called HERV-E, which may have converted one of the

  • genes that codes for amylase in our pancreas into a salivary gland version.

  • This isn't just a weird quirk, either.

  • Although it's not confirmed, one hypothesis suggests this salivary amylase might help

  • us taste sugary foods better and identify good sources of nutrition. So it could play

  • a big role in your everyday life.

  • Scientists are always interested in learning more about endogenous retroviruses, because

  • they can tell us a lot about why humans look and act like humans. But these days, they're

  • also interested in how we can use HERVs to understand specific diseases.

  • For example, some evidence suggests that HERV-W might be involved in multiple sclerosis, and

  • HERV-K might contribute to the pathway that leads to ALS.

  • These things are just now being investigated, but scientists hope that understanding these

  • viruses will eventually lead to cures for both of these neurological diseases.

  • And that's just the tip of the iceberg.

  • At the end of the day, human genetics are complicated, and there are a lot of variables

  • that shaped our species and our adaptations.

  • But the next time you taste a sugary food, recover from a cold, or hear that a friend

  • delivered a healthy babywell, you might have some ancient viruses to thank.

  • If you want to learn more about how you went from a clump of cells to a fully-grown baby,

  • you might want to check out the pregnancy episode of our podcast, SciShow Tangents.

  • Tangents is a weekly, lightly competitive podcast hosted by some of the people who have

  • brought you SciShow over the years: Stefan Chin, Sam Schultz, Ceri Riley, and myself.

  • In every episode, we show off our best science facts, try and stump our co-hosts with lies,

  • and always end up talking a little bit about butts.

  • I'm not kidding, there's a butt segment.

  • It's a great time, and I always learn a lot from it.

  • If you want to join us, you can listen to SciShow Tangents by searching for it

  • anywhere you get your podcasts.

  • {♫Outro♫}

{♫Intro♫}

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B1 中級 武漢肺炎 新型冠狀病毒 新冠肺炎 COVID-19

塑造人類的病毒 (The Viruses That Shaped Humanity)

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    林宜悉 發佈於 2021 年 01 月 14 日
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