Name: 人類を形作ったウイルス (The Viruses That Shaped Humanity)
Uploaded: 2021-01-14T10:28:41.000Z
Duration: 11 min
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：新型コロナウイルス,新型肺炎,COVID-19

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

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 unique — maybe 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

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

mutations — which 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.

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

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

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

That means those viruses' DNA was passed to their host's offspring — and 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 instructions — ones 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 code — eight percent! — and they've

had a significant influence on how our bodies work.

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

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.

This retrovirus marked your cells as pluripotent and kept them in that state — preventing

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

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

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

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 development — maybe 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

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

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