Articles, Blog

The Viruses That Shaped Humanity

January 16, 2020

{♫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 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 out where these changes came from. 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. 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 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. 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 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 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 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 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 pregnancy
— it 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 work — or 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 against… microbes 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 mammals — including humans
— have 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 baby — well, 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♫}


  • Reply greatgreen109 January 16, 2020 at 1:10 am


  • Reply Cody Bell January 16, 2020 at 1:11 am


  • Reply cole olson January 16, 2020 at 1:11 am

    Forth and this is a lit channel

  • Reply Jay January 16, 2020 at 1:11 am

    Jesus Christ appeared to me in a radical way and changed my life forever

  • Reply jokesonu420 January 16, 2020 at 1:11 am


  • Reply James Weaver January 16, 2020 at 1:11 am


  • Reply Tammy Piercy January 16, 2020 at 1:11 am


  • Reply Anime Lover January 16, 2020 at 1:11 am


  • Reply Richard Lin January 16, 2020 at 1:12 am

    I've never been this early before.

  • Reply Shumatsu Chan January 16, 2020 at 1:12 am

    Whoa that's really spoopy

  • Reply Duwalage Pasindu Chamodya Gunaratna January 16, 2020 at 1:12 am

    please help me i haven't felt a emotion a very long time

  • Reply Scarlett Chesser January 16, 2020 at 1:12 am


  • Reply Taw Meh2 January 16, 2020 at 1:12 am

    YouTube:7 comments

    Me:can I see them

    YouTube:hell no
    This is not original

  • Reply Jade D January 16, 2020 at 1:13 am

    The viruses shape us like mold

  • Reply Sophie Robinson January 16, 2020 at 1:13 am

    Retroviruses! Yay!

  • Reply linkin543210 January 16, 2020 at 1:14 am

    Retroviruses were cool before everyone else.

  • Reply Student Gaming January 16, 2020 at 1:17 am

    Real life parasyte lol

  • Reply Patty hates her life lol January 16, 2020 at 1:18 am

    retroviruses aren't like the other girls

  • Reply William Cyparski January 16, 2020 at 1:18 am

    Hank for President!

  • Reply RhysThePiece January 16, 2020 at 1:20 am


  • Reply Beau Martin January 16, 2020 at 1:20 am


  • Reply DANDAN THE DANDAN January 16, 2020 at 1:21 am

    I can see a sci-fi horror novel about an ancient retrovirus being artificially reanimated in order to turn people into zombies being written.

  • Reply Glenn Griffon January 16, 2020 at 1:22 am

    so basically the viruses are responsible for making humanity. God is an infectious disease.

  • Reply honey January 16, 2020 at 1:23 am

    this is fascinating

  • Reply Nethmi Samadhi January 16, 2020 at 1:23 am

    Superb video 👍

  • Reply Cinderball January 16, 2020 at 1:24 am

    Retroviruses: doing to evolution what your classmate did to your art project when you left the computer open during break.

  • Reply Deniss January 16, 2020 at 1:25 am

    So thats why my parents called me a parasite!

  • Reply Jeffrey Siebrecht January 16, 2020 at 1:25 am

    Not all mammals. There are non-placental mammals.

  • Reply Plipp the First January 16, 2020 at 1:25 am

    But foetuses ARE kinda like viruses attached to a host, Hank.

  • Reply ElZamo92 January 16, 2020 at 1:27 am

    What about CRISPR?

  • Reply Half_Centaur January 16, 2020 at 1:27 am

    I want to devise a virus

    to bring dire straits to your environment

    crush your corporations with a mild touch

    trash your whole computer system and revert you to papyrus

  • Reply Christopher Alcantar January 16, 2020 at 1:28 am


  • Reply tru7hhimself January 16, 2020 at 1:29 am

    interesting. but you didn't even mention transposons being degenerated retroviruses and accelerating our evolution by making it easier to duplicate and delete genes as well as aiding with crossover during meiosis.

  • Reply smdanny1 January 16, 2020 at 1:31 am

    wait, so theoretically it's possible for virus/pathogens/bites by spiders/snakes/bats to give me special abilities and superpowers like super healing factors???

  • Reply Sofía T Garcia January 16, 2020 at 1:32 am

    I love the way you present Hank! Thanks, I've been enjoying you for years.

  • Reply Emanresu56 January 16, 2020 at 1:32 am

    I'd love a YouTube channel about viruses. Is there one?

  • Reply Hououin Kyouma January 16, 2020 at 1:32 am

    Is humanity a complex virus

  • Reply John Faria January 16, 2020 at 1:33 am


  • Reply ursaltydog January 16, 2020 at 1:34 am

    HERV-E then has gone cray-cray by making us want sugar instead of good sources of nutrition..

  • Reply Keepercraft January 16, 2020 at 1:34 am

    You don't build computer only from software, and you need hardware.
    Organism look similar, only DNA don't work. You need mitochondria, cell hardware and other bacteria.

  • Reply eric gauthier January 16, 2020 at 1:34 am

    Speaking of retrovirus, hsv-1 have infected over 60%of the population with an asymptomatic behavior… maybe we'll benefit from it in a couple of century who know?

  • Reply Eduard Qualls January 16, 2020 at 1:37 am

    Almost 8 billion humans and, even with viruses, there's less genetic diversity among us than in your average, 35-or-so member chimpanzee troupe.
    Looks like we've been a species of cousin tuppers for a long time.
    [Look up tup, Joe — it'll broaden your swearing vocabulary, and make you giggle every time someone says, "Tupperware."]

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