Articles, Blog

Where Did Viruses Come From?

December 6, 2019

Thanks to Curiosity Stream for supporting
PBS Digital Studios. The Earth never shook beneath their feet. We’ve never found their remains in the rocks. And by some standards, they’re not even
alive. They’re just bits of protein and genetic
information that might give you a sniffle for a couple of days Or worse. But they’re also proof that even the very
smallest things can have an outsize impact on the history of life. I’m talking, of course, about those tiny
genetic burglars that you all have been asking about: viruses. There’s no fossil record of viruses in the
conventional sense. They’re just too small and fragile to be
preserved in rock. But there are fossils of viruses, of sorts,
preserved in the DNA of the hosts that they’ve infected. Including you. And, yeah, I mean, me too. To some extent I guess. But this molecular fossil trail can help us
understand where viruses came from, and how they evolved with the rest of us. And it can even help us tackle the biggest
question of all: Are viruses alive? The key to the viruses’ success is their
simplicity. In general, they consist of a bit of genetic
information, either DNA or RNA, wrapped in a capsule of protein. Many are small, of course, on the order
of tens of nanometers, while others are surprisingly big. But they all rely on infecting some sort of
host to reproduce and survive. We think that viruses have been around as
long as life itself, partly because they can infect all forms of life: bacteria, archaea,
and eukaryotes. And because they’re so simple, some scientists
think they evolved alongside, or even before, the earliest cells. But without real fossils, how can we know
the history of viruses? Enter the science of paleovirology. This is a young field within paleontology,
because it’s built on another emerging field: genomics. In order to look for traces of ancient viruses,
experts have to study the genomes of their hosts. It makes sense when you think about how viruses
actually work. Viruses have to infect a host cell to access
the machinery that it uses to replicate its DNA, and then hijack that machinery in order
to reproduce. Which is, like, when I say it out loud such a scumbag move The host cell is forced to manufacture new
viruses, which then leave and look for new hosts to infect. Except…the virus and the host don’t always
part ways entirely. Sometimes, the genome of the virus can become
integrated into the DNA of the host. And as long as it doesn’t cause a mutation
that damages the host cell, that bit of viral information may stay there indefinitely. And, if this happens in a cell that forms
sperm or eggs, then the viral genome can actually be inherited, passed on to the host’s offspring
with the rest of its genome. So in this way, the viral genome becomes a
sort of molecular fossil. And those ancient bits of viral information
can also shed light on how old viruses are. That’s because, ordinarily, viruses change
really quickly. That’s why you have to get a new flu shot
every year. A virus mutates so fast that, after only a
few hundred years, not much of the original genome may be left. However! If that DNA is integrated into its host, then
it can only mutate as fast as the host does. And since hosts reproduce more slowly than
viruses, their mutation rate is slower too. All this means that the viral gene will be
preserved, though not perfectly, for way, way longer than a virus that’s just floating
around out there on its own. Now, scientists can use this to help figure
out the age of virus fossils. And they do it the same way they study the
evolution of other genes: by lining up comparable sequences from different organisms, and comparing
them. If a sequence of viral DNA is found in two
different animals, then they probably both got it from a common ancestor. And that means the virus has to be at least
as old as that ancestor. So, for example, circoviruses are a group
of viruses that are known to cause stomach problems in dogs. And scientists once thought that circoviruses
had been around for less than 500 years. But traces of these viruses have been found
in the genomes of dogs, and also cats, and even pandas. So the viruses must date back to before those
mammals last shared a common ancestor, which might be as much as 68 million years ago,
in the late Cretaceous Period. So, what’s the oldest evidence of viruses? Well, one study in 2011 looked at the history
of bracoviruses, which specifically infect wasps. And it found evidence to suggest that the
group these viruses belong to, could be as old the insects themselves, dating back to
the Carboniferous Period, 310 million years ago. But other research has brought the history
of viruses even closer to home. Research in 2009 dated a gene found in mammals,
called CGIN1, to the early days of mammal evolution, between 125 and 180 million years
ago. And that gene is thought to have originally
come from a virus, because parts of it resemble a type of RNA virus called a retrovirus. And guess what. You’re a mammal. So. some retrovirus infected a sperm or egg cell
in one of our mammal ancestors millions of years ago, and now a gene derived from it
is in you. And again, yeah probably me too Scientists don’t think this gene has much
of a function, but they do think it’s just one of many examples of how viruses have left
their mark on our own DNA. In fact, it’s been estimated that 8 percent
of the human genome includes sequences that originally came from viruses. So paleovirology has helped us date the evolution
of viruses back hundreds of millions of years. But that doesn’t bring us much closer to
when we think viruses first originated, billions of years ago. Now, there are a few different models for
where viruses came from, and they’re still hotly debated by scientists. So, just be prepared if you pick a side, One model is known as the virus-first model,
and it holds that, since viruses are so much simpler than cellular life, they must have
evolved first. This would mean that viruses are older than
the oldest single-celled organisms. They’d be relics of a time when all life
was made up of simple, self-replicating units, probably made of RNA, which preyed on more complex
life forms as they evolved. But there’s also what’s known as the escape
hypothesis. This model suggests that viruses evolved after
cells, from within their own genes. See, our genomes contain pieces that can actually
copy and paste themselves from one part of our DNA to another. So, some experts think that if one of those
pieces became able to make itself a nice coat of protein, it could easily escape the cell
and become a virus. The third model hinges on the discovery of
so-called giant viruses. The first one, discovered in 2003, was named
Mimivirus — short for mimicking microbe. And these things are huge by virus standards,
around 750 nanometers across. That’s bigger than some bacteria. Now fortunately, they only infect amoebas, so
you don’t have to worry about them. At least yet. Now, Mimiviruses have way more genes than
normal viruses do, including some genes that can be used to make protein — which viruses
are not supposed to be able to do. But Mimiviruses still depend on their hosts
to reproduce, so what are all those genes doing in there? Some scientists think those genes are leftovers
from a time when some groups of viruses were bigger, more complex, and more like cellular
life. This model suggests that viruses were once
free-living and then developed a symbiotic relationship with another organism. And then over time that relationship became
parasitic. Which sometimes happens The more dependent they became on their hosts
to replicate, the more complexity the viruses lost. Or at least, so the thinking goes. But recent research has cast doubt on this
idea, known as the regressive model, at least where Mimivirus is concerned. Some scientists argue that the extra genes
in Mimivirus are just random leftovers that it picked up from its hosts over the eons. Now, these different models all put different
spins on the big question: Are viruses alive? Now I said at the beginning that paleovirology
can help us tackle this question. And it can. But the answer depends a lot on who you
ask.. Many scientists are content to just put viruses
in a sort of gray area of semi-living things. But others are determined to figure out whether
they have a place on the tree of life. And if so, where. To answer the question of whether viruses
are alive, we need to agree on a definition of life. It’s generally agreed that life can reproduce,
make energy for itself, maintain a stable environment within its cells, and can evolve,
among other things Viruses can reproduce, but not on their own. And we’ve already talked about how viruses
can evolve. But they have no way to produce energy. And they can’t control their internal environment. And that’s why they occupy such a gray area:
because the answer to some questions is yes, others no. It has been suggested that, while viruses
don’t occupy their own branch of the tree of life, they might be thought of as vines
that wrap around it. Which is an elegant image. If also maybe a little creepy one But either way, viruses are here. They’re in our DNA. They make us sick, sometimes very badly. So there’s no denying that they have a place
in the greater picture of what life on Earth is like. For good or for ill. Thanks for joining me today, and you’re
welcome for not making a joke about going viral or whatever. And thanks also to Curiosity Stream for continuing
to support PBS Digital Studios. With CuriosityStream you can stream documentary
films, and programs about science, nature, and history, including Curiosity Stream originals! One show you might like? Rapidly Evolving Human, which explores how
changes to our genetic code have made us who we are today, and why we continue to evolve. You can learn more at, and when you sign up, use the code EONS. Now, what do you want to learn about? And you know we read these comments, because that’s
where this episode came from! So leave a comment below, and if you haven’t
already, go to and subscribe.


  • Reply Electrono9 November 13, 2019 at 12:50 pm

    Where viruses come from starts at 4:45

  • Reply James Hudson November 14, 2019 at 3:36 am

    6:50 humans can't make energy for themselves. guess we're not alive either.

  • Reply Ben M November 14, 2019 at 7:46 pm

    Do I understand correctly what viruses are and do? Here's my perception: Viruses are self-driven to replicate, and they use external self-powered tools (cells) to manufacture copies of themselves. This allows viruses to stay simpler and be more flexible in taking control of and harnessing more types of external manufacturing resources. Their conservation of energy enables them to more efficiently find and enter cells, direct manufacturing, and leave cells to replicate exponentially. Doing so maximizes their success in propagating their key encoding into perpetuity. The classification of "being alive" requires an entity to be driven to and successful at propagating its key encoding via some form of being "animate" or mobile. The propagation doesn't require consciousness or decision-making. Viruses are driven to and are preeminently successful at replicating themselves. They do so by using adaptation and mobility to leverage external tools and resources. If these things are true, how can viruses not be considered living entities when they are singularly dedicated to replicating themselves just as much as, and more capably than, say, molds are?

  • Reply E :3 November 14, 2019 at 9:40 pm

    I've been asking and looking up forever where STDs originate from and how a person initially gets it (apart from being infected by it via dexual activity). Like are there people who are born with it? Or is it something that develops suddenly initially.

  • Reply JOTM222 November 15, 2019 at 2:11 am

    Pathogens in general: Enters body
    Immune System: Tough guy finger snapping

  • Reply 天楽 November 15, 2019 at 7:55 am

    So basically….
    We are like virus:
    We can reproduce, but one can not do it alone.
    We can evolve, obviously.
    Produce no energy but wastes a ton.
    No control over internal environment, being bodily or emotional.
    I see a pattern here……

  • Reply super peter November 15, 2019 at 10:52 am

    F my sianc teaher i new we yas we wer mamal

  • Reply fadrium November 15, 2019 at 11:52 am

    No virus fosil? Well insect traped in amber usually has bacteria and virus traped in it.

  • Reply The DDV Show November 15, 2019 at 8:23 pm

    The question of life is a philosophical one, not a scientific one. Yes, viruses are alive.

    A living entity is one that is self-sustaining and whose actions are self-motivated.

  • Reply Kaiser SwagHelm II November 16, 2019 at 12:48 am

    1k Anti-VaxTards disliked this work of art!

  • Reply Jaydeep Vipradas November 16, 2019 at 9:20 am

    A. Viruses somehow "knows our genes", not just individual but also as some community.
    B. They can patch our genes.
    A and B together implies that viruses are global phenomenon, not local.
    It appears, during evolution we took shortcuts. Or our consciousness and emotions are so strong that we hurt our genes. This gives opportunities for viruses to patch.
    Evolution of viruses could be emergence. Set of people behaving in certain ways can create vibrations yielding shortcomings at genetic level. As far extent of life can go in the universe, feeble part of the universe could respond to set of vibrations, as nothing goes to waste in the universe. This reaction could be a genetic code yielding viruses. That's why we could be seeing certain community is prone to some viruses and some is immune.

  • Reply Mandoon November 16, 2019 at 10:25 am

    I'm simple, am I a virus?

  • Reply Ren November 17, 2019 at 2:25 am

    Our definition of 'Life' is limited…there's a lot more out in the universe…but could we recognize it as life, when we can't even recognize a virus as such?

  • Reply D3F4ULT INPUT November 17, 2019 at 2:33 am

    They just smol rocks

  • Reply Jin Wentao November 17, 2019 at 6:43 am

    retrotransposons are 40% of human genome

  • Reply Vincent Cardona November 17, 2019 at 1:13 pm

    Ohhhh. This is why bacteria and viruses are different

  • Reply Zack Irahza November 18, 2019 at 10:40 am

    Its okay guys.
    I have my Hazmat suit on stand by at all times…

  • Reply Bryon Feliksa November 20, 2019 at 1:37 am

    Viruses drive evolution.

  • Reply tom gaming November 20, 2019 at 2:36 am

    im going viral now

  • Reply PresidentFlip November 20, 2019 at 5:27 am

    this guy looks like Lip Gallagher

  • Reply Juni Post November 20, 2019 at 6:52 am

    notices who narrator is and nearly spit takes The blooming daylights is he doing here?!

  • Reply Jeremy Kradolfer November 20, 2019 at 8:47 am

    The cell exists to create, as the virus exist to destroy! Muahaha ? ?

  • Reply Andre Lee November 20, 2019 at 11:37 am

    What's this host's name? He's cute.

  • Reply US Gaming November 20, 2019 at 12:34 pm

    I got A+ because of this video

  • Reply Jing Wang November 20, 2019 at 10:23 pm

    All viruses come from hackers. Everyone knows that.

  • Reply Alex Castañeda November 21, 2019 at 5:15 am

    Por qué el vírus herpes no se puede erradicar del cuerpo con antivírales, y el sida si se puede tratar con la vacuna recien descubierta en los USA

  • Reply Mahadev Parmekar November 21, 2019 at 6:58 pm

    Did Thanos kill 50% viruses when he snapped, or was he also confused whether they are living or non-living? ..?

  • Reply Luis Rodriguez November 21, 2019 at 11:24 pm

    I think we have found Lip Gallagher lol.

  • Reply inuyasha November 22, 2019 at 5:38 am

    All we need is a T Virus and we got zombies.

  • Reply Rizk Marshall November 23, 2019 at 6:03 am

    It’s called googooviriulbidodo Island????

  • Reply Roger Mathew November 23, 2019 at 4:53 pm

    Viruses came from JOE

  • Reply James is a James November 24, 2019 at 5:41 am

    How if, virus doesn’t exist???

  • Reply Fritz November 25, 2019 at 10:18 pm

    If you have DNA or RNA you are alive.

  • Reply pj h November 27, 2019 at 5:30 am

    really?you too?

  • Reply Parkourior November 27, 2019 at 4:22 pm

    I be like 8% dead

  • Reply libetop November 27, 2019 at 7:29 pm

    well i feel even dumber now

  • Reply Vu Ngo December 1, 2019 at 5:14 am

    So if i havnt gotten a flu shot in over 10 years and gets sick once a year if that , ill be likely to die cuz of a virus?

  • Reply Wbannie Dylan December 1, 2019 at 1:30 pm

    Why does the viruses look like the long bois in JSaB

  • Reply Mathew Hamel December 1, 2019 at 6:08 pm

    I believe that every virus exists fir the pure effort of eliminating the weak in the same way that lions and other like predators hunt.. The weak the old and the sick..
    Its called natural population control..
    Its why i do not and never will be vaccinated again. I had no choice as a child. Now, i submit myself to the pure nature of planet earth…

  • Reply vinod k December 1, 2019 at 6:34 pm

    I hate viruses and cockroaches

  • Reply kshitij singh December 1, 2019 at 10:53 pm

    Ross geller wants to know your location

  • Reply Joe Wright December 2, 2019 at 3:09 pm

    But does that mean, that technically, we're part virus?

  • Reply Natasja Teerling December 4, 2019 at 6:21 pm

    In my view on viruses: they are a kind of artificial intelligence: learning, adapting. It's a programmed piece of DNA.
    That leaves the question: who were the programmers? This is a piece of genetic technology…

  • Reply Wilhelm Vermeulen December 4, 2019 at 7:54 pm

    Always like to watch episodes on evolution. Love your chanel also. Please do a episode on protists. That would be, awesome. Thanks

  • Reply The flamingsword December 5, 2019 at 3:25 am

    I like to think of them as natural machines that can self replicate, like stars. Stars are in no way alive, yet they have a mechanism for creating more, similar, copies themselves. Because of this, they still currently exist and there are many of them.

  • Reply Robert Miller December 5, 2019 at 11:03 am

    They came from dirty vag

  • Reply Paul Jackson December 5, 2019 at 6:03 pm

    I'm interested in the content, but do I ever hate the stand-up comedy style of these videos.

  • Reply GD Fuzion December 6, 2019 at 12:39 am

    me in school: bad grades
    Me: They Havent seen the best of mr.

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