Placeholder Image

字幕列表 影片播放

  • Around 33 million years ago, in the late Oligocene Epoch, a new virus emerged and infectedsome

  • kinda mammal.

  • As were all well aware at this point, viral infections happenand spreadall the

  • time.

  • But this time was different.

  • It would turn out to be the beginning of a viral outbreak that left its mark in the genomes

  • of almost every order of mammals around today -- the start of a huge, ancient pandemic,

  • one that touched many different species, spanned the globe, and lasted for more than 15 million

  • years!

  • Yes.

  • Imagine: A pandemic that lasted 15 million years.

  • And we know this pandemic happened because, in 2001, biologists discovered part of this

  • ancient virus trapped in our DNA -- a molecular fossil entombed in our own genome.

  • This fossil was not the actual virus itself, but a fragment of its genetic material.

  • Scientists named this DNA sequence ERV-Fc, and it’s just one of a growing number of

  • viral fossils that have been discovered in the genomes of modern animals.

  • And, I know you have questions about this, because I definitely do.

  • Like, what was this virus?

  • How did it end up in our genome?

  • What disease did it cause, if any?

  • And is it...doing anything in there now?

  • While we don’t have all of the answers yet, the legacy of this ancient pandemic is just

  • one of the many stories hidden in our very own DNA, just waiting to be discovered.

  • Whether theyre animals, plants, or fungi, all living organisms have at least one thing

  • in common: Their DNA is riddled with the remains of ancient viruses - and humans are no exception.

  • These viral leftovers are known as Endogenous RetroViruses, or ERVs.

  • And most of them are the remnants of ancient retroviruses that infected the egg or sperm

  • cells of our mammal ancestors.

  • Those kinds of viruses insert themselves into their host’s DNA as part of their normal

  • life cycle.

  • And when this happens in reproductive cells, that viral sequence can be passed down from

  • parent to offspring.

  • Now, this doesn’t happen with every viral infection.

  • In fact, viruses embedding in the genome of their host is pretty rare.

  • Because everything has to go just right for this to happen.

  • First, the virus has to integrate into the DNA of sperm or egg cells at just the right

  • spots to not kill those cells or disrupt how they work.

  • Then, those cells have to be the lucky few that actually end up forming a fertilized

  • egg, which is what allows the ERV to be passed down to the next generation.

  • And then, it has to keep getting passed on - even as those organisms change and evolve

  • over time.

  • But over millions of years, these rare events have happened often enough to leave a viral

  • fossil record behind.

  • In some ways, it’s actually a lot like the regular kinds of fossilization that we talk

  • about here: it requires specific conditions and only happens some of the time.

  • Nonetheless!, ERVs are so common in our DNA that, by most estimates, they make up around

  • 8% of our total genome.

  • That’s about four times higher than the proportion of our genome that’s made up

  • of genes - the sequences that actually code for proteins.

  • Yes, you heard that right, but just in case you didn’t get it I’m gonna say it again:

  • In terms of DNA content, youre about 8% virus and 1 to 2% genes.

  • So what I'm saying is....

  • You are more virus than you are genes!

  • Now, figuring out where each bit of that viral DNA came from isn’t easy.

  • Many of these sequences have built up so many mutations and been shuffled around so much

  • that theyre really fragmented and hard to make sense of.

  • But in the case of ERV-Fc, that sequence tells the story of an ancient, global, multi-species

  • pandemic.

  • Or, maybe more accurately, a series of epidemics, played out over a huge span of time.

  • And, while ERV-Fc was first identified in 2001, it wasn’t until 2016 that a group

  • of biologists at Boston College was able to piece together the history of that particular

  • ancient virus.

  • They searched the genomes of 50 modern species of mammals to identify fossilised sequences

  • of ERV-Fc and found them embedded in the DNA of over half of those species.

  • Those sequences were even shared between species as distantly related as humans and aardvarks.

  • Now, when a biologist sees this kind of pattern, the first thing they try to figure out is

  • whether there’s some simple evolutionary explanation for it.

  • Like, did the virus infect some early mammal that was the common ancestor of many different

  • groups, so that this molecular fossil was just passed down to all of its descendants?

  • In this case, the answer turned out to be no - it’s a little more complicated than

  • that.

  • Instead of one infection event, it looked like this virus independently infected different

  • mammal species and embedded itself in their genomes multiple times.

  • For example, 12 of the mammals studied, including humans, pandas, and dogs, all have more than

  • one genetically distinct sequence of ERV-Fc in their genomes.

  • This suggests that there were many independent infection events by different strains of the

  • virus as it evolved while spreading around the world.

  • So, we actually have two different fossils of ERV-Fc in our DNA and, as far as we know,

  • they don't do anything right now.

  • The older one comes from an infection in the common ancestor of all great apes, sometime

  • between about 20 million and 16 million years ago.

  • And the younger one we got from the common ancestor of all African great apes, between

  • about 16 million and 9 million years ago.

  • As for how widespread this pandemic was, it’s likely that the virus made it to every continent

  • except Antarctica and Australia, which were geographically isolated at the time.

  • And this may explain why no Australian marsupials in the study, like wallabies and Tasmanian

  • devils, were found to have any of that virus in their genomes.

  • So, those are some of the things that we can figure out about this ancient pandemic.

  • But there are other things that we just can’t, because the fossil record of this virus is

  • incomplete.

  • After all, there are probably species that lost their sequences of ERV-Fc over time.

  • And others were probably infected, but the virus didn’t embed itself in their DNA successfully.

  • And some lineages that might have carried the viral fossil have justgone extinct.

  • So, were missing pieces of the puzzle, which makes some of the details of the viruses

  • spread a mystery.

  • But, by using that method known as the molecular clock, and by building a family tree of the

  • virus strains across mammal species, researchers think that the virus originated around

  • 33 million years ago.

  • And from there, it spread around the worldfor at least 15 million years.

  • While we don’t know what species the virus infected first, we can tell that it started

  • jumping from group to group pretty quickly.

  • There’s evidence of at least 26 independent transmissions between species.

  • And some of these events look pretty weird, because the jumps weren’t always between

  • closely related species.

  • For example, the sequence of this virus found in dolphin DNA is closest to those found in

  • rodents and rabbits.

  • Now, this doesn’t mean a dolphin caught the virus directly from a rabbit, or vice

  • versa.

  • Although I personally would love to know what that would look like.

  • Like I said, the fossil record of this virus is incomplete, so were probably missing

  • some steps there.

  • All we have to go on is what we know about how viruses jump between species today, like

  • by one species preying or scavenging on another.

  • Beyond that, we just don’t know.

  • But one of the reasons that the virus was so successful at hopping between species may

  • have been its ability to swap genes with other strains, and with other totally different

  • viruses, over the course of the outbreak.

  • This is known as viral recombination, and it can happen when two different viruses infect

  • the same animal at the same time.

  • This lets them splice elements of their genomes together as they replicate within the same

  • cell.

  • And it happens naturally all the time.

  • In fact, it looks like this is probably what happened to allow SARS-CoV-2 - the virus that

  • causes COVID-19 - to spread so successfully.

  • There’s good evidence that most of this virus is similar to coronaviruses seen in

  • bats, but a bit of the protein that helps it enter its target cells looks more similar

  • to another one found in a pangolin.

  • We still don’t know what host animal was infected by both viruses, though.

  • Now, recombination can also happen when a virus picks up genes from viruses that had

  • already embedded some of their sequences in the host’s genome.

  • And researchers have found evidence of multiple recombinations over the history of ERV-Fc.

  • So it mightve gotten the genes that helped it jump between species from other viruses

  • and other strains.

  • This seems even more likely because one of those recombinations is in the protein that

  • helped the ancient virus enter its target cells.

  • Which sounds...familiar.

  • But what kind of virus was ERV-Fc?

  • And how deadly was it?

  • Well, we do know that the virus infected mammals, replicated in their cells, and occasionally

  • became embedded in their genomes.

  • And we can look at its closest living viral relatives to try to figure out what kind of

  • infection it might have caused.

  • Based on its genetic structure, ERV-Fc is an ancient member of a genus called Gammaretroviruses.

  • Today, this group includes the Murine leukemia virus in mice and Feline leukemia virus in

  • cats.

  • And one of the most common effects of this group of viruses is oncogenesis, or the formation

  • of cancer in their hosts.

  • So, it’s possible that the ERV-Fc virus caused similar illnesses - inducing cancers.

  • But because we don’t know what kinds of cells it infected, we can't tell whether it

  • was likely to have caused leukemias or some other kind of cancer.

  • And we might never know.

  • As the genomes of more species are sequenced, our ability to untangle the histories of these

  • viral fossils will only improve.

  • Because, ERV-Fc isn’t the only viral fossil in our DNA.

  • There are mannny others in the human genome, whose stories are waiting to be told.

  • And without the record in our genome, we’d never have known this epic, ancient pandemic

  • happened.

  • And these ancient remnants don’t just help us understand what happened in the deep past.

  • They show us that viruses have jumped between species for millions of years - and still

  • do.

  • And they also help us understand, and predict, how new viruses behave and spread today.

  • Which, I think we can all agree, is pretty important.

  • From one mammal millions of years ago, to our DNA today, the story of ERV-Fc reminds

  • us that pandemics are part of the history of life on our planet.

  • And, whether we like it or not, theyll be part of our future, too.

  • Now, while ERV-Fc is a viral fossil, there’s other ancient viruses that are still actively

  • infecting us - find out more in our episode, “The Two Viruses That Weve Had For Millions

  • of Years”.

  • Also - I just sanitized my hands - high fives to this month’s Eontologists: Sean Dennis,

  • Jake Hart, Annie & Eric Higgins, John Davison Ng, and Patrick Seifert!

  • Become an Eonite by supporting us at patreon.com/eons.

  • Eonites get fun perks like submitting a joke for me to read

  • like this one. Which I have not seen before so

  • Ok this is from Bethie Kennedy.

  • What is a dinosaur's least favorite reindeer?

  • Comet!

  • That feels like too soon to me

  • Is that too soon?

  • And as always thanks for joining me in the Konstantin Haase studio.

  • Subscribe at youtube.com/eons for more adventures in deep time

Around 33 million years ago, in the late Oligocene Epoch, a new virus emerged and infectedsome

字幕與單字

影片操作 你可以在這邊進行「影片」的調整,以及「字幕」的顯示

B2 中高級 美國腔

持续了1500万年的大流行(The Pandemic That Lasted 15 Million Years)

  • 13 1
    joey joey 發佈於 2021 年 05 月 03 日
影片單字