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  • 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.

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  • Now, what do you want to learn about?

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B1 中級 新型コロナウイルス 新型肺炎 COVID-19

ウイルスはどこから来たのか? (Where Did Viruses Come From?)

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    robert に公開 2021 年 01 月 14 日
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