B1 中級 511 タグ追加 保存
動画の字幕をクリックしてすぐ単語の意味を調べられます!
単語帳読み込み中…
字幕の修正報告
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 curiositystream.com/eons, 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 youtube.com/eons and subscribe.

コツ:単語をクリックしてすぐ意味を調べられます!

読み込み中…

Where Did Viruses Come From?

511 タグ追加 保存
robert 2018 年 7 月 15 日 に公開
お勧め動画
  1. 1. クリック一つで単語を検索

    右側のスプリクトの単語をクリックするだけで即座に意味が検索できます。

  2. 2. リピート機能

    クリックするだけで同じフレーズを何回もリピート可能!

  3. 3. ショートカット

    キーボードショートカットを使うことによって勉強の効率を上げることが出来ます。

  4. 4. 字幕の表示/非表示

    日・英のボタンをクリックすることで自由に字幕のオンオフを切り替えられます。

  5. 5. 動画をブログ等でシェア

    コードを貼り付けてVoiceTubeの動画再生プレーヤーをブログ等でシェアすることが出来ます!

  6. 6. 全画面再生

    左側の矢印をクリックすることで全画面で再生できるようになります。

  1. クイズ付き動画

    リスニングクイズに挑戦!

  1. クリックしてメモを表示

  1. UrbanDictionary 俚語字典整合查詢。一般字典查詢不到你滿意的解譯,不妨使用「俚語字典」,或許會讓你有滿意的答案喔