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 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 viruses' success is their simplicity. In general, they consist of a bit of genetic information, either DNA or RNA, wrapped in a protein capsule. 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 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 as the insects themselves, dating back to the Carboniferous Period, 310 million years ago.

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 protein coat, 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, not yet. Now, Mimiviruses have way more genes than normal viruses do, including some genes that can be used to make proteins -- 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. 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 life on Earth.

It's 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 Curiosity Stream, you can stream documentary films and programs about science, nature,

and history, including Curiosity Stream originals! One show you might like?  Rapidly Evolving Humans, 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.

Where Did Viruses Come From?

PBS Eons

바이러스와 그 영향에 대한 소개

바이러스는 기존의 화석 기록은 없지만 생명의 역사에 큰 영향을 미칠 수 있는 작은 유전적 요소입니다.

바이러스는 '작은 유전자 도둑'으로 묘사되며 그 영향에 대해 논의합니다.

전통적인 화석 기록은 없지만 바이러스는 숙주의 DNA에 흔적을 남깁니다.

바이러스의 생존 여부에 대한 질문이 제기됩니다.

바이러스의 성공 비결은 단백질 캡슐에 유전 정보를 감싸고 숙주를 감염시켜 번식하는 단순성에 있습니다.

고생물학 및 바이러스 게놈 통합

고생물학은 유전체학을 활용하여 숙주의 게놈을 통해 고대 바이러스를 연구합니다.

바이러스는 숙주 세포의 메커니즘을 탈취하여 복제하고 때로는 숙주의 DNA에 자신의 게놈을 통합하기도 합니다.

바이러스 게놈은 숙주의 DNA에 통합되어 자손에게 유전되어 분자 화석처럼 작용할 수 있습니다.

숙주 DNA에 통합된 바이러스 DNA는 돌연변이가 느리게 일어나므로 자유 부유 바이러스보다 더 오래 보존됩니다.

과학자들은 다양한 유기체의 DNA 염기서열을 비교하여 바이러스의 나이를 파악함으로써 바이러스의 진화를 연구합니다.

가장 오래된 바이러스와 데이트하기

연구에 따르면 브라코바이러스는 곤충만큼이나 오래된(3억 1천만 년) 바이러스일 수 있다고 합니다.  

포유류의 유전자(CGIN1)는 1억 2,500만~1억 8,000만 년 전으로 거슬러 올라가는데, 레트로바이러스에서 유래한 것으로 추정됩니다.

인간 게놈의 약 8%는 바이러스에서 유래합니다.

'바이러스 우선' 모델은 바이러스가 세포보다 먼저 진화했다고 가정합니다.

'탈출 가설'은 바이러스가 세포 유전자 내에서 진화했다는 가설입니다.

'퇴행 모델'은 바이러스가 한때 자유롭게 살다가 기생하는 존재가 되었다고 가정합니다.

최근 연구에서는 특히 미미바이러스와 관련하여 회귀 모델에 의문을 제기하고 있습니다.

과학자들은 바이러스의 분류에 대해 논쟁을 벌이고 있으며, 일부는 바이러스를 반생명체로 간주하기도 합니다.

생명체의 정의에는 번식, 에너지 생산, 안정된 환경 유지, 진화 등이 포함되는데, 바이러스는 일부 기준을 충족하지만 모든 기준을 충족하지는 않습니다.

바이러스는 생명나무를 감싸고 있는 '덩굴'로 묘사됩니다.

바이러스는 모호한 분류에도 불구하고 생명에 미치는 영향은 부인할 수 없습니다.

Where Did Viruses Come From?

Where Did Viruses Come From?

스크립트