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.