Name: The Pandemic That Lasted 15 Million Years
Uploaded: 2021-05-03T04:28:01.000Z
Duration: 10 min 7 s
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Around 33 million years ago, in the late Oligocene Epoch, a new virus emerged and infected … some

As we're all well aware at this point, viral infections happen … and spread … all the

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

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.

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 they're 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

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

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

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

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

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

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, you're about 8% virus and 1 to 2% 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 they're 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

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

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

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,

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

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 just … gone extinct.

So, we're missing pieces of the puzzle, which makes some of the details of the viruses'

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

And from there, it spread around the world … for at least 15 million years.

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

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The Pandemic That Lasted 15 Million Years

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