Sometime in the distant past, between about 3 million and 1.4 million years ago, a hominin
in Africa had a meal that would affect humanity forever.
This hominin probably consumed the meat of another primate -- specifically, one belonging
to the species from which chimpanzees and bonobos both evolved.
We don’t know if the hominin hunted down that ancestral-chimp or just scavenged its
remains, but it’s safe to assume that removing the meat from its bones was not a clean task.
While tearing up the carcass, it would’ve gotten blood on its hands, and maybe some
of it got into cuts or sores on its skin.
And this scenario likely played out before we see the control of fire in the archaeological
record, so that ancestral-chimp meat would’ve been eaten raw.
Now, while this part of the story is speculative, we think we know what the outcome was.
And we can use that end-point to come up with hypotheses about what might’ve happened
to get us there.
And one of those hypotheses is that this ancient hominin may have gotten more than it bargained
for in that fateful meal.
It might have contracted a virus — one that’s stayed with humanity for millions of years,
even before our species existed, and which has been passed down to us.
That virus was herpes.
The virus that we commonly call herpes is just one member of a very large group of viruses
known as herpesviruses, which are a very old lineage of DNA viruses.
There are lots of different kinds of herpesvirus, and they’ve been infecting vertebrates and
codiverging with them for millions of years.
As each species split off from its common ancestor, the viruses did, too.
This means the family tree of the virus mirrors the family tree of their hosts.
And in primates, the family tree of herpesviruses mostly matches up with the part of the primate
tree that includes monkeys, apes, and humans.
Scientists think this pattern of co-diverging goes back at least 44 million years, when
New World and Old World Monkeys split off from their common ancestor.
So, basically, there’s one kind of herpesvirus that’s specific to one species of primate,
and each virus split off from the herpesvirus family tree when the primate split off from
its own tree.
But of course, humans are a special kind of primate.
We’re the only primate we know of to be infected by two kinds of herpesviruses: There’s
HSV1, or oral herpes, which causes cold sores, and there’s HSV2, which causes genital herpes.
Both viruses are transmitted by direct, skin-to-skin contact with the infected area, or with a
sore, or with saliva, or with genital secretions that are infected with the virus.
And while these viruses mostly don’t cause symptoms, when they do, it’s generally in
the form of painful blisters in the affected area.
Now, our closest living relatives, chimps, have their own herpesvirus, called ChHV1.
And in them, it causes oral symptoms just like HSV1 does in us.
But it turns out, the virus that causes genital herpes in humans is actually more closely
related to chimp herpes than it is to our own cold-sore virus.
Which is weird, since these related viruses infect different hosts.
So, the simplest explanation for this is that, at some point in the distant past, ChHV1 -- the
chimp virus -- just switched hosts.
It went from some early, proto-chimp to one of our hominin relatives -- possibly through
the pathway I just described, with a hominin handling the butchered remains of a primate.
And from there, the virus went on to evolve into what’s now one of the most common sexually
transmitted infections in humans.
That means the origin of our HSV2 -- genital herpes -- was as a zoonotic disease - one
that moved from an animal into, in this case, a human relative.
And that puts it in pretty infamous company, like HIV, classic SARS, MERS, Ebola, and,
yes, probably also SARS-CoV-2, the coronavirus that causes COVID-19, although we don’t
know yet what its original animal host was.
Now, figuring out how and when viruses are transmitted and switch between hosts is hard,
even in living species.
It takes the efforts of scientists from lots of different fields, and often many long years
of work.
So, imagine trying to do all that in organisms that lived millions of years ago.
Two groups of scientists have taken a crack at figuring out the history of herpesviruses
in humans, and their results are pretty intriguing.
They think that, in order for herpes to get from that ancestral chimp to us, it must have
involved at least one, and maybe more than one, of our extinct cousins.
To try to figure out how and when all this happened, they started with the modern viruses
themselves.
In 2014, a group of researchers looked at the family tree of primate herpes and came
up with 10 scenarios that could explain the relationship between the two human viruses
and the chimp virus.
All 10 of these scenarios were pretty simple and required just one of two possible events:
either a single transmission event between different species, or one instance of the
ancestral virus splitting into two lineages within the same host.
So, to test these scenarios, they used a new computational method that combined different
models of evolution, selection, and molecular clock dating.
And, the scenario that best fit the dates for when each herpesvirus split off from its
ancestor was that HSV1 - human oral herpes - and chimp herpes diverged around 6 million
years ago, which is around the time we shared our last common ancestor with chimps.
So it seems that we didn’t get infected with the oral herpes virus from some chimp
ancestor -- instead, we inherited it, from an ancestor we share with chimps.
Which means the oral herpes virus has always been specific to our lineage.
But!
That’s not the case for the virus that causes genital herpes!
HSV2 seems to have split from the chimp virus much more recently, between 3 million and
1.4 million years ago.
And these researchers think it happened by jumping from a proto-chimp into an extinct
species in our genus, Homo.
So, that study gives us a when and a where for the origin of human genital herpes - sometime
in the late Pliocene or early Pleistocene Epoch.
But if you’ve ever watched any of our videos on human evolution, you know that this was
a pretty crowded time for hominins in Africa.
Lots of our relatives were around then, possibly interacting with proto-chimps and with each
other, and developing all sorts of interesting behaviors, like maybe hunting and scavenging.
So, with this in mind, in 2017, another group of scientists picked up where the first group
left off.
They wanted to figure out which of those hominins was most likely the one that got infected
with chimp herpes, and passed it down to us as genital herpes, today.
This group started by gathering data on extinct hominins in Africa: where they were found
and when they lived.
But they couldn’t do the same for chimpanzees, because there are no chimp fossils from before
500,000 years ago.
So, instead they used the modern ranges of chimps and bonobos, combined with data on
where tropical rainforests would’ve been in the past, since that’s where ancestral
chimps would’ve lived.
Then they added data about where hominin fossils were located in space and time relative to
those ancient rainforests.
This let them come up with a bunch of potential pathways for the virus that would eventually
give rise to HSV2, to get from ancestral chimps to Homo erectus.
And they focused on Homo erectus because the virus would’ve only had to make it to erectus
to have ended up in us, since they’re thought to be our direct ancestors.
After doing a whole bunch of math, the team found the two most likely pathways for transmission.
One was from ancestral chimps to Homo habilis to Paranthropus boisei to Homo erectus.
The second was a little more direct: from the ancestral chimps to Paranthropus to Homo
erectus.
So either way, Paranthropus boisei seems to be a key step in the transmission chain.
And experts think it would’ve been easy for a hominin to pick up the virus by processing
and eating the raw, infected meat of some proto-chimp, or maybe through the bite of
a chimp during a hunt.
Both Homo habilis and Paranthropus boisei are known from sites with stone tools and
tool-marked bones.
All they would’ve needed was a small open wound, either on their hands or in their mouths,
to serve as the entry point for the virus.
So the transmission would’ve happened through contact with an open sore or a mucous membrane,
not in the digestive tract itself.
From there, we know that Paranthropus boisei and Homo erectus overlapped in time, at sites
in Kenya, and that Homo erectus almost certainly hunted and butchered prey.
And the researchers even suggest that hominins might have hunted each other.
Like, maybe Homo erectus hunted and ate Paranthropus, which is how that link in the transmission
chain could’ve happened.
Now this research does also suggest that the virus could have been transmitted by mating,
which I’m sure has crossed your mind.
But they think the hunting pathway was more likely.
Either way, once it got into Homo erectus, the virus stayed there for generations, passed
on either through mating or from mother to offspring.
And it remained there when we diverged from Homo erectus as our own species, Homo sapiens.
As for why HSV2 became genital herpes, instead of staying an oral herpes, well, that might
have to do with the evolutionary process known as niche partitioning.
The cold sore virus, HSV1, that we had already inherited from a distant ancestor was already
occupying the niche of infecting the mouth.
That virus came first.
So, HSV2 might’ve changed venues in order to avoid competition with HSV1 - even though
both viruses can infect both locations, they’re just better adapted for one or the other.
Now, it’s worth pointing out that both of these studies are based on models and probability.
So, they’re really interesting ways for us to draw insights about our evolutionary
history, but they’re still hypotheses.
Data that we collect in the future could contradict them, that’s just how science works.
But when we’re studying our evolutionary history, whether in paleoanthropology or paleovirology,
sometimes the best we can do is to use the frameworks that we see in the modern world,
and try to apply them to the past.
Like, we know that modern viruses can jump between species when they come in contact
with each other.
And we can build evolutionary trees for viruses, to try to retrace the genetic steps that lead
back to the origin of a virus.
So, in the case of herpes, that series of steps seems to have involved not just our
probable direct ancestor, Homo erectus, but also our hominin cousin, Paranthropus boisei.
Who knows whether we’ll ever be able to test this hypothesis in the future.
But we can say for sure that viruses have been transmitted between species in the distant
past, like with that hominin that may have feasted on some dead primate more than a million
years ago.
And it’s been happening ever since.
Sometimes the virus that’s passed along is relatively mild, like herpes often is.
Sometimes, it’s much worse.
But for now, the thing to keep in mind is that this has been happening to us, and our
ancestors, for millions of years.
And we’re still here.
Thanks to this month’s Eontologists: Patrick Seifert, Jake Hart, Jon Davison Ng, Sean Dennis,
and Steve!
You can join them by pledging your support at patreon.com/eons.
Also thanks for joining me today in the Konstantin Haase studio.
If you want to join us for more adventures in deep time, just go to youtube.com/eons
and subscribe.