HOST: Sometime in the distant past, between about 3 million and 1.4 million years ago, a hominid in Africa had a meal that would affect humanity forever.

This hominid 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 hominid 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 have 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 have been eaten raw.

Now while this part of the story is speculative, we think we know what the outcome was, and we could use that endpoint to come up with hypotheses about what might have happened to get us there.

And one of those hypotheses is that this ancient hominid may have gotten more than it bargained for in that fateful meal.

It might have contracted a virus, one that 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 herpes viruses, which are a very old lineage of DNA viruses.

There are lots of different kinds of herpes virus, and they've been infecting vertebrates and co-diverging with them for millions of years.

As each species split off from its common ancestor, the viruses did too.

This means that the family tree of the virus mirrors the family tree of their hosts.

And in primates, the family tree of herpes viruses mostly matches up with a part of their 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 herpes virus that's specific to one species of primate, and each virus split off from the herpes virus family tree when their 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 herpes viruses.

There is HSV-1, or oral herpes, which causes cold sores, and there's HSV-2, which causes genital herpes.

Both viruses can be 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 herpes virus called ChHV-1.

And in them, it causes oral symptoms just like HSV-1 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, ChHV-1, the chimp virus, just switched hosts.

It went from some early proto-chimp to one of our hominid relatives, possibly through the pathway I just described, with a hominid 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 HSV-2, genital herpes, was 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 that in organisms that lived millions of years ago.

Two groups of scientists have taken a crack at figuring out the history of herpes viruses in humans, and the 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 this all 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 herpes virus split off from its ancestor was that HSV-1-- human oral herpes-- and chimp herpes diverged about 6 million years ago, which is also 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 with a virus that causes genital herpes.

HSV-2 seems to have split from the chimp virus much more recently, between 3 million and 1.4 million years ago.

And these researchers think that 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 in human evolution, you know that this was a pretty crowded time for hominids 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 hominids 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 hominids 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 have been in the past, since that's where ancestral chimps would have lived.

Then they added data about where hominid 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 HSV-2 to get from ancestral chimps to Homo erectus.

And they focused on Homo erectus because the virus would only have 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 pathway went 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 have been easy for a hominid 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 have 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 have 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 hominids might have hunted each other.

Like maybe Homo erectus hunted an ape Paranthropus, which is how that link in the transmission chain could have happened.

Now the 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 the virus 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 HSV-2 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 HSV-1 that we had already inherited from a distant ancestor was already occupying the niche of infecting the mouth.

That virus came first.

So HSV-2 might have changed venues in order to avoid competition with HSV-1, even though both viruses can affect 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 are really interesting ways 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 just 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 hominid 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 hominid 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.