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  • 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 wouldve 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 wouldve 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 mightve 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 virusone 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 theyve 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.

  • Were 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 youve 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 wouldve been in the past, since that’s where ancestral

  • chimps wouldve 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 wouldve only had to make it to erectus

  • to have ended up in us, since theyre 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 wouldve 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 wouldve 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 wouldve 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 couldve 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 mightve changed venues in order to avoid competition with HSV1 - even though

  • both viruses can infect both locations, theyre 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, theyre really interesting ways for us to draw insights about our evolutionary

  • history, but theyre still hypotheses.

  • Data that we collect in the future could contradict them, that’s just how science works.

  • But when were 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 well 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 were 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.

Sometime in the distant past, between about 3 million and 1.4 million years ago, a hominin

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我们已经存在了数百万年的两种病毒(The Two Viruses That We’ve Had For Millions of Years)

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    joey joey 發佈於 2021 年 05 月 02 日
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