Why Does Evolution Keep Inventing Mind Control?

What if your thoughts, fears and desires weren't entirely your own? What if something living inside your body could change your behaviour, without you ever knowing? Professor Hannah Fry and Michael Stevens (Vsauce) enter the disturbing world of parasites. From worms that grow inside crickets to viruses that alter animal behaviour, they explore the extraordinary lengths parasites will go to in order to survive and reproduce. But how does one organism evolve the ability to manipulate another? How do parasites overcome the body's defences? Along the way, Hannah and Michael investigate 500 million year old fossils, parasites that hitchhike on mosquitoes, the strange science of rabies and the unsettling relationship between cats, rats and a parasite that may even influence our own behaviour. ------------------- For more information about Cancer Research UK, their research, breakthroughs and how you can support them, visit https://cancerresearchuk.org/restisscience Cancer Research UK is a registered charity in England and Wales (1089464), Scotland (SC041666), the Isle of Man (1103) and Jersey (247). A company limited by guarantee. Registered company in England and Wales (4325234) and the Isle of Man (5713F). Registered address: 2 Redman Place, London, E20 1JQ. ------------------- Find The Rest Is Science all over the internet by ⁠⁠clicking here.⁠⁠ ------------------- Video Producer: Adam Thornton + Oli Oakley + Jack Meek Animator: Sam Benson Video & Social: Bex Tyrrell Producer: Simona Rata + Lucy Lipscombe Senior Producer: Lauren Armstrong-Carter Chief Digital Officer: Samuel Oakley Exec Producer: Neil Fearn Learn more about your ad choices. Visit podcastchoices.com/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:01Michael Stevens?

  Hello and welcome to The Rest Is Science. I'm Michael Stevens.

• 0:04Hannah Fry?

  And I'm Hannah Fry. Today, Michael, I thought I thought I'd introduce you to my new housemates. I've I've got them right here. There's 6,000 of them.

• 0:12Michael Stevens?

  Oh.

• 0:13Hannah Fry?

  They are, they're all teeny tiny little assassins. That's, that's what I've, that's what I've invited in to live with me. They are known as Trichogramma evenisans.

• 0:24Michael Stevens?

  Oh, I've heard they're cute.

• 0:25Hannah Fry?

  Here they are. They all live in here. And they are a parasitic wasp that I have willingly and lovingly invited into my home.

• 0:34Michael Stevens?

  Okay, so hold on. You're holding up a white bag that's all sealed up. Are they like freeze dried?

• 0:39Hannah Fry?

  Their little eggs are in here. They're all.

• 0:41Michael Stevens?

  Oh, they're eggs.

• 0:43Hannah Fry?

  Mm hmm.

• 0:44Michael Stevens?

  Okay, so I've got a lot of questions.

• 0:46Hannah Fry?

  Hmm. Okay, go on.

• 0:48Michael Stevens?

  First of all, parasitic to what? To humans?

• 0:52Hannah Fry?

  No, to clothes moths.

• 0:55Michael Stevens?

  Oh.

• 0:57Hannah Fry?

  Mmm.

• 0:58Michael Stevens?

  So can you get these? First, how does it work? So you've got a bunch of parasitic wasp eggs. Do you just sprinkle them all over your clothes?

• 1:07Hannah Fry?

  No, you like, it comes in a little sachet like this, and then you pop this in the bottom of the wardrobe. And then the, all of the little wasps will emerge. They're teeny, teeny tiny, by the way, like, they're so small, you can't even really see them.

• 1:19Michael Stevens?

  You won't feel like your house is full of wasps.

• 1:21Hannah Fry?

  Oh, no. We're not talking about yellow jackets here, you know? This isn't like, all of a sudden, you're eating jam and there's a swarm around you. No, this is like, mini, mini, mini, mini, mini little things. And they emerge, so you sort of pop this inside your wardrobe, they emerge from the packet, they go off, they do their thing. And then, very soon, a few weeks, all the, all the moss are gone. It's great. I've done, so I've done, this is like the second or third round that I've done over many years.

• 1:48Michael Stevens?

  Oh, okay. And it's worked every time, but you do need to periodically re-wasp your home.

• 1:53Hannah Fry?

  And then when they arrive in the post, they've got this little label on them saying, "Live cargo."

• 1:58Michael Stevens?

  Wow.

• 2:00Hannah Fry?

  Yeah. Do you want to see a picture of what they look like? Hang on.

• 2:02Michael Stevens?

  Yeah.

• 2:03Hannah Fry?

  And when, when they're up close.

• 2:04Michael Stevens?

  They've got these golden yellow bodies, wings and bright red eyes.

• 2:09Hannah Fry?

  Yeah, they're pretty, aren't they? They're really pretty.

• 2:11Michael Stevens?

  Oh, and they are tiny. They're only a fiftieth of an inch long, which means they're like half a millimeter long.

• 2:21Hannah Fry?

  Right.

• 2:22Michael Stevens?

  That is smaller than I imagined. Like, that is a dime.

• 2:27Hannah Fry?

  Yeah, yeah, yeah.

• 2:28Michael Stevens?

  That is a point bug.

• 2:29Hannah Fry?

  Right, totally. I mean, even the, even the moth eggs that they're going into are, I mean, like, the size of a full stop on a sentence.

• 2:37Michael Stevens?

  Right.

• 2:37Hannah Fry?

  The thing is, right, so they, they're very effective. They're really effective, these things. I'm, I'm a big fan of them. But also, and I say this as their employer, they are sort of unimaginative in the way that they are parasitic in, across another species. They want the body, they want, they want sort of the, the body of the moth egg. They go in and they eat it and then they leave. And then they, they don't really have a lot of vision. I think that's what I'm saying here. Because if you're going to be a parasite, there's, there's some pretty big parasites to, to live up to here. And these parasitic wasps are, they're kind of low down on the pecking order of what, what potentially parasites can do. Because there are other parasites that manage to not just get into the egg and eat it and then leave,

• 3:19Hannah Fry?

  but properly get into the body of the other creature. So this summer, I was on holiday in France.

• 3:31Hannah Fry?

  This episode is brought to you by Cancer Research UK.

• 3:34Michael Stevens?

  We know that DNA holds a lot of information, but here's a way to visualize it. If we encoded all the movies ever made in all of history into DNA, that clump of DNA would be smaller than a sugar cube.

• 3:49Hannah Fry?

  It is packed with data. And so Cancer Research UK scientists are analyzing cancer DNA to understand what makes every cancer different. They're even searching through what was once considered genetic junk.

• 4:02Michael Stevens?

  That's because uncovering these differences could lead to more personalized and precise ways to prevent, detect and treat the disease.

• 4:10Hannah Fry?

  Over the last 50 years, Cancer Research UK's work has helped to double cancer survival in the UK. And today, it's continuing to save and improve lives around the world.

• 4:20Michael Stevens?

  For more information about Cancer Research UK, their research, breakthroughs, and how you can support them, visit cancerresearchuk.org/RestIsScience.

• 4:31Speaker 02

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• 6:00Speaker 04

  This episode is brought to you by Google Chrome. You think you know a browser, but Gemini and Chrome, that's new. It can help you with practically anything on the web, like restoring a vintage motorcycle from a 50-page restoration block, or finally break down that long article you've had open for weeks. Gemini and Chrome is here for it. Ready to make anything online make sense? There's no place like Chrome. Check responses set up required. Compatibility and availability varies. 18 plus.

• 6:30Hannah Fry?

  And there was, there was a cricket in my sister's bathroom. So she, she got the cricket, she trapped it in a glass, and then she used the side of the glass to squish the cricket, right? Look, we're not proud of it, but we kill insects sometimes, okay? Anyway, as she did that, she got her comeuppance. Because as she squished the side of the glass against the cricket's body, out of the cricket emerged this horrific, long worm that was living inside of the body of this cricket.

• 7:08Michael Stevens?

  I'm not sure I want to open this WhatsApp image you've sent me.

• 7:13Hannah Fry?

  Do you know what? I was preparing for this, right? I was preparing for this, for chatting to you while I was eating my lunch, and I had to stop eating my lunch because it was just a bit too much.

• 7:25Michael Stevens?

  Wow. Okay, I am looking at the picture, and that is a very long worm.

• 7:30Hannah Fry?

  Isn't it?

• 7:32Michael Stevens?

  It's very out of focus, but still, I can see the, the general shape of the parasite and the cricket.

• 7:40Hannah Fry?

  Mm hmm. So this is called, it's called a hairworm. They can grow up to 30 centimeters long.

• 7:46Michael Stevens?

  Hmm.

• 7:47Hannah Fry?

  And they, they do this inside of a cricket. These worms.

• 7:52Michael Stevens?

  How long is your hair?

• 7:54Michael Stevens?

  Your hair is probably about 30 centimeters long. Maybe longer.

• 8:00Hannah Fry?

  Might be a bit longer, but yeah, probably. About that. Yeah, yeah.

• 8:03Michael Stevens?

  There you go. That's like a monster hairworm.

• 8:07Michael Stevens?

  Millions of them all over your head.

• 8:09Hannah Fry?

  Isn't that horrific? Isn't that horrific, that idea? Yeah, that probably is about 30 centimeters long. Oh, gosh. Anyway, these things, these worms, they breed in water. And so what happens is, they're like, they deliberately sort of go in water, and they're breeding in water. And then they get eaten by little bugs that live in the water. Fine, they're happy with that, they're cool with that. And then those bugs, in turn, get eaten by crickets. That is where they're like, right, I'm happy, I'm where I wanted to be. They kind of grow bigger and bigger and bigger. They change the behavior of the cricket.

• 8:46Michael Stevens?

  Oh.

• 8:46Hannah Fry?

  So the cricket ends up making a beeline for like ponds, you know, like lakes, rivers, whatever, dives in, at which point, boom, this worm emerges from, from the body of the cricket, back in to restart its life cycle all over again.

• 9:05Michael Stevens?

  Does the cricket survive?

• 9:07Hannah Fry?

  Sometimes. Sometimes it does. Sometimes it does. Yeah. I'm, I'm going to say not every time. And also, I don't think that the, I don't think that the worm particularly is that fussed about whether it, whether it manages to make it or not. But here is the thing, okay? So these are two examples of where one species' life cycle totally depends on the other. And with the hairworm, it's one where essentially, evolution has come up with mind control.

• 9:40Hannah Fry?

  Here's my question for today, okay? How on earth, how on earth and why on earth can it possibly be the case that one organism ends up linking their evolution to another in this parasitic way? That's what I'm gonna, that's what I'm gonna do today, okay? Cause it is, it is completely wild.

• 10:01Michael Stevens?

  Yeah, I love that. I wrote that down, the hairworm. Cause I love when behavior is changed by a parasite. Cause it really brings in deeper questions about identity and desire and want. I don't know if crickets have them, but look, maybe we'll wind up going this far today. If I eat a whole bunch of sugar, I feel different. And the sugar isn't alive, but it changes who I am.

• 10:29Hannah Fry?

  Mm hmm.

• 10:30Michael Stevens?

  It's in a way a parasite that is controlling my mind. It's changing how I think and react. And that's to say nothing for medicines and drugs.

• 10:39Hannah Fry?

  Do you remember that fun idea about, um, did we domesticate wheat? Or did wheat domesticate us?

• 10:45Michael Stevens?

  Yeah.

• 10:45Hannah Fry?

  Because if you think about it, if you were, um, if you were wheat and you were like, we want to take over the world. So what we're gonna do is we're gonna trick another species into mowing down large ecosystems, into growing us, into breeding us, into making us one of the most abundant plants on the entire planets. Uh, you know, there's sort of a perspective shift right there, right?

• 11:10Michael Stevens?

  I know. And, and while we're this perspective shifted, I could also bring up the old chestnut that the, our entire species, or let me say, rephrase this, that humans themselves are kind of just a machine built by DNA to make more DNA.

• 11:29Hannah Fry?

  Mm hmm.

• 11:29Michael Stevens?

  So who's in charge here?

• 11:32Hannah Fry?

  Right, who is ultimately in charge? Okay, so these are, you are, you are, as ever, Michael, you are nailing every single one of the, of the areas that I wanted to chat to you about a little bit more. The first thing though, the, where I wanna start is I wanna start with the oldest evidence of parasites. So, okay, it's really difficult when you're looking in the fossil record to be absolutely sure that you've spotted a parasite. But the oldest sort of confirmed evidence that we have of it is 512 million years ago. So this is, you know, this is predating the dinosaurs by 300 million years. Okay, this is like li- literally at, at the point where life starts to get complex.

• 12:12Michael Stevens?

  Yeah.

• 12:13Hannah Fry?

  The complex life starts to emerge. Uh, it looks like there were, there were parasites. Okay, so this is in 2020. There's researchers in Northwest University in China, and they have got these fossils that are these animals that have two shells. So a little bit like a clam. These are called Neobolus wulongquingenesis. It's gonna be a lot of me saying Latin words in this, um, in this episode.

• 12:37Michael Stevens?

  My favorite kind of episode.

• 12:39Hannah Fry?

  Exactly. I think it is for everybody. Okay, so you've got a little picture, um, that I've just sent you there. The main animal here that you're looking at, the kind of clam, is it's got a soft animal inside of this hard shell, but it's also got this tentacle that, that creates a water current to help suck in food and particles. Okay, so it's like, it's sort of, it's changing the fluid dynamics to get water in.

• 13:02Michael Stevens?

  So let me, let me describe these pictures I'm looking at. I'm looking at like a, a fossil in kind of an orangish rock of, which just looks like a seashell, but it does have some like globs, some tubular globs that are radiating out from the edges. And then I've also been sent a cartoon of what they may have looked like alive. And they look like clams, like a classic clam shape, but they've got these tubes, maybe four or five of them or more, um, uh, radiating across their shells with little like feathery things coming out of them.

• 13:38Hannah Fry?

  Right, exactly. So you've got, so the clam has got a little tentacle. It's like waving around in the water and helping to direct flow into the clam so it can eat. And then the little tubes, those are your parasites. So they are growing on the side of this hard shell.

• 13:54Michael Stevens?

  Oh, they aren't part of the clam.

• 13:56Hannah Fry?

  Correct, they are all there like hoovering up, using the clam to sort of direct all the food in and then sort of parasitically just hoofing up anything that comes by that's kind of good and juicy and tasty. Not doing any of the work, just taking all of the credit. That's, um, that's essentially what those guys are doing.

• 14:13Michael Stevens?

  That makes, it makes a lot of sense that that would be one of the earliest forms of parasitism. Because if you've got a clam that's just like, its feelers are, are directing flows of water to concentrate food near its mouth, quote unquote, then sure, other organisms are gonna want to live there.

• 14:32Hannah Fry?

  Yeah, I'm, I'm gonna get me some of that, thank you.

• 14:35Michael Stevens?

  And hey, why don't I just clamp onto this shell?

• 14:37Hannah Fry?

  Yes, please.

• 14:38Michael Stevens?

  Yeah, this is my home.

• 14:40Hannah Fry?

  Exactly. So the thing that's, the thing that's really interesting about these ones. So on, on the one hand, they're not that interesting because they're living on the outside of the shell, right? These are not parasites that have managed to penetrate the body of the organism. It's, it's, it's sort of attacking.

• 14:57Michael Stevens?

  And so, hold on. What makes them parasites then?

• 15:02Hannah Fry?

  We know that they're parasites. The reason why we are sure that they are parasites is because, two reasons. First, all of the tubes, they face the same direction. They're towards the incoming food. So you can be sure that they're stealing the food, basically, from, from this clam. And the second thing is they took a bunch of measurements. They've got loads and loads and loads of these. They took all these measurements of the size of the shells and those that didn't have any tubes were bigger overall. And those that had loads and loads of tubes were, were much smaller, much weaker. So it's this host parasite relationship.

• 15:36Michael Stevens?

  It wasn't a symbiotic relationship where they both lived their best life together. The parasites really were harming the host.

• 15:45Hannah Fry?

  Exactly.

• 15:45Michael Stevens?

  Which reminds me of one of my favorite definitions of a parasite, which is that it's a predator that kills its prey in units of less than one.

• 15:55Michael Stevens?

  It doesn't just kill and eat the whole thing. It just sits there and says, your life is going to be less. It's going to be, you're gonna be hungrier and you're gonna grow less. And that's, that's my modus operandi.

• 16:08Hannah Fry?

  Right, totally. Cause that is the definition. It's where, because you do get these, these relationships, these symbiotic relationships, as you describe, where you have two species that are very, very closely interwoven with one another. There's something called commensalism, which is where one creature benefits and the other creature just doesn't really, like, it doesn't really affect them at all. So it's sort of like a, a plus zero, right? If you're sort of keeping, keeping score.

• 16:35Michael Stevens?

  Good for one, indifferent for the other.

• 16:37Hannah Fry?

  So this might be things like the little mites that live in your, in your eyebrows, right? There's mutualism where both receive the benefits. This is like plus plus. Gut bacteria fall into this category, although the, sort of the, the relationship does vary quite massively between different species. But broadly speaking, we are much better off that we have so much bacteria living inside of our gut. And they also are much better off that they're living inside of our gut than they would be otherwise. Parasitism is where it's a plus minus, where one of them is definitely benefiting and the other one is definitely being harmed.

• 17:13Michael Stevens?

  You know, I thought of those face mites as soon as you said commensalism. I'm gonna, I'm gonna go ahead and, and reveal a cool little fact that I've been saving for a future Vsauce video right now. And it's that you can easily find them by putting a piece of sellotape, Scotch tape on your face and then covering it in darkness. They come out when it's dark and they'll get stuck on the tape. Okay, so like put a bag over your head or, or go in a dark room. Take off the tape and look at it under a microscope and you can just find these guys. But here's what's cool. They hide in our pores and in our skin face down, okay?

• 17:50Hannah Fry?

  Butts up.

• 17:50Michael Stevens?

  Which means their butts are up, which means that almost all of us are butt heads. When you look at someone's face, you are seeing one human face and thousands of bug butts.

• 18:02Hannah Fry?

  Michael, you, you honestly are one of a kind. Absolutely. How your mind works, I will never, I'm endlessly jealous of it.

• 18:09Michael Stevens?

  It's not my mind, Hannah, it's reality.

• 18:13Michael Stevens?

  We gotta, we gotta face up, we gotta butt up to the reality of what we are.

• 18:19Hannah Fry?

  Just a whole plethora of bug butts.

• 18:21Michael Stevens?

  But these mites don't bother us.

• 18:23Hannah Fry?

  They don't bother us.

• 18:24Michael Stevens?

  It's a, it's a plus for them and a, and a nothing, a zero for us.

• 18:28Hannah Fry?

  Right, exactly. And the other thing about those, those bugs is that they're living on the surface of our skins. I mean, they're sort of burrowing into our pores and things, but they're, but they're not like penetrating our, you know, our, our skin properly. And, and finding the first parasite that has managed to do that is, is difficult, right? So there is, um, there's another example. This is another clam-like creature around the same time. This is called Linarsonia, I think. Look, I'm sorry if you are related to Linarsonia. If this is one of your favorite clams, um, I'm really sorry for the way I'm pronouncing this, but I'm giving it a good go. So this one is from, from 520 million years ago. And this one also has a worm, but the worm is inside the sort of mantle cavity,

• 19:13Hannah Fry?

  right? Inside the shell of this clam.

• 19:15Michael Stevens?

  Oh.

• 19:15Hannah Fry?

  The only thing is you do find them, but you can't be sure it's parasite because you just don't have the same evidence in, in, as, as the, the other example. But we do know, right, that like, basically as soon as complex life emerged, some dudes were trying to sort of get in through the back door, get into other dudes and like, take whatever the hell that they could get.

• 19:39Michael Stevens?

  You can't blame them.

• 19:40Hannah Fry?

  Can't blame them, can't blame them. But you know what? It is, it is so rare being able to do this, being able to successfully get into another creature's body and make that your sort of domain where you live, is so rare and so hard. So in 2016, uh, two scientists, Sarah Weisman and Arman Kuris, they went through the entire animal kingdom, looking for every occasion where parasitism, parasites basically, had independently evolved. Okay. So we're not talking about how many parasites there are in total. We're talking about moments when some creature worked out that this was a good strategy.

• 20:18Michael Stevens?

  On its own, right.

• 20:19Hannah Fry?

  On its own, right. The first time that it did it. And they found at least 223 times.

• 20:26Michael Stevens?

  Oh, yeah.

• 20:27Hannah Fry?

  Incredibly rare.

• 20:29Michael Stevens?

  It's delicate because not only do you need to, uh, successfully not be repelled by the host, you also need to not harm the, the host because that's your home.

• 20:42Hannah Fry?

  While you're in it, yeah. And then, I mean, you, you, you're getting to my list here, right? This is, this is what you've got to do. This is why it's so hard. You've got, you've got to find a host, you've got to gain access to it, you've got to survive its defenses, you've got to steal enough resources to keep it alive while you reproduce. And then somehow you've got to get your offspring into the next host. I mean, that is, that's crazy hard.

• 21:07Michael Stevens?

  My goodness.

• 21:08Hannah Fry?

  It's crazy hard. Cause the other thing is you've got that list, right? Get into a host, gain access to it, survive its defenses, steal enough resources to reproduce, then get into another host. If you fail on any of them, it's game over. You know, it's sort of the end of the experiment. Um, you've got to, you've got to successfully manage every single one of that, that, those sort of checkboxes. But if you do manage to do it, if you manage to be a parasite, the prize, oh my God, Michael, it is extraordinary. This is like.

• 21:38Michael Stevens?

  It's worth the squeeze.

• 21:40Hannah Fry?

  This is sunlit uplands. This is worth a squeeze. This is utopic, okay? Cause your host does all the work for you, you know, it finds your food, it digests it for you. It, it ends up sort of making all your complicated molecules for you. It's, keeps you at the right temperature. So, so the prize is pretty great.

• 21:57Michael Stevens?

  Well, maybe I should try this.

• 22:02Hannah Fry?

  Okay. Right. Here's a question of how on earth do you get in? And how on earth does this work? And why on earth does this keep happening? I'm going to come to it after the break.

• 22:18Speaker 04

  This episode is brought to you by Google Chrome. You think you know a browser, but Gemini and Chrome, that's new. It can help you with practically anything on the web, like restoring a vintage motorcycle from a 50-page restoration block, or finally break down that long article you've had open for weeks. Gemini and Chrome is here for it. Ready to make anything online make sense? There's no place like Chrome. Check responses set up required compatibility and availability varies 18 plus.

• 22:43Speaker 05

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• 23:48Hannah Fry?

  All right. We're talking about how parasites managed to get in. And this is tricky. This is, I think in many ways, this is, this is the trickiest part. Because from a parasite's point of view, the body is, is sort of built as a fortress, you know, your, your skin is waterproof. It's like armor. You've got mucus that traps things. You've got stomachs that are full of acid with, you know, enzymes that are supposed to dismantle biological material. And even if you manage to get through all of that, there is this, you know, mobile security force in your immune system that is, its entire job is to like hunt down things that are not you and then go in and kill them. And so there are a few different ways

• 24:30Hannah Fry?

  that parasites have worked out how to get around all of those defenses. The first is that, uh, you can kind of hitch a ride on another creature to make a hole for you. So this is a virus rather than a parasite, although, although it is a sort of parasitic virus, right?

• 24:50Michael Stevens?

  Ah, okay. I, I, go on because yeah, I made a note about like, should we consider a virus a parasite? Does a parasite have to be its own living organism? Because that's debatable with viruses. I personally think they should fall on the life spectrum. But are there viruses that aren't parasitic in any way? Or are they all?

• 25:09Hannah Fry?

  No, no, that or because that, that, they're all parasitic. Because the thing is, is that for a virus to reproduce itself, it has to hijack, uh, the sort of the, the, uh, function of a cell.

• 25:22Michael Stevens?

  That's right.

• 25:22Hannah Fry?

  In order to reproduce. And so by definition, it's getting inside of its host. The host might be a single cell rather than, than a larger individual. But yes, all viruses are parasites, but not all parasites are viruses. So I guess with this whole mind control thing, I mean, there are examples, I'll give you one in a second, of where viruses end up changing people's behavior. But, but I, I guess overall, the mind control thing, I'm talking about more complex organisms. Okay. One way that you can do this is you can get somebody else to make the hole for you.

• 25:53Michael Stevens?

  Oh, the, the hole, the hole in the host that you get to enter through.

• 25:57Hannah Fry?

  Climb on in. And that is exactly what malaria does with mosquitoes. So you have an infected mosquito and when it bites you, it's piercing the skin, right? It's like literally ripping a hole in, in the host. And it deposits this plasmodium, the parasite, into your tissue. But it doesn't always manage to get it straight into your bloodstream, which is where the parasite wants to be. So it just sort of dumps it into your skin. And then these microscopic parasites start moving through the tissue until they enter a blood vessel. And then they're like, way, free ride all the way over to the liver. Um, but the mosquito is the thing that has done the difficult thing. The mosquito is the one that has dumped the parasite into the host where it wants to be. Malaria though,

• 26:41Hannah Fry?

  it isn't even the most shameless example of this, even with mosquitoes. So there's, um, uh, something called a botfly. Have you ever come across these? They're horrific.

• 26:50Michael Stevens?

  I've heard about it. In moments when I'm looking up the grossest things I can imagine. But I don't remember exactly what they do. So you better tell me.

• 27:00Hannah Fry?

  Genuinely horrific. Okay. So the, the adult female botfly, what she does, she catches mosquitoes and then she glues her eggs onto its body.

• 27:12Michael Stevens?

  Oh.

• 27:13Hannah Fry?

  And then she lets it go. And then that mosquito goes off, lands on a human or another mammal, uh, to, to drink their blood. And then the bot, botfly larva, they emerge and then they penetrate into the, to the host's skin. Um, and then they sort of, I mean, it's absolutely disgusting. We end up coming breaking out later. It's, it's genuinely horrific. Anyway, don't Google it if it's late at night. Okay. That's, that's all I'm saying.

• 27:42Michael Stevens?

  Yeah, right. I didn't realize that the botfly larva traveled to us on mosquitoes.

• 27:52Hannah Fry?

  Mm hmm.

• 27:53Michael Stevens?

  Is that the only way they can get on you?

• 27:55Hannah Fry?

  I think the botfly doesn't really care about what it sticks it to. It's just going to be a blood sucking insect. So it doesn't have to be a mosquito.

• 28:05Michael Stevens?

  And then the larva get inside whoever was bit by the mosquito and they've got some free food and they can come out later.

• 28:14Hannah Fry?

  When they're ready.

• 28:15Michael Stevens?

  Yeah.

• 28:17Hannah Fry?

  Horrifying, isn't it horrifying? Um, but, but the point about that though, is that what the botfly has done is it's not only a parasite in the sense of it's sort of parasitically borrowing the flesh of a mammal to raise its young. It's also borrowing the kind of delivery service of a mosquito to get into the host in the first place. Um, sort of, it's outsourced, it's break in basically. So that's one way you can do it is you can, you can kind of punch a hole or, or use another creature to punch a hole. The other way that you can get into a host is to use the door that the host already has, by which I mean our mouths.

• 28:56Michael Stevens?

  Yeah. Like what happens with the cricket and the hairworm.

• 29:00Hannah Fry?

  What? Yeah. Mm hmm. Exactly. It eats something that has got the hairworm inside it and that's how it sort of ends up getting in.

• 29:08Michael Stevens?

  So the, by the way, at this, I thought this back when you first mentioned this, the hairworm is like a turducken parasite. It's like, I'm going to get in a bug and then that bug's going to get inside a cricket and then I'm ready. I need, I need, I need two layers above me before I'm home.

• 29:23Hannah Fry?

  Well, I mean, same as the botfly. It's like, I'm going to get on a, on a bug and then that bug is going to take me to the host and then, then, and then, right?

• 29:31Michael Stevens?

  Well, it's not quite turducken because the mosquito doesn't get inside me.

• 29:35Hannah Fry?

  True.

• 29:35Michael Stevens?

  It just.

• 29:36Hannah Fry?

  Well, it sort of does. It punctures you, but yes.

• 29:38Michael Stevens?

  It does. I guess it's temporarily inside of me. Yeah.

• 29:40Hannah Fry?

  Yeah.

• 29:40Michael Stevens?

  But it's not all wrapped up like a little turducken.

• 29:43Hannah Fry?

  No, true.

• 29:44Michael Stevens?

  Anyway, if, if you, if you're in charge of the scientific naming organizations, clearly you gotta hire me because I've got taxonomy, uh, distinctions.

• 29:55Hannah Fry?

  Wait, I, I, turducken is in like, you put the duck inside the turkey.

• 29:59Michael Stevens?

  You put a, a, a duck inside a, no, sorry. What, or I think you put a chicken inside a duck inside a turkey.

• 30:06Hannah Fry?

  Amazing. Have you ever done it?

• 30:08Michael Stevens?

  No. It feels a little, a little too cruel. It's like, I'm going to eat you, but I'm not going to rub your nose in it.

• 30:15Hannah Fry?

  I mean, every time you put egg on chicken though.

• 30:19Michael Stevens?

  Yeah, like a chicken omelet. Sometimes I feel like maybe I'll keep them separate on the same plate.

• 30:26Hannah Fry?

  It just sounds like there's going to be a lot of bones. That's what it sounds like to me. It's like to, to get to the meat that's in the middle. There's a lot of hacking.

• 30:33Michael Stevens?

  Yeah. Well, you remove the bones bef, when you do the wrapping. Yeah. You take out the bones. So you've just got the, the boneless.

• 30:42Hannah Fry?

  Like breast meat. Hmm.

• 30:43Michael Stevens?

  Yeah. Yeah. I don't think you even have to really separate the, the cuts of meat. You just de-bone it. Now it's can be turned into like a meatball, an unground meatball and shoved inside a larger unground meatball.

• 30:58Hannah Fry?

  Honestly, you Americans, you really are the absolute height of, of culinary excellence, aren't you?

• 31:05Michael Stevens?

  Is it, it can't just be an American thing, but it's certainly today culturally is like an American thing. The Canadians epic meal time did their whole like seven layers where they started with a small bird and went all the way up to a whole hog roast. And these are, this is where we went while talking about parasites.

• 31:28Hannah Fry?

  I'm just looking at pictures of them now. Can I just say, no?

• 31:33Michael Stevens?

  You're looking at pictures of what? Turducken?

• 31:35Hannah Fry?

  Turducken. Yeah. It's, it's either that or botfly. And, um, I've got the same response to both.

• 31:42Michael Stevens?

  But the point is that you can, you can have another organism put you in your host or you can get in through the host's existing holes.

• 31:51Hannah Fry?

  You can. Exactly. Exactly. So like in their nose or in their mouth, whatever. Okay. The other, the only other option, right? Is if you are not being eaten or not being kind of punched in. The only other option is that you can, there are some parasites that just sort of dispense with all of that subtlety altogether and they just kind of go through you. Hookworms are an example of this. Hookworms are wild. Do you know how hookworms end up getting into your intestine? It's, it's crazy.

• 32:16Michael Stevens?

  I think I've heard about something that just comes from the ground up through your bare feet.

• 32:23Hannah Fry?

  Exactly. Exactly.

• 32:24Michael Stevens?

  Is that a hookworm?

• 32:26Hannah Fry?

  That's a hookworm. That's exactly it. So you can walk on them and they penetrate your skin, but then when they're inside, they want to get to your intestine. Okay? That's where they want to get. But the moment they're in your foot, let's first get into the bloodstream. We'll go around the bloodstream until we get to the heart and it's like, next stop here. Thank you very much. I'll get off at the heart. Then when they get to the heart, they're like, okay, well, we'll go through to the lung. Okay. Then when they're in the lung, they're like, right, nearly there. Climb up the windpipe.

• 32:57Hannah Fry?

  Yes.

• 32:57Michael Stevens?

  We nearly.

• 32:58Hannah Fry?

  Get into the throat, get swallows, get into the intestine.

• 33:02Michael Stevens?

  Oh, I didn't know that.

• 33:05Hannah Fry?

  I know.

• 33:05Michael Stevens?

  I, I assumed they took a faster route, which is I guess like capillaries to bloodstream to intestines.

• 33:12Hannah Fry?

  Right. But then, but then you can't guarantee you're going to stop off at the intestine, can you?

• 33:17Michael Stevens?

  Right. But you're guaranteed to go to the heart.

• 33:19Hannah Fry?

  You're guaranteed to go to the heart. And then when you're at the heart, you can be like, whoop, let's go on the other loop. Thank you.

• 33:24Michael Stevens?

  Lungs.

• 33:25Hannah Fry?

  It's like, it's like changing on the circle line, isn't it?

• 33:28Michael Stevens?

  Wow.

• 33:29Hannah Fry?

  Okay. So that's how, that's how they get in. Okay. Now, I mean, getting in, you've got, you've got all of these defenses that are going on. So you have to, and this is like, you know, these things are incredibly finely tuned to sneak on under the, um, immune responses of various creatures. And, and yet they have managed to do this. I mean, you can sort of see how, you can see the evolutionary pressure in that situation very easily, that if you are trying some tactic that the immune system shuts down, it's not going to go anywhere. Whereas if you manage to find something that's like a trick that happens to work, you're way more likely to survive. So, um, you know, you can kind of see how that, that pressure leads for them to be able to, to duck under the radar of, of, of these systems. But then

• 34:14Hannah Fry?

  the problem is somehow or other, you've got to get back out of your host and you've got to get back into the next version, you know? And this is your sort of hairworm getting back into the water via the cricket. Okay. So it's like.

• 34:27Michael Stevens?

  By changing the cricket's behavior. Yeah.

• 34:29Hannah Fry?

  By changing the cricket's behavior. Exactly. Because otherwise it's just in a very comfortable prison. It's sort of very nice, but it's got no, no way of, you know, sending on its, its offspring to, for the next generation. Now, the thing is, you sort of mentioned this right at the beginning about how when you eat sugar, you become a slightly different person.

• 34:45Michael Stevens?

  Yeah.

• 34:46Hannah Fry?

  And actually you're totally right. There are all kinds of things that change our behavior. If you think about when you get flu, um, you get hot, you get tired, you want to lay down, right? You're, you're doing that to yourself, right? Your body is doing that to yourself to fight the virus.

• 35:01Michael Stevens?

  Right.

• 35:01Hannah Fry?

  Um, when you eat sugar, I mean, how does sugar affect you? To make you grumpy?

• 35:05Michael Stevens?

  It makes me really irritable. It makes me really impatient and grumpy.

• 35:09Hannah Fry?

  Right. Okay. And I think that's the thing is that like all of our bodies, all biological bodies have, they kind of come out of the box with these behavioral switches that are already fitted. Um, and, uh, all the parasite has to do is to, to learn what those switches are, learn what molecules affect what behavior, and then off you go, easy peasy. Rabies is a really, really good example of this. So rabies is a virus rather than a parasite. But what rabies does, it, to reach another animal, it needs to get infected saliva into another animal's skin. And so late into the infection, it, uh, it spreads into the salivary

• 35:54Hannah Fry?

  glands, but it also ends up getting through the brain and makes certain creatures unusually excitable and aggressive so that they will bite another, which means that the virus itself has like, uh, worked out how to sort out its own delivery mechanism by tweaking the behavior of an individual who ends up being infected by it.

• 36:19Michael Stevens?

  Well, yeah. And that's where things get really spooky because infecting the salivary glands so that you can pass to another organism through a bite feels really mechanical. But to then also infect the brain and make the animal more aggressive so it's more likely to bite, that seems to be changing something deeper about the identity and personality of the animal.

• 36:43Hannah Fry?

  Right, completely. You don't have a distinct end and beginning. If you believe that your, uh, that your behavior, that your, your choices, your actions, your reactions are an entirely conscious decision that you're making at every moment in time, I'm really sorry to break it to you, but quite a lot of this comes down to the molecules that are floating around in your body. You know, you see this with antidepressants that boost serotonin. Suddenly people are just actually way more, you know, way more relaxed, way less self-critical, all of these things. It, it's just a single molecule that's sort of driving your behavior. And what happens is these parasites manage to work out how to tap into exactly those molecules. So I think, I think the clearest illustration

• 37:28Hannah Fry?

  of this, I mean, it's all, by the way, phenomenally complex, right? And we don't always have clear mechanistic answers for this, then this, then this, then this. And that's how it all works, like rabies, for example. But there are some where we have like a much better idea of what exactly is going on. And there's, uh, one called the spongy moth caterpillar, right? It's this really cute little caterpillar. It spends most of its day in the, uh, you know, in the forest, lower down. You get them in Germany and the Germans have a word for what ends up happening to them. They call it wipfelkrankheit, which means tree top disease.

• 38:04Michael Stevens?

  Walter Cronkite?

• 38:05Hannah Fry?

  Wipfelkrankheit. Wipfelkrankheit. Okay. That was my G C German coming out. Thank you.

• 38:11Michael Stevens?

  Walter Cronkite was a famous American news broadcaster.

• 38:14Hannah Fry?

  Was he? Okay. Yeah. Well, it was named after him.

• 38:18Michael Stevens?

  I think he was famously the one who famously announced the assassination of John F. Kennedy.

• 38:24Hannah Fry?

  Oh.

• 38:25Michael Stevens?

  To, to the world through television. Yeah.

• 38:27Hannah Fry?

  Was he climbed up the top of a tree when he did it?

• 38:30Michael Stevens?

  No, he was on, he was on television.

• 38:33Hannah Fry?

  Okay, so what these, what happens to these caterpillars is if they're infected by this parasite, if these caterpillars are infected, what they do is they, they normally have this, this life cycle where they spend most of their time down at the ground, but they do have this life cycle where they'll climb up the tree a little bit and then they'll like go through molting and sort of emerge as a beautiful moth. But when they're infected, they do something really different. Instead of kind of hanging around down the ground, they climb up the vegetation really, really, really, really, really high up in the treetops where they die. And then when their bodies sort of disintegrate,

• 39:13Hannah Fry?

  they end up like spilling this virus all over the leaves below for other caterpillars to come and get infected. Um, now that is a really clear example of how the behavior of this caterpillar is directly impacted by the parasite that um, that, that has infected it. But we know that this whole process um, is uh, we know that their, their molting process is controlled by this particular hormone, which is called uh, twenty E, twenty hydroxy disone. Okay. Now, when these hormones rise in the caterpillar, the caterpillar changes state, it stops feeding, it sort of shifts

• 39:54Hannah Fry?

  its development, it's, it starts molting. This parasite carries a gene called EGT and EGT messes up that hormone. It basically goes in, adds a sugar to it, makes it not work. So it's like directly tapping into the hormone that is already working in the caterpillar's body. And it's like very specific in the way that it's targeting it. So in twenty eleven, uh, Kelly Hoover and colleagues, they did this really beautiful um, experiment where they deleted EGT from the virus, they deleted that tiny little bit of the virus that, that makes this enzyme that mucks up

• 40:38Hannah Fry?

  this protein or this hormone. And they gave it to caterpillars and the caterpillars that were infected with this modified parasite still died, they still didn't make it, but they don't climb up the top of the tree.

• 40:54Michael Stevens?

  Wow.

• 40:55Hannah Fry?

  They don't climb. And they also, there's the other experiments that they've done. Oh, by the way, you put EGT back in, they start climbing again, right? So it's like, it's definitely, that's definitely what it is. But they also have done these experiments with these caterpillars where they put them in vertical tubes and they put the light at the top. And when they're infected, the caterpillars are just climbing towards the light.

• 41:14Michael Stevens?

  Mm hmm.

• 41:15Hannah Fry?

  Uh, climbing upwards. But if you put the light at the bottom, the caterpillars will climb downwards. So it's not that the caterpillar is like, I must get to the top of this tree. It's just like the caterpillar is like, whoa, it's just, I'm heading towards the light.

• 41:28Michael Stevens?

  Towards light.

• 41:29Hannah Fry?

  And it looks like the hairworm, the horrible little thing that goes inside of a cricket, is doing the same thing. It's just changing the sensitivity of the cricket to light. So not, not like off you go, off you go find some water. You, the, the cricket which is infected is no better at finding water when it's sort of masked off than otherwise. It's not like it's sort of sensing it or smelling it or anything.

• 41:55Michael Stevens?

  I was wondering how they found the water. Yeah.

• 41:57Hannah Fry?

  It's all about light, all about light.

• 41:59Michael Stevens?

  Huh. Man, you know, I've heard about these like, you know, zombie animals infected with, by mind controlling viruses, but I'd never really questioned how the virus or parasite does it. And when you get into how it happens, it just makes all living things seem so much more like elaborate chemical reactions, puppets being pulled by molecular strings and the puppet master can be anyone who has the right molecules.

• 42:27Hannah Fry?

  Right. It, it, you have absolutely nailed it. You have completely nailed it. We are, and I think it raises all these questions about free will as well, about like, how much are we actually defining and controlling our own behavior, as well as the question that you raised a moment ago of like, where do we begin and end, you know? Like, how can you say that this is like a distinct entity, it's distinct organism, when, I mean, in our bodies, right? We barely exist without our, the bacteria that, that makes up, um, makes us up, the bug butts all over our faces. It's just really kind of, you know, difficult to draw the lines here and there.

• 43:04Michael Stevens?

  I know. I mean, we, we, we've drawn the lines. We say things like, oh, I'm freaking out. My amygdala is pumping out a bunch of cortisol, but that's me doing it because the amygdala is always part of my brain. But like, at what point do you say, nah, it's just along for the ride, along with all your other parts? And you're all getting along most of the time, but sometimes some of them act up and you change and you feel differently, but you are the fictitious combined behaviors of all these parts that found themselves together when you were born and are gonna have to make the best of it.

• 43:41Hannah Fry?

  Honestly, the longer I live, the more I think that we are just a fictitious. Exactly as you've just described it, that this idea that you are a thing is just a fiction.

• 43:52Michael Stevens?

  It's, it's, the, the one that freaks me out the most lately is remembering things. When I go, oh yeah, what was that actor's name? And I can't think of it, but then it pops into my head. And I'm like, who the heck put that there? I mean, remembering is such a normal thing that we all do, and yet I don't get how it's happening. When I remember, okay, yesterday, what did I have for dinner? It somehow just shows up, usually as a visual. And I'm like, who in there did that? Who like went in, found the file and then put it into my awareness and was like, here's what you had. Um, you had, I had Indian food. Okay. Like, how did that happen?

• 44:34Hannah Fry?

  There's um, there's an amazing bit of work, neuroscience work that was done about ten years ago, maybe a little bit longer, about how memory and imagination are the exact same parts of the brain.

• 44:45Michael Stevens?

  Hmm.

• 44:46Hannah Fry?

  That you think that you're recalling something from a filing cabinet, but actually you are creating a story with like, you get the kind of gist of it that gets filed away, but um, but the rest of it, you're just, you're creating a story.

• 45:00Michael Stevens?

  Yeah. And you know, this is something I want to learn more about. It's that there's some kind of feeling we have, a feeling of, wow, my camera was really shaking when I was using my eraser. Look at that. I'm erasing a note on my notepad and it's like causing an earthquake. I've, I've been curious about, is there a name for the feeling of this is real? Cause like, when things happen in my dream, I remember them later and I, I'm like, but that didn't really happen. And when I imagine something at a whole cloth, like a elephant that can fly, I'm aware that like that thought in my head is not real. But when I remember things from the past, I'm also creating

• 45:44Michael Stevens?

  a visual, but yet my body accepts that like, oh, no, no, this isn't like a daydream. This is like, this really happened in the past. And it feels different. It has a different quality to it. So it almost feels like a memory and an imagination are the same thing with a different coat of lacquer on them so that we can distinguish, oh yeah, that, that's actually what happened yesterday versus that is what could have happened yesterday, you know?

• 46:11Hannah Fry?

  I want to do a proper dive into this. We should, we should definitely put this on the list of future episodes because I want to learn more about it. I want, I have a few friends who, who have worked on this in the past. I want to go and talk to them. I want to like, get their, get their take on it. This is, I want to do a full episode on it.

• 46:26Michael Stevens?

  Yeah. Okay. We should do that because we've, we have talked in the past about that feeling of not being able to discern, wait a second, has this happened before or did I just dream that it happened? So that's a place where that quality has eroded or it's malfunctioning and you can't quite remember, wait a second, did I dream that or did it, did it happen? And I think that's fascinating that that can happen to us.

• 46:48Hannah Fry?

  Yeah, I totally agree. Um, shall we finish up with my favorite parasite?

• 46:52Michael Stevens?

  Oh, please. Yeah. What is it?

• 46:54Hannah Fry?

  Everyone's got one, surely by now. After, if you've survived this far through with our hairworms, our tapeworms and our hookworms. Okay, it's toxoplasmosis gondii.

• 47:03Michael Stevens?

  Ooh, wait. So toxic plasmosis, this is like the cat poop thing?

• 47:07Hannah Fry?

  It's um, it's, I think, one of the best stories of all time ever. So it's this eukaryotic parasite. So it's like a single cell. It um, it's very happy when it is inside the gut of a cat. That's his, that's where it wants to be. And it comes out of the gut of the cat in the cat poop. Right?

• 47:28Michael Stevens?

  Right.

• 47:29Hannah Fry?

  Problem is now it's on the ground in cat poop and somehow it needs to get back into the gut of another cat. And it does that by going via a rat. So rats will eat anything. They're like, you know, they're disgusting. And so eventually it's, it's kind of common that, you know, rats will eat a bit of cat litter. That's fine. So now it's inside of a rat and now toxoplasmosis has to get into a cat by getting the rat into a cat. And the way that it does this is, as we've seen before, by altering the behavior of the cat.

• 48:06Michael Stevens?

  Whoa.

• 48:07Hannah Fry?

  So normally you take a rat, you take a rat into like a big, wide open room and the rat is like, I'm not going in the middle. Thank you very much. I want to scurry around the outside. I'm, I'm no fool. I don't want to get eaten. If you put cat urine in a big, open space and a normal rat will be like, absolutely no way. Thank you very much. I'm not going anywhere near that. If you put a rat into a room that has been infected by toxoplasmosis with cat urine right in the middle of the room, the rat is just like, ah, let me at him. Just makes an absolute beeline, like runs straight over. It's like, yeah, give me the cat urine, right? Loves it.

• 48:44Michael Stevens?

  Whoa.

• 48:45Hannah Fry?

  Loves it. Anyway.

• 48:47Michael Stevens?

  The rat to put itself in danger so it will be more likely to be eaten by a rat.

• 48:52Hannah Fry?

  Precisely.

• 48:52Michael Stevens?

  Sorry, by eaten by a cat.

• 48:54Hannah Fry?

  Precisely. But not only does it just say, oh, love the smell of cat urine, off I go. There is some evidence, okay? This is like, I would say maybe a little bit controversial, but there is some evidence that the way that it works is it changes the same part of the rat's brain as sexual attraction. So that actually what it's doing is making.

• 49:18Hannah Fry?

  Is making the rat sexually attracted to the cat. Okay, is that not the best thing that you've ever? I'm not actually sure how they ran that experiment. I should have looked into it a bit deeper. But in my head, I like to imagine that the rat is sitting in an MRI scanner and they're just showing it pictures of cats and it's like, hmm, that one is, oh.

• 49:34Michael Stevens?

  Yeah, yeah. And it's like reproductive pathways are lighting up like, oh, yeah, that's my type, cats.

• 49:42Hannah Fry?

  This is what happens, basically irrespective of the precise mechanism that it has changed inside of the rat, rats will launch themselves at cats if they have been infected with this particular parasite. The cat will eat the rat, gets back into the cat gut, happy days.

• 49:56Michael Stevens?

  Wow.

• 49:57Hannah Fry?

  The reason, by the way, this is the reason why you have to be extremely careful about handling cat poop or cat litter if you are pregnant, because toxoplasmosis can end up having like really serious impact on your unborn baby. But there is, there's all these studies on, because humans do get infected with it too, right? This is not an unusual thing. We live very closely with cats.

• 50:22Hannah Fry?

  But there have been these studies now, okay, some of this gets a little bit on the, you know, a little bit dodgy, right?

• 50:27Michael Stevens?

  Okay.

• 50:28Hannah Fry?

  In terms of like reproducibility and whatever, but, but let me just tell you them anyway, okay? Because some studies have found that if you're infected with toxoplasmosis, it makes men enjoy the smell of cat way more.

• 50:43Michael Stevens?

  Hmm. Let's see if we can reproduce this study. I want to try it.

• 50:47Hannah Fry?

  There was one study that looked at the connection between being infected with the parasite and being involved in a road traffic accident and found that you are about twice as likely to be involved. Now, look, I'm saying all of this stuff with a raised eyebrow, right? Because I think that we get into quite shaky science in terms of like the quality of these particular studies.

• 51:06Michael Stevens?

  Yeah, and we're not really sure what kind of conclusions to really draw. Is there a cause here? Or is it just a correlation? Could be spurious, but it could also be that it really is affecting your cognition.

• 51:17Hannah Fry?

  I mean, it definitely affects the behavior. I mean, look, parasites affect the behavior of their host. They are perfectly capable of doing that. It is not mechanistically impossible, but we just don't have like the hard, the hard evidence here, I think. But I think going back to the earlier point, I mean, that's my favorite parasite. You can, you can have that one, pop it on your top trump cards for parasites. But, but I think going back to your point here earlier, Richard Dawkins had this idea of an extended phenotype, that a gene's phenotype doesn't necessarily sort of end at the physical boundary of the creature, the organism that's carrying it. Because if a parasite's gene reliably changes

• 52:01Hannah Fry?

  another animal's behavior in a way that helps the parasite to reproduce, well, then that other animal's behavior has become part of the, of the parasite's phenotype, right? Of like the way that they sort of present themselves.

• 52:13Michael Stevens?

  Right.

• 52:14Hannah Fry?

  And I think this ultimately sort of answered the question that I set out at the beginning of like, why does evolution keep reinventing mind control? It's, it's partly because you can't think of like hard boundaries between organisms. It's sort of, it's much, much, much more nuanced than that. But it's also that it's not because minds are easy to control necessarily. It's more that once evolution has managed to put one organism inside of another, everything the host already knows and does, knows and does, everything that host, everything that the host already knows and does, then becomes a potential resource for that organism, including its behavior.

• 52:59Michael Stevens?

  Yeah.

• 52:59Hannah Fry?

  And that, I think, is, is the reason why mind control is on the table.

• 53:04Michael Stevens?

  And it's why we do this show. You guys have infected our minds and we, wait, who's the parasite here? Is it us or them?

• 53:14Hannah Fry?

  I'm, I'm not sure. I'm not sure. I think we are. I think it's us.

• 53:18Michael Stevens?

  I think there's an argument both ways. And so I think that the, the true answer is that it's symbiotic. We are both better together.

• 53:26Hannah Fry?

  We certainly are. We certainly are. And we'll be better together later on this week as well with our usual episode of Filonotes. But in the meantime, thank you very much for joining us.

• 53:35Michael Stevens?

  See you later.

• 53:46Speaker 03

  Sam's Club is the club for every holiday club. Guess who's joining the club this season? Walmart Plus. Yep. Sam's Club Plus membership now comes with Walmart Plus at no extra cost. Two memberships, one great price. Just 120 bucks a year can save you time and over $700. Want to get in on the membership duo of the century? Visit sam'sclub.com. Come join us. Sam's Club.

• 54:06Speaker 07

  Annual plan only. Auto renews. T and C apply. Estimate based on average. Combine Walmart Plus and Sam's Club Plus savings among members of both for one plus year versus non-member or club member pricing slash fees. Excludes membership fees. See Walmart Plus terms and Sam's Club terms for more info. When you shop pickup at Kroger, you can expect the savings you love and fresh groceries selected just for you. Our associates are committed to getting every detail right, carefully handpicking your items, checking for quality and freshness, and packing your order with care. Because bringing you fresh, quality groceries is what we do best. And right now, enjoy $20 off your first online order of $75 or more. Restrictions apply. See site for details. Kroger, fresh for everyone.

• 54:46Michael Stevens?

  This holiday, all plans, even unlimited, are just $15 a month. A holiday deal this good obviously deserves an enormously expensive holiday song, like All I Want... Right. Okay, we can afford that. You know what? Forget the music budget. We're going to stick with whatever's in the public domain. Give it a try at mintmobile.com/switch.

• 55:04Speaker 06

  Upfront payment of $45 for three months, $90 for six months, or $180 for 12-month plan required. $15 per month equivalent. Taxes and fees extra. New customer offer for initial plan term only. Greater than 50GB may slow when network is busy. See terms.