Daniel and Kelly’s Extraordinary Universe - The discovery of parasite life cycles

Episode Date: May 7, 2026

Parasites can have amazingly complicated life cycles - moving through a series of host species, and looking quite different while residing within each type of host. Frankly, this stumped scientists for a long time. In today's episode, Daniel and Kelly take a stroll through the history of science to explore how we finally started to figure parasite life cycles out.See omnystudio.com/listener for privacy information.

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Howdy, Extraordinaries? In the past on the show, we've talked about the tortuous paths that parasites take to complete their life cycles. Trematoed parasites, for example, can go from snails to fish, to birds, and then back again. And at each step along the way,

they look pretty darn different. than they did in the prior stage. Frankly, it's amazing we ever figured these life cycles out at all. And today we're going to talk about how we went about doing that and some of the mistakes we made along the way. Welcome to Daniel and Kelly's complicated universe. Hi, I'm Daniel. I study particle physics and aliens. Hi, I'm Kelly Weiner-Smith.

I study parasites and space, and I don't know why Daniel is so excited about space aliens when there are creatures with such alien life cycles living right here. Well, I do have a very different emotional reaction to aliens and parasites. For example, I would be terrified to know that there are any parasites living inside of me, but I'd be kind of tickled if I found out that aliens had, like, taken up life within me. No way. Are you serious?

What they came from? Because if they come from Earth, they're gooey and gross. And if they come from outer space, they're fascinating. Grosser. And gosh, I hope they are. In fascinating ways, right? Oh, my goodness. You are inconsistent.

Absolutely. I have a purely emotional and non-rational response to being invaded personally. All right. Yeah, that's fine. I can let that slide. So my question for you today is not about which parasite is the grossest, but which parasitologist is the grossest. You're the president of the American Society for Parasitology. Is there a group of folks in that society?

who are the grossest. Oh, all right. How can I answer your question, but not answer your question, is what I'm thinking right now. All right. So first of all, a clarification. I'm president of the Helmynthological Society of Washington, which is local, but I'm president-elect of the American Society of parasitologists. I won't be president until July.

Yeah, yeah. Glad we cleared that up. I was talking to someone who studies space medicine, and he was saying that he actually got, I think, a master's degree in parasitology. before he got into space medicine. And he ended up spending a bunch of time being like the flight surgeon for NASA. And he was showing me all of these like amazing photos of him

in the vomit comet where like astronauts were like throwing up on either side of him. And he was just sitting there like taking notes like a total badass. And anyway, I'm not sure if he knew I was a parasitologist or not because I was there in my capacity as the author of a city on Mars. And he was talking about how, you know, he met a bunch of parasitologists. And parasitologists were clearly the degenerates of the medical field.

And so I would just like to say that all of us, we're all the stinky kids on the bus. All the parasitologists are. And so I'm just going to go ahead and, you know, throw myself in there with everybody else. I'm not calling anyone out. And that's fascinating that you're not the only parasitologist who then transitioned to thinking and studying about space. Yeah. Right? This guy was a parasitologist and now he works on space medicine.

Is that because he discovered space parasites? and he's been keeping them a secret? No. Are we going to start a conspiracy theory right here today on the pod? Yes. Yes, that's right, because we're trying to edge out the Sasquatch Chronicles. So, yes, that's right. He started studying parasites and then he found out about the alien parasites

and then he had to work on space. And that's what I was doing too. And that's why I split my time between these two fields. There's actually a really fun TV show that involves aliens that are brain parasites that come and infect all the high-level politicians. and that's why our politics have gone so crazy over the last few years. It's a really fun show. It's called Brain Dead.

That would explain a lot. All right, but today we're not talking about fictional parasites, conspiracy parasites, or even parasitical memes that infect the American population and make them not believe in science. We're talking about real parasites, how amazing they are, how bizarre they are, how complicated they are, and how we figured it all out. And look, y'all can't blame me for talking about parasites all the time because I'm just answering a listener's request here. So let's go ahead and listen to what Boris wanted to know about. Hey, Kelly and Daniel. Since I listened to the episode about killyfish and the trematodes that

affect them, I've been wondering, how did we figure out the life cycle of this parasite with the four separate stages? Did someone just find a cyst in the brain of a fish and think that it looks similar to something that they found in a snail? Do these different stages even look similar to each other? And a more general question would be, how did we discover similar life cycles in other parasites in the first place? Thank you. I totally agree with Boris. I feel like every time you describe a parasite life cycle, it's amazing and hilarious. And I wonder, like, how did this thing evolve where, you know, it has to be like in a snail and then picked up by this kind of eagle and then dropped on a rock at this velocity and then stepped on by a rabbit which walked under a tree or something?

You know, it's so specific. And I wonder not only how did this evolve, but how did we ever figure this out? It still blows my mind that we figured it out. And there are still life cycles. Like, you know, when I go to the American Society of Paracetologist Conference, you'll regularly hear someone be like, and we found a parasite in this animal, and we have no idea where it is when it's outside of this one. Like, is it in sharks?

Maybe. But we haven't found it yet. And so there's, you know, loads of life cycles that we have not figured out yet. but the ones that are important for humans, largely we have. Well, that's reassuring, I suppose. Yeah, yeah, I suppose as well. So today we are going to go through the history, and we're going to talk about how parasitologists figured it out.

And unfortunately, we're also going to talk about how this group of degenerates were one of the last groups of scientists to be holding on to the idea of spontaneous generation, which we will talk about a little bit more in a moment here. But spontaneous generation briefly, just being the idea that, like, life, just kind of pops up from stuff. Like abiogenesis. Yeah, exactly. That's another word for it. Right.

Yeah, yeah. Very ancient idea. Aristotle thought this is one of the four ways that things came into being. So, like, way back to Aristotle, probably even earlier than that. But it's still sort of relevant for the conversation about, like, how life on Earth began originally, because we had a period when there was no life, and then we had a period where there was life. And so clearly, life started from non-life at least once. Yes, but let's not get too far off track.

Yes, you're right, you're right. Okay. So here's, I want to tell you where we're going to end, because I think it will be important for understanding the path that we take. All right. So which parasite are we talking about today? So we're going to talk about two kinds of parasites. One kind are trematodes and one kind are tapeworms. Now, you may remember because, let me finish. Let me finish. You might remember how much fun we had on the zombie fish episode. where I geeked out about the work that I was doing for my Ph.D. Killyfish. Killingish, yes. Yes.

Right. Okay, I do remember. Okay, and that's a trematode. And another kind of trematode that maybe you've heard of is the one that causes the disease called schistosomyasis. And again, we're going to stay as far away from jargon as we can today. But what's a trematode? Is it a quark? Is it a lepton?

Is it a boson? What is it? Oh, yeah. Okay. So it is a multicellular parasite. So it's not like a bacteria, not a virus. and it is kind of flat, usually, and a little bit wide. You probably have never seen it, but it looks different in each stage of its life.

I had described what the adult looks like, and so I don't feel like going into descriptions of what it looks like is going to help you that much, but it's a multicellular parasite that tends to live inside the body of its hosts. And it's a category of parasites, like there are many species that are trematodes? Yes, many species that are turntones. And a tapeworm is an example of a trematode because the tapeworm is multicellular and flat, right? Yeah. So you're right that a tapeworm is multicellular and flat, but it is a different kind of parasite. Oh, I know. I was just feeling like I understood it. I know, I know. But so,

yeah, gosh, would it help if I called trematodes flukes? Flukes and tapeworms to those words? Yeah. Flukes, I like that. All right. Okay. So trematodes, we're going to call them flukes. Tapeworms, it's a word more people are familiar with. We're going to stick with tapeworms. But they're different things, tapeworms and flukes. Okay. Yep. They're groups of parasites. many different species, and I'm going to give you an example of their life cycle that's very like general. Okay. There's a lot of variability, but we're not going to worry about that for this episode.

I'm just going to give you a general life cycle for each of them, and we're going to talk about how scientists sort of stumbled upon the answers over time. And the reason we're doing two at once is because you think we're smart enough to understand two things at once or because their stories are entangled. I did go back and forth on this. Not because I doubt your intelligence, but because, you know, you know, you know, You know, two life cycles is a lot to keep in your head. But their stories are, to some extent, entangled.

And I wanted to go through the story chronologically. And it's not like we figured one out in one year and another out in another year. We've picked up, like, pieces over time as we went. Right. Because if you're figuring things out, you don't know necessarily, like, which pieces connect to which. And sometimes you can get them messed up, I guess. Okay, that makes sense. Yeah.

So tell us about these life cycles and prepare to be grossed out, everybody. Yeah. All right. Flukes. Parasite 1. Yes. Daniel wants me to do Parasite 1, Beetle 1 or whatever.

Okay, Parasite 1. These are flukes. I'm telling you about the sheep liver fluke in case you care, but this is a general life cycle. Okay, so here's what happens. The adults are living near the livers of like sheep and sometimes they can get into humans. Ooh.

They make eggs. Those eggs move into our intestines and get pooped out into the environment. All right. So now they're out of us and they're in a pile of poop somewhere. They're in a pile of poop somewhere. Hopefully they get into water. And usually they do. There's rain that like, you know, passes through the hayfields, brings them into a creek or something like that. Hopefully from the perspective of the parasite, not from the perspective of like somebody who's rooting for humans to have clean drinking water. Yes, quite right, quite right. Yes. And which, and like me, who's rooting for humans to have clean drinking water. But yes, from the perspective of the parasite. All right, so it gets into the water. That egg hatches.

And out of the egg comes a little swimming parasite that has like cilia. So it's got little hairs on the side and those hairs help it swim. And it swims until it finds a snail. A snail. A snail. And it burrows inside of the snail. And then it forms a sack. And inside of that sack, it starts producing loads and loads and loads of this stage

that crawls out of the snake. that crawls out of the snail and starts swimming around on its own. So this is incredible. You've got this tiny little thing with hairs that swims, and now you've got a sack with all of this,

like these writhing little things inside. So they're like heads with tails attached to them. So it looks nothing like the sack. It looks nothing like the little swimming thing with hairs. These all look very different. And what about the snail? You just sort of yada, yada, yada, yada

over the snail. Does it survive this? So usually often what happens to snail, is they get castrated. That's what happened. That's what happened in the zombie fish episode we were talking about. Maybe you should have yada yada yada. And the energy that they would have used to make their own babies now goes towards the production of these parasites.

Sad. Yeah. I don't know in particular if this snow gets castrated. I would guess that it does. You don't know. It sounds like you don't care. I mean, you're just like totally on the side of the parasite here.

I mean, I'm not. You're rooting for poop in the water. You don't care what happens to the castrated snail. All right. That's fine. president of the Parasite Society. It's a lobbying group. I get it. I mean, okay, I don't have to love these creatures to have respect for the amazing ability of

them to like shape shift shift like this and manage to make out a living in all of these crazy environments. In today's economy, this is very impressive, yes. In today's economy. Yeah, okay. So, so that sack produces a bunch of stages that are like heads with tails and the heads with tails emerge from the snail. They swim off, they find something like vegetation, and then they form a cyst. So they essentially excrete a protective outer layer that protects them from drying out. And then they can wait there for a long time until they accidentally get eaten by a sheep or a person. Oh. And then they migrate back to near the liver again.

And the cycle starts again. So they're in three different organisms. Sheep, snail, plant. All those three play a role in the cycle. of this fluke. Yes. They're not taking any energy from the plant. They're just kind of laying on the plant in the hope that the sheep will eat it. But yes, they are interacting with three different organisms. Wow. Sheep snail plant. And they've got that stage where they're living in the water also. So those are like four crazy environments that they have to survive in, which blows my mind. All right. So sheep, water,

snail plant. Sheep water, snail plant. Okay. Okay. Parasite one. Sheep water. snail plant. All right. Parasite 2. These are tapeworms. Tapeworms, I think, unfortunately, all of us can shut our eyes and imagine what a tapeworm looks like. We're going to keep this life cycles a lot easier. You can imagine a tapeworm in the human gut. They've got a head that helps them, like, hold on to our intestines. They produce this long strand. Parts of it pop off. And when it pops off, they've got a bunch of eggs in those strands. And either those segments or just the eggs themselves pass out into the feces. And then they end up sometimes in areas where there's bad sanitation, they end up in the grass.

And then they get eaten by an animal like a pig or a cow. So they don't have to go through a water stage at all. They're just like feces to grass to cow. Yes. Yeah. So let's do feces to grass to cow. Let's forget about pigs right now. Feces to grass to cow.

Okay. And then after they're eaten by a cow, they hatch out of their egg, and then they produce these cysts. And so they're like, they get bigger, and you can find these cysts in the cow. Like, so for example, before we ended up having much better sanitation, butchers in the United States used to butcher cattle, and they would see all of these cysts inside the cows. And so they called this, quote, measly beef when it happened. So actually, throughout history, doctors, butcher, etc., would find cysts like this. And for a while, they didn't know they were parasites.

They were just like, gosh, I wonder what happens to cows that makes them have these weird sacks of fluids in them. And it took a while before they realized those weird sacks of fluids are another organism inside of them. But they were calling it measly, which sounds negative, so they already knew this wasn't good. Yeah, they weren't inclined towards eating. Right, okay. The measly beef.

Okay. But sometimes, instead of having lots of these spots, you just have like one or two and maybe a it. And so if you don't cook your meat through well enough, a human eats beef that's infected by these cysts and they consume it, the parasite then comes out of the cyst and you get the adult stage again. And do you need to go through the cow? Like what happens if you go through the human and then you go out into the feces and that ends up on a plant which the human eats? Can that go straight through the human? Great question. Since we are talking about just the one that infects cows, the answer is nothing. If you consume the eggs,

Nothing. It will die. Okay. And that's the end for the tapeworm. Amazing to me that these parasites are so complicated and they rely on every single stage. You can't skip a step. Yeah, that's what makes it so complicated. Like, there had been experiments where people would like take parasite eggs, you know, not from humans that were infected by tapeworms, but from other animals that were infected by tapeworms.

And they'd give the eggs to like an uninfected, I don't know, rabbit or something. And then the rabbit wouldn't get the tapeworm. And they'd be like, why doesn't this make me? more tapeworms, and it was because it had to go through another species. Incredible. Yeah, and so, okay, so quickly, some reasons why this is complicated. So, you know, as Daniel said, we've got sheep, water, snails, plants. Yeah.

And in the snails, there's, like, three different stages in the snail. So it's like crazy, right? Okay, so they're complicated, last to figure out. The eggs, when you, you know, eventually we find them, but they're tiny and hard to find. And so for a long time, we don't see any eggs at all. And so spontaneous generation seems kind of normal. And the stages don't look similar. And to us, like, you know, today we, it's not too surprising to us that you can have organisms that look different at different stages of their life.

But that was not what people were expecting back then. And so that right there was a massive mental roadblock. The idea that like something that looks one way in a pig could be the same thing that is in a human, even though it looks totally different, that like just did not make any sense at the time. Right, that makes sense. I mean, like humans look very similar over their life cycle, right? Babies are a little weird, but you can see all the bits there. Yeah. And it's true for lots of different things out there in the world. So I guess it makes sense that it would blow our minds a little bit that something could look totally different in a different life cycle. Yeah, I mean, can you think of any like livestock or plants where the baby form looks so different from an adult? Well, what about birds? Like, eggs look pretty different from birds. But that's an egg. Like we understood sperm and. eggs existed. But like, you know, a baby bird, you see it and you're like, that's a bird. Like, it's an ugly bird, but it's a bird. Or maybe it's an adorable bird depending on who you are.

But like, okay, let's take a break. And when we come back, we'll talk about sort of the best guesses at how parasites arose. So, you know, if someone gets a tapeworm, how the heck did they get that tapeworm? We'll talk about what scientists thought. Your husband is not who you think he is. Your body is not what you saw it was. Your identity is formed by a secret history. I'm Danny Shapiro,

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And Sam Ramos on the IHeart Radio app, Apple Podcasts, wherever you get your podcast. Okay, we're back and we're learning about parasites today. Not only are we learning about parasites, we're learning about how we learned about parasites. And I love these meta-conversations because the history of science is fascinating. It shows you what it's like to work on the cutting edge of human ignorance, where we don't already know the path forward, where things are confusing. So try to strip away everything you think you know about science. to put yourself in the minds of these early thinkers and join them in these questions.

Yes. So let's go through the mental and frankly emotional journey that they went through because there was a lot of like, as far as I can tell, name calling during this process. I'm sure parasitologists today are much better behaved, right, Kelly? Oh, man, a lot of my job involves. Let's move on. So, all right. She pleads the fifth, folks. So tell us where this mental journey begins. Okay, so taxonomous, which are people who, like, identify, like, this is one species, this is another species, and, you know, this is where it belongs on the tree of life. They had seen a lot of parasites.

And so they had found the parasites that are, like, in the cows for the tapeworms. And they were like, that's a species. And then they had, unfortunately, recovered pieces of parasite from the poop of humans and been like, that's a different species. Because they look very different. Yeah, because they look very different. So essentially, we had categorized about. twice as many parasites has actually existed because we needed to put those life cycles together, but we didn't know that. It's like saying heads is a coin, tails is a coin. We have two kinds

of coins. We have two kinds of coins. That's right. Exactly. And one of the things that was stumping people was that the version that you see in cows doesn't have any reproductive organs. The version you see in humans does. And that was really keeping them from, you know, sort of connecting things. But all right, so where did they think the parasites were coming from? And the answer is spontaneous generation. Spontaneous generation. Wow, that sounds so magical. It sounds so magical, but so this is like a very early idea.

So, like, you can find ancient Egyptians talking about intestinal worms because, you know, there are some really big parasitic worms that you just kind of can't miss when you have them. Like the parasites die, you defecate them out, and you're like, holy crow, there was a giant worm living inside me. Holy crow. Holy crow. What is the hieroglyph for Holy crow?

was a giant worm living inside of me. I want to see that one. It's a good question. I just don't want to make, you know, Matt, our audio guy, have to hit the bleak button too many times in this episode. So they thought that you end up with intestinal worms because of some interaction between food that's in your stomach that's decomposing, some wounds you might have in your intestine,

and maybe a fever that you happen to have. And so this, like, confluence of things. create an intestinal worm. And so essentially it's not a worm giving birth to another worm. It's just the right combination of things coming together to give you a worm. So they were cool with life beginning from non-life all the time. Yes. Yeah. I mean, they thought their sacred scarred beetles arose from the balls of dung that they were making.

Wow. And so, yeah, yeah, they thought life was arising from not life. I mean, some cultures also had this extra layer of, like, a vital force bringing into the world life. And so there was a bit of a, like, maybe a spiritual thing laid on top, depending on what culture you're talking about. Like, different groups had different sort of subtle ways of thinking about it. So then I'm going to ask you a question. You're almost certainly not prepared to answer. Oh, yay.

What did the ancient Egyptians think about the possibility of aliens? because if they imagine that life arises all the time from nothing, when they look out into the universe, do they imagine, wow, life could be beginning here and there and everywhere? Homework question for Daniel! Okay, all right. I don't know the answer to that one. All right, I'll look into that one.

Okay. So bring us back to parasites. Okay, so moving away from the ancient Egyptians, in like the 1750s and the 1820s, I was able to find quotes from people that were essentially saying, like, hey, look, if you look in someone's gut, you get like some mucus that falls off, and then that mucus sort of coagulates, and that starts to become a worm, and then the worm grows in epithelium, which is like, you know, skin cells on the outside or something.

And it just, lots of scientists seemed totally okay with this idea that, like, some stuff in your gut interacts in a way that results in the production of these worms. And at this point, what was our understanding of reproduction like in humans, right? mid-1700s, we already understood like sperm and eggs and all that stuff, right? Yeah, that's right. And so for quote-unquote higher organisms like humans and dogs and stuff like that, and I didn't mean to sound quite so condescending. I do feel pretty good about being a human.

But we were pretty comfortable with the idea that, like, you know, when we make sperm with eggs, that is the thing that makes babies. But we weren't able to explain things like, where do the worms come from? We also weren't able to explain, like, when a piece of rotting meat sits out and maggots show up, where the heck do those come from? And so there were just like a lot of things that we couldn't figure out either because we hadn't put together like complicated life cycles or because we couldn't see, for example, eggs that were laid by flies. And so we were sort of thrown off the track. And does it make sense to you in that situation to imagine, okay, there's some kinds of life which require sperm and eggs and other kinds of life can just arise whenever. and wherever. Wouldn't it have made more sense to say all of life comes from sperm and eggs?

We just haven't figured out the sort of cycle yet for some of it? Absolutely. I guess that's easy in hindsight to imagine. Right. And it's not like nobody was saying that. So there were some people who were saying like this kind of doesn't make sense. Probably we just haven't like figured it out yet. And there were some people, I think this was like a quote from the 1800s that I found it. They were arguing that like this idea of spontaneous generation is. is quote, unphilosophical.

But so, you know, the problem is they, like, their best argument was, nah, ah. And like, and well, I think, nah, ah, is a pretty good argument. Having not actually been able to see the eggs, you've got this idea that is like, I don't know, maybe as old as time that these things just kind of emerge on their own. And it's a tough idea to unseat, especially when you, you know, efforts to unseat it have not really succeeded. And a lot of this is happening in the era before experiments are very popular.

And so maybe let's jump into one of the first experiments that helped overturn this stuff. So in 1668, you get Francesco Redi, who was court physician to the Duke of Tuscany. And he did a bunch of amazing experiments in general. But he was not convinced by this idea of spontaneous generation. And so he did some very careful observations staring at like dead snakes and hunks of carcasses. and he noticed that like, hey, the kinds of flies that are coming off of these carcasses are the same kinds of flies that landed on the carcasses earlier. Like he noticed that not all flies are the same species. And so he took careful observations.

He noticed that the maggots on the meat turned into these like eggshell shaped things. These are the pupil stages, not actually eggs, but just like a young intermediate stage for insects. He collected those up. And he noticed that the flies that hatched out of those were the same kind of flies as the one that were hovering over it earlier. And he's like, I bet they were laying eggs. So he knew that flies came from maggots. And his question was, where did maggots come from? Did they spontaneously appear in the meat or are they eggs laid by earlier flies?

I believe he knew that maggots show up in meat. Yeah. And then he was the first one to be like, hold on. These maggots turned into pupa. and out of the pupa come the flies. Right. And you always get the same kind of fly from the same kind of pupa.

And you always get the same kind of fly as the kind of fly that was hovering around earlier. So that's probably there's eggs we can't see. You know, I am nostalgic for the time when you could do incredible earth-shaking science just by like watching a hunk of meat for a few days and noticing like maggots turn into flies. Yes.

Like I can tell you that just from taking out my trash. I know, right? But at the time, it was revolutionary to be like, I am just going to sit here and watch these chunks of meat decay very carefully. But then, so then he coupled his observations with experiments because he said belief would be vain without the confirmation of experiment, a man after my own heart. And so he took some jars. He put a couple different kinds of rotting meat in a couple different sets of jars. And he had three different treatments. He had a treatment that was completely open so flies could go into the jar.

Okay. He had a treatment that was covered in gauze, so air could go into the jar in case there was some, like, I don't know, vital force or the heavens that needed access to the decaying meat. The heavens needed access, but a jar was too much for the heavens to overcome. Nice. Well, these things were complicated. And so, and then there was another jar that was just, like, corked shut, like completely closed. Nothing could get in there. Not even the heavens. And so he noticed that the only place you got maggots on the meat.

was the jar that was completely open. And so after that, he concluded, okay, maggots, this is not a spontaneous generation thing. The maggots are not arising from the meat, plus some interaction with something else. The flies are coming in there. They're laying the eggs. The maggots are baby flies.

And this is science, right? Let the data speak. That's right. Let the data speak. But as a man who was driven by data, he says, okay, so I have definitively shown that maggots come from flies, but I do not know.

what's happening with those intestinal worms. I cannot figure out, you know, where the eggs would be coming from, how they got there. Like nobody, you know, those tapeworms can be meters long. No one is swallowing something like that, right? So how is it getting in there? It's got to be spontaneous generation. So even though he pushed the list of organisms that he thought arose by spontaneous generation back, he was comfortable at the end saying, but you know what?

Intestinal parasites? Probably still spontaneous generation. What? That totally surprises me. I was about to ask you if this killed the idea of spontaneous generation for once and for all. And when I was reading the history, I was like, bam, good job, Riddy. But no, he was like, he just pushed it a little farther back. Fascinating.

So it's really a sticky idea. It's a very sticky idea, right? And so then five years later, you get Anton von Luey. You know, the number of times I have to say this guy's name, you'd think I would get it right eventually. But no, you're wrong. He published his first paper in philosophical transactions of the Royal Society. And since I totally avoided saying the name altogether, this is the guy who was making very careful observations with his microscope. He wasn't the first one to look through a microscope, but he was the first one to describe what he called animacules, which are the little critters moving around.

Like in water, he was like looking at the bacteria on his teeth. He was looking at his sperm moving. And actually, he did not believe in spontaneous generation. So do you remember we were talking about Parasite 1, the fluke, that you find. in the sheep, there had been an outbreak in the sheep. And so he was like, all right, I'm going to go to the field where those sheep are infected. And I'm going to try to find what looks like mini versions of those parasites because they should be out there. Right.

But he was looking for something that looked, you know, it wasn't going to look like that because they looked different. He was thinking baby birds look like bigger birds. And so baby flukes should look like bigger flukes. But they don't. They look like little cysts on the plants. Exactly. Right. And so he couldn't find it.

And Lewin Hook was interested in these parasites. I didn't realize that so much of this foundational biology about where life comes from is motivated by parasitology. Yes. Wow. I mean, like, it was one of many things he was interested in looking at. He probably didn't spend nearly as much time looking for parasite eggs as he did looking at his teeth. But like, I'll take it. I was excited that he thought about parasites at all.

I was like, that's going in our talk. Yeah. Yeah. So then about 20 years later, another scientist starts looking at the stage of the flukes, which is parasite one. And so it goes, humans, water, snail, plant, sheep. Right. And so he had found eggs. So he had found the stage that leaves human butt and goes into water and next needs to hatch and find a snail.

And so, yes, it couldn't go right back into the human. There was a lot left to be uncovered. But now he at least knows that there's eggs, which really makes it seem like, why would you bother making eggs if you arise by spontaneous generation? And so the pieces are starting to come together. It's always so easy to put this together in hindsight, right? When you know what the story is. Remember, these folks understood much less than we do. And we're telling the story in a very linear way

when there was like lots of branching, different ways of exploration and thinking about these questions. Yes, right. So then you get this guy, Spalenzani, who lived between 1729 and 1799, and he noticed like, okay, you've got all these tiny little critters that are moving around in the world,

water. He called them infusorians. And he was like, you know, if you heat the water up and you kill them and then you cap it, they don't come back. So it seems like they're like coming from the air. They're not arising by spontaneous generation either. But a bunch of people said of his argument like, well, when you heated up the air, you messed up the conditions that you needed for spontaneous generation. And so a bunch of people weren't convinced by this argument. But other people would say like, okay, the infusorians are now clearly not arising by spontaneous. I love that. That's such a Dr. Seuss name.

Yes. I was going to say it sounds Dr. Whoish also. But, okay, so the infusorians, infus, I'll say it a couple different ways. And, you know, it'll be right, one of them, maybe. On average. On average. Right. So maggots, not coming from spontaneous generation, whatever Spalzani was studying, not coming

by spontaneous generation. but parasitic worms still an open question. And so the Royal Academy of Sciences in Copenhagen announces there's going to be a contest. And they want you to submit essays on the subject of where do parasitic worms come from? And whoever writes the best essay is going to get a prize. And after the break, we're going to hear

about the answers in the best essays. Your husband is not who you think he is. Your body is not what you saw it was. Your identity is formed by a secret history. I'm Danny Shapiro, and these are just a few of the stunning stories I'll be exploring on the 14th season of family secrets. And just then, we felt the plain turn in the air, so much so that the bags that were under people's seats just kind of flew into the aisle. Each week, we dive head first into the complex power of secrecy. how it shapes our identities and relationships

and how it ultimately can reveal to us our truest selves. My daughter, she's pretending she doesn't know, but is trying to cook and feed me and keep me alive because I wasn't eating anything and me pretending like everything was fine. He kind of shoved me out of the way and said, move. And he went out the front door and he jumped in a car and drove off and that was the last time I saw him.

Listen to season 14 of Family Secrets on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast. May is Mental Health Awareness Month, and your 20s, they can feel like a lot. On the psychology of your 20s podcast, we unpack the anxiety, the overthinking, the heartbreak, the identity crisis, all of it that comes with being in your 20s. Because if you've ever thought, is anybody else feeling this way, they definitely are. I feel like my 20s was a process of checking off everything that I was not good at to get to what I was good at. Oftentimes we take everything a little bit too seriously and we get lost in things that we later on decide weren't even important to us to begin when.

There was a large chunk of my 20s that I was just so wanting to be out of that phase out of my skin. And I just like really regret not living in the present more. Each week we break down the science behind what you're going through and give you real tools to navigate it. Your 20s aren't about having it all figured out. They're about understanding yourself just a little bit better. Listen to the psychology of your 20s on the IHeart radio app, Apple Podcasts, or wherever you get your podcasts. The story I've told myself about love or relationships can then shape my behavior, and that can lead me to sabotage the possibility of connection.

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So we're back and we're hearing about parasites in Copenhagen. And why were the Danes so interested in parasites, Kelly? I know that currently in Denmark, there are more Danish pigs than Danish people. Was that true back then? I don't know. But I will say that as we're going to keep moving through this story, two of the most important players in my mind are going to be Danes. I mean, I think everybody

cares about parasites, right? Like parasites are killing people wherever you go. They're killing your livestock. Frankly, everybody should do. Why would you even ask why people would care about parasites, Daniel? I'm flabbergasted. I knew. It's just Copenhagen just kind of came out of the blue there. But I am looking forward to hearing you pronounce some Danish names. I'm not. I'm not. All right. So it's 1780. The Royalties. Academy of Science in Copenhagen has put out a prize and that prize is going to go to the best essay on the subject of the origin of parasitic worms. And there are two winners. And the essays from both of those winners say that parasitic worms come from spontaneous generation.

No. What? Still. Come on people. I know. After all of this. observations, lots of taxonomy. Okay, but 10 years later, we start to see a crack in the clouds and light starts to come through because Dane, Peter Christian, Abelgard, pretty good, probably.

Pretty good. Yeah, that's all right. Yeah. He makes this beautiful observation. And just like we've talked about with other amazing scientists who have made great observations, this one is going to be completely ignored. But still, I want to talk about it because it's great. So he was looking at a small fish and he noticed a tapeworm in its intestine. And he was like, whoa, that tapeworm looks a lot like the tapeworm that I see in birds. A lot of the time, parasites look completely different depending on which host they're in. Right. So when you're in the sheep or in the snail or in the water, you look totally different. Totally. But if you're looking super closely, there are a few characteristics that are consistent.

So, for example, maybe it'll be like you have a sucker in a very particular place, and it stays in that place in each stage of your life cycle. So if you are looking close enough, there are clues, but you have to be looking real close. All right. And Abeldsgaard should have gone with Peter. Really should have gone with Peter. Do you know, in Danish, there are vowels that don't exist in English.

And so you just can't map them to English at all. And this double A is actually an A with a little circle over it, which is pronounced more like an O than an A. But anyway, that's just an aside. Can you say the name? It's probably something like, I mean, I have a terrible accent in Danish, but it's probably Peter Creston, Abel Gort. Oh, nice. Okay. Way better.

All right, that's good. I'm going to have you say a name that comes later, and I'm not even going to try. I'm glad we've got outlines so you can look at it. So, all right, this guy notices a tapeworm that is in a fish, and he's like, wow, parts of the this look a lot like a tapeworm that I've seen in a bird. And so he takes a bunch of fish that are infected and feeds them to the birds. And then, and I'm sorry to say this, dissects the birds. And he finds the tapeworm that he was expecting in those birds. And he's like, this is probably the first life cycle experiment that we know about. This was the first instance where someone was

purposefully like, can I get the tapeworm that's living in this thing? to end up in this thing, and then what does it look like when it goes from one thing to another? How does it change in appearance? And did he just get lucky? Like, how did he know if the thing living in organism one then needed to be in organism two? Is he just lucky? Probably partly, yes. But I'm guessing it was a couple things.

One, he was a very careful observer. And for whatever reason, he had been looking at a bunch of these worms. And two, he probably knew a lot about natural history. and it probably occurred to him that these fish often get eaten by these birds because he knew that these birds are fish-eating birds. Got it. And so it would make sense that the tapeworm could get from one place to another. And so, I don't know, maybe that's what's happening. I mean, that's a real leap of faith, given that nobody has had that, like, thought before as far as I know.

Well, what a tremendous advance. With a moment of discovery, surely he was fetid and internationally and won all sorts of prizes, right? Totally forgotten. Oh, no. Yes. And as far as I can tell, darkness for maybe 20 or 30 years on the topic of life cycles of parasites, until Mudevig Bogenes in Germany. Yep, now apologizing to all the German.

Kelly's doing the, like, international tour of apologies today. So anyway, a German scientist was dissecting a freshwater snail. So we were just talking about someone who was working on a tapeworm life cycle. Yeah. Did amazing work. We're going back to Parasite 1. Flukes. These are the flukes.

Okay. So now, when you are in the snail, you get the sack, and the sack makes the free living stage that swims away. Yeah. So he was dissecting a freshwater snail, and he noticed that free swimming stage crawling out of the sack. Oh. And he was like, holy cow, that sack looks like. it's producing that free swimming stage, but they look nothing alike. Right. Not even a little bit alike. Right. And so he makes some more careful observations and he publishes it and the guy who

runs the journal he published it in said, quote, observations of this kind make one dizzy. Moments of understanding. I mean, maybe you need to be a parasitologist to like have your mind blown by that. But no, also like, I mean, what I think about how amazing it was to be the first one to be like, Whoa, whoa, whoa, those sacks, that's where you get the parasites from. They're coming out of the snails and they're being made in those sacks. Like, that was really exciting. And so now we're starting to pick up the pace kind of. All right.

Okay, so maybe it takes another 10 or so years. But you start getting a bunch of, again, I think mostly German scientists who are now looking at trematode eggs. And so remember, the eggs, they need to hatch. and then you get that little stage that can swim. They've got hairs, and they go off in search of the snail. So they start looking at Trematoed eggs, and they watch them, and they see them hatch. And then they see this little stage pop out and start to swim away. And they're like, whoa, hey, something's happening there.

Those eggs aren't just like ending up in the environment and doing nothing. Those eggs are active, and they're going off in search of something. And then in 1835, you get this guy, Carl von Sebald, who lives in present-day Poland, And he observes that little stage that hatches and moves around. When it dies, it sort of degrades into that sack thing that we were talking about. Okay. And he's like, oh, oh, you get the thing that moves around in the water. And then you get the sack thing.

Then the sack thing we know makes those swimming things. And now you're starting to get the pieces starting to come together. Right. Yeah. And in 1842, we get another amazing contribution by Adain. Daniel, take it away. This looks like Johann Yepetus Smith. Smith is not a very Danish name. And then Strenstrup.

Awesome. So he wrote a book called On the Alternation of Generations. I'm going to leave the rest of the name off. But basically, he looks at the life cycles of a number of different organisms, trematodes being one of them, jellyfish being another, that have life stages that look very different in different states. and he's like, hey, everybody, we need to come to terms with the fact that you can have life stages where sometimes animals at one point in their life look nothing like their parents. But eventually, they're going to look like their parents and the cycle's going to start again. So, for example, jellyfish have a stage where they look like a polyp so they look like they're rooted to

the ground and they kind of look like an umbrella that's been turned inside out by the wind. And then they start like sort of having a bunch of extra like umbrella top. and those sort of pop off and become jellyfish. Wow. And the jellyfish produce eggs and it starts over again. But those umbrella tops don't really look anything like jellyfish. Yeah. And so he suddenly got people thinking, oh, my gosh, okay, you can have these complete life cycles

where we have to totally change our conception of what, like, birth needs to look like. Yeah. Babies don't look like their parents. They look totally different. And now this really gets the ball rolling. I wonder if that makes it hard for the parents to love their kids. Because, you know, like, there's something about seeing a little baby that looks a little bit like you. And, oh, look, their fingers are so cute.

They're just like mine. But if you give birth to, like, a little blob that looks nothing like you, do you think parasite parents are like, ew, gross? Or do you think they still think they're cute? So here's the thing. As far as I can tell any of the organisms that have this strategy, have this strategy where they essentially release their offspring into the environment and never see them again. And so it doesn't come on. That's one kind of parenting. That's right.

That's right. Foreign strategy. All right. So you're not advocating for the parasite strategy of parenting. Absolutely not. No. No. I overinvested in my children, maybe.

And that's how I like it to be. So this amazing Danish scientist whose name I'm not going to try to pronounce gets a lot of details right. He doesn't actually get all of the life cycle details for Trematodes right. We don't need to go into the details of what he got wrong. But he really got the ball rolling. Yeah. One of the balls he got rolling was a man named Friedrich Kuchenmeister in Germany. Now, Daniel, do you also speak German?

I only speak a tiny bit of German, but enough to know what Kuchenmeister means. What does it mean? It's basically like Master Baker. I think it's like Cake Master, basically. Oh, well, that is going to become ironic a little bit later on in the story. So hold on. Amazing. So he's a German physician, and he starts working on experimental and,

infections to close life cycles. And so first thing he does is he says, okay, he's working on the tapeworms. And he is like, all right, there have been some experiments where we've tried, like, feeding parts of tapeworms to other organisms. But, like, you've got to know something about the natural history, right? So if you have cis in rabbits, feed them to foxes. Because in nature, foxes eat rabbits. Makes sense.

And so he feeds sys from rabbits to foxes. Bam, he gets the tapeworm. You're so excited about giving tapeworms. to these foxes. I mean, I'm not, but like, he didn't. Anybody think about the poor foxes. Well, you're ruining this for me, Daniel. He's figuring out the life cycle. This is amazing. You know those times when you're like, Daniel, it's ridiculous that you would sacrifice all of Earth to learn the secrets of physics. That's what you're feeling right now.

Tushay. Tushay. Okay. I do really like foxes. They are very cute. I'm sorry about, thank you to all the foxes who have given their lives to science. Same goes for the mice and the cat that I'm about to talk about. So next, he got cysts from mice and he fed them to a cat. He dissected the cat. Bam, another life cycle. She says with sadness in her voice. And empathy.

And empathy. Thank you. That's right. Sadness and empathy. And these life cycle experiments just start taking off. People start doing a lot more. And now we used to have, you know, like the list of parasites that were found as cysts in, hey, it turns out a lot of prey animals.

and the list of parasites that were living as long, stringy tapeworms in the guts of predators, turns out those aren't two separate species. You lump them together. That's the life cycle of one tapeworm. Heads plus tails equals one coin, actually. That's right. And so we were talking about a parasite that goes from humans to cows. Kuchenmeister was interested in a parasite that goes from humans to pigs.

And he happened to have pig cysts. that he was able to get his hands on, and he had access to a convict who was on death row. Oh. And he was allowed to slip the cysts into lukewarm food that the convict was being fed. And then after, without the convict knowing. And then after.

Was it a cake? No, it was soup or sausage because it's Germany. And so, my apologies to Germans. And so whose sausage is delicious when it's not infected by parasite. And so after the convict was killed, he dissected or autopsied the convict and found the parasites within. And so he was able to complete the life cycle that way in humans as well by doing something that I would argue not so ethical. But anyway, so a flurry of life cycle experiments went forward and we have since figured out a bunch of tapeworm life cycles.

Thanks to Bratworth, Meister. Thanks to Mr. Bratworth. And it took a little while longer for us to figure out all of the details of the trematode lifecycle. As you could tell from the description at the beginning, there's a lot more steps involved. Yeah. But eventually we were able to figure all of it out. So it took until like 1879 to 1882 for us to figure out all of the different steps of the life cycle. So it took a while to figure out, for example, that it forms these little sys on grass that then need to get eaten by the sheep.

and it took a while to figure out exactly what snail that free swimming stage was looking for. But eventually we figured out the life cycle for the sheep liver fluke. And we figured out a bunch of life cycle since. And it's not always easy, but we do it. And the key insight is that you can't assume that the babies look just like the parents, right? Yes. And once you release that assumption, now you're looking for this thing to have a different shape and a different appearance in different parts of its life cycle. That's really cool.

It's really cool. But once you've unlocked some of the piece, it allows you to move much more quickly because now you know, like for the tapeworms, for example, you know that you're looking for cysts that look a certain way and you know that you're probably looking for a predator of that animal. So if you know about the natural history and you know about like predator prey relationships, you have a pretty reasonable guess at what's going to be next. So yeah, like the more knowledge that we built up, the easier it became to try to figure this kind of stuff out. But it still was complicated. And there were still a lot of hard problems that needed to be solved. Things like it took a while for us to figure out that some diseases are transmitted by mosquitoes that essentially inject the parasites into your blood. And that could be a whole other episode if anyone wants me to go there. But, yeah, I know. It's crazy. And so these parasite life cycles weren't solved specifically to address the question of spontaneous generation.

But when we did figure out these parasite life cycles, we were able to say, okay, clearly it's not spontaneous generation. We now know how it's done. It feels like spontaneous generation is like the explanation of the gaps. It's like anything where we don't understand its reproduction, we'll just assume it's coming out of nothing. Were there experiments done to support spontaneous generation? Any early experiments whose results were confusing that encourage these thoughts? Or is it always just the things we can't explain? Oh, that's a great question.

So I remember reading that people essentially would come up with lists of like, well, here is how you make mice. and here is how you make this organism. And so they had like observed a number of different cases where these combination of factors resulted in mice. And I think they just had an observation where, oh, a couple of different times when I went into the granary, for example, underneath the pile of rags, I found mice. And that's probably how you get mice. And of course, that's not how you get mice.

But like they, you know, they had observations that they thought could be best explained by that. And, you know, every time you see. rotting meat or every time you see fruit flies arise on fruits and you can't explain it any other way, you probably say, well, there you go. What more evidence do you need? Like there weren't any fruit flies in my kitchen before. Now I have rotting fruit and there's thousands of them. Where else did they come from, Kelly? But it's not just anecdotal evidence. They actually were experiments by like John Needham who tried to isolate this stuff and show that when you like boil mutton gravy

and put it in sealed flasks, you still get microorganisms. And so this supported the idea that it came from the broth. But the issue, of course, is that they didn't really remove all of the microorganisms, didn't really kill all of it. So it's an experimental issue. Yeah. And I think that's an important thing to recognize when science changes its paradigm. You have to go back and explain all the previous experiments, right?

You can't just be like, look, this one seems really convincing. You have to explain everything. And that's why it's hard for science to change. That's why it's hard for new ideas to knock off established paradigms. Yeah, no, absolutely. And it's good that it's hard. Like, it should take a lot of careful thoughts and careful experiment. And one experiment that has a different result shouldn't, you know, wipe out all of the careful work that had come before.

Yeah, exactly. For all the conspiracy theories about scientists defending the dogma, it's really just like all the evidence that needs to be accounted for. And it is hard to have a new idea come in and explain everything else that we've seen. And so, yeah, it should be hard to change directions. Otherwise, we'd be like whipsawing every day as a new paper comes out. Yeah, exactly. Well, I really appreciate that Boris asked this question and gave me the opportunity to dig into the history of my field and to talk about parasites and be able to say, it's not my fault. We have another episode about parasites.

And so, Boris, did I answer your question? And thank you so much for sharing your question and your curiosity with us. And I think this is evidence that interest in parasites is not spontaneous. It's not just appearing randomly the population. You're seeding it, Kelly, by talking about the amazing parasite science. Oh, yeah. So I laid like an egg in Boris's brain and let's stop. Yeah, okay.

I really enjoyed this episode. And of course, you cover my question and much more. I learned learning about the history of science. It's very interesting to see how, on one hand, we've learned so much in changed so much over the last few hundred years, like we used to solve open problems by writing essays. And on the other hand, it all happened incrementally. And it's cool to see examples of this. Like, we made progress because someone was curious and decided to do an experiment with meat in a jar. Really enjoyed the episode. Thank you.

Whoops. Boris's initial question asked about how we figured out parasite life cycles in general, but it also asked about how we figured out the life cycle for a specific parasite. and I neglected to go into that detail. Back on the January 8th, 2026 episode of DKEU, I told you all about my PhD research studying a trematode parasite called Uhaplurcus Californiances or UHA. And I told you all about the life cycle,

where each day during the summer, thousands of the swimming stage of the parasite emerge from snails and then hope to run into their next host, California killifish. And if they're lucky enough to do that, and they run into that host, they burrow through and they move up to the,

the brain. Then they form this protective outer wall and they wait there to be consumed by the next host in their life cycle, a predatory bird. And from their location atop the control center of the fish, the parasite seems to be able to manipulate the fish's behavior, getting the fish to dart around a bunch, which might be attracting the attention of predatory birds, making the fish more likely to get eaten and making the parasites more likely to make it to their next home. Once inside the bird, the parasites burst out of their protective shell, find a mate, and make eggs that get mixed up with the bird poop, and will hopefully accidentally be consumed by a snail so the cycle can start again.

And that hopefully is from the perspective of the parasite, of course, not the snail. The snail gets castrated by the parasite, so it's obviously hoping to avoid that. Anyway, to learn more about this crazy system, check out our zombie fish episode. And so after that episode was released, Boris, one of our... Extraordinaries, wrote in to ask how the heck that life cycle was figured out in the first place. And so the answer is that in 1950, W.E. Martin published this paper explaining how he did just that. So first, he noticed that one of the actually many trematode species that swim out of the snails looked like a closely related species that he had read about, and that species tended to go off

in search of fish after leaving the snail. So he said, okay, I think I have a new species. species here, and I need to find the next host in the life cycle, and so I probably should go and search the fish next. And he found that in 100% of the California killifish, there was a parasite on its brain that had certain features that were very similar to this one parasite that was coming out of the snail. So that was a good start. Probably it was going from snail to California killyfish. Then it was experiment time. The stage of the parasite found in the fish was fed to birds, and the birds became infected. And when the experiment was done on rats,

they didn't become infected. So the parasite goes from snail to California killifish to predatory bird. Martin then assumes that the snails must eat the parasite eggs because that's what happens to other parasites that looked like the one that he was working with and he assumed that similar parasites are doing the similar thing.

All right, so that was a lot of work to figure out the life cycle, but Martin had a lot of things going for him at the outset. He knew that the thing emerging from the snail was a trematode, and he could use published descriptions of other trematodes to try to figure out if this was a parasite that was new to science or not. And if it was new to science, which it turned out it was, he could figure out who its relatives were based on the appearance of the parasite.

And today we would use genetic information as well to figure that out. And once he knew who the relatives were, he had a lot of cues about the parasite's life cycle, because related parasites tend to have pretty similar life cycles. So rather than search every single animal in the salt marsh and try to figure out if the parasites within them kind of looked like parasites that you found in the snail,

he could start by just looking at the fish because he had a pretty good idea that that's where he was going to find the parasite he was looking for. And after the trematoda ends up in a host like a fish, he knew that usually the next host in the life cycle was a predator of the fish. And in these salt marshes, a very common predator, are marsh birds. So Martin had a lot of resources at his disposal.

And while this was still really hard work, he was working at a time when a lot of trematode life cycles had already been figured out. Okay, until next time, thanks. Thanks everybody for listening. Please go and do us a favor and rate the show on whatever podcast app you're using. It really helps people find us. Daniel and Kelly's Extraordinary Universe is edited by the amazing Matt Kesselman. He really is a wizard. You can also find us online on Blue Sky,

Instagram, and X, D&K Universe. Come engage with us. You can email us at questions at Danielandkelly.org. We really do want to hear from you. And you can find our website, www. www. danielandkelly.org, where you'll also find an invitation to join our Discord where everybody comes and talks about the amazing universe. And we also have the most amazing moderators. This is an I-Heart podcast. Thanks for joining us. Another podcast from some SNL,

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