The discovery of parasite life cycles

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 https://omnystudio.com/listener for privacy information.

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

• 0:00Daniel Whiteson?

  This is an iHeart Podcast. Guaranteed Human.

• 0:04Kelly Weinersmith?

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• 1:13Kelly Weinersmith?

  Howdy, extraordinaries! In the past, on the show, we've talked about the tortuous paths that parasites take to complete their life cycles. Trematode 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.

• 1:57Daniel Whiteson?

  Hi, I'm Daniel. I study particle physics and aliens.

• 2:01Kelly Weinersmith?

  Hi, I'm Kelly Wiener-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.

• 2:13Daniel Whiteson?

  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.

• 2:28Kelly Weinersmith?

  No way. Are you serious? Why does it matter what planet they came from?

• 2:34Daniel Whiteson?

  Because if they come from Earth, they're gooey and gross. And if they come from outer space, they're fascinating.

• 2:40Kelly Weinersmith?

  But they could be gooier and grosser. And gosh, I hope they are.

• 2:44Daniel Whiteson?

  In fascinating ways, right?

• 2:46Kelly Weinersmith?

  Oh, my goodness. You are inconsistent.

• 2:49Daniel Whiteson?

  Absolutely. I have a purely emotional and non rational response to being invaded personally.

• 2:55Kelly Weinersmith?

  All right. Yeah, that's fine. I can, I can let that slide.

• 3:00Daniel Whiteson?

  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?

• 3:16Kelly Weinersmith?

  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 Helminthological Society of Washington, which is local, but I'm president-elect of the American Society of Parasitologists. I won't be president till July. Yeah, yeah.

• 3:33Daniel Whiteson?

  Glad we cleared that up.

• 3:35Kelly Weinersmith?

  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, and he was talking about how, uh, you know, he met a bunch of parasitologists and parasitologists were clearly the degenerates of the medical field.

• 4:13Kelly Weinersmith?

  And so, uh, so I would just like to say that, uh, all of us, we're all the stinky kids, uh, on the bus. All the parasitologists are. And so I'm gonna, I'm just gonna go ahead and, and, you know, throw myself in there with everybody else. I'm not calling anyone out.

• 4:31Daniel Whiteson?

  And that's fascinating that you're not the only parasitologist who then transitioned to thinking and studying about space.

• 4:37Kelly Weinersmith?

  Yeah.

• 4:38Daniel Whiteson?

  Right? This guy was a parasitologist and now he works on space medicine. Is that because he discovered space parasites and he'd been keeping them a secret?

• 4:45Kelly Weinersmith?

  No.

• 4:46Daniel Whiteson?

  Are we gonna start a conspiracy theory right here today on the pod?

• 4:49Kelly Weinersmith?

  Yes. Yes, that's right. Cause we're, we're trying to edge out the Sasquatch Chronicles. So, uh, 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, that's what I was doing too. And that's why I split my time between these two fields. Yeah.

• 5:05Daniel Whiteson?

  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.

• 5:18Kelly Weinersmith?

  That would explain a lot.

• 5:21Daniel Whiteson?

  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.

• 5:37Kelly Weinersmith?

  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.

• 5:48Speaker 03

  Hey Kelly and Daniel, since I listened to the episode about gillfish and the trematodes that infect 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.

• 6:20Daniel Whiteson?

  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?

• 6:47Kelly Weinersmith?

  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 Parasitologists 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.

• 7:12Daniel Whiteson?

  Well, that's reassuring, I suppose.

• 7:14Kelly Weinersmith?

  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.

• 7:42Daniel Whiteson?

  Like abiogenesis.

• 7:43Kelly Weinersmith?

  Yeah, exactly. That's another word for it.

• 7:45Daniel Whiteson?

  Right.

• 7:46Kelly Weinersmith?

  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.

• 7:55Daniel Whiteson?

  But 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.

• 8:06Kelly Weinersmith?

  Yes, but let's not get too far off track. But 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.

• 8:18Daniel Whiteson?

  All right. So which parasite are we talking about today?

• 8:21Kelly Weinersmith?

  So we're, we're going to talk about two kinds of parasites. One kind are tramehtodes and one kind are tapeworms. Now you may remember.

• 8:31Daniel Whiteson?

  I don't.

• 8:32Kelly Weinersmith?

  Because, no, well, no, I, 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 PhD.

• 8:43Daniel Whiteson?

  Killifish.

• 8:44Kelly Weinersmith?

  Killifish, yes.

• 8:45Daniel Whiteson?

  Yes.

• 8:45Kelly Weinersmith?

  Right.

• 8:46Daniel Whiteson?

  Okay, I do remember.

• 8:47Kelly Weinersmith?

  Okay, and that's a tramehtode. And another kind of tramehtode that maybe you've heard of is the one that causes the disease called schistosomiasis. And again, we're not, we're going to stay as far away from jargon as we can today.

• 8:59Daniel Whiteson?

  But what's a tramehtode? Is it a quark? Is it a lepton? Is it a boson? What is it?

• 9:04Kelly Weinersmith?

  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 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.

• 9:30Daniel Whiteson?

  And it's a category of parasites? Like there are many species that are tramehtodes?

• 9:34Kelly Weinersmith?

  Yes, many species that are.

• 9:35Daniel Whiteson?

  And a tapeworm is an example of a tramehtode? Because a tapeworm is multicellular and flat, right?

• 9:40Kelly Weinersmith?

  Yeah, so you're right that a tapeworm is multicellular and flat, but it is a different kind of parasite.

• 9:45Daniel Whiteson?

  No.

• 9:47Daniel Whiteson?

  Just feeling like I understood.

• 9:48Kelly Weinersmith?

  I know, I know. But so, yeah, gosh, would it help if I called tramehtodes flukes? Flukes and tapeworms? Do those words?

• 9:55Daniel Whiteson?

  Yeah. Flukes, I like that.

• 9:58Kelly Weinersmith?

  All right, okay. So tramehtodes, we're going to call them flukes. Tapeworms, it's a word more people are familiar with. We're going to stick with tapeworms.

• 10:05Daniel Whiteson?

  But they're different things.

• 10:06Kelly Weinersmith?

  Yes.

• 10:06Daniel Whiteson?

  Tapeworms and flukes. Okay.

• 10:08Kelly Weinersmith?

  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.

• 10:15Daniel Whiteson?

  Okay.

• 10:15Kelly Weinersmith?

  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.

• 10:26Daniel Whiteson?

  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?

• 10:32Kelly Weinersmith?

  I did go back and forth on this, not because I doubt, not because I doubt your intelligence, but because, 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.

• 10:53Daniel Whiteson?

  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.

• 11:00Kelly Weinersmith?

  Yeah.

• 11:00Daniel Whiteson?

  So tell us about these life cycles and prepare to be grossed out, everybody.

• 11:05Kelly Weinersmith?

  Yeah. All right. Flukes. Parasite one.

• 11:08Daniel Whiteson?

  Yes.

• 11:08Kelly Weinersmith?

  Daniel wants me to do parasite one, beetle one, or whatever. Okay, parasite one. These are flukes.

• 11:14Daniel Whiteson?

  Yeah.

• 11:14Kelly Weinersmith?

  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.

• 11:30Daniel Whiteson?

  Ooh.

• 11:31Kelly Weinersmith?

  They make eggs. Those eggs move into our intestines and get pooped out into the environment.

• 11:38Daniel Whiteson?

  All right. So now they're out of us and they're in a pile of poop somewhere.

• 11:42Kelly Weinersmith?

  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 hay fields, brings them into a creek or something like that.

• 11:52Daniel Whiteson?

  Hopefully, from the perspective of the parasite, not from the perspective of like somebody who's rooting for humans to have clean drinking water.

• 12:00Kelly Weinersmith?

  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, like so it's got little hairs on the side and those hairs help it swim. And it swims until it finds a snail.

• 12:23Daniel Whiteson?

  A snail.

• 12:24Kelly Weinersmith?

  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 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.

• 12:59Daniel Whiteson?

  And what about the snail? You just sort of yada, yada, yada it over the snail? Does it survive this?

• 13:04Kelly Weinersmith?

  Oh, so usually often what happens to snails is they get castrated. That's what happens. It, that's what happened in the zombie fish episode we were talking about.

• 13:14Kelly Weinersmith?

  And, and the energy that they would have used to make their own babies now goes towards the production of these parasites.

• 13:19Daniel Whiteson?

  Sad.

• 13:20Kelly Weinersmith?

  Yep. I don't know in particular if this snail gets castrated. I would guess that it does.

• 13:25Daniel Whiteson?

  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.

• 13:30Kelly Weinersmith?

  I mean, I'm not.

• 13:30Daniel Whiteson?

  You're rooting for poop in the water. You don't care what happens to the castrated snail. All right, that's fine. You're president of the parasite society. It's a lobbying group. I get it.

• 13:40Kelly Weinersmith?

  I mean, okay, I don't have to love these creatures to have respect for the amazing ability of them to like

• 13:48Daniel Whiteson?

  Yes.

• 13:49Kelly Weinersmith?

  shapeshift like this and manage to make out a living in all of these crazy environments.

• 13:54Daniel Whiteson?

  In today's economy, this is very impressive. Yes.

• 13:56Kelly Weinersmith?

  In today's economy. Yeah. Okay. So, so they, 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 like excrete a protective outer layer that protects them from like drying out. And then they can wait there for a long time until they accidentally get eaten by a sheep or a person. And then they migrate back to near the liver again and the cycle starts again.

• 14:29Daniel Whiteson?

  So they're in three different organisms. Sheep, snail, plant. All those three play a role in the cycle of this. Luke.

• 14:38Kelly Weinersmith?

  Yes, they're not taking any energy from the plants. 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.

• 14:49Daniel Whiteson?

  Wow.

• 14:49Kelly Weinersmith?

  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.

• 14:59Daniel Whiteson?

  All right, so sheep, water, snail, plant.

• 15:01Kelly Weinersmith?

  Sheep, water, snail, plant.

• 15:03Daniel Whiteson?

  Okay.

• 15:04Kelly Weinersmith?

  Okay, parasite one, sheep, water, snail, plant. All right. Parasite two, 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, this life cycle is 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.

• 15:48Kelly Weinersmith?

  And then they get eaten by an animal like a pig or a cow.

• 15:52Daniel Whiteson?

  So they don't have to go through a water stage at all. They're just like feces to grass to cow.

• 15:58Kelly Weinersmith?

  Yes, yeah, so let's do feces to grass to cow. Let's forget about pigs right now. Feces to grass to cow.

• 16:04Daniel Whiteson?

  Okay.

• 16:04Kelly Weinersmith?

  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, they called this quote measly beef when it happened. So actually throughout history, doctors, butchers, et cetera, 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.

• 16:43Kelly Weinersmith?

  And it took a while before they realized those weird sacks of fluids are another organism inside of them.

• 16:50Daniel Whiteson?

  But they were calling it measly, which sounds negative. So they already knew this wasn't good.

• 16:54Kelly Weinersmith?

  Yeah, they didn't. They weren't inclined towards eating.

• 16:57Daniel Whiteson?

  Right, okay.

• 16:58Kelly Weinersmith?

  The measly beef.

• 16:59Daniel Whiteson?

  Okay.

• 16:59Kelly Weinersmith?

  But sometimes, instead of having lots of these spots, you just have like one or two and maybe you miss it.

• 17:04Daniel Whiteson?

  Uh huh.

• 17:05Kelly Weinersmith?

  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 thing comes out of the cyst and you get the adult stage again.

• 17:16Daniel Whiteson?

  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?

• 17:26Kelly Weinersmith?

  Great question. Since we are talking about just the one that infects cows, the answer is nothing.

• 17:32Daniel Whiteson?

  Wow.

• 17:33Kelly Weinersmith?

  If you consume the eggs, nothing. It will die.

• 17:37Daniel Whiteson?

  Okay.

• 17:38Kelly Weinersmith?

  And that's the end for the tapeworm.

• 17:40Daniel Whiteson?

  Amazing to me that these parasites are so complicated and they rely on every single stage. You can't skip a step.

• 17:46Kelly Weinersmith?

  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 more tapeworms? And it was because it had to go through another species. And

• 18:08Daniel Whiteson?

  Incredible.

• 18:09Kelly Weinersmith?

  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.

• 18:17Daniel Whiteson?

  Yeah.

• 18:18Kelly Weinersmith?

  And in the snails, there's like three different stages.

• 18:20Daniel Whiteson?

  Right.

• 18:21Kelly Weinersmith?

  In the snail. So it's like crazy, right? Okay, so they're complicated, lots 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.

• 18:34Daniel Whiteson?

  Hmm.

• 18:35Kelly Weinersmith?

  And the stages don't look similar.

• 18:38Daniel Whiteson?

  Right.

• 18:38Kelly Weinersmith?

  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.

• 19:04Daniel Whiteson?

  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.

• 19:11Kelly Weinersmith?

  Yeah.

• 19:11Daniel Whiteson?

  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.

• 19:20Kelly Weinersmith?

  Yeah, I mean, can you think of any like livestock or plants where the baby form looks so different from an adult's?

• 19:27Daniel Whiteson?

  Well, what about birds? Like, eggs look pretty different from birds.

• 19:31Kelly Weinersmith?

  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.

• 20:06Speaker 02

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• 21:13Speaker 05

  Hello, hello. I'm Malcolm Gladwell, host of Smart Talks with IBM. I spoke with IBM's Ana Paula Assis about what digital sovereignty actually means in the age of AI.

• 21:24Speaker 06

  AI now moves from somebody operating a system to the technology actually making decisions. So you have to have a much better visibility of what AI is touching, data it is accessing and where.

• 21:40Speaker 05

  Hear the full conversation wherever you get your podcasts.

• 21:45Speaker 02

  Hi, it's Kathleen Griffith, co-host of the Unshakeables podcast. Listen in as Ron and Kelly Moore share their story.

• 21:53Daniel Whiteson?

  I'm Ron Moore.

• 21:55Speaker 02

  And I'm Kelly Moore. And starting our own business in our 50s was a little scary, but exciting.

• 22:03Daniel Whiteson?

  The biggest advantage was our community. The whole thing about business is relationships.

• 22:08Speaker 02

  We'll say team a lot, not just our internal team, it's the team of our vendors, the team of our clients.

• 22:15Daniel Whiteson?

  We help the Veterans Resource Center and we give scholarships. I'm doing this for my heart, for my children, for you, for your children's family. I don't want anything back. And I tell a lot of people, don't give back to me, pay it forward.

• 22:29Speaker 02

  If you work to give, you just see the world differently.

• 22:33Daniel Whiteson?

  I'm gonna help as many people as we can. I can't take it with me.

• 22:36Speaker 02

  To hear more from Ron and Kelly, listen to the Unshakeables wherever you get your podcasts.

• 22:54Daniel Whiteson?

  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.

• 23:25Kelly Weinersmith?

  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.

• 23:34Daniel Whiteson?

  I'm sure parasitologists today are much better behaved, right, Kelly?

• 23:38Kelly Weinersmith?

  Oh, man, a lot of my job involves, uh, let's move on. So, uh, all right.

• 23:44Daniel Whiteson?

  She pleads the fifth, folks. So tell us where this mental journey begins.

• 23:49Kelly Weinersmith?

  Okay, so taxonomists, which are people who like identify like, oh, 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.

• 24:14Daniel Whiteson?

  Because they look very different.

• 24:15Kelly Weinersmith?

  Yeah, because they look very different. So essentially, we had categorized about twice as many parasites as actually existed, because we needed to put those life cycles together, but we didn't know that.

• 24:25Daniel Whiteson?

  It's like saying heads is a coin, tails is a coin. We have two kinds of coins.

• 24:29Kelly Weinersmith?

  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.

• 24:51Daniel Whiteson?

  Spontaneous generation. Wow, that sounds so magical.

• 24:54Kelly Weinersmith?

  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.

• 25:15Daniel Whiteson?

  Holy crow.

• 25:16Kelly Weinersmith?

  Holy crow.

• 25:17Daniel Whiteson?

  What is the hieroglyph for holy crow, there was a giant worm living inside of me? I want to see that one.

• 25:22Kelly Weinersmith?

  It's a good question. I just don't want to make, you know, Matt, our audio guy, have to hit the bleep 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 creates an intestinal worm.

• 25:52Kelly Weinersmith?

  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.

• 26:00Daniel Whiteson?

  So they were cool with life beginning from non-life, like, all the time?

• 26:06Kelly Weinersmith?

  Yes. Yeah. I mean, they thought their sacred scarab beetles arose from the balls of dung that they were making.

• 26:14Daniel Whiteson?

  Wow.

• 26:14Kelly Weinersmith?

  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.

• 26:36Daniel Whiteson?

  So then I'm gonna ask you a question you're almost certainly not prepared to answer.

• 26:39Kelly Weinersmith?

  Oh, yay.

• 26:43Daniel Whiteson?

  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?

• 26:57Kelly Weinersmith?

  Homework question for Daniel. I don't know the answer to that one.

• 27:03Daniel Whiteson?

  All right, I'll look into that one.

• 27:04Kelly Weinersmith?

  Okay.

• 27:05Daniel Whiteson?

  So bring us back to parasites.

• 27:06Kelly Weinersmith?

  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 an epithelium, which is like, you know, skin cells on the outside or something. And, 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.

• 27:38Daniel Whiteson?

  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?

• 27:47Kelly Weinersmith?

  Yeah, that's right. And so for quote unquote, higher organisms like humans and dogs and stuff like that, and I, you know, I didn't mean to sound quite so condescending. I do feel pretty good about being a human, but like, we were pretty comfortable with the idea that like, you know, when we mix 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

• 28:32Kelly Weinersmith?

  of thrown off the track.

• 28:33Daniel Whiteson?

  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?

• 28:50Kelly Weinersmith?

  Absolutely.

• 28:52Daniel Whiteson?

  I guess that's easy in hindsight to imagine.

• 28:55Kelly Weinersmith?

  Right. And it's not like, nobody was saying that. So that 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 quote, unphilosophical. But so that, you know, the problem is they, like, their best argument was, nah-ah. And like, and, and while I think nah-ah is a pretty good argument, having not actually been able to see the eggs, you've, 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

• 29:40Kelly Weinersmith?

  succeeded. And this is, 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 Radee, 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.

• 30:24Daniel Whiteson?

  Okay.

• 30:25Kelly Weinersmith?

  And so he took careful observations. He noticed that the maggots on the meat turned into these like eggshell shaped things. These are the pupal stages, not actually eggs, but just like an, you know, 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 was were hovering over it earlier. And he's like, I bet they were laying eggs.

• 30:49Daniel Whiteson?

  So he knew that flies came from maggots. And his question was, where did maggots come from? Do they spontaneously appear in the meat? Or are they eggs laid by earlier flies?

• 30:59Kelly Weinersmith?

  I believe he knew that maggots show up in meat.

• 31:02Daniel Whiteson?

  Yeah.

• 31:02Kelly Weinersmith?

  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.

• 31:10Daniel Whiteson?

  Right.

• 31:10Kelly Weinersmith?

  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.

• 31:23Daniel Whiteson?

  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.

• 31:33Kelly Weinersmith?

  Yes.

• 31:33Daniel Whiteson?

  Like, I can tell you that just from taking out my trash.

• 31:36Kelly Weinersmith?

  I know, right? But at the time, it was revolutionary to be like, I am just gonna 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 of different kinds of rotting meat in a couple of 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.

• 32:05Daniel Whiteson?

  Okay.

• 32:05Kelly Weinersmith?

  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.

• 32:13Daniel Whiteson?

  The heavens needed access, but a jar was too much for the heavens to overcome. Nice.

• 32:20Kelly Weinersmith?

  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, and 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.

• 32:52Daniel Whiteson?

  And this is science, right? Let the data speak.

• 32:54Kelly Weinersmith?

  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.

• 33:06Daniel Whiteson?

  Right.

• 33:07Kelly Weinersmith?

  I cannot figure out, you know, where the eggs would be coming from, how they got there. Like nobody, you know, those, 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.

• 33:33Daniel Whiteson?

  Well, that totally surprises me. I was about to ask you if this killed the idea of spontaneous generation for once and for all.

• 33:40Kelly Weinersmith?

  I know. And when I was reading the history, I was like, bam, good job, Rudy. But no, he was like, he's just pushed it a little farther back.

• 33:47Daniel Whiteson?

  Fascinating. So it's really a sticky idea.

• 33:50Kelly Weinersmith?

  It's a very sticky idea, right? And so then five years later, you get Anton von Lue. 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 like, 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.

• 34:31Kelly Weinersmith?

  So do you remember we were talking about parasite one, 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.

• 34:47Daniel Whiteson?

  Right.

• 34:47Kelly Weinersmith?

  But he was looking for something that looks, you know, it wasn't going to look like that because they look different.

• 34:52Daniel Whiteson?

  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.

• 35:00Kelly Weinersmith?

  Exactly. Right. And so he couldn't find it.

• 35:02Daniel Whiteson?

  And Lewin Hook was interested in these parasites. He didn't realize that so much of this foundational biology about where life comes from is motivated by parasitology.

• 35:11Kelly Weinersmith?

  Yes.

• 35:12Daniel Whiteson?

  Wow.

• 35:12Kelly Weinersmith?

  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.

• 35:25Daniel Whiteson?

  Yeah.

• 35:26Kelly Weinersmith?

  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.

• 35:41Daniel Whiteson?

  Right.

• 35:41Kelly Weinersmith?

  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.

• 36:06Daniel Whiteson?

  It's always so easy to put this together in hindsight, right? When you know what the story is. But 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.

• 36:21Kelly Weinersmith?

  Yes, right. So then you get this guy Spalanzani, who lived between 17 29 and 1799 and he noticed like, okay, you've got all these tiny little critters that are moving around in the 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.

• 37:00Daniel Whiteson?

  Wow.

• 37:01Kelly Weinersmith?

  But other people would say like, okay, the infusorians are now clearly not arising by spontaneous.

• 37:08Daniel Whiteson?

  That's such a Dr. Seuss name.

• 37:10Kelly Weinersmith?

  Yes, I was gonna say it sounds Doctor Who ish also, but uh, okay, so the infusorians, I'll say it a couple of different ways and you know, it'll be right, one of them maybe.

• 37:21Daniel Whiteson?

  On average.

• 37:21Kelly Weinersmith?

  On average, right. So maggots, not coming from spontaneous generation. Whatever Spalzoni was studying, not coming by spontaneous generation. But parasitic worms, still an open question. And so

• 37:34Daniel Whiteson?

  Right.

• 37:34Kelly Weinersmith?

  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.

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• 39:10Speaker 05

  Hello, hello. I'm Malcolm Gladwell, host of Smart Talks with IBM. Imagine your business depends on a system you didn't build and don't fully control. For years, it works perfectly. It's fast, reliable, affordable. So over time, more and more of your operations rely on it. Then one day, the terms change. What once felt flexible now feels difficult to leave. Moving to another provider or bringing those capabilities back under your own control could take years, cost millions, and disrupt the business in the process. That's the challenge of digital sovereignty. Businesses today rely on all sorts of third-party systems.

• 39:52Speaker 05

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• 40:36Speaker 02

  Hi, it's Kathleen Griffith, co-host of the Unshakeables podcast. Listen in as Ron and Kelly Moore share their story. I'm Ron Moore. And I'm Kelly Moore. And starting our own business in our 50s was a little scary, but exciting. The biggest advantage was our community. The whole thing about business is relationships. We'll say team. A lot, not just our internal team. It's the team of our vendors, the team of our clients.

• 41:06Speaker 04

  We help the Veterans Resource Center and we give scholarships. I'm doing this for my heart, for my children, for you, for your children's family. I don't want anything back. And I tell a lot of people, don't give back to me, pay it forward.

• 41:20Speaker 02

  If you work to give, you just see the world differently.

• 41:24Speaker 04

  I'm gonna help as many people as we can. I can't take it with me.

• 41:28Speaker 02

  To hear more from Ron and Kelly, listen to The Unshakeables wherever you get your podcasts.

• 41:46Daniel Whiteson?

  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?

• 41:59Kelly Weinersmith?

  Uh, 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. Yeah, frankly, everybody should do. Why, why would you even ask why people would care about parasites, Daniel? I'm flabbergasted.

• 42:22Daniel Whiteson?

  I knew it just Copenhagen just kind of came out of the blue there. But I am looking forward to hearing you pronounce some Danish names.

• 42:29Kelly Weinersmith?

  I'm not, I'm not. All right, so it's 1780. The Royal 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.

• 42:55Kelly Weinersmith?

  Spontaneous generation.

• 42:56Daniel Whiteson?

  No, what? Still? Come on, people.

• 42:59Kelly Weinersmith?

  I know, after all of this. So this idea is still holding fast, even though we've got eggs now and we're like, you know, there's been a lot of 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 Abldgaard, pretty good, probably.

• 43:20Daniel Whiteson?

  Pretty good. Yeah, it's all right.

• 43:22Kelly Weinersmith?

  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.

• 43:54Daniel Whiteson?

  Right. So when you're in the sheep or in the snail or in the water, you look totally different.

• 43:59Kelly Weinersmith?

  Totally.

• 43:59Daniel Whiteson?

  Okay.

• 44:00Kelly Weinersmith?

  But if you're looking super closely.

• 44:02Daniel Whiteson?

  Yeah.

• 44:03Kelly Weinersmith?

  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.

• 44:20Daniel Whiteson?

  All right.

• 44:21Kelly Weinersmith?

  And Abldgaard should have gone with Peter. Really should have gone with Peter.

• 44:27Daniel Whiteson?

  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.

• 44:42Kelly Weinersmith?

  Can you say the name?

• 44:43Daniel Whiteson?

  It's probably something like, I mean, I have a terrible accent in Danish, but it's probably Peter Christian Abldgaard.

• 44:50Kelly Weinersmith?

  Oh, nice. Okay, way better. All right, that's good. I'm gonna have you say a name that comes later, and I'm not even gonna 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 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.

• 45:16Daniel Whiteson?

  Oh.

• 45:17Kelly Weinersmith?

  And he finds the tapeworm that he was expecting in those birds.

• 45:22Daniel Whiteson?

  Okay.

• 45:22Kelly Weinersmith?

  And he's like, this is probably the first life cycle experiment that we know about. This was the first instance where someone was purposely 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?

• 45:39Daniel Whiteson?

  And did he just get lucky? Like, how did he know that the thing living in organism one then needed to be in organism two? Was he just lucky?

• 45:46Kelly Weinersmith?

  Probably partly, yes. But I'm guessing it was a couple of 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.

• 46:07Daniel Whiteson?

  Got it.

• 46:08Kelly Weinersmith?

  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,

• 46:20Daniel Whiteson?

  Right.

• 46:20Kelly Weinersmith?

  thought before, as far as I know.

• 46:21Daniel Whiteson?

  Well, what a tremendous advance, what a moment of discovery. Surely he was feted internationally and won all sorts of prizes, right?

• 46:28Kelly Weinersmith?

  Totally forgotten.

• 46:30Daniel Whiteson?

  Oh.

• 46:34Kelly Weinersmith?

  Yes, and as far as I can tell, darkness for maybe 20 or 30 years on the topic of life cycles of parasites, until Ludwig Bojanis in Germany. Yep, now apologizing to all the German. Kelly's doing the, like, international tour of apologies today. So, anyway, a German scientist

• 46:57Daniel Whiteson?

  Yeah.

• 46:57Kelly Weinersmith?

  was dissecting a freshwater snail. So we were just talking about someone who was working on a tapeworm life cycle.

• 47:03Daniel Whiteson?

  Yeah.

• 47:03Kelly Weinersmith?

  Did amazing work. We're going back to parasite one.

• 47:06Daniel Whiteson?

  Flukes.

• 47:07Kelly Weinersmith?

  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.

• 47:16Daniel Whiteson?

  Yep.

• 47:16Kelly Weinersmith?

  So he was dissecting a freshwater snail and he'd noticed that free swimming stage crawling out of the sack.

• 47:24Daniel Whiteson?

  Oh.

• 47:25Kelly Weinersmith?

  And he was like, holy cow, that sack looks like it's producing that free swimming stage, but they look nothing alike.

• 47:34Daniel Whiteson?

  Right.

• 47:34Kelly Weinersmith?

  Not even a little bit alike.

• 47:36Daniel Whiteson?

  Right.

• 47:36Kelly Weinersmith?

  And he, so he makes some more careful observations and he publishes it and the, the guy who runs the journal he published it in said, quote, "Observations of this kind make one dizzy."

• 47:48Daniel Whiteson?

  Moments of understanding.

• 47:50Kelly Weinersmith?

  I mean, maybe you need to be a parasitologist to like have your mind blown by that. But no, also, I like, I mean, when 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, that was really exciting. And so now we're starting to pick up the pace, kind of.

• 48:14Daniel Whiteson?

  All right.

• 48:14Kelly Weinersmith?

  Okay. So maybe, 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 trame toad 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 trame toad 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, whoa, hey, something's happening there. Those eggs aren't just like.

• 48:48Daniel Whiteson?

  All right.

• 48:48Kelly Weinersmith?

  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 Siebold, 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.

• 49:09Daniel Whiteson?

  Okay.

• 49:10Kelly Weinersmith?

  And he's like, oh, oh, that 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 swimmy things. And now you're starting to get the pieces starting to come together. And in 1842, we get another amazing contribution by a Dane. Daniel, take it away.

• 49:32Daniel Whiteson?

  This looks like Johan Jepers Smith. Smith is not a very Danish name. And then Strain Strup.

• 49:39Kelly Weinersmith?

  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, trimatodes being one of them, jellyfish being another, that have life stages that look very different in different stages. 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 gonna look like their parents and the cycle's gonna 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.

• 50:23Kelly Weinersmith?

  And then they start like sort of having a bunch of extra like umbrella tops and those sort of pop off and become jellyfish. And the jellyfish produce eggs and it starts over again. But those umbrella tops don't really look anything like jellyfish. 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. Babies don't look like their parents. They look totally different. And now this really gets the ball rolling.

• 50:54Daniel Whiteson?

  I wonder if that makes it hard for the parents to love their kids. As 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?

• 51:13Kelly Weinersmith?

  So, 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 up.

• 51:26Daniel Whiteson?

  That's one kind of parenting.

• 51:28Kelly Weinersmith?

  That's right. That's right. Foreign strategy, but.

• 51:31Daniel Whiteson?

  All right. So you're not advocating for the parasite strategy parenting?

• 51:34Kelly Weinersmith?

  Absolutely not. No, no, I over invest 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 tracheotodes right. We don't need to go into the details of what he got wrong, but he really got the ball rolling.

• 51:56Daniel Whiteson?

  Yeah.

• 51:57Kelly Weinersmith?

  One of the balls he got rolling was a man named Friedrich Kuchenmeister in Germany. Now, Daniel, do you also speak German?

• 52:05Daniel Whiteson?

  I only speak a tiny bit of German, but enough to know what Kuchenmeister means.

• 52:09Kelly Weinersmith?

  What does it mean?

• 52:10Daniel Whiteson?

  It's basically like master baker. I think it's like cake master, basically.

• 52:14Kelly Weinersmith?

  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 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 cysts in rabbits, feed them to foxes, because in nature, foxes eat rabbits.

• 52:48Daniel Whiteson?

  Makes sense.

• 52:48Kelly Weinersmith?

  And so he feeds cysts from rabbits to foxes. Bam! He gets the tapeworm.

• 52:54Daniel Whiteson?

  You're so excited about giving tapeworms to these foxes.

• 52:57Kelly Weinersmith?

  I mean, I'm not, but like, he did it.

• 52:59Daniel Whiteson?

  Won't anybody think about the poor foxes?

• 53:01Kelly Weinersmith?

  Well, you're ruining this for me, Daniel. He's figuring out the life cycle. This is amazing.

• 53:07Daniel Whiteson?

  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.

• 53:15Kelly Weinersmith?

  Touche, touche. 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.

• 53:28Daniel Whiteson?

  Yes.

• 53:28Kelly Weinersmith?

  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.

• 53:40Daniel Whiteson?

  And empathy.

• 53:41Kelly Weinersmith?

  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.

• 54:08Daniel Whiteson?

  Heads plus tails equals one coin, actually.

• 54:11Kelly Weinersmith?

  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.

• 54:31Daniel Whiteson?

  Oh.

• 54:31Kelly Weinersmith?

  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-

• 54:42Daniel Whiteson?

  Was it a cake?

• 54:43Kelly Weinersmith?

  No, it was soup or sausage, because it's Germany. And so, and so my apologies to Germans. And so, whose sausage is delicious when it's not infected by parasites. 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.

• 55:21Daniel Whiteson?

  Thanks to Bratwurstmeister.

• 55:23Kelly Weinersmith?

  Thanks to Mr. Bratwurst. And it took a little while longer for us to figure out all of the details of the Trima Toad life cycle. As you could tell from the description at the beginning, there's a lot more steps involved.

• 55:36Daniel Whiteson?

  Yeah.

• 55:37Kelly Weinersmith?

  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 cysts 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 cycles since. And it's not always easy, but we do it.

• 56:08Daniel Whiteson?

  And the key insight is that you can't assume that the babies look just like the parents, right?

• 56:13Kelly Weinersmith?

  Yes.

• 56:13Daniel Whiteson?

  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.

• 56:21Kelly Weinersmith?

  It's really cool, but what like, once you've unlocked some of the pieces, 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.

• 56:44Daniel Whiteson?

  Right.

• 56:44Kelly Weinersmith?

  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.

• 56:54Daniel Whiteson?

  Yeah.

• 56:54Kelly Weinersmith?

  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.

• 57:04Daniel Whiteson?

  Ooh, mosquitoes.

• 57:06Kelly Weinersmith?

  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.

• 57:24Daniel Whiteson?

  It feels like spontaneous generation is like the explanation of the gaps. It's like.

• 57:27Kelly Weinersmith?

  Yeah.

• 57:28Daniel Whiteson?

  Anything where we don't understand it's 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 encouraged these thoughts? Or was it always just the things we can't explain?

• 57:43Kelly Weinersmith?

  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.

• 58:10Daniel Whiteson?

  Yeah.

• 58:11Kelly Weinersmith?

  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 fruit 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?

• 58:38Daniel Whiteson?

  But it's not just anecdotal evidence. They actually were experiments by like John Needham, who tried to isolate this stuff and showed 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.

• 59:02Kelly Weinersmith?

  Yeah.

• 59:02Daniel Whiteson?

  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.

• 59:20Kelly Weinersmith?

  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.

• 59:33Daniel Whiteson?

  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. So.

• 59:52Kelly Weinersmith?

  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.

• 1:00:14Daniel Whiteson?

  And I think this is evidence that interest in parasites is not spontaneous. It's not just appearing randomly in the population. You're seeding it, Kelly, by talking about the amazing parasite science.

• 1:00:23Kelly Weinersmith?

  Oh, yeah. So I laid like an egg in Boris's brain and let's stop. Okay.

• 1:00:32Speaker 03

  I really enjoyed this episode, and of course, you covered 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 and 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.

• 1:01:03Kelly Weinersmith?

  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 8, 2026, episode of DKEU, I told you all about my PhD research studying a trematode parasite called Yuaplorcus californiensis, or Yuha. 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 brain. Then they form

• 1:01:44Kelly Weinersmith?

  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 that hopefully is from the perspective of the parasite, of course, not the snail. The snail gets castrated by the

• 1:02:28Kelly Weinersmith?

  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 here, and I need to find the next host in

• 1:03:11Kelly Weinersmith?

  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 killifish. 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

• 1:03:54Kelly Weinersmith?

  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

• 1:04:36Kelly Weinersmith?

  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 trematode 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 Trima Toad life cycles had already been figured out.

• 1:05:21Kelly Weinersmith?

  Okay, until next time. Thanks.

• 1:05:30Daniel Whiteson?

  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.

• 1:05:38Kelly Weinersmith?

  Daniel and Kelly's extraordinary universe is edited by the amazing Matt Kesselman.

• 1:05:42Daniel Whiteson?

  He really is a wizard. You can also find us online on Blue Sky, Instagram and X, D and K Universe. Come engage with us.

• 1:05:52Kelly Weinersmith?

  You can email us at questions at danielandkelly.org. We really do want to hear from you.

• 1:05:58Daniel Whiteson?

  And you can find our website, www.danielandkelly.org, where you'll also find an invitation to join our Discord, where everybody comes and talks about the amazing universe.

• 1:06:09Kelly Weinersmith?

  And we also have the most amazing moderators. This is an iHeart Podcast. Thanks for joining us.

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