Short Wave - Parasites Have Haunted Us For Millions Of Years

Episode Date: October 24, 2025

Parasites have roamed the Earth for a long time. They were here before the dinosaurs: The oldest fossils are more than 500 million years old. Today on the show, Regina G. Barber speaks with paleontologists Karma Nanglu and Danielle de Carle about a mysterious fossil called the “Riddler,” and the oldest-ever leech fossil. They share stories of the enduring power of parasitism through the ages and why the clues to prehistoric mysteries may be tucked in a basement.Interested in more science behind rare fossils? Email us your question at [e-mail] to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

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While you're listening, take a second, leave us a review, and share us with your friends. Okay, on to our show. You're listening to Shortwave from NPR. Parasites scare me. I think of corticeps, the parasitic fungus that inspired the video game turned TV series, The Last of Us, the one that in real life bursts out of the head of ants and controls them when they're dead. or I think of people finding a tapeworm in their bodies after eating raw meat. Those have haunted me since childhood.

And even more unsettling, I was reminded recently by a paleontologist that parasites have been around a lot longer than I had thought. We have fossils as old as about 515, 516 million years old that have evidence of parasites on them. And these are kind of small, worm-like animals, building tubes on top of these shelled creatures called brachio pods in order to basically steal the food that they're drawing into their mouths. Carmen Nanglu's that paleontologist.

He's at the University of California Riverside, and he says trying to trace how far back Parasize popped up is hard work. They're rare to find in fossils. Among other things, they tend to be made of only soft tissue that doesn't preserve well in rock. Sometimes researchers get clues that only give a small portion of the whole mystery. And like the world's greatest detective Batman, And karma had to deal with a mysterious character.

But in this case, it was a fossil that nobody could figure out. They had this weird marking on them. It looked pretty much like a perfect sort of stylized question mark. We had no explanation at the time for what this thing could possibly be. They did have a name for it, though, the riddler, like the bad man villain. So it just sort of sat there in the back of our minds, kind of like burrowing away at them for, you know, over the course of maybe like two, three years since we first found them. And we kept on shelving it because we weren't sure what it was,

but we knew we'd come back to it because it was super interesting. Karma's detective skills even helped a mystery outside of his specialty. A few years ago, while at the University of Toronto, he stumbled upon some fossils that he suspected could be a rare first. At first, his colleagues didn't think he was on to something. Then after looking deeper into specimens from the same dig site, they realized... We found the first leech fossil that has ever been recorded. That's Danielle DeCarl, a paleontologist at the University of Toronto.

Together with her and karma on today's show, we get into an ancient haunt, parasites. We talk about what the riddler tells us about the power of parasites through the ages, and why breakthroughs in our fossil record may be tucked away in a basement. I'm Regina Barber, and you're listening to Shortwave, the science podcast from NPR. All right, for the lowdown on these creepies, these parasites, this Halloween season, our dynamic duo is Carmen Nanglu and Danielle DeCarl. Danielle, let's start with the basics. What are the qualifications for being a parasite? Oh, yeah. So parasitism is a type of symbiosis. And symbiosis is basically just a situation where you have individuals from two or more species that live together in very close association for a long period of time. And they're adapted for this purpose. And parasitism is like a specific form of symbiosis where you have one organism that lives.

lives inside or on another organism that's called the host. And the parasite gains nutrients at the expense of the host, and it has some kind of adaptation for doing so. So it's either eating little bits of the host or it's kind of stealing nutrients from the host. Yeah. Okay. So I've heard of a few kinds of parasite fossils, like the ones that live on other organisms and steal their food. Like, what kinds of parasites can be found in these fossil records? Yeah. There's tons of different fossil parasite worms. There are actually fossils of insects in amber. I think specifically a group of insects called plant hoppers. And we can actually see nematodes emerging from them in this amber fossil. So we have not just the host. We also have the parasite. And in fact, the parasite is actually

leaving the body. It looks basically like if you took a photo of that happening in a modern environment. So, Karma, I want to talk about the riddler, this mystery you solved. Like, take me back. You're looking at all these images of ancient mollusks, basically. with question marks on them, and you're like, I have no idea what this is. Yeah, we were stewing on this for about two, three years. So once in a while, I would basically go back to the paper, or go back to those photos, rather, and do a bit of a search. And I feel like I was just basically, like, kind of like an investigation, you're knocking

off lists of animals. List of suspects, right? Exactly, suspects. And that's not just other animals, but it's like, could it be a feature of the animal's own gut? Could it be something about their gills? So we kept on looking at photos of these kinds of structures. Couldn't be any of them. and then eventually I wound up with these parasitic modern worms that build tubes that are very, very similar.

I mean, which is so cool, and it makes it a kind of aquatic worm called a spionid. So in the study you said that this also shows the behavior of this really old spionid or parasitic worm and actually show behavior in fossils is really, really rare. What do you mean by that? Like, how are you seeing the behavior of these parasitic worms on ancient mollusks? Yeah, no, totally. Well, you know, the first thing to remember is for an animal to enter the fossil record is super rare. Most things are never going to enter the fossil record at all.

And then to have the association is doubly rare. And then what's cool about this, the shape of this trace specifically, is it's highly characteristic. And when we look at their modern-day relatives, the spionids who produce these kinds of traces, we actually know quite a lot about their behavior and how they produce the trace, because spionids on modern day oysters and muscles and other kinds of commercially important bivalves, you know, for eating, are have been really well characterized. And so we kind of know what must have happened. The larva must have landed on one of these shells.

It bore in, sort of dissolved away a little bit of the shell, built itself a little home, and then gradually elaborated this long tube out of which the adult worm would have stuck its face and its tentacles to feed on water. Now that you figured out that the gridler, it's an ancient parasite, what else can that tell us? Like, now that we know more about this ancestor, how can that help us understanding that parasite now?

Yeah, you know, 480 million years ago, this group of worms was living inside of basically small clam shells, completely content as can be. Over the course of basically every major mass extinction, this group has continued to be successful to the point that we still are studying this modern group of organisms. So it tells you something about parasitic lifestyles, how resilient they can be. Because they basically still exist in our ocean today. Totally, yeah. Everyone likes to talk about dinosaurs, but, you know, these worms are, you know, a little bit innocuous, but they're still doing their thing. Yeah, and Danielle, you helped karma on some other research. We're talking about, again, this mystery you found this really rare fossil, this clue to like leach evolution. And normally when we think about leeches, we think they are parasites, right?

But your study is kind of unique, right? Yeah. So like you said, when we think about leeches, most of the time we think about ones that feed on our blood, right, or the blood of other vertebrates. But today, there are lots of wiches that don't do that. So some of them, they'll swallow basically anything that can fit in their mouths or they'll bite off chunks of, like, dead bodies and things like this. And then we have other leeches in the kind of modern biota, I suppose. So they'll suck on the bodily fluids from things like crabs and shrimp. They'll kind of attach and stick their mouth parts in the little membranous parts between the hard plates.

Oh my gosh. And they'll suck out the bodily fluids in that way. So the fossil that we found, it wasn't found alongside any real large vertebrates. So we think that instead of parasitizing vertebrates, which is sort of the prevailing hypothesis for what the oldest leeches did, we think that our fossil instead would have either preyed on other animals by kind of swallowing them whole, or it might have been a parasite of invertebrates, of larger animals like trilobites. Yeah, and what's fun is like this fossil wasn't hidden away or newly on Earth. You both found out it existed from a published paper about a fossil site in Wisconsin.

Yeah, they also included photos of some fossils that were beautifully preserved, but they weren't entirely sure yet exactly what they might be. Okay. Another mystery. Karma saw one of them. Yes, another mystery. So Karma saw one and he thought it might be a leech. So he brought it to myself and our other co-author, Raphael Oama,

who's also a leech scientist. And he was very excited about how this might be a leech. And instantly, Rafa and I both were like, no, I don't think so. Karma, how did you feel when they rejected you like that? I felt crushed. Absolutely crushed, which is what happened. Because, you know, it looked like a dynamite thing. And leeches I know don't have a fossil record.

I'm not a leech scientist myself, but I knew what a huge discovery that would be. But, no, they were pretty upfront about that, which is what you want in a scientific colleague, so that's great. How did you all confirm? Like what was the process in being like, yes, this is the first leech fossil. After we rejected karma and we threw his first hunch out the window, we decided to get in touch with Andrew and Lauren. And they were kind enough to share some photos of a lot of animals that they or a lot of fossils from the Waukesha site that they hadn't yet identified, which included some other segmented worms. And one that immediately left out at us was this one that we ended up describing as macromyze. on as the first fossil leech. And this one did, in Karma's defense, have lots of similarities to that first fossil.

But it wasn't the one he showed you. It had a little bit as what you're telling me. Yeah, it had a little, a little bit of extra detail in there that sort of made us feel really confident. And the first thing in particular that we noticed was that big sucker that it has at the posterior end, which is something that's a huge hallmark of leeches today as well. So, karma, you were right. Like, do you feel vindicated? Like, in the end, it was a different fossil, but your suspicion of these things being leeches, you're right. I feel like vindicated sounds too competitive. I feel really scientifically satisfied that there was.

Is that too professional? You're so Canadian. Well, yeah, I know. Danielle's Canadian too, so, you know, she's giving me grief. But, you know, it's courage, right? Like, you had the courage to be wrong. And because of that, we stumbled upon this amazing discovery. If you hadn't had the courage to be wrong, we'd all.

still be leachless. We'd be leachless. That's true. Daniel, what does this tell us about modern-day leeches, that now we have a fossil record? Yeah, it tells us a couple things. First of all, it tells us that this group of animals, that leeches are like 200 million years older than we thought they were initially. It also tells us that despite the kind of prevailing sort of hypotheses at the time, it was

unlikely that the first leech fed on vertebrae blood. Instead, we think it was either a parasite of invertebrates or it was a predator. And it also kind of tells us a little bit about the habitat of leeches. So as Karma kind of mentioned earlier, most leeches today are either aquatic living in freshwater or there are terrestrial ones. But we also have some kind of marine leeches. And the prevailing wisdom before this new discovery was that those marine leeches represented sort of a single origin of of a recolonization of the oceans, right? Like the ancestor of this one lineage moved back into the oceans and gave rise to a whole bunch of all the marine leeches that we see today.

I've been covering paleontology stories for a couple years now. And one of the things that always sticks with me is just how many fossils are just sitting around. Daniel, I'm going to ask you first, like, how do you see the fossil record and, like, of many animals or anything kind of changing in the future with so much data not even looked at? yet. Yeah, I think there's huge potential for discovering new fossils in collections that already exist. I'm sure there are lots of projects just like The Ridler, you know, the generations of scientists never, they never found that one paper from the 70s that really kind of blew everything open, right? And they just kind of put those things in a drawer and they're waiting to be rediscovered by new generations of people. I think as well with new technologies, you know, again, with this,

the Ridler, one of the technologies that they use to sort of get a lot more information was CT scanning, things like this, right? So if we sort of subject the fossils that we do know about to further scrutiny, maybe we'll find even more organisms, like inside ones that we already knew, whether their bodies are physically there or whether there are traces of that activity. I think there's huge potential. I love that. I'm so glad I asked you that question.

Karma, what do you think? If you think about the history of life, basically, like a movie that's going through, basically the fossil record, we're not getting a complete picture. So imagine watching that movie, but you're squinting the entire time. But once in a while, you get to open your eyes and see the whole picture. And then you go back to squinting. And so this site in Wisconsin called Waukesha is one of these sites of exceptional preservation where when watching the movie, that's the whole tape of life,

you get to open your eyes for a very brief moment and get the totality of the picture, which includes all the soft tissues, things like leeches and stuff. Thank you, Danielle. Thank you, Karma, for coming to talk to me. this has been amazing. Oh, likewise, thank you so much for having us. This has been really fun. Yeah, anytime. It's always fun. This episode was produced by Rachel Carlson

and edited by showrunner Rebecca Ramirez. Tyler Jones checked the facts, and Jimmy Keeley was the audio engineer. Beth Donovan is our vice president of podcasting. I'm Regina Barber. Thank you for listening to Shortwave from NPR.