Short Wave - Love Fruit? Thank (Dinosaur) Mass Extinction

Episode Date: March 14, 2025

Move over, TikTokers. It's time to shine a spotlight on some of the earliest influencers around: dinosaurs. When these ecosystem engineers were in their heyday, forest canopies were open and seeds were small. But around the time most dinosaurs were wiped out, paleontologists noticed an interesting shift in the fossil record: Seeds got bigger — much bigger. There was a fruit boom. Did the death of these dinosaurs have something to do with it? And who are the modern day equivalent of dinosaur influencers? To find out, host Emily Kwong talks to Chris Doughty, an ecologist at Northern Arizona University.Tell us what other tales of dino past you want us to regale you with by emailing us at [e-mail]! Listen 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

You're listening to Shortwave from NPR. Today, we are going back in time all the way to the Cretaceous period. Here's what it looked like. It would have been a very bright forest. They would have been very open. You have a lot of pine trees. A time that Christopher Doty loves because of this one kind of dinosaur. The seropods are the biggest terrestrial animal that's ever walked the earth.

Chris uses big data sets to understand ecosystems. He's an associate professor of eco-informatics at Northern Arizona University. And he told me that saropods were so big. They acted like ecosystem engineers. They can knock down trees. They can distribute nutrients. They can move seeds. They do a lot of really important things.

And big animals tend to do that differently than small animals. And the way saropods move nutrients and seeds around was through their poop. Yeah, and their bodies. The decaying corpse of a sarapod. That's true. That's not quite as fun to think about. But yeah, it's like the circle of life. Maybe there was like a Bufasa sarapod who is like, my son, you'll be the grass someday.

That's right. Yeah, those same elements have cycled through your son that were in a sarapod someday probably. Those saropods were herbivores eating plants. And the plant seeds from this time were actually kind of small, like a centimeter across. And that was fine for them because of all this light in the understory. And so there is no evolutionary reason for seeds to get big. But what happened was after the dinosaurs went extinct, the forest got really dark. When the asteroid hit, it wiped out as much as 75% of all life on Earth.

And suddenly there were no longer these big dinosaurs to knock down the trees. So the trees grew lush and tall, and the understory between the forest floor and the tree canopy, got roughly 20% darker. For a little seed, that's trouble. You know, these little seeds, they need light. And paleontologists observed something weird in the fossil record around this time. Not long after the asteroid hit, seeds suddenly got bigger. Much bigger.

And there was this sudden boom in fruit, the edible plant ovaries we love today. But why? And what did the death of the dinosaurs have to do with it? Today on the show, how the mass extinction of the dinosaurs may have led to the fruit and vegetable aisle you see today. and how we're living in another time of mass extinction, where the dinosaurs are us. I'm Emily Kwong, and you're listening to Shorewave, the science podcast from NPR.

So, Chris, let's talk about mass extinctions. The world has gone through five of them. How do you know something as a mass extinction? That's right. So species go extinct all the time, and it's not unusual. It's just part of nature. But certain things happen that drastically accelerate this process,

that make it so, you know, 80, 90% of species go extinct. So what distinguishes a mass extinction from just a normal extinction is many more species are going extinct and it's global. All right. So you have this theory that the mass extinction got rid of these dinosaurs that were changing the forest and knocking down trees. And that meant that the forest understory was getting darker. How did you go about testing this idea that that dark understory then led to bigger? seeds and fruits. Yeah. So it's not like you can go back and check. You can't set up a wildlife cam. No, no, no. Yeah. It is difficult to try to piece these together. So the first starting point was we had colleagues that literally scraped off fossils from leaves right before the extinctions

and pretty much right after the extinctions. And we could test the chemistry of those leaves. And we could actually say how much light these leaves got. And so for one of the first times, we were able to accurately quantify, you know, how much less light. was in this understory after the dinosaurs went extinct. And then what we were able to do was go into rainforest and study how plants grow. And so, you know, from that perspective, we could see how tall would you get at a given seed size? And it's a very strong correlation. So if you're small, you develop into a tiny little seedling.

And if you're a big seed, you develop into a big seedling. And then we can go even further and look at modern megafauna like forest elephants and see, you know, how do they open up the understory? And so then what we do is we take, you know, all this information going from fossil records to how plants grow, to how seeds grow, to how force elephants affect forests, and we put it into a model. You can think about like a computer simulation. It's like roller coaster tycoon. I don't know that one. But, yeah, so like, you know, like, you know, SimCity or something. Sim dinosaurs.

Yeah, that's the same genre of you're taking all this real world data and you're putting it into a model and you're saying like, what if. Exactly. Yeah. And then you just allow, you know, millions of years. of ecology to happen. And so basically you just kind of input this into our model and then you see what happens. So yeah, what's the verdict? The verdict is. Yeah, it worked. So, you know, basically what we saw was the darker understory basically created an evolutionary advantage for bigger seeds. And so those bigger seeds then created fruit that would entice bigger animals to eat them and these bigger animals would

distribute the fruits farther. And the trees that are moved farther away from their mother tree are very happy because you don't have to put up with the same diseases that your mother has. You don't have to be under her shade. And so that's an evolutionary incentive as well. So basically they're bribing these animals with fruit to move them around. In the time immediately following the mass extinction of the dinosaurs, did that eventually give rise to the ancestors of primates? Yes.

Our primate ancestors developed because they were good at climbing trees and eating fruit. And so had there not been fruit, you wouldn't have this incentive to climb up in the tree and eat fruit. So the death of the dinosaurs was our boon? It was. It was, yes. Well, I feel bad for them. Okay, did the model ever show this trend happening in the other direction? Yes.

At one point, all the seed sizes were getting bigger. And then about 35 million years ago, these seed sizes started getting smaller. And it was a big mystery as to, you know, what's driving this? You know, and what our model explained was that it was actually the evolution of these really large mammals now. Like who? Who were the new influencers? You know, you had all sorts of like, you know, weird-looking mammal groups that, you know, grew enormous that are kind of the, some of the relatives of what we see now out on the African savannah. So they'd get really big.

And then they'd start opening up these understories as well. And so long, those big seeds were no longer way better than the small seeds. And so the seed sites started to decrease. But then, you know, boom, seed size goes up again several million years after that. Woolly mammoths, saber-tooth tigers, giant sloths. What happened to those mammals and what happened to the plants? That's right. So that's another really interesting extinction event because it overlaps with us.

So this is commonly referred to as the sixth extinction. There is a lot of tree species that co-evolved with these big animals. to have them move their seeds around. So think of the avocado, think of chocolate, two of our favorite plant species. You know, those co-evolved with these, you know, now extinct South American elephants. So that's why you kind of see this increase in seed size over time more recently. Yeah. I mean, animals are clearly such a huge factor in shaping ecosystems as engineers, as influencers in a way.

And we are currently in another mass extinction, the Holocene. who are the influencers of today? That's right. Yeah. So, you know, I'm really interested in trying to predict the future with some of these models, not just thinking about the past. And so, you know, one of the fun things we did in this paper was say, all right, we went through another mass extinction recently of these really large, important animals. What do we predict for future seed size? And without these large mammals like, you know, the gomp the fears, mammoths, giant ground sloths, you know, these forest understories are a lot darker. And so our model would say, okay, well, it's dark. This. the seeds want to grow bigger again. But the interesting thing was we couldn't actually say that because, you know, there's another enormous ecosystem engineer that, you know, is dominant on the planet now, which is us. Ah, we're quite a bit of a variable to add to the model.

Exactly. Yeah. So we've taken on the ecological roles of some of these now extinct animals. Oh, how so? Through logging and, you know, creating farms, creating grasslands. Like, you know, it's not uncommon for, you know, larger animals to create grazing lawns. we just call it agriculture. You know, they're excellent at distributing nutrients across the landscape.

We're not so good at moving nutrients, but we dig up a lot of, you know, elements and, you know, spread it across the planet. And so a lot of these big animals used to consume a lot of this vegetation that, you know, when they went extinct, you got a big increase in fire. Right. So, yeah, there's a lot of interesting ecological changes that occurred once these animals went extinct. Yeah. And I actually have an interesting little tidbit.

for, you know, we are King Kong. And I don't mean that as a metaphor. I mean that we're metabolically King Kong. Wait, what do you mean by that? Humans, you know, use other energy. We heat our houses. You know, we have fires. And so if you figure out how much energy we use, like, by burning oil on top of, like, everything we eat, you could predict what size animal would be. And so we would be eight billion King Kong's, we're metabolically eight billion King Kong running around the planet. And so, you know, we in the Western world consume a lot more energy than typical global citizen. We might be the ultrasaurus. You know, I haven't done that actual calculation.

We might even be bigger than King Kong. So we're really changing this long-term evolutionary trajectory of the planet in interesting ways. But, of course, you know, who knows where we're going to be in, you know, a thousand years versus a million years? That's something that our models cannot predict, unfortunately. And considering the way the dinosaurs went, I just, I don't feel great of it. about this. So with all this in mind, how do you hope people think about the kind of like wax and wane of the understory in relationship to mass extinctions? Because we're in one right now. Yeah. I mean, I'd like people to reflect on our ecosystems co-evolved with these big animals.

They're used to having these big animals both as nutrient distributors, you know, affecting forest structure. They play really critical roles and now they're missing. And so that's kind of our motivating factor for creating these models is to, you know, say something about our future world on big spatial scales over long periods of time. Thank you so much for coming on the show. Oh, thank you so much for having me. This episode was produced by Burley McCoy and edited by our showrunner Rebecca Ramirez. It was fact-checked by Tyler Jones.

The audio engineer was Robert Rodriguez. Beth Donovan is our senior director and Colin Campbell is our senior vice president of podcasting strategy. I'm Emily Kwong. Thank you for listening. to Shortwave, Science Podcast from NPR.