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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 sauropods 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 ecoinformatics at Northern Arizona University. And he told me that sauropods 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 sauropods move nutrients and seeds around was through their poop.
Yeah, and their bodies. The decaying corpse of a sauropod. 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 Mufasa sauropod who is like, my son, you'll be the grass someday. That's right. Yeah. The same elements have cycled through your sun that were in a sauropod someday. Those sauropods 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 was 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. At Radiolab, we love nothing more than nerding out about science, neuroscience, chemistry. But we do also like to get into other kinds of stories. Stories about policing or politics, country music, hockey.
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So, Chris, let's talk about mass extinctions. The world has gone through five of them. How do you know something is 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. 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 how much less light was in this understory after the dinosaurs went extinct. And then what we were able to do was go into rainforests and study how plants grow. And so 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, 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 forest elephants affect forests, and we put it into a model. You can think about like a computer simulation. It's like Rollercoaster Tycoon.
I don't know that one, but yes, like, you know, like, you know, SimCity or something. SimDinosaurs. Yeah, that's the same genre. 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. 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 the...
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 savanna. So they'd get really. big, and then they'd start opening up these understories as well. And so 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-toothed 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 was 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 I'm really interested in...
trying to predict the future with some of these models, not just thinking about the past. And so 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. The seeds want to grow bigger again. But the interesting thing was we couldn't actually say that because there's another enormous ecosystem engineer that is dominant on the planet now, which is us. We're quite 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, it's not uncommon for larger animals to create grazing lawns. We just call it agriculture. They're excellent at distributing nutrients across the landscape. We're not so good at moving nutrients, but we dig up a lot. 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. So yeah, there's a lot of...
Interesting ecological changes that occurred when these, these animals went extinct. Yeah. And I actually have a, I have an interesting little, little tidbit for, you know, we're, 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 um 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 8 billion King Kongs. We're metabolically 8 billion King Kongs 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. 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, who knows where we're going to be in a thousand years versus a million years. That's something that our models cannot predict, unfortunately. And considering the way the dinosaurs went, I don't feel great 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 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 like 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, a science podcast from NPR. These days, there's so much news, it can be hard to keep up with what it all means for you, your family, and your community.
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