Unexplainable - The trees of death

Episode Date: December 3, 2025

Way back when forests first evolved on Earth... they might have triggered one of the biggest mass extinctions in the history of the planet. (Originally aired in 2024) Guests: Charles Ver Straeten, curator of sedimentary rocks at the New York State Museum; Lisa Amati, curator of invertebrate paleontology at the New York State Museum; Thomas Algeo, professor of geochemistry at the University of Cincinnati For show transcripts, go to ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unxtranscripts⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠ For more, go to ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unexplainable⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠ And please email us! ⁠⁠⁠[e-mail]⁠⁠⁠ We read every email. Support Unexplainable (and get ad-free episodes) by becoming a Vox Member today: ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/members⁠⁠⁠⁠⁠⁠⁠⁠ Thank you! Learn more about your ad choices. Visit podcastchoices.com/adchoices

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I saw my friend on the other side of the street. I was heading to school with the kids. I let go of Mom's hand to wave. I had already forgotten their lunches. I ran over to hug her. She came out of nowhere. And then... It stopped.

Sometimes the moments that never happen matter most. Volvo's automatic emergency braking helps ensure a safe ending for everyone. Learn more at VolvoCars.a. at the far end of town, where the backroads wind past the little diners and campgrounds closed for the season. Into the woods we go. I feel like it's just starting to be spring. I can feel it. I caught.

Just when you think you've missed it, turn left into an unmarked driveway. Pass the fences and the no trespassing signs. All right. Ah, another chain link fence. and a much more secure padlock. And you'll finally arrive at a huge, empty gravel lot. It's definitely nothing living inside that area. An old quarry long out of use.

A lot of gravel. Some birds, too. But look carefully at the dusty ground. You might just see the rocks that transported Chuck into the past. Just thinking about time, like geologists think about time in millions of years, hundreds of millions of years, by looking in the sedimentary rocks. All this stuff is big scale. It's, you know, only astronomers think in a much bigger scale. I love this.

They're not the oldest rocks in the world at all, but they are mighty old. Mighty old. We're walking across this quarry floor, and my eye just unconsciously picked up on an odd pattern in the quarry floor. It looked like a shallow little gutter that kind of meandered back and forth a little bit while it went out mostly in one direction. And it's like, what is that? And we started looking around and followed them,

and three of these all met at one place. along with eight more of these gutter things that had gotten wider as we walked towards where they met. And it didn't take too long before we realized, all of us individually realized, wow, that is where a tree stood 385 million years ago. Holy shit. That is unbelievable. So this was one tree. Are we saying that this is literally two tree, like the star formations are like the basis of these two trees? Yeah.

The impressions of the roots, well preserved. Like the footprints almost. Yeah, it would be like footprints, but the impressions of the roots where they were in the ground. Do you know how big that tree was? Do you know what you mean? Yeah. No, I mean, literally, I think it looks like the trees that we can see on the other side of the fence. Tree-sized.

Tree-sized. We could stand where a tree stood 385 million years ago and see by the impressions of the roots where all the other trees in this forest stood. It looks so magical. It's like, yeah, it feels hard to wrap your head around, especially when it's, you know, I don't know, when you're looking at what it is now. I mean, so many individual lives have lived on this planet. Everything died, left behind something, but most of everything decays, is broken down, dissolved. It's all about Earth's life and a lot of time.

Such an awesome, awesome, awesome thing to imagine a forest from that long ago. Way back. Way, way back, when these trees stood tall, forests like this one exploded across the planet, radically transforming the earth beneath their roots, the air rippling through their leaves. You can see the aftermath of this explosion in the world all around. us, but nowhere more clearly than these ghostly footprints etched into the dusty ground. I'm Meredith Hoffnot, and this is unexplainable. So this is the paleontology collection at the New York State Museum.

We are walking through a room that looks kind of like the warehouse at the end of Raiders of the Lost Ark. The fossil collection has lots of stuff, sitting in the aisles and on tables, carts are full. Lisa Amadi cares for hundreds of thousands of fossils here in her job as the state paleontologist. Lisa and geologists like Chuck Verstraten piece together fragments of an ancient past that they can't ever study directly. Whenever I'm working with the fossils, I'm picturing where they lived. I'm picturing how they lived. And it's like not just thinking about the rock and its character, not just thinking about the fossils and what. what they tell me. It's a mystery novel.

It's an adventure novel. It's a puzzle. It's everything all in one. We don't have the answers, which is what makes it fun. Our story starts in a time before trees, about 420 million years ago at the beginning of the Devonian period. Earth's continents were bunched up in the southern hemisphere, and the land they carried would have been unrecognizable.

There really wasn't much of anything on land at all. In fact, nothing lived on land except for some tiny plants and the ancestors of things like insects. There were very few signs of life. All of them weird. This thing's 12 feet tall. So what could it possibly be? It can't be a plant. It can't be an animal.

Turns out it's a fungus. Whoa, this is a fungus. Oh, and we call it fungzil. This was a landscape of barren rock and gravel, buffeted by winds and rains, the loneliest alien planet. But underwater was a different world. It was so much life in the oceans. The Devonian is known as the age of fishes, because there were so many kinds of fish. All of the major groups of fish were alive during the day.

in the Devonian period. There were jaulous fish that looked like dune worms crossed with horseshoe crabs. There were placoderms, these fish plated in armor with huge Siberian tiger fangs. Sharks spread through the oceans at this time, as well as lobed finned fishes with bony little limbs. And the abundance of life in the ocean was just beginning to make its way onto the land. It started to evolve from algae to be able to survive in a wet environment but out of the water. At the beginning of the Devonian, your tallest plants were only about the reach between your thumb and the end of your thumb and the end of your index finger, you know, four or five, maybe six inches.

And once plants on land got a foothold, they started evolving faster and faster. By 35 million years later, if we're talking about the Cairo quarry, Everything had evolved through stages of those simple little plants to things that began to become more complex, began to grow into shrubbery-sized plants, and then by the middle of the Devonian, you had trees. You had trees. Big portions of New York were underwater, and the Appalachian Mountains were just forming, and they were huge, really tall mountains. Andy's scale mountains, perhaps, out there in New England. England. And in the tropical

floodplains between the mountains and the sea, the Kero Quarry Forest flourished with the strangest trees. Broccoli-headed palm-like trees, draping evergreen ancestors, club moss gone giant. It's like

a furry telephone pole. And also smaller plants. Little things like liverworts. Liverworts are just cool. If you don't know what one is, you should look one up. Teensy little leaflet-like things. The land now had these rich habitats full of resources and shelter, like an empty house, ready to be a home.

And so the first lobefin fishes evolved little finny legs and crawled up from the water into the forests. These were our ancestors, the first vertebrates on land, the beginning of all the amphibians, all the birds, the dinosaurs, and us. And that's about where we are here in New York. We had everything, vertebrates, invertebrates, and plants on land at this time. So this is one of the first full forests on the planet Earth. That's unbelievable.

Totally. It's crazy. What do you think is so in, I don't want to say the word enchanting. I'm not sure if that's right, but just like enthralling about the K. Rosa. Why do you think it like sparked so many people's imaginations? It's the oldest perspective. It's the oldest sense we have of first forests, where you can actually look out across the forest floor

and see where all these trees stood. But this is a big perspective. It's really the Devonian period when life first really colonized the land to a large scale. It was that Devonian period. The tropical floodplains of upstate New York were a perfect place for these forests to take root. Monsoons rolled off the towering Appalachian Mountains,

drowning the marshes and swamp lands under epic floods. It was wet, but also it was warm, ideal for plants starting to figure out how to live, on land. And then, explosion. And it's actually literally called the Devonian plant explosion. Life had found a new home on the land, and there was no stopping it. Plants raced across this barren, rocky world, an explosion of biodiversity, transforming the world from gray and blue to green and blue into a planet we'd more or less recognize today. And in doing so, they might have caused one of the biggest mass extinctions

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By the end of the Devonian, once teeming seas, were now a graveyard. The carnage of one of the biggest mass extinctions in the history of the planet, an extinction as violence. as the one that killed the dinosaurs. Nearly three quarters of the species on earth died in this slowly unfolding catastrophe.

And the oceans suffered worst of all. Corpses littered the sea floor. The age of fishes was over. It takes a certain degree of imagination to see that things were different in the past. And the deeper back you go into Earth history the more bizarre they are and the more foreign they are to what we're familiar with. Geologist Thomas Algeo studies ancient apocalypses.

I've effectively delved into all of the mass extinction events in Earth history. Our planet has nurtured life for billions of years. It's been a remarkably stable place to live and thrive. And in that long history, there have only been five, times when the world was flung into such chaos that a majority of the species on Earth were wiped out before they had a chance to adapt. And for that to happen, there has to be some kind of trigger, some kind of external trigger that upsets the system. And Thomas thinks that this mass extinction in the oceans was triggered by the trees. The Devonian plant explosion changed the face of our planet.

But it also changed how the planet worked, how the air, the soil, and the water all interacted. These first forests did what forests still do today. They pulled carbon dioxide from the atmosphere and released oxygen. But this scale of photosynthesis was something totally new. Trees basically remade the atmosphere, dropping carbon dioxide levels by maybe as much as 90% and doubling the oxygen. And that is a big increase. That brings us up close to modern oxygen levels.

This giant swing is one of just a few times living beings have disrupted the atmosphere so dramatically. These first forests upended the climate and plunged the earth into an ice age that enveloped the world for tens of millions of years. But an Ice Age alone doesn't necessarily lead to mass extinction. There was a second punch coming for the age of fishes, a wave of death, spreading out from the land. The roots of these first forests dug deep into the earth, breaking up rock and releasing minerals. The trees then died, decomposed, and formed new layers of carbon-rich soil. As the forests grew, so did the soil, which made it easier for younger plants to take root and help those forests grow, which then led to even more soil.

And there's a positive feedback there. But eventually, the nutrients are going to leak out into the groundwater system. They'll get flushed out of the soil, into the groundwater. The groundwater runs into a local river basin somewhere. The river carries the nutrients down to the ocean. Soils don't just help plants grow on land. They also help plants grow in the water. And so as these trees flourished,

they helped fertilize huge blooms of algae, with devastating consequences for the seas. As those massive algal blooms died and decayed, it robbed the waters of dissolved oxygen. Without dissolved oxygen in the water, Marine life couldn't breathe. Huge dead zones spread across the world's oceans, enveloping vibrant coral reef communities in a vacuous cloud of death. Suffocating the age of fishes.

And that's effectively what I think happened during the late Devonian. For the asteroid that killed the dinosaurs, there is clear, direct evidence. A giant impact crater in the Gulf of Mexico, and a layer of extraterrestrial dust flung across the globe. It's very testable. But the Devonian plants are different because you're looking, the evidence is indirect. You see big changes in climate and environmental conditions, but you can't simply say those are due to the influence of land plants.

They could be due to other triggers. So did trees pull the trigger on this mass extinction? Or did they set the world up? Did they put it in front of the gun and set the stage for some other trigger, like a volcanic eruption? Thomas is determined to find out. Well, what's really needed is a better record

of the evolution of land plants themselves. But if you find a fossil in one place, it doesn't necessarily tell you anything about what's going on in another place. So for some kind of global pattern, you literally have to go out and analyze the changes in dozens or maybe hundreds of different locations in order to be confident you're looking at a global signal.

There are countless fossil forests around the world, like the one here in Cairo, that could shed more light on the Devonian plant explosion. But they're buried under earth and rock, still waiting to be discovered. So there was a time before forests. and then the world was never the same again. It's an insane story. As I walk across the quarry floor, the distance to the past seems permeable.

I can retrace the steps of my amphibian ancestors. Imagine them scampering from tree to tree. This forest was their first home on land. The beginning of life as we know it, the reason I'm even here. Millions of years later. In that light, the carved lines of ancient roots seem like etchings in a stone shrine, a monument commemorating these trees, these heroes that filled this fresh spring air and breathing with oxygen.

There's also a darkness here around the edges, a somber memorial to a mass extinction that killed the majority of life on earth, a tombstone to the age of fishes, marking of violent, revolution that overturned the planet. Such a double-edged sword. Yeah, to think about like life, finding new homes to be as explosive, as violent as an asteroid. Yeah, yeah, absolutely. Wow.

But the past feels closest when I think about the present. We are now the life transforming Earth's climate and eco-fuscary. systems. We're releasing carbon into the air that these first trees and their descendants buried in the ground so long ago. We are on the brink of the six mass extinction in the history of the planet. Maybe the second one ever caused by living beings. And I can't help but wonder, how will we be remembered? All right. It's hard to leave. This is such a magical place. Take a selfie with me. Absolutely.

This episode was reported, produced, and scored by me, Meredith Totanat. It was edited by Jorge Just with help from Brian Resnick. Christian Ayala did the mix, and we collaborated together on the sound design. Melissa Hirsch checked the facts. Mandy Nguyen is lost in a good way. Bird Pinkerton is on the hunt, and Noam Hassanfeld got stuck in a black hole. Or maybe he was there all along. Special thanks to Bill Stein, Chris Berry, Lucas Pollock, Serita Morris, and the town of Cairo.

And a huge thank you to Burl Barajas for making this adventure so magical. And also, thank you, Milo Chestnut, for sharing your wonder and curiosity. I love you guys. I couldn't have made this story without you. You can also support us and all of Vox's journalism by joining our membership program. You can go to Vox.com slash members to sign up. And if you do, you'll be helping make this place run. You also get unlimited access to all the phenomenal reporting on Vox.com.

You'll get exclusive newsletters and you'll get all of our podcasts ad-free. Unexplainable is a part of the Vox Media Podcast Network. We will see you next week.