Pod Insights Unexplainable The trees of death About this episode 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
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Transcript 3,526 words · 10 speakers
Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.
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1:21 Speaker 02 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.
1:33 Speaker 02 I feel like it's just starting to be spring. I can feel it.
1:38 Speaker 02 Just when you think you've missed it, turn left into an unmarked driveway. Past the fences and the no trespassing signs. All right, another chain link fence. And a much more secure padlock.
1:57 Speaker 02 And you'll finally arrive.
2:02 Speaker 02 At a huge, empty gravel lot. It's definitely nothing living inside that area. An old quarry, long out of use.
2:16 Speaker 02 Lot of gravel.
2:20 Speaker 02 Some birds, too.
2:23 Speaker 02 But look carefully at the dusty ground. You might just see the rocks that transported Chuck into the past.
2:31 Speaker 03 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, yeah.
3:01 Speaker 03 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?
3:24 Speaker 03 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.
3:50 Speaker 02 Wow.
3:52 Speaker 03 That is where a tree stood 385 million years ago.
3:57 Speaker 02 Holy shit. Unbelievable.
4:03 Speaker 02 So this was one tree.
4:06 Speaker 02 Are we saying that this is literally two trees? Like the star formations are like the basis of these two trees?
4:15 Speaker 03 Yeah. The impressions of the roots, well-preserved.
4:19 Speaker 02 Like the footprints, almost?
4:22 Speaker 03 Yeah, it'd be like footprints, but the impressions of the roots where they were in the ground.
4:27 Speaker 02 You know how big that tree was?
4:30 Speaker 04 Do you know what I mean?
4:31 Speaker 02 Yeah, no, I mean, literally, I think it looks like the trees that we can see on the other side of the fence.
4:35 Speaker 04 Tree-sized.
4:36 Speaker 02 Tree-sized.
4:39 Speaker 03 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.
4:53 Speaker 04 It looks so magical. It's like, yeah, it feels hard to wrap your head around, especially when it, I don't know, when you're looking at what it is now.
5:06 Speaker 03 I mean, so many individual lives have lived on this planet.
5:16 Speaker 03 Everything died, left behind something, but most of everything decays, is broken down, dissolved.
5:27 Speaker 03 It's all about Earth, life, and a lot of time.
5:35 Speaker 03 Such an awesome, awesome, awesome thing to imagine a forest from that long ago.
6:02 Speaker 02 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.
6:34 Speaker 02 I'm Meredith Hocknull, and this is Unexplainable.
6:45 Speaker 05 So this is the paleontology collection at the New York State Museum.
6:51 Speaker 05 All right. We are walking through a room that looks kind of like the warehouse at the end of Raiders of the Lost Ark.
7:00 Speaker 05 The fossil collection has lots of stuff, sitting in the aisles and on tables, carts are full.
7:07 Speaker 02 Lisa Amati cares for hundreds of thousands of fossils here, and her job as the state paleontologist. Lisa and geologists like Chuck Verstratton piece together fragments of an ancient past that they can't ever study directly.
7:24 Speaker 05 Whenever I'm working with the fossils, I'm picturing where they lived. I'm picturing how they lived.
7:29 Speaker 03 And it's like, not just thinking about the rock and its character, not just thinking about the fossils and what they tell me.
7:38 Speaker 05 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.
7:49 Speaker 02 Our story starts in a time before trees, about 420 million years ago, at the beginning of the Devonian period.
8:00 Speaker 02 Earth's continents were bunched up in the southern hemisphere, and the land they carried would have been unrecognizable.
8:08 Speaker 03 There really wasn't much of anything on land at all.
8:13 Speaker 05 In fact, nothing lived on land, except for some tiny plants and the ancestors of things like insects.
8:20 Speaker 02 There were very few signs of life, all of them weird.
8:26 Speaker 05 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 Fungzilla.
8:39 Speaker 02 This was a landscape of barren rock and gravel, buffeted by winds and rains. The loneliest alien planet.
8:50 Speaker 02 But underwater was a different world.
8:55 Speaker 03 It was so much life in the oceans.
8:59 Speaker 02 The Devonian is known as the Age of Fishes because there were so many kinds of fish.
9:07 Speaker 03 All of the major groups of fish were alive during the Devonian period.
9:12 Speaker 02 There were jawless 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 fin fishes with bony little limbs. And the abundance of life in the ocean was just beginning to make its way onto the land.
9:41 Speaker 05 Plants started to evolve from algae to be able to survive in a wet environment out of the water.
9:47 Speaker 03 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, five, maybe six inches.
9:59 Speaker 02 And once plants on land got a foothold, they started evolving faster and faster.
10:05 Speaker 03 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.
10:31 Speaker 05 Big portions of New York were underwater, and the Appalachian Mountains were just forming, and they were huge, really tall mountains.
10:40 Speaker 03 Andy's Scale Mountains, perhaps, out there in New England.
10:44 Speaker 02 And in the tropical floodplains between the mountains and the sea, the Cairo Quarry forest flourished with the strangest trees: broccoli-headed, palm-like trees, draping evergreen ancestors, club moss gone giant.
11:00 Speaker 05 It's like a furry telephone pole.
11:03 Speaker 02 And also smaller plants.
11:05 Speaker 03 Little things like liverworts.
11:07 Speaker 05 Liverworts are just cool. If you don't know what one is, you should look one up.
11:10 Speaker 03 Teensy, little leaflet-like things.
11:13 Speaker 02 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 lobe fin 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.
11:49 Speaker 02 And that's about where we are here in New York. We had everything, vertebrates, invertebrates, and plants on land at this time.
12:06 Speaker 02 So this is one of the first full forests on the planet Earth.
12:14 Speaker 02 Unbelievable.
12:16 Speaker 02 Totally. Crazy.
12:21 Speaker 02 What do you think is so, um, and I want to say the word enchanting, I'm not sure if that's right, but just like enthralling about the Cairo site? Why do you think it like sparked so many people's imaginations?
12:38 Speaker 03 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.
12:56 Speaker 03 But this is a big perspective.
12:59 Speaker 03 It's really the Devonian period when life first really colonized the land to a large scale. It was that Devonian period.
13:14 Speaker 02 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 swamplands 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 in explosion of biodiversity,
13:55 Speaker 02 transforming the world from gray and blue to green and blue, into a planet we'd more or less recognize today.
14:05 Speaker 02 And in doing so, they might have caused one of the biggest mass extinctions in the history of the world.
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17:19 Speaker 01 This episode is brought to you by Indeed. You're ready to level up your business, but the pressure to find the right person to make that happen is intense. Indeed-sponsored jobs can help carry the load. It boosts your job post to help it reach quality talent fast. Cut through the chaos with Indeed-sponsored jobs. Get a $75 sponsored job credit at indeed.com/podcast. Terms and conditions apply. Hiring now? Then this is a job for Indeed-sponsored jobs.
17:54 Speaker 02 The story of life exploding on the land is also the story of life dying in the oceans.
18:04 Speaker 02 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 violent 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 seafloor. The age of fishes was over.
18:40 Speaker 07 It takes a certain degree of imagination to see that things were different in the past. And the deeper back you go into Earth's history, the more bizarre they are and the more foreign they are to what we're familiar with.
18:55 Speaker 02 Geologist Thomas Algeo studies ancient apocalypses.
19:00 Speaker 07 I've effectively delved into all of the mass extinction events in Earth's history.
19:06 Speaker 02 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.
19:30 Speaker 07 And for that to happen, there has to be some kind of trigger, some kind of external trigger that upsets the system.
19:38 Speaker 02 And Thomas thinks that this mass extinction in the oceans was triggered by the trees.
19:48 Speaker 02 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.
20:26 Speaker 07 And that is a big increase. That brings us up close to modern oxygen levels.
20:32 Speaker 02 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,
21:16 Speaker 02 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.
21:40 Speaker 07 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.
22:00 Speaker 02 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.
22:18 Speaker 07 As those massive algal blooms died and decayed, it robbed the waters of dissolved oxygen.
22:26 Speaker 02 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.
22:45 Speaker 07 And that's effectively what I think happened during the Late Devonian.
22:58 Speaker 02 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.
23:12 Speaker 07 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.
23:31 Speaker 02 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.
23:50 Speaker 07 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.
24:15 Speaker 02 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.
24:46 Speaker 02 So there was a time before forests, and then the world was never the same again.
24:52 Speaker 08 It's an insane story.
25:01 Speaker 02 As I walk across the quarry floor, the distance to the past seems permeable.
25:08 Speaker 08 Unbelievable.
25:10 Speaker 02 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. And 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 I'm breathing with oxygen.
25:46 Speaker 02 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 a violent revolution that overturned the planet.
26:04 Speaker 08 Wow, such a double-edged sword.
26:07 Speaker 02 Yeah, to think about like, life finding new homes to be as explosive, as violent as an asteroid.
26:18 Speaker 08 Yeah, yeah, absolutely. Wow.
26:23 Speaker 02 But the past feels closest when I think about the present.
26:28 Speaker 02 We are now the life transforming Earth's climate and ecosystems. We're releasing carbon into the air that these first trees and their descendants buried in the ground so long ago.
26:44 Speaker 02 We are on the brink of the sixth 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?
27:09 Speaker 02 All right. It's hard to leave. This is such a magical place.
27:13 Speaker 09 Come take a selfie with me.
27:15 Speaker 02 Absolutely.
27:35 Speaker 02 This episode was reported, produced, and scored by me, Meredith Thottnott. It was edited by Jorge Just with help from Brian Resnick. Christian Ayla 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 Pollack, Sarita Morris, and the town of Cairo.
28:15 Speaker 02 And a huge thank you to Brill 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/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.
28:58 Speaker 02 Unexplainable is a part of the Vox Media Podcast Network. We will see you next week.
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