The trees of death

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! ⁠⁠⁠[email protected]⁠⁠⁠ 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

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:01Speaker 00

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• 0:49Speaker 01

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• 1:21Speaker 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.

• 1:33Speaker 02

  I feel like it's just starting to be spring. I can feel it.

• 1:38Speaker 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.

• 1:57Speaker 02

  And you'll finally arrive.

• 2:02Speaker 02

  At a huge, empty gravel lot. It's definitely nothing living inside that area. An old quarry, long out of use.

• 2:16Speaker 02

  Lot of gravel.

• 2:20Speaker 02

  Some birds, too.

• 2:23Speaker 02

  But look carefully at the dusty ground. You might just see the rocks that transported Chuck into the past.

• 2:31Speaker 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.

• 3:01Speaker 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?

• 3:24Speaker 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.

• 3:50Speaker 02

  Wow.

• 3:52Speaker 03

  That is where a tree stood 385 million years ago.

• 3:57Speaker 02

  Holy shit. Unbelievable.

• 4:03Speaker 02

  So this was one tree.

• 4:06Speaker 02

  Are we saying that this is literally two trees? Like the star formations are like the basis of these two trees?

• 4:15Speaker 03

  Yeah. The impressions of the roots, well-preserved.

• 4:19Speaker 02

  Like the footprints, almost?

• 4:22Speaker 03

  Yeah, it'd be like footprints, but the impressions of the roots where they were in the ground.

• 4:27Speaker 02

  You know how big that tree was?

• 4:30Speaker 04

  Do you know what I mean?

• 4:31Speaker 02

  Yeah, no, I mean, literally, I think it looks like the trees that we can see on the other side of the fence.

• 4:35Speaker 04

  Tree-sized.

• 4:36Speaker 02

  Tree-sized.

• 4:39Speaker 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.

• 4:53Speaker 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.

• 5:06Speaker 03

  I mean, so many individual lives have lived on this planet.

• 5:16Speaker 03

  Everything died, left behind something, but most of everything decays, is broken down, dissolved.

• 5:27Speaker 03

  It's all about Earth, life, and a lot of time.

• 5:35Speaker 03

  Such an awesome, awesome, awesome thing to imagine a forest from that long ago.

• 6:02Speaker 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.

• 6:34Speaker 02

  I'm Meredith Hocknull, and this is Unexplainable.

• 6:45Speaker 05

  So this is the paleontology collection at the New York State Museum.

• 6:51Speaker 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.

• 7:00Speaker 05

  The fossil collection has lots of stuff, sitting in the aisles and on tables, carts are full.

• 7:07Speaker 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.

• 7:24Speaker 05

  Whenever I'm working with the fossils, I'm picturing where they lived. I'm picturing how they lived.

• 7:29Speaker 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.

• 7:38Speaker 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.

• 7:49Speaker 02

  Our story starts in a time before trees, about 420 million years ago, at the beginning of the Devonian period.

• 8:00Speaker 02

  Earth's continents were bunched up in the southern hemisphere, and the land they carried would have been unrecognizable.

• 8:08Speaker 03

  There really wasn't much of anything on land at all.

• 8:13Speaker 05

  In fact, nothing lived on land, except for some tiny plants and the ancestors of things like insects.

• 8:20Speaker 02

  There were very few signs of life, all of them weird.

• 8:26Speaker 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.

• 8:39Speaker 02

  This was a landscape of barren rock and gravel, buffeted by winds and rains. The loneliest alien planet.

• 8:50Speaker 02

  But underwater was a different world.

• 8:55Speaker 03

  It was so much life in the oceans.

• 8:59Speaker 02

  The Devonian is known as the Age of Fishes because there were so many kinds of fish.

• 9:07Speaker 03

  All of the major groups of fish were alive during the Devonian period.

• 9:12Speaker 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.

• 9:41Speaker 05

  Plants started to evolve from algae to be able to survive in a wet environment out of the water.

• 9:47Speaker 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.

• 9:59Speaker 02

  And once plants on land got a foothold, they started evolving faster and faster.

• 10:05Speaker 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.

• 10:31Speaker 05

  Big portions of New York were underwater, and the Appalachian Mountains were just forming, and they were huge, really tall mountains.

• 10:40Speaker 03

  Andy's Scale Mountains, perhaps, out there in New England.

• 10:44Speaker 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.

• 11:00Speaker 05

  It's like a furry telephone pole.

• 11:03Speaker 02

  And also smaller plants.

• 11:05Speaker 03

  Little things like liverworts.

• 11:07Speaker 05

  Liverworts are just cool. If you don't know what one is, you should look one up.

• 11:10Speaker 03

  Teensy, little leaflet-like things.

• 11:13Speaker 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.

• 11:49Speaker 02

  And that's about where we are here in New York. We had everything, vertebrates, invertebrates, and plants on land at this time.

• 12:06Speaker 02

  So this is one of the first full forests on the planet Earth.

• 12:14Speaker 02

  Unbelievable.

• 12:16Speaker 02

  Totally. Crazy.

• 12:21Speaker 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?

• 12:38Speaker 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.

• 12:56Speaker 03

  But this is a big perspective.

• 12:59Speaker 03

  It's really the Devonian period when life first really colonized the land to a large scale. It was that Devonian period.

• 13:14Speaker 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,

• 13:55Speaker 02

  transforming the world from gray and blue to green and blue, into a planet we'd more or less recognize today.

• 14:05Speaker 02

  And in doing so, they might have caused one of the biggest mass extinctions in the history of the world.

• 14:14Noam Hassenfeld?

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• 14:21Noam Hassenfeld?

  All right, one of life's great mysteries. You're listening to music in the living room. You gotta grab something from the kitchen. You don't want to take the speaker with you, but you also don't want to miss that beat drop. And this is where I introduce you to the magic of Sonos. Sonos sent me a couple of speakers to try out, so I've got to move two here in the living room. And then I walk down the hall.

• 14:46Noam Hassenfeld?

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• 15:03Noam Hassenfeld?

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• 15:36Noam Hassenfeld?

  head outside for a minute.

• 15:39Noam Hassenfeld?

  It's super easy and yeah, pretty nice out. Visit sonos.com and build your system. Sonos, love the sound, feel the system.

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• 17:19Speaker 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.

• 17:54Speaker 02

  The story of life exploding on the land is also the story of life dying in the oceans.

• 18:04Speaker 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.

• 18:40Speaker 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.

• 18:55Speaker 02

  Geologist Thomas Algeo studies ancient apocalypses.

• 19:00Speaker 07

  I've effectively delved into all of the mass extinction events in Earth's history.

• 19:06Speaker 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.

• 19:30Speaker 07

  And for that to happen, there has to be some kind of trigger, some kind of external trigger that upsets the system.

• 19:38Speaker 02

  And Thomas thinks that this mass extinction in the oceans was triggered by the trees.

• 19:48Speaker 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.

• 20:26Speaker 07

  And that is a big increase. That brings us up close to modern oxygen levels.

• 20:32Speaker 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,

• 21:16Speaker 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.

• 21:40Speaker 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.

• 22:00Speaker 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.

• 22:18Speaker 07

  As those massive algal blooms died and decayed, it robbed the waters of dissolved oxygen.

• 22:26Speaker 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.

• 22:45Speaker 07

  And that's effectively what I think happened during the Late Devonian.

• 22:58Speaker 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.

• 23:12Speaker 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.

• 23:31Speaker 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.

• 23:50Speaker 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.

• 24:15Speaker 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.

• 24:46Speaker 02

  So there was a time before forests, and then the world was never the same again.

• 24:52Speaker 08

  It's an insane story.

• 25:01Speaker 02

  As I walk across the quarry floor, the distance to the past seems permeable.

• 25:08Speaker 08

  Unbelievable.

• 25:10Speaker 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.

• 25:46Speaker 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.

• 26:04Speaker 08

  Wow, such a double-edged sword.

• 26:07Speaker 02

  Yeah, to think about like, life finding new homes to be as explosive, as violent as an asteroid.

• 26:18Speaker 08

  Yeah, yeah, absolutely. Wow.

• 26:23Speaker 02

  But the past feels closest when I think about the present.

• 26:28Speaker 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.

• 26:44Speaker 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?

• 27:09Speaker 02

  All right. It's hard to leave. This is such a magical place.

• 27:13Speaker 09

  Come take a selfie with me.

• 27:15Speaker 02

  Absolutely.

• 27:35Speaker 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.

• 28:15Speaker 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.

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  Unexplainable is a part of the Vox Media Podcast Network. We will see you next week.