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To keep building your STEM skills beyond this video, check out Brilliant.org/SciShow. That link will give you 20% off an annual premium subscription! [♪ INTRO] Picture what happens during a mass extinction.

I’ll guess that you’re thinking of full-blown catastrophes, like volcanic eruptions or asteroids raining fire from the sky. And there are extinctions that looked like that. But what if I told you that one mass extinction may have been caused by trees?

It’s called the Late Devonian Extinction, and it might make you look at your favorite forest in a different light. To take a quick step back here: It’s worth knowing that extinctions happen all the time, in the grand scheme of things. Species are always evolving and disappearing.

The difference between regular, garden variety extinctions and a mass extinction is scale. Scientists usually consider something a mass extinction when global biodiversity drops by 75% or more over the course of the extinction event. So, the number of species on Earth seriously tanks.

And the Late Devonian Extinction definitely meets that criteria. This event happened from 375 to 359 million years ago — so, about 16 million years total. And scientists are still working out exactly how it unfolded.

In a typical mass extinction scenario, you’d usually picture… mass extinctions – as in, lots more things than normal going extinct all at once. But in the Late Devonian, it’s possible that instead of more things dying out, fewer new things were evolving, so anything that did go extinct left a dramatically big void. In any case, they do know that the number of species on Earth started to tumble, and it didn’t stop tumbling until the very end of the Devonian.

In particular, this crash hit ocean ecosystems, hard. Animals like stromatoporoids, which were reef-building sponges, disappeared completely, as did the shelled animals that depended on these ecosystems. Our friends the trilobites suffered major losses in the Late Devonian, too: Only one group survived, and they never returned to their pre-Devonian glory.

Armor-headed fishes called placoderms were also seriously affected. Here, only two species made it through the extinction event, and even they went extinct not long after. Overall, for ocean ecosystems, this extinction was the second-largest biodiversity collapse in the fossil record.

Meanwhile, ecosystems on land were… pretty much fine, like they threw down a big old “skip” card in UNO. So, what gives? Well, the reason life on land wasn’t as affected by this extinction might be because the organisms living on land were the ones that caused it!

Specifically, the culprit may have been trees, which were beginning to evolve around this time. See, before this extinction event, land plants weren’t very diverse. They were dependent on bodies of water for reproduction, so they couldn’t live far from shorelines.

And they weren’t very good at moving water around their little plant bodies, so their stems couldn’t get very tall. That is, until trees evolved. Trees are kind of the crabs of the plant world: Lots of different kinds of plants keep evolving into tree shapes.

It’s a strategy, not a family grouping. What makes a tree a tree is, very basically, having sturdy, thick stalks or trunks, and really strong root systems that can reach deeper water sources and break up sediment as they grow into the earth. Right around when plants figured out how to get big like this, they also started forming seeds with hard shells.

These seeds retained water, so the baby plants inside could stay viable far from rivers and oceans. Being big and watertight now meant plants could go a lot of new places. So, all of a sudden, these plants start showing up in the fossil record, all over the place.

And these Devonian forests may have been bad news for life in the oceans, for two reasons. The first is that their deep roots would have churned up layers of sediment, releasing minerals that used to be bound up in rock. The roots themselves would also host nitrogen-fixing bacteria, which turn regular nitrogen from the air into compounds that plants can use, like nitrates and ammonia.

Those minerals and compounds would have soaked into the water supply and been carried away by things like streams, melting snow, and rainwater. And when they’d inevitably end up in the ocean, they would have cause an explosive growth of algae. Except, those booming algae populations wouldn’t have been sustainable, so thick mats of dead plant matter would have soon fallen to the ocean floor all at once, too quickly to be decomposed.

They would have basically choked out anything living underneath them. And there go the trilobites… We have evidence that this happened during the Late Devonian Extinction, too. These mats of dead algae produced layers at the bottom of the ocean with basically no oxygen in them, and these layers eventually fossilized into rock.

But that isn’t the only clue we have leading back to our leafy friends. Big plants also photosynthesize more, and the first trees would have been sucking so much carbon dioxide out of the air that the amount of it in the atmosphere would have plummeted. And since less carbon dioxide means less heat trapped in the atmosphere, this meant the planet would have cooled down.

Which we also have evidence for. In fact, the average temperature during the Late Devonian Extinction dropped between 3 and 6 degrees Celsius, which would have made the ocean way too cold for some marine organisms to deal with. Now, the tricky thing about both these lines of evidence is that they seem compelling… but neither of them tell us that trees caused this extinction for sure.

Technically, something else could have caused the algae to bloom and die, or the global temperature drop. So, some scientists favor other ideas about the Late Devonian, from meteor impacts to volcanic eruptions. And unless we find a fossil tree holding a literal smoking gun, the research community may never all agree what happened.

But even if they weren’t the main cause of the Late Devonian Extinction, trees were probably playing some role in it. That means the success of one kind of organism likely resulted in the collapse of other ecosystems, Game of Thrones style! So, ultimately, the Late Devonian Extinction is a great reminder that all life is connected, for better and for worse.

No matter where we all grow, we all have shared roots. Thanks for watching this SciShow video and thank you to Brilliant for supporting it! Brilliant is an online learning platform that offers guided problem-solving based courses in math, science, and engineering.

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