Name: How Plants Caused the First Mass Extinction
Uploaded: 2021-05-01T09:49:16.000Z
Duration: 10 min 30 s
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In the middle of the Cambrian Period, about 500 million years ago, the face of our planet

There was land, but there weren't any plants or animals living on it..anywhere.

Instead, the dry land was rocky and barren, with no shrubs or trees or grasses.

But, clinging to the rocks and thin ancient soils was life - just a paper-thin film of

These microbes were most likely the only terrestrial life around, and had been for several billion

Scientists think that these ancient microbial films were probably made up of cyanobacteria

And each bacterium was likely doing what cyanobacteria do today - sending out tiny filaments of cells

from the main bacterial mat to start new colonies.

So, the fact is, for a good chunk of Earth's history, cyanobacteria had a monopoly on the

But life on land was about to get a little more crowded.

And those newcomers would end up changing the world.

Their arrival would make the world colder, and fast, and it would drain much of the oxygen

Eventually, it would help cause a massive extinction event, in which around 85% animal

species, including a quarter of marine animal families, disappeared from the planet forever.

This environmental catastrophe is known today as the End-Ordovician Extinction Event, and

it was the first of what we often call the Big Five mass extinctions in the history of

So, what could've caused such a massive, global calamity?

Well, scientists think it may have been kicked off by the world's first, tiny terrestrial

Now, we don't know exactly what the first terrestrial plant on Earth was.

But we have a good idea of what it looked like, and how it lived.

Unlike animals, plants tend to leave behind a terrible fossil record.

You might get a leaf or a stem, but rarely a whole plant.

So the earliest fossil record of land plants isn't parts of their bodies -- it's their

spores, the particles that ancient plants used to reproduce.

Pollen didn't exist when plants first made the move onto land.

But there were the spores like those you'd see today on a moss or a fern.

Back in the 1990s, scientists found lots of plant spores in rocks from Saudi Arabia and

These spores were dated to 462 million years ago - during that cooling event that took

And they could tell they came from land plants, and not aquatic plants, because the spores

had a thick covering that all land plant spores have today.

This covering protects the spores as they deal with environmental stressors, like wind

And aquatic plants don't have that, because they don't need it in their environment,

And this covering is also what allows spores to fossilize, along with the fact that they

are produced in huge quantities in a variety of habitats.

In 2010, even older spores were found in Argentina and dated to 470 million years ago.

But paleontologists think that the arrival of plants on land actually happened even earlier,

based on dates produced by the method known as the molecular clock.

By looking at the average number of changes in DNA over time, scientists can calculate

when a type of organism evolved on Earth.

And this method puts plants on land at least 515 million years ago, right in the middle

And it looks like land plants started diversifying almost as soon as they left the oceans.

The fossil spores in Argentina weren't just from one kind of plant, but from at least

5 different kinds - a little community of Ordovician plants.

It's hard to know what those plants were based on spores, but scientists can tell that

they were non-vascular, meaning that they didn't have the system of roots and tubes

that many modern plants use to move water and nutrients around.

Paleobotanists are still debating what exactly the first type of land plant actually was,

but they agree that it was small and moss-like, probably some kind of green algae or liverwort.

And these were pioneering little plants, venturing from the water into conditions where they

Scientists think that these early plants probably clung to rocks near the water.

There, they released their spores, taking advantage of the tide to disperse those spores,

like their ancestors had done for generations, and gradually transitioning from aquatic to

Over time, through natural selection, they acquired adaptations for life on land, like

hard-walled spores and waxy coverings called cuticles that allowed them to become more

And it looks like their tendency to cling to rocks is what would have spelled disaster for

Today, the scientific name for living material that clings to rocks is cryptogamic cover.

And this cover doesn't just sit there; it interacts with rocks, wearing them down over

time and releasing minerals, like phosphorus, potassium, and iron.

Scientists have used modern cryptogamic covers to see how the first plants might have worn

By growing moss on rocks and measuring the minerals released, they found that moss-covered

rocks released 60 times more phosphorus than rocks without moss.

Once it's freed from the rocks, the phosphorus gets washed away by rainfall, traveling over

landscapes and eventually flowing into the oceans.

And geologists have found evidence of this very phenomenon in the deep past.

In rock formations in modern-day New Mexico and Texas, they found phosphorus in deposits

dating to the late Ordovician Period, when the American Southwest was underwater, and

just as plants were getting a foothold on land.

And those ancient deposits spelled doom for ocean animal life.

That's because phosphorus is one of the nutrients that plants need for growth, but

it's usually in short supply; plants can only get it from the breaking down of rocks.

So a major influx of phosphorus into the oceans would have caused an explosion of marine plants,

After algae bloom, they eventually die, and are broken down by bacteria.

And this process uses up a lot of the oxygen in water.

As a result, the ocean becomes oxygen poor, or hypoxic, or even anoxic, where there's

And since marine animals need oxygen, they can't survive.

But that's not the only change that was caused by the phosphorus runoff.

A hypoxic ocean can also cool the climate.

Because, carbon needs to bind with oxygen to cycle out of the ocean and into the atmosphere

But when ocean water is hypoxic, the carbon just gets buried in sediments and stays there.

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How Plants Caused the First Mass Extinction

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