How Plants Caused the First Mass Extinction

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In the middle of the Cambrian Period, about 500 million years ago, the face of our planet
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looked completely different.
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There was land, but there weren’t any plants or animals living on it..anywhere.
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Instead, the dry land was rocky and barren, with no shrubs or trees or grasses.
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But, clinging to the rocks and thin ancient soils was life - just a paper-thin film of
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microbes.
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These microbes were most likely the only terrestrial life around, and had been for several billion
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years.
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Scientists think that these ancient microbial films were probably made up of cyanobacteria
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and maybe some of the first fungi.
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And each bacterium was likely doing what cyanobacteria do today - sending out tiny filaments of cells
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from the main bacterial mat to start new colonies.
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So, the fact is, for a good chunk of Earth’s history, cyanobacteria had a monopoly on the
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terrestrial environment.
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But life on land was about to get a little more crowded.
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And those newcomers would end up changing the world.
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Their arrival would make the world colder, and fast, and it would drain much of the oxygen
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out of the world’s oceans.
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Eventually, it would help cause a massive extinction event, in which around 85% animal
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species, including a quarter of marine animal families, disappeared from the planet forever.
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This environmental catastrophe is known today as the End-Ordovician Extinction Event, and
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it was the first of what we often call the Big Five mass extinctions in the history of
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our planet.
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So, what could’ve caused such a massive, global calamity?
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Well, scientists think it may have been kicked off by the world’s first, tiny terrestrial
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plants.
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Now, we don’t know exactly what the first terrestrial plant on Earth was.
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But we have a good idea of what it looked like, and how it lived.
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Unlike animals, plants tend to leave behind a terrible fossil record.
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You might get a leaf or a stem, but rarely a whole plant.
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So the earliest fossil record of land plants isn’t parts of their bodies -- it’s their
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spores, the particles that ancient plants used to reproduce.
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Pollen didn’t exist when plants first made the move onto land.
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But there were the spores like those you’d see today on a moss or a fern.
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Back in the 1990s, scientists found lots of plant spores in rocks from Saudi Arabia and
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the Czech Republic.
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These spores were dated to 462 million years ago - during that cooling event that took
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place in the Ordovician Period.
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And they could tell they came from land plants, and not aquatic plants, because the spores
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had a thick covering that all land plant spores have today.
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This covering protects the spores as they deal with environmental stressors, like wind
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or flowing water.
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And aquatic plants don’t have that, because they don’t need it in their environment,
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which tends to be less harsh.
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And this covering is also what allows spores to fossilize, along with the fact that they
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are produced in huge quantities in a variety of habitats.
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In 2010, even older spores were found in Argentina and dated to 470 million years ago.
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But paleontologists think that the arrival of plants on land actually happened even earlier,
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based on dates produced by the method known as the molecular clock.
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By looking at the average number of changes in DNA over time, scientists can calculate
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when a type of organism evolved on Earth.
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And this method puts plants on land at least 515 million years ago, right in the middle
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of the Cambrian Period.
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And it looks like land plants started diversifying almost as soon as they left the oceans.
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The fossil spores in Argentina weren’t just from one kind of plant, but from at least
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5 different kinds - a little community of Ordovician plants.
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It’s hard to know what those plants were based on spores, but scientists can tell that
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they were non-vascular, meaning that they didn’t have the system of roots and tubes
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that many modern plants use to move water and nutrients around.
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Paleobotanists are still debating what exactly the first type of land plant actually was,
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but they agree that it was small and moss-like, probably some kind of green algae or liverwort.
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And these were pioneering little plants, venturing from the water into conditions where they
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were at risk of drying out.
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Scientists think that these early plants probably clung to rocks near the water.
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There, they released their spores, taking advantage of the tide to disperse those spores,
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like their ancestors had done for generations, and gradually transitioning from aquatic to
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terrestrial life.
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Over time, through natural selection, they acquired adaptations for life on land, like
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hard-walled spores and waxy coverings called cuticles that allowed them to become more
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fully terrestrial.
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And it looks like their tendency to cling to rocks is what would have spelled disaster for
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life in the oceans.
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Today, the scientific name for living material that clings to rocks is cryptogamic cover.
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And this cover doesn’t just sit there; it interacts with rocks, wearing them down over
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time and releasing minerals, like phosphorus, potassium, and iron.
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Scientists have used modern cryptogamic covers to see how the first plants might have worn
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rocks down 500 million years ago.
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By growing moss on rocks and measuring the minerals released, they found that moss-covered
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rocks released 60 times more phosphorus than rocks without moss.
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Once it’s freed from the rocks, the phosphorus gets washed away by rainfall, traveling over
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landscapes and eventually flowing into the oceans.
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And geologists have found evidence of this very phenomenon in the deep past.
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In rock formations in modern-day New Mexico and Texas, they found phosphorus in deposits
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dating to the late Ordovician Period, when the American Southwest was underwater, and
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just as plants were getting a foothold on land.
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And those ancient deposits spelled doom for ocean animal life.
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That’s because phosphorus is one of the nutrients that plants need for growth, but
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it’s usually in short supply; plants can only get it from the breaking down of rocks.
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So a major influx of phosphorus into the oceans would have caused an explosion of marine plants,
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in the form of huge algal blooms.
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After algae bloom, they eventually die, and are broken down by bacteria.
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And this process uses up a lot of the oxygen in water.
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As a result, the ocean becomes oxygen poor, or hypoxic, or even anoxic, where there’s
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no oxygen left.
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And since marine animals need oxygen, they can’t survive.
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But that’s not the only change that was caused by the phosphorus runoff.
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A hypoxic ocean can also cool the climate.
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Because, carbon needs to bind with oxygen to cycle out of the ocean and into the atmosphere
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as carbon dioxide.
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But when ocean water is hypoxic, the carbon just gets buried in sediments and stays there.
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In the geologic record, buried organic carbon with no oxygen shows up as black shales.
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And there are extensive black shale deposits in places like China and northern Africa,
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dating to the late Ordovician.
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So, a cooler climate and an oxygen-poor ocean could certainly have been behind the major
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extinction of ocean life.
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Now, in fairness to the plants, experts know that there were other things going on that
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likely contributed to the extinction event.
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Namely, it was also a time of massive tectonic activity.
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New mountains were forming, like the Appalachians, and huge volcanic eruptions took place as
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the tectonic plates of the supercontinent Gondwana moved and folded against each other.
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Some researchers even suspect that all of the gases spewed out by those volcanoes cooled
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the Earth, causing “volcanic winters.”
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Plus, acid rain likely caused rock weathering of the new mountains, which removed even more
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carbon from the atmosphere and drove even more global cooling.
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But, what stands out in the geologic record is how sudden this cold snap was.
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Starting around 488 million years ago, the planet began to cool.
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And the temperature continued to drop over the next 44 million years — which is pretty
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fast in geologic terms.
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So, something else must have been at work to cause that amount of cooling in such a
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short timeframe.
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And, based on the evidence, and modern experimental work, it looks like that trigger might’ve
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been plants moving onto land.
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But, there’s no need to hate on plants because of all of the downstream effects that came
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with their big terrestrial transition.
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Sure, the first land plants were the spark that wreaked havoc on ocean biodiversity,
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but they also paved the way for all the terrestrial life that came after.
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Because, those tiny plants set up the conditions for more sophisticated terrestrial life to
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evolve.
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They built up a rich soil base through death and decomposition.
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And they gradually flooded the atmosphere with oxygen.
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And over time, the plants themselves took over the land.
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Their roots became longer to tap deeper for nutrients.
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Vascular tissue began to carry water and minerals around the plants, supporting the growth of
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much bigger plants.
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Later, huge changes, like the evolution of flowering plants, transformed the vegetation
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on Earth into the ancestors of the plants that we see today and use for food.
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If it weren’t for the pioneering little plants that got a foothold on land half a
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billion years ago, our planet might still be barren, rocky, and populated by nothing
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but microbial films.
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So maybe we can give them a pass for getting the ball rolling on the world’s first mass
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extinction
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If you want to learn more about cyanobacteria, Journey to the Microsmos has a wonderful video
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