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  • In the middle of the Cambrian Period, about 500 million years ago, the face of our planet

  • looked completely different.

  • 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

  • microbes.

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

  • years.

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

  • and maybe some of the first fungi.

  • 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

  • terrestrial environment.

  • 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

  • out of the world’s oceans.

  • 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

  • our planet.

  • So, what couldve caused such a massive, global calamity?

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

  • plants.

  • 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

  • the Czech Republic.

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

  • place in the Ordovician Period.

  • 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

  • or flowing water.

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

  • which tends to be less harsh.

  • 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

  • of the Cambrian Period.

  • 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

  • were at risk of drying out.

  • 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

  • terrestrial life.

  • 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

  • fully terrestrial.

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

  • life in the oceans.

  • 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

  • rocks down 500 million years ago.

  • 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,

  • in the form of huge algal blooms.

  • 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

  • no oxygen left.

  • 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

  • as carbon dioxide.

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

  • In the geologic record, buried organic carbon with no oxygen shows up as black shales.

  • And there are extensive black shale deposits in places like China and northern Africa,

  • dating to the late Ordovician.

  • So, a cooler climate and an oxygen-poor ocean could certainly have been behind the major

  • extinction of ocean life.

  • Now, in fairness to the plants, experts know that there were other things going on that

  • likely contributed to the extinction event.

  • Namely, it was also a time of massive tectonic activity.

  • New mountains were forming, like the Appalachians, and huge volcanic eruptions took place as

  • the tectonic plates of the supercontinent Gondwana moved and folded against each other.

  • Some researchers even suspect that all of the gases spewed out by those volcanoes cooled

  • the Earth, causingvolcanic winters.”

  • Plus, acid rain likely caused rock weathering of the new mountains, which removed even more

  • carbon from the atmosphere and drove even more global cooling.

  • But, what stands out in the geologic record is how sudden this cold snap was.

  • Starting around 488 million years ago, the planet began to cool.

  • And the temperature continued to drop over the next 44 million yearswhich is pretty

  • fast in geologic terms.

  • So, something else must have been at work to cause that amount of cooling in such a

  • short timeframe.

  • And, based on the evidence, and modern experimental work, it looks like that trigger mightve

  • been plants moving onto land.

  • But, there’s no need to hate on plants because of all of the downstream effects that came

  • with their big terrestrial transition.

  • Sure, the first land plants were the spark that wreaked havoc on ocean biodiversity,

  • but they also paved the way for all the terrestrial life that came after.

  • Because, those tiny plants set up the conditions for more sophisticated terrestrial life to

  • evolve.

  • They built up a rich soil base through death and decomposition.

  • And they gradually flooded the atmosphere with oxygen.

  • And over time, the plants themselves took over the land.

  • Their roots became longer to tap deeper for nutrients.

  • Vascular tissue began to carry water and minerals around the plants, supporting the growth of

  • much bigger plants.

  • Later, huge changes, like the evolution of flowering plants, transformed the vegetation

  • on Earth into the ancestors of the plants that we see today and use for food.

  • If it weren’t for the pioneering little plants that got a foothold on land half a

  • billion years ago, our planet might still be barren, rocky, and populated by nothing

  • but microbial films.

  • So maybe we can give them a pass for getting the ball rolling on the world’s first mass

  • extinction

  • Do you want more Eons content? Be sure to follow Eons on social media!

  • You can find us on Instagram, Twitter, and Facebook.

  • And you can join me on Instagram at fossil_librarian.

  • If you want to learn more about cyanobacteria, Journey to the Microsmos has a wonderful video

  • that nicely compliments this one.

  • If you haven’t watched their videos yet, you are really missing out.

  • Each episode uses incredible footage to take you on a dive into the tiny, unseen world

  • that surrounds us!

  • Check them out at youtube.com/microsmos

  • Gotta thank this month’s cool Eontologists: Patrick Seifert, Jake Hart, Jon Davison Ng,

  • Sean Dennis, and Steve!

  • Remember all pledge levels have access to our Discord, so come nerd out with us by becoming

  • an Eonite at patreon.com/eons.

  • Also thank you for joining me in the Konstantin Haase Studio

  • And if you like what we do here, subscribe at youtube.com/eons.

In the middle of the Cambrian Period, about 500 million years ago, the face of our planet

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植物如何导致第一次大灭绝(How Plants Caused the First Mass Extinction)

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    joey joey 發佈於 2021 年 05 月 01 日
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