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 could’ve 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, causing “volcanic 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 years — which 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 might’ve 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,
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Love this channel!!
Thank you for introducing me to this series on PBS, I will prob now watch all 3 seasons. Awesome content.
Very interesting! Ty for sharing
What a great video! Definitely following it on youtube.
Jokes on you I already watched this yesterday lol
Thanks