All right.

There's a really good site right here.

We can take a look at this boundary layer.

All right.

So we're just a little bit outside of downtown Salt Lake City, and we're on this trail.

Why are we here?

What are we looking at?

Well, this layer here represents the worst mass extinction that ever happened on Earth.

♪ [Roaring] These rocks were deposited during the Great Dying 252 million years ago.

♪ Gabriel-Philip Santos: It might not look like much, but this geologic boundary marks the single greatest catastrophe that life on Earth ever faced.

On one side of the boundary are rock layers from the late Permian Period, the final chapter of the Paleozoic, the era of ancient life, and on the other side of the boundary are rock layers from the Early Triassic Period, the first chapter of the Mesozoic, the fossil record's middle era.

♪ Kallie Moore: An apocalypse occurred at this boundary between the Permian and Triassic that wiped out more than 80% of all species, altering the course of evolution in ways that we're still feeling the effects of today 252 million years later.

♪ So even though the transition from the Late Permian to the Early Triassic represents only a brief moment in geologic time, crossing the boundary feels like traveling to an entirely different, much bleaker world.

♪ Michelle Barboza-Ramirez: The Late Permian's flourishing ecosystems teeming with diverse species suddenly vanish almost entirely from the fossil record, and in the desolation of the Early Triassic that follows, all that remains are just a few surviving branches of a tree of life that has otherwise withered away.

♪ Blake de Pastino: The fossil record of this time tells the story of a cataclysm that paleontologists call "The Great Dying," the closest that complex life on Earth ever came to total annihilation and an event that our ancient relatives barely survived.

It was a complex event involving a geologic, climatic, and ecological cascade that is still being pieced together by scientists reading the rocks at places like this, at the literal end of an era.

Benjamin Burger: These rocks are 252 million years old, so they're like a little time capsule.

Gabriel: But what we can say is that the Great Dying was set in motion by a force deep within the planet itself under the northern reaches of Pangea thousands of kilometers away from here in what's now Siberia.

Blake: If things had gone just a little differently at that thin boundary for our direct ancestors, if they had been just a little less scrappy and a little less lucky, they could have easily joined the majority of animal life on Earth and succumbed to the worst ever mass extinction.

♪ [Growls] ♪ [Projector chattering] ♪ Kallie: The biodiversity of planet Earth is balanced by two opposing forces-- the origin of species, speciation; and the loss of species, extinction-- both of which happen constantly in the ecological background.

The concept that species come and go might seem pretty obvious, seeing as many of us grew up learning about the long-gone days of mammoths, dinosaurs, and dodos or the numerous species facing extinction today, but in reality, the idea that species can vanish entirely from the face of the Earth is actually a relatively recent one.

The concept of extinction only became established in Western science at the turn of the 19th century, most notably by a French naturalist named Georges Cuvier, sometimes known as the Father of Paleontology.

Before Cuvier, the prevailing European view of the natural world was a much more static one, rooted in Christian theology-- life on Earth was perfect and unchanging.

The species of today had existed since the start and would always exist.

They had been made deliberately by a Creator, and He wouldn't allow them to be unmade.

The idea of a species dying out completely-- removed like a chess piece from the board of creation-- was so outlandish that it wasn't even seriously entertained by most pre-19th century thinkers, and even when the remains of strange ancient creatures were found, it was assumed that living members of these species still existed somewhere in the world, including by Thomas Jefferson, who had a personal fascination with fossils and believed that species like mammoths and giant sloths would likely be found living somewhere in the American West, but Cuvier argued that the fossil record contained the bones of species clearly distinct from living ones and that, in many cases, they were just too big to have stayed hidden.

These were animals that were not alive anymore from a time when planet Earth must have been a wildly different place than it is now.

Cuvier's concept of extinction was a revolution in the way that we saw the natural world.

It was no longer stable and unchanging.

This was a world in flux, and it had a dramatic and mysterious history.

♪ Over the more than 200 years of studying the fossil record since Cuvier's time, our understanding of that history-- and just how profoundly it has been shaped by extinction-- has only become more radical because now we know that not only can species go extinct but that, on at least 5 separate occasions in the history of complex life, most species suddenly did.

The most famous of the Big 5 by far is also the most recent of them, the End-Cretaceous Extinction at the close of the Mesozoic Era some 66 million years ago.

You probably know it as the time that a gigantic asteroid impact ended the reign of the dinosaurs, but despite it being the most well-known mass extinction event, as well as the most sudden and dramatic, it wasn't actually the worst one.

That honor goes to the Great Dying at the end of the Permian Period, when our planet suffered a level of death that it had never seen before and thankfully has never seen since.

In our journey through the Permian so far, we've watched ecosystems get increasingly impressive, dynamic, and layered, but just like a house of cards, the more complex and multi-dimensional an ecosystem gets, the more dramatic its collapse will be, and unlike the End-Cretaceous asteroid impact, the Great Dying wasn't caused by an external threat descending from above but by an internal threat rising from below.

Blake: Some of my colleagues on "Eons" described the Great Dying as the "Murder on the Orient Express" of mass extinctions.

Like, everyone did it.

Mm-hmm.

Mm-hmm.

Every ecological factor on that train tried to kill life on Earth.

Mm-hmm.

There were a lot of variables there.

So can you walk me through everything that was going on that triggered the Great Dying?

Yeah, there were, of course, many culprits in this extinction event, but there was a main trigger, and that was the eruption of a large amount of lava in what is now Russia called the Siberian Traps.

♪ Michelle: Around 252 million years ago, deep within the Earth, a superheated plume of molten rock formed inside the planet's mantle layer.

As convection currents carried it higher and higher, this plume pushed against and melted the planet's crust until eventually it broke through entirely in northern Pangea in what would one day be Siberia, triggering some of the largest volcanic eruptions in history.

Pia Viglietti: So we're talking about an area the size of Western Europe over 2,000 feet tall.

This happened over the course of a million years.

This put tons and tons of carbon dioxide into the atmosphere and other greenhouse gases.

Michelle: The consequences of these eruptions were felt on every inch of the supercontinent of Pangea and even the depths of the oceans that surrounded it because these eruptions did much more than just spew out an obscene amount of lava.

They also set off a chain reaction that turned this regional disaster into a global crisis of unprecedented proportions.

See, what made the eruptions so especially devastating was their location.

That eruption was situated underneath a huge basin of carboniferous coal.

Michelle: Permian Siberia contained vast fossil fuel deposits that had formed in the extensive tropical swamps of the Carboniferous Period right before the Permian began.

As its name suggests, the Carboniferous is famous for being the enormous amount of carbon dioxide that was pulled from the atmosphere by its plants.

As these plants died and fell into the murky depths of the swamps, they didn't decompose but became buried and compressed underground, locking away all that carbon in a solid form, over a trillion tons of coal.

Millions of years later at the end of the Permian, when these ancient coal deposits came into contact with red hot molten rock from the End-Permian eruptions, they ignited, and once they were on fire, no force on Earth could put them out.

Gabriel: Now, one of the most painful lessons that our species has learned over the last century is that when you burn a lot of coal really fast, the planetary effects are pretty dire.

Suddenly, releasing all that stored carbon back into the atmosphere triggers rapid and intense global warming, and 252 million years ago, this same process set in.

The combination of emissions released directly from the eruptions, as well as from the unchecked coal combustion, raised atmospheric carbon dioxide levels 6-fold according to some estimates.

Now, if that was a little bit higher, it would be lethal to all air-breathing terrestrial animals.

Gabriel: The runaway greenhouse effect that followed was intense, increasing ambient temperatures to around 35-40 degrees Celsius-- beyond the ability of most ecosystems to tolerate for long.

Pia: We also see evidence of dead zones where rising temperatures just made parts of the globe inhospitable for all life, particularly at the equators.

Gabriel: Plus, the massive volcanism and coal combustion also spewed out other pollutants, like sulfur dioxide, triggering planet-wide acid rain with a pH level equivalent to vinegar.

The Great Dying literally burned the world, and from the seas to the trees, no ecosystem was safe.

Pia: So globally, what we saw on land, this triggered a number of extreme climatic events like mega storms, flooding, droughts, wildfires.

In the oceans, rising temperatures caused ocean acidification and anoxia, which means the oceans lose their oxygen.

The ocean extinction was truly devastating.

It's believed that 8 out of every 10 marine species went extinct.

Some of the oldest features of the ancient world's marine life vanished forever, like trilobites and horn corals, for example.

These groups had lived in the oceans of planet Earth for hundreds of millions of years, some of them dating all the way back to the Cambrian period when complex animal life first became common.

Oh, my goodness.

♪ Probably my favorite invert.

Trilobites.

Oh, my God.

They're just beautiful.

Look at these.

So the specimens in this drawer are trilobites from the Cambrian Period of Earth history.

OK.

I think they're around about 525 million years old or so, and that's right around the time when trilobites first show up in the fossil record.

Blake: They had lasted almost the entire Paleozoic, facing every major challenge that the world had thrown at them, including two of the Big 5 mass extinctions already, so it's a testament to the severity of the Great Dying that it finally proved a match for even these hardiest of marine animals.

Blake: What was the last straw?

Why couldn't they make it through the Great Dying if they'd made it through a quarter billion years of other mass extinction events?

It's probably ultimately in a way bad luck.

Late Permian trilobites are animals that mostly were living in sort of shallow ocean environments, and those environments were very strongly affected by all the bad things that happen at the end of the Permian.

Blake: And even for groups that made it through the extinction event, most were whittled down to just a few lucky survivors.

Take ammonoids, for example, the ancient shelled relatives of squid and octopuses.

They had been common and diverse in the waters of the Permian but experienced a bottleneck so severe that as few as 3 lineages survived across the Permian-Triassic boundary.

Ammonoid history is really a series of ups and downs, waxings and wanings of their diversity.

So there's several groups of Paleozoic ammonoids that all go extinct at the time of the Great Dying.

However, one group of ammonites, the Ceratitida, were doing something different apparently and were sort of on an upswing, and so they're the main group of ammonites that survives into the Triassic, and it's essentially animals like this then that are the progenitors of all the rest of the Mesozoic ammonoids.

Many other marine animal groups passed through similar diversity bottlenecks over the course of the extinction event, from invertebrates like reef-building sponges and corals to fishes In fact, for millions of years after the extinction, not a single reef has been found in the fossil record.

It's the biggest so-called reef gap in all of history and a clear sign of a world in ecological turmoil.

Gabriel: Above the water line, things were just as bad.

We see just a complete extinction of a lot of plants that were living at this time.

♪ Gabriel: Late Permian Pangea was already hot and dry in many places for part of the year at least, so the sudden temperature spike was enough to push much of the supercontinent into an almost unsurvivable hellscape.

Blistering temperatures, drought, and acid rain triggered an ecological collapse on land that took down the majority of all terrestrial species with it, and it began with the plants that formed the base of most Permian food chains.

Pangea's forests were hit by periods of acid rain that not only damaged their leaves and other tissues but also upset the chemical balance of the soil.

Meanwhile, the runaway greenhouse effect spiked temperatures by 10 degrees Celsius, pushing all species outside of the conditions they were adapted for.

Benjamin: A lot of the plants from the Carboniferous Period of time have a mechanism of reproduction that requires water, things like ferns and horsetails and those types of plants, so the loss of water, freshwater that they could live in really was devastating.

Gabriel: We know from studies of modern plants that at temperatures of around 35-40 degrees Celsius the heat stress is often enough for photosynthesis to slow down or even stop.

In Australia and China, increased charcoal in rock layers dating to the time of the extinction suggests that the heat, drought, and aridity triggered a spike in wildfires during the extinction, too, and for a while afterwards, we see the complete opposite.

There's no charcoal at all.

Benjamin: So it's crazy what's happening.

We have massive forest fires that are occurring because of the climate change.

After that, we had this period where there is no forests anymore.

The vegetation at that time was no higher than, like, your knee, so all the forests, all the trees were gone.

Gabriel: The wildfires that tore through forests during the Great Dying would have further sped up the runaway greenhouse effect by releasing even more carbon dioxide into the atmosphere, and the loss of so much vegetation meant that not only was there now more carbon dioxide being emitted, but there was also less capacity for natural carbon sinks to draw that CO2 back down.

It was a feedback loop that only made the Great Dying even greater.

Michelle: Beyond scorching temperatures, raging wildfires, and acid rain, there's also evidence that a much less visible, but no less devastating, factor may have led to the collapse of Permian forests, radiation.

See, volcanic eruptions also release gases known for their ability to deplete the planet's ozone layer, the protective shield against the worst of the sun's ultraviolet radiation, and the scale of the Siberian Traps eruptions would have released enough of these gases to severely weaken this shield, temporarily bathing the surface of the planet in a dangerously high level of UV exposure.

Benjamin: And that can be seen in some of these studies that have looked at pollen through this zone, and they noticed that there's a lot of mutations in the pollen.

It just doesn't look healthy, and that's been attributed to a lack of an ozone during this period.

Michelle: This was yet another mechanism that probably contributed to the near-total global deforestation at the end of the Permian, not by killing the trees outright, but by affecting them at the cellular and molecular level, putting a hold on their ability to reproduce, but at least one group seems to have flourished as the Permian forests collapsed--fungi.

A thin layer of extremely abundant fungal remains is widespread in geological formations that date to the mass extinction.

They're evidence of a fungal spike across Pangea as decomposers exploded to feast on the abundance of dead vegetation.

This fungal spike was relatively short-lived, just a few thousand years or so judging by the thickness of the layer in the fossil record.

But the weird thing-- and this is something that I think it took me a while to understand-- what we're starting to see is that after a while there's no food left for the decomposers to decompose.

Most of the time during an extinction event, it's what suddenly disappears from the fossil record that's most telling, but for the forests of the Permian, it's what suddenly shows up in the fossil record that actually tells the story of their dramatic decline-- spikes of charcoal, mutated pollen, and global fungal blooms.

♪ Blake: The collapse of the forests and increasing heat, drought, and aridity pulled the ecological rug out from under the feet of the Late Permian animal life.

The effects of the Permo-Triassic mass extinction, or the Great Dying, on land were really profound.

So as a starting point, you can say about 70% of terrestrial vertebrate species that we know of at the time went extinct.

So a very large die-off of animals.

Blake: As we've seen in earlier episodes, the Permian saw many land animal lineages rise from humble beginnings to some of the largest and most specialized animals that the terrestrial world had ever seen, but when disaster struck, these hard-won traits were actually a liability.

Smaller and more generalist species tend to do better than bigger and more specialized ones in chaotic times.

Pia: If you are dependent on a specific prey species or plant and that goes extinct, you're in trouble.

Blake: Populations of big herbivores from both the reptile and synapsid side of the family tree collapsed alongside their habitats as the vegetation they relied on withered, burned, and rotted away.

Take the lumbering armored pareiasaurs, for example.

They were a group of herbivorous reptiles that had been abundant in the Late Permian, finding enormous success munching their way through the plant life of Pangea.

There's research that colleagues and I have done actually modeling how disturbances to food chains can cause extinction, essentially that cascading process, and one of the things that we found is really the most effective way to kill off a really big swath of the community is to remove the plants.

start removing the plants, and everything else unravels.

On our proto-mammal side of the family tree, the picture was pretty similar-- bigger and more specialized species died out en masse, suddenly wiping out many lineages that had been at the top of their game like the dicynodonts, for example.

Kenneth Angielczyk: Dicynodonts in general are a very diverse group, particularly in the Permian period of Earth history.

So there is about 150 dicynodont species that have been named over the years, and they come in a really wide range of shapes and sizes.

So the smallest dicynodonts are things like this little guy.

This is a little skull of an animal called Emydorhinus.

It would be a roughly guinea pig-sized animal.

Wow.

And the very biggest dicynodonts get up to the size of a large rhino or a small elephant.

Blake: But dicynodonts were almost entirely erased as the vegetation they relied on disappeared, Kenneth: Only basically 4 lineages or kind of family groups of dicynodonts actually survive.

Blake: And while some of these strange cousins of ours survived for the remainder of the Triassic Period before eventually dying out too, they'd never recapture the level of diversity that they'd had in the Permian.

Large carnivorous therapsids were also hit hard during the Great Dying, like the saber-toothed gorgonopsians, who had been at the top of the Late Permian food chain.

Kenneth: Apex predators occupy in a way a very precarious position in their communities.

Because of the way energy transfer scales, as you move up the food chain, you need a very large base of primary producers like plants and smaller food sources to sort of supply smaller animals to support a small number of apex predators, and when things start going bad, all of that makes you vulnerable.

The complex, multidimensional ecosystems that had painstakingly evolved over the course of the Permian came crashing down, and it would take millions of years for the biodiversity of planet Earth to recover.

Kallie: The tree of life had been brutally pruned.

In total, over 80% of species were now extinct, and whole ancient branches were gone forever.

Life was down but not fully out because from the desperate survivors of the Great Dying would emerge some of the most successful animal dynasties of all time.

The dinosaurs would soon take the ecological spotlight in the new Mesozoic Era that had begun, and eventually, the mammals would follow.

See, also among the survivors was an obscure group of small generalist therapsids that reached up to around the size of a large meerkat called cynodonts.

Kenneth: They were a group that wasn't terribly diverse and relatively rare in the Permian, but they really don't show much drop-off in diversity as you go through that extinction event.

Really?

They just sort of go through and do their thing, and when they get to the Triassic and ecosystems are sort of reorganizing, they really start to diversify.

Blake: Cynodonts are the only group of Permian synapsids that have survived until today in the form of their direct descendants, us mammals, and the question of why our cynodont ancestors just happened to be among the small fraction of life that survived is hard to know for sure.

So they're relatively advanced animals for their time.

They have relatively complicated teeth that allows them to eat a variety of kinds of food.

They have a little bit larger brain, so probably a little more adaptable and behaviorally flexible.

A lot of the early cynodonts can construct burrows to live in and potentially hibernate to get out of bad conditions, so that kind of generalist ecology is a good way to survive extinctions.

If you can be sort of good at almost everything, no matter what nature throws at you, you're sort of prepared for it.

But perhaps dumb luck and sheer chance played a role, too.

While the worst catastrophe of all time set the stage for the rise of us mammals by taking out a lot of the competition, it would be a mistake to assume that things were just destined to work out that way.

Natural history is messy, full of unpredictable situations, butterfly effects, and changes of fortune that were far from inevitable and could have gone down very differently, so if the events of the End-Permian could be replayed, who knows if the Great Dying might have been the end of our story, too?

Blake: I know you're a scientist, but can I ask you to speculate with me?

I am dying to know what would-- maybe dying is too strong a word.

Those burrowing generalists were our direct ancestors, right?

What if things had gone differently for them?

How would the world be a different place now?

Would we be here?

Well, the answer's easy.

No.

We would've not seen the evolution of true mammals, and we would not be here.

♪ Gabriel: Sometimes, we have a tendency to separate ourselves from nature.

We forget that we're a part of Earth's history, that we are part of geologic history, and even though Salt Lake City is just a few minutes away, so many people may not realize that when they run past here that they're running over a marker of one of the worst events in our history, and admittedly, it is kind of unremarkable at first When you walk past.

It looks like every other rock that you may walk by or cycle by, but it reminds us that we could easily not be here and also just shows us that, again, life is so resilient.

♪ More than anything, though, this thin boundary is a reminder that while biodiversity painstakingly develops over vast stretches of geologic time, extinction takes just a blink of an evolutionary eye.

♪ "Eons" "Life and Death on Pangea" is available on Amazon Prime video ♪