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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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

We also see evidence

of dead zones where

rising temperatures just made parts

of the globe inhospitable

for all life, particularly at the equators.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

If you are dependent

on a specific prey species

or plant and that goes extinct,

you're in trouble.

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.

But dicynodonts were almost entirely erased

as the vegetation they relied on disappeared,

Only basically 4 lineages

or kind of family groups of dicynodonts

actually survive.

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.

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.

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.

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.

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?

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.

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.