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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.