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252 million years ago,

in the dying days of the Permian Period,

life on Earth was brought to its knees.

In the northern reaches of the supercontinent Pangea

in what's now Siberia,

colossal volcanic eruptions

burst through the planet's crust,

and they didn't stop erupting

for hundreds of thousands of years.

An area nearly the size of Western Europe

was blanketed in lava,

and the knock-on effects of these eruptions

baked the planet into a hellscape that,

for most of the species of the Permian,

was not survivable.

Blake de Pastino: On land, relentless heat, drought,

and acid rain collapsed whole ecosystems

and scorched the straggling survivors.

More than 70% of terrestrial species

were lost forever.

Life in the oceans was devastated, too,

as the water rapidly heated, acidified,

and was stripped of its oxygen.

Over 80% of all marine species went extinct

in an evolutionary blink of an eye.

Michelle Barboza-Ramirez: An entire ancient world

of diverse living creatures

was brought to a sudden and brutal end by this event,

a catastrophe known in the study of deep time

simply as the Great Dying.

The Great Dying was the single worst mass extinction

in the entire history of life on Earth,

a level of death that has no known equal

in the fossil record,

even more severe than the asteroid impact

that ended the reign of the dinosaurs.

It was perhaps the closest that complex life ever came

to being extinguished altogether.

Gabriel-Philip Santos: But what really made the Great Dying

at the end of the Permian such a disaster was that

in the Permian Period itself,

life had been thriving in ways

that it had never thrived before.

The Permian was a chapter when ancient life on Earth

conquered new realms for the first time,

faced and overcame new ecological challenges,

and pioneered entirely new ways of living.

These species rose and fell

long before the first blade of grass sprouted,

the first dinosaur hatched, or the first flower bloomed.

And the vastness of time that separates us

makes them seem both strange and familiar to our eyes.

Kallie Moore: But the story of their evolutionary ascent

and, in particular, of their catastrophic fall

is one of the most important and dramatic

in all of natural history--

a story about how a chain of geological, climatic,

and evolutionary events erased an ancient world

and created the modern one we know today.

And it's a story about us.

Our direct ancestors were shaped by the Permian

and somehow survived

the disastrous way that it ended,

when few other species could.

But to understand the full story

of the time that almost everything died,

we'd have to start tens of millions of years earlier,

before the Permian Period even began,

in an ancient swampy forest here,

in what's now Joggins, Nova Scotia,

at a time when planet Earth

would have looked... oddly familiar.

The history

of complex life on Earth

is split into three main eras,

each composed of multiple periods of time.

Currently, we're in the Cenozoic Era,

or the era of recent life,

the heyday of flowering plants, birds, and mammals.

In fact, you might also sometimes hear

the Cenozoic Era simply called the "age of mammals."

Before that was the Mesozoic, or life's middle era,

famous for its dinosaurs,

and so sometimes called the "age of reptiles."

And even further back was the Paleozoic,

the era of ancient life,

when species were far older and stranger

and rarely got the attention they deserve.

Now, the final chapter of the Paleozoic

was the Permian Period.

It was brought to a close by the Great Dying,

and it began with the formation

of the supercontinent Pangea.

We'll get back to that a little later,

but to set the scene,

let's rewind just a bit further to the end of the period,

just before the Permian,

the Carboniferous Period.

While the Paleozoic seems so far in the distant past

that Earth would barely be recognizable,

at first glance,

the resemblance would actually be

pretty uncanny at times.

Because the way our planet looks now,

blue, green, and white

is actually pretty rare for Earth.

And the last time it had that classic Earth look

was during the Late Paleozoic Ice Age,

when the planet had glacial ice covering large areas of land,

as it does now.

And for most of that ice age,

the world even had distinct continents,

like it does today.

The supercontinent of Pangea,

on which the story of the Permian would play out,

had yet to fully form,

though it would soon enough.

It's hard to imagine our world looking any different,

but this combination of features is an unusual occurrence

from a deep time perspective,

and a connection we share

with those ancient days of the planet's history.

For the last few days, we've been here in Boston

at the Museum of Comparative Zoology in...

Harvard.

Sorry, Boston.

It's been really fun,

because I've been able to go into the collection

and see some really amazing fossils.

It was this museum kid's dream come true to be in there.

Yeah. From the outside, it's like,

"Oh, that's just another rock."

And then someone thought,

"Oh, let me crack this thing open.

Oh, I wonder if there's anything inside."

And lo and behold,

an early tetrapod from the Late Carboniferous.

At the peak of this ancient ice age,

near the end of the Carboniferous Period,

the world didn't just look geologically familiar,

it had started to look ecologically familiar, too.

Christina Byrd: What we're looking at here

are some plant fossils from the Carboniferous.

In particular, we're finding a lot of ferns.

See, while the world was in a rare deep freeze

with huge glaciers covering much of the southern hemisphere,

the area around the equator was covered

in lush, humid tropical swamps and rainforests,

teeming with complex life.

In earlier periods of the Paleozoic,

complex life had been found only in the oceans,

where it had originally evolved.

But eventually, plants and invertebrates

colonized the land,

and by the Carboniferous Period,

we vertebrates had joined them, too.

Thanks to the head start that invertebrates got over us

in moving onto land, though,

the Carboniferous is often remembered

as the golden age of giant bugs,

like Arthropleura, for example,

a nearly 2.5-meter long, 50-kilogram millipede relative

that had few, if any, competitors or predators

to worry about in the swamps of the Carboniferous.

And ruling the skies was Meganeura,

a pigeon-sized dragonfly relative,

and one of the largest known insects to ever fly.

The vertebrate invasion of land

was still only in its earliest stages,

and we couldn't yet challenge the dominance of the giant bugs.

But our arrival meant

that the swampy equatorial wetlands of the Carboniferous

were among the earliest ecosystems on land

to broadly resemble the ones we have today,

with those three groups--

plants, invertebrates, and vertebrates--

all coexisting and interacting

as they have ever since.

Basically, back in the Carboniferous

with things moving onto land and evolving onto land,

we're seeing the start

of what today's rainforests started as

and what they will eventually become.

But these ecosystems had some big differences, too,

because during the ancient ice age,

life on land was still finding its feet.

So, where exactly are we?

We're at the Joggins Fossil Cliffs.

It's a UNESCO World Heritage Site.

And it's 15 kilometers of beach exposure that's

the Late Carboniferous or Pennsylvanian.

And it's the best Carboniferous exposure

in the world.

I've wanted to go to Joggins

for a very long time,

because I really just love the Carboniferous.

When I was a kid,

the very first fossils I collected

were Carboniferous fossils.

I even collected little, tiny Crinoid stems

from the playground gravel when I was in grade school.

And I still have them.

When we first pulled up for the very first shoot here,

I was, like, shaking the back of the car,

and I was just like, "Let me out, let me go see it."

You see, the stars

of the Carboniferous fossil record are its plants,

fossils that fuel the modern world

as we know it today.

So, if we were able to go back 300 million years ago,

what would it look like?

300 million years ago,

Nova Scotia was located at the equator,

so it was a tropical rainforest

with a lot of rivers flowing through.

So, the best environment to think of today would be

the Amazon rainforest.

The swamps were dominated

by towering club mosses,

relatives of horsetails and ferns,

all of which reproduced by using spores

that needed to find their way to moisture

to become fertilized and grow.

Spores were an ancient feature in these plants.

They'd been using them

since their small and spongy ancestors

first colonized the land.

And by the Carboniferous, they'd grown and diversified

to create complex rainforests,

which included giants like Lepidodendron

that had evolved into a towering tree-like form

over 50 meters tall.

Oh, my gosh, look. It's quite big.

Holy smokes.

And it's in that thick sandstone layer there.

And then if we move over, there's a second stump.

There's another one!

These are visible in the cliffs

for about three to four years,

and then they weather out.

And because it's a forest,

new stumps will be visible in different parts.

Seeing the Lepidodendron tree stumps in the cliffs

was pretty amazing.

We walked for quite some time,

and everybody was kind of like,

"Are we actually going to see any?"

And then all of a sudden,

Dr. Jade Atkins was like, "Oh, look. There they are."

And everybody in the crew-- everybody was like

"There they are, there they are."

Knowing that these trees

would have been flooded and buried

305 million years ago or so,

and here they are just eroding out of the coast today

is really special.

So, we found a really cool fossil on the beach here.

Can you tell us what it is?

So, this would be part of a stump

of one of those scale trees,

so Lepidodendron or Sigillaria.

Cool.

Each hole here would be a secondary branch in life.

The carbon dioxide drawn down from the atmosphere

and incorporated into

these photosynthesizing swamp plants

is what gives the Carboniferous its name,

the coal-bearing age.

No, you can still see plant in it.

Yes. Yes, in most of it, you can still see plant.

As they died, they sank into the murky depths

generation after generation,

where the conditions kept them from decomposing.

And as the glaciers of the Late Paleozoic Ice Age

waxed and waned,

these coastal swamps and their drowned plant material

were regularly submerged,

when the melting ice raised sea levels.

This plant material then formed peat that was buried,

and after enough time,

heat, and pressure, eventually transformed into coal.

The same vast coal deposits

that we would eventually find, dig up, and burn

to power the world

were created here in the swamps of the Carboniferous Period

over 300 million years ago.

So, what was life like when the coal layer

was first being laid down?

So, this was a really lovely time

for animals that might breathe through their skin.

So, amphibians, but also insects

were having a great time at this time.

Everybody was getting very big,

and it was just a really moist rainforest environment.

That allows amphibians to be more widespread,

because they are reliant on water

to lay their gelatinous eggs.

They cannot lay them in a dry environment.

One group of amphibious creatures

called temnospondyls

was especially widespread and abundant in these swamps.

Over the course of the Carboniferous,

they flourished and diversified

into a range of different sizes and ecological niches.

Some early temnospondyls were small and salamander-like,

hunters of insects and other tiny prey

in the forest undergrowth,

like Dendrerpeton or "Tree-creeper," for example.

Dendrerpeton is one of the oldest members

of a lineage that probably contains today's amphibians.

So, the frogs, salamanders, and caecilians that we know,

this is one of their oldest known relatives, probably.

Dendrerpeton is named for the way its fossils

have often been found,

preserved in the hollowed-out stumps

of swamp trees that individuals crawled into

only to become trapped and buried.

Others, like the big and powerful Eryops

from the very end of the Carboniferous,

were more crocodile-like.

What is that?

This is Eryops megacephalus.

Now, Eryops first evolved in the Late Carboniferous,

but it didn't really get to this kind of size

until the Permian.

The lush swamps of the Carboniferous were

ideal for amphibians,

and in their damp, waterlogged paradise,

they thrived.

Some even began spending so much time on land

that they became almost fully terrestrial,

reliant on watery habitats

for only one last big part of life--

reproduction.

But among both the plants and vertebrates

of the Late Carboniferous,

a revolution was underway.

New groups had arisen

that could break their ties to water entirely.

I have spent my entire adult life

in the American West,

which I love.

And that means I've spent a lot of time

hiking in the woods among pine trees

and spruce and fir.

And over that time, I've become enamored

with a particular tree, the Ponderosa pine.

Part of what I love about them is the scent that they produce

a certain time of year.

I'll be out on a hike,

and the woods smell like butterscotch and vanilla.

And whenever I smell that and I'm on a hike,

I know that spring is coming.

Alongside the ferns, horsetail relatives

and club mosses of the swamps were another group of plants

known scientifically as gymnosperms.

Rather than reproducing like those other plants did

via spores, which are tiny naked reproductive cells

exposed to the world,

they had evolved seeds.

Today, I'm at the Montana Conservation Seedling Nursery,

and I met a fellow named Benjamin Jones,

whose title is Grower of Seeds.

And his job is to turn these little, tiny seeds

into giant Ponderosa pine trees.

Benjamin Jones: At the nursery, we grow

about a million seedlings a year

for anything from wildfire rehabilitation

to just increasing wildlife habitat.

And you've been working here how long?

18 years.

So, you personally are responsible

for growing 18 million trees, give or take.

I'll take partial credit.

That's amazing.

You must be very proud. Yeah.

So I understand you're going to put me to work today.

What are we going to be doing?

We're going to sow Ponderosa pine

into a styro block using a shutter box.

Okay, while I'm doing this, explain to me,

why are seeds so important?

Why are they so successful?

Seeds are sort of an all-inclusive package.

They contain their own nutrients,

they have their own moisture content,

and they will only germinate

when conditions are ideal.

Having seeds had helped early seed plants

gain an increasing foothold in parts of the rainforests

over the course of the Carboniferous.

It allowed them to tolerate

the occasional seasonal dry spell,

while spore-producing plants struggled to reproduce.

And by the Late Carboniferous,

some of the gymnosperms were beginning to look

kind of like the ones we have today.

The earliest conifer trees had evolved by this time,

with many of the same traits as today's species.

They were woody, with needle-like leaves

that retain water,

and cones containing seeds and pollen

that rely less on water for reproduction.

These adaptations helped primitive conifers

and other gymnosperms thrive

in drier areas of forest, where they had an advantage.

But plants were not the only ones

that had stumbled on this same general solution

for surviving in drier environments.

Around that same time,

vertebrates had given rise

to a new group that evolved a similar trick of their own--

the amniotes, with their shelled eggs.

Unlike the naked, gelatinous eggs

of their amphibious ancestors and cousins

that had to hatch in an aquatic environment,

these amniotic eggs provided

a self-contained aquatic environment internally.

The embryo was encased in an outer shell

and an amniotic membrane

that protected the developing young,

just like the seeds of gymnosperms.

When this trait first arose,

it was just a weird reproductive ability,

albeit a useful one in areas of forests

that seasonally dried out.

Yet this bizarre and unusual trait

would go down in deep time

as one of the single most game-changing

evolutionary innovations ever,

because it was the final push needed

for vertebrates to become fully terrestrial.

And it gave rise to one of the greatest animal dynasties

the world ever saw--

the amniotes.

So, amniotes lay eggs. Mm-hmm.

What makes these eggs such a big deal

in the grand story of life?

Eggs were basically

a key innovation of amniotes,

and it allowed amniotes

to break their ties with water,

so that they could reproduce on land

and not have to rely on going back to the water,

like amphibians do.

All living and extinct reptiles, birds, and mammals

are amniotes.

And even though we mammals mostly don't lay eggs anymore,

we've essentially internalized our amniotic egg,

losing the shell but keeping the amniotic membrane

that surrounds the offspring inside the mother,

though this extension of the process wouldn't develop

for more than another 100 million years.

These earliest amniotes in the Carboniferous are

who every vertebrate that isn't a fish or an amphibian--

from humans to hummingbirds to humpback whales--

can trace our ancestry to.

The rise of eggs and seeds played out around the same time

in similar places, and for similar reasons, too.

But they also have one more thing in common.

They've both left lasting legacies

on the world ever since.

So, you would say, like, the egg is what led

to the success of, like, amniotes through time?

I would definitely say the egg is

one of the key things

that allowed amniotes to become so successful.

And the egg definitely came before the chicken, then?

Well, that's still up for debate.

And among one lineage of amniotes,

an event was about to take place

that seemed subtle at first

but had consequences that still define

the modern world today--

one ancestral population branched into two.

We don't know exactly how it happened.

Perhaps one group was cut off from another

by an environmental barrier,,

like a newly-formed river,

setting them on different evolutionary journeys.

Or maybe one group changed their behavior,

becoming active at a different time of day,

a different level of the forest canopy,

or hunting a different type of insect prey,

eventually drifting apart

from the other group ecologically.

But we do know

that the early species from each side

still looked pretty much identical at first glance.

Yet from one side of the family split

would come all reptiles--

from crocodiles to turtles to snakes to dinosaurs,

including birds.

And from the other side would eventually come all mammals--

from manatees to mammoths to bats and wombats to us.

During the Late Carboniferous,

it was really hard to tell the difference

between these two groups,

and it wasn't until later on

in their evolutionary history where we see

the defining traits of these two groups

starting to evolve.

Think of all the iconic differences

between both sides of the family tree today--

our skin, our teeth, our behaviors, our diets.

At this point, none of these differences exist yet.

The only reason we can tell them apart

is because their fossil skulls

have some subtle differences in structure

that had begun to distinguish the two sides

from one another.

As we'll see over the course of the Permian to come,

the changes in sauropsid and synapsid anatomy

will eventually become clear as they

continue on their parallel evolutionary journeys.

So, by this time,

egg-laying amniotes and seed plants,

including gymnosperms,

had independently developed similar solutions

to the same problem in the same environments,

reproducing during times when land became dry land.

And while they were only ecological side characters

compared to their more water-reliant cousins

in the Carboniferous swamps,

that was all about to change.

As the Carboniferous drew to a close,

those oddball seed- and egg-producing species

found themselves in the right place

at the right time,

and with the right traits

to rise to prominence as the Permian approached.

This opportunity came

in the form of a sudden ecological shift

around 305 million years ago,

an event known

as the Carboniferous Rainforest Collapse.

Just before the Carboniferous Period

came to a close, the planet started to dry out and warm up,

which reshaped ecosystems on land.

The big swampy rainforests of the equator

began to break up into isolated pockets,

giving way to drier and more open environments.

Forests of conifers and other seed plants

began to spread, replacing the wet swamps

that had been so prevalent before.

And big forests of seed plants, like conifers,

have stuck around in much the same way ever since.

These forests provide habitats for countless species

and represent one of the planet's major stores of carbon.

It's hard to imagine the world without them,

but they might have stayed on the ecological margins

if the Late Carboniferous had played out

just a little differently.

Instead, they became a familiar feature of the modern world

that we can trace directly back

to a 300-million-year-old climate shift

in the era of ancient life.

And we, too, can trace the story

of our rise to the same climate shift,

because it was also the moment

that the two lineages of amniotes

found themselves perfectly placed

to move into the new drier niches

that were now available.

So, as we start to move into the Permian,

things start to get drier.

So, we start to move into an environment

where amniotes--

so animals that have the water with them in their egg--

start to thrive,

because they're not tied to water

the way the amphibians are.

Those animals actually had an opportunity to grow

and diversify into these new niches

that were opening

because the old systems were starting to fail

with this change in climate.

And this led to a rapid diversification of amniotes

into all sorts of shapes and sizes during the Permian.

And the Carboniferous Rainforest Collapse was

just part of a longer-term trend in the Late Paleozoic climate

that would suit both these groups.

The world was heating up,

and the ice age was beginning to end.

Glaciers started to recede from the higher latitudes

as the deep freeze ended,

allowing species to spread from the fragments of the swamps

into these new regions reclaimed from under the ice.

The Carboniferous had ended,

and the Permian had begun hot and dry,

and it would only become more so over time.

And it wasn't the only planetary-scale trend

in the Early Permian shaping the evolution of life,

and, quite literally, shaping the world itself.

The continents we know today

had gradually been fused into one by plate tectonics--

the movement of chunks of the Earth's crust and mantle

over geologic time.

By the Early Permian, these forces had created

a single huge, continuous landmass--Pangea--

which literally means "all the lands."

Pangea had been hundreds of millions of years

in the making,

and on it would play out the final chapter

of the Paleozoic Era.

A chapter that would begin with bands of ancient wanderers

leaving the shrinking swamps of the equator

to venture into the new habitats that Pangea had to offer--

ecological opportunities beyond count,

open to any species

adventurous and hardy enough to survive the journey.

But Pangea brought with it new challenges for life, too.

The ancient world's weather and climate

entered a new phase--

one marked by intense seasonal extremes.

It's fair to say the way the Permian Period began--

with the collapse of the rainforest,

the end of an ice age, and the formation of Pangea--

was pretty chaotic.

But as we'll see,

it's nothing compared to the way the Permian will conclude,

with the Great Dying

and the end of the Paleozoic world.