All language subtitles for Nova.S53E04.Mammal.Origins.2026.1080p.AMZN.WEB-DL.DDP2.0.H.264-RAWR_track3_[eng]

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Mammals.

They live in almost every corner
of the planet.

Thousands of different species.

From tiny desert creatures

to giant ocean beasts

to us.

But when did our shared history
begin?

You might think
the start of the mammal story

was the extinction
of the dinosaurs

some 66 million years ago...

...but the story actually begins
much, much earlier...

We have the very earliest
ancestors of mammals.

Those animals
are four times as old

as the oldest T. rex.

...in a time when mysterious
animals ruled.

They looked very different
from anything alive today.

In some ways,

they looked like reptiles.

But it's from here
that mammals took shape...

They were our ancestors.

...surviving a tumultuous period

in Earth's history.

It became apparent

that there were
two extinction events.

Our ancestors made it through

for very specific reasons.

We had very specific adaptations
as a group,

and that allowed us to make it
past these extinction events.

What was the secret
to our ancestors' survival

in the face
of deadly catastrophes?

"Mammal Origins."

Right now, on "NOVA."

Mammals are

spectacularly diverse.

They not only survive,

but thrive
in all kinds of environments.

Mammals have basically
taken over the world.

There's mammals
all over the land.

There's mammals that fly
in the air.

There's mammals
that swim in the oceans.

They range in size

from tiny elephant shrews

to gargantuan 200-ton whales.

Yet all mammal species--
more than 6,000 of them--

share many
of the same basic traits.

They have hair or fur

and four chambers
in their hearts.

Mammals produce milk
for their young.

And they are all warm-blooded,

a trait that has long puzzled
scientists.

The creatures
that mammals evolved from

were cold-blooded,

and so it's a really key thing

in the history of mammals,

figuring out
how and when and why

our ancestors
changed their metabolism

and started warming up
their bodies.

So, where does
the mammal story begin?

Where do we come from?

Rewind the clock
270 million years,

to a time
even before the dinosaurs.

Back then, our planet was home
not only

to the ancestors of the
dinosaurs and other reptiles,

but to another group,
also now long gone.

Among them, creatures that would
one day give rise to mammals:

the therapsids.

Paleontologists have
known about them for decades,

but in 2023,

some of the oldest known remains
of these enigmatic animals

were discovered in South Africa.

This skull
is 266 million years old,

which is really interesting,

because it's one of
the oldest fossils we have.

The snout is here,
in this direction.

You can also clearly see
its teeth.

This creature would have
been the size of a large dog,

but with sprawling limbs
like a reptile

and a cold-blooded metabolism.

So what is it
that has scientists convinced

it's not a reptile?

Here is the eye.

And here, behind the eye,

there's a pit called
the temporal fossa,

which is an attachment point

for the muscle
that closes the jaw.

It's a defining characteristic
of therapsids.

Dinosaurs, crocodiles,
and many reptiles

have two of these pits,
called temporal fossae,

on each side of their skulls.

But therapsids have
just one pit behind each eye.

It's the place
where the jaw muscles

that control biting and chewing
are anchored.

And the only other group
that has one pit behind each eye

is our group: mammals.

This anatomical similarity
was one of the first clues

that led scientists to suspect

that mammals and therapsids

are part
of the same evolutionary line.

But that's not
the only evidence.

BENOÎT
You can see another
characteristic

that distinguishes therapsids
from other animals,

and that's their teeth.

Therapsids have
specialized teeth,

incisors for nipping

and back teeth
known as "cheek teeth."

The two groups of teeth

are separated
by rather impressive canines.

Just like mammals today,

including us humans.

These shared features
helped convince scientists

that therapsids
gave rise to mammals.

BENOÎT
However,

if you came across
any of them here today,

you wouldn't realize

that they were
the ancestors of mammals,

because they looked a lot
like reptiles.

Some were as big as rhinoceroses

and others as small as mice.

Some were carnivores,
others herbivores.

There wasn't just one type
of therapsid, but lots.

In fact, six main groups of
therapsids roamed ancient Earth,

and each group included
many different species,

including the 2023 fossil,

which may be from a group
called the biarmosuchians,

the most ancient and primitive
therapsids found so far.

Many of the other groups
have more specialized traits,

like dinocephalians,

recognizable by their heads--

thick skulls with bumps.

And dicynodonts,

which are distinguished
by a horny beak and tusks.

Gorgonopsians are characterized
by their oversized canines,

some even larger
than the teeth of a T. Rex.

Therocephalians
had smaller jaws and teeth,

but were more upright and agile.

Finally, the cynodonts,

many of which were small,
burrowing creatures.

Altogether,
there were perhaps hundreds,

if not thousands
of therapsid species

among these
groups.

They were
hugely varied

in their
appearance and size,

much like mammals
are today.

One group
would lead

to the emergence
of mammals on Earth

225 million
years ago,

eventually evolving
and then likely passing down

the most iconic mammal features:

hair,

warm blood,

and lactation.

But which group was it?

Therapsid fossils have
been found on every continent,

but one part of South Africa
is especially rich in clues.

The Karoo is one of the most

important fossil sites
in the world.

90% of fossils
found in these rocks

are therapsids,
the ancestors of mammals.

Many thousands of fossils

have been discovered
in the Karoo,

and every field trip delivers
more fossils and new species.

This has been the case

since the first therapsid fossil
was found here

in the 1830s.

In the 1900s, during apartheid,

access to dig sites was mostly
limited to white scientists.

But today,

the government
has opened up access

and is supporting new projects.

Now discoveries are on the rise.

But why have so many therapsid
fossils been found here?

What was this place like

hundreds of millions
of years ago

that allowed therapsids
not only to thrive,

but to be preserved as stone?

Well, over here,
you can see these rocks

and these ball structures,

they form underwater.

And accompanying
with these things,

we find fossilized fishes

and also fossilized wood,

which didn't grow here,

but it was transported in
by the rivers.

We also find
lots of ripple marks.

Fossil evidence suggests
a watery past.

But to fully understand

what the Karoo of the
therapsids' time looked like,

we need to go back
300 million years,

to a time when all
of today's continents

existed as one landmass:

Pangaea.

What was to become South Africa

was much closer
to the South Pole

and under an ice cap.

By 265 million years ago,

the Earth was warming up.

The ice that covered the Karoo
had melted,

forming an inland sea.

Over the next few million years,

the Karoo Sea
filled with sediments.

It nourished lush vegetation
that grew in the new wetlands.

This was the environment

where therapsids were first
preserved in the Karoo.

The remains of the animals
that we are finding there,

the fossils,

were once living on the banks
of these rivers.

But what was life like
for these ancient therapsids?

Were any mammal-like traits
beginning to appear?

One of the most exciting
discoveries

was made in the mid-1980s

by South African fossil
collector John Nyaphuli.

John was my preparator

at the National Museum in
Bloemfontein, where I worked.

He and I spent many, many years

out living in a tent,

collecting fossils.

It's not easy to find fossil.

But your eyes must be trained.

You can't find fossil anywhere.

You must go to the right areas,

because geology
must first tell you

that there was,
there's good place to trace.

And you must love the thing,
and then you can find them.

In 1984, he said to me,

"I found some big bone
in the rock."

So I went to look at it,
and it was big bone.

It was sort of this size.

John started opening it up,

and we found that
by the middle of the day,

we had a complete skull
of an animal.

There were some teeth
sticking out.

It went on into the ground,
and for several months,

we started excavating it.

It took 23 years to expose
that skeleton fully.

But it was a completely
new species of animal,

it had never been found,

and it was a very
primitive dinocephalian.

This is the most complete
dinocephalian skeleton

that's yet been found
in the world.

Dinocephalians were a group
of early therapsids.

This remarkable specimen
was more than nine feet long.

The name "dinocephalian"

comes from the Greek
for "terrible heads,"

because they had
especially thick skulls--

in some areas, more than
four inches of solid bone.

But what was all that extra
skull thickness used for?

In the French Alps,
scientists are looking for clues

with a powerful X-ray source
called a synchrotron.

Here, in a ring larger
than seven soccer fields,

electrons circulate
at almost the speed of light.

VINCENT FERNANDEZ
The synchrotron radiation
is so intense

that it produces 10,000 billion
times more intense X-rays

than a hospital
or laboratory scanner.

With this very intense signal,
we're able to see

through the thickness of the
fossil

and produce beautiful images.

Paleontologist Vincent Fernandez

is scanning another
dinocephalian skull

found in the Karoo.

His colleague, paleontologist
Julien Benoît,

is doing the analysis.

BENOÎT:
Here you can see,
that's the brain cavity.

And it's very small compared
to the large size of the skull,

and in particular, you have
that thickness of bone.

This is all bone.
Wow.

This is not rock.

Why is the bone so thick here?

Back at the University
of the Witwatersrand,

Julien uses
the synchrotron scans

to reconstruct a 3-D model

of the organs that were
once inside the skull.

You can see the brain in green.

What's really interesting
about this scan

is that we can also
very clearly see

this animal's inner ear.

It's shown here in purple.

This organ is used for hearing,

but also for balance,

particularly these
three semicircular canals.

One here, a second here,

and the third.

Let me move the image--
it's here.

The orientation of these
ear canals in the skull

suggests
that this dinocephalian's head

would usually point down,

a bit like that of a goat
or a sheep today.

And if mammals like these
are any guide,

that might mean that
the dinocephalians also engaged

in social behaviors
like ramming.

As the sun rose
on ancestral Earth,

dinocephalians awoke.

Living in a swampy landscape
where food was abundant,

they thrived,

great herds of them
perhaps living together

and interacting as a group.

With behaviors similar
to some of today's mammals,

could these dinocephalians

have been the mammal ancestors?

Fossils found in the Karoo
reveal

that 265 million years ago--

nearly 40 million years before

the appearance of mammals--

five therapsid groups

were living here.

And the dinocephalians

were likely the most dominant.

This is the skull
of Anteosaurus,

a particularly menacing type
of dinocephalian.

BENOÎT:
The top of the snout is there,

and the back of the skull
is there.

And this was a therapsid,

because you can see
this temporal fenestra here.

It's very big,

so the jaw muscle must have
been incredibly powerful.

And it is a dinocephalian

because of the thickness
of the bones

that make up
the whole skull.

But it's also very different

from the other species
of thick-skulled dinocephalians

Julien studies.

BENOÎT:
That canine here
is beautifully recurved,

which is really a signature
of carnivorous animals.

These Anteosaurus
were large creatures,

likely predators,

some 20 feet long
when fully grown.

But as successful
as they might have been,

the dinocephalians weren't
the only therapsid group.

They were just one part
of a complex ecosystem

that included
other therapsid groups.

Based on fossils
found in the Karoo,

we know that many species

from all five groups
of therapsids

existed alongside each other.

Like dicynodonts.

Equipped with unique beaks,

these therapsids had their own
specialized feeding habits,

able to efficiently tear
and clip plant material.

Carnivorous therapsids,

such as gorgonopsians,
would have been a threat.

But the most fearsome
was still the Anteosaurus.

This dinocephalian beast
was a true apex predator

in the ancient
Middle Permian Karoo.

Therapsids lived here
about 265 million

to 125 million years ago.

Within that time, they came up

against more than one
global catastrophe

that threatened to wipe out
nearly all life on Earth.

And today, the Karoo
still bears the scars.

For more than 15 years,

paleontologist Mike Day
and his team have been

accumulating evidence of
these large-scale extinctions.

If you were to go
a little further south,

into slightly older rocks,

you'd find all those
typical Middle Permian faunas:

the large dinocephalians,

the big therocephalians,
and lots of dicynodonts.

But by the time you get to
this sandstone behind us here,

they've all gone extinct.

Victims of a mass extinction
event

called the Permian Extinction.

Fossil evidence suggests

that for as long
as eight million years,

life on Earth was
under tremendous pressure.

And as more and more fossils
have been found

and plotted
along the rock record,

scientists now believe

that within
these eight million years,

there were actually
two distinct extinction events.

Now, both of these
mass extinctions

affected therapsids badly,

but there were survivors.

And those survivors include
the ancestors of mammals.

But what caused the first
of these huge events,

a global catastrophe known
as the Capitanian crisis?

In terms of a smoking gun
here in the Karoo,

we haven't really found
any good evidence

for what caused
this mass extinction here.

We do know
that in Southeast China,

there was huge outpourings
of lava

at about the same time,

so it's very likely
that that had something to do

with the Capitanian extinction
here in the Karoo.

Around 260 million years ago,

far away from the Karoo,

at the other end of Pangaea,

Earth's surface was volatile.

In what was to become China,
immense molten lava flows

spread across
around 100,000 square miles,

releasing vast quantities

of carbon dioxide, methane,
and sulfur

into the atmosphere.

The oceans became acidified
and depleted of oxygen,

and the climate changed.

This is most likely the event

that caused
the Capitanian crisis.

It was disastrous for life,

particularly for large animals.

But we know that at least

some therapsids survived.

Otherwise, we wouldn't be here.

But which ones?

And how?

During the crisis,

environmental disturbances
like acid rain

and a rapidly changing climate

likely ravaged the vegetation,

causing the extinction

of the large plant-eating
dinocephalians.

Their carnivorous cousins

eventually suffered
the same fate.

Earth's first giants,
the dinocephalians,

did not survive to become
our direct ancestors.

But what about
the other four groups

alive in the Karoo at this time,

the gorgonopsians,
with their huge teeth,

the upright and agile
therocephalians,

the beaked dicynodonts,

and the primitive
biarmosuchians?

Did the rest of them make it?

The Karoo fossil record
reveals the survivors.

This is the Rubidge Collection
of Fossils.

It was amassed largely through
the enthusiasm

of my grandfather from
the 1930s.

These fossils were collected
by Sidney Rubidge

from sediment
above a volcanic ash layer

dated to the end
of the Capitanian crisis.

They show that in the Karoo,

all therapsid groups other
than the dinocephalians made it,

but only some of
the smallest species of each.

The largest were lost.

The question then becomes,
which of these remaining groups

would give rise
to the mammal line?

One of the most
interesting fossils

in the Rubidge Collection

is a member of
the gorgonopsian group.

Gorgonopsians were the
apex predators of the time.

With the dinocephalians gone,

they became
top of the food chain,

and the new species that evolved
were much larger in size.

And this one is called
Rubidgea atrox.

These large gorgonopsians

might have been
three-and-a-half meters long.

They probably had
a more leathery skin,

something like a rhino
or an elephant.

They're characterized by
having very large canine teeth

and large incisors
for flesh eating.

Only small species
of gorgonopsians

survived the Capitanian crisis,

but the Rubidge Collection shows

that as the ecosystem recovered
and food returned,

these small gorgonopsians
not only survived, but thrived.

Over time, they evolved to be
the size of modern-day bears.

And in the fossil record,
there is evidence

that some new traits
might have been developing.

A gorgonopsian fossil
originally found in the 1940s

recently provided new clues

to how they might have lived.

Inside this skull
are the remains

of an encounter
with another animal.

BENOÎT
Here is a tooth stuck
in its snout,

and this tooth
isn't just any tooth,

it's the tooth
of another gorgonopsian.

The same species.

And you can even see

that it survived the fight,

because the wound
around the tooth has healed.

For Julien,

this is a sign of how these
animals may have interacted.

These animals fought
among themselves,

biting each other on the snout,

exactly as mammals do today

to determine dominance
within groups

and also to find mates
and claim territories.

Until this discovery,

the teeth
of carnivorous therapsids

had been considered
solely hunting weapons.

And although not direct evidence
of mammal ancestry,

using them
for more social interactions,

like competition,

is certainly
a characteristic seen

in many mammals alive today.

But the gorgonopsians

weren't the only ones
developing new behaviors.

The dicynodonts were adapting

in the post-extinction world,
as well.

Perhaps the most varied
of the therapsids,

they came in a wide range
of sizes,

some as small as guinea pigs.

The only known therapsid group
with beaks,

some also had tusks.

What was it about the dicynodont
group that helped it survive?

A fossil in South Africa
might hold the answer.

Here we see a new behavior
in the late Permian,

where animals were starting
to burrow underground

and create these
really interesting spirals

with a chamber underneath,
where they would live.

We can see it here because it's
actually preserved as a cast.

This cast is natural.

A flood poured sediment
into a hollow burrow,

which hardened into rock
over time.

But what were these burrows
used for?

How were they an advantage?

To see
through this mass of rock,

a dicynodont burrow cast

was sent to the synchrotron.

And hidden inside,

fossilized remains.

This is the burrow that was
discovered

with the dicynodont.

Removing the rock reveals
a spectacular find.

We have the adult dicynodont.

We can see, like,

right in the middle,
this big eye socket

and teeth in front.

The beak would be there,
as well.

That's not all
this ancient burrow was hiding.

There was another,
more unexpected find.

It's like it's a,

it's a baby version
of the same species.

So basically,
it's telling us

that dicynodont,
they were using burrows

kind of as nurseries
to take care of their babies.

It appears that a semblance

of today's mammalian
parent-child bond

may have existed
260 million years ago,

but it's an idea that scientists
are still exploring.

What we do know
is that these burrows

allowed dicynodonts to survive
tough post-crisis conditions.

That kind of group sharing
behavior

has a lot
of different advantages.

If you're in a burrow,
you're protected from predators.

You're protected from the sun.

It's, it's pretty good to be
in a burrow

if it's hot outside--
hot and dry.

But how well did the other
therapsid groups fare?

After the Capitanian crisis,

not all therapsid groups
had the same fate.

The dinocephalians
disappeared completely.

The gorgonopsians emerged
as the apex predators.

As for the therocephalians,

they took second place
to their larger cousins.

The biarmosuchians

were still
holding on,

but also remained
the most primitive.

It was
the dicynodonts

who adapted best

to the changing
environments.

But as some groups
struggled,

the picture grew
more complicated.

A new group
emerged

in a post-crisis therapsid boom,

taking the number
of existing therapsid groups

back up to five:

the cynodonts.

They shared a common ancestor
with the therocephalians,

and it's possible that they were

actually just a sub-group
of therocephalians themselves.

But one thing
is certain:

one of
the secrets

to their success

was their size.

Cynodonts generally were
quite small.

You could hold most of them
in your arms.

They also had relatively
larger brains

and more specialized teeth.

With the Karoo ecosystem
having recovered

from the Capitanian crisis,

the five groups of therapsids
stabilized.

Cynodonts thrived, feeding off
insects and other small animals.

Meanwhile, dicynodonts continued
with their plant-eating diets

and still used burrows
as their homes.

But this wasn't
always an advantage.

Dicynodonts were also the prey
of choice for therocephalians.

These larger predators
were perhaps able to hunt down

entire dicynodont families.

Even so, gorgonopsians remained

the apex predators.

But, at the end
of the Permian period,

another crisis.

In the rock layers of the Karoo
dating to the period

after 252 million years ago,

most big dicynodonts
and gorgonopsians disappear,

signaling
another extinction crisis.

Was it something local
to the Karoo

or more global?

Since the 1990s,

scientists have suspected
that the Siberian Traps,

enormous volcanic eruptions
in what is today's Russia,

were responsible.

In 2023, an international team
of scientists

published a project
testing the idea.

When scientists studied
the rocks in Siberia,

they found that there was

an increase in mercury

in the rocks that they sampled.

So there was a mercury spike,

and they found this associated

with those volcanic eruptions.

Mercury is often associated
with volcanoes.

As a volcano erupts,
spewing out lava,

it also releases mercury vapor
into the atmosphere.

This vapor can be transported
across continents

by currents and winds.

But we did not know

whether we had such a spike in
the Karoo Basin of South Africa.

If found, that spike
could be evidence

that these Siberian eruptions

were responsible
for the climate changes

that caused the extinction event
seen in the Karoo.

So our team went to the field
and we collected samples,

and we ran the analyses,

and we found that indeed,

we are finding a mercury spike
in South Africa.

The therapsid extinctions
in the Karoo

252 million years ago

were indeed part
of a global crisis.

In the northeastern corner
of Pangaea,

in what is now Siberia,

one of the biggest volcanic
eruptions

life on Earth has ever seen

began.

The scale of this event would
dwarf the Capitanian crisis.

The lava spread across nearly
three million square miles,

an area almost as large
as the U.S. Lower 48.

The eruptions lasted
for at least 200,000 years.

Consequences for life
were terrible.

On land, around 70% of species
disappeared.

In the seas,
the toll was even heavier.

Changes in acidity
and oxygen levels

wiped out
as many as 95% of species.

The greenhouse effect
also went into overdrive,

causing temperatures
to rise dramatically.

Paleontologists have dubbed
this event "the Great Dying,"

and it marked the end
of the Permian period.

This was the closest
life has ever come

to completely dying out.

Our ancestors,

they were there when it happened
and they made it through.

And if they didn't,
then we wouldn't be here today.

This crisis heralded the start
of a new geological era.

It transformed the planet's
climate and landscape,

including in the Karoo.

But what was it like
to be there?

And what did this mean
for the many therapsids

that called this part
of the world home?

At the Iziko South African
Museum,

a dicynodont fossil sheds light
on the Karoo's climate.

So here we have the skull
of the animal.

The head.

There's the outline of one
of its eyes, front of its nose.

And then coming around,
we have the front foot.

The fingers are missing.

Then, if you follow it along,

here's the beautifully
articulated

complete spine of the animal.

And the animal's
kind of spread-eagle.

This unusual position
can tell us

how this dicynodont died.

One of the ways in which

an animal dies of exhaustion

is actually by collapsing,

and collapsing with its feet
spread out

in this very distinctive
spread-eagle pose.

In the Triassic Karoo,
therapsids struggled.

Hunger, thirst, and exhaustion
were all common threats.

What adaptations
made it possible

for any of them to survive?

In Bethulie, east of the Karoo,

the remains of a species of
dicynodont known as Lystrosaurus

have given us clues.

This is an area
that I like to call

the Lystrosaurus killing fields.

I've been able to map
the actual embedded skeletons

throughout this entire area.

And I have 600
in situ skeletons already.

So originally,
there must have been

literally many thousands.

These dicynodonts represent
95% of the therapsid fossils

discovered in this area
from this time period.

But why were there so many here?

Paleontologist Jennifer Botha
has discovered a clue hidden

in the microstructure
of their bones.

What you are looking at here

is a cross-section
through the bone.

The organic components
of the bone have disappeared,

but the position
of all the bone fibers

have remained in place.

So we can tell certain things
about how the animal grew.

I can see these lines

running down through here
through the bone.

And they represent
yearly growth marks.

So if I count
the number of growth marks,

I can tell how old
the animal was when it died.

But how can the age at death
shed light

on why Lystrosaurus
was so prolific in the Karoo

during the Triassic
after the Permian crisis?

Here I have a Lystrosaurus
species from the Permian

on the left

and a Lystrosaurus species
from the Triassic

on the right.

And highlighted in green
are the number of growth marks

I have been able to count.

So I can tell that this animal

was at least eight years old
when it died.

In comparison,
this Triassic specimen

was only at least two years old
when it died.

And very importantly,

this is a large
Lystrosaurus Permian species,

and this is the largest known
Triassic specimen.

So even the largest
Triassic specimens

that we have ever discovered

are not fully grown.

They are at most
two or three years old.

After the crisis,
Lystrosauruses were dying young.

So what explains
their large population?

In order to be so abundant,

they had to be breeding
quite abundantly.

This suggests that the
birth rate increased rapidly.

During periods of drought,

times were tough
for all the animals here.

Predators, who depended
on the herbivores for food,

went hungry.

The carnivorous gorgonopsians

succumbed to the hostile
conditions and vanished.

And so did the biarmosuchians.

The dicynodonts
were dying young,

but their rapid reproduction
rates and their burrowing habits

helped them survive
this period's climate chaos.

The therocephalians
initially made it,

but as time went on,
they, too, eventually died out.

The cynodonts survived
this tumultuous period, barely.

Only two groups remained:

the dicynodonts

and the cynodonts.

The fossil record shows just how
successful the dicynodonts were.

This is what this
animal looked like.

It was about 50 to 70
centimeters long,

so about the size
of a small pig.

Dicynodonts literally swarmed

during the first five million
years of the Triassic period.

So much so that they
rediversified

and gave rise
to large herbivores,

as illustrated here
by this skull,

which belonged to an animal
that must have been roughly

the size of a modern wildebeest,
weighing several hundred kilos.

And these giant dicynodonts
repopulated the world.

They spread
across the whole of Pangaea.

Fossil remains of dicynodonts
have been found

on every continent.

But the dicynodonts
weren't the only group

that found a way to adapt
to their new environment.

Paleontologist Roger Smith
has found a fossil

that reveals
that some of the cynodonts

had discovered
some of the same tricks.

Here we have
an underground burrow cast

made into the ancient floodplain
by a cynodont.

The architecture
of the cynodont burrow

is a tube going into the ground,
and when it gets to the end,

it forms this round
terminal chamber.

This doesn't have a skeleton
in it.

If it did have a skeleton--

and we do have skeletons
in these--

they're curled up generally

with their back to the outside,

with the tail curled around
against the wall.

This burrow cast could well be

a behavioral adaptation of
the cynodonts to go underground

to escape the extremes of heat
during the day and night,

and perhaps to even breed.

Being able to burrow,

that was a major
survival strategy.

Being able to hide away.

And that is probably one
of the things

that enabled our ancestors
to endure.

The same adaptations
that helped the dicynodonts

emerge from the first crisis
eight million years before

also helped
the cynodonts survive.

As the ecosystem
recovered from the crisis,

the cynodonts led
a relatively stable existence.

But according
to the fossil record,

the dicynodonts really thrived.

Freed from the
gorgonopsian threat,

they took full advantage
of available ecological niches

and grew many times larger.

Meanwhile, the therapsids

weren't the only creatures
doing well.

Others were starting to appear.

The top predators
of the late Triassic

are things called
pseudosuchians.

They're sort of the cousins
of dinosaurs.

They look a bit
like modern crocs.

They're covered in body armor
along their back.

Soon to be found in almost
every corner of the globe,

they are evidence

of yet another planet-wide
environmental change:

one that would greatly impact

the therapsids
and their mammal descendants.

232 million-year-old
plant fossils

show new flora was developing:

the type of plants
that only exist

in extremely rainy climates.

Heavy rainfall
was a planet-wide phenomenon,

and paleontologists have
dubbed this period

"the Carnian pluvial episode."

Over about two million years,

the climate changed
from extreme to extreme:

cold to hot,

dry to wet.

In the Karoo,
the desert turned green.

But only one therapsid lineage
would survive this time.

Would it be the thriving
and vastly varied dicynodonts

or the newly emerged cynodonts?

Which would give rise
to all of mammal-kind?

To us?

The remaining therapsids
had to contend

with new creatures dominating
the Karoo.

Dinosaurs, like the ones
in front of me,

soon become the most dominant
large-bodied animals

on the landscape.

The therapsids
and the other animals

that were once so common
on landscapes

become much more rare.

And instead of seeing therapsids
eating plants,

we'd be seeing dinosaurs
like this one

browsing on the vegetation.

It was also a time
when new characteristics

that would prove helpful
for survival

were appearing.

And one in particular,
a very mammal-like quality,

may have been
the biggest advantage of all:

a metabolism
that kept the body warm.

Being warm-blooded
is really important.

It means that we can go outside
when it's wintertime.

It means that we can be active
in the morning

or when the sun's not out.

You're not
like a cold-blooded animal

that's at the mercy of the sun.

And that's really unusual.

It's a superpower.

Modern-day mammals
are warm-blooded.

When did this trait emerge?

And how?

It is really challenging

to figure out
if a fossil species

was warm-blooded
or cold-blooded.

You can't just stick
a thermometer into them

and see how their body
temperature changes

over the day.

So you have to look for clues
in the fossils.

There's one
very interesting clue,

this opening
in the top of the head

for what's called a third eye.

It's, it's a bunch of cells
that sense light.

It's thought
that the third eye

helped
ancient cold-blooded therapsids

regulate their daily activities.

In modern species,
the third eye only exists

among cold-blooded animals.

Therefore, the disappearance
of the third eye may be linked

to the appearance
of warm-blooded metabolism.

My colleagues and I decided
to look

for when the therapsids lost
this third eye.

Julien and his team have
studied fossils

from across the reign
of the therapsids,

all the way back
to almost 270 million years ago,

in the Permian period,
before the Great Dying.

In the Permian, almost all
therapsids had a third eye.

Then, when we enter
the Triassic,

the first Triassic species
still had a third eye.

But a little later,

we begin to encounter species
that no longer have

a hole at the top
of their skull.

They no longer had a third eye.

By 240 million years ago,
the third eye

had all but vanished
from the fossil record

of one therapsid group,
the cynodonts.

This isn't definitive proof
of warm blood,

but scientists
have found another clue.

BENOÎT
The most decisive argument
was the study of the inner ear.

The warmer the internal
body temperature of an animal,

the runnier
their ear canal fluid becomes

and the thinner
the tubes can be.

We were able to measure changes

in the size of the canals
in cynodonts

and determine precisely
that the body temperature

had increased
by ten degrees

exactly here,

between this
Triassic animal

and this animal

233 million
years ago.

Evidence suggests
that the cynodonts

were warm-blooded during
the Carnian pluvial episode.

There was a time
of, of fairly rapid change

in climate and in precipitation,

and being warm-blooded

would have been beneficial
at that time,

because if you can control
your own body temperature,

that can protect you
from the whims of the weather.

What did this mean
for therapsids

that were not warm-blooded?

This Carnian rainfall event had

a huge impact on the therapsids.

By about 200 million years ago,

the dicynodonts disappear
from the fossil record.

There was one small group
that came out on top,

the cynodonts.

And this group is the one
that gave rise to mammals.

It was the small size
of these cynodonts,

their ability to dig burrows,
being able to grow fast,

being able to eat
lots of different foods.

That's what helped
make them through.

And then on the other side,
they would have found

this wide-open world,

a land of new frontiers
and opportunities.

230 million years ago,
the cynodonts were just starting

to adapt to this new world.

How did they give rise
to mammals

just five million years later?

When did
the other key mammal traits,

lactation and hair,

appear in their evolution?

Hair is the
insulating layer

that allows mammals
to retain heat.

The rarity of hair or fur
in the fossil record

has made it difficult
for scientists to find exactly

when this mammal trait evolved.

But there is one type of hair
that leaves a mark:

whiskers.

And although mammals
are not the only animals

with whisker-like features
today,

they're a sure sign
that hair has evolved.

We can track the evolution
of whiskers

thanks to traces left behind

by the nerve
which makes them sensitive.

Whiskers help mammals
detect small changes

in the world around them,

find prey,
and even communicate.

They're incredibly sensitive
to external surroundings.

This is because

they are connected directly
to the brain

by the trigeminal nerve.

It's evidence of how this nerve
emerged from cynodont skulls

that has revealed when whiskers
first appeared in this line.

In animals that lived
around 230 million years ago,

a single hole appears,

which is exactly what we find
in modern mammals.

This single hole
is the passageway

for the trigeminal nerve,

which innervates the whiskers,
located here,

in this cavity.

This creature had whiskers

just after the Carnian pluvial
episode,

around the same time
warm-bloodedness appeared.

Coincidence?

Or could they be linked?

Typically, traits evolve
at different rates

over millions of years

unless they are somehow
connected.

The final missing piece
in the puzzle

is the appearance of lactation,

mothers producing milk
to feed their young.

When did that appear?

Lactation is tough to spot
in the fossil record,

so scientists have instead
turned to DNA for answers,

with tantalizing results.

The evolution of a single gene
seems to be related

to all three mammalian traits.

The MSX2 gene controlled

not only the disappearance
of the third eye

and the appearance of hair,

but also the appearance
of mammary glands,

and, with that, lactation.

This is the most
characteristic trait of mammals.

The evolution of MSX2
contributed to the emergence

of these mammalian features
in cynodonts,

which they then passed on
to their mammal descendants,

giving them
the defining characteristics

of all mammal-kind.

But it's not the only gene
responsible

for big evolutionary changes.

Whether it's turning
fish fins into limbs

or developing a spoken language,

there is a set
of key regulator genes

that have a profound impact

on development
and evolutionary change.

At the boundary between the
Triassic and Jurassic periods,

the dicynodont group
had gone extinct

and the cynodonts were the
last surviving therapsids.

So at last,
we have an answer.

This is the group that
gave rise to the mammal line

around 225 million years ago,

not long before the other
therapsid lineages ended.

The very first true mammals

were tiny.

And if you saw one,
it would have looked

like a little mouse
or a little shrew.

They would have been covered
in hair.

They would have had molar teeth.

They would have fed their babies
milk.

You would recognize them
as mammals,

but you would probably
just think they were

some kind of little rodent
scurrying around.

It wasn't easy
for the newly evolved mammals,

as the dinosaurs reigned

for roughly
the next 160 million years.

The fates of dinosaurs
and mammals

have always been intertwined.

And while dinosaurs took over
the world,

mammals went the other way.

They went small.

They evolved
stronger jaw muscles

so they could bite harder.

They evolved a keener sense
of hearing.

All these things
would have helped them

survive in a world controlled
by massive, enormous dinosaurs

that could have smashed
one of these little mammals

with a single footstep.

As had been the case
for the therapsids

in the extinction events
they faced,

it would be primarily
the smallest of the mammals

that would survive
the next major crisis:

the asteroid strike
that wiped out the dinosaurs

66 million years ago.

Our ancestors made it
through that boundary

for very specific reasons.

We had very specific adaptations
as a group

that allowed us to make it
past these extinction events.

Over millions of years
of evolution,

cynodonts,
these small burrowing animals,

survived multiple crises,

adapting and changing to fit
each new environment.

They gave rise to the mammals
some 200 million years ago,

at a time when the dinosaurs

were just beginning their reign
on Earth.

Mammals were not born

as a result of the extinction
of the dinosaurs,

but had appeared
millions of years before.

For 160 million years,

they lived
alongside these giants.

And only once the dinosaurs
were extinct

could the surviving mammals
rise,

diversifying
into the spectacular variety

we see today.

We and all mammals alive today,

we are the descendants
of survivors.

So ultimately,

we come from this line

that's been small, adaptable,

a line that's been resilient,
that could handle everything

that nature's been able
to throw at us

over hundreds of millions
of years.

With more than 270 million years

of shared evolution
running through our veins...

...we are therapsids...

...just one of the thousands
of modern mammal species

that are their descendants.

Long live the therapsids.