Nova (1974–…): Season 46, Episode 19 - Rise of the Mammals - full transcript

A new trove of fossils reveals how mammals took over after an asteroid wiped out the dinosaurs 66 million years ago.

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It's the worst day in the
history of life on Earth.

A cataclysm that ends
the age of the dinosaurs.

♪♪

If they hadn't met
their destruction,

our story never
would have begun.

From the ashes,

a band of survivors rises
to become bigger...

nimbler...

and smarter
than any that came before.

Our tribe... the mammals.

It was only after
we lose the dinosaurs



that mammals really explode

into the diversity of forms
that we see today.

♪♪

What happened after the impact?

♪♪

How did life surge back?

The clues have been elusive.

You could go your entire career

and not find a skull
from this time period.

That's how rare they are.

But now comes
a remarkable discovery.

There's just nothing like
picking up something

and finding out
it's something amazing.

It's like winning the lottery.



Bam! We hit it big.

An unprecedented trove
of fossils

reveals a turning point
in the history of life.

That moment of rapid mammal
evolution

is effectively the trigger

to our existence here
on planet Earth.

♪♪

"Rise of the Mammals"...
right now, on "NOVA."

♪♪

We live in the age of mammals.

All across the globe,

they thrive in an astonishing
array of forms...

♪♪

From the great beasts
of the forests

to giants that sound
the ocean depths...

♪♪

To us,

mammals dominate the planet.

But it wasn't always this way.

For 150 million years,

mammals lived in the shadows
of the dinosaurs.

Then, a catastrophe
gave them an opening.

For years, we've known just bits
and pieces

about how mammals seized
the moment

and made our story possible.

But now that's changing.

♪♪

An extraordinary discovery
in Colorado

is revealing an unparalleled
collection of fossils

from the time when mammals
began their reign.

♪♪

This hoard lay hidden
in plain sight

until a young fossil hunter made
a once-in-a-lifetime discovery.

♪♪

I grew up in a town
of about 80 people,

basically the middle of nowhere
in southwestern North Dakota.

♪♪

Tyler Lyson's home might look
like other outposts

scattered
across the American West,

but this place is unique.

Right by my hometown,
about a mile away,

are badlands that preserve
the very last moment of time

when dinosaurs were alive.

As a child, I would run around
the badlands

and find dinosaur bone
after dinosaur bone,

just tons of dinosaur bones
just tumbling out of the hills.

♪♪

Known as the Hell Creek
formation,

this was the first place T. rex
fossils were discovered

a century ago.

♪♪

Ever since, bone diggers have
been searching

for more traces
of that lost world.

♪♪

Even as a boy,
Tyler was in the thick of it.

Years ago, I met a kid named
Tyler Lyson.

And he was this gangly
12-year-old kid.

By the time he was 13,
he was going out by himself

and finding amazing fossils.

He found an entire dinosaur
when he was, like, 14 years old.

Tyler even discovered
a new species

of feathered dinosaur.

He became sort
of this dinosaur wunderkind.

It was a made-for-TV story.

High school sophomore
Tyler Lyson has walked

just about every inch
of this land,

and has already headed
20 dinosaur excavations.

His most spectacular find?

A dinosaur fossil
with preserved skin and muscle.

He kept finding more fossils,

and soon became an expert
in ancient turtles.

He became fascinated
by a well-known pattern

in where dinosaurs were
and where they weren't.

I remember as a child crossing
over this band

and getting above that area
and not finding any dinosaurs.

The distinct layer seemed
to mark their disappearance.

So, this always fascinated me.

Why, above that boundary,
were there no more dinosaurs?

♪♪

What happened here?

The boundary layer,
Tyler learned,

was the signature of an event
that changed the world forever.

♪♪

During the age of reptiles,

dinosaurs tower over mammals.

Other reptiles
rule the skies and seas.

They take on a wide range
of shapes and lifestyles.

Many are gigantic.

Mammals had been around
for millions of years.

But in all of that time,
most grew no bigger than a rat.

Then, 66 million years ago,
the cataclysm.

♪♪

A chunk of rock
over six miles wide

tears through the atmosphere
and slams into the planet.

The asteroid blasts a crater
20 miles deep

and over 100 miles wide

in the Yucatan Peninsula.

It showers the planet
with molten rock

and white-hot beads of glass.

And this broils the surface
of the Earth.

Creatures on the surface roast.

Skies across the globe darken.

Fire gives way to frost.

You have intense global cooling.

You're talking about
immediate global winter.

Deprived of sunlight,
plants wither.

Lush forests collapse.

So the animals that rely
on those plants for food,

the animals that rely
on those animals

that rely on plants for food,
they would all suffer.

♪♪

You really are having the
entire food chain fall apart.

All of the dinosaurs,
except the birds, perish.

Three out of four species

that were alive on the planet
before the impact

died.

And that's just the species.

About 90% of the creatures

that were alive
before the impact

are dead afterwards.

We think about
the loss of dinosaurs,

but dinosaurs weren't the only
animals to suffer from this.

Many, many groups of organisms
took a very serious hit,

including mammals.

♪♪

It's a really bad time
for life on Earth.

Yet some things survive, right?

Including some of our earliest,
earliest ancestors.

♪♪

We don't know much
about these creatures.

♪♪

None was bigger than a squirrel.

And their size and scarcity
have made their fossils

hard to come by.

Yet somehow, this band of
survivors and their descendants

will claim the Earth.

♪♪

Today, there are over 6,000
species of mammals.

♪♪

And they range in size
from the tiny pygmy jerboa,

which weighs about
as much as a coin,

to the largest animal to have
ever lived, the blue whale,

which is about 180 tons,
the size of 15 school buses.

Over the last 66 million years,

they've evolved an incredible
diversity of forms.

♪♪

Mammals today live in almost
every environment on the planet,

from the tops
of the highest mountains

to living in the trees,
to living in the water.

♪♪

They thrive in scorching deserts

and across the frozen tundra.

Mammals even patrol
the depths of every ocean

and the moonlit reaches
of the sky.

♪♪

All this diversity
can be traced back

to those survivors
of the extinction.

♪♪

What happened in the wake
of that catastrophic impact?

That's what I'm interested in.

♪♪

How long did it take
for ecosystems to recover?

For habitats to reform?

For life to get big again?

♪♪

The answers require fossils.

But such relics are rare.

I've spent much of the last
35 years looking at rocks

from the first million years

after the extinction
of the dinosaurs.

I've studied the fossil plants
from that time period.

And through that entire
35 years,

I think I've found

one or two fossil mammal pieces.

And I'm looking.

I'm out there looking, and I
know how rare those things are.

For years, Tyler and others

search for these scarce
specimens

in North Dakota.

He targets the rock
above the boundary layer...

Sediments laid down
just after the impact.

♪♪

Fossil hunters had occasionally
found mammal traces here,

but Tyler has no such luck.

Small animals get broken up
into fragments.

So, when you're looking
for an early mammal,

you're looking
for effectively the tooth

of a mouse-sized animal.

You're looking for something
that's about the size

of the head of a pin.

And they're excruciatingly hard
to find.

♪♪

Every year, I go back
to my hometown,

and every year, I spend time
above the boundary

looking for fossils.

And I've found so few fossils,
it's, it's pathetic.

♪♪

And I was starting
to think about, like,

"Well, where's the next area?"

I was getting ready to give up.

But then, Tyler gets a job...
and another shot...

In Colorado.

He joins up with Ian Miller,
an expert in plant fossils

at the Denver Museum
of Nature and Science.

Ian shares an interest in the
extinction and its aftermath,

and knows Colorado
is a great place to search.

♪♪

We have this really unparalleled
record of this period of time

here in Western North America.

But the best place
is right here.

Together, they start exploring
locations around the state.

♪♪

One, called Corral Bluffs,

offers 400 feet of exposed rock
rising above the boundary.

Tyler begins by scanning
the ground for bits of bone.

That's how we're trained
to find fossils...

Looking for actual scraps
of bone on the surface,

or to see bones
sticking out of the ground.

But at Corral Bluffs,
this approach yields

just a few scattered fragments.

I really didn't find
a whole lot.

You can only look for fossils
for so long

before you get really frustrated
and decide to go home.

What Tyler doesn't know

is that a museum volunteer

had already found the key
to his quest

in this same spot.

♪♪

Long before Tyler arrived
in Colorado,

Sharon Milito set off to look
for fossils

from the plateau
atop the bluffs.

When we first came out,

we were just looking
for whatever we could find.

The important thing we wanted
to find

were mammal jaws
and mammal teeth,

but they were elusive.

So, one day,
I was walking along in an area

I'd been many times before,

and I saw this white, round rock

sitting there on kind of
its own little pedestal.

♪♪

And I picked it up
and looked at it,

and as soon as I looked at it,
I saw these teeth

that were just smiling out
at me.

And I just almost
had a heart attack.

She'd found part
of a mammal skull...

One that could fit in the palm
of her hand.

I was so excited,

because it was really, really
well preserved.

♪♪

She brought the fossil
to the museum,

where it was catalogued
and filed away.

♪♪

It was years
before Tyler happened across it.

I was downstairs, just looking
through the collection,

pulling open drawers, and there,
sitting right in the front,

was the palate of a mammal.

And I was absolutely astonished;
I couldn't believe it.

And so I was sort of thinking
to myself, like,

"If there's this complete
of a skull here,

there's got to be more."

Sharon's fossil had been found
in a particular kind of rock

called a concretion.

Sometimes, concretions can form
around organic material,

like bone.

And if conditions are right,

they protect the fossil inside
for eons.

Sharon's skull
was incredibly lucky

because it showed that,
yes, there are fossils inside,

and this is the type of rock
to look for.

♪♪

Tyler decides to change his game

and look for concretions
to crack open.

It was absolutely
a light bulb moment.

That was the game changer.

♪ It was a beautiful day ♪

♪ The sun beat down ♪

♪ I had the radio on ♪

♪ I was drivin' ♪

♪ The trees went by ♪

♪ Me and Del were singin'
"Little Runaway" ♪

♪ I was flyin' ♪

♪ Yeah, runnin' down a dream ♪

♪ That never would come to me ♪

Tyler started
to put these pieces together,

thinking about this new lens
to look for these concretions.

♪ Runnin' down a dream ♪

Just saying, like,
"We're going to go down there,

blow the doors off
this thing."

Nobody believes him, right?

And even me, who, his colleague,

I'm just, like, "That sounds
good, that sounds good, man."

They hit the bluffs,

their eyes peeled
for promising rocks.

♪♪

There's a whole hillside
of choices.

One catches Tyler's eye.

I see this rock,
this concretion on the ground,

the very first one that I pick
up, and I crack it,

and it was amazing.

I just found a mammal skull!

And that was just the beginning.

It was, it was crazy
the way it happened.

I mean, you could go
your entire career

as a mammal paleontologist

and not find a skull
from this time period...

That's how rare they are.

We found, I think,

five or six mammal skulls

within about a ten-minute
time span.

We were just laughing
on the outcrop.

Nothing like that's ever
happened to me before.

I mean, amazing.

They'd cracked the case...

♪♪

And unearthed a trove of time
capsules filled with clues.

♪♪

It was like opening a door
into a new world.

In the months that follow,
Tyler and Ian scour the bluff

and discover
thousands of fossils,

including plants,

reptiles, and dozens of mammals.

All of them lived here
just after the mass extinction.

Most of the time, you know,

we're pretty happy
if we find some nice teeth

from an area that's that close
to the boundary.

Having this kind of record
is really rare.

You got the other side
a little bit, too?

They had found their quarry
at last.

But the really hard work
now lay ahead...

Figuring out how mammals and
their world rose from the ashes.

♪♪

What creatures
had they unearthed?

What was their strange world
like?

♪♪

And how did they establish
a foothold

in a planetary wasteland?

♪♪

To answer these questions,

Tyler and Ian needed to assemble
a crack team of experts.

The fossils are not just the
biological remains of animals.

They are also buried
in a geological system,

and so no one person can really
understand all the data

that we can get from a fossil.

We need experts

to really be able
to piece together

what the entire ecosystem
would have looked like.

Each fossil contains
a wealth of information,

but it's hidden in the rock.

The first step is to free it.

So, they reach out
to a virtuoso.

I love working on fossils.

It's so rewarding
when you start a fossil and,

that you really can't see.

It's, it's almost
like a piece of art, sometimes,

and it also is contributing
to scientific knowledge.

Bob Masek spent his early career
working in factories,

but he found he had a knack

for releasing captives
from rocks.

When I was little,
I used to work on models.

So, I did learn to do
detailed work.

It was sort of in me.

My contribution as a preparator

is to make the fossil readable
so it can be used for science.

And you can imagine, you have
to be incredibly careful

that you don't break your
fossils while you're doing that,

or even shape them
with your own ideas

of what they should look like.

The painstaking process can take
months for a single fossil.

But it reveals secrets held
tight for millions of years.

As you're working,
you do start to realize

that this was a living animal,
a living thing.

I'm bringing it sort of
back to life.

♪♪

One by one, the fossils emerge.

Each is exquisitely preserved.

But what are they,
and what do they reveal

about the mammals' rise?

♪♪

Stephen Chester is a vertebrate
paleontologist

who specializes in mammals.

So when I made this discovery,
I needed to bring him on board

to help me identify all these
mammals that I was finding.

Stephen examines each fossil
to find details that reveal

what it's related to.

Not an easy job so far back
on the mammalian family tree.

We're talking about species
that can be

very difficult to identify,

because they don't look exactly
like what you might expect

for, for living mammals.

♪♪

It's a common problem
in tracing ancestry.

Consider how different
an elephant is from a hyrax,

or a manatee.

You might not think
that they are closely related.

They are incredibly different
animals.

But their DNA shows us

that they are actually
really closely related.

♪♪

Descendants of a common ancestor

don't always look like each
other, or the ancestor,

because they've evolved
in their own directions

for millions of years.

But they're bound to share
some common traits.

That's why Stephen examines
each skull feature by feature.

His first task is to figure out

what kind of mammal
they've unearthed.

All mammals share
a few key features.

They're warm-blooded,

grow hair,

and have bigger brains
than other animals their size.

They also give their young
a jump start in life

by nursing them.

But how they have these babies
sets them apart.

Some, like kangaroos and
opossums, use a pouch

to bring their embryos to term.

A handful... platypuses
and echidnas... lay eggs.

But what about the biggest group
of mammals alive today...

Those that carry their offspring
in the mother's womb?

Like us,

they're called placentals.

And they include the mammal
Stephen examines

from Corral Bluffs.

It's one of the oldest placental
fossils ever discovered.

A creature called loxolophus.

It's not yet clear if it has
any living descendants.

But the fossil hints
at a highly adaptable creature.

This mammal is probably
about the size of a skunk.

It has teeth in the front
of its mouth

that are probably good
for tearing meat,

but the back of its mouth
has other teeth

that would be better
at processing plants.

It's probably eating
a mixed diet.

This versatility would have been
a great advantage

in a recovering world,

where the types of food
were probably limited.

Team member Karen Cuevas

finds other insights
about how it lived

by peering inside the fossil.

When I first heard

about how mammals became
what they are today,

I was really excited to see

how much we could deduce
from these fossils

and paint this picture
from so many million years ago.

One of the things that we can do
with the CT scan

is view the inner anatomy.

CT reconstructions
give us insight

into tissues
that aren't preserved,

and teach us a lot

about what those animals
were really like

as living, breathing creatures.

The data help the team imagine
what the living animal was like.

Based on CT scans, this animal
had a hole underneath its eye.

A passage for nerves
from sensitive whiskers.

It also has a large portion
of its brain

dedicated to the sense of smell.

♪♪

These senses would have been key
to its lifestyle

in this new world.

Details from related species
suggest how it walked.

We would assume that this animal
was moving around on the ground

on all fours, probably something
like a raccoon.

It's a lot to have learned

about one of the creatures
that roamed the new world.

But Corral Bluffs is about
to reveal something even bigger.

The discovery here extends

well beyond
any one particular fossil.

Here, we're dealing with the
recovery of an entire ecosystem.

♪♪

If the team can determine
the age of each fossil,

they can start to see how
the mammals evolved over time,

and how their world changed
alongside them.

When we find some interesting
fossils,

then the next thing that we do
is spend a huge amount of effort

trying to figure out
exactly how old they are.

But getting accurate dates on
fossil sites is incredibly hard.

To estimate the age
of their fossils,

Tyler and Ian reach out
to specialists.

Geologists Will Clyde
and Anthony Fuentes.

Their goal: determine the age
of each layer in the rock

by building a timeline
up the cliff.

To do it, they'll rely on clues
forged by planetary forces.

♪♪

Earth is surrounded
by a magnetic field,

which creates north and south
magnetic poles.

These may seem permanent,
but they're not.

From time to time,
the field reverses.

And so what is today

the magnetic north for the Earth

sometimes flips and becomes
the magnetic south.

♪♪

Some rocks can record the
direction of the magnetic field

at the time they form.

As these build up
on top of each other,

they can capture a record
of magnetic reversals over time.

The date of each flip
has been firmly established

in rocks across the world.

Four, five...

Four, five.

So if we can find

certain flip-flop in
the polarity in the rocks here,

then we know the date
of that change.

Six-four.

To the east.

To look for these flips,

they collect rocks
along the bottom of the bluffs,

where the impact boundary is,

and work their way up.

They then bring each sample back
to their lab

to measure
its magnetic orientation.

Near the bottom of the bluffs,

they find a flip
from 350,000 years after impact,

and trace it across the land.

But a single date isn't enough.

If you get one date
in a pile of rocks,

you know where that one date is.

But above it, you don't know how
fast the rocks are accumulating.

There could be a ton of rock
in a short period of time,

or it could be a small amount of
rock in a long period of time.

♪♪

Most of the bluffs,
and the team's discoveries,

lie above the flip
they've found.

They need a second one higher up
to bracket the dates

of their fossils.

They work their way up,
one layer after another.

As we went up the hill,

you know, you're sampling,
you're sampling,

but there's no flip,
there's nothing,

and we're just running
out of rock.

They test samples
right up to the top.

Still...

No flip.

No way to place
their discoveries in time.

♪♪

Then, they find a small outcrop

perched above
the rest of the bluffs.

It's one last chance
to catch the flip.

♪♪

And it delivers.

♪♪

They've found
the second date they need.

That allowed us to place
all these fossils

in this incredible time frame.

We had the whole thing,

and that was a shock.

We didn't expect that.

We thought we might be missing
a part of the record,

but it's all there.

♪♪

The towering face of the bluffs

chronicles the first one million
years of the recovery.

If you were to choose

any one-million-year sliver
of time

in the four-and-a-half-
billion-year history of Earth,

it's an enormously lucky event
to have one that happens

right after a giant asteroid
impact.

It's like winning the lottery.

♪♪

Now all the extraordinary
fossils of Corral Bluffs

can be placed in time.

But what story do they tell?

Another fossil
reveals a turning point.

♪♪

In the middle part
of the bluffs,

I found a much larger skull.

This creature lived 300,000
years into the recovery.

What's interesting about it

is that it has
these really large teeth.

Mammals today use teeth like
these to grind leaves and stems.

It suggests this creature dined
mostly on plants.

And this is really the first
major specialization that we see

in the mammal fossil record

right after the extinction
of the dinosaurs.

Just after the impact,
the mammals that survived

had been opportunistic
scavengers.

But now, mammals were
specializing in eating plants

and seizing a niche
long held by dinosaurs.

♪♪

It's a major milestone.

But how did it happen?

The answer could lie in the
story of the plants themselves.

And Colorado's rocks
are packed with clues.

A lot of leaves.

But just two species right here.

Some leaves are
so beautifully preserved,

they look as if they had
just fallen.

♪♪

But other telling fossils are
invisible to the naked eye.

Like pollen.

A thimble-sized chunk of rock,

it might have 100,000
pollen grains in it.

Each of the different kinds
of trees, the little shrubs,

produce different kinds of
pollen based on who they are.

Ian and scientists around
the world have documented

the microscopic record
from this time.

♪♪

Take a chunk of rock,

you dissolve it,

you take all the pollen spores
out of it,

put it on a slide, and you see
many different types of plants.

And then you can sort of paint
this image

of what these worlds were like.

♪♪

And so right at that mark
where we see the evidence

of this extraterrestrial impact,

the pollen just dives right off.

Everything seems to die.

After the impact,

one primitive form of life
resurges,

and it's a sign
of a devastated world.

Right after the boundary,

there's an explosion
of fungal spores.

The thing is that fungus grows
on things that are dead.

So most of the world
must have been rotting.

Forests destroyed
by heat and darkness

decompose
beneath this fungal carpet.

But then some plants rebound.

♪♪

Following immediately on top
of the fungal world,

we see a spike in fern spores.

♪♪

Ferns are this group of plants
that come back after disasters

before the rest of the forest
is able to re-establish itself.

First thing that comes back
are a blanket of ferns.

And that probably lasts
for a few thousand years.

♪♪

The world is green again,

but ferns are a limited source
of food.

New options arise
as the landscape evolves,

and long-dormant seeds
spring to life...

A new forest.

One of the big surprises
was this incredible abundance

of palm pollen.

And we get tons of palm fronds...
Huge, beautiful palm fronds.

They're so dominant, it feels
like the world is all palms.

And it probably lasts
about 100,000 years.

What I'm thinking right now is
that if you were a mammal,

there would have been a big
canopy of palms above your head.

But there still wasn't
that great source of food.

The habitat looks lush,
but still lacks diversity.

That, in turn, limits
the variety of animals,

as a close look
at the team's fossils reveals.

When there's an extinction,

it kind of ripples from the
bottom of the ecosystem,

starting from the plants,
up into all of the other levels.

Gussie Maccracken deciphers
what happened

to some of the smallest
creatures:

insects.

Insects don't fossilize well,

but what they do to plants can.

Insect damage on a leaf
can tell us

about the ecology
of the ecosystem in the past

in a way that no other types
of fossils can.

Some insects are generalists,

and feed on many types
of leaves.

They carve signature marks that
can be preserved in fossils.

♪♪

Simple holes,

or chunks bitten
from the leaf's edge.

♪♪

But other kinds of damage
are caused by insects

that specialize
in certain plants.

They leave these wild patterns

and we can tell things
like if it's a moth,

or if it's a fly,

or sometimes a beetle.

Before the asteroid struck,

the insect world was filled
with specialists.

But that changes abruptly.

During recovery, a lot of the
specialist insect damage types,

like from leaf miners
and galling insects,

just disappear.

♪♪

The specialists were doomed.

In a world with only
a few kinds of plants,

insects that can live on
anything have an advantage

over pickier eaters.

It echoes what happened
to the mammals.

Only the generalists survive.

At the time
of an extinction event,

any species that's highly
dependent on a single lifestyle

or a single location
or a single food source

is likely going to go extinct
when those ecosystems collapse.

♪♪

But in time, new plants arise

and chances to specialize
return.

The forests become
more and more diverse.

We get different kinds of trees
growing on the landscape.

♪♪

300,000 years
after its collapse,

the forest has recovered.

♪♪

And it's right there
when the animals can specialize

in eating certain kinds
of plants.

And get bigger.

These new plant-eaters
were now larger

than mammals had ever been

in over 100 million years
on the planet.

♪♪

They've shattered their record
in just a fraction of that time.

And their growth spurt
was just beginning.

In the very upper part
of the bluffs,

I started to find a whole array
of large and diverse mammals.

One of the mammals that we found
is called taeniolabis.

This is about 80 pounds,
so really a large animal.

It was ten times the size

of the omnivore
they'd found earlier.

It had these really big
incisors,

much like modern-day beavers
have.

We think this animal was living
in or near the rivers,

because we only find this animal
in riverine sediments.

♪♪

Near this fossil,

Tyler finds the skull
of an even bigger mammal.

This is a large animal
that was about 100 pounds,

so almost the size of a wolf.

And this animal had really big,
flat back teeth,

and it seems to be eating seeds
and other plants.

In less than a million years
since the extinction,

mammals have exploded
from just a couple of pounds.

Some are 50 times bigger.

It would take nearly 20 million
years to make such a leap again.

But what might have fueled
this dramatic change?

An intriguing clue
lay just a few feet

from the beasts' remains.

Ian was leading a group of teens

studying at the museum
for the summer.

Probably about...
I'd say about the same...

We were doing all kinds
of different things,

and it wasn't a terribly
exciting site.

A lot of fossils people hand you
are just sticks.

So, a lot of people are just,
like,

"Oh, this is
an incredible fossil!"

I'm, like,
"Yeah, it's just a stick,"

you know, "throw that back."

But this one student,
her name is Aeon.

We're thinking about moving on,
and going somewhere else,

and she hands me this rock
and she's, like, "What's this?"

Did you get something?
I don't know.

And so I took it up,
looked at it in the light,

and Io and behold,

it was the middle part
of a bean pod.

Um, I found this legume
two seconds ago

from this rock
that's sitting in my lap.

Right here, 700,000 years
into the recovery.

This is the earliest record
of the legume family ever

in the fossil record.

So that's pretty cool.

Aeon's legume was alive
right at the time

when mammals' size
was exploding.

A new food for a new world.

♪♪

Legumes are very special plants.

Today, there are almost
20,000 species of them,

including peanuts,
lentils, and soybeans.

Their signature pods
are packed with nutrients.

♪♪

Legumes are like

these protein bars for
the mammals on the landscape,

and at that same moment, we see
all these new and large mammals,

and we think
that they're taking advantage

of this new food source.

Taken together, the many fossils
of Corral Bluffs

suggest plants were helping
to drive mammal evolution.

And a final piece of evidence
reveals how plants, in turn,

were being driven
by something else.

♪♪

Ian uncovers clues to one of the
planet's most powerful forces...

Climate.

He zeroes in on the shape
of the leaves he's unearthed.

The more species you have
with serrations,

the cooler it is;

the more smooth leaves you have,
the warmer it is.

And so we can use that to tell
temperature in the past.

♪♪

The size of the leaves
can reveal rainfall.

If you've got a really big leaf,

it means that you're growing
in a world that rained a lot,

and so the forest
was really dense.

Most of the fossil leaves
from Corral Bluffs

are smooth and large.

Together, they paint a picture
of a very different Colorado

65 million years ago.

♪♪

We're probably looking at a
world that's more like Florida.

It's pretty wet, pretty humid,
and pretty darn warm.

This climate shaped the plants
and the creatures

that evolved here.

♪♪

No organism evolves or lives
in isolation, right?

Everything about them really
reflects their interactions

with their environment,
with other species.

We really need to understand
those relationships.

We want to build up a picture of
how life evolved in this period.

♪♪

In the first two years after
Tyler and Ian made their find,

their team collected
over 1,000 vertebrate fossils.

♪♪

Some 6,000 plants.

♪♪

And the end is nowhere in sight.

♪♪

The discovery at Corral Bluffs
is remarkable for many reasons.

We have the whole ecosystem,

and we have it
at different intervals of time.

I mean, we're dealing
with the entire story here.

So far, that story includes

16 different mammal species
they've found.

It's an extraordinary record
that unites plants, animals,

climate, and time

in a single picture.

♪♪

So, what Tyler and Ian have
discovered is a Rosetta Stone.

And Corral Bluffs is telling us
something very important.

It's telling us about ourselves,
how we got to be here.

♪♪

The team's discoveries,
along with other evidence,

trace the beginning
of the mammals' fateful rise.

♪♪

Few survived the impact winter.

But as the climate warmed,
plants regained lost ground.

Small creatures endured,
eating whatever they could find.

Forests recovered,

and fueled mammals
as they exploded in size

and took on new forms...

All in less
than a million years.

♪♪

In the ages that followed,
mammals seized empty niches.

Our ancestors scaled the trees.

Others took to the air,

or became giants of the seas.

Mammals rose
to dominate the planet.

♪♪

All this diversity

sprang from the band
of humble creatures

that emerged from the cataclysm.

♪♪

They inherited a whole world

and they had the planet
to play in.

And that moment
of rapid mammal evolution

is effectively the trigger

to our existence here
on planet Earth.

♪♪

By unveiling the start
of the mammals' explosion,

Corral Bluffs seems destined
to join the ranks

of other celebrated
fossil sites,

like Tyler's back yard
in North Dakota.

I can trace those same questions

that I'm asking today
out on the bluffs

all the way back to when I was
six or seven years old,

running around the badlands
looking for fossils.

By pursuing those questions,

Tyler and his colleagues
are helping us understand

how the planet recovered
from catastrophe.

They're illuminating
the dawn of a new world.

Our world.

♪♪