All language subtitles for 7068BCB8827150041F74328B609BC49A_eng

This is a story about
you and me and your dog.

There was a time not
long ago, before dogs.

They didn't exist.

Now there are big ones, small
ones, smugglers, guardians, hunters.

Every kind of dog you
could possibly want.

How did that happen?

It's not just dogs.

Where did all the different kinds
of living creatures come from?

The answer is a transforming
power that sounds like something...

straight out of a fairy tale or
myth, but it's no such thing.

Let's go back across 30,000
years to a time before dogs...

when our ancestors lived in the
endless winter of the last ice age.

Our ancestors were
Wanderers living in small bands.

They slept beneath the stars.

The sky was their storybook,
calendar, an instruction manual for living.

It told them when the
bitter colds would come...

when the wild
grains would ripen...

when the herds of caribou
and bison would be on the move.

Their idea of home
was Earth itself.

But they lived in fear of
other hungry creatures.

The mountain lions and the bears that
competed with them for the same prey.

And the wolves that threatened
to carry off and devour...

the most vulnerable among them.

All the wolves want
to get at the bone...

but most are too frightened
to come close enough.

Their fear is due to high levels
of stress hormones in their blood.

It's a matter of survival.

Because coming too close
to humans can be fatal.

But a few wolves, due
to natural variations...

have lower levels
of those hormones.

This makes them
less afraid of humans.

This wolf has discovered what a
branch of his ancestors figured out...

some 15,000 years ago.
An excellent survival strategy.

Domestication, humans.

Let the humans do the
hunting, don't threaten them...

and they'll let you
scavenge their garbage.

You'll eat more regularly,
you'll leave more offspring...

and those offspring will
inherit your disposition.

This selection for tameness would
be reinforced with each generation...

until that line
of wild wolves...

evolves into dogs.

You might call this
"survival of the friendliest."

Then as now, this was a
good deal for the humans too.

The scavenging dogs
weren't just a sanitation squad.

They worked security.

As this interspecies
partnership continued over time...

the dogs appearance
changed also.

Cuteness became a
selective advantage.

The more adorable you were,
the better chance you had...

to live and pass on your
genes to another generation.

What began as an
alliance of convenience...

became a friendship
that deepened over time.

To see what happens next...

let's leave our
distant ancestors...

of some 20,000 years ago
to visit the more recent past...

during an intermission
in the Ice Age.

This break in the
climate starts a revolution.

Instead of wandering,
people are settling down.

There's something new
in the world: villages.

People still hunt and gather, but now
they also produce food and clothing.

Agriculture.

The wolves have traded their
freedom in exchange for a steady meal.

They've given up their
right to choose a mate.

Now the humans choose for them.

They consistently kill off the
dogs that can't be trained...

the ones that bite
the feeding hand.

And they breed the
dogs that please them.

They nurture those dogs
that do their bidding...

hunting, herding, guarding,
hauling, and keeping them company.

From every litter...

the humans select the
puppies they like best.

Over the generations,
the dogs evolve.

This kind of evolution is
called artificial selection...

or breeding.

Turning wolves into
dogs was the first time...

we humans took evolution
into our own hands.

And we've been
doing it ever since...

to shape all the plants and
animals that we depend on.

In a blink of cosmic time,
just 15- or 20,000 years...

we turned gray wolves into all
the kinds of dogs we love today.

Think of it. Every breed
of dog you've ever seen...

was sculpted by human hands.

Many of our best friends,
the most popular breeds...

were created in only
the last few centuries.

The awesome power of evolution
transformed the ravenous wolf...

into the faithful shepherd...

who protects the herd
and drives the wolf away.

Artificial selection turned
the wolf into the shepherd...

and the wild grasses
into wheat and corn.

In fact, almost every plant
and animal that we eat today...

was bred from a wild,
less-edible ancestor.

If artificial selection can
work such profound changes...

in only 10 or 15,000 years,
what can natural selection do...

operating over
billions of years?

The answer is all the
beauty and diversity of life.

How does it work?

Our ship of the imagination can
take us anywhere in space and time...

even to the hidden
microcosmos...

where one kind of life can
be transformed into another.

Come with me.

May not seem like it...

but we've been living in an ice
age for the last two million years.

This just happens to be
one of the long intermissions.

For most of those two million years,
the climate has been cold and dry.

The North Polar ice cap
extended much farther south...

than it does today.

In one of those long,
cold glacial periods...

when the winter sea ice
stretched from the North Pole...

all the way down to
what is now Los Angeles...

great bears roamed the
frozen wastes of Ireland.

This might look like
an ordinary bear...

but something extraordinary
is happening inside her.

Something that will give
rise to a new species.

In order to see it, we'll need to
descend down to a much smaller scale...

to the cellular level...

so that we can explore the
bear's reproductive system.

We'll take the subclavian
artery through the heart.

Almost there.

Those are some of her eggs.

To see what's going on in one of
them, we'll have to get even smaller.

We'll have to shrink down
to the molecular level.

Our ship of the imagination
is now so small...

you could fit a million of
them into a grain of sand.

See those guys over there
strutting along those girders?

They are proteins
called kinesin.

These kinesin are part
of the transport crew...

that's busy moving
cargo around the cell.

How alien they seem.

And yet these tiny creatures,
and beings like them...

are a part of every living cell,
including the ones inside you.

If life has a sanctuary...

it's here in the nucleus
which contains our DNA.

The ancient scripture
of our genetic code.

And it's written in a
language that all life can read.

DNA is a molecule shaped
like a long twisted ladder...

or double helix.

The rungs of the ladder are made of
four different kinds of smaller molecules.

These are the letters
of the genetic alphabet.

Particular arrangements
of those letters...

spell out the instructions
for all living things...

telling them how to
grow, move, digest...

sense the environment,
heal, and reproduce.

The DNA double helix
is a molecular machine...

with about 100 billion
parts called atoms.

There are as many atoms in a
single molecule of your DNA...

as there are stars
in a typical galaxy.

The same is true
for dogs and bears...

and every living thing.

We are, each of
us, a little universe.

The DNA message handed
down from cell to cell...

and from generation
to generation is copied...

with extreme care.

The birth of a new DNA molecule
begins when an unwinding protein...

separates the two strands
of the double helix...

breaking the rungs apart.

Inside the liquid
of the nucleus...

the molecular letters of
the genetic code float freely.

Each strand of the helix
copies its lost partner...

resulting in two
identical DNA molecules.

That's how life reproduces
genes and transmits them...

from one generation to the next.

When a living cell
divides in two...

each one takes away with it
a complete copy of the DNA.

A specialized protein
proofreads to make sure...

that only the right letters are accepted
so that the DNA is accurately copied.

But nobody's perfect.

Occasionally, a proofreading
error slips through...

making a small, random
change in the genetic instructions.

A mutation has occurred
in the bear's egg cell.

A random event as tiny as this
one can have consequences...

on a far grander scale.

That mutation altered the
gene that controls fur color.

It will affect the production
of dark pigment in the fur...

of the bear's offspring.

Most mutations are harmless.

Some are deadly.

But a few, purely by chance,
can give an organism...

a critical advantage
over the competition.

A year has passed. Our
bear is now a mother.

And as a result
of that mutation...

one of her two cubs was
born with a white coat.

When the cubs get old enough
to venture out on their own...

which bear is more likely...

to be able to sneak up
on unsuspecting prey?

The brown bear can be seen
against the snow a mile away.

The white bear
prospers and passes on...

its own particular set of genes.

This happens repeatedly.

Over succeeding generations...

the gene for white fur spreads through
the entire population of Arctic bears.

The gene for dark fur loses
out in the competition for survival.

Mutations are entirely
random and happen all the time.

But the environment
rewards those...

that increase the
chance for survival.

It naturally selects the living things
that are better suited to survive.

And that selection is
the opposite of random.

The two populations of bears
separated and over thousands of years...

evolved other characteristics
that set them apart.

They became different species.

That's what Charles Darwin
meant by "the origin of species."

An individual bear
doesn't evolve.

The population of bears
evolves over generations.

If the Arctic ice
continues to dwindle...

due to global warming, the
polar bears may go extinct.

They'll be replaced
by brown bears...

better adapted to the
now defrosted environment.

This is a different story
from the one about the dogs.

No breeder guided these changes.

Instead, the environment
itself selects them.

This is evolution by
natural selection...

the most revolutionary
concept in the history of science.

Darwin first presented the
evidence for this idea in 1859.

The uproar it caused
has never subsided.

Why?

We all understand the
twinge of discomfort...

at the thought that we share a
common ancestor with the apes.

No one can embarrass
you like a relative.

Our closest ones,
the chimpanzees...

they frequently behave
inappropriately in public.

There's an understandable human
need to distance ourselves from them.

A central premise
of traditional belief...

is that we were created
separately from all the other animals.

It's easy to see why this idea has
taken hold. It makes us feel special.

But what about our kinship with the
trees? How does that make you feel?

Okay, here's a segment of the oak
tree's DNA. Think of it like a barcode.

The instructions written in the code of
life tell the tree how to metabolize sugar.

Now let's compare it with the
same section of my own DNA.

The DNA doesn't lie.

This tree and me,
we're long-lost cousins.

And it's not just the trees.

If you go back far enough,
you'll find that we share...

a common ancestor
with the butterfly...

gray wolf...

mushroom...

shark...

bacterium...

sparrow.

What a family.

Other parts of the barcode
vary from species to species.

That's what makes the difference
between an owl and an octopus.

Unless you have
an identical twin...

there's no one else in the universe
with the exact same DNA as you.

Within other species,
the genetic differences...

provide the raw material
for natural selection.

The environment selects
which genes survive and multiply.

When it comes to the genetic instructions
for life's most basic functions...

say, digesting sugars, we and
other species are almost identical.

That's because those
functions are so basic to life...

they evolved before the various
life-forms branched off from each other.

This is our tree of life.

Science has made it possible
for us to construct this family tree...

for all the species
of life on Earth.

Close genetic relatives occupy
the same branch of the tree...

while more distant
cousins are farther away.

Each twig is a living species.

And the trunk of the tree
represents the common ancestors...

of all life on Earth.

The stuff of life
is so malleable...

that once it got started,
the environment molded it...

into a staggering
variety of forms...

10,000 times more than
we can possibly show here.

Biologists have cataloged...

a half a million different
kinds of beetles alone.

Not to mention the
numberless varieties of bacteria.

There are many
millions of living species...

of animals and plants, most of
them still unknown to science.

Think of that. We have
yet to make contact...

with most of the
forms of terrestrial life.

That's how many kinds of life
there are on this tiny planet alone.

The tree of life extends
its feelers in all directions...

finding and exploiting what
works, creating new environments...

and opportunities for new forms.

The tree of life is three
and a half billion years old.

That's plenty of time to develop
an impressive repertoire of tricks.

Evolution can disguise
an animal as a plant...

taking thousands of generations
to contrive an elaborate costume...

that fools predators into looking
elsewhere for someone to eat.

Or it can disguise a
plant as an animal...

evolving blossoms that take
on the appearance of a wasp...

the orchid's way of fooling
real wasps into pollinating it.

This is the awesome shape-shifting
power of natural selection.

Among the dense, tangled
limbs of the vast tree of life...

you are here.

One tiny branch among
countless millions.

Science reveals that
all life on Earth is one.

Darwin discovered the
actual mechanism of evolution.

The prevailing belief
was that the complexity...

and variety of life must be the
work of an intelligent designer...

who created each of these millions
of different species separately.

Living things are just
too intricate, it was said...

to be the result of
unguided evolution.

Consider the human eye, a
masterpiece of complexity.

It requires a cornea,
iris, lens, retina...

optic nerves, muscles...

let alone the brain's elaborate
neural network to interpret images.

It's more complicated than any device
ever crafted by human intelligence.

Therefore, it was argued, the
human eye can't be the result...

of mindless evolution.

To know if that's true, we
need to travel across time...

to a world before
there were eyes to see.

In the beginning,
life was blind.

This is what our world looked
like four billion years ago...

before there were
any eyes to see.

Until a few hundred million
years passed, and then, one day...

there was a microscopic copying
error in the DNA of a bacterium.

This random mutation gave that microbe
a protein molecule that absorbed sunlight.

Want to know what the world looked
like to a light-sensitive bacterium?

Take a look at the
right side of the screen.

Mutations continued
to occur at random...

as they always do in any
population of living things.

Another mutation caused a
dark bacterium to flee intense light.

What is going on here?

Night and day.

Those bacteria that
could tell light from dark...

had a decisive advantage
over the ones that couldn't.

Why? Because the daytime
brought harsh, ultra violet light...

that damages DNA.

The sensitive bacteria
fled the intense light...

to safely exchange
their DNA in the dark.

They survived in
greater numbers...

than the bacteria that
stayed at the surface.

Over time, those light-sensitive proteins
became concentrated in a pigment spot...

on the more advanced,
one-celled organism.

This made it possible
to find the light...

an overwhelming advantage...

for an organism that
harvests sunlight to make food.

Here's a flatworm's-eye
view of the world.

This multi-celled organism
evolved a dimple in the pigment spot.

The bowl-shaped depression...

allowed the animal to
distinguish light from shadow...

to crudely make out
objects in its vicinity...

including those to eat
and those that might eat it...

a tremendous advantage.

Later, things became a little
clearer. The dimple deepened...

and evolved into a socket
with a small opening.

Over thousands of generations...

natural selection was
slowly sculpting the eye.

The opening contracted to a pinhole covered
by a protective transparent membrane.

Only a little light
could enter the tiny hole

but it was enough to
paint a dim image...

on the sensitive inner surface of
the eye. This sharpened the focus.

A larger opening would
have let in more light...

to make a brighter image
but one that was out of focus.

This development launched the
visual equivalent of an arms race.

The competition needed
to keep up to survive.

But then a splendid new
feature of the eye evolved...

a lens that provided both
brightness and sharp focus.

In the eyes of primitive fish...

the transparent gel near
the pinhole formed into a lens.

At the same time, the pinhole
enlarged to let in more and more light.

Fish could now
see in high-def...

both close up and far away.

And then something
terrible happened.

Have you ever noticed that
a straw in a glass of water...

looks bent at the
surface of the water?

That's because light bends
when it goes from one medium...

to another, say
from water to air.

Our eyes originally
evolved to see in water.

The watery fluid
in those eyes...

neatly eliminated the
distortion of that bending effect.

But for land animals, the light
carries images from dry air...

into their still-watery eyes.

That bends the light rays
causing all kinds of distortions.

When our amphibious ancestors
left the water for the land...

their eyes, exquisitely
evolved to see in water...

were lousy for
seeing in the air.

Our vision has never
been as good since.

We like to think of our
eyes as state-of-the-art...

but 375 million years later...

we still can't see things
right in front of our noses...

or discern fine details in near
darkness the way fish can.

When we left the water, why
didn't nature just start over again...

and evolve us a new set of eyes
that were optimal for seeing in the air?

Nature doesn't work that way.

Evolution reshapes existing
structures over generations...

adapting them
with small changes.

It can't just go back to the drawing
board and start from scratch.

At every stage of its
development, the evolving eye...

functioned well enough to provide
a selective advantage for survival.

And among animals
alive today, we find eyes...

at all these stages
of development.

And all of them function.

The complexity
of the human eye...

poses no challenge to
evolution by natural selection.

In fact, the eye and all of biology
makes no sense without evolution.

Some claim that evolution is just a
theory as if it were merely an opinion.

The theory of evolution,
like the theory of gravity...

is a scientific fact.

Evolution really happened.

Accepting our kinship with all life
on Earth is not only solid science.

In my view, it's also a
soaring spiritual experience.

Because evolution is blind...

it cannot anticipate or
adapt to catastrophic events.

The tree of life has
some broken branches.

Many of them were severed in
the five greatest catastrophes...

that life has ever known.

Somewhere, there's a memorial...

to the multitude of lost
species, the Halls of Extinction.

Come with me.

Welcome to the
Halls of Extinction.

A monument to the broken
branches on the tree of life.

For every single one of the
millions of species alive today...

perhaps a thousand
others have perished.

Most of them died out in everyday
competition with other life-forms.

But many of them were swept
away in vast cataclysms...

that overwhelmed the planet.

In the last 500 million years,
this has happened five times.

Five extinctions
devastated life on Earth.

The worst happened
250 million years ago...

at the end of an era
known as the Permian.

Trilobites were armored
animals that hunted...

in great herds
across the seafloor.

They were among the first animals
to evolve image-forming eyes.

Trilobites had a good long
run, some 270 million years.

Earth was once the
planet of the trilobites.

But now they're
all gone. Extinct.

The last of them were
swept from life's stage...

along with countless other species in
an unparalleled environmental disaster.

The apocalypse began
in what is now Siberia...

with volcanic eruptions on a scale
unlike anything in human experience.

Earth was very different then...

with one single supercontinent
and one great ocean.

Relentless floods
of fiery lava...

engulfed an area larger
than Western Europe.

The pulsing eruptions went on
for hundreds of thousands of years.

The molten rock ignited coal
deposits and polluted the air...

with carbon dioxide and
other greenhouse gases.

This heated the Earth...

and stopped the ocean
currents from circulating.

Noxious bacteria bloomed...

but nearly everything
else in the seas died.

The stagnant waters belched deadly
hydrogen sulfide gas into the air...

which suffocated most
of the land animals.

Nine in 10 of all species
on the planet went extinct.

We call it the Great Dying.

Life on Earth came so
near to being wiped out...

that it took more than 10
million years to recover.

But new life-forms
slowly evolved...

to fill the openings left
by the Permian holocaust.

Among the biggest
winners were the dinosaurs.

Now the Earth was their planet.

Their reign continued
for over 150 million years.

Until it too came crashing
down in another mass extinction.

Life on Earth has taken
quite a beating over the eons.

And yet it's still there. The
tenacity of life is mind-boggling.

We keep finding it where
no one thought it could be.

That nameless corridor?

That's for another day.

I know an animal that can live
in boiling water or in solid ice.

It can go 10 years
without a drop of water.

It can travel naked
in the cold vacuum...

and intense radiation of
space and will return unscathed.

The tardigrade, or water bear.

It's equally at home atop
the tallest mountains...

and in the deepest
trenches of the sea.

And in our own backyards,
where they live among the moss...

in countless numbers.

You've probably never noticed
them because they're so small.

About the size of a pinpoint.

But they're tough.

The tardigrades have survived
all five mass extinctions.

They've been in business
for a half a billion years.

We used to think that life was
finicky, that it would only take hold...

where it was not
too hot, not too cold...

not too dark or salty
or acidic or radioactive.

And whatever you do,
don't forget to add water.

We were wrong. As the hardy
tardigrade demonstrates...

life can endure conditions that would
mean certain death for us humans.

But differences between
us and life found...

in even the most extreme
environments on our planet...

are only variations on a single
theme, dialects of a single language.

The genetic code of Earth life.

But what would life
be like on other worlds?

Worlds with a completely
different history...

chemistry and evolution
from our planet?

There's a distant world
I wanna take you to.

A world far different from our
own, but one that may harbor life.

If it does, it promises
to be unlike anything...

we've ever seen before.

Clouds and haze completely
hide the surface of Titan...

Saturn's giant moon.

Titan reminds me
a little bit of home.

Like Earth, it has an
atmosphere that's mostly nitrogen.

But it's four times denser.

Titan's air has
no oxygen at all.

And it's far colder than anywhere
on Earth. But still, I wanna go there.

We have to descend through a
couple hundred kilometers of smog...

before we can even
see the surface.

But hidden beneath lies a
weirdly familiar landscape.

Titan is the only other world in
the solar system where it ever rains.

It has rivers and coastlines.

Titan has hundreds of lakes. One of them
larger than Lake Superior in North America.

Vapor rising from the lakes
condenses and falls again as rain.

The rain feeds rivers...

which carve valleys into the
landscape, just like on Earth.

But with one big difference.

On Titan, the
seas and the rain...

are made not of water,
but of methane and ethane.

On Earth, those
molecules form natural gas.

On frigid Titan, they're liquid.

Titan has lots of water but
all of it is frozen hard as rock.

In fact, the landscape and mountains
are made mainly of water ice.

At hundreds of
degrees below zero...

Titan is far too cold for
water to ever be liquid.

Astrobiologists since Carl
Sagan have wondered...

if life might swim in
Titan's hydrocarbon lakes.

The chemical
basis for such life...

would have to be entirely
different from anything we know.

All life on Earth depends on liquid water,
and Titan's surface has none of that.

But we can imagine
other kinds of life.

There might be creatures that
inhale hydrogen instead of oxygen.

And exhale methane
instead of carbon dioxide.

They might use acetylene instead
of sugar as an energy source.

How could we find
out if such creatures...

rule a hidden empire
beneath the oil-dark waves?

We're diving down deep
into the Kraken Sea...

named for the mythic
Norse sea monster.

Even if there is one of those down
there, we probably couldn't see it.

It's so dark.

If you took all the oil
and natural gas on Earth...

it would amount to but a tiny
fraction of Titan's reserves.

Let's turn on some lights.

We're now 200 meters
beneath the surface.

Did you see something?
Over there, by that vent.

Maybe it was just my imagination.
I guess we'll have to come back...

if we want to find out for sure.

There's one last
story I want to tell you.

And it's the greatest
story science has ever told.

It's the story of
life on our world.

Welcome to the Earth
of four billion years ago.

This was our planet before life.

Nobody knows
how life got started.

Most of the evidence from
that time was destroyed...

by impact and erosion.

Science works on the frontier
between knowledge and ignorance.

We're not afraid to
admit what we don't know.

There's no shame in that.

The only shame is to pretend
that we have all the answers.

Maybe someone watching this...

will be the first to solve the
mystery of how life on Earth began.

The evidence from
living microbes...

suggest that their earliest ancestors
preferred high temperatures.

Life on Earth may have arisen in hot
water around submerged volcanic vents.

In Carl Sagan's
original Cosmos series...

he traced the unbroken
thread that stretches...

directly from the
one-celled organisms...

of nearly four billion
years ago to you.

Four billion years
in 40 seconds.

From creatures who had
yet to discern day from night...

to beings who are
exploring the cosmos.

Those are some of the
things that molecules do...

given four billion
years of evolution.