Cosmos: Possible Worlds (2014–…): Season 1, Episode 2 - Some of the Things That Molecules Do - full transcript
Host Neil deGrasse Tyson is on a voyage to explore the relatedness of all livings things and the possible evolution of life in the cosmos. Go on a journey to discover how artificial selection turned the wolf into canine breeds and how natural selection sculpted the complex human eye. Later, Tyson visits the grand Hall of Extinction, a monument to all the broken branches on the tree of life. Retrace the story of life on Earth and the unbroken thread that stretches from the first one-celled organisms to human life.
TYSON: This is a story about
you and me and your dog.
[ANIMAL HOWLING]
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.
"Some of the Things That Molecules Do"
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.
[WOLF GROWLING]
[SNARLING]
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."
[CHUCKLES]
[DOG WHIMPERS]
Then as now, this was a good
deal for the humans too.
The scavenging dogs
weren't just a sanitation squad.
They worked security.
[DOG BARKING]
[WOLF GROWLING]
[DOG BARKING]
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.
[DOGS BARKING]
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.
[GROWLING]
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.
[HEART BEATING]
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?
[BIRDS CHIRPING]
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, ultraviolet 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 flatworms-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 Hails 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.
[RUMBLING AND EXPLOSIONS]
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
1O 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.
SAGAN: Those are some of the things
that molecules do...
...given four billion years of evolution.
you and me and your dog.
[ANIMAL HOWLING]
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.
"Some of the Things That Molecules Do"
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.
[WOLF GROWLING]
[SNARLING]
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."
[CHUCKLES]
[DOG WHIMPERS]
Then as now, this was a good
deal for the humans too.
The scavenging dogs
weren't just a sanitation squad.
They worked security.
[DOG BARKING]
[WOLF GROWLING]
[DOG BARKING]
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.
[DOGS BARKING]
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.
[GROWLING]
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.
[HEART BEATING]
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?
[BIRDS CHIRPING]
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, ultraviolet 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 flatworms-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 Hails 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.
[RUMBLING AND EXPLOSIONS]
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
1O 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.
SAGAN: Those are some of the things
that molecules do...
...given four billion years of evolution.