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, snugglers, 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, 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 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.