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.
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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 of them 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;
the domestication of 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 15,000 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 10,000 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, 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 bar code.
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 bar code 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 catalogued
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 mechanisof 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 of 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 the 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 mass extinctions that devastated life on Earth.
The worst one of all happened some 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 ten 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 ten 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 ten 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 want to 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 want to 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.
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