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TYSON: This is a story about
you and me and your dog.
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[ANIMAL HOWLING]
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There was a time not long ago,
before dogs.
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They didn't exist.
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Now there are big ones, small ones,
smugglers, guardians, hunters.
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Every kind of dog you could possibly want.
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How did that happen?
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It's not just dogs.
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Where did all the different kinds
of living creatures come from?
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The answer is a transforming power
that sounds like something...
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...straight out of a fairy tale or myth,
but it's no such thing.
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"Some of the Things That Molecules Do"
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Let's go back across 30,000 years
to a time before dogs...
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...when our ancestors lived
in the endless winter of the last ice age.
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Our ancestors were wanderers
living in small bands.
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They slept beneath the stars.
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The sky was their storybook, calendar,
an instruction manual for living.
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It told them when the bitter colds
would come...
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...when the wild grains would ripen...
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...when the herds of caribou and bison
would be on the move.
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Their idea of home was Earth itself.
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But they lived in fear
of other hungry creatures.
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The mountain lions and the bears
that competed with them for the same prey.
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And the wolves that threatened
to carry off and devour...
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...the most vulnerable among them.
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[WOLF GROWLING]
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[SNARLING]
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All the wolves want to get at the bone...
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...but most are too frightened
to come close enough.
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Their fear is due to high levels
of stress hormones in their blood.
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It's a matter of survival.
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Because coming too close to humans
can be fatal.
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But a few wolves, due to
natural variations...
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...have lower levels of those hormones.
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This makes them less afraid of humans.
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This wolf has discovered what a branch
of his ancestors figured out...
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...some 15,000 years ago.
An excellent survival strategy.
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Domestication, humans.
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Let the humans do the hunting,
don't threaten them...
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...and they'll let you
scavenge their garbage.
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You'll eat more regularly,
you'll leave more offspring...
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...and those offspring
will inherit your disposition.
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This selection for tameness would be
reinforced with each generation...
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...until that line of wild wolves...
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...evolves into dogs.
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You might call this
"survival of the friendliest."
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[CHUCKLES]
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[DOG WHIMPERS]
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Then as now, this was a good
deal for the humans too.
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The scavenging dogs
weren't just a sanitation squad.
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They worked security.
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[DOG BARKING]
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[WOLF GROWLING]
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[DOG BARKING]
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As this interspecies partnership
continued over time...
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...the dogs' appearance changed also.
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Cuteness became a selective advantage.
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The more adorable you were,
the better chance you had...
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...to live and pass on your genes
to another generation.
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What began as an alliance
of convenience...
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...became a friendship
that deepened over time.
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To see what happens next...
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...let's leave our distant ancestors...
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...of some 20,000 years ago
to visit the more recent past...
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...during an intermission in the Ice Age.
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This break in the climate
starts a revolution.
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Instead of wandering,
people are settling down.
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There's something new
in the world: villages.
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People still hunt and gather, but now
they also produce food and clothing.
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Agriculture.
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The wolves have traded their freedom
in exchange for a steady meal.
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They've given up their right
to choose a mate.
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Now the humans choose for them.
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They consistently kill off the dogs
that can't be trained...
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...the ones that bite the feeding hand.
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And they breed the dogs that please them.
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[DOGS BARKING]
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They nurture those dogs
that do their bidding...
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...hunting, herding, guarding, hauling,
and keeping them company.
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From every litter...
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...the humans select the puppies
they like best.
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Over the generations, the dogs evolve.
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This kind of evolution is called
artificial selection...
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...or breeding.
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Turning wolves into dogs
was the first time...
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...we humans took evolution
into our own hands.
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And we've been doing it ever since...
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...to shape all the plants and animals
that we depend on.
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In a blink of cosmic time,
just 15- or 20,000 years...
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...we turned gray wolves into
all the kinds of dogs we love today.
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Think of it.
Every breed of dog you've ever seen...
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...was sculpted by human hands.
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Many of our best friends,
the most popular breeds...
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...were created in only
the last few centuries.
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The awesome power of evolution
transformed the ravenous wolf...
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...into the faithful shepherd...
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...who protects the herd
and drives the wolf away.
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[GROWLING]
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Artificial selection
turned the wolf into the shepherd...
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...and the wild grasses
into wheat and corn.
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In fact, almost every plant
and animal that we eat today...
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...was bred from a wild,
less-edible ancestor.
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If artificial selection can work
such profound changes...
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...in only 10 or 15,000 years,
what can natural selection do...
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...operating over billions of years?
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The answer is all the beauty
and diversity of life.
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How does it work?
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Our ship of the imagination
can take us anywhere in space and time...
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...even to the hidden microcosmos...
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...where one kind of life
can be transformed into another.
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Come with me.
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May not seem like it...
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...but we've been living in an ice age
for the last two million years.
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This just happens to be
one of the long intermissions.
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For most of those two million years,
the climate has been cold and dry.
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The North Polar ice cap
extended much farther south...
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...than it does today.
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In one of those long,
cold glacial periods...
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...when the winter sea ice
stretched from the North Pole...
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...all the way down
to what is now Los Angeles...
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...great bears roamed
the frozen wastes of Ireland.
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This might look like an ordinary bear...
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...but something extraordinary
is happening inside her.
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Something that will give rise
to a new species.
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In order to see it, we'll need to
descend down to a much smaller scale...
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...to the cellular level...
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...so that we can explore
the bear's reproductive system.
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We'll take the subclavian
artery through the heart.
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[HEART BEATING]
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Almost there.
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Those are some of her eggs.
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To see what's going on in one of them,
we'll have to get even smaller.
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We'll have to shrink down
to the molecular level.
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Our ship of the imagination
is now so small...
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...you could fit a million of them
into a grain of sand.
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See those guys over there
strutting along those girders?
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They are proteins called kinesin.
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These kinesin
are part of the transport crew...
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...that's busy moving
cargo around the cell.
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How alien they seem.
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And yet these tiny creatures,
and beings like them...
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...are a part of every living cell,
including the ones inside you.
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If life has a sanctuary...
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...it's here in the nucleus
which contains our DNA.
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The ancient scripture of our genetic code.
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And it's written in a language
that all life can read.
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DNA is a molecule
shaped like a long twisted ladder...
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...or double helix.
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The rungs of the ladder are made of
four different kinds of smaller molecules.
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These are the letters
of the genetic alphabet.
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Particular arrangements
of those letters...
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...spell out the instructions
for all living things...
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...telling them how to
grow, move, digest...
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...sense the environment, heal,
and reproduce.
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The DNA double helix
is a molecular machine...
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...with about 100 billion
parts called atoms.
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There are as many atoms
in a single molecule of your DNA...
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...as there are stars in a typical galaxy.
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The same is true for dogs and bears...
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...and every living thing.
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We are, each of us, a little universe.
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The DNA message handed down
from cell to cell...
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...and from generation to generation
is copied...
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...with extreme care.
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The birth of a new DNA molecule
begins when an unwinding protein...
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...separates the two strands
of the double helix...
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...breaking the rungs apart.
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Inside the liquid of the nucleus...
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...the molecular letters of
the genetic code float freely.
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Each strand of the helix
copies its lost partner...
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...resulting in two
identical DNA molecules.
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That's how life reproduces
genes and transmits them...
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...from one generation to the next.
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When a living cell divides in two...
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...each one takes away with it
a complete copy of the DNA.
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A specialized protein
proofreads to make sure...
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...that only the right letters are accepted
so that the DNA is accurately copied.
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But nobody's perfect.
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Occasionally,
a proofreading error slips through...
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...making a small, random change
in the genetic instructions.
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A mutation has occurred
in the bear's egg cell.
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A random event as tiny as this one
can have consequences...
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...on a far grander scale.
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That mutation altered the gene
that controls fur color.
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It will affect the production
of dark pigment in the fur...
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...of the bear's offspring.
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Most mutations are harmless.
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Some are deadly.
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But a few, purely by chance,
can give an organism...
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...a critical advantage
over the competition.
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A year has passed.
Our bear is now a mother.
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00:13:49,579 --> 00:13:51,832
And as a result of that mutation...
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...one of her two cubs
was born with a white coat.
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When the cubs get old enough
to venture out on their own...
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...which bear is more likely...
196
00:14:01,007 --> 00:14:04,432
...to be able to sneak up
on unsuspecting prey?
197
00:14:04,594 --> 00:14:07,894
The brown bear can be seen
against the snow a mile away.
198
00:14:08,056 --> 00:14:10,730
The white bear prospers and passes on...
199
00:14:10,892 --> 00:14:13,611
...its own particular set of genes.
200
00:14:13,770 --> 00:14:16,193
This happens repeatedly.
201
00:14:16,356 --> 00:14:17,983
Over succeeding generations...
202
00:14:18,149 --> 00:14:23,451
...the gene for white fur spreads through
the entire population of Arctic bears.
203
00:14:23,613 --> 00:14:28,961
The gene for dark fur
loses out in the competition for survival.
204
00:14:30,620 --> 00:14:34,750
Mutations are entirely random
and happen all the time.
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00:14:34,916 --> 00:14:36,634
But the environment rewards those...
206
00:14:36,793 --> 00:14:38,966
...that increase the chance for survival.
207
00:14:39,128 --> 00:14:43,804
It naturally selects the living things
that are better suited to survive.
208
00:14:43,966 --> 00:14:46,640
And that selection
is the opposite of random.
209
00:14:50,139 --> 00:14:54,189
The two populations of bears
separated and over thousands of years...
210
00:14:54,352 --> 00:14:57,777
...evolved other characteristics
that set them apart.
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00:14:57,939 --> 00:15:01,364
They became different species.
212
00:15:02,693 --> 00:15:07,665
That's what Charles Darwin meant
by "the origin of species."
213
00:15:08,449 --> 00:15:10,827
An individual bear doesn't evolve.
214
00:15:10,993 --> 00:15:15,840
The population of bears evolves
over generations.
215
00:15:16,666 --> 00:15:18,714
If the Arctic ice continues to dwindle...
216
00:15:18,876 --> 00:15:22,631
...due to global warming,
the polar bears may go extinct.
217
00:15:22,797 --> 00:15:24,720
They'll be replaced by brown bears...
218
00:15:24,882 --> 00:15:27,556
...better adapted
to the now defrosted environment.
219
00:15:29,178 --> 00:15:31,977
This is a different story
from the one about the dogs.
220
00:15:32,140 --> 00:15:34,643
No breeder guided these changes.
221
00:15:34,809 --> 00:15:38,063
Instead, the environment
itself selects them.
222
00:15:38,229 --> 00:15:40,857
This is evolution by natural selection...
223
00:15:41,023 --> 00:15:44,618
...the most revolutionary concept
in the history of science.
224
00:15:44,777 --> 00:15:48,873
Darwin first presented the evidence
for this idea in 1859.
225
00:15:49,031 --> 00:15:52,706
The uproar it caused has never subsided.
226
00:15:52,869 --> 00:15:54,997
Why?
227
00:16:02,712 --> 00:16:04,589
[BIRDS CHIRPING]
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00:16:04,755 --> 00:16:06,849
We all understand
the twinge of discomfort...
229
00:16:07,008 --> 00:16:09,932
...at the thought that we share
a common ancestor with the apes.
230
00:16:10,845 --> 00:16:13,473
No one can embarrass you like a relative.
231
00:16:13,639 --> 00:16:15,562
Our closest ones, the chimpanzees...
232
00:16:15,725 --> 00:16:18,604
...they frequently behave inappropriately
in public.
233
00:16:18,769 --> 00:16:22,694
There's an understandable human need
to distance ourselves from them.
234
00:16:22,857 --> 00:16:25,406
A central premise of traditional belief...
235
00:16:25,568 --> 00:16:29,198
...is that we were created separately
from all the other animals.
236
00:16:29,363 --> 00:16:34,585
It's easy to see why this idea has
taken hold. It makes us feel special.
237
00:16:35,077 --> 00:16:41,130
But what about our kinship with the trees?
How does that make you feel?
238
00:16:49,592 --> 00:16:54,644
Okay, here's a segment of the oak
tree's DNA. Think of it like a barcode.
239
00:16:54,805 --> 00:17:00,107
The instructions written in the code of
life tell the tree how to metabolize sugar.
240
00:17:00,269 --> 00:17:03,148
Now let's compare it with
the same section of my own DNA.
241
00:17:08,986 --> 00:17:10,283
The DNA doesn't lie.
242
00:17:10,446 --> 00:17:14,246
This tree and me, we're long-lost cousins.
243
00:17:14,408 --> 00:17:15,910
And it's not just the trees.
244
00:17:16,077 --> 00:17:18,830
If you go back far enough,
you'll find that we share...
245
00:17:18,996 --> 00:17:21,966
...a common ancestor with the butterfly...
246
00:17:22,124 --> 00:17:26,546
"Gray Wolf... ...mushroom...
...shark...
247
00:17:26,712 --> 00:17:29,473
...bacterium... ...sparrow.
248
00:17:29,632 --> 00:17:31,009
What a family.
249
00:17:31,175 --> 00:17:34,395
Other parts of the barcode vary
from species to species.
250
00:17:34,554 --> 00:17:38,149
That's what makes the difference
between an owl and an octopus.
251
00:17:38,307 --> 00:17:40,355
Unless you have an identical twin...
252
00:17:40,518 --> 00:17:45,024
...there's no one else in the universe
with the exact same DNA as you.
253
00:17:45,189 --> 00:17:47,408
Within other species,
the genetic differences...
254
00:17:47,567 --> 00:17:50,741
...provide the raw material
for natural selection.
255
00:17:50,903 --> 00:17:54,999
The environment selects which genes survive
and multiply.
256
00:17:55,157 --> 00:17:58,661
When it comes to the genetic instructions
for life's most basic functions...
257
00:17:58,828 --> 00:18:03,504
...say, digesting sugars, we and
other species are almost identical.
258
00:18:03,666 --> 00:18:06,966
That's because those functions
are so basic to life...
259
00:18:07,128 --> 00:18:11,053
...they evolved before the various
life-forms branched off from each other.
260
00:18:11,215 --> 00:18:14,014
This is our tree of life.
261
00:18:14,176 --> 00:18:17,726
Science has made it possible
for us to construct this family tree...
262
00:18:17,888 --> 00:18:20,232
...for all the species of life on Earth.
263
00:18:20,391 --> 00:18:23,520
Close genetic relatives occupy
the same branch of the tree...
264
00:18:23,686 --> 00:18:27,065
...while more distant cousins
are farther away.
265
00:18:27,231 --> 00:18:30,360
Each twig is a living species.
266
00:18:31,611 --> 00:18:34,785
And the trunk of the tree
represents the common ancestors...
267
00:18:34,947 --> 00:18:37,666
...of all life on Earth.
268
00:18:37,825 --> 00:18:40,044
The stuff of life is so malleable...
269
00:18:40,202 --> 00:18:42,921
...that once it got started,
the environment molded it...
270
00:18:43,080 --> 00:18:46,129
...into a staggering variety of forms...
271
00:18:46,292 --> 00:18:50,968
...10,000 times more than
we can possibly show here.
272
00:18:52,965 --> 00:18:54,387
Biologists have cataloged...
273
00:18:54,550 --> 00:18:57,645
...a half a million different
kinds of beetles alone.
274
00:19:01,182 --> 00:19:04,652
Not to mention the numberless varieties
of bacteria.
275
00:19:05,186 --> 00:19:07,564
There are many millions
of living species...
276
00:19:07,730 --> 00:19:11,325
...of animals and plants,
most of them still unknown to science.
277
00:19:11,484 --> 00:19:14,533
Think of that.
We have yet to make contact...
278
00:19:14,695 --> 00:19:18,245
...with most of the forms
of terrestrial life.
279
00:19:18,407 --> 00:19:23,379
That's how many kinds of life there are
on this tiny planet alone.
280
00:19:24,288 --> 00:19:27,041
The tree of life extends
its feelers in all directions...
281
00:19:27,208 --> 00:19:30,257
...finding and exploiting what works,
creating new environments...
282
00:19:30,419 --> 00:19:33,093
...and opportunities for new forms.
283
00:19:36,300 --> 00:19:39,725
The tree of life is three
and a half billion years old.
284
00:19:39,887 --> 00:19:44,893
That's plenty of time to develop
an impressive repertoire of tricks.
285
00:19:48,187 --> 00:19:52,112
Evolution can disguise an animal
as a plant...
286
00:19:57,697 --> 00:20:01,543
...taking thousands of generations
to contrive an elaborate costume...
287
00:20:01,701 --> 00:20:06,582
...that fools predators into looking
elsewhere for someone to eat.
288
00:20:06,747 --> 00:20:10,468
Or it can disguise a plant as an animal...
289
00:20:10,626 --> 00:20:13,630
...evolving blossoms that take on
the appearance of a wasp...
290
00:20:13,796 --> 00:20:17,642
...the orchid's way of fooling
real wasps into pollinating it.
291
00:20:19,677 --> 00:20:24,934
This is the awesome shape-shifting power
of natural selection.
292
00:20:31,147 --> 00:20:35,653
Among the dense, tangled limbs
of the vast tree of life...
293
00:20:35,818 --> 00:20:37,866
...you are here.
294
00:20:38,028 --> 00:20:42,078
One tiny branch among countless millions.
295
00:20:42,241 --> 00:20:47,998
Science reveals that all
life on Earth is one.
296
00:20:50,291 --> 00:20:53,761
Darwin discovered the actual mechanism
of evolution.
297
00:20:53,919 --> 00:20:56,513
The prevailing belief
was that the complexity...
298
00:20:56,672 --> 00:21:00,518
...and variety of life must be the work
of an intelligent designer...
299
00:21:00,676 --> 00:21:04,726
...who created each of these millions
of different species separately.
300
00:21:04,889 --> 00:21:08,359
Living things are just
too intricate, it was said...
301
00:21:08,517 --> 00:21:12,067
...to be the result of unguided evolution.
302
00:21:12,229 --> 00:21:17,611
Consider the human eye,
a masterpiece of complexity.
303
00:21:20,529 --> 00:21:26,127
It requires a cornea, iris, lens, retina...
304
00:21:26,285 --> 00:21:28,413
...optic nerves, muscles...
305
00:21:28,579 --> 00:21:33,585
...let alone the brain's elaborate
neural network to interpret images.
306
00:21:34,126 --> 00:21:39,599
It's more complicated than any device
ever crafted by human intelligence.
307
00:21:39,757 --> 00:21:43,853
Therefore, it was argued,
the human eye can't be the result...
308
00:21:44,011 --> 00:21:46,389
...of mindless evolution.
309
00:21:46,555 --> 00:21:50,230
To know if that's true,
we need to travel across time...
310
00:21:50,392 --> 00:21:54,863
...to a world before
there were eyes to see.
311
00:22:14,416 --> 00:22:17,295
In the beginning, life was blind.
312
00:22:19,755 --> 00:22:23,510
This is what our world looked like
four billion years ago...
313
00:22:23,676 --> 00:22:26,555
...before there were any eyes to see.
314
00:22:26,720 --> 00:22:31,601
Until a few hundred million years passed,
and then, one day...
315
00:22:31,767 --> 00:22:35,692
...there was a microscopic copying error
in the DNA of a bacterium.
316
00:22:35,855 --> 00:22:41,112
This random mutation gave that microbe
a protein molecule that absorbed sunlight.
317
00:22:41,277 --> 00:22:44,907
Want to know what the world looked like
to a light-sensitive bacterium?
318
00:22:45,072 --> 00:22:48,372
Take a look at the right
side of the screen.
319
00:22:49,368 --> 00:22:51,462
Mutations continued to occur at random...
320
00:22:51,620 --> 00:22:55,716
...as they always do in any population
of living things.
321
00:22:57,418 --> 00:23:02,640
Another mutation caused a dark bacterium
to flee intense light.
322
00:23:03,299 --> 00:23:05,301
What is going on here?
323
00:23:05,467 --> 00:23:06,844
Night and day.
324
00:23:07,011 --> 00:23:09,639
Those bacteria that could
tell light from dark...
325
00:23:09,805 --> 00:23:12,479
...had a decisive advantage
over the ones that couldn't.
326
00:23:12,641 --> 00:23:16,441
Why? Because the daytime brought
harsh, ultraviolet light...
327
00:23:16,604 --> 00:23:18,698
...that damages DNA.
328
00:23:19,273 --> 00:23:22,026
The sensitive bacteria
fled the intense light...
329
00:23:22,192 --> 00:23:24,866
...to safely exchange
their DNA in the dark.
330
00:23:25,029 --> 00:23:26,656
They survived in greater numbers...
331
00:23:26,822 --> 00:23:29,245
...than the bacteria
that stayed at the surface.
332
00:23:30,117 --> 00:23:34,588
Over time, those light-sensitive proteins
became concentrated in a pigment spot...
333
00:23:34,747 --> 00:23:37,876
...on the more advanced,
one-celled organism.
334
00:23:38,334 --> 00:23:41,554
This made it possible to find the light...
335
00:23:41,712 --> 00:23:43,180
...an overwhelming advantage...
336
00:23:43,339 --> 00:23:46,969
...for an organism
that harvests sunlight to make food.
337
00:23:53,724 --> 00:23:57,319
Here's a flatworms-eye view of the world.
338
00:23:57,478 --> 00:24:01,949
This multi-celled organism
evolved a dimple in the pigment spot.
339
00:24:02,107 --> 00:24:03,609
The bowl-shaped depression...
340
00:24:03,776 --> 00:24:07,451
...allowed the animal to distinguish
light from shadow...
341
00:24:07,613 --> 00:24:10,412
...to crudely make out objects
in its vicinity...
342
00:24:10,574 --> 00:24:14,750
...including those to eat
and those that might eat it...
343
00:24:14,912 --> 00:24:17,131
...a tremendous advantage.
344
00:24:17,790 --> 00:24:21,465
Later, things became a little clearer.
The dimple deepened...
345
00:24:21,627 --> 00:24:24,801
...and evolved into a socket
with a small opening.
346
00:24:24,964 --> 00:24:26,637
Over thousands of generations...
347
00:24:26,799 --> 00:24:30,849
...natural selection
was slowly sculpting the eye.
348
00:24:32,054 --> 00:24:37,231
The opening contracted to a pinhole covered
by a protective transparent membrane.
349
00:24:37,393 --> 00:24:39,680
Only a little light could
enter the tiny hole
350
00:24:39,692 --> 00:24:41,990
but it was enough to
paint a dim image...
351
00:24:42,147 --> 00:24:46,744
...on the sensitive inner surface of
the eye. This sharpened the focus.
352
00:24:46,902 --> 00:24:49,121
A larger opening
would have let in more light...
353
00:24:49,279 --> 00:24:52,874
...to make a brighter image
but one that was out of focus.
354
00:24:53,033 --> 00:24:57,880
This development launched
the visual equivalent of an arms race.
355
00:25:09,425 --> 00:25:13,180
The competition needed to keep up
to survive.
356
00:25:13,345 --> 00:25:17,270
But then a splendid new feature
of the eye evolved...
357
00:25:17,433 --> 00:25:22,610
...a lens that provided both brightness
and sharp focus.
358
00:25:23,272 --> 00:25:24,774
In the eyes of primitive fish...
359
00:25:24,940 --> 00:25:28,615
...the transparent gel
near the pinhole formed into a lens.
360
00:25:28,777 --> 00:25:32,782
At the same time, the pinhole enlarged
to let in more and more light.
361
00:25:32,948 --> 00:25:35,827
Fish could now see in high-def...
362
00:25:35,993 --> 00:25:39,839
...both close up and far away.
363
00:25:39,997 --> 00:25:42,591
And then something terrible happened.
364
00:25:43,751 --> 00:25:46,379
Have you ever noticed
that a straw in a glass of water...
365
00:25:46,545 --> 00:25:48,513
...looks bent at the surface of the water?
366
00:25:48,672 --> 00:25:51,346
That's because light bends
when it goes from one medium...
367
00:25:51,508 --> 00:25:54,261
...to another, say from water to air.
368
00:25:54,428 --> 00:25:58,478
Our eyes originally evolved
to see in water.
369
00:25:58,640 --> 00:26:00,938
The watery fluid in those eyes...
370
00:26:01,101 --> 00:26:03,820
...neatly eliminated the distortion
of that bending effect.
371
00:26:06,482 --> 00:26:10,658
But for land animals,
the light carries images from dry air...
372
00:26:10,819 --> 00:26:13,743
...into their still-watery eyes.
373
00:26:13,906 --> 00:26:18,503
That bends the light rays
causing all kinds of distortions.
374
00:26:19,161 --> 00:26:22,131
When our amphibious ancestors
left the water for the land...
375
00:26:22,289 --> 00:26:25,293
...their eyes,
exquisitely evolved to see in water...
376
00:26:25,459 --> 00:26:27,553
...were lousy for seeing in the air.
377
00:26:27,711 --> 00:26:30,385
Our vision has never been as good since.
378
00:26:30,547 --> 00:26:33,266
We like to think of our eyes
as state-of-the-art...
379
00:26:33,425 --> 00:26:35,723
...but 375 million years later...
380
00:26:35,886 --> 00:26:38,605
...we still can't see things
right in front of our noses...
381
00:26:38,764 --> 00:26:42,985
...or discern fine details in near darkness
the way fish can.
382
00:26:43,143 --> 00:26:46,522
When we left the water,
why didn't nature just start over again...
383
00:26:46,688 --> 00:26:50,864
...and evolve us a new set of eyes
that were optimal for seeing in the air?
384
00:26:51,026 --> 00:26:52,778
Nature doesn't work that way.
385
00:26:52,945 --> 00:26:56,199
Evolution reshapes existing structures
over generations...
386
00:26:56,365 --> 00:26:58,413
...adapting them with small changes.
387
00:26:58,575 --> 00:27:02,751
It can't just go back to the drawing board
and start from scratch.
388
00:27:02,913 --> 00:27:06,087
At every stage of its development,
the evolving eye...
389
00:27:06,250 --> 00:27:10,175
...functioned well enough to provide
a selective advantage for survival.
390
00:27:10,337 --> 00:27:13,716
And among animals alive today,
we find eyes...
391
00:27:13,882 --> 00:27:16,260
...at all these stages of development.
392
00:27:19,680 --> 00:27:21,682
And all of them function.
393
00:27:23,475 --> 00:27:25,398
The complexity of the human eye...
394
00:27:25,561 --> 00:27:28,110
...poses no challenge to evolution
by natural selection.
395
00:27:28,272 --> 00:27:33,745
In fact, the eye and all of biology
makes no sense without evolution.
396
00:27:33,902 --> 00:27:39,204
Some claim that evolution is just a theory
as if it were merely an opinion.
397
00:27:39,366 --> 00:27:42,245
The theory of evolution,
like the theory of gravity...
398
00:27:42,411 --> 00:27:44,413
...is a scientific fact.
399
00:27:44,580 --> 00:27:47,003
Evolution really happened.
400
00:27:47,166 --> 00:27:51,717
Accepting our kinship with all life
on Earth is not only solid science.
401
00:27:51,879 --> 00:27:56,476
In my view,
it's also a soaring spiritual experience.
402
00:28:04,057 --> 00:28:06,059
Because evolution is blind...
403
00:28:06,226 --> 00:28:10,356
...it cannot anticipate or adapt
to catastrophic events.
404
00:28:11,440 --> 00:28:13,909
The tree of life
has some broken branches.
405
00:28:14,067 --> 00:28:17,867
Many of them were severed
in the five greatest catastrophes...
406
00:28:18,030 --> 00:28:20,283
...that life has ever known.
407
00:28:20,449 --> 00:28:22,292
Somewhere, there's a memorial...
408
00:28:22,451 --> 00:28:27,298
...to the multitude of lost species,
the Hails of Extinction.
409
00:28:27,456 --> 00:28:29,129
Come with me.
410
00:28:43,639 --> 00:28:46,483
Welcome to the Halls of Extinction.
411
00:28:47,643 --> 00:28:52,399
A monument to the broken branches
on the tree of life.
412
00:29:02,616 --> 00:29:06,541
For every single one
of the millions of species alive today...
413
00:29:06,703 --> 00:29:09,832
...perhaps a thousand others have perished.
414
00:29:09,998 --> 00:29:13,844
Most of them died out in everyday
competition with other life-forms.
415
00:29:14,002 --> 00:29:17,472
But many of them were swept away
in vast cataclysms...
416
00:29:17,631 --> 00:29:19,383
...that overwhelmed the planet.
417
00:29:20,092 --> 00:29:24,518
In the last 500 million years,
this has happened five times.
418
00:29:26,431 --> 00:29:30,436
Five extinctions devastated life on Earth.
419
00:29:31,144 --> 00:29:35,524
The worst happened
250 million years ago...
420
00:29:35,691 --> 00:29:40,367
...at the end of an era
known as the Permian.
421
00:29:57,004 --> 00:29:59,098
Trilobites were
armored animals that hunted...
422
00:29:59,256 --> 00:30:02,135
...in great herds across the seafloor.
423
00:30:02,301 --> 00:30:06,556
They were among the first animals to evolve
image-forming eyes.
424
00:30:09,433 --> 00:30:13,654
Trilobites had a good long run,
some 270 million years.
425
00:30:13,812 --> 00:30:17,077
Earth was once the planet
of the trilobites.
426
00:30:17,089 --> 00:30:20,366
But now they're all gone.
Extinct.
427
00:30:20,527 --> 00:30:22,996
The last of them
were swept from life's stage...
428
00:30:23,155 --> 00:30:29,413
...along with countless other species
in an unparalleled environmental disaster.
429
00:30:33,999 --> 00:30:37,629
[RUMBLING AND EXPLOSIONS]
430
00:30:38,211 --> 00:30:42,091
The apocalypse
began in what is now Siberia...
431
00:30:42,257 --> 00:30:48,765
...with volcanic eruptions on a scale
unlike anything in human experience.
432
00:31:01,151 --> 00:31:03,404
Earth was very different then...
433
00:31:03,570 --> 00:31:07,746
...with one single supercontinent
and one great ocean.
434
00:31:07,908 --> 00:31:10,331
Relentless floods of fiery lava...
435
00:31:10,494 --> 00:31:13,794
...engulfed an area larger
than Western Europe.
436
00:31:13,955 --> 00:31:18,426
The pulsing eruptions went on for hundreds
of thousands of years.
437
00:31:18,585 --> 00:31:22,089
The molten rock ignited coal deposits
and polluted the air...
438
00:31:22,255 --> 00:31:25,805
...with carbon dioxide
and other greenhouse gases.
439
00:31:25,967 --> 00:31:27,219
This heated the Earth...
440
00:31:27,386 --> 00:31:31,357
...and stopped the ocean currents
from circulating.
441
00:31:45,946 --> 00:31:47,948
Noxious bacteria bloomed...
442
00:31:48,115 --> 00:31:51,836
...but nearly everything else
in the seas died.
443
00:31:51,993 --> 00:31:56,669
The stagnant waters belched
deadly hydrogen sulfide gas into the air...
444
00:31:56,832 --> 00:32:00,382
...which suffocated most
of the land animals.
445
00:32:13,181 --> 00:32:16,811
Nine in 10 of all species
on the planet went extinct.
446
00:32:17,853 --> 00:32:20,948
We call it the Great Dying.
447
00:32:33,702 --> 00:32:36,626
Life on Earth came so near
to being wiped out...
448
00:32:36,788 --> 00:32:40,088
...that it took more than
1O million years to recover.
449
00:32:40,250 --> 00:32:42,298
But new life-forms slowly evolved...
450
00:32:42,461 --> 00:32:46,261
...to fill the openings left
by the Permian holocaust.
451
00:32:53,972 --> 00:32:57,226
Among the biggest winners
were the dinosaurs.
452
00:32:57,392 --> 00:33:00,111
Now the Earth was their planet.
453
00:33:00,270 --> 00:33:04,320
Their reign continued
for over 150 million years.
454
00:33:04,483 --> 00:33:09,364
Until it too came crashing down
in another mass extinction.
455
00:33:09,821 --> 00:33:12,415
Life on Earth has taken quite a beating
over the eons.
456
00:33:12,574 --> 00:33:17,831
And yet it's still there.
The tenacity of life is mind-boggling.
457
00:33:17,996 --> 00:33:20,590
We keep finding it
where no one thought it could be.
458
00:33:25,420 --> 00:33:27,798
That nameless corridor?
459
00:33:28,465 --> 00:33:31,059
That's for another day.
460
00:33:36,348 --> 00:33:41,149
I know an animal that can live
in boiling water or in solid ice.
461
00:33:41,311 --> 00:33:44,485
It can go 10 years without a drop of water.
462
00:33:44,648 --> 00:33:46,650
It can travel naked in the cold vacuum...
463
00:33:46,816 --> 00:33:51,196
...and intense radiation of space
and will return unscathed.
464
00:33:51,363 --> 00:33:53,786
The tardigrade, or water bear.
465
00:33:53,949 --> 00:33:56,202
It's equally at home
atop the tallest mountains...
466
00:33:56,368 --> 00:33:58,837
...and in the deepest trenches of the sea.
467
00:33:58,995 --> 00:34:01,714
And in our own backyards,
where they live among the moss...
468
00:34:01,873 --> 00:34:04,171
...in countless numbers.
469
00:34:04,334 --> 00:34:07,304
You've probably never noticed them
because they're so small.
470
00:34:07,462 --> 00:34:09,464
About the size of a pinpoint.
471
00:34:09,631 --> 00:34:10,974
But they're tough.
472
00:34:11,132 --> 00:34:14,978
The tardigrades have survived
all five mass extinctions.
473
00:34:15,136 --> 00:34:17,810
They've been in business
for a half a billion years.
474
00:34:17,973 --> 00:34:21,273
We used to think that life was finicky,
that it would only take hold...
475
00:34:21,434 --> 00:34:24,233
...where it was not too
hot, not too cold...
476
00:34:24,396 --> 00:34:27,650
...not too dark or salty
or acidic or radioactive.
477
00:34:27,816 --> 00:34:30,945
And whatever you do,
don't forget to add water.
478
00:34:31,111 --> 00:34:34,490
We were wrong.
As the hardy tardigrade demonstrates...
479
00:34:34,656 --> 00:34:39,036
...life can endure conditions that would
mean certain death for us humans.
480
00:34:39,202 --> 00:34:41,204
But differences between us
and life found...
481
00:34:41,371 --> 00:34:43,999
...in even the most extreme environments
on our planet...
482
00:34:44,165 --> 00:34:49,547
...are only variations on a single theme,
dialects of a single language.
483
00:34:49,713 --> 00:34:51,807
The genetic code of Earth life.
484
00:34:56,886 --> 00:34:59,548
But what would life be
like on other worlds?
485
00:34:59,560 --> 00:35:02,234
Worlds with a completely
different history...
486
00:35:02,392 --> 00:35:05,612
...chemistry and evolution from our planet?
487
00:35:07,856 --> 00:35:11,326
There's a distant world
I wanna take you to.
488
00:35:11,901 --> 00:35:17,158
A world far different from our own,
but one that may harbor life.
489
00:35:17,324 --> 00:35:21,295
If it does,
it promises to be unlike anything...
490
00:35:21,453 --> 00:35:24,127
...we've ever seen before.
491
00:35:43,224 --> 00:35:46,444
Clouds and haze completely hide
the surface of Titan...
492
00:35:46,603 --> 00:35:48,731
...Saturn's giant moon.
493
00:35:48,897 --> 00:35:51,491
Titan reminds me a little bit of home.
494
00:35:51,650 --> 00:35:54,529
Like Earth, it has an atmosphere
that's mostly nitrogen.
495
00:35:54,694 --> 00:35:56,742
But it's four times denser.
496
00:35:56,905 --> 00:35:59,533
Titan's air has no oxygen at all.
497
00:35:59,699 --> 00:36:06,048
And it's far colder than anywhere on Earth.
But still, I wanna go there.
498
00:36:08,375 --> 00:36:11,845
We have to descend through
a couple hundred kilometers of smog...
499
00:36:12,003 --> 00:36:14,597
...before we can even see the surface.
500
00:36:14,756 --> 00:36:19,603
But hidden beneath
lies a weirdly familiar landscape.
501
00:36:27,394 --> 00:36:31,649
Titan is the only other world
in the solar system where it ever rains.
502
00:36:31,815 --> 00:36:35,285
It has rivers and coastlines.
503
00:36:39,656 --> 00:36:45,288
Titan has hundreds of lakes. One of them
larger than Lake Superior in North America.
504
00:36:45,453 --> 00:36:49,253
Vapor rising from the lakes condenses
and falls again as rain.
505
00:36:50,792 --> 00:36:53,011
The rain feeds rivers...
506
00:36:53,795 --> 00:36:59,393
...which carve valleys into the landscape,
just like on Earth.
507
00:37:00,969 --> 00:37:02,937
But with one big difference.
508
00:37:03,096 --> 00:37:05,474
On Titan, the seas and the rain...
509
00:37:05,640 --> 00:37:09,690
...are made not of water,
but of methane and ethane.
510
00:37:09,853 --> 00:37:12,527
On Earth, those molecules form natural gas.
511
00:37:14,107 --> 00:37:17,532
On frigid Titan, they're liquid.
512
00:37:22,991 --> 00:37:27,622
Titan has lots of water
but all of it is frozen hard as rock.
513
00:37:27,787 --> 00:37:32,543
In fact, the landscape and mountains
are made mainly of water ice.
514
00:37:32,709 --> 00:37:34,882
At hundreds of degrees below zero...
515
00:37:35,044 --> 00:37:38,548
...Titan is far too cold
for water to ever be liquid.
516
00:37:40,759 --> 00:37:44,104
Astrobiologists since Carl Sagan
have wondered...
517
00:37:44,262 --> 00:37:47,983
...if life might swim
in Titan's hydrocarbon lakes.
518
00:37:49,893 --> 00:37:51,645
The chemical basis for such life...
519
00:37:51,811 --> 00:37:55,441
...would have to be entirely different
from anything we know.
520
00:37:55,607 --> 00:38:00,488
All life on Earth depends on liquid water,
and Titan's surface has none of that.
521
00:38:00,653 --> 00:38:02,951
But we can imagine other kinds of life.
522
00:38:03,114 --> 00:38:07,164
There might be creatures that
inhale hydrogen instead of oxygen.
523
00:38:07,327 --> 00:38:10,422
And exhale methane
instead of carbon dioxide.
524
00:38:10,580 --> 00:38:14,050
They might use acetylene
instead of sugar as an energy source.
525
00:38:14,209 --> 00:38:16,553
How could we find out if such creatures...
526
00:38:16,711 --> 00:38:21,012
...rule a hidden empire
beneath the oil-dark waves?
527
00:38:36,523 --> 00:38:39,868
We're diving down deep
into the Kraken Sea...
528
00:38:40,026 --> 00:38:43,246
...named for the mythic
Norse sea monster.
529
00:38:46,074 --> 00:38:49,749
Even if there is one of those down there,
we probably couldn't see it.
530
00:38:49,911 --> 00:38:52,209
It's so dark.
531
00:38:53,331 --> 00:38:56,756
If you took all the oil
and natural gas on Earth...
532
00:38:56,918 --> 00:39:00,764
...it would amount to but a tiny fraction
of Titan's reserves.
533
00:39:04,259 --> 00:39:06,603
Let's turn on some lights.
534
00:39:10,682 --> 00:39:14,607
We're now 200 meters beneath the surface.
535
00:39:18,356 --> 00:39:22,987
Did you see something?
Over there, by that vent.
536
00:39:23,152 --> 00:39:27,077
Maybe it was just my imagination.
I guess we'll have to come back...
537
00:39:27,240 --> 00:39:29,493
...if we want to find out for sure.
538
00:39:32,954 --> 00:39:36,333
There's one last story I want to tell you.
539
00:39:36,499 --> 00:39:40,549
And it's the greatest story
science has ever told.
540
00:39:44,924 --> 00:39:49,100
It's the story of life on our world.
541
00:40:11,034 --> 00:40:13,878
Welcome to the Earth
of four billion years ago.
542
00:40:14,829 --> 00:40:17,833
This was our planet before life.
543
00:40:17,999 --> 00:40:20,548
Nobody knows how life got started.
544
00:40:20,710 --> 00:40:23,133
Most of the evidence
from that time was destroyed...
545
00:40:23,296 --> 00:40:25,674
...by impact and erosion.
546
00:40:25,840 --> 00:40:29,515
Science works on the frontier
between knowledge and ignorance.
547
00:40:29,677 --> 00:40:31,896
We're not afraid to admit
what we don't know.
548
00:40:32,055 --> 00:40:33,728
There's no shame in that.
549
00:40:33,890 --> 00:40:37,861
The only shame is to pretend
that we have all the answers.
550
00:40:38,019 --> 00:40:39,441
Maybe someone watching this...
551
00:40:39,604 --> 00:40:44,826
...will be the first to solve the mystery
of how life on Earth began.
552
00:40:52,075 --> 00:40:53,793
The evidence from living microbes...
553
00:40:53,952 --> 00:40:57,422
...suggest that their earliest ancestors
preferred high temperatures.
554
00:40:58,081 --> 00:41:02,712
Life on Earth may have arisen in hot water
around submerged volcanic vents.
555
00:41:07,548 --> 00:41:09,926
In Carl Sagan's original Cosmos series...
556
00:41:10,093 --> 00:41:12,767
...he traced the unbroken thread
that stretches...
557
00:41:12,929 --> 00:41:15,182
...directly from the
one-celled organisms...
558
00:41:15,348 --> 00:41:19,649
...of nearly four billion years ago to you.
559
00:41:19,811 --> 00:41:23,111
Four billion years in 40 seconds.
560
00:41:23,272 --> 00:41:26,617
From creatures
who had yet to discern day from night...
561
00:41:26,776 --> 00:41:31,498
...to beings who are exploring the cosmos.
562
00:42:19,412 --> 00:42:22,791
SAGAN: Those are some of the things
that molecules do...
563
00:42:22,957 --> 00:42:26,461
...given four billion years of evolution.