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NEIL DEGRASSE TYSON:
This is a story about you
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and me and your dog.
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(wolf howls)
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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, snugglers,
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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
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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,
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but it's no such thing.
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¶ ¶
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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
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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,
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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
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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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(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 of them are too
frightened to come close enough.
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Their fear is due to high levels
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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
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what a branch of his ancestors
figured out
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some 15,000 years ago:
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an excellent survival strategy;
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the domestication of 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
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would be reinforced
with each generation
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until that line of wild wolves
evolves into...
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dogs.
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You might call this
"survival of the friendliest."
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(chuckles)
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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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(wolf growling, dog barks)
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(growling, barking continues)
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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,
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but now they also produce
food and clothing...
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agriculture.
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The wolves have traded
their freedom
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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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(barking)
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They nurture those dogs
that do their bidding:
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hunting, herding,
guarding, hauling,
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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 15,000 or 20,000 years,
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we turned gray wolves into
all the kinds of dogs
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we love today.
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Think of it.
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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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(snarling)
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The awesome power of evolution
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transformed the ravenous wolf
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into the faithful shepherd...
(barks)
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...who protects the herd
and drives the wolf away.
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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 10,000 or 15,000 years,
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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
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and time,
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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DEGRASSE TYSON:
May not seem like it,
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but we've been living
in an ice age
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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,
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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,
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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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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,
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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
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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 spell out
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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, 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
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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
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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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00:13:21,884 --> 00:13:26,688
That mutation altered the gene
that controls fur color.
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00:13:26,722 --> 00:13:29,691
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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00:13:42,404 --> 00:13:44,372
A year has passed.
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00:13:44,406 --> 00:13:46,207
Our bear is now a mother.
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00:13:46,242 --> 00:13:47,942
And as a result
of that mutation,
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00:13:47,977 --> 00:13:51,613
one of her two cubs was born
with a white coat.
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00:13:51,647 --> 00:13:54,349
When the cubs get old enough
to venture out on their own,
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which bear is more likely
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00:13:56,685 --> 00:14:00,855
to be able to sneak up
on unsuspecting prey?
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00:14:00,890 --> 00:14:04,125
The brown bear can be seen
against the snow a mile away.
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00:14:04,159 --> 00:14:07,028
The white bear prospers
and passes on
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00:14:07,046 --> 00:14:08,997
its own particular set
of genes.
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00:14:09,031 --> 00:14:11,866
This happens repeatedly.
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00:14:11,901 --> 00:14:14,035
Over succeeding generations,
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00:14:14,069 --> 00:14:16,638
the gene for white fur
spreads through
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00:14:16,672 --> 00:14:19,307
the entire population
of Arctic bears.
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00:14:19,341 --> 00:14:22,043
The gene for dark fur loses out
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in the competition
for survival.
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00:14:26,849 --> 00:14:30,718
Mutations are entirely random
and happen all the time.
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But the environment
rewards those
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00:14:32,721 --> 00:14:34,989
that increase the chance
for survival.
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00:14:35,024 --> 00:14:37,392
It naturally selects
the living things
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00:14:37,426 --> 00:14:40,061
that are better suited
to survive.
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00:14:40,095 --> 00:14:42,897
And that selection
is the opposite of random.
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The two populations of bears
separated,
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and over thousands of years,
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evolved other characteristics
that set them apart.
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They became different species.
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That's what
Charles Darwin meant
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00:15:00,983 --> 00:15:04,619
by "the origin of species."
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00:15:04,653 --> 00:15:07,188
An individual bear
doesn't evolve;
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00:15:07,222 --> 00:15:11,125
the population of bears evolves
over generations.
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00:15:12,828 --> 00:15:14,896
If the Arctic ice
continues to dwindle
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due to global warming,
the polar bears may go extinct.
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They'll be replaced
by brown bears,
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00:15:20,669 --> 00:15:23,805
better adapted to
the now defrosted environment.
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00:15:23,839 --> 00:15:28,309
This is a different story
from the one about the dogs.
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00:15:28,344 --> 00:15:30,812
No breeder guided these changes.
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00:15:30,846 --> 00:15:34,315
Instead, the environment itself
selects them.
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00:15:34,350 --> 00:15:36,985
This is evolution
by natural selection,
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00:15:37,019 --> 00:15:40,788
the most revolutionary concept
in the history of science.
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00:15:40,823 --> 00:15:44,959
Darwin first presented the
evidence for this idea in 1859.
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00:15:44,994 --> 00:15:48,997
The uproar it caused
has never subsided.
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00:15:49,031 --> 00:15:51,032
Why?
255
00:15:56,055 --> 00:15:58,056
(birds chirping)
256
00:15:58,090 --> 00:16:00,191
We all understand
the twinge of discomfort
257
00:16:00,225 --> 00:16:03,461
at the thought that we share
a common ancestor with the apes.
258
00:16:03,495 --> 00:16:06,831
No one can embarrass you
like a relative.
259
00:16:06,865 --> 00:16:09,033
Our closest ones,
the chimpanzees,
260
00:16:09,068 --> 00:16:11,703
they frequently behave
inappropriately in public.
261
00:16:11,737 --> 00:16:13,972
There's an understandable
human need
262
00:16:14,006 --> 00:16:16,207
to distance ourselves
from them.
263
00:16:16,241 --> 00:16:18,476
A central premise
of traditional belief
264
00:16:18,510 --> 00:16:20,478
is that we were
created separately
265
00:16:20,512 --> 00:16:22,480
from all the other animals.
266
00:16:22,514 --> 00:16:25,249
It's easy to see why
this idea has taken hold.
267
00:16:25,284 --> 00:16:28,086
It makes us feel... special.
268
00:16:28,120 --> 00:16:32,190
But what about our kinship
with the trees?
269
00:16:32,224 --> 00:16:34,625
How does that make you feel?
270
00:16:42,334 --> 00:16:45,520
Okay, here's a segment
of the oak tree's DNA.
271
00:16:45,571 --> 00:16:47,638
Think of it like a bar code.
272
00:16:47,740 --> 00:16:50,341
The instructions written
in the code of life
273
00:16:50,376 --> 00:16:53,344
tell the tree
how to metabolize sugar.
274
00:16:53,379 --> 00:16:56,647
Now let's compare it with
the same section of my own DNA.
275
00:17:01,887 --> 00:17:03,788
The DNA doesn't lie.
276
00:17:03,822 --> 00:17:07,125
This tree and me--
we're long-lost cousins.
277
00:17:07,159 --> 00:17:09,127
And it's not just the trees.
278
00:17:09,161 --> 00:17:12,030
If you go back far enough,
you'll find that we share
279
00:17:12,064 --> 00:17:14,065
a common ancestor with...
280
00:17:14,099 --> 00:17:15,400
the butterfly...
281
00:17:15,434 --> 00:17:17,035
gray wolf...
282
00:17:17,069 --> 00:17:18,436
mushroom...
283
00:17:18,470 --> 00:17:20,071
shark...
284
00:17:20,105 --> 00:17:21,539
bacterium...
285
00:17:21,573 --> 00:17:22,874
sparrow.
286
00:17:22,908 --> 00:17:24,442
What a family!
287
00:17:24,476 --> 00:17:27,979
Other parts of the bar code vary
from species to species.
288
00:17:28,013 --> 00:17:31,215
That's what makes the difference
between an owl and an octopus.
289
00:17:31,250 --> 00:17:33,451
Unless you have
an identical twin,
290
00:17:33,485 --> 00:17:35,753
there's no one else
in the universe
291
00:17:35,788 --> 00:17:38,423
with the exact same DNA as you.
292
00:17:38,457 --> 00:17:40,658
Within other species,
the genetic differences
293
00:17:40,692 --> 00:17:44,062
provide the raw material
for natural selection.
294
00:17:44,096 --> 00:17:47,999
The environment selects which
genes survive and multiply.
295
00:17:48,033 --> 00:17:50,101
When it comes
to the genetic instructions
296
00:17:50,202 --> 00:17:52,170
for life's
most basic functions--
297
00:17:52,204 --> 00:17:54,972
say, digesting sugars--
we and other species
298
00:17:55,007 --> 00:17:57,008
are almost identical.
299
00:17:57,042 --> 00:18:00,144
That's because those functions
are so basic to life,
300
00:18:00,179 --> 00:18:03,081
they evolved before
the various life-forms
301
00:18:03,115 --> 00:18:05,083
branched off from each other.
302
00:18:05,117 --> 00:18:07,552
This is our Tree of Life.
303
00:18:07,586 --> 00:18:09,087
Science has made it possible
304
00:18:09,121 --> 00:18:11,255
for us to construct
this family tree
305
00:18:11,290 --> 00:18:13,257
for all the species
of life on Earth.
306
00:18:13,292 --> 00:18:17,095
Close genetic relatives occupy
the same branch of the tree,
307
00:18:17,129 --> 00:18:20,731
while more distant cousins
are farther away.
308
00:18:20,766 --> 00:18:22,934
Each twig is a living species.
309
00:18:24,937 --> 00:18:28,106
And the trunk of the tree
represents the common ancestors
310
00:18:28,140 --> 00:18:30,942
of all life on Earth.
311
00:18:30,976 --> 00:18:33,344
The stuff of life
is so malleable
312
00:18:33,378 --> 00:18:36,314
that once it got started,
the environment molded it
313
00:18:36,348 --> 00:18:39,617
into a staggering
variety of forms--
314
00:18:39,651 --> 00:18:43,454
10,000 times more than
we can possibly show here.
315
00:18:45,457 --> 00:18:47,592
Biologists have catalogued
316
00:18:47,626 --> 00:18:51,129
a half a million different
kinds of beetles alone.
317
00:18:53,832 --> 00:18:56,434
Not to mention
the numberless varieties
318
00:18:56,468 --> 00:18:58,102
of bacteria.
319
00:18:58,137 --> 00:19:00,471
There are many millions
of living species
320
00:19:00,506 --> 00:19:02,874
of animals and plants,
most of them
321
00:19:02,908 --> 00:19:05,276
still unknown to science.
322
00:19:05,310 --> 00:19:08,379
Think of that--
we have yet to make contact
323
00:19:08,413 --> 00:19:11,716
with most of the forms
of terrestrial life.
324
00:19:11,750 --> 00:19:14,152
That's how many kinds
of life there are
325
00:19:14,186 --> 00:19:15,920
on this tiny planet alone.
326
00:19:17,923 --> 00:19:20,458
The Tree of Life extends
its feelers in all directions,
327
00:19:20,492 --> 00:19:22,059
finding and exploiting
what works,
328
00:19:22,094 --> 00:19:23,794
creating new environments
329
00:19:23,829 --> 00:19:26,063
and opportunities
for new forms.
330
00:19:28,233 --> 00:19:32,970
The Tree of Life is three
and a half billion years old.
331
00:19:33,005 --> 00:19:35,173
That's plenty of time
to develop
332
00:19:35,207 --> 00:19:38,376
an impressive repertoire
of tricks.
333
00:19:40,779 --> 00:19:43,414
Evolution can
disguise an animal
334
00:19:43,448 --> 00:19:45,883
as a plant...
335
00:19:50,389 --> 00:19:52,356
...taking thousands
of generations
336
00:19:52,391 --> 00:19:55,526
to contrive
an elaborate costume
337
00:19:55,561 --> 00:19:57,428
that fools predators
into looking elsewhere
338
00:19:57,462 --> 00:19:59,931
for someone to eat.
339
00:19:59,965 --> 00:20:03,935
Or it can disguise a plant
as an animal,
340
00:20:03,969 --> 00:20:07,071
evolving blossoms that take on
the appearance of a wasp--
341
00:20:07,105 --> 00:20:11,108
the orchid's way of fooling
real wasps into pollinating it.
342
00:20:12,144 --> 00:20:14,212
This is the awesome
343
00:20:14,246 --> 00:20:17,648
shape-shifting power
of natural selection.
344
00:20:24,389 --> 00:20:29,060
Among the dense, tangled limbs
of the vast Tree of Life...
345
00:20:29,094 --> 00:20:31,229
you are here.
346
00:20:31,263 --> 00:20:35,533
One tiny branch
among countless millions.
347
00:20:35,567 --> 00:20:40,571
Science reveals that all life
on Earth is one.
348
00:20:43,242 --> 00:20:46,410
Darwin discovered the actual
mechanisof evolution.
349
00:20:46,445 --> 00:20:49,447
The prevailing belief
was that the complexity
350
00:20:49,481 --> 00:20:52,116
and variety of life
must be the work
351
00:20:52,150 --> 00:20:55,086
of an intelligent designer,
who created each
352
00:20:55,120 --> 00:20:58,256
of these millions
of different species separately.
353
00:20:58,290 --> 00:21:01,759
Living things are just
too intricate, it was said,
354
00:21:01,793 --> 00:21:05,329
to be the result
of unguided evolution.
355
00:21:05,364 --> 00:21:07,798
Consider the human eye,
356
00:21:07,833 --> 00:21:10,067
a masterpiece of complexity.
357
00:21:13,739 --> 00:21:15,840
It requires a cornea,
358
00:21:15,874 --> 00:21:19,310
iris, lens, retina,
359
00:21:19,344 --> 00:21:21,712
optic nerves, muscles,
360
00:21:21,747 --> 00:21:24,448
let alone the brain's
elaborate neural network
361
00:21:24,483 --> 00:21:27,385
to interpret images.
362
00:21:27,419 --> 00:21:30,288
It's more complicated
than any device
363
00:21:30,322 --> 00:21:33,391
ever crafted
by human intelligence.
364
00:21:33,425 --> 00:21:35,059
Therefore, it was argued,
365
00:21:35,093 --> 00:21:37,128
the human eye
can't be the result
366
00:21:37,162 --> 00:21:39,463
of mindless evolution.
367
00:21:39,498 --> 00:21:43,501
To know if that's true,
we need to travel across time
368
00:21:43,535 --> 00:21:47,171
to a world before
there were eyes to see.
369
00:21:55,063 --> 00:21:57,431
¶ ¶
370
00:22:03,572 --> 00:22:06,140
DEGRASSE TYSON:
In the beginning,
371
00:22:06,174 --> 00:22:07,775
life was blind.
372
00:22:09,845 --> 00:22:11,979
This is what
our world looked like
373
00:22:12,014 --> 00:22:13,981
four billion years ago,
374
00:22:13,983 --> 00:22:17,651
before there were
any eyes to see.
375
00:22:17,686 --> 00:22:20,721
Until a few hundred million
years passed,
376
00:22:20,756 --> 00:22:22,356
and then, one day,
377
00:22:22,391 --> 00:22:24,425
there was a microscopic
copying error
378
00:22:24,459 --> 00:22:25,993
in the DNA of a bacterium.
379
00:22:26,028 --> 00:22:28,729
This random mutation
gave that microbe
380
00:22:28,764 --> 00:22:31,532
a protein molecule
that absorbed sunlight.
381
00:22:31,566 --> 00:22:33,351
Want to know what
the world looked like
382
00:22:33,402 --> 00:22:35,069
to a light-sensitive bacterium?
383
00:22:35,103 --> 00:22:38,356
Take a look at the right side
of the screen.
384
00:22:38,407 --> 00:22:42,009
Mutations continued
to occur at random,
385
00:22:42,077 --> 00:22:45,279
as they always do in any
population of living things.
386
00:22:47,265 --> 00:22:49,383
Another mutation
387
00:22:49,418 --> 00:22:53,287
caused a dark bacterium
to flee intense light.
388
00:22:53,372 --> 00:22:55,789
What is going on here?
389
00:22:55,791 --> 00:22:57,525
Night and day.
390
00:22:57,592 --> 00:22:59,593
Those bacteria that could
tell light from dark
391
00:22:59,595 --> 00:23:02,229
had a decisive advantage
over the ones that couldn't.
392
00:23:02,264 --> 00:23:04,065
Why?
393
00:23:04,099 --> 00:23:07,234
Because the daytime brought
harsh, ultraviolet light
394
00:23:07,269 --> 00:23:09,203
that damages DNA.
395
00:23:09,237 --> 00:23:12,773
The sensitive bacteria
fled the intense light
396
00:23:12,791 --> 00:23:15,309
to safely exchange
their DNA in the dark.
397
00:23:15,343 --> 00:23:17,244
They survived
in greater numbers
398
00:23:17,312 --> 00:23:19,713
than the bacteria that
stayed at the surface.
399
00:23:19,748 --> 00:23:22,817
Over time, those
light-sensitive proteins
400
00:23:22,851 --> 00:23:25,453
became concentrated
in a pigment spot
401
00:23:25,487 --> 00:23:28,756
on the more advanced,
one-celled organism.
402
00:23:28,790 --> 00:23:31,959
This made it possible
to find the light,
403
00:23:31,993 --> 00:23:34,095
an overwhelming advantage
404
00:23:34,129 --> 00:23:37,431
for an organism that harvests
sunlight to make food.
405
00:23:44,539 --> 00:23:47,441
Here's a flatworm's-eye view
of the world.
406
00:23:47,476 --> 00:23:49,543
This multi-celled organism
407
00:23:49,644 --> 00:23:51,946
evolved a dimple
in the pigment spot.
408
00:23:51,980 --> 00:23:54,248
The bowl-shaped depression
409
00:23:54,282 --> 00:23:57,618
allowed the animal to
distinguish light from shadow
410
00:23:57,636 --> 00:24:00,988
to crudely make out objects
in its vicinity,
411
00:24:01,022 --> 00:24:04,959
including those to eat
and those that might eat it...
412
00:24:05,060 --> 00:24:07,595
a tremendous advantage.
413
00:24:07,629 --> 00:24:10,264
Later, things became
a little clearer.
414
00:24:10,298 --> 00:24:11,999
The dimple deepened
415
00:24:12,033 --> 00:24:14,768
and evolved into a socket
with a small opening.
416
00:24:14,803 --> 00:24:17,138
Over thousands of generations,
417
00:24:17,172 --> 00:24:20,357
natural selection
was slowly sculpting the eye.
418
00:24:22,160 --> 00:24:25,312
The opening contracted
to a pinhole covered
419
00:24:25,347 --> 00:24:27,648
by a protective
transparent membrane.
420
00:24:27,682 --> 00:24:30,151
Only a little light
could enter the tiny hole,
421
00:24:30,185 --> 00:24:32,553
but it was enough
to paint a dim image
422
00:24:32,571 --> 00:24:35,122
on the sensitive inner surface
of the eye.
423
00:24:35,157 --> 00:24:37,024
This sharpened the focus.
424
00:24:37,058 --> 00:24:39,360
A larger opening
would have let in more light
425
00:24:39,394 --> 00:24:42,963
to make a brighter image
but one that was out of focus.
426
00:24:42,998 --> 00:24:45,899
This development launched
the visual equivalent
427
00:24:45,901 --> 00:24:48,536
of an arms race.
428
00:24:58,947 --> 00:25:03,517
The competition needed
to keep up to survive.
429
00:25:03,552 --> 00:25:07,254
But then a splendid new feature
of the eye evolved,
430
00:25:07,289 --> 00:25:12,259
a lens that provided both
brightness and sharp focus.
431
00:25:12,261 --> 00:25:14,695
In the eyes of primitive fish,
432
00:25:14,729 --> 00:25:18,432
the transparent gel near
the pinhole formed into a lens.
433
00:25:18,466 --> 00:25:21,202
At the same time,
the pinhole enlarged
434
00:25:21,236 --> 00:25:23,270
to let in more and more light.
435
00:25:23,305 --> 00:25:25,739
Fish could now see in high-def,
436
00:25:25,741 --> 00:25:29,426
both close up and far away.
437
00:25:29,477 --> 00:25:33,080
And then
something terrible happened.
438
00:25:33,114 --> 00:25:36,584
Have you ever noticed that
a straw in a glass of water
439
00:25:36,618 --> 00:25:39,086
looks bent at the surface
of the water?
440
00:25:39,120 --> 00:25:41,855
That's because light bends
when it goes from one medium
441
00:25:41,890 --> 00:25:44,758
to another,
say from water to air.
442
00:25:44,793 --> 00:25:48,529
Our eyes originally evolved
to see in water.
443
00:25:48,563 --> 00:25:51,432
The watery fluid
in those eyes neatly
444
00:25:51,466 --> 00:25:55,236
eliminated the distortion
of that bending effect.
445
00:25:56,238 --> 00:25:58,405
But for land animals,
446
00:25:58,440 --> 00:26:00,808
the light carries images
from dry air
447
00:26:00,842 --> 00:26:03,911
into their still-watery eyes.
448
00:26:03,945 --> 00:26:05,479
That bends the light rays,
449
00:26:05,513 --> 00:26:09,483
causing all kinds
of distortions.
450
00:26:09,517 --> 00:26:12,253
When our amphibious ancestors
left the water for the land,
451
00:26:12,287 --> 00:26:15,489
their eyes, exquisitely evolved
to see in water,
452
00:26:15,523 --> 00:26:17,825
were lousy for seeing
in the air.
453
00:26:17,859 --> 00:26:20,494
Our vision has never been
as good since.
454
00:26:20,528 --> 00:26:23,430
We like to think of our eyes
as state-of-the-art,
455
00:26:23,465 --> 00:26:26,333
but 375 million years later,
456
00:26:26,368 --> 00:26:28,769
we still can't see things
right in front of our noses
457
00:26:28,803 --> 00:26:31,438
or discern fine details
in near darkness
458
00:26:31,473 --> 00:26:33,340
the way fish can.
459
00:26:33,375 --> 00:26:34,842
When we left the water,
460
00:26:34,876 --> 00:26:37,011
why didn't nature
just start over again
461
00:26:37,045 --> 00:26:38,646
and evolve us a new set of eyes
462
00:26:38,680 --> 00:26:40,981
that were optimal
for seeing in the air?
463
00:26:41,016 --> 00:26:43,083
Nature doesn't work that way.
464
00:26:43,118 --> 00:26:46,487
Evolution reshapes existing
structures over generations,
465
00:26:46,521 --> 00:26:48,689
adapting them
with small changes.
466
00:26:48,723 --> 00:26:50,357
It can't just go back
to the drawing board
467
00:26:50,392 --> 00:26:52,192
and start from scratch.
468
00:26:52,227 --> 00:26:55,763
At every stage of its
development, the evolving eye
469
00:26:55,797 --> 00:26:57,498
functioned well enough
470
00:26:57,532 --> 00:27:00,668
to provide a selective
advantage for survival.
471
00:27:00,702 --> 00:27:03,537
And among animals alive today,
we find eyes
472
00:27:03,571 --> 00:27:06,740
at all these stages
of development.
473
00:27:10,011 --> 00:27:13,314
And all of them function.
474
00:27:13,348 --> 00:27:15,482
The complexity
of the human eye poses
475
00:27:15,517 --> 00:27:17,618
no challenge to evolution
by natural selection.
476
00:27:17,652 --> 00:27:22,122
In fact, the eye and all
of biology makes no sense
477
00:27:22,157 --> 00:27:23,791
without evolution.
478
00:27:23,825 --> 00:27:26,460
Some claim that evolution
is just a theory,
479
00:27:26,494 --> 00:27:29,647
as if it were merely
an opinion.
480
00:27:29,698 --> 00:27:32,399
The theory of evolution,
like the theory of gravity,
481
00:27:32,434 --> 00:27:34,501
is a scientific fact.
482
00:27:34,536 --> 00:27:37,237
Evolution really happened.
483
00:27:37,272 --> 00:27:39,673
Accepting our kinship
with all life on Earth
484
00:27:39,708 --> 00:27:41,742
is not only solid science.
485
00:27:41,776 --> 00:27:46,947
In my view, it's also
a soaring spiritual experience.
486
00:27:53,722 --> 00:27:56,557
Because evolution is blind,
487
00:27:56,591 --> 00:28:00,828
it cannot anticipate or adapt
to catastrophic events.
488
00:28:00,862 --> 00:28:04,665
The Tree of Life
has some broken branches.
489
00:28:04,699 --> 00:28:08,168
Many of them were severed in
the five greatest catastrophes
490
00:28:08,203 --> 00:28:09,737
that life has ever known.
491
00:28:09,771 --> 00:28:11,739
Somewhere, there's a memorial
492
00:28:11,773 --> 00:28:14,742
to the multitude
of lost species,
493
00:28:14,776 --> 00:28:16,910
the Halls of Extinction.
494
00:28:16,945 --> 00:28:19,380
Come with me.
495
00:28:30,575 --> 00:28:34,078
Welcome to the Halls
of Extinction,
496
00:28:34,112 --> 00:28:38,916
a monument to the broken
branches on the Tree of Life.
497
00:28:49,094 --> 00:28:52,796
For every single one
of the millions of species
498
00:28:52,831 --> 00:28:57,067
alive today, perhaps
a thousand others have perished.
499
00:28:57,135 --> 00:28:59,570
Most of them died out
in the everyday competition
500
00:28:59,604 --> 00:29:01,138
with other life-forms.
501
00:29:01,172 --> 00:29:04,742
But many of them were swept away
in vast cataclysms
502
00:29:04,776 --> 00:29:06,977
that overwhelmed the planet.
503
00:29:06,979 --> 00:29:09,413
In the last 500 million years,
504
00:29:09,447 --> 00:29:12,116
this has happened five times.
505
00:29:12,150 --> 00:29:17,321
Five mass extinctions
that devastated life on Earth.
506
00:29:17,355 --> 00:29:22,593
The worst one of all happened
some 250 million years ago,
507
00:29:22,660 --> 00:29:25,145
at the end of an era
508
00:29:25,196 --> 00:29:27,631
known as the Permian.
509
00:29:44,015 --> 00:29:46,750
Trilobites were
armored animals that hunted
510
00:29:46,785 --> 00:29:49,253
in great herds
across the seafloor.
511
00:29:49,287 --> 00:29:51,955
They were among
the first animals to evolve
512
00:29:51,990 --> 00:29:54,391
image-forming eyes.
513
00:29:56,961 --> 00:30:00,997
Trilobites had a good long run,
some 270 million years.
514
00:30:00,999 --> 00:30:04,535
Earth was once
the planet of the trilobites.
515
00:30:04,537 --> 00:30:07,971
But now they're all gone,
extinct.
516
00:30:08,006 --> 00:30:10,707
The last of them were swept
from life's stage
517
00:30:10,742 --> 00:30:12,810
along with countless
other species
518
00:30:12,844 --> 00:30:17,047
in an unparalleled
environmental disaster.
519
00:30:21,886 --> 00:30:25,556
(explosions rumbling)
520
00:30:25,590 --> 00:30:29,193
The apocalypse
began in what is now Siberia,
521
00:30:29,227 --> 00:30:33,864
with volcanic eruptions
on a scale unlike anything
522
00:30:33,898 --> 00:30:36,066
in human experience.
523
00:30:48,513 --> 00:30:51,048
Earth was very different then,
524
00:30:51,082 --> 00:30:55,385
with one single supercontinent
and one great ocean.
525
00:30:55,420 --> 00:30:57,888
Relentless floods of fiery lava
526
00:30:57,922 --> 00:31:00,691
engulfed an area larger than
Western Europe.
527
00:31:00,725 --> 00:31:03,393
The pulsing eruptions
went on for hundreds
528
00:31:03,428 --> 00:31:05,562
of thousands of years.
529
00:31:05,597 --> 00:31:07,164
The molten rock
530
00:31:07,198 --> 00:31:09,900
ignited coal deposits
and polluted the air
531
00:31:09,934 --> 00:31:12,903
with carbon dioxide
and other greenhouse gases.
532
00:31:12,937 --> 00:31:14,838
This heated the Earth
533
00:31:14,873 --> 00:31:17,975
and stopped the ocean currents
from circulating.
534
00:31:32,991 --> 00:31:35,292
Noxious bacteria bloomed,
535
00:31:35,326 --> 00:31:39,463
but nearly everything else
in the seas died.
536
00:31:39,497 --> 00:31:43,066
The stagnant waters belched
deadly hydrogen sulfide gas
537
00:31:43,101 --> 00:31:44,534
into the air,
538
00:31:44,569 --> 00:31:47,104
which suffocated most
of the land animals.
539
00:32:00,118 --> 00:32:04,454
Nine in ten of all species
on the planet went extinct.
540
00:32:04,489 --> 00:32:06,556
We call it...
541
00:32:06,591 --> 00:32:08,592
The Great Dying.
542
00:32:21,205 --> 00:32:23,824
Life on Earth came so near
to being wiped out
543
00:32:23,875 --> 00:32:27,678
that it took more than
ten million years to recover.
544
00:32:27,712 --> 00:32:30,013
But new life-forms
slowly evolved
545
00:32:30,048 --> 00:32:33,717
to fill the openings left
by the Permian holocaust.
546
00:32:41,492 --> 00:32:44,361
Among the biggest winners
were the dinosaurs.
547
00:32:44,395 --> 00:32:47,197
Now the Earth was their planet.
548
00:32:47,199 --> 00:32:51,468
Their reign continued
for over 150 million years.
549
00:32:51,502 --> 00:32:54,104
Until it, too,
came crashing down
550
00:32:54,138 --> 00:32:57,040
in another mass extinction.
551
00:32:57,075 --> 00:32:59,660
Life on Earth has taken quite
a beating over the eons.
552
00:32:59,711 --> 00:33:02,512
And yet it's still there.
553
00:33:02,547 --> 00:33:05,349
The tenacity of life
is mind-boggling.
554
00:33:05,366 --> 00:33:08,168
We keep finding it where
no one thought it could be.
555
00:33:12,657 --> 00:33:15,959
That nameless corridor?
556
00:33:15,994 --> 00:33:17,978
That's for another day.
557
00:33:23,501 --> 00:33:27,070
I know an animal that can live
in boiling water
558
00:33:27,105 --> 00:33:29,239
or in solid ice.
559
00:33:29,273 --> 00:33:31,641
It can go ten years
without a drop of water.
560
00:33:31,676 --> 00:33:34,311
It can travel naked
in the cold vacuum
561
00:33:34,345 --> 00:33:38,849
and intense radiation of space
and will return unscathed.
562
00:33:38,883 --> 00:33:41,318
The tardigrade, or water bear.
563
00:33:41,352 --> 00:33:44,421
It's equally at home
atop the tallest mountains
564
00:33:44,455 --> 00:33:46,256
and in the deepest trenches
of the sea.
565
00:33:46,290 --> 00:33:49,326
And in our own backyards,
where they live among the moss
566
00:33:49,360 --> 00:33:51,828
in countless numbers.
567
00:33:51,863 --> 00:33:53,230
You've probably
never noticed them
568
00:33:53,264 --> 00:33:54,731
because they're so small.
569
00:33:54,766 --> 00:33:56,366
About the size of a pinpoint.
570
00:33:56,401 --> 00:33:58,101
But they're tough.
571
00:33:58,136 --> 00:34:02,406
The tardigrades have survived
all five mass extinctions.
572
00:34:02,440 --> 00:34:05,175
They've been in business
for a half a billion years.
573
00:34:05,209 --> 00:34:07,544
We used to think
that life was finicky,
574
00:34:07,578 --> 00:34:09,513
that it would only take hold
where it was
575
00:34:09,547 --> 00:34:11,515
not too hot, not too cold,
576
00:34:11,549 --> 00:34:15,285
not too dark or salty
or acidic or radioactive.
577
00:34:15,319 --> 00:34:18,088
And whatever you do,
don't forget to add water.
578
00:34:18,122 --> 00:34:19,856
We were wrong.
579
00:34:19,891 --> 00:34:21,191
As the hardy tardigrade
demonstrates,
580
00:34:21,225 --> 00:34:23,393
life can endure conditions
581
00:34:23,428 --> 00:34:26,630
that would mean certain death
for us humans.
582
00:34:26,664 --> 00:34:28,465
But differences between us
and life found
583
00:34:28,499 --> 00:34:31,034
in even the most extreme
environments on our planet
584
00:34:31,069 --> 00:34:34,037
are only variations
on a single theme,
585
00:34:34,072 --> 00:34:36,606
dialects of a single language.
586
00:34:36,641 --> 00:34:39,409
The genetic code of Earth life.
587
00:34:44,348 --> 00:34:46,883
But what would life be like
on other worlds?
588
00:34:46,918 --> 00:34:49,920
Worlds with a completely
different history,
589
00:34:49,954 --> 00:34:52,956
chemistry and evolution
from our planet?
590
00:34:55,426 --> 00:34:58,962
There's a distant world
I want to take you to--
591
00:34:58,996 --> 00:35:01,398
a world far different
from our own,
592
00:35:01,432 --> 00:35:04,768
but one that may harbor life.
593
00:35:04,802 --> 00:35:08,672
If it does, it promises
to be unlike anything
594
00:35:08,706 --> 00:35:11,141
we've ever seen before.
595
00:35:27,892 --> 00:35:31,261
Clouds and haze completely hide
the surface of Titan,
596
00:35:31,295 --> 00:35:33,263
Saturn's giant moon.
597
00:35:33,297 --> 00:35:35,866
Titan reminds me
a little bit of home.
598
00:35:35,868 --> 00:35:39,369
Like Earth, it has an atmosphere
that's mostly nitrogen.
599
00:35:39,403 --> 00:35:41,538
But it's four times denser.
600
00:35:41,572 --> 00:35:44,441
Titan's air has
no oxygen at all.
601
00:35:44,475 --> 00:35:46,943
And it's far colder
than anywhere on Earth.
602
00:35:46,978 --> 00:35:50,580
But still...
I want to go there.
603
00:35:51,749 --> 00:35:54,251
We have to descend
604
00:35:54,285 --> 00:35:56,720
through a couple hundred
kilometers of smog
605
00:35:56,754 --> 00:35:59,723
before we can even
see the surface.
606
00:35:59,757 --> 00:36:03,293
But hidden beneath lies
a weirdly familiar landscape.
607
00:36:11,002 --> 00:36:14,470
Titan is the only other world
in the solar system
608
00:36:14,472 --> 00:36:15,906
where it ever rains.
609
00:36:15,940 --> 00:36:19,342
It has rivers and coastlines.
610
00:36:24,282 --> 00:36:26,349
Titan has hundreds of lakes.
611
00:36:26,384 --> 00:36:29,920
One of them larger than
Lake Superior in North America.
612
00:36:29,954 --> 00:36:31,922
Vapor rising from the lakes
613
00:36:31,956 --> 00:36:34,157
condenses and falls again
as rain.
614
00:36:34,192 --> 00:36:38,094
The rain feeds rivers,
615
00:36:38,129 --> 00:36:41,431
which carve valleys
into the landscape,
616
00:36:41,465 --> 00:36:43,466
just like on Earth.
617
00:36:45,736 --> 00:36:47,804
But with one big difference.
618
00:36:47,838 --> 00:36:50,540
On Titan, the seas and the rain
619
00:36:50,575 --> 00:36:54,477
are made not of water
but of methane and ethane.
620
00:36:54,512 --> 00:36:57,447
On Earth, those molecules
form natural gas.
621
00:36:57,514 --> 00:37:00,317
On frigid Titan,
622
00:37:00,351 --> 00:37:02,352
they're liquid.
623
00:37:07,658 --> 00:37:09,659
Titan has lots of water,
624
00:37:09,694 --> 00:37:12,395
but all of it is frozen
hard as rock.
625
00:37:12,430 --> 00:37:14,798
In fact, the landscape
and mountains
626
00:37:14,832 --> 00:37:17,300
are made mainly of water ice.
627
00:37:17,335 --> 00:37:19,536
At hundreds of degrees
below zero,
628
00:37:19,570 --> 00:37:23,473
Titan is far too cold
for water to ever be liquid.
629
00:37:23,507 --> 00:37:25,675
(rainfall, distant thunder)
630
00:37:25,710 --> 00:37:28,912
Astrobiologists since
Carl Sagan have wondered
631
00:37:28,946 --> 00:37:32,382
if life might swim
in Titan's hydrocarbon lakes.
632
00:37:34,719 --> 00:37:36,653
The chemical basis for such life
633
00:37:36,687 --> 00:37:40,090
would have to be entirely
different from anything we know.
634
00:37:40,124 --> 00:37:42,926
All life on Earth
depends on liquid water.
635
00:37:42,960 --> 00:37:45,262
And Titan's surface
has none of that.
636
00:37:45,296 --> 00:37:47,831
But we can imagine
other kinds of life.
637
00:37:47,865 --> 00:37:50,433
There might be creatures
that inhale hydrogen
638
00:37:50,468 --> 00:37:51,935
instead of oxygen.
639
00:37:51,969 --> 00:37:55,372
And exhale methane
instead of carbon dioxide.
640
00:37:55,473 --> 00:37:58,908
They might use acetylene instead
of sugar as an energy source.
641
00:37:58,943 --> 00:38:01,361
How could we find out
if such creatures
642
00:38:01,412 --> 00:38:05,915
rule a hidden empire
beneath the oil-dark waves?
643
00:38:21,165 --> 00:38:24,434
We're diving down deep
into the Kraken Sea,
644
00:38:24,468 --> 00:38:27,470
named for the mythic
Norse sea monster.
645
00:38:30,841 --> 00:38:33,043
Even if there is
one of those down there,
646
00:38:33,077 --> 00:38:34,744
we probably couldn't see it.
647
00:38:34,779 --> 00:38:37,314
It's so dark.
648
00:38:37,348 --> 00:38:41,751
If you took all the oil
and natural gas on Earth,
649
00:38:41,786 --> 00:38:45,522
it would amount to but a tiny
fraction of Titan's reserves.
650
00:38:49,026 --> 00:38:50,960
Let's turn on some lights.
651
00:38:55,266 --> 00:38:58,702
We're now 200 meters
beneath the surface.
652
00:39:01,972 --> 00:39:04,507
Did you see something?
653
00:39:04,542 --> 00:39:07,744
Over there, by that vent.
654
00:39:07,778 --> 00:39:10,246
Maybe it was just
my imagination.
655
00:39:10,281 --> 00:39:12,248
I guess we'll have to come back
656
00:39:12,283 --> 00:39:15,285
if we want to find out
for sure.
657
00:39:17,521 --> 00:39:21,157
There's one last story
I want to tell you.
658
00:39:21,192 --> 00:39:25,362
And it's the greatest story
science has ever told.
659
00:39:29,700 --> 00:39:33,136
It's the story of life
on our world.
660
00:39:55,593 --> 00:39:58,695
Welcome to the Earth
of four billion years ago.
661
00:39:58,729 --> 00:40:02,399
This was our planet before life.
662
00:40:02,433 --> 00:40:05,502
Nobody knows
how life got started.
663
00:40:05,536 --> 00:40:07,904
Most of the evidence
from that time was destroyed
664
00:40:07,938 --> 00:40:10,540
by impact and erosion.
665
00:40:10,574 --> 00:40:14,344
Science works on the frontier
between knowledge and ignorance.
666
00:40:14,378 --> 00:40:16,312
We're not afraid to admit
what we don't know.
667
00:40:16,347 --> 00:40:18,648
There's no shame in that.
668
00:40:18,682 --> 00:40:22,519
The only shame is to pretend
that we have all the answers.
669
00:40:22,553 --> 00:40:24,220
Maybe someone watching this
670
00:40:24,255 --> 00:40:26,756
will be the first
to solve the mystery
671
00:40:26,791 --> 00:40:29,192
of how life on Earth began.
672
00:40:36,767 --> 00:40:38,601
The evidence
from living microbes
673
00:40:38,619 --> 00:40:40,537
suggest that their
earliest ancestors
674
00:40:40,571 --> 00:40:42,272
preferred high temperatures.
675
00:40:42,306 --> 00:40:45,208
Life on Earth may have
arisen in hot water
676
00:40:45,242 --> 00:40:47,544
around submerged volcanic vents.
677
00:40:52,316 --> 00:40:54,784
In Carl Sagan's original
Cosmos series,
678
00:40:54,819 --> 00:40:57,587
he traced the unbroken thread
that stretches
679
00:40:57,621 --> 00:40:59,689
directly from
the one-celled organisms
680
00:40:59,723 --> 00:41:02,258
of nearly four billion
years ago...
681
00:41:02,293 --> 00:41:04,561
to you.
682
00:41:04,595 --> 00:41:07,730
Four billion years
in 40 seconds.
683
00:41:07,765 --> 00:41:11,301
From creatures who had yet
to discern day from night
684
00:41:11,335 --> 00:41:15,138
to beings who are
exploring the cosmos.
685
00:41:15,172 --> 00:41:18,174
¶ ¶
686
00:42:04,288 --> 00:42:07,290
SAGAN: Those are some of
the things that molecules do
687
00:42:07,324 --> 00:42:10,793
given four billion years
of evolution.
688
00:42:10,828 --> 00:42:10,793
Captioned by
Media Access Group at WGBH
access.wgbh.org