All language subtitles for Through.the.Wormhole.S08E02.Can.We.Cheat.Death.iNTERNAL.720p.HDTV.x264-DHD[ettv]

Are we born to die?

For millennia, we've tried
to outsmart our own mortality...

Only to find that death
is the greatest certainty.

But for the first time
in history,

that may be about to change.

Science is unraveling
the mysteries of aging,

discovering an animal

that comes back from the dead,

and turning our quest
for eternal life on its head.

Are we about to enter
the age of immortality?

Or is death necessary
for the survival of our species?

Can we cheat death?

Space, time, life itself.

The secrets of the cosmos

lie through the wormhole.

Captions by vitac...
www.Vitac.Com

captions paid for by
discovery communications

when will I die?

It's a question
I've asked myself.

I'm sure you've thought
about it, too.

From the day we are born,

the clock of our lives marks

our inevitable decline...

Ticking inexorably
toward our final hour.

Medical science has made
incredible advances

in prolonging life.

For most of human history,

people were lucky
to survive into their 40s.

Now, people often break 100.

The oldest person we know of

lived to celebrate
her 122nd birthday.

And as we probe deeper into
the fundamental biology of life,

we are now questioning

whether death really has
to come at all,

whether we can... Stop the clock

David Sinclair likes
to live in the fast Lane.

But he knows our bodies,

like cars, can't push
the needle forever.

Well, I've been interested
in studying aging

since I can remember.

I think it was
around the age of 4

when I realized that my parents
would one day die,

and I would, too,

and that seemed
extremely tragic.

And I also thought
that if we could figure out

why we age
and what we could do about it,

that would have a really big
beneficial impact on the world.

David spends a lot of time
thinking about old age.

But he's interested in more
than the wrinkles on our skin.

As a professor of genetics
at Harvard,

he asks what role
our genes play in aging.

An aging body is a lot
like a rusty old car.

Rust, or oxidation,
is what causes everything,

including our DNA,
to break down.

David believes
we may soon be able

to repair our failing bodies

as easily as a mechanic can
spruce up a rusty body panel.

David's research focuses

on our body's own set
of repair mechanics...

A group of genes
called sirtuins.

As our bodies age,

they accumulate
old or senescent cells.

And longevity genes prevent
those cells from accumulating.

This would be similar,
in an old car,

to the mechanics removing
rusty old parts

and replacing them
with new ones.

But these repair genes
begin to fall silent

as we pass our 30s and 40s.

So if we could find a way

of getting these longevity genes
working harder,

this would be a way
to keep our bodies healthier

and working for longer.

If our cells have
the tools to restore

and rebuild themselves, why
can't they do that forever...

In other words, stop aging?

Sinclair has dedicated
his career

to finding ways to help
our repair mechanisms

keep going well past middle age.

It's driven him deep into
the inner mechanics of the cell.

In order for a human cell
to function,

it requires close collaboration

between two distinct parts...

The nucleus,
the cell's control center,

and its tiny power engines
called mitochondria.

Every task a cell performs
relies on precise communication

between these two.

Imagine this kitchen
is a young cell.

Inside it are the nucleus
and mitochondria,

two chefs working in concert.

Our genes are
just like a recipe book.

In a young cell, the nucleus
and the mitochondria,

they're reading
the recipe perfectly,

and they're really
working well together.

A delicious meal
is taking shape.

But over time, the communication

and coordination
between the chefs gets worse.

In an old cell, the nucleus
and the mitochondria,

they're throwing in things
that don't belong.

They're leaving things out.
They don't communicate well.

One chef might be trying
to pass a spoon to the other,

and the other one is
not paying attention.

It's as if the chefs
are spilling on the recipe book

and losing that ability
to read it.

DNA is the recipe of life.

But over time,

wear and tear makes
the recipe hard to read,

and our cells lose the ability
to prepare the meal.

Whether it's damage
or changes in the environment,

the cell eventually loses
the memory

of which genes should be on
and which genes should be off,

and that, we think,
causes aging.

David has been combing
through aging cells,

trying to see if he can identify

which vital ingredient
they lack.

And now he thinks
he's found the molecule

that will get our cellular
kitchen back on track.

It's called N.A.D.

It's a molecule
that's essential for life.

Without it, you'd be dead
in about 20 seconds.

And, as we age,
the levels of this molecule

go down steadily to the point
where you have

about half the levels
of what you once had.

David wondered if there
was a way to restore N.A.D.

To more youthful levels.

He injected mice
with a special compound

that boosts production of N.A.D.

The effects were dramatic.

This mouse is
very youthful and energetic.

You can see the ears are still
nice and young-looking,

and the fur isn't turning gray.

As you can see here,
this mouse is going gray.

She's losing hair.

Her ears are getting wrinkled.

Her spine is starting to bend.

Well, what's amazing
about these mice

is that they were born
on the same day.

What we saw was
a rapid-aging reversal

in their muscles,

from a mouse that's equivalent
to a 60-year-old human

back to a 20-year-old.

If we could apply this
to humans,

imagine what a city street
might look like

50 years from now.

Almost everyone
would appear youthful,

free of the illness
and frailties of age.

David's research

may one day slow down and
even reverse aging in people.

We're not talking here
about being older for longer.

It's quite the opposite.

It's about being
younger for longer.

But David's work only
treats the symptoms of aging.

It doesn't attack
the root of the problem...

The gradual decay of our DNA.

Japanese scientist Shin Kubota
thinks we can turn that tide.

My dream is to live
with my families

of different generations

so that we can enjoy
our great-great grandchildren.

The sea is where all life began,

and with every empty shell
it leaves behind,

it seems to prove
that all lives must end.

But shin knows otherwise.

Shin studies
a tiny marine animal

called Turritopsis Dohrnii,

or the immortal jellyfish.

This creature has figured out
how to cheat death.

At shin's lab
in the town of Shirahama

is the world's
only captive population

of immortal animals.

First discovered
in the Mediterranean in 1988,

this jellyfish is no bigger
than your fingernail.

Immortal jellyfish is
very cute, small jellyfish.

Diameter is
about several millimeter.

But after showing
these creatures

such tender, loving care,

shin does the unthinkable.

He chops them into pieces.

Try to kill this jellyfish,

you will be surprised.

A remarkable thing happens.

The jellyfish does not die
beneath shin's hand.

Instead, it seems to be reborn.

The bell reabsorbs the tentacles
and becomes a gelatinous blob.

I call this stage the meatball.

The meatball becomes a polyp,

and soon it matures
into an adult jellyfish.

The jellyfish shows
very remarkable transformation.

The jellyfish's secret
is a process called

cellular transdifferentiation.

The cells in any fragment
of the animal

can figure out
what body parts are missing,

then retrofit themselves
to grow back the entire body.

If we could determine
how do the jellyfish rejuvenate,

we could borrow its techniques.

Imagine shin were attacked
by a mutant sea monster.

It's Japan, after all,
so it's not such a crazy idea.

If the only thing left of shin

were the tip
of one little finger...

Transdifferentiation could
turn those finger cells

into brain cells,

heart cells, muscle cells.

Shin would come back
from the dead.

It seems like science fiction,

but jellyfish and humans
have surprisingly similar genes.

If a jellyfish can
learn this trick,

perhaps we can, as well.

From the ruins
of a single animal,

hundreds of identical jellyfish
have sprouted.

It's a feat unequaled in nature,

but perhaps not for long.

If we could unlock the genetic
secrets of this jellyfish,

we could regenerate
aging organs,

perhaps even entire bodies.

I believe, one day,

that humans can become immortal

using the same process
as a jellyfish.

The promise of immortality
always seemed a fantasy,

but the infinite regeneration

of the jellyfish
is biological reality.

So can we get there
with our biology?

Some people believe we will,

and sooner than you think.

Death has always been
a fact of life.

Why?

Because our bodies wear down.

Our hearts won't beat forever.

Our muscles gradually weaken.

Our brains lose their edge.

But it doesn't have
to be that way.

Our bodies have
built-in technology

to repair themselves...
Stem cells.

These cells have the potential

to replenish any organ
in the body

if we can coax them
into doing it.

Cell biologist Larry Goldstein
thinks that if we want

to extend our life-spans,

we'll have to leap past
a major obstacle...

The slow decline of our minds.

I had the unfortunate experience

of helping to care for my mother

when she developed
Alzheimer's disease

in her late 60s.

It was a terrible experience,

and I hate the idea
of anybody else

having to go through that
with their family members

and their loved ones.

50% of us will develop
Alzheimer's disease

by the time we're 85 years old.

It does no good to age to 85

and be physically healthy

if your brain doesn't function.

At the Sanford consortium
of regenerative medicine,

Larry keeps an hourglass
on his desk.

It reminds him of all the people
who are running out of time.

But Larry thinks
he's found the key

to fighting
debilitating diseases of aging

like Alzheimer's... Stem cells.

The problem is that
we don't really understand

what goes wrong in brain cells
that have the disease.

And we're using stem cells
to try to tackle that problem

in a unique way.

Stem cells are the body's
original building blocks.

They have the potential
to become any type of cell.

In the primordial furnace
of the womb,

our bodies use
these raw materials

much like a master glassblower,

sculpting beautifully intricate,
finished organs

from shapeless, raw ingredients.

You can think about stem cells

as being like
biological raw material,

just like these glass pellets
are the raw material

that you can throw
into a really hot oven

and use to make anything at all
that you'd like to make.

So, let's see how this works.

In you go. Ooh, that's hot.

As we grow from an embryo,

our bodies prepare
and shape stem cells

for the adult roles
they will need to fill.

Each stem cell decides
what adult cell type to become

based on cues
from the cells around it.

Stem cells respond

to biochemical signals
from other cells.

And so a heart cell started out
as a stem cell,

and it got instructions
to become a heart cell.

You can imagine
putting a heart together

with different vessel cells

or different muscle cells.

A great glassblower can put all
these different shapes together

to make something
beautiful at the end.

- Cool enough to touch.
- Amazing.

It's incredible, really,
when you think about it.

Millions and millions
of stem cells get together,

respond to the right sorts
of biochemical signals,

and make this heart.

The power of stem cells
to create organs from scratch

has drawn the interest
of scientists for decades.

The goal is
to harness this resource

to rebuild and replace
failing tissue.

But there's a problem...

By the time we're fully grown,

we've exhausted our supply
of embryonic stem cells,

and the few stem cells
we have left

are not nearly as flexible.

There are so-called
adult stem cells,

which are partially formed.

They're committed to make one
of the types of adult tissues.

Skin stem cells make skin.

Brain stem cells
make brain tissues.

But the important point

is that
it's partially committed.

It has part of a shape
that commits it

to do some things
and not others.

Only embryonic stem cells

have the flexibility
to become any cell in the body.

But harvesting them
from embryonic tissue

has proved politically
controversial.

Then, in 2006,

scientists made
a stunning discovery.

Adult stem cells can be recycled
back to embryonic form

by modifying
just four of their genes.

They become what are called
induced pluripotent stem cells,

or IPS cells.

So the discovery of IPS cell

was like the shot heard
'round the world.

It was a true revolution
in our understanding

of what cells could do,

and how simply it could be done.

Like throwing glass
back into the furnace,

we can now restore cells
to their embryonic state

and create raw materials

we could use
to stay eternally young.

You could make this
into brain cells,

or heart cells, or skin cells,
or what have you.

We've used genetic trickery
to take, for example,

a skin cell from my arm,

trick it using genetic elements

into becoming a cell

like an embryonic stem cell
in its abilities.

Larry is now using
these rebooted cells

to uncover what goes wrong
in Alzheimer's.

He starts with skin cells
of people with the disease

and changes them
into brain cells.

Then, he can study exactly
what's going wrong

in those cells and develop
new drugs to treat them.

Right now, we have zero drugs

that alter the course
of Alzheimer's disease.

I do know that if we want
to make it past 85 or 90

on a regular basis,

we're going to have
to solve the problem.

Larry hopes
his work on Alzheimer's is

the beginning
of something bigger.

He thinks if we stored
our stem cells

while we're young and healthy,

we could use them later
to replace our aging organs.

It's incredible really
when you think about it...

The ability
to someday build an organ

from stem cells like this
will be a huge breakthrough.

In the future, we may be able

to extend our lives
by recreating

our failing hearts, lungs,
and kidneys.

But what happens to our society
when people live longer?

Are we turning nature's plan
upside down?

One scientist
has simulated a world

where no one dies of old age,

and he's reached
a startling conclusion.

If our organs have
the power to regenerate,

why do they allow us
to age and die?

Plants, bugs, animals...
It happens to us all.

Perhaps it's because

death is programmed into us
by evolution.

Old animals must die
to free up space for the young.

So, if we cheat death,

if we regenerate
the sweetness of youth...

Will evolution end?

It's taken humans
more than 1,000 years

and many lifetimes
to master the game of chess.

That's a puzzle for Brazilian
evolutionary biologist

André Martins.

Wouldn't it be better
if one person

could live long enough
to acquire

that mastery all
by him or herself?

For quite a while,
scientists thought

that evolution could
not explain aging.

When someone die, in average,
they will leave less children,

and that's a clear disadvantage

from the evolutionary
point of view.

Evolution is supposed to favor

the individuals
best adapted for survival.

So why doesn't it let us
survive forever?

Why are we denied
the ultimate adaptation?

André created
a computer simulation

to find out.

I have always find the evolution
problem fascinating.

I thought it could explain
not how but why we age.

André' simulation uses
game theory,

the math of competitive
situations.

He thought he could find out

whether dying
actually serves a purpose

by staging a competition.

So, what the computer model
does is simulate the competition

between mortals and immortals.

When you see blue, it means
that that region of space,

there is a mortal living there.

And if it's red, there is
an immortal living there.

They fill out every space

and they start competing
for the limited resources there.

To picture André' simulation,

imagine a head-to-head contest

between two very different
soccer teams.

The immortal players, in red,
do not wear out with time.

But the mortals, in blue,

will eventually become
too tired to play.

By pitting the two
against each other

in a game lasting
many generations,

André thought he could find out

just how much advantage
immortality conveys.

Every time I run the simulation,

the same thing happens.

The advantage of not dying
makes a huge difference,

and the immortals start winning.

Like soccer,
life is a competitive struggle.

If you compete more effectively,
you control the game.

But André' simulation contains
another life-like factor...

Environmental change.

Just like in the real world,

André' virtual players face
constantly shifting conditions,

like new diseases, predators,

and climate change.

While immortals can
struggle through them,

not all mortals make it.

However, when they die,

they are replaced
with younger, stronger players.

In any evolutionary process,

if you are mutating
and adapting,

the new generations will be
slightly better than older ones

because they are
the ones that survive.

Since the mortals can evolve,

this gives them an advantage

that allow us then
to get back in the field.

In evolution,
each new generation

has some tricks
the last one didn't.

And the faster
you get them on the field,

the faster the species improves.

The impact,
after many generations,

is huge.

First, there is
no advantage for any side.

And then, slowly
the blue mortals start winning.

And after a while,
they can actually drive

the red team, the immortals,
to extinction.

In the end,
the mortal population wins

because the immortals
can't adapt.

They're doomed.

This suggests that aging
has a biological goal.

André believes, to keep pace
with a changing world,

we need new generations
with new adaptations.

And we must make room for them,

which makes mortality necessary.

The mortal can drive
the immortals to extinction.

It was much easier
than what I was expecting.

From evolutionary point of view
for the species,

it is a good thing to die.

Yeah!

From a personal point of view,
it's not a good thing at all.

We owe our very existence
to the wonders of evolution,

even though it has designed us
to get out of the way.

But what if we could sidestep
natural evolution,

manipulate the genes
that make us die?

Could we evolve ourselves
to live forever?

At the core of every living cell

is a single
fabulously complex molecule...

DNA.

Its genetic code
is the source of life.

But DNA is also
the reason we die.

As we age, our DNA code
gets slowly jumbled,

until it can no longer
keep us alive.

If we can undo those mistakes,

we can stop death's shadow
from creeping up on us.

Professor Jennifer Doudna thinks
we're on the brink

of being able to fix
all the errors in our DNA

and make ourselves
immune to disease and decay.

It's been an amazing decade
or more in science

when it's been possible
to sequence

not only the entire human genome

but now many human genomes.

And what's really on the horizon

is the opportunity
to rewrite that information.

Over the past few years,
Jennifer has developed

a revolutionary
DNA-editing technique

which has turned the dream
of modifying our genes

into practical reality.

What's exciting right now
is that we have a chance

to make changes to the DNA

at the level of a single letter

in the more
than 3 billion letters

in the human cell.

So this is a wonderful moment,

when a technology
has become available

for that kind of precision
genome engineering.

In the past,
DNA editing has been

a crude, hit-and-miss affair.

Imagine trying to fix
a pair of eyeglasses

without being able to see them

and with tools that are
far too big for the job.

We didn't dare
to edit human genes that way.

But Jennifer
and her collaborator,

Emmanuelle Charpentier,
have developed a new,

far more precise tool

that makes DNA surgery
not only possible,

but easy.

It's called CRISPR,

and it places
almost god-like powers

in the hands of humanity.

Like many scientific wonders,

CRISPR was pioneered
by nature herself

inside bacteria.

We began our project
with the goal of understanding

how bacteria fight
viral infection.

But we recognized
that molecules involved

in those processes in bacteria

could be harnessed
as a technology

for rewriting the DNA in cells.

You could think
about the DNA of a bacterium

as a house

and the invading viral DNA
as a burglar.

Jennifer
and Emmanuelle discovered

that bacteria have a built-in
surveillance system.

They can't prevent
the first attack...

But a system called CRISPR

takes a precise snapshot
of the viral DNA,

the same way
a security camera captures

a picture of a burglar.

In a real bacterial cell,

the CRISPR sequence is the way

that the cell records images
in the form of DNA

that represent foreign invaders

and keep it for future reference

to protect the cell
from those same invaders.

When the intruder
shows up a second time,

the bacterial cell
recognizes the invader

from stored surveillance images

and calls the cops.

In the bacterial cell,
those cops are the enzyme cas9.

So when the house
pulls out a picture

from the surveillance camera...

And identifies
a potential invader,

that's the CRISPR system
using the enzyme cas9

to find and destroy foreign DNA.

The cas9 molecule acts
like a pair of scissors

precisely guided by CRISPR

to a matching site
on the virus' DNA.

It then cuts the DNA
in that spot,

destroying the virus'
ability to reproduce.

The threat to the bacterium
is eliminated.

But Jennifer realized

she could adapt
this natural mechanism

into a pinpoint method
for gene editing,

accurate down to
a single letter of DNA.

This genome-editing technology
provides the opportunity

both to identify

genetic mutations
that cause disease,

but also to actually correct
those mutations.

The ability to
precisely fix errors in our DNA

brings the promise
of extending our lives,

lifting the shadow
that death casts over us.

We could use CRISPR to stamp out
hereditary diseases

like cystic fibrosis
and sickle cell anemia

or simply make better humans,

ones with extra-strong bones,

low risk of Alzheimer's,

or built-in resistance
to cancer.

But a dark side looms.

Could this technology be used
to create

genetically engineered embryos
for a super race,

an elite population
of designer babies

with enhanced intelligence,
beauty, or strength?

I, myself, came
to this realization over time,

and one of the things
that really influenced me

was a dream that I had
in which I was being asked

to explain the CRISPR technology
to someone in a dark room,

and when that person
turned around,

it was the profile of Hitler.

I still feel chills when I think
about that moment in my dream,

when I really felt strongly
that this technology needs

to be handled with caution.

Humans genetically
engineered to live longer

are no longer the stuff
of science fiction.

The more we study our genome,

the more opportunities
we will see to improve it.

In fact, scientists
are already hunting

for the genetic secret
to living for 200 years

in the DNA of another animal.

In 2007, fishermen caught
a bowhead whale

off the coast of Alaska.

Embedded in its flesh,

they found a harpoon tip
dating to the 1890s.

No mammal on earth lives
longer than the bowhead.

Some of them live past 200.

If we are looking for the
genetic tools to cheat death,

all that we need is
probably already out there,

in nature's vast pool of DNA.

We just have to find it.

Harvard scientist
George church believes

we need to probe
the vast pool of animal DNA

to find the secret
of longer life.

One of the mysteries of aging
is why it is

that some animals like mice
live for 2 1/2 years

because of their
genetic program,

while other animals like bowhead
whales live for 200 years

because of a different program.

There's a wide range
of life spans

across all organisms in nature,

but there is
a consistent trend...

Large animals tend
to live longer.

You could think of this
in terms of a swimming race...

Between long-distance swimmers
and sprinters.

In this analogy,
the bowhead whale long-live...

They're like
the marathon white-cap swimmers,

and the mice are like
the red-cap sprinters.

Big animals move slowly
but last for the long haul.

Small guys race along
and burn out quickly.

But George doesn't think
size explains

a bowhead whale's
long life span.

He thinks the secret lies
in how it takes care of its DNA.

Researchers
recently found evidence

that the bowhead excels

at repairing damage
to its genes.

In humans, DNA damage is
a major cause of aging.

George thinks we could
repair that damage using

the powerful new CRISPR
gene-editing technique.

We're harnessing
vast amounts of information

gathered on long-lived animals

and converting it
into a gene therapy

that can test whether
we can do aging reversal

in large animals and humans.

Most of us die because
our organs fail...

Our hearts, our lungs,
livers, our brains.

But George thinks
we can build better organs

by going back
and removing genetic defects

from the cells
they are made from.

To test this idea,

George looked
at the genomes of pigs.

We are using editing methods

to engineer cells
that are capable

of producing pig embryos.

Scientists have discovered that,

over the millennia,
pig DNA has been contaminated

by 62 different viruses.

George decided to use it
as a test case

to see if gene-editing

could delete
these tiny contaminants.

What we did was unprecedented,

which was change 62
pig viral genes simultaneously

so that now the pigs are,
for the first time, virus-free.

With sperm and egg cleansed

of ancient contaminants,

George made his own pig embryos

using in vitro fertilization.

Once the pig embryos
are implanted,

we no longer have to keep
going back to the embryo stage.

Then, the pigs breed
just like regular pigs would.

It's just that they
no longer produce viruses.

It also gives us the opportunity

to make more healthy organs

which are resistant to viruses,
cancer, and aging.

Ultimately,
George wants to apply

this technique to humans

and to a much broader range
of genetic defects,

deleting harmful genes

and adding in beneficial ones

we find in long-lived animals
like the bowhead whale.

Today, we can test for genetic
defects in unborn children,

but we lack the means
to cure them.

So when you go
to a modern obstetrics clinic

like this, and you're concerned
about genetic disease,

the mother and the father can
get a readout of their genome

and make decisions that might
involve termination.

George believes that,
in the near future,

parents won't have to face
these difficult choices.

The gene-editing techniques
he's pioneering in animals

will soon be applied
to the genes of human parents

before they
ever conceive a child.

In the near future,
you'll be able to edit

either the mother
or father's body DNA

so that the sperm
or egg does not contain

the very serious disease

and never even makes it
into an embryo.

Further into the future,

we'll even add DNA modifications
so that children are born

not only healthy but
with long-lived super organs.

Today, living for 200 years
is beyond our reach.

But with precision editing
in the sperm and egg,

our genes may soon have

the staying power of the genes
of the bowhead whale.

Just as our ancestors

had a life expectancy
of 45 years,

today it's more like 90.

Mothers today can expect
their children to live

into their 100s.

Imagine a future where people

over 100 years old continue
contributing to society

as if they were
in their 30s again,

where their vast experience
is valued,

and ageism is
a thing of the past.

But this man thinks
our quest to cheat death

threatens the fabric
of civilization itself.

He says it's time
for us to embrace death.

A century ago,

the average life expectancy
of an American was around 52.

With genetic engineering,

living to 150 will
fairly soon be the new norm.

Now, that sounds great.

Who doesn't want more time
to grow, to love, to create?

But what happens
if we keep pushing the limit...

300, 500, maybe forever?

You'd think a world
where death is a rarity

would be a utopia.

But would it?

Stephen cave is
a professional philosopher.

He thinks a lot about death.

You might say it's what gets him
out of bed in the morning.

Most people, most of the time,
are running away from death.

We're terrified of death

even though it's one of life's
great certainties,

along with taxes.

Most people are in denial of it.

But Stephen is
a little different.

He's actually running
towards death.

As he sees it,
our civilization depends on it.

I think it's important
that we actually face up

to mortality,
that actually life is richer

and better and more valuable

when we recognize
that it is limited.

But that awareness
of our own mortality causes

an intense anxiety.

To deal with it,
we repress and deny it,

and even lie to ourselves.

Social psychologists call it
terror management theory.

But what do we do?
How do we cope with that terror?

Well, we tell ourselves stories
that deny the reality of death,

that tell us that somehow
we can keep going forever.

In other words,
confront someone with the fact

that they're going to die,

and they will believe any story
claiming we can live forever.

So every civilization
has some story

about why we don't need
to fear death.

Now this might be a religion,
for example,

that promises that if we believe
we can live forever in heaven.

Or it might be patriotism
or nationalism

that promises we can live on
as part of this greater whole.

Imagine these
identically dressed runners

represent one subgroup
of human society.

And now suppose that
the group splits in two,

with each half taking different
paths through the city.

The two groups remain
indistinguishable

from one another,

until one passes
through a graveyard.

The group who have been
running past the cemetery

will unconsciously have been
reminded about death,

and because of that they will
be in a different state of mind

when they finish their run.

When the members of
the group join back together,

they meet a pair of individuals
who belong to a new group.

The members of the group
who saw the tombstones

aren't interested
in the outsiders.

Those who never saw
the reminders

of the dead
are more open-minded.

Over 400 studies have shown
merely thinking about death

causes us to pull tighter
into our social groups.

This may sound like a bad thing,

but Stephen argues that,
on balance,

over the course of history,
it has not been.

Fear of death is the engine
that drives us

to create tight-knit societies,

and from that culture and
civilization itself are born.

So much of what we've built is
about helping us to live longer.

And if we can't stay
alive physically, you know,

if buildings and medicine

and science
doesn't do it for us,

then we've got art
and culture and religion

that helps to carry us forward,

even after bodily death.

Stephen believes
all civilizations exist

to defend us
against the fear of death.

Without the fear of death,

humanity as we know it
would cease to exist.

If we all woke up tomorrow
and found ourselves immortal,

then there would be profound
changes in our society.

Religion would lose
its unique selling point.

We wouldn't need to sign up

in order
to get eternity in heaven

if we already had eternity
here on earth.

The inevitability
that our lives will one day end

has spurred us
to leave a legacy,

to create literature,
art, technology,

everything
that makes us who we are.

Rather than paralyzing
us with fear,

Stephen believes
knowledge of our demise

can help us live.

You can see life as like a book.

Just as a book
is bounded by its covers,

so our lives are bounded
by beginning and end.

And even though a book
is limited by its cover,

still it can encompass
fantastic adventures

and distant landscapes
and exotic figures.

The characters in a book,

they're not afraid
of you reaching the last page.

And so we shouldn't worry

about whether our story
is long or short,

whether it's a comic story
or an epic.

We should just focus
on making it a good story.

We've been so busy striving
to cheat death,

we forgot to ask ourselves
if we really want to live

in a world filled
with immortals.

As science allows life
to last longer,

it may also grow sweeter,

free from disease,
physical infirmity,

and the slow decay of the mind.

But nothing needs
to last forever.

To the young,
death's shadow brings fear.

But to a man of my age,
it brings vigor,

the desire to do what I love
while I still have time.

Oh, I'm happy to cheat death,

but only as long
as my passion for life remains.