Wonders of Life (2013–…): Season 1, Episode 5 - Home - full transcript

This creature...

is a wonder of nature.

BIRDSONG

Its biology is hard-wired
to the heavens.

BUZZING

It has an exquisitely sensitive eye

that locks onto the sun

and allows it to navigate its way
across the face of the planet.

In a sense,

it has an instinctive understanding
of its place in the solar system.

A tiny insect brain



joined to the movements of the sun
and the planets.

This connection steers the monarch
and millions of its brethren

as they make one of the longest
migrations of any butterfly species.

They're heading for these trees
known locally as the oyamel,

or sacred firs.

Some of the butterflies began their
journey over 4,000 kilometres away,

that's 2,500 miles,

up here in the north-eastern
United States and Canada.

And over the autumn and the winter,

they've migrated south
across the United States

and arrived here, in central Mexico.

Incredibly, no butterfly
has ever learned this route.

It can't have,

because it takes at least three
generations to make the round trip.



Instead,
the homing instinct is carried

on a river of genetic information
that flows through each butterfly.

The allure of this place
to the butterflies,

this sense of belonging,

is a deep feeling we all share.

We even have a word for it - home.

Every living thing that we know
to exist is found on this one rock.

So, what is it about our planet

that makes it such a rich,
colourful, living world?

I want to show you why our world

is the only habitable planet
we know of anywhere in the universe.

Now, the answer depends
on the presence of a handful

of precious ingredients
that make our world a home.

SQUAWKING

'In the beginning, God created
the heaven and the earth.

'And the earth was without
form and void.

'And darkness was upon
the face of the deep.'

SQUAWKING

Home is such an evocative word.

I mean, it will mean something
to you.

The place you went to school,
the place you live,

the place where your kids
had their first Christmas.

But in a scientific sense,
what does it mean?

It means...that the ingredients
are there for you to live.

An atmosphere,

food, water.

You need the temperature
to be right.

Home is the place that has
the things you need for your biology

and chemistry to work.

And it's no less evocative for that.

YELLING AND WHINNYING

This is Mexico.

A country rich in the ingredients
that set our world apart.

It's not a bad place to come

because, with about 1% of
the land surface area of our planet,

it's home to 12% of the species.

There are 26,000 plant species here,

there are 700 species of reptiles

and 400 species of mammals.

It's also been home to some
of the world's great civilisations.

The Maya built their temples
out there in the forest here

for thousands
and thousands of years.

NATIVE SINGING

Mexico is bursting with life.

And if you know where to look,

hidden inside these creatures

are clues that tell how this planet
became their home.

First stop is in the southeast
of the country.

An area covered in thick jungle.

The Yucatan's a strip
of essentially pure limestone

that separates the Caribbean
from the Gulf of Mexico.

And it's got all the ingredients
you might think you need

for a rich and diverse ecosystem.

The tropical sun warms the forest,

delivering precious energy
to each and every leaf.

Oxygen escapes
from the plants and trees,

which is breathed in
by the forest animals.

And where they can, each of them

draws deeply from the region's
hidden water supply.

But there are some
of the ingredients you need

to grow this tropical forest

that are far more important
than others.

You might think that this place
would be awash with water.

It does rain a lot
and it's incredibly humid.

But actually,
there are no surface rivers at all

on the Yucatan Peninsula

because the water just seeps
into the porous limestone.

That's where these things come in.
These are cenotes.

They're caverns dissolved
out of the limestone by the rain.

And they collect water.

And they play a vital role
in the ecosystem.

I mean, the forest changes
when you get around a cenote.

Just listen to that.

RIBBITING

Those are frogs.

And you don't hear those frogs
anywhere else in the forest,

just around the cenotes.

The cenotes are flooded caves

that have been cut off
from the outside world

for thousands of years.

Lilies, troglodytic fish,
even the occasional turtle,

all thrive around the openings
of these freshwater wells.

As I head deeper into the cave,

the temperature drops
and the light fades.

One by one, the ingredients
I depend upon begin to disappear.

Yet even here,
far from the soil and air,

strangely-coloured algae
still find a home in the water.

If there's one thing that unites
every form of life in the cenote,

in fact, every form of life
out there in the forests,

in fact, every form of life
we've ever discovered

anywhere on planet Earth,

it's that it has to be wet.

Only on our home does water
run freely between the skies,

oceans, rivers and on,
into every living thing.

MARIACHI MUSIC PLAYS

CAR HORN BEEPS

SHE SPEAKS IN NATIVE TONGUE

To understand why life and water
are so intertwined,

we need to look a little deeper

into one of the strangest
substances we know.

ANIMATED CHATTER

Now, I may be
a bit of a middle-aged academic,

but I can still do the odd
experiment every now and again.

So what I'm doing is I'm charging up
this Perspex rod.

So giving it an electric charge
by rubbing it on the fleece.

Now, watch what happens...

when I put the rod
next to a stream of water.

You see that?

Look at that.

The electric field,
the electric charge,

is bending the water towards it.

Now, the reason for that,

the reason that water behaves
in that way

when it's passing through
an electric field,

is exactly the same reason that
it is vital for all life on Earth.

Water is a polar molecule,

which means it responds
to electric charge.

Its polarity comes about

because of the structure
of water molecules themselves.

Now, water is H2O,

two hydrogens and one oxygen atom
bound together.

So two hydrogen atoms
approach oxygen.

Now, oxygen's got a cloud
of eight electrons around it,

so when the hydrogens come in,
then what happens

is the electrons get dragged
over here, around the oxygen.

So you end up with an electron cloud
around here and, to some extent,

pretty isolated,
positively-charged protons out here.

So you get a net positive charge
over here

and the electron cloud
with its negative charge over here,

so you get
what's called a polar molecule.

And that's why,
when you bring a charged Perspex rod

close to water molecules,
they bend towards it.

BIRDSONG

Water's polar nature means that
although its molecules are simple,

together, they form a subtle,
endlessly complex liquid.

A home in which
one tiny creature thrives.

There he is. Look at that.

That...is a pond skater.

A predator that floats
on the surface of the water

and actually uses the surface
of the water to sense its prey.

Pond skaters are vicious predators

that live for most of their lives
on the surface.

Tiny hairs on their legs
provide a large area

that spreads their weight.

Their middle legs
thrust them forward.

Hind legs are employed to steer.

They're so well adapted to life
in this flat world

that they even sense
their sexual partners

through tiny vibrations
in the water's surface.

The reason it can do that is
the result of a complex interaction

between adaptions
in the animal itself

and the physics and the chemistry

of the surface of water.

Water molecules are polar.

And that means that water molecules
themselves can bond together.

So you can get a hydrogen
with its slight positive charge

getting close to the oxygen
of another water molecule

with its slight negative charge
and bonding to it.

You can build up quite large,

in fact, VERY large structures
in liquid water.

This is what gives water
its unique ability

to form a surface habitat
for the pond skaters.

Clumps of H2O stick together,

keeping the surface under tension.

Forming a chorus
of water molecules,

all joined together
by hydrogen bonds.

Then a pond skater comes along
and it puts its legs or its...

dangly things into the water
and pushes it down,

bends the surface of the water.

Now, the water doesn't like that

because a bend in the water
is increasing its surface area.

It's increasing its energy.

It's making it harder for all
the molecules to bond together

with the hydrogen bonds.
So they try to push back.

They exert a force
on the pond skater's leg

because they want to bond
as much as they can.

And that's how pond skaters stay
on the surface of the water.

Hydrogen bonds do far more

than just give the pond skaters
a place to live.

They're fundamental to all life.

I've heard it said that
we won't truly understand biology

until we understand water.

These are...very thin tubes
of glass.

They're about a millimetre
in diameter.

And if I dip one
into the surface of this river...

..can you see that the water
just climbs up the tube?

It pulls itself up, quite literally,
against the force of gravity.

Now, in trees, there are tubes which
are about half the diameter of this,

perhaps about half a millimetre
or even less.

And they are called xylem.

And they allow the tree to lift
water up through the root system

because the water molecules
strongly attract each other

and are strongly attracted
to the sides of the tubes.

So when you look at trees like that,
which are very high,

and you ask yourself the question,

"How do they get the water from
the roots to the top of the tree?",

a big part of that
is capillary action,

which is down to the polar nature
of water.

One of water's most important
qualities

is its ability to dissolve and carry

all manner of substances
around the living world.

Because its molecules
are very small and polar,

water is a tremendously effective
solvent.

Those molecules can get
in amongst other substances,

salts and sugars, for example,
and disperse them, if you like,

in that sea of hydrogen bonds.

Within every one of us,

water is constantly flowing
around each and every cell.

Blood plasma is over 90% water.

And in it are dissolved
everything I need to live -

oxygen, the nutrients from food,
everything -

distributed around my body
in rivers of water.

We live on a beautiful blue anomaly
of a world.

The only planet we know with
a surface drenched in liquid water.

The story of how each drop ended up
here has been hard to fathom.

Largely because it happened
so long ago,

there's very little direct evidence.

But back in the Yucatan jungle,

clues to how it turned up
can still be found.

Every civilisation on the Yucatan,

be it the modern Mexicans
or the Mayans,

had to get their water
from those deep wells, the cenotes.

And I've got a completely unbiased
map of the larger cenotes here,

which I'm going to overlay
on the Yucatan.

Look at that.
They lie in a perfect arc,

centred around
a very particular village,

which is...there,

and it's called Chicxulub.

Now, to a geologist,
there are very few natural events

that can create a structure,
such a perfect arc as that.

All the evidence
points to just one explanation.

You're looking at what's left
of a gigantic asteroid strike.

One that wiped out three-quarters
of all plant and animal species

when it hit the Earth
65 million years ago.

You may think that impacts
from space are a thing of the past.

A thing that only happened to
the dinosaurs, but that's not true.

About 55 million kilograms of rock
hits the Earth every year.

And around 2% of that is water.

This hints that at least some
of Earth's water arrived from space.

Late in 2010,
these glimpses of comet Hartley 2

arrived back on Earth.

They were sent by NASA's
deep-impact probe.

From its surface,
dust and ice spray into space.

Analysis of this water found it had
a very similar mixture of isotopes

to the water in our own oceans.

This was the first firm evidence

that icy comets must have
contributed

to the formation
of our world's oceans.

Earth began life as a molten hell.

Its internal heat drove off
any trace of moisture.

But soon, the planet cooled
and the first clouds grew.

Then, 4.2 billion years ago,

a deluge,
the like of which the solar system

had never seen before or since,
rained down.

THUNDERCLAP

And again,
thanks to those hydrogen bonds,

water's boiling point is high enough
to have allowed it to remain

on the surface of the Earth
to the present day.

So from quite early in its history,

our home has been able to hang on
to this most vital of ingredients.

But to trace the origin
of the next ingredients,

you have to look beyond
our planet...

..to our nearest star.

And the rays of light
it sends our way.

This is the train from Los Mochis
to Chihuahua,

which inexplicably leaves
at 6:00am in the morning.

Um...the local name for this area
in all the guidebooks

is the Land of Turtles.

Beautifully romantic name for
this place on the Sea of Cortez.

But we just found out it's probably
more likely to have been called

the Land of Spinach-type Vegetables.

So we're going from
the Land of Spinach-type Vegetables

to Chihuahua,

which is
the Land of Very Small Dogs.

One of the great railway journeys
of the world.

TRAIN HOOTS

Almost all life depends on the
energy that the sun sends our way.

But the sun is a far-from-benevolent
companion

because its radiant rain can be
as dangerous as it is nourishing.

We're still round about sea level
now

and the sun is quite low in the sky.

It's about 7:00am,
so it's not been up long.

I'm going to measure the amount
of UV radiation

falling on every square centimetre
with this,

a digital, ultraviolet radiometer.

At the moment, it says
there's about 22 microwatts

per square centimetre
falling on my skin.

But as we climb in altitude,
then that UVB light

is going to have to travel through
less and less of the atmosphere,

so less of it is going to be
absorbed.

And sure enough,
as the miles pass by

and we head
into the mountainous interior,

the meter readings start to go up.

Now it's about 10:00am, so the sun's
significantly higher in the sky.

The train's also climbed
quite a bit in altitude.

Now...

..we're getting nearly 250
microwatts per square centimetre.

So that's about a factor of ten
higher.

And that's just because the UVB has
had significantly less atmosphere

to travel through, from the top of
the Earth's atmosphere down to me.

That's more than enough
to burn unprotected skin

in just a few minutes.

And that's because
what arrived from the sun

is far more
than just the stuff we can see.

Beyond the visible, the higher
energy part of the spectrum,

there's ultraviolet light,
particularly UVB,

which does get through the Earth's
atmosphere and gets to the surface.

Now, UVB can be beneficial to life.

We use it to produce vitamin D,
for example.

But because it's higher energy,
it can also be extremely damaging.

It can damage DNA, it can burn our
skin as well as give us a suntan,

and, of course, ultimately,
it can give us skin cancer.

WHISTLE HOOTS

If ultraviolet light is a problem

for life on Earth to deal with
today,

then the physicists might raise

an interesting problem
for the biologists.

Because we know
that 3.5 billion years ago,

when life on Earth began,

although the sun was much dimmer
in the visible part of the spectrum,

it was significantly brighter
in the ultraviolet.

The young sun seems like a paradox.

It was fainter to the eye,

perhaps 30% less bright
than the sun we enjoy today,

yet rich in deadly ultraviolet.

Inside, the core was spinning
much faster,

which created more
electromagnetic heating

of the plasma on its surface.

And this plasma emitted more energy,

not in the lower visible
frequencies,

but in the higher frequencies.

Like X-rays...

and ultraviolet.

It seems as if just as life was
getting settled on its wet home,

the faint young sun was making it
tough to survive near the surface.

This is the top of Copper Canyon, so
the summit of the railway journey.

It's about 2,200 metres,
which is about...

somewhere between
7,000 and 8,000 feet.

So I'll take a UV reading
of the sun.

It's actually reading about 260 now.

Now, if you remember, at midday,
down at sea level,

we were getting readings around 260.

So although the sun has dropped
in the sky,

so the sunlight and the UV are
coming through much more atmosphere,

that's been compensated for
by the thinness of the air up here.

I'm getting more UV now
than I would have been

at the same time of day
at sea level.

It's hard to be sure,

but we think that
it's these kinds of radiation levels

that early life had to deal with.

Because back then,
the sun's ultraviolet output

was significantly stronger.

So I think it is fair to say

that that could have posed
a significant threat

to the development of early
life on Earth.

WHINNYING

ANIMATED CHATTER

Today, life has painted the surface
of our home

in all the colours of the rainbow.

From greens to blues,

reds to yellows,

oranges and violets.

And the origin of all life's hues
can be traced back

to the way it interacts
with sunlight.

I'm a particle physicist, so
I'm allowed to think of everything

in terms of the interactions
of particles.

So I would picture the light
from the sun

as being really a rain of particles.

Photons, they're called,
particles of light

of different energies, raining down
on the surface of the Earth.

The blue ones are
the highest-energy photons,

the red ones are
the lowest-energy photons

and all the colours of the rainbow
in the middle

are just simply photons
of different energies.

SHE SPEAKS IN NATIVE TONGUE
Oh, thank you.

Wow.

For this, the chilli salsa which
I see as red, there are pigment

molecules in there that are
absorbing the blue photons,

the blue light from the sun.

The red ones,
it doesn't interact with,

so they bounce back into my eye,
and that is why I see it as red.

The same with the green chilli,

but in this case the red photons
are interacting, doing something,

talking to pigments in here,

and what I am seeing are the green
photons and some of the blue photons

coming into my eye, mixing up,
allowing me to see that as green.

Pigments bring colour to the world.

The planet is painted by genes,

honed by billions of years
of evolution.

'Some colours warn of danger...'

This stuff is on fire, I tell you!

'..or attract pollinators.'

Pigments are one of the ways
that life has evolved

to take on the sun's powerful
ultraviolet light.

This little guy is called
a bombardier beetle.

If I just grab him...

His name comes
from his unique defence mechanism.

He produces two chemicals. One
of them you might have heard of -

hydrogen peroxide. The other one
is something called hydroquinone,

and when you scare him,

both those chemicals are injected
into a little chamber in his body.

It raises the temperature
to the boiling point of water,

and increases the pressure,

squirting a hot and noxious chemical
out of its rear.

A clever way to defend yourself.

But this is just one of the ways
this character uses chemistry

to increase the chance of survival.

The bombardier beetle and me,

and in fact every living thing
you can see, are exposed to

the same threat on the high plains
of Mexico, the high-energy

ultraviolet photons raining down
on this landscape from the sun.

If they hit DNA in my skin,
for example, they damage the DNA.

So that must be prevented.

Me and my friend, the beetle, have
both reached the same solution -

you see that the beetle is brown
and black.

My skin, when it is exposed
to the sun, is going brown.

I am producing a pigment called
melanin, and so is the beetle.

Melanin is a very simple molecule,

it's just a ring of carbon atoms
with a few extra bits bolted on,

but the sea of electrons behaves
in a very specific way.

When a high-energy ultraviolet
photon from the sun

hits one of those electrons, it
very quickly dissipates that energy.

That potentially threatening photon
has been absorbed

and all its energy has been
dissipated away as heat.

Melanin is so efficient,

over 99.9% of the harmful
ultraviolet radiation is absorbed.

So melanin is protecting

both my skin and my friend,
the bombardier beetle,

from the potentially harmful effects
of the sun.

From the start,

life had to evolve strategies for
coping with the energetic young sun.

Life is nothing if not resourceful.

Pigments are the way
that living things interact with

the radiation from the sun. So why
just use them to dissipate energy,

to protect?

Why not use them to harness
that energy for its own ends?

That is exactly what life did.

In doing so, it transformed
our planet by introducing

a wonderful new ingredient.

Earth has an atmosphere
unlike any other planet

we know of in the universe.

Only in the air on our world
do fires burn.

Only on our world has a gas
been released which allowed

complex life to evolve.

What makes our home unique
is its oxygen-rich atmosphere.

Deep in a cave in the hills
of Tabasco, you can find a hint

of what living planet without oxygen
might be like.

This is one of the more unique
environments on our planet.

This cave is full of sulphur,
you can see it in the water.

You can see that milky colour
flowing through the cave.

That is dissolved sulphur.

It is coming
from hydrogen-sulphide gas,

the source of which
is actually not entirely known.

The hydrogen sulphide
is toxic to me.

It has another rather alarming
effect on this hellhole.

It is a bad-smelling gas,

but it is also a gas that drives
the oxygen out,

so as you go on into the cave,
you get less and less oxygen.

In a sense, some of the chemistry,

the biochemistry that takes place
in the dark of this cave system,

could be very similar
to the chemistry

and biochemistry that occurred
when our planet was very young.

For the first half of its history,

Earth was without oxygen
in the atmosphere.

But incredibly, in this echo
of the past, which I can only visit

for a few minutes, there are forms
of life that are completely at home.

Look at that!

There they are,
cities of sulphur-eating bacteria

living off
the hydrogen-sulphide gas.

Colonies of extremophiles,

organisms living off a very
different environment of gases

to the one that we are used to
on the surface.

They are a window
on a much earlier time.

Because without oxygen,
the ancestors of these extremophiles

were the only forms of life
our planet could support.

Understanding how Earth developed

an atmosphere rich in oxygen
has taken centuries.

The secret lies
with ancient bacteria.

In 1676, a Dutchman
called Antonie Leeuwenhoek

was trying to find out
why pepper is spicy.

See, they thought that there were
little spikes on peppercorns

that dug into your tongue.

He was using the microscope,

which had been discovered
about 60 years before,

but inexplicably, had never been
used for anything useful before.

He put the peppercorns on there
and looked down
and he couldn't see anything,

so he thought
he would grind them up,

dissolve them in water
and have a look. When he did that,

he didn't see anything interesting
in the peppercorns,

but he found that there were
little animals swimming around.

He said that he estimated

you could line about 100
of the "wee little creatures" -

those are his words - on the length
of a single coarse sand grain.

What Leeuwenhoek thought were
animals were, in all probability,

not animals at all.

Although he didn't know it
at the time,

he had discovered a whole new domain
of life.

Bacteria.

They are by far the most numerous
organisms on the Earth.

In fact, there are more bacteria
on our planet than

there are stars
in the observable universe.

And there is one kind of bacteria
more numerous than all the rest.

One of the most striking structures
I can see on this slide is

a kind of blue-green filament
which is a little colony

of a type of bacteria
called cyanobacteria.

These things are incredibly
important organisms.

Fossilised cyanobacteria had
been found as far back

as 3.5 billion years ago.

And at some point,
around 2.4 billion years ago,

they became the first living things
to use pigments

to split water apart
and use it to make food.

This evolutionary invention was
incredibly complex.

Even its name is a mouthful -
oxygenic photosynthesis.

It starts with a photon from the sun

hitting that green pigment,
chlorophyll.

Chlorophyll takes that energy
and uses it

to boost electrons up a hill,
if you like.

And when they get to the top, they
cascade down a molecular waterfall,

and the energy is used
to make something called ATP,

which is potentially
the energy currency of life.

This little molecular machine is
called photosystem II,

and it makes energy for the cell
from sunlight.

But when the electrons reach
the bottom of that waterfall,

they enter photosystem I.

They meet some more chlorophyll,

which is hit by another photon
from the sun,

and that energy raises the electrons
up again,

and forces them onto carbon dioxide,

turning that carbon dioxide
eventually into sugars,

into food for the cell.

Now, why all this complexity?

Why do you need
these two photosystems

joined together in this way,

just to get some electrons and make
sugar and a bit of energy out of it?

It's because

only when life coupled these
two biological machines together

that it could split water apart
and turn it into food.

But it wasn't easy.

The thing is that water is
extremely difficult to split,

so for a leaf to do it,
for a blade of grass to do it,

just using a trickle of light from
the sun, is extremely difficult.

In fact, the task is SO complex
that, unlike flight or vision,

which have evolved separately
many times during our history,

oxygenic photosynthesis
has only evolved once.

Every tree, every plant,
every blade of grass on the planet,

everything that carries out
oxygenic photosynthesis today

does it in EXACTLY the same way.

And the structures inside every leaf
that do that

look remarkably similar
to cyanobacteria.

In other words, the descendants
of one cyanobacterium

that worked out, for some reason,

how to couple those complex
molecular machines together

in some primordial ocean,
billions of years ago,

are still present on the Earth
today.

The cyanobacteria changed
the world...

..turning it green.

And that had
a wonderful consequence.

With this new way of living,

life released oxygen
into the atmosphere of our planet

for the first time. And in doing so,

over hundreds of millions of years,

it eventually completely transformed
the face of our home.

And as the oxygen levels grew

the stage was set for the arrival
of ever more complex creatures.

But on Earth, the emergence
of complex life required

a rather more intangible ingredient.

Something that you can't see,
touch or smell,

and yet you pass through every day.

Late January,

and the monarch butterflies have
found their way home.

They've entered a hibernation state,
huddling together for warmth.

But they're only here at all
thanks to one of the most accurate

biological clocks found in nature.

These are the pine
and oyamel forests, high altitude,

about, what, three hours north-west
of Mexico City,

and one of the few wintering grounds
of the monarch butterflies,

as you can see.

But there is a colony
of millions of monarchs

somewhere due north of here,

so if I head off into the forest

then hopefully this will just be
a taster of what's to come.

To find the butterflies, I need
to get an accurate bearing on them.

And to do this I need a timepiece.

If you don't have a compass,

how can you tell which direction is
north and which direction is south?

Well, you can use the sun.

The sun rises in the east,
sets in the west,

and at midday, in the northern
hemisphere, it's due south.

But what if it ISN'T midday?

Well, there's an old trick,
which is to use a watch.

See, it's about three
in the afternoon now,

and if you line the hour hand
of your watch up with the sun,

then, in the northern hemisphere,

the line in between the hour hand
and 12 o'clock

will point due south.

Which means north is that way.

For thousands of miles
on their way here,

the monarchs have faced
the same problem.

To make their way south, it's
no good simply following the sun.

Because, as the day progresses,

the sun's position
drifts across the sky.

Somehow they have to correct
for this.

They use what's called
a time-compensated sun compass.

They measure the position of the sun
every day, using their eyes,

but it's also thought
they can measure the position

even when it's cloudy, by using
the polarisation of the light.

Having locked onto the sun, their
brain then corrects for its movement

across the sky by using one of
nature's most accurate timepieces.

By combining the information
from their precise clocks

and their eyes,
they can navigate due south.

That ability to orientate
themselves is, I think,

one of the most remarkable things
I've seen.

The biological clocks
that have brought the monarchs home

are not unique to butterflies.

Almost all life shares
in these circadian rhythms.

They're an evolutionary consequence
of living on a spinning rock.

Our world turns on its axis once
every 24 hours, giving us a day.

It's on a billion-kilometre journey
around the sun,

and each orbit gives us a year.

We live inside a celestial clock,

one that has been ticking away
for over 4.5 billion years.

And that's a full third
of the age of the universe.

This is the final ingredient
that our home has provided.

Time.

Take the horse.

Like all complex living things,
it's here because our planet

has been stable enough
for long enough

to allow evolution time to play.

The horse is the animal
whose family tree

we know with the highest precision.

So it's possible to lay out
just one unbroken chain of life

that stretches back
nearly four billion years.

Animals that are recognisably
horselike have

been around for a long time -

something like 55 million years.

You then have to jump quite a lot
to something like 225 million years

if you want to ask the question,
where is the earliest mammal?

And it's this thing, which looks
something like a little shrew.

535 million.

This is the point when complex life
really began to explode

in the oceans.

You then have to sweep back
a long, long time to find the next

evolutionary milestone, arguably
the most important milestone -

the emergence of the complex self,
the eukaryote.

And then, you have to step back
a long way in time.

You have to step back
all the way to here,

the emergence of the prokaryote,
the first life form.

And so,
we have this beautiful long line.

We can trace my friend,
the horse, and his ancestry

back to the events that happened
3.5, 3.6, 3.7 billion years ago

on the primordial Earth.

Looking back over that vast
sweep of time,

you could ask yourself the question,
well, do you need 3.5 billion years

to go from a simple form of life
to something as complex as a horse?

Well, the answer to that question
is, we don't know for sure.

It seems that you need vast expanses
of time, but do you need

those big gaps from the simple cell
to the complex cell,

do you need the gap
from the complex cell

to the evolution
of multicellular life?

We don't know.

We only have one example.

There is only one planet
where we've been able to study

the evolution of life,
and it's this one.

And Earth has been an interesting
mixture of stability and upheaval.

It's had an environment

that's never completely conspired
to wipe out life,

but it's constantly thrown it
challenges.

The deep time that our planet has
given life

has allowed it to grow from a tiny
seed of genetic possibility

to the planet-wide web of complexity
we are part of today.

Only a few of us have ever stepped
outside of this world.

But those that have discovered
something rather wonderful.

'For all the people back on Earth,

'the crew of Apollo 8 has a message
that we would like to send to you.'

On Christmas Eve 1968,
my first Christmas Eve,

the Apollo 8 spacecraft entered
the darkness

on the far side of the moon.

'In the beginning, God created
the heaven and the earth.

'And the earth was without form.'

The three astronauts,
Borman, Lovell and Anders,

became the first human beings
in history

to lose sight of the Earth.

'And God said, let there be light.

'And there was light. And God saw
the light, that it was good.'

When they emerged
from the dark side of the moon,

and the Earth rose into view,
they chose to broadcast

their culture's creation story
back to the inhabitants of Earth.

And, just like the Aztecs
and the Mayans

and every civilisation before them,

it told of the origins
of their home.

'And God called the dry land Earth,

'and the gathering together
of the waters called He seas.

'And God saw that it was good.'

It must be innately human, the
desire to understand how our home

came to be the way that it is.

And seen from lunar orbit
against the blackness of space,

the Earth is a fragile world,

but seen by science, it's a world

that's been crafted and shaped by
life over almost four billion years.

So we're on our way to understanding

how we came to be here, but as the
Apollo astronauts discovered,

the journey of discovery has already
delivered much more

than just the facts,
because it's given us

a powerful perspective on the
intricacy and beauty of our home.

'From the crew of Apollo 8, we
close with good night, good luck,

'a merry Christmas,
and God bless all of you,

'all of you on the good Earth.'

Subtitles by Red Bee Media Ltd