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This creature...


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is a wonder of nature.


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BIRDSONG


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Its biology is hard-wired
to the heavens.


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BUZZING


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It has an exquisitely sensitive eye


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that locks onto the sun


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


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In a sense,


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


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A tiny insect brain


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joined to the movements of the sun
and the planets.


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This connection steers the monarch
and millions of its brethren


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


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


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or sacred firs.


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


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that's 2,500 miles,


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


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And over the autumn and the winter,


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they've migrated south
across the United States


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and arrived here, in central Mexico.


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Incredibly, no butterfly
has ever learned this route.


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It can't have,


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


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Instead,
the homing instinct is carried


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


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The allure of this place
to the butterflies,


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this sense of belonging,


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is a deep feeling we all share.


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We even have a word for it - home.


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


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So, what is it about our planet


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


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I want to show you why our world


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


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Now, the answer depends
on the presence of a handful


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of precious ingredients
that make our world a home.


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SQUAWKING


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


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'And the earth was without
form and void.


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


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SQUAWKING


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Home is such an evocative word.


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I mean, it will mean something
to you.


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


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the place where your kids
had their first Christmas.


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But in a scientific sense,
what does it mean?


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


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An atmosphere,


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food, water.


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You need the temperature
to be right.


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


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and chemistry to work.


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And it's no less evocative for that.


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YELLING AND WHINNYING


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This is Mexico.


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


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It's not a bad place to come


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


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it's home to 12% of the species.


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There are 26,000 plant species here,


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there are 700 species of reptiles


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and 400 species of mammals.


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


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The Maya built their temples
out there in the forest here


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for thousands
and thousands of years.


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NATIVE SINGING


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Mexico is bursting with life.


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And if you know where to look,


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hidden inside these creatures


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are clues that tell how this planet
became their home.


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First stop is in the southeast
of the country.


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An area covered in thick jungle.


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The Yucatan's a strip
of essentially pure limestone


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that separates the Caribbean
from the Gulf of Mexico.


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


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for a rich and diverse ecosystem.


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The tropical sun warms the forest,


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delivering precious energy
to each and every leaf.


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Oxygen escapes
from the plants and trees,


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which is breathed in
by the forest animals.


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And where they can, each of them


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draws deeply from the region's
hidden water supply.


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But there are some
of the ingredients you need


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to grow this tropical forest


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that are far more important
than others.


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


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


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But actually,
there are no surface rivers at all


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on the Yucatan Peninsula


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because the water just seeps
into the porous limestone.


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


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


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And they collect water.


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And they play a vital role
in the ecosystem.


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


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Just listen to that.


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RIBBITING


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Those are frogs.


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


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just around the cenotes.


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The cenotes are flooded caves


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that have been cut off
from the outside world


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for thousands of years.


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Lilies, troglodytic fish,
even the occasional turtle,


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all thrive around the openings
of these freshwater wells.


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As I head deeper into the cave,


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the temperature drops
and the light fades.


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


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Yet even here,
far from the soil and air,


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


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


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


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


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anywhere on planet Earth,


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it's that it has to be wet.


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


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oceans, rivers and on,
into every living thing.


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MARIACHI MUSIC PLAYS


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CAR HORN BEEPS


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SHE SPEAKS IN NATIVE TONGUE


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To understand why life and water
are so intertwined,


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we need to look a little deeper


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into one of the strangest
substances we know.


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ANIMATED CHATTER


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


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


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


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


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Now, watch what happens...


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


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You see that?


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Look at that.


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The electric field,
the electric charge,


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is bending the water towards it.


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Now, the reason for that,


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the reason that water behaves
in that way


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when it's passing through
an electric field,


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


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Water is a polar molecule,


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which means it responds
to electric charge.


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Its polarity comes about


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because of the structure
of water molecules themselves.


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Now, water is H2O,


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two hydrogens and one oxygen atom
bound together.


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So two hydrogen atoms
approach oxygen.


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


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so when the hydrogens come in,
then what happens


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


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


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pretty isolated,
positively-charged protons out here.


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So you get a net positive charge
over here


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and the electron cloud
with its negative charge over here,


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so you get
what's called a polar molecule.


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


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close to water molecules,
they bend towards it.


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BIRDSONG


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


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together, they form a subtle,
endlessly complex liquid.


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A home in which
one tiny creature thrives.


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There he is. Look at that.


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That...is a pond skater.


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A predator that floats
on the surface of the water


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


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Pond skaters are vicious predators


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that live for most of their lives
on the surface.


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Tiny hairs on their legs
provide a large area


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that spreads their weight.


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Their middle legs
thrust them forward.


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Hind legs are employed to steer.


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


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that they even sense
their sexual partners


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through tiny vibrations
in the water's surface.


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


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between adaptions
in the animal itself


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and the physics and the chemistry


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00:14:00,480 --> 00:14:02,280
of the surface of water.


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Water molecules are polar.


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And that means that water molecules
themselves can bond together.


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00:14:12,880 --> 00:14:16,520
So you can get a hydrogen
with its slight positive charge


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00:14:16,520 --> 00:14:20,120
getting close to the oxygen
of another water molecule


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with its slight negative charge
and bonding to it.


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You can build up quite large,


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00:14:26,360 --> 00:14:29,760
in fact, VERY large structures
in liquid water.


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This is what gives water
its unique ability


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to form a surface habitat
for the pond skaters.


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Clumps of H2O stick together,


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00:14:44,920 --> 00:14:47,680
keeping the surface under tension.


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00:14:49,280 --> 00:14:52,280
Forming a chorus
of water molecules,


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00:14:52,280 --> 00:14:55,280
all joined together
by hydrogen bonds.


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00:14:59,640 --> 00:15:04,960
Then a pond skater comes along
and it puts its legs or its...


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00:15:04,960 --> 00:15:09,040
dangly things into the water
and pushes it down,


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00:15:09,040 --> 00:15:11,400
bends the surface of the water.


195
00:15:11,400 --> 00:15:13,440
Now, the water doesn't like that


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00:15:13,440 --> 00:15:17,000
because a bend in the water
is increasing its surface area.


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00:15:17,000 --> 00:15:18,840
It's increasing its energy.


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00:15:18,840 --> 00:15:22,200
It's making it harder for all
the molecules to bond together


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00:15:22,200 --> 00:15:25,000
with the hydrogen bonds.
So they try to push back.


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00:15:25,000 --> 00:15:28,240
They exert a force
on the pond skater's leg


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00:15:28,240 --> 00:15:31,320
because they want to bond
as much as they can.


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00:15:31,320 --> 00:15:34,880
And that's how pond skaters stay
on the surface of the water.


203
00:15:39,880 --> 00:15:41,520
Hydrogen bonds do far more


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00:15:41,520 --> 00:15:44,480
than just give the pond skaters
a place to live.


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00:15:45,640 --> 00:15:49,120
They're fundamental to all life.


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00:15:52,800 --> 00:15:56,760
I've heard it said that
we won't truly understand biology


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00:15:56,760 --> 00:15:59,480
until we understand water.


208
00:16:05,200 --> 00:16:11,120
These are...very thin tubes
of glass.


209
00:16:11,120 --> 00:16:15,120
They're about a millimetre
in diameter.


210
00:16:15,120 --> 00:16:19,680
And if I dip one
into the surface of this river...


211
00:16:21,800 --> 00:16:26,160
..can you see that the water
just climbs up the tube?


212
00:16:26,160 --> 00:16:30,120
It pulls itself up, quite literally,
against the force of gravity.


213
00:16:30,120 --> 00:16:34,880
Now, in trees, there are tubes which
are about half the diameter of this,


214
00:16:34,880 --> 00:16:37,600
perhaps about half a millimetre
or even less.


215
00:16:37,600 --> 00:16:40,080
And they are called xylem.


216
00:16:40,080 --> 00:16:44,520
And they allow the tree to lift
water up through the root system


217
00:16:44,520 --> 00:16:47,760
because the water molecules
strongly attract each other


218
00:16:47,760 --> 00:16:51,280
and are strongly attracted
to the sides of the tubes.


219
00:16:51,280 --> 00:16:55,760
So when you look at trees like that,
which are very high,


220
00:16:55,760 --> 00:16:57,560
and you ask yourself the question,


221
00:16:57,560 --> 00:17:01,120
"How do they get the water from
the roots to the top of the tree?",


222
00:17:01,120 --> 00:17:03,680
a big part of that
is capillary action,


223
00:17:03,680 --> 00:17:07,120
which is down to the polar nature
of water.


224
00:17:12,280 --> 00:17:14,920
One of water's most important
qualities


225
00:17:14,920 --> 00:17:17,640
is its ability to dissolve and carry


226
00:17:17,640 --> 00:17:20,880
all manner of substances
around the living world.


227
00:17:23,920 --> 00:17:27,560
Because its molecules
are very small and polar,


228
00:17:27,560 --> 00:17:31,400
water is a tremendously effective
solvent.


229
00:17:31,400 --> 00:17:35,200
Those molecules can get
in amongst other substances,


230
00:17:35,200 --> 00:17:39,400
salts and sugars, for example,
and disperse them, if you like,


231
00:17:39,400 --> 00:17:42,080
in that sea of hydrogen bonds.


232
00:17:43,800 --> 00:17:45,600
Within every one of us,


233
00:17:45,600 --> 00:17:50,040
water is constantly flowing
around each and every cell.


234
00:17:53,080 --> 00:17:56,040
Blood plasma is over 90% water.


235
00:17:56,040 --> 00:17:59,560
And in it are dissolved
everything I need to live -


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00:17:59,560 --> 00:18:03,440
oxygen, the nutrients from food,
everything -


237
00:18:03,440 --> 00:18:08,680
distributed around my body
in rivers of water.


238
00:18:12,160 --> 00:18:15,920
We live on a beautiful blue anomaly
of a world.


239
00:18:15,920 --> 00:18:22,080
The only planet we know with
a surface drenched in liquid water.


240
00:18:27,840 --> 00:18:32,200
The story of how each drop ended up
here has been hard to fathom.


241
00:18:34,360 --> 00:18:36,680
Largely because it happened
so long ago,


242
00:18:36,680 --> 00:18:38,880
there's very little direct evidence.


243
00:18:46,400 --> 00:18:48,480
But back in the Yucatan jungle,


244
00:18:48,480 --> 00:18:52,200
clues to how it turned up
can still be found.


245
00:18:54,680 --> 00:18:56,920
Every civilisation on the Yucatan,


246
00:18:56,920 --> 00:18:59,800
be it the modern Mexicans
or the Mayans,


247
00:18:59,800 --> 00:19:03,680
had to get their water
from those deep wells, the cenotes.


248
00:19:03,680 --> 00:19:08,160
And I've got a completely unbiased
map of the larger cenotes here,


249
00:19:08,160 --> 00:19:11,080
which I'm going to overlay
on the Yucatan.


250
00:19:15,520 --> 00:19:19,800
Look at that.
They lie in a perfect arc,


251
00:19:19,800 --> 00:19:23,200
centred around
a very particular village,


252
00:19:23,200 --> 00:19:26,680
which is...there,


253
00:19:26,680 --> 00:19:29,480
and it's called Chicxulub.


254
00:19:29,480 --> 00:19:33,560
Now, to a geologist,
there are very few natural events


255
00:19:33,560 --> 00:19:39,280
that can create a structure,
such a perfect arc as that.


256
00:19:42,800 --> 00:19:46,600
All the evidence
points to just one explanation.


257
00:19:50,320 --> 00:19:54,360
You're looking at what's left
of a gigantic asteroid strike.


258
00:19:57,040 --> 00:20:01,960
One that wiped out three-quarters
of all plant and animal species


259
00:20:01,960 --> 00:20:06,240
when it hit the Earth
65 million years ago.


260
00:20:06,240 --> 00:20:09,320
You may think that impacts
from space are a thing of the past.


261
00:20:09,320 --> 00:20:13,600
A thing that only happened to
the dinosaurs, but that's not true.


262
00:20:13,600 --> 00:20:18,560
About 55 million kilograms of rock
hits the Earth every year.


263
00:20:18,560 --> 00:20:21,760
And around 2% of that is water.


264
00:20:23,200 --> 00:20:28,480
This hints that at least some
of Earth's water arrived from space.


265
00:20:32,160 --> 00:20:36,600
Late in 2010,
these glimpses of comet Hartley 2


266
00:20:36,600 --> 00:20:38,240
arrived back on Earth.


267
00:20:39,880 --> 00:20:43,840
They were sent by NASA's
deep-impact probe.


268
00:20:43,840 --> 00:20:48,000
From its surface,
dust and ice spray into space.


269
00:20:49,920 --> 00:20:54,560
Analysis of this water found it had
a very similar mixture of isotopes


270
00:20:54,560 --> 00:20:56,840
to the water in our own oceans.


271
00:21:00,480 --> 00:21:02,400
This was the first firm evidence


272
00:21:02,400 --> 00:21:04,720
that icy comets must have
contributed


273
00:21:04,720 --> 00:21:07,520
to the formation
of our world's oceans.


274
00:21:23,920 --> 00:21:26,920
Earth began life as a molten hell.


275
00:21:28,000 --> 00:21:32,280
Its internal heat drove off
any trace of moisture.


276
00:21:34,720 --> 00:21:39,480
But soon, the planet cooled
and the first clouds grew.


277
00:21:41,680 --> 00:21:44,960
Then, 4.2 billion years ago,


278
00:21:44,960 --> 00:21:47,800
a deluge,
the like of which the solar system


279
00:21:47,800 --> 00:21:52,040
had never seen before or since,
rained down.


280
00:21:52,040 --> 00:21:53,480
THUNDERCLAP


281
00:22:12,040 --> 00:22:15,280
And again,
thanks to those hydrogen bonds,


282
00:22:15,280 --> 00:22:19,800
water's boiling point is high enough
to have allowed it to remain


283
00:22:19,800 --> 00:22:23,840
on the surface of the Earth
to the present day.


284
00:22:23,840 --> 00:22:26,480
So from quite early in its history,


285
00:22:26,480 --> 00:22:31,120
our home has been able to hang on
to this most vital of ingredients.


286
00:22:31,120 --> 00:22:33,960
But to trace the origin
of the next ingredients,


287
00:22:33,960 --> 00:22:36,800
you have to look beyond
our planet...


288
00:22:38,600 --> 00:22:40,680
..to our nearest star.


289
00:22:43,000 --> 00:22:45,920
And the rays of light
it sends our way.


290
00:22:47,360 --> 00:22:49,800
This is the train from Los Mochis
to Chihuahua,


291
00:22:49,800 --> 00:22:52,600
which inexplicably leaves
at 6:00am in the morning.


292
00:22:52,600 --> 00:22:56,440
Um...the local name for this area
in all the guidebooks


293
00:22:56,440 --> 00:22:58,520
is the Land of Turtles.


294
00:22:58,520 --> 00:23:02,320
Beautifully romantic name for
this place on the Sea of Cortez.


295
00:23:02,320 --> 00:23:05,440
But we just found out it's probably
more likely to have been called


296
00:23:05,440 --> 00:23:08,200
the Land of Spinach-type Vegetables.


297
00:23:08,200 --> 00:23:11,440
So we're going from
the Land of Spinach-type Vegetables


298
00:23:11,440 --> 00:23:13,720
to Chihuahua,


299
00:23:13,720 --> 00:23:16,520
which is
the Land of Very Small Dogs.


300
00:23:17,920 --> 00:23:20,680
One of the great railway journeys
of the world.


301
00:23:29,560 --> 00:23:31,080
TRAIN HOOTS


302
00:23:35,800 --> 00:23:40,160
Almost all life depends on the
energy that the sun sends our way.


303
00:23:41,800 --> 00:23:45,400
But the sun is a far-from-benevolent
companion


304
00:23:45,400 --> 00:23:50,920
because its radiant rain can be
as dangerous as it is nourishing.


305
00:24:03,280 --> 00:24:05,320
We're still round about sea level
now


306
00:24:05,320 --> 00:24:07,200
and the sun is quite low in the sky.


307
00:24:07,200 --> 00:24:10,000
It's about 7:00am,
so it's not been up long.


308
00:24:10,000 --> 00:24:12,320
I'm going to measure the amount
of UV radiation


309
00:24:12,320 --> 00:24:15,320
falling on every square centimetre
with this,


310
00:24:15,320 --> 00:24:18,680
a digital, ultraviolet radiometer.


311
00:24:21,720 --> 00:24:26,200
At the moment, it says
there's about 22 microwatts


312
00:24:26,200 --> 00:24:30,160
per square centimetre
falling on my skin.


313
00:24:30,160 --> 00:24:33,640
But as we climb in altitude,
then that UVB light


314
00:24:33,640 --> 00:24:36,800
is going to have to travel through
less and less of the atmosphere,


315
00:24:36,800 --> 00:24:39,240
so less of it is going to be
absorbed.


316
00:24:45,320 --> 00:24:47,600
And sure enough,
as the miles pass by


317
00:24:47,600 --> 00:24:50,200
and we head
into the mountainous interior,


318
00:24:50,200 --> 00:24:53,040
the meter readings start to go up.


319
00:25:18,160 --> 00:25:21,960
Now it's about 10:00am, so the sun's
significantly higher in the sky.


320
00:25:21,960 --> 00:25:25,040
The train's also climbed
quite a bit in altitude.


321
00:25:25,040 --> 00:25:27,200
Now...


322
00:25:29,000 --> 00:25:32,560
..we're getting nearly 250
microwatts per square centimetre.


323
00:25:32,560 --> 00:25:34,800
So that's about a factor of ten
higher.


324
00:25:34,800 --> 00:25:39,480
And that's just because the UVB has
had significantly less atmosphere


325
00:25:39,480 --> 00:25:43,640
to travel through, from the top of
the Earth's atmosphere down to me.


326
00:25:49,160 --> 00:25:52,200
That's more than enough
to burn unprotected skin


327
00:25:52,200 --> 00:25:53,600
in just a few minutes.


328
00:25:55,040 --> 00:25:57,240
And that's because
what arrived from the sun


329
00:25:57,240 --> 00:26:01,120
is far more
than just the stuff we can see.


330
00:26:05,040 --> 00:26:08,800
Beyond the visible, the higher
energy part of the spectrum,


331
00:26:08,800 --> 00:26:12,200
there's ultraviolet light,
particularly UVB,


332
00:26:12,200 --> 00:26:16,120
which does get through the Earth's
atmosphere and gets to the surface.


333
00:26:16,120 --> 00:26:19,480
Now, UVB can be beneficial to life.


334
00:26:19,480 --> 00:26:23,040
We use it to produce vitamin D,
for example.


335
00:26:23,040 --> 00:26:26,440
But because it's higher energy,
it can also be extremely damaging.


336
00:26:26,440 --> 00:26:30,920
It can damage DNA, it can burn our
skin as well as give us a suntan,


337
00:26:30,920 --> 00:26:34,520
and, of course, ultimately,
it can give us skin cancer.


338
00:26:34,520 --> 00:26:37,280
WHISTLE HOOTS


339
00:26:37,280 --> 00:26:39,120
If ultraviolet light is a problem


340
00:26:39,120 --> 00:26:41,120
for life on Earth to deal with
today,


341
00:26:41,120 --> 00:26:43,000
then the physicists might raise


342
00:26:43,000 --> 00:26:45,600
an interesting problem
for the biologists.


343
00:26:45,600 --> 00:26:48,480
Because we know
that 3.5 billion years ago,


344
00:26:48,480 --> 00:26:50,400
when life on Earth began,


345
00:26:50,400 --> 00:26:54,600
although the sun was much dimmer
in the visible part of the spectrum,


346
00:26:54,600 --> 00:26:58,440
it was significantly brighter
in the ultraviolet.


347
00:27:03,040 --> 00:27:05,880
The young sun seems like a paradox.


348
00:27:06,920 --> 00:27:08,760
It was fainter to the eye,


349
00:27:08,760 --> 00:27:13,000
perhaps 30% less bright
than the sun we enjoy today,


350
00:27:13,000 --> 00:27:15,720
yet rich in deadly ultraviolet.


351
00:27:17,320 --> 00:27:21,280
Inside, the core was spinning
much faster,


352
00:27:21,280 --> 00:27:24,240
which created more
electromagnetic heating


353
00:27:24,240 --> 00:27:26,160
of the plasma on its surface.


354
00:27:28,680 --> 00:27:31,520
And this plasma emitted more energy,


355
00:27:31,520 --> 00:27:33,720
not in the lower visible
frequencies,


356
00:27:33,720 --> 00:27:36,200
but in the higher frequencies.


357
00:27:37,640 --> 00:27:39,560
Like X-rays...


358
00:27:39,560 --> 00:27:41,840
and ultraviolet.


359
00:27:47,240 --> 00:27:51,920
It seems as if just as life was
getting settled on its wet home,


360
00:27:51,920 --> 00:27:57,760
the faint young sun was making it
tough to survive near the surface.


361
00:28:09,120 --> 00:28:13,040
This is the top of Copper Canyon, so
the summit of the railway journey.


362
00:28:13,040 --> 00:28:15,960
It's about 2,200 metres,
which is about...


363
00:28:15,960 --> 00:28:18,800
somewhere between
7,000 and 8,000 feet.


364
00:28:20,120 --> 00:28:23,480
So I'll take a UV reading
of the sun.


365
00:28:23,480 --> 00:28:26,320
It's actually reading about 260 now.


366
00:28:26,320 --> 00:28:29,360
Now, if you remember, at midday,
down at sea level,


367
00:28:29,360 --> 00:28:32,080
we were getting readings around 260.


368
00:28:32,080 --> 00:28:34,880
So although the sun has dropped
in the sky,


369
00:28:34,880 --> 00:28:38,360
so the sunlight and the UV are
coming through much more atmosphere,


370
00:28:38,360 --> 00:28:42,280
that's been compensated for
by the thinness of the air up here.


371
00:28:42,280 --> 00:28:44,560
I'm getting more UV now
than I would have been


372
00:28:44,560 --> 00:28:46,880
at the same time of day
at sea level.


373
00:28:50,360 --> 00:28:51,760
It's hard to be sure,


374
00:28:51,760 --> 00:28:55,680
but we think that
it's these kinds of radiation levels


375
00:28:55,680 --> 00:28:58,080
that early life had to deal with.


376
00:28:58,080 --> 00:29:01,720
Because back then,
the sun's ultraviolet output


377
00:29:01,720 --> 00:29:04,080
was significantly stronger.


378
00:29:08,240 --> 00:29:10,560
So I think it is fair to say


379
00:29:10,560 --> 00:29:13,480
that that could have posed
a significant threat


380
00:29:13,480 --> 00:29:16,160
to the development of early
life on Earth.


381
00:29:16,160 --> 00:29:18,600
WHINNYING


382
00:29:18,600 --> 00:29:21,360
ANIMATED CHATTER


383
00:29:23,720 --> 00:29:27,160
Today, life has painted the surface
of our home


384
00:29:27,160 --> 00:29:29,760
in all the colours of the rainbow.


385
00:29:31,640 --> 00:29:34,360
From greens to blues,


386
00:29:34,360 --> 00:29:36,160
reds to yellows,


387
00:29:36,160 --> 00:29:38,480
oranges and violets.


388
00:29:40,840 --> 00:29:44,640
And the origin of all life's hues
can be traced back


389
00:29:44,640 --> 00:29:47,840
to the way it interacts
with sunlight.


390
00:29:50,200 --> 00:29:53,120
I'm a particle physicist, so
I'm allowed to think of everything


391
00:29:53,120 --> 00:29:56,680
in terms of the interactions
of particles.


392
00:29:56,680 --> 00:29:59,040
So I would picture the light
from the sun


393
00:29:59,040 --> 00:30:02,720
as being really a rain of particles.


394
00:30:02,720 --> 00:30:05,520
Photons, they're called,
particles of light


395
00:30:05,520 --> 00:30:09,200
of different energies, raining down
on the surface of the Earth.


396
00:30:09,200 --> 00:30:12,160
The blue ones are
the highest-energy photons,


397
00:30:12,160 --> 00:30:14,840
the red ones are
the lowest-energy photons


398
00:30:14,840 --> 00:30:16,920
and all the colours of the rainbow
in the middle


399
00:30:16,920 --> 00:30:19,920
are just simply photons
of different energies.


400
00:30:19,920 --> 00:30:24,360
SHE SPEAKS IN NATIVE TONGUE
Oh, thank you.


401
00:30:24,360 --> 00:30:25,720
Wow.


402
00:30:26,760 --> 00:30:31,800
For this, the chilli salsa which
I see as red, there are pigment


403
00:30:31,800 --> 00:30:35,080
molecules in there that are
absorbing the blue photons,


404
00:30:35,080 --> 00:30:36,400
the blue light from the sun.


405
00:30:36,400 --> 00:30:38,600
The red ones,
it doesn't interact with,


406
00:30:38,600 --> 00:30:42,800
so they bounce back into my eye,
and that is why I see it as red.


407
00:30:42,800 --> 00:30:44,400
The same with the green chilli,


408
00:30:44,400 --> 00:30:48,560
but in this case the red photons
are interacting, doing something,


409
00:30:48,560 --> 00:30:50,640
talking to pigments in here,


410
00:30:50,640 --> 00:30:55,040
and what I am seeing are the green
photons and some of the blue photons


411
00:30:55,040 --> 00:30:58,760
coming into my eye, mixing up,
allowing me to see that as green.


412
00:31:02,400 --> 00:31:05,640
Pigments bring colour to the world.


413
00:31:05,640 --> 00:31:07,960
The planet is painted by genes,


414
00:31:07,960 --> 00:31:11,160
honed by billions of years
of evolution.


415
00:31:15,600 --> 00:31:17,840
'Some colours warn of danger...'


416
00:31:17,840 --> 00:31:19,560
This stuff is on fire, I tell you!


417
00:31:23,080 --> 00:31:24,520
'..or attract pollinators.'


418
00:31:41,880 --> 00:31:44,840
Pigments are one of the ways
that life has evolved


419
00:31:44,840 --> 00:31:47,680
to take on the sun's powerful
ultraviolet light.


420
00:32:17,160 --> 00:32:20,760
This little guy is called
a bombardier beetle.


421
00:32:20,760 --> 00:32:23,080
If I just grab him...


422
00:32:28,680 --> 00:32:31,560
His name comes
from his unique defence mechanism.


423
00:32:32,800 --> 00:32:36,080
He produces two chemicals. One
of them you might have heard of -


424
00:32:36,080 --> 00:32:40,120
hydrogen peroxide. The other one
is something called hydroquinone,


425
00:32:40,120 --> 00:32:42,120
and when you scare him,


426
00:32:42,120 --> 00:32:46,040
both those chemicals are injected
into a little chamber in his body.


427
00:32:47,440 --> 00:32:50,640
It raises the temperature
to the boiling point of water,


428
00:32:50,640 --> 00:32:52,560
and increases the pressure,


429
00:32:52,560 --> 00:32:56,080
squirting a hot and noxious chemical
out of its rear.


430
00:32:59,400 --> 00:33:01,280
A clever way to defend yourself.


431
00:33:03,400 --> 00:33:06,840
But this is just one of the ways
this character uses chemistry


432
00:33:06,840 --> 00:33:09,960
to increase the chance of survival.


433
00:33:12,520 --> 00:33:14,200
The bombardier beetle and me,


434
00:33:14,200 --> 00:33:18,080
and in fact every living thing
you can see, are exposed to


435
00:33:18,080 --> 00:33:22,040
the same threat on the high plains
of Mexico, the high-energy


436
00:33:22,040 --> 00:33:26,160
ultraviolet photons raining down
on this landscape from the sun.


437
00:33:29,360 --> 00:33:33,640
If they hit DNA in my skin,
for example, they damage the DNA.


438
00:33:33,640 --> 00:33:35,160
So that must be prevented.


439
00:33:37,880 --> 00:33:41,680
Me and my friend, the beetle, have
both reached the same solution -


440
00:33:41,680 --> 00:33:45,720
you see that the beetle is brown
and black.


441
00:33:45,720 --> 00:33:48,720
My skin, when it is exposed
to the sun, is going brown.


442
00:33:48,720 --> 00:33:54,120
I am producing a pigment called
melanin, and so is the beetle.


443
00:33:54,120 --> 00:33:56,120
Melanin is a very simple molecule,


444
00:33:56,120 --> 00:33:59,680
it's just a ring of carbon atoms
with a few extra bits bolted on,


445
00:33:59,680 --> 00:34:04,560
but the sea of electrons behaves
in a very specific way.


446
00:34:04,560 --> 00:34:07,520
When a high-energy ultraviolet
photon from the sun


447
00:34:07,520 --> 00:34:12,600
hits one of those electrons, it
very quickly dissipates that energy.


448
00:34:12,600 --> 00:34:15,960
That potentially threatening photon
has been absorbed


449
00:34:15,960 --> 00:34:20,160
and all its energy has been
dissipated away as heat.


450
00:34:23,200 --> 00:34:24,920
Melanin is so efficient,


451
00:34:24,920 --> 00:34:30,520
over 99.9% of the harmful
ultraviolet radiation is absorbed.


452
00:34:32,920 --> 00:34:35,360
So melanin is protecting


453
00:34:35,360 --> 00:34:40,000
both my skin and my friend,
the bombardier beetle,


454
00:34:40,000 --> 00:34:42,800
from the potentially harmful effects
of the sun.


455
00:35:04,120 --> 00:35:05,360
From the start,


456
00:35:05,360 --> 00:35:10,040
life had to evolve strategies for
coping with the energetic young sun.


457
00:35:14,520 --> 00:35:16,560
Life is nothing if not resourceful.


458
00:35:16,560 --> 00:35:20,240
Pigments are the way
that living things interact with


459
00:35:20,240 --> 00:35:26,160
the radiation from the sun. So why
just use them to dissipate energy,


460
00:35:26,160 --> 00:35:27,520
to protect?


461
00:35:27,520 --> 00:35:31,080
Why not use them to harness
that energy for its own ends?


462
00:35:31,080 --> 00:35:33,400
That is exactly what life did.


463
00:35:37,960 --> 00:35:43,080
In doing so, it transformed
our planet by introducing


464
00:35:43,080 --> 00:35:45,720
a wonderful new ingredient.


465
00:35:56,120 --> 00:35:59,280
Earth has an atmosphere
unlike any other planet


466
00:35:59,280 --> 00:36:01,160
we know of in the universe.


467
00:36:06,800 --> 00:36:11,360
Only in the air on our world
do fires burn.


468
00:36:15,360 --> 00:36:19,800
Only on our world has a gas
been released which allowed


469
00:36:19,800 --> 00:36:22,240
complex life to evolve.


470
00:36:29,520 --> 00:36:34,760
What makes our home unique
is its oxygen-rich atmosphere.


471
00:36:40,920 --> 00:36:45,320
Deep in a cave in the hills
of Tabasco, you can find a hint


472
00:36:45,320 --> 00:36:49,040
of what living planet without oxygen
might be like.


473
00:37:00,400 --> 00:37:03,840
This is one of the more unique
environments on our planet.


474
00:37:05,680 --> 00:37:10,520
This cave is full of sulphur,
you can see it in the water.


475
00:37:10,520 --> 00:37:14,000
You can see that milky colour
flowing through the cave.


476
00:37:14,000 --> 00:37:16,040
That is dissolved sulphur.


477
00:37:16,040 --> 00:37:18,720
It is coming
from hydrogen-sulphide gas,


478
00:37:18,720 --> 00:37:22,240
the source of which
is actually not entirely known.


479
00:37:26,320 --> 00:37:29,560
The hydrogen sulphide
is toxic to me.


480
00:37:29,560 --> 00:37:33,720
It has another rather alarming
effect on this hellhole.


481
00:37:35,960 --> 00:37:37,640
It is a bad-smelling gas,


482
00:37:37,640 --> 00:37:40,640
but it is also a gas that drives
the oxygen out,


483
00:37:40,640 --> 00:37:44,360
so as you go on into the cave,
you get less and less oxygen.


484
00:37:48,920 --> 00:37:51,920
In a sense, some of the chemistry,


485
00:37:51,920 --> 00:37:56,680
the biochemistry that takes place
in the dark of this cave system,


486
00:37:56,680 --> 00:38:00,360
could be very similar
to the chemistry


487
00:38:00,360 --> 00:38:04,000
and biochemistry that occurred
when our planet was very young.


488
00:38:06,880 --> 00:38:09,000
For the first half of its history,


489
00:38:09,000 --> 00:38:11,560
Earth was without oxygen
in the atmosphere.


490
00:38:15,960 --> 00:38:19,480
But incredibly, in this echo
of the past, which I can only visit


491
00:38:19,480 --> 00:38:24,040
for a few minutes, there are forms
of life that are completely at home.


492
00:38:26,160 --> 00:38:28,360
Look at that!


493
00:38:30,160 --> 00:38:33,000
There they are,
cities of sulphur-eating bacteria


494
00:38:33,000 --> 00:38:35,920
living off
the hydrogen-sulphide gas.


495
00:38:42,360 --> 00:38:44,840
Colonies of extremophiles,


496
00:38:44,840 --> 00:38:49,960
organisms living off a very
different environment of gases


497
00:38:49,960 --> 00:38:52,840
to the one that we are used to
on the surface.


498
00:38:57,760 --> 00:39:00,880
They are a window
on a much earlier time.


499
00:39:05,360 --> 00:39:09,680
Because without oxygen,
the ancestors of these extremophiles


500
00:39:09,680 --> 00:39:13,320
were the only forms of life
our planet could support.


501
00:39:29,720 --> 00:39:31,760
Understanding how Earth developed


502
00:39:31,760 --> 00:39:35,560
an atmosphere rich in oxygen
has taken centuries.


503
00:39:37,280 --> 00:39:40,560
The secret lies
with ancient bacteria.


504
00:39:53,800 --> 00:39:58,720
In 1676, a Dutchman
called Antonie Leeuwenhoek


505
00:39:58,720 --> 00:40:03,400
was trying to find out
why pepper is spicy.


506
00:40:03,400 --> 00:40:06,880
See, they thought that there were
little spikes on peppercorns


507
00:40:06,880 --> 00:40:09,240
that dug into your tongue.


508
00:40:09,240 --> 00:40:11,040
He was using the microscope,


509
00:40:11,040 --> 00:40:13,760
which had been discovered
about 60 years before,


510
00:40:13,760 --> 00:40:17,440
but inexplicably, had never been
used for anything useful before.


511
00:40:17,440 --> 00:40:20,720
He put the peppercorns on there
and looked down
and he couldn't see anything,


512
00:40:20,720 --> 00:40:22,720
so he thought
he would grind them up,


513
00:40:22,720 --> 00:40:26,000
dissolve them in water
and have a look. When he did that,


514
00:40:26,000 --> 00:40:28,880
he didn't see anything interesting
in the peppercorns,


515
00:40:28,880 --> 00:40:33,280
but he found that there were
little animals swimming around.


516
00:40:33,280 --> 00:40:35,040
He said that he estimated


517
00:40:35,040 --> 00:40:38,240
you could line about 100
of the "wee little creatures" -


518
00:40:38,240 --> 00:40:43,200
those are his words - on the length
of a single coarse sand grain.


519
00:40:45,080 --> 00:40:48,560
What Leeuwenhoek thought were
animals were, in all probability,


520
00:40:48,560 --> 00:40:50,400
not animals at all.


521
00:40:52,280 --> 00:40:54,800
Although he didn't know it
at the time,


522
00:40:54,800 --> 00:40:58,320
he had discovered a whole new domain
of life.


523
00:41:02,640 --> 00:41:04,720
Bacteria.


524
00:41:12,560 --> 00:41:16,320
They are by far the most numerous
organisms on the Earth.


525
00:41:17,640 --> 00:41:21,120
In fact, there are more bacteria
on our planet than


526
00:41:21,120 --> 00:41:24,960
there are stars
in the observable universe.


527
00:41:28,040 --> 00:41:33,440
And there is one kind of bacteria
more numerous than all the rest.


528
00:41:37,520 --> 00:41:40,200
One of the most striking structures
I can see on this slide is


529
00:41:40,200 --> 00:41:45,040
a kind of blue-green filament
which is a little colony


530
00:41:45,040 --> 00:41:49,040
of a type of bacteria
called cyanobacteria.


531
00:41:52,920 --> 00:41:56,800
These things are incredibly
important organisms.


532
00:42:02,400 --> 00:42:06,520
Fossilised cyanobacteria had
been found as far back


533
00:42:06,520 --> 00:42:08,880
as 3.5 billion years ago.


534
00:42:11,920 --> 00:42:16,360
And at some point,
around 2.4 billion years ago,


535
00:42:16,360 --> 00:42:20,120
they became the first living things
to use pigments


536
00:42:20,120 --> 00:42:23,320
to split water apart
and use it to make food.


537
00:42:26,560 --> 00:42:30,480
This evolutionary invention was
incredibly complex.


538
00:42:30,480 --> 00:42:36,240
Even its name is a mouthful -
oxygenic photosynthesis.


539
00:42:39,160 --> 00:42:42,800
It starts with a photon from the sun


540
00:42:42,800 --> 00:42:45,880
hitting that green pigment,
chlorophyll.


541
00:42:45,880 --> 00:42:49,440
Chlorophyll takes that energy
and uses it


542
00:42:49,440 --> 00:42:53,320
to boost electrons up a hill,
if you like.


543
00:42:53,320 --> 00:42:58,080
And when they get to the top, they
cascade down a molecular waterfall,


544
00:42:58,080 --> 00:43:01,840
and the energy is used
to make something called ATP,


545
00:43:01,840 --> 00:43:06,000
which is potentially
the energy currency of life.


546
00:43:06,000 --> 00:43:10,040
This little molecular machine is
called photosystem II,


547
00:43:10,040 --> 00:43:14,080
and it makes energy for the cell
from sunlight.


548
00:43:14,080 --> 00:43:17,120
But when the electrons reach
the bottom of that waterfall,


549
00:43:17,120 --> 00:43:19,200
they enter photosystem I.


550
00:43:19,200 --> 00:43:21,440
They meet some more chlorophyll,


551
00:43:21,440 --> 00:43:24,320
which is hit by another photon
from the sun,


552
00:43:24,320 --> 00:43:27,480
and that energy raises the electrons
up again,


553
00:43:27,480 --> 00:43:30,440
and forces them onto carbon dioxide,


554
00:43:30,440 --> 00:43:34,720
turning that carbon dioxide
eventually into sugars,


555
00:43:34,720 --> 00:43:36,560
into food for the cell.


556
00:43:36,560 --> 00:43:39,280
Now, why all this complexity?


557
00:43:39,280 --> 00:43:42,000
Why do you need
these two photosystems


558
00:43:42,000 --> 00:43:44,080
joined together in this way,


559
00:43:44,080 --> 00:43:48,440
just to get some electrons and make
sugar and a bit of energy out of it?


560
00:43:52,560 --> 00:43:54,000
It's because


561
00:43:54,000 --> 00:43:57,520
only when life coupled these
two biological machines together


562
00:43:57,520 --> 00:44:01,160
that it could split water apart
and turn it into food.


563
00:44:02,440 --> 00:44:04,000
But it wasn't easy.


564
00:44:05,360 --> 00:44:09,320
The thing is that water is
extremely difficult to split,


565
00:44:09,320 --> 00:44:12,480
so for a leaf to do it,
for a blade of grass to do it,


566
00:44:12,480 --> 00:44:16,440
just using a trickle of light from
the sun, is extremely difficult.


567
00:44:20,360 --> 00:44:25,840
In fact, the task is SO complex
that, unlike flight or vision,


568
00:44:25,840 --> 00:44:29,960
which have evolved separately
many times during our history,


569
00:44:29,960 --> 00:44:34,640
oxygenic photosynthesis
has only evolved once.


570
00:44:37,120 --> 00:44:42,520
Every tree, every plant,
every blade of grass on the planet,


571
00:44:42,520 --> 00:44:47,040
everything that carries out
oxygenic photosynthesis today


572
00:44:47,040 --> 00:44:49,400
does it in EXACTLY the same way.


573
00:44:49,400 --> 00:44:53,240
And the structures inside every leaf
that do that


574
00:44:53,240 --> 00:44:57,200
look remarkably similar
to cyanobacteria.


575
00:45:01,600 --> 00:45:05,600
In other words, the descendants
of one cyanobacterium


576
00:45:05,600 --> 00:45:08,040
that worked out, for some reason,


577
00:45:08,040 --> 00:45:11,920
how to couple those complex
molecular machines together


578
00:45:11,920 --> 00:45:15,320
in some primordial ocean,
billions of years ago,


579
00:45:15,320 --> 00:45:18,360
are still present on the Earth
today.


580
00:45:36,400 --> 00:45:39,400
The cyanobacteria changed
the world...


581
00:45:40,680 --> 00:45:42,760
..turning it green.


582
00:45:49,120 --> 00:45:51,880
And that had
a wonderful consequence.


583
00:45:58,400 --> 00:46:00,240
With this new way of living,


584
00:46:00,240 --> 00:46:04,040
life released oxygen
into the atmosphere of our planet


585
00:46:04,040 --> 00:46:07,680
for the first time. And in doing so,


586
00:46:07,680 --> 00:46:11,400
over hundreds of millions of years,


587
00:46:11,400 --> 00:46:16,400
it eventually completely transformed
the face of our home.


588
00:46:20,160 --> 00:46:22,520
And as the oxygen levels grew


589
00:46:22,520 --> 00:46:26,840
the stage was set for the arrival
of ever more complex creatures.


590
00:46:28,440 --> 00:46:32,920
But on Earth, the emergence
of complex life required


591
00:46:32,920 --> 00:46:35,120
a rather more intangible ingredient.


592
00:46:39,120 --> 00:46:43,240
Something that you can't see,
touch or smell,


593
00:46:43,240 --> 00:46:46,000
and yet you pass through every day.


594
00:46:54,520 --> 00:46:56,280
Late January,


595
00:46:56,280 --> 00:47:00,040
and the monarch butterflies have
found their way home.


596
00:47:02,360 --> 00:47:06,720
They've entered a hibernation state,
huddling together for warmth.


597
00:47:10,080 --> 00:47:14,040
But they're only here at all
thanks to one of the most accurate


598
00:47:14,040 --> 00:47:17,080
biological clocks found in nature.


599
00:47:36,360 --> 00:47:41,200
These are the pine
and oyamel forests, high altitude,


600
00:47:41,200 --> 00:47:44,320
about, what, three hours north-west
of Mexico City,


601
00:47:44,320 --> 00:47:48,080
and one of the few wintering grounds
of the monarch butterflies,


602
00:47:48,080 --> 00:47:50,240
as you can see.


603
00:47:50,240 --> 00:47:53,400
But there is a colony
of millions of monarchs


604
00:47:53,400 --> 00:47:55,440
somewhere due north of here,


605
00:47:55,440 --> 00:47:57,680
so if I head off into the forest


606
00:47:57,680 --> 00:48:02,440
then hopefully this will just be
a taster of what's to come.


607
00:48:05,560 --> 00:48:10,240
To find the butterflies, I need
to get an accurate bearing on them.


608
00:48:10,240 --> 00:48:13,720
And to do this I need a timepiece.


609
00:48:15,320 --> 00:48:17,000
If you don't have a compass,


610
00:48:17,000 --> 00:48:20,880
how can you tell which direction is
north and which direction is south?


611
00:48:20,880 --> 00:48:22,440
Well, you can use the sun.


612
00:48:22,440 --> 00:48:25,360
The sun rises in the east,
sets in the west,


613
00:48:25,360 --> 00:48:29,840
and at midday, in the northern
hemisphere, it's due south.


614
00:48:29,840 --> 00:48:31,640
But what if it ISN'T midday?


615
00:48:31,640 --> 00:48:35,320
Well, there's an old trick,
which is to use a watch.


616
00:48:35,320 --> 00:48:38,360
See, it's about three
in the afternoon now,


617
00:48:38,360 --> 00:48:41,600
and if you line the hour hand
of your watch up with the sun,


618
00:48:41,600 --> 00:48:43,600
then, in the northern hemisphere,


619
00:48:43,600 --> 00:48:48,240
the line in between the hour hand
and 12 o'clock


620
00:48:48,240 --> 00:48:50,920
will point due south.


621
00:48:50,920 --> 00:48:54,000
Which means north is that way.


622
00:48:59,880 --> 00:49:02,480
For thousands of miles
on their way here,


623
00:49:02,480 --> 00:49:05,240
the monarchs have faced
the same problem.


624
00:49:06,440 --> 00:49:11,080
To make their way south, it's
no good simply following the sun.


625
00:49:12,640 --> 00:49:14,400
Because, as the day progresses,


626
00:49:14,400 --> 00:49:17,280
the sun's position
drifts across the sky.


627
00:49:21,440 --> 00:49:24,440
Somehow they have to correct
for this.


628
00:49:45,920 --> 00:49:49,800
They use what's called
a time-compensated sun compass.


629
00:49:51,480 --> 00:49:55,240
They measure the position of the sun
every day, using their eyes,


630
00:49:55,240 --> 00:49:58,080
but it's also thought
they can measure the position


631
00:49:58,080 --> 00:50:02,200
even when it's cloudy, by using
the polarisation of the light.


632
00:50:03,800 --> 00:50:08,400
Having locked onto the sun, their
brain then corrects for its movement


633
00:50:08,400 --> 00:50:13,280
across the sky by using one of
nature's most accurate timepieces.


634
00:50:13,280 --> 00:50:17,120
By combining the information
from their precise clocks


635
00:50:17,120 --> 00:50:21,640
and their eyes,
they can navigate due south.


636
00:50:23,320 --> 00:50:27,200
That ability to orientate
themselves is, I think,


637
00:50:27,200 --> 00:50:29,880
one of the most remarkable things
I've seen.


638
00:50:36,840 --> 00:50:40,360
The biological clocks
that have brought the monarchs home


639
00:50:40,360 --> 00:50:42,760
are not unique to butterflies.


640
00:50:44,680 --> 00:50:48,680
Almost all life shares
in these circadian rhythms.


641
00:50:50,560 --> 00:50:54,760
They're an evolutionary consequence
of living on a spinning rock.


642
00:51:01,200 --> 00:51:07,920
Our world turns on its axis once
every 24 hours, giving us a day.


643
00:51:12,480 --> 00:51:15,760
It's on a billion-kilometre journey
around the sun,


644
00:51:15,760 --> 00:51:18,600
and each orbit gives us a year.


645
00:51:21,600 --> 00:51:24,360
We live inside a celestial clock,


646
00:51:24,360 --> 00:51:29,800
one that has been ticking away
for over 4.5 billion years.


647
00:51:31,320 --> 00:51:35,160
And that's a full third
of the age of the universe.


648
00:51:50,400 --> 00:51:55,200
This is the final ingredient
that our home has provided.


649
00:51:55,200 --> 00:51:56,920
Time.


650
00:52:06,800 --> 00:52:08,760
Take the horse.


651
00:52:08,760 --> 00:52:13,720
Like all complex living things,
it's here because our planet


652
00:52:13,720 --> 00:52:16,400
has been stable enough
for long enough


653
00:52:16,400 --> 00:52:18,960
to allow evolution time to play.


654
00:52:33,160 --> 00:52:36,560
The horse is the animal
whose family tree


655
00:52:36,560 --> 00:52:38,920
we know with the highest precision.


656
00:52:44,000 --> 00:52:48,600
So it's possible to lay out
just one unbroken chain of life


657
00:52:48,600 --> 00:52:51,840
that stretches back
nearly four billion years.


658
00:52:55,680 --> 00:52:59,120
Animals that are recognisably
horselike have


659
00:52:59,120 --> 00:53:01,720
been around for a long time -


660
00:53:01,720 --> 00:53:04,360
something like 55 million years.


661
00:53:04,360 --> 00:53:09,040
You then have to jump quite a lot
to something like 225 million years


662
00:53:09,040 --> 00:53:13,120
if you want to ask the question,
where is the earliest mammal?


663
00:53:13,120 --> 00:53:17,160
And it's this thing, which looks
something like a little shrew.


664
00:53:17,160 --> 00:53:19,200
535 million.


665
00:53:19,200 --> 00:53:22,880
This is the point when complex life
really began to explode


666
00:53:22,880 --> 00:53:24,920
in the oceans.


667
00:53:24,920 --> 00:53:28,840
You then have to sweep back
a long, long time to find the next


668
00:53:28,840 --> 00:53:33,360
evolutionary milestone, arguably
the most important milestone -


669
00:53:33,360 --> 00:53:37,200
the emergence of the complex self,
the eukaryote.


670
00:53:37,200 --> 00:53:41,160
And then, you have to step back
a long way in time.


671
00:53:42,320 --> 00:53:47,800
You have to step back
all the way to here,


672
00:53:47,800 --> 00:53:51,560
the emergence of the prokaryote,
the first life form.


673
00:53:51,560 --> 00:53:55,720
And so,
we have this beautiful long line.


674
00:53:55,720 --> 00:53:59,920
We can trace my friend,
the horse, and his ancestry


675
00:53:59,920 --> 00:54:06,600
back to the events that happened
3.5, 3.6, 3.7 billion years ago


676
00:54:06,600 --> 00:54:08,840
on the primordial Earth.


677
00:54:15,080 --> 00:54:18,560
Looking back over that vast
sweep of time,


678
00:54:18,560 --> 00:54:24,000
you could ask yourself the question,
well, do you need 3.5 billion years


679
00:54:24,000 --> 00:54:29,040
to go from a simple form of life
to something as complex as a horse?


680
00:54:31,160 --> 00:54:35,520
Well, the answer to that question
is, we don't know for sure.


681
00:54:35,520 --> 00:54:40,840
It seems that you need vast expanses
of time, but do you need


682
00:54:40,840 --> 00:54:44,560
those big gaps from the simple cell
to the complex cell,


683
00:54:44,560 --> 00:54:47,320
do you need the gap
from the complex cell


684
00:54:47,320 --> 00:54:50,160
to the evolution
of multicellular life?


685
00:54:50,160 --> 00:54:51,440
We don't know.


686
00:54:53,840 --> 00:54:55,680
We only have one example.


687
00:54:55,680 --> 00:54:58,840
There is only one planet
where we've been able to study


688
00:54:58,840 --> 00:55:01,400
the evolution of life,
and it's this one.


689
00:55:02,760 --> 00:55:07,920
And Earth has been an interesting
mixture of stability and upheaval.


690
00:55:07,920 --> 00:55:09,840
It's had an environment


691
00:55:09,840 --> 00:55:13,120
that's never completely conspired
to wipe out life,


692
00:55:13,120 --> 00:55:16,120
but it's constantly thrown it
challenges.


693
00:55:19,320 --> 00:55:22,880
The deep time that our planet has
given life


694
00:55:22,880 --> 00:55:27,480
has allowed it to grow from a tiny
seed of genetic possibility


695
00:55:27,480 --> 00:55:32,200
to the planet-wide web of complexity
we are part of today.


696
00:55:42,000 --> 00:55:46,120
Only a few of us have ever stepped
outside of this world.


697
00:55:47,400 --> 00:55:51,600
But those that have discovered
something rather wonderful.


698
00:55:53,880 --> 00:55:57,000
'For all the people back on Earth,


699
00:55:57,000 --> 00:56:01,600
'the crew of Apollo 8 has a message
that we would like to send to you.'


700
00:56:01,600 --> 00:56:06,600
On Christmas Eve 1968,
my first Christmas Eve,


701
00:56:06,600 --> 00:56:09,360
the Apollo 8 spacecraft entered
the darkness


702
00:56:09,360 --> 00:56:11,160
on the far side of the moon.


703
00:56:11,160 --> 00:56:16,160
'In the beginning, God created
the heaven and the earth.


704
00:56:16,160 --> 00:56:18,680
'And the earth was without form.'


705
00:56:18,680 --> 00:56:22,080
The three astronauts,
Borman, Lovell and Anders,


706
00:56:22,080 --> 00:56:24,880
became the first human beings
in history


707
00:56:24,880 --> 00:56:27,600
to lose sight of the Earth.


708
00:56:27,600 --> 00:56:30,720
'And God said, let there be light.


709
00:56:30,720 --> 00:56:36,040
'And there was light. And God saw
the light, that it was good.'


710
00:56:37,040 --> 00:56:39,960
When they emerged
from the dark side of the moon,


711
00:56:39,960 --> 00:56:43,520
and the Earth rose into view,
they chose to broadcast


712
00:56:43,520 --> 00:56:47,760
their culture's creation story
back to the inhabitants of Earth.


713
00:56:47,760 --> 00:56:50,640
And, just like the Aztecs
and the Mayans


714
00:56:50,640 --> 00:56:53,440
and every civilisation before them,


715
00:56:53,440 --> 00:56:56,480
it told of the origins
of their home.


716
00:56:56,480 --> 00:56:59,720
'And God called the dry land Earth,


717
00:56:59,720 --> 00:57:03,920
'and the gathering together
of the waters called He seas.


718
00:57:03,920 --> 00:57:06,840
'And God saw that it was good.'


719
00:57:06,840 --> 00:57:13,200
It must be innately human, the
desire to understand how our home


720
00:57:13,200 --> 00:57:15,920
came to be the way that it is.


721
00:57:15,920 --> 00:57:19,800
And seen from lunar orbit
against the blackness of space,


722
00:57:19,800 --> 00:57:22,080
the Earth is a fragile world,


723
00:57:22,080 --> 00:57:24,720
but seen by science, it's a world


724
00:57:24,720 --> 00:57:29,440
that's been crafted and shaped by
life over almost four billion years.


725
00:57:31,440 --> 00:57:33,600
So we're on our way to understanding


726
00:57:33,600 --> 00:57:37,040
how we came to be here, but as the
Apollo astronauts discovered,


727
00:57:37,040 --> 00:57:40,240
the journey of discovery has already
delivered much more


728
00:57:40,240 --> 00:57:42,800
than just the facts,
because it's given us


729
00:57:42,800 --> 00:57:47,920
a powerful perspective on the
intricacy and beauty of our home.


730
00:57:49,400 --> 00:57:54,440
'From the crew of Apollo 8, we
close with good night, good luck,


731
00:57:54,440 --> 00:57:58,320
'a merry Christmas,
and God bless all of you,


732
00:57:58,320 --> 00:58:01,200
'all of you on the good Earth.'


733
00:58:08,840 --> 00:58:12,040
Subtitles by Red Bee Media Ltd