Wonders of Life (2013) s01e05 Episode Script
Home
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 meansthat 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.
Thatis 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 arevery 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 isthere, 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.
Umthe 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.
'
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 meansthat 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.
Thatis 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 arevery 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 isthere, 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.
Umthe 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.
'