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00:00This creature is a wonder of nature.
00:16Its biology is hardwired to the heavens.
00:24It has an exquisitely sensitive eye that locks onto the sun and allows it to navigate
00:30its way across the face of the planet.
00:37In a sense, it has an instinctive understanding of its place in the solar system.
00:44A tiny insect brain joined to the movements of the sun and the planets.
00:53This connection steers the monarch and millions of its brethren as they make one of the longest
00:59migrations of any butterfly species.
01:09They're heading for these trees known locally as the oyamel, or sacred firs.
01:14Some of the butterflies began their journey over 4,000 kilometres away, that's 2,500
01:20miles, up here in the northeastern United States and Canada.
01:24And over the autumn and the winter, they've migrated south across the United States and
01:30arrived here in central Mexico.
01:33Incredibly, no butterfly has ever learned this route.
01:38It can't have, because it takes at least three generations to make the round trip.
01:43Instead, the homing instinct is carried on a river of genetic information that flows
01:49through each butterfly.
01:54The allure of this place to the butterflies, this sense of belonging, is a deep feeling
02:00we all share.
02:02We even have a word for it, home.
02:11Every living thing that we know to exist is found on this one rock.
02:19So what is it about our planet that makes it such a rich, colourful, living world?
02:26I want to show you why our world is the only habitable planet we know of anywhere in the
02:31universe.
02:32Now, the answer depends on the presence of a handful of precious ingredients that make
02:38our world a home.
03:04Home is such an evocative word.
03:28I mean, it will mean something to you.
03:31The place you went to school, the place you live, the place where your kids had their
03:35first Christmas.
03:36But in a scientific sense, what does it mean?
03:42It means that the ingredients are there for you to live.
03:49An atmosphere, food, water.
03:53You need the temperature to be right.
03:58Home is the place that has the things you need for your biology and chemistry to work.
04:06It's no less evocative for that.
04:21This is Mexico, a country rich in the ingredients that set our world apart.
04:29It's not a bad place to come because about 1% of the land surface area of our planet
04:34is home to 12% of the species.
04:38There are 26,000 plant species here, there are 700 species of reptiles and 400 species
04:44of mammals.
04:45And it's also been home to some of the world's great civilisations.
04:48The Maya built their temples out there in the forest here for thousands and thousands
04:53of years.
04:57Mexico is bursting with life.
05:01And if you know where to look, hidden inside these creatures are clues that tell how this
05:07planet became their home.
05:16Our next stop is in the south-east of the country, an area covered in thick jungle.
05:23The Yucatan is a strip of essentially pure limestone that separates the Caribbean from
05:28the Gulf of Mexico.
05:29And it's got all the ingredients you might think you need for a rich and diverse ecosystem.
05:37The tropical sun warms the forest, delivering precious energy to each and every leaf.
05:45Oxygen escapes from the plants and trees, which is breathed in by the forest animals.
05:55And where they can, each of them draws deeply from the region's hidden water supply.
06:03But there are some of the ingredients you need to grow this tropical forest that are
06:09far more important than others.
06:26You might think that this place would be awash with water, and it does rain a lot, and it's
06:30incredibly humid.
06:32But actually, there are no surface rivers at all on the Yucatan Peninsula, because the
06:37water just seeps into the porous limestone.
06:39And that's where these things come in.
06:42These are cenotes, they're caverns dissolved out of the limestone by the rain.
06:48And they collect water, and they play a vital role in the ecosystem.
06:52I mean, the forest changes when you get around a cenote.
06:56Just listen to that.
07:00Those are frogs, and you don't hear those frogs anywhere else in the forest, just around
07:06the cenotes.
07:19The cenotes are flooded caves that have been cut off from the outside world for thousands
07:25of years.
07:36Lillies, troglodytic fish, even the occasional turtle, all thrive around the openings of
07:51these fresh water wells.
07:55As I head deeper into the cave, the temperature drops, and the light fades.
08:09One by one, the ingredients I depend upon begin to disappear.
08:17Yet even here, far from the soil and air, strangely coloured algae still find a home
08:24in the water.
08:40There's one thing that unites every form of life in this cenote.
08:45In fact, every form of life out there in the forest.
08:48In fact, every form of life we've ever discovered, anywhere on planet Earth.
08:54It's that it has to be wet.
09:00Only on our home does water run freely between the skies, oceans, rivers, and on into every
09:07living thing.
09:45To understand why life and water are so intertwined, we need to look a little deeper into one of
09:51the strangest substances we know.
09:59Now I may be a bit of a middle-aged academic, but I can still do the odd experiment every
10:03now and again.
10:04So what I'm doing is I'm charging up this perspex rod, so giving it an electric charge
10:08by rubbing it on a fleece.
10:10Now, watch what happens when I put the rod next to a stream of water.
10:17You see that?
10:19Look at that.
10:20The electric field, the electric charge, is bending the water towards it.
10:25Now, the reason for that, the reason that water behaves in that way when it's passing
10:31through an electric field, is exactly the same reason that it is vital for all life
10:36on Earth.
10:38Water is a polar molecule, which means it responds to electric charge.
10:55Its polarity comes about because of the strength of the electric field.
11:02Its polarity comes about because of the structure of water molecules themselves.
11:09Now, water is H2O, two hydrogens and one oxygen atom bound together.
11:15So, two hydrogen atoms approach oxygen.
11:19Now, oxygen's got a cloud of eight electrons around it.
11:22So when the hydrogens come in, then what happens is the electrons get dragged over here.
11:28What happens is the electrons get dragged over here, around the oxygen.
11:32So you end up with an electron cloud around here.
11:35And, to some extent, pretty isolated, positively charged protons out here.
11:41So, you get a net positive charge over here, and the electron cloud with its negative charge
11:47over here.
11:48So, you get what's called a polar molecule.
11:52And that's why when you bring a charged perspex rod close to water molecules,
11:56they bend towards it.
12:12Water's polar nature means that, although its molecules are simple,
12:16together they form a subtle, endlessly complex liquid.
12:21A home in which one tiny creature thrives.
12:52There he is. Look at that.
12:55That is a pond skater, a predator that floats on the surface of the water
13:01and actually uses the surface of the water to sense its prey.
13:07Pond skaters are vicious predators that live for most of their lives on the surface.
13:14Tiny hairs on their legs provide a large area that spreads out.
13:20Their middle legs thrust them forward.
13:23Hind legs are employed to steer.
13:32They're so well adapted to life in this flat world
13:35that they even sense their sexual partners through tiny vibrations in the water's surface.
13:42The reason it can do that is the result of a complex interaction
13:46between adaptions in the animal itself
13:49and the physics and the chemistry of the surface water.
13:57Now, water molecules are polar,
13:59and that means that water molecules themselves can change.
14:05Now, water molecules are polar,
14:07and that means that water molecules themselves can bond together.
14:12So you can get a hydrogen with its slight positive charge
14:16getting close to the oxygen of another water molecule
14:20with its slight negative charge and bonding to it.
14:24And you can build up quite large, in fact very large, structures in liquid water.
14:34This is what gives water its unique ability to form a surface habitat for the pond skaters.
14:42Clumps of H2O stick together, keeping the surface under tension,
14:49forming a chorus of water molecules, all joined together by hydrogen bonds.
14:59Then a pond skater comes along and it puts its legs
15:03or its dangly things into the water
15:07and pushes it down, bends the surface of the water.
15:11And the water doesn't like that because a bend in the water is increasing its surface area,
15:16it's increasing its energy,
15:18it's making it harder for all the molecules to bond together with the hydrogen bonds.
15:23So they try to push back, they exert a force on the pond skater's leg
15:28because they want to bond as much as they can.
15:31And that's how pond skaters stay on the surface of the water.
15:39Hydrogen bonds do far more than just give the pond skaters a place to live.
15:45They're fundamental to all life.
15:52I've heard it said that we won't truly understand biology until we understand water.
16:02These are very thin tubes of glass.
16:08They're about, what, a millimetre in diameter.
16:12And if I dip one into the surface of this river,
16:19you see that the water just climbs up the tube.
16:23It pulls itself up, quite literally, against the force of gravity.
16:27Now, in trees there are tubes which are about half the diameter of this,
16:31perhaps about half a millimetre or even less.
16:34And they are called xylem,
16:36and they allow the tree to lift water up through the root system
16:41because the water molecules strongly attract each other
16:44and are strongly attracted to the sides of the tubes.
16:48So when you look at trees like that, which are very high,
16:52and you ask yourself the question,
16:54how do they get the water up from their roots to the top of the tree?
16:57A big part of that is capillary action,
17:00which is down to the polar nature of water.
17:08One of water's most important qualities
17:11is its ability to dissolve and carry all manner of substances around the living world.
17:18Because its molecules are very small and polar,
17:21water is a tremendously effective solvent.
17:25Those molecules can get in amongst other substances,
17:29salts and sugars, for example,
17:31and disperse them, if you like, in that sea of hydrogen bonds.
17:38Within every one of us, water is constantly flowing,
17:43Blood plasma is over 90% water,
17:46and in it are dissolved everything I need to live.
17:50Oxygen, the nutrients from food,
17:53everything distributed around my body in rivers of water.
18:02Within every one of us, water is constantly flowing,
18:06around each and every cell.
18:10We live on a beautiful blue anomaly of a world,
18:15the only planet we know,
18:18with a surface drenched in liquid water.
18:27The story of how each drop ended up here has been hard to fathom.
18:33Largely because it happened so long ago,
18:36there's very little direct evidence.
18:46But back in the Yucatan jungle,
18:48clues to how it turned up can still be found.
18:54Every civilisation on the Yucatan,
18:56be it the modern Mexicans or the Mayans,
18:59had to get their water from those deep wells, the cenotes.
19:04And I've got a map, a completely unbiased map,
19:07of the larger cenotes here, which I'm going to overlay on the Yucatan.
19:16Look at that. They lie in a perfect arc,
19:20centred around a very particular village,
19:24which is there, and it's called Chichilub.
19:29Now, to a geologist, there are very few natural events
19:34that can create a structure such a perfect arc as that.
19:43All the evidence points to just one explanation.
19:50You're looking at what's left of a gigantic asteroid strike.
19:56One that wiped out three quarters of all plant and animal species
20:01when it hit the Earth 65 million years ago.
20:05You may think that impacts from space are a thing of the past,
20:09a thing that only happens to the dinosaurs,
20:11but that's not true either.
20:13About 55 million kilograms of rock hits the Earth every year,
20:18and around 2% of that is water.
20:22This hints that at least some of Earth's water arrived from space.
20:31Late in 2010, these glimpses of Comet Hartley 2 arrived back on Earth.
20:39They were sent by NASA's Deep Impact Probe.
20:43From its surface, dust and ice spray into space.
20:48Analysis of this water found it had a very similar mixture of isotopes
20:53to the water in our own oceans.
20:58This was the first firm evidence that icy comets must have contributed
21:03to the formation of our world's oceans.
21:18CRASH
21:23Earth began life as a molten hell.
21:28Its internal heat drove off any trace of moisture.
21:34But soon, the planet cooled and the first clouds grew.
21:41Then, 4.2 billion years ago,
21:44a deluge the like of which the solar system
21:47had never seen before or since rained down.
21:52CRASH
22:11And again, thanks to those hydrogen bombs,
22:15water's boiling point is high enough to have allowed it to remain
22:19on the surface of the Earth to the present day.
22:23So from quite early in its history,
22:25our home has been able to hang on to this most vital of ingredients.
22:30But to trace the origin of the next ingredient,
22:33you have to look beyond our planet.
22:38To our nearest star.
22:42And the rays of light it sends our way.
22:45This is the train from Los Mochis to Chihuahua,
22:48which inexplicably leaves at 6am in the morning.
22:51The local name for this area, in all the guidebooks,
22:55is the Land of Turtles,
22:57a beautifully romantic name for this place on the Sea of Cortez.
23:01But actually, we just found out that it's probably more likely
23:04that it's been called the Land of Spinach-Type Vegetables.
23:07So we're going from the Land of Spinach-Type Vegetables to Chihuahua,
23:13which is the Land of Very Small Dogs.
23:16One of the great railway journeys of the world.
23:35Almost all life depends on the energy that the sun sends our way.
23:42But the sun is a far from benevolent companion,
23:45because its radiant rain can be as dangerous as it is nourishing.
24:03We're still round about sea level now,
24:05and the sun is quite low in the sky.
24:07It's about 7am, so it's not been up long.
24:10I'm going to measure the amount of UV radiation
24:12falling on every square centimetre with this,
24:15a digital ultraviolet radiometer.
24:21At the moment, it says there's about 22 microwatts
24:26per square centimetre falling on my skin.
24:30But as we climb in altitude,
24:32then that UVB light is going to have to travel
24:35through less and less of the atmosphere,
24:37so less of it's going to be absorbed.
24:41And sure enough, as the miles pass by
24:45and we head into the mountainous interior,
24:48the metre readings start to go up.
25:11Now it's about ten o'clock in the morning,
25:13so the sun's significantly higher in the sky.
25:16The train's also climbed quite a bit in altitude.
25:19And now...
25:23we're getting nearly 250 microwatts per square centimetre,
25:27so that's about a factor of ten higher.
25:29And that's just because the UVB has had
25:31significantly less atmosphere to travel through.
25:36That's more than enough to burn unprotected skin
25:39in just a few minutes.
25:42And that's because what arrives from the sun
25:45is far more than just the stuff we can see.
25:52Beyond the sun,
25:54there's a lot of other stuff we can see,
25:57and that's why we're here.
26:00Beyond the visible, the higher energy part of the spectrum,
26:04there's ultraviolet light, particularly UVB,
26:07which does get through the Earth's atmosphere
26:10and gets to the surface.
26:12Now UVB can be beneficial to life.
26:15We use it to produce vitamin D, for example.
26:18But because it's higher energy,
26:20it can also be extremely damaging.
26:22It can damage DNA, it can burn our skin,
26:25as well as give us a skin rash.
26:27It can damage DNA, it can burn our skin,
26:30as well as give us a sun tan.
26:32And of course, ultimately, it can give us skin cancer.
26:38Now if ultraviolet light is a problem
26:40for life on Earth to deal with today,
26:42then the physicists might raise
26:44an interesting problem for the biologists.
26:46Because we know that 3.5 billion years ago,
26:49when life on Earth began,
26:51although the sun was much dimmer
26:53in the visible part of the spectrum,
26:55it was significantly brighter in the ultraviolet.
27:03The young sun seems like a paradox.
27:07It was fainter to the eye,
27:09perhaps 30% less bright than the sun we enjoy today,
27:13yet rich in deadly ultraviolet.
27:17Inside, the core was spinning much faster,
27:21which created more electromagnetic heating
27:24of the plasma on its surface.
27:29And this plasma emitted more energy
27:31not in the lower visible frequencies,
27:34but in the higher frequencies,
27:37like X-rays
27:40and ultraviolet.
27:48It seems as if just as life was getting settled on its wet home,
27:52the faint young sun was making it tough to survive near the surface.
28:08This is the top of Copper Canyon,
28:10so the summit of the railway journey.
28:12It's about 2,200 metres,
28:14which is about, what, somewhere between 7,000 and 8,000 feet.
28:19So I'll take a UV reading of the sun.
28:23It's actually reading about 260 now.
28:26Now, if you remember, at midday down at sea level,
28:29we were getting readings around 260.
28:32So although the sun has dropped in the sky,
28:35so the sunlight and the UV are coming through much more atmosphere,
28:38that's been compensated for by the thinness of the air up here.
28:42I'm getting more UV now
28:44than I would have been at the same time of day at sea level.
28:49It's hard to be sure,
28:51but we think that it's these kinds of radiation levels
28:55that early life had to deal with,
28:57because back then, the sun's ultraviolet output
29:01was significantly stronger.
29:07So I think it is fair to say
29:09that that could have posed a significant threat
29:12to the development of early life on Earth.
29:16Today, life has painted the surface of our home
29:20in all the colours of the rainbow.
29:25From greens to blues, reds to yellows,
29:29oranges and violets.
29:34And the origin of all life's hues
29:36can be traced back to the way ancient people lived.
29:40And the origin of all life's hues
29:43can be traced back to the way it interacts with sunlight.
29:49I'm a particle physicist,
29:51so I'm allowed to think of everything
29:53in terms of the interactions of particles.
29:56So I would picture the light from the sun
29:59as being really a rain of particles,
30:02photons they're called,
30:04particles of light of different energies
30:06raining down on the surface of the Earth.
30:09The blue ones are the highest energy photons,
30:12the red ones are the lowest energy photons,
30:14and all the colours of the rainbow in the middle
30:17are just simply photons of different energies.
30:21Oh, thank you.
30:24Wow.
30:26So for this, for Chile salsa, which I see as red,
30:30then there are pigment molecules in there
30:32that are absorbing the blue photons,
30:34the blue light from the sun.
30:36The red ones, it doesn't interact with,
30:38so they bounce back into my eye,
30:40and that's why I see it as red.
30:42Same for the green chili.
30:44But in this case, the red photons are interacting,
30:47they're doing something,
30:48they're talking to the pigments in here,
30:50and what I'm seeing are the green photons
30:53and some of the blue photons coming into my eye,
30:55mixing up, and allowing me to see that as green.
31:02Pigments bring colour to the world.
31:05A planet painted by genes,
31:08honed by billions of years of evolution.
31:16Some colours warn of danger.
31:18This stuff's on fire, I tell you.
31:23Or attract pollinators.
31:36Pigments are one of the ways that life has evolved
31:39to take on the sun's powerful ultraviolet light.
32:06This little guy is called a bombardier beetle.
32:10If I just grab him...
32:18His name comes from his unique defence mechanism.
32:23He produces two chemicals, one of them, you might call it,
32:26is a chemical that's used to kill animals.
32:31And when you scare him,
32:32both those chemicals are injected into a little chamber in his body,
32:37which raises the temperature to the boiling point of water
32:41and increases the pressure,
32:43squirting a hot, noxious chemical out of its rear.
32:49What a clever way to kill a beetle.
32:53Squirting a hot, noxious chemical out of its rear.
32:59What a clever way to defend yourself.
33:03But this is just one of the ways this character uses chemistry
33:07to increase its chances of survival.
33:12The bombardier beetle and me,
33:14and in fact every living thing you can see,
33:17are exposed to the same threat here on the high plains of Mexico.
33:21It's the high-energy ultraviolet photons
33:24raining down on this landscape from the sun.
33:29If they hit DNA in my skin, for example,
33:32then they'll damage the DNA, so that's got to be prevented.
33:37Me and my friend the beetle have both reached the same solution.
33:42You see that the beetle is brown and black.
33:46My skin, when it's exposed to the sun, is going brown
33:50producing a pigment called melanin, and so is the beetle.
33:54Now melanin is a very simple molecule,
33:56it's just a ring of carbon atoms with a few extra bits bolted on,
34:00but its sea of electrons behaves in a very specific way.
34:04So when a high-energy ultraviolet photon from the sun
34:07hits one of those electrons,
34:09it very, very quickly dissipates that energy.
34:12That potentially threatening photon has been absorbed
34:16and all its energy has been dissipated away as heat.
34:23Melanin is so efficient that over 99.9%
34:27of the harmful ultraviolet radiation is absorbed.
34:32So melanin is protecting both my skin and my friend the bombardier beetle
34:39from the potentially harmful effects of the sun.
34:47MUSIC PLAYS
35:02From the start, life had to evolve strategies
35:06for coping with the energetic young sun.
35:10Now, life is nothing if not resourceful,
35:12and pigments are the way that living things interact
35:16with the radiation from the sun.
35:18So why just use them to dissipate energy, to protect?
35:23Why not use them to harness that energy for its own ends?
35:27Well, that's exactly what life did.
35:31And in doing so, it transformed our planet
35:35by introducing a wonderful new ingredient.
35:50Earth has an atmosphere unlike any other planet we know of in the universe.
35:56Only in the air on our world do fires burn.
36:04Only on our world has a gas been released
36:08which allowed complex life to evolve.
36:15And it's only on our planet,
36:17which has a gas that's been released,
36:19which has allowed complex life to evolve.
36:26What makes our home unique is its oxygen-rich atmosphere.
36:38Deep in a cave in the hills of Tabasco,
36:41you can find a hint of what a living planet without oxygen might be like.
36:56Well, this is one of the more unique environments on our planet.
37:02This cave is full of sulfur.
37:05You can see it in the water.
37:07You can see that milky color of the lake that's flowing through the cave.
37:10That's dissolved sulfur.
37:12And it's coming from hydrogen sulfide gas,
37:15the source of which is actually not entirely known.
37:19The hydrogen sulfide is toxic to me,
37:22and it has another rather alarming effect on this hellhole.
37:29It's a bad-smelling gas,
37:31but it's also a gas that drives the oxygen out.
37:34So certainly, as you go on into the cave system,
37:36you get less and less oxygen.
37:42I suppose, to be honest,
37:45I suppose, in a sense,
37:47some of the chemistry and the biochemistry
37:50that takes place in the dark in these cave systems
37:54could be very similar to the chemistry and biochemistry
37:58that occurred when our planet was very young.
38:04For the first half of its history,
38:06Earth was without oxygen in its atmosphere.
38:10But incredibly, in this echo of the past,
38:13which I can only visit for a few minutes,
38:16there are forms of life that are completely at home.
38:22Wow, look at that.
38:24There they are, cities of sulfur-eating bacteria
38:28living off the hydrogen sulfide gas.
38:31Colonies of extremophiles,
38:34organisms that are living off a very different environment of gases
38:39to the one that we're used to on the surface.
38:47They are a window on a much earlier time.
38:50In the early part of the 20th century,
38:52there was a time when there was no gas at all.
38:56They are a window on a much earlier time.
39:03Because without oxygen,
39:05the ancestors of these extremophiles
39:08were the only forms of life our planet could support.
39:26Understanding how Earth developed an atmosphere rich in oxygen
39:31has taken centuries.
39:34And the secret lies with ancient bacteria.
39:39In 1676, a Dutchman called Antoine Leeuwenhoek
39:43was trying to find out why pepper is spicy.
39:48See, they thought that there were little spikes on peppercorns
39:52that dug into your tongue,
39:54and that was the secret.
39:56But it wasn't.
39:58It wasn't.
40:00It wasn't.
40:02It wasn't.
40:04It wasn't.
40:06It wasn't.
40:08It wasn't.
40:10It wasn't.
40:12It wasn't.
40:14It wasn't.
40:16It wasn't.
40:18It wasn't.
40:20It wasn't.
40:22It wasn't.
40:24It wasn't.
40:26It wasn't.
40:28It wasn't.
40:30It wasn't.
40:32It wasn't.
40:34It was estimated you could line about 100 of the wee little creatures,
40:38those are his words, up along the length of a single coarse sand grain.
40:44What Leeuwenhoek thought were animals
40:47were in all probability not animals at all.
40:53Although he didn't know it at the time,
40:55he had discovered a whole new domain of life.
41:00Bacteria.
41:10They are by far the most numerous organisms on the Earth.
41:14In fact, there are more bacteria on our planet
41:18than there are stars in the observable universe.
41:25And there is one kind of bacteria
41:28more numerous than all the rest.
41:35One of the most striking structures I can see on this slide
41:38is a kind of a blue-green filament,
41:41which is a little colony of a type of bacteria called cyanobacteria.
41:51These things are incredibly important organisms.
41:59Fossilised cyanobacteria have been found
42:03as far back as 3.5 billion years ago.
42:10And at some point, around 2.4 billion years ago,
42:14they became the first living things to use pigments
42:18to split water apart and use it to make food.
42:23This evolutionary invention was incredibly complex.
42:27Even its name is a mouthful.
42:30Oxygenic photosynthesis.
42:36It starts with a photon from the sun
42:39hitting that green pigment, chlorophyll.
42:43Chlorophyll takes that energy
42:45and uses it to boost electrons up a hill, if you like.
42:50And when they get to the top,
42:52they cascade down a molecular waterfall,
42:55and the energy is used to make something called ATP,
42:58which is essentially the energy currency of life.
43:02This little molecular machine is called Photosystem II,
43:06and it makes energy for the cell from sunlight.
43:10But when the electrons reach the bottom of that waterfall,
43:13they enter Photosystem I.
43:15They meet some more chlorophyll,
43:17which is hit by another photon from the sun,
43:20and that energy raises the electrons up again
43:24and forces them onto carbon dioxide,
43:27turning that carbon dioxide eventually into sugars,
43:31into food for the cell.
43:33Now, why all this complexity?
43:35You know, why do you need these two photosystems
43:38joined together in this way,
43:40just to get some electrons to make food for the cell?
43:44Why not get some electrons
43:46and make sugar and a bit of energy out of it?
43:53It's because only when life coupled
43:55these two biological machines together
43:58that it could split water apart and turn it into food.
44:03But it wasn't easy.
44:05The thing is that water is extremely difficult to split,
44:10so for a leaf to do it, for a blade of grass to do it
44:13or for a trickle of light from the sun is extremely difficult.
44:20In fact, the task is so complex
44:23that unlike flight or vision,
44:26which have evolved separately many times during our history,
44:30oxygenic photosynthesis has only evolved once.
44:37Every tree, every plant, every blade of grass on the planet,
44:42everything that carries out oxygenic photosynthesis today
44:46does it in exactly the same way.
44:49And the structures inside every leaf that do that
44:53look remarkably similar to cyanobacteria.
45:01In other words, the descendants of one cyanobacterium
45:06that worked out for some reason
45:08how to couple those complex molecular machines together
45:11in some primordial ocean billions of years ago
45:15are still present on the Earth today.
45:36The cyanobacteria changed the world.
45:41Turning it green.
45:49And that had a wonderful consequence.
45:58With this new way of living, life released oxygen
46:02into the atmosphere of our planet for the first time.
46:06And in doing so, over hundreds of millions of years,
46:11it eventually completely transformed the face of our home.
46:19And as the oxygen levels grew,
46:21the stage was set for the arrival of ever more complex creatures.
46:28But on Earth, the emergence of complex life
46:31required a rather more intangible ingredient.
46:37Something that you can't see, touch or smell,
46:42and yet you pass through every day.
46:53Late January, and the monarch butterflies have found their way home.
47:00They've entered a hibernation state,
47:04huddling together for warmth.
47:09But they're only here at all
47:11thanks to one of the most accurate biological clocks found in nature.
47:34These are the pine and oyamel forests, high altitude,
47:38about, what, three hours northwest of Mexico City,
47:42and one of the few wintering grounds of the monarch butterflies, as you can see.
47:47But there is a colony of millions of monarchs,
47:50somewhere due north of here.
47:52So if I head off into the forest,
47:55then hopefully this will just be a taster of what's to come.
48:04To find the butterflies, I need to get an accurate bearing on them.
48:09And to do this, I need a timepiece.
48:14If you don't have a compass,
48:16how can you tell which direction's north and which direction's south?
48:19Well, you can use the sun.
48:21The sun rises in the east, sets in the west,
48:24and at midday in the northern hemisphere, it's due south.
48:28But what if it isn't midday?
48:31Well, there's an old trick, which is to use a watch.
48:34See, it's about three in the afternoon now,
48:37and if you line the hour hand of your watch up with the sun,
48:41then in the northern hemisphere,
48:43a line in between the hour hand and 12 o'clock will point due south,
48:50which means that north is that way.
48:54For thousands of miles on their way here,
48:57the monarchs have faced the same problem.
49:00To make their way south, it's no good simply following the sun,
49:07because as the day progresses,
49:09the sun's position drifts across the sky.
49:16Somehow, they have to correct this.
49:21Somehow, they have to correct for this.
49:25They use what's called a time-compensated sun compass.
49:50They measure the position of the sun every day
49:54using their eyes, but it's also thought
49:56they can measure the position even when it's cloudy
49:59by using the polarisation of the light.
50:03Having locked on to the sun,
50:05their brain then corrects for its movement across the sky
50:09by using one of nature's most accurate timepieces.
50:13By combining the information from their precise clocks and their eyes,
50:19they can navigate due south.
50:23That ability to orientate themselves
50:25is, I think, one of the most remarkable things I've seen.
50:36The biological clocks that have brought the monarchs home
50:40are not unique to butterflies.
50:44Almost all life shares in these circadian rhythms.
50:50They're an evolutionary consequence of living on a spinning rock.
51:02Our world turns on its axis once every 24 hours, giving us a day.
51:12It's on a billion-kilometre journey around the sun,
51:16and each orbit gives us a year.
51:22We live inside a celestial clock,
51:25one that has been ticking away for over 4.5 billion years,
51:31and that's a full third of the age of the universe.
51:47This is the final ingredient that our home has provided.
51:53Time.
52:04Take the horse.
52:06Like all complex living things,
52:08it's here because our planet has been stable enough for long enough
52:13to allow evolution time to play.
52:31The horse is the animal whose family tree we know with the highest precision.
52:38So it's possible to lay out just one unbroken chain of life
52:43that stretches back nearly 4 billion years.
52:51Animals that are recognisably horse-like have been around for a long time.
52:57Something like 55 million years.
53:00You then have to jump quite a lot to something like a million years.
53:05You have to jump quite a lot to something like 225 million years
53:09if you want to ask the question, where is the earliest mammal?
53:13And it's this thing which looks something like a little shrew.
53:17535 million.
53:19This is the point when complex life really began to explode in the oceans.
53:25You then have to sweep back a long, long time
53:28to find the next evolutionary milestone,
53:31arguably the most important milestone,
53:34the complex cell, the eukaryote.
53:37And then you have to step back a long way in time.
53:42You have to step back all the way to here,
53:47the emergence of the prokaryote, the first life form.
53:51And so we have this beautiful long line.
53:55We can trace my friend the horse and his ancestry
54:00back to events that happened 3.5, 3.6, 3.7 billion years ago
54:07on the primordial Earth.
54:15And looking back over that vast sweep of time,
54:19you could ask yourself the question,
54:21well, do you need 3.5 billion years
54:24to go from a simple form of life to something as complex as a horse?
54:31Well, the answer to that question is we don't know for sure.
54:35It seems that you need vast expanses of time,
54:40but do you need those big gaps from the simple cell to the complex cell?
54:45Do you need the gap from the complex cell
54:48to the evolution of multicellular life?
54:50We don't know.
54:54We only have one example.
54:56There's only one planet where we've been able to study
54:59the evolution of life, and it's this one.
55:03And Earth has been an interesting mixture of stability and upheaval.
55:08It's had an environment that's never completely conspired
55:12to wipe out life, but it's constantly thrown it challenges.
55:20The deep time that our planet has given life
55:23has allowed it to grow from a tiny seed of genetic possibility
55:28to the planet-wide web of complexity we're part of today.
55:43Only a few of us have ever stepped outside of this world,
55:47but those that have discovered something rather wonderful.
55:53For all the people back on Earth,
55:56the crew of Apollo 8 has a message that we would like to send to you.
56:02On Christmas Eve 1968, my first Christmas Eve,
56:06the Apollo 8 spacecraft entered the darkness on the far side of the moon.
56:11In the beginning, God created the heaven and the earth,
56:16and the earth was without form.
56:19And the three astronauts, Borman, Lovell and Anders,
56:22became the first human beings in history to lose sight of the earth.
56:28And God said, let there be light.
56:31And there was light.
56:33And God saw the light.
56:35And it was good.
56:37When they emerged from the dark side of the moon
56:40and the earth rose into view,
56:42they chose to broadcast their culture's creation story
56:46back to the inhabitants of Earth.
56:48And just like the Aztecs and the Mayans and every civilisation before them,
56:53it told of the origins of their home.
56:57And God called the dry land Earth.
57:00And the gathering together of the waters called East Sea.
57:04And God saw that it was good.
57:07It must be innately human,
57:10the desire to understand how our home came to be the way that it is.
57:16And seen from lunar orbit against the blackness of space,
57:19the earth is a fragile world.
57:22But seen by science, it's a world that's been crafted
57:25and shaped by life over almost four billion years.
57:31So we're on our way to understanding how we came to be here.
57:34But as the Apollo astronauts discovered,
57:36the journey of discovery has already delivered much more than just the facts,
57:41because it's given us a powerful perspective
57:44on the intricacy and beauty of our home.
57:49To the crew of Apollo 8,
57:51we close with good night,
57:53good luck,
57:55a merry Christmas,
57:57and God bless all of you,
57:59all of you on the good Earth.
58:07Still to come tonight on BBC HD,
58:09high drama and plenty of twitchy moments.
58:12It's all change in Dancing on the Edge, coming up next.