0:00:07 > 0:00:09This creature...
0:00:09 > 0:00:11is a wonder of nature.
0:00:11 > 0:00:13BIRDSONG
0:00:15 > 0:00:20Its biology is hard-wired to the heavens.
0:00:20 > 0:00:23BUZZING
0:00:24 > 0:00:27It has an exquisitely sensitive eye
0:00:27 > 0:00:28that locks onto the sun
0:00:28 > 0:00:33and allows it to navigate its way across the face of the planet.
0:00:37 > 0:00:39In a sense,
0:00:39 > 0:00:43it has an instinctive understanding of its place in the solar system.
0:00:44 > 0:00:46A tiny insect brain
0:00:46 > 0:00:50joined to the movements of the sun and the planets.
0:00:53 > 0:00:57This connection steers the monarch and millions of its brethren
0:00:57 > 0:01:02as they make one of the longest migrations of any butterfly species.
0:01:09 > 0:01:12They're heading for these trees known locally as the oyamel,
0:01:12 > 0:01:14or sacred firs.
0:01:14 > 0:01:18Some of the butterflies began their journey over 4,000 kilometres away,
0:01:18 > 0:01:20that's 2,500 miles,
0:01:20 > 0:01:23up here in the north-eastern United States and Canada.
0:01:23 > 0:01:25And over the autumn and the winter,
0:01:25 > 0:01:29they've migrated south across the United States
0:01:29 > 0:01:33and arrived here, in central Mexico.
0:01:33 > 0:01:37Incredibly, no butterfly has ever learned this route.
0:01:37 > 0:01:38It can't have,
0:01:38 > 0:01:42because it takes at least three generations to make the round trip.
0:01:42 > 0:01:45Instead, the homing instinct is carried
0:01:45 > 0:01:50on a river of genetic information that flows through each butterfly.
0:01:53 > 0:01:56The allure of this place to the butterflies,
0:01:56 > 0:01:58this sense of belonging,
0:01:58 > 0:02:01is a deep feeling we all share.
0:02:01 > 0:02:05We even have a word for it - home.
0:02:10 > 0:02:15Every living thing that we know to exist is found on this one rock.
0:02:18 > 0:02:20So, what is it about our planet
0:02:20 > 0:02:24that makes it such a rich, colourful, living world?
0:02:26 > 0:02:28I want to show you why our world
0:02:28 > 0:02:32is the only habitable planet we know of anywhere in the universe.
0:02:32 > 0:02:35Now, the answer depends on the presence of a handful
0:02:35 > 0:02:40of precious ingredients that make our world a home.
0:02:58 > 0:03:00SQUAWKING
0:03:09 > 0:03:14'In the beginning, God created the heaven and the earth.
0:03:14 > 0:03:18'And the earth was without form and void.
0:03:18 > 0:03:21'And darkness was upon the face of the deep.'
0:03:23 > 0:03:25SQUAWKING
0:03:25 > 0:03:29Home is such an evocative word.
0:03:29 > 0:03:31I mean, it will mean something to you.
0:03:31 > 0:03:34The place you went to school, the place you live,
0:03:34 > 0:03:37the place where your kids had their first Christmas.
0:03:37 > 0:03:40But in a scientific sense, what does it mean?
0:03:44 > 0:03:48It means...that the ingredients are there for you to live.
0:03:48 > 0:03:51An atmosphere,
0:03:51 > 0:03:52food, water.
0:03:52 > 0:03:55You need the temperature to be right.
0:03:57 > 0:04:02Home is the place that has the things you need for your biology
0:04:02 > 0:04:05and chemistry to work.
0:04:05 > 0:04:07And it's no less evocative for that.
0:04:11 > 0:04:13YELLING AND WHINNYING
0:04:21 > 0:04:22This is Mexico.
0:04:22 > 0:04:26A country rich in the ingredients that set our world apart.
0:04:28 > 0:04:30It's not a bad place to come
0:04:30 > 0:04:34because, with about 1% of the land surface area of our planet,
0:04:34 > 0:04:37it's home to 12% of the species.
0:04:37 > 0:04:40There are 26,000 plant species here,
0:04:40 > 0:04:42there are 700 species of reptiles
0:04:42 > 0:04:44and 400 species of mammals.
0:04:44 > 0:04:47It's also been home to some of the world's great civilisations.
0:04:47 > 0:04:51The Maya built their temples out there in the forest here
0:04:51 > 0:04:54for thousands and thousands of years.
0:04:54 > 0:04:56NATIVE SINGING
0:04:56 > 0:05:00Mexico is bursting with life.
0:05:00 > 0:05:02And if you know where to look,
0:05:02 > 0:05:04hidden inside these creatures
0:05:04 > 0:05:09are clues that tell how this planet became their home.
0:05:14 > 0:05:18First stop is in the southeast of the country.
0:05:18 > 0:05:20An area covered in thick jungle.
0:05:22 > 0:05:26The Yucatan's a strip of essentially pure limestone
0:05:26 > 0:05:29that separates the Caribbean from the Gulf of Mexico.
0:05:29 > 0:05:32And it's got all the ingredients you might think you need
0:05:32 > 0:05:35for a rich and diverse ecosystem.
0:05:36 > 0:05:39The tropical sun warms the forest,
0:05:39 > 0:05:43delivering precious energy to each and every leaf.
0:05:44 > 0:05:48Oxygen escapes from the plants and trees,
0:05:48 > 0:05:51which is breathed in by the forest animals.
0:05:55 > 0:05:58And where they can, each of them
0:05:58 > 0:06:01draws deeply from the region's hidden water supply.
0:06:03 > 0:06:06But there are some of the ingredients you need
0:06:06 > 0:06:08to grow this tropical forest
0:06:08 > 0:06:10that are far more important than others.
0:06:25 > 0:06:28You might think that this place would be awash with water.
0:06:28 > 0:06:31It does rain a lot and it's incredibly humid.
0:06:31 > 0:06:35But actually, there are no surface rivers at all
0:06:35 > 0:06:36on the Yucatan Peninsula
0:06:36 > 0:06:40because the water just seeps into the porous limestone.
0:06:40 > 0:06:43That's where these things come in. These are cenotes.
0:06:43 > 0:06:47They're caverns dissolved out of the limestone by the rain.
0:06:47 > 0:06:49And they collect water.
0:06:49 > 0:06:52And they play a vital role in the ecosystem.
0:06:52 > 0:06:55I mean, the forest changes when you get around a cenote.
0:06:55 > 0:06:57Just listen to that.
0:06:57 > 0:06:59RIBBITING
0:06:59 > 0:07:01Those are frogs.
0:07:01 > 0:07:04And you don't hear those frogs anywhere else in the forest,
0:07:04 > 0:07:06just around the cenotes.
0:07:19 > 0:07:20The cenotes are flooded caves
0:07:20 > 0:07:23that have been cut off from the outside world
0:07:23 > 0:07:26for thousands of years.
0:07:44 > 0:07:48Lilies, troglodytic fish, even the occasional turtle,
0:07:48 > 0:07:53all thrive around the openings of these freshwater wells.
0:08:00 > 0:08:01As I head deeper into the cave,
0:08:01 > 0:08:06the temperature drops and the light fades.
0:08:09 > 0:08:14One by one, the ingredients I depend upon begin to disappear.
0:08:18 > 0:08:22Yet even here, far from the soil and air,
0:08:22 > 0:08:26strangely-coloured algae still find a home in the water.
0:08:40 > 0:08:44If there's one thing that unites every form of life in the cenote,
0:08:44 > 0:08:47in fact, every form of life out there in the forests,
0:08:47 > 0:08:51in fact, every form of life we've ever discovered
0:08:51 > 0:08:53anywhere on planet Earth,
0:08:53 > 0:08:56it's that it has to be wet.
0:08:59 > 0:09:04Only on our home does water run freely between the skies,
0:09:04 > 0:09:08oceans, rivers and on, into every living thing.
0:09:08 > 0:09:11MARIACHI MUSIC PLAYS
0:09:16 > 0:09:18CAR HORN BEEPS
0:09:43 > 0:09:45SHE SPEAKS IN NATIVE TONGUE
0:09:48 > 0:09:52To understand why life and water are so intertwined,
0:09:52 > 0:09:54we need to look a little deeper
0:09:54 > 0:09:57into one of the strangest substances we know.
0:09:57 > 0:10:00ANIMATED CHATTER
0:10:02 > 0:10:04Now, I may be a bit of a middle-aged academic,
0:10:04 > 0:10:07but I can still do the odd experiment every now and again.
0:10:07 > 0:10:10So what I'm doing is I'm charging up this Perspex rod.
0:10:10 > 0:10:13So giving it an electric charge by rubbing it on the fleece.
0:10:13 > 0:10:15Now, watch what happens...
0:10:15 > 0:10:20when I put the rod next to a stream of water.
0:10:20 > 0:10:22You see that?
0:10:22 > 0:10:24Look at that.
0:10:24 > 0:10:26The electric field, the electric charge,
0:10:26 > 0:10:28is bending the water towards it.
0:10:28 > 0:10:31Now, the reason for that,
0:10:31 > 0:10:33the reason that water behaves in that way
0:10:33 > 0:10:36when it's passing through an electric field,
0:10:36 > 0:10:40is exactly the same reason that it is vital for all life on Earth.
0:10:50 > 0:10:52Water is a polar molecule,
0:10:52 > 0:10:56which means it responds to electric charge.
0:11:04 > 0:11:06Its polarity comes about
0:11:06 > 0:11:10because of the structure of water molecules themselves.
0:11:12 > 0:11:14Now, water is H2O,
0:11:14 > 0:11:17two hydrogens and one oxygen atom bound together.
0:11:17 > 0:11:21So two hydrogen atoms approach oxygen.
0:11:21 > 0:11:25Now, oxygen's got a cloud of eight electrons around it,
0:11:25 > 0:11:27so when the hydrogens come in, then what happens
0:11:27 > 0:11:32is the electrons get dragged over here, around the oxygen.
0:11:32 > 0:11:36So you end up with an electron cloud around here and, to some extent,
0:11:36 > 0:11:41pretty isolated, positively-charged protons out here.
0:11:41 > 0:11:45So you get a net positive charge over here
0:11:45 > 0:11:48and the electron cloud with its negative charge over here,
0:11:48 > 0:11:52so you get what's called a polar molecule.
0:11:52 > 0:11:55And that's why, when you bring a charged Perspex rod
0:11:55 > 0:11:58close to water molecules, they bend towards it.
0:12:02 > 0:12:03BIRDSONG
0:12:12 > 0:12:16Water's polar nature means that although its molecules are simple,
0:12:16 > 0:12:21together, they form a subtle, endlessly complex liquid.
0:12:22 > 0:12:26A home in which one tiny creature thrives.
0:12:55 > 0:12:57There he is. Look at that.
0:12:57 > 0:13:00That...is a pond skater.
0:13:00 > 0:13:04A predator that floats on the surface of the water
0:13:04 > 0:13:08and actually uses the surface of the water to sense its prey.
0:13:08 > 0:13:11Pond skaters are vicious predators
0:13:11 > 0:13:15that live for most of their lives on the surface.
0:13:17 > 0:13:20Tiny hairs on their legs provide a large area
0:13:20 > 0:13:22that spreads their weight.
0:13:24 > 0:13:28Their middle legs thrust them forward.
0:13:28 > 0:13:31Hind legs are employed to steer.
0:13:36 > 0:13:39They're so well adapted to life in this flat world
0:13:39 > 0:13:42that they even sense their sexual partners
0:13:42 > 0:13:46through tiny vibrations in the water's surface.
0:13:50 > 0:13:54The reason it can do that is the result of a complex interaction
0:13:54 > 0:13:57between adaptions in the animal itself
0:13:57 > 0:14:00and the physics and the chemistry
0:14:00 > 0:14:02of the surface of water.
0:14:05 > 0:14:07Water molecules are polar.
0:14:07 > 0:14:12And that means that water molecules themselves can bond together.
0:14:12 > 0:14:16So you can get a hydrogen with its slight positive charge
0:14:16 > 0:14:20getting close to the oxygen of another water molecule
0:14:20 > 0:14:24with its slight negative charge and bonding to it.
0:14:24 > 0:14:26You can build up quite large,
0:14:26 > 0:14:29in fact, VERY large structures in liquid water.
0:14:34 > 0:14:37This is what gives water its unique ability
0:14:37 > 0:14:41to form a surface habitat for the pond skaters.
0:14:42 > 0:14:44Clumps of H2O stick together,
0:14:44 > 0:14:47keeping the surface under tension.
0:14:49 > 0:14:52Forming a chorus of water molecules,
0:14:52 > 0:14:55all joined together by hydrogen bonds.
0:14:59 > 0:15:04Then a pond skater comes along and it puts its legs or its...
0:15:04 > 0:15:09dangly things into the water and pushes it down,
0:15:09 > 0:15:11bends the surface of the water.
0:15:11 > 0:15:13Now, the water doesn't like that
0:15:13 > 0:15:17because a bend in the water is increasing its surface area.
0:15:17 > 0:15:18It's increasing its energy.
0:15:18 > 0:15:22It's making it harder for all the molecules to bond together
0:15:22 > 0:15:25with the hydrogen bonds. So they try to push back.
0:15:25 > 0:15:28They exert a force on the pond skater's leg
0:15:28 > 0:15:31because they want to bond as much as they can.
0:15:31 > 0:15:34And that's how pond skaters stay on the surface of the water.
0:15:39 > 0:15:41Hydrogen bonds do far more
0:15:41 > 0:15:44than just give the pond skaters a place to live.
0:15:45 > 0:15:49They're fundamental to all life.
0:15:52 > 0:15:56I've heard it said that we won't truly understand biology
0:15:56 > 0:15:59until we understand water.
0:16:05 > 0:16:11These are...very thin tubes of glass.
0:16:11 > 0:16:15They're about a millimetre in diameter.
0:16:15 > 0:16:19And if I dip one into the surface of this river...
0:16:21 > 0:16:26..can you see that the water just climbs up the tube?
0:16:26 > 0:16:30It pulls itself up, quite literally, against the force of gravity.
0:16:30 > 0:16:34Now, in trees, there are tubes which are about half the diameter of this,
0:16:34 > 0:16:37perhaps about half a millimetre or even less.
0:16:37 > 0:16:40And they are called xylem.
0:16:40 > 0:16:44And they allow the tree to lift water up through the root system
0:16:44 > 0:16:47because the water molecules strongly attract each other
0:16:47 > 0:16:51and are strongly attracted to the sides of the tubes.
0:16:51 > 0:16:55So when you look at trees like that, which are very high,
0:16:55 > 0:16:57and you ask yourself the question,
0:16:57 > 0:17:01"How do they get the water from the roots to the top of the tree?",
0:17:01 > 0:17:03a big part of that is capillary action,
0:17:03 > 0:17:07which is down to the polar nature of water.
0:17:12 > 0:17:14One of water's most important qualities
0:17:14 > 0:17:17is its ability to dissolve and carry
0:17:17 > 0:17:20all manner of substances around the living world.
0:17:23 > 0:17:27Because its molecules are very small and polar,
0:17:27 > 0:17:31water is a tremendously effective solvent.
0:17:31 > 0:17:35Those molecules can get in amongst other substances,
0:17:35 > 0:17:39salts and sugars, for example, and disperse them, if you like,
0:17:39 > 0:17:42in that sea of hydrogen bonds.
0:17:43 > 0:17:45Within every one of us,
0:17:45 > 0:17:50water is constantly flowing around each and every cell.
0:17:53 > 0:17:56Blood plasma is over 90% water.
0:17:56 > 0:17:59And in it are dissolved everything I need to live -
0:17:59 > 0:18:03oxygen, the nutrients from food, everything -
0:18:03 > 0:18:08distributed around my body in rivers of water.
0:18:12 > 0:18:15We live on a beautiful blue anomaly of a world.
0:18:15 > 0:18:22The only planet we know with a surface drenched in liquid water.
0:18:27 > 0:18:32The story of how each drop ended up here has been hard to fathom.
0:18:34 > 0:18:36Largely because it happened so long ago,
0:18:36 > 0:18:38there's very little direct evidence.
0:18:46 > 0:18:48But back in the Yucatan jungle,
0:18:48 > 0:18:52clues to how it turned up can still be found.
0:18:54 > 0:18:56Every civilisation on the Yucatan,
0:18:56 > 0:18:59be it the modern Mexicans or the Mayans,
0:18:59 > 0:19:03had to get their water from those deep wells, the cenotes.
0:19:03 > 0:19:08And I've got a completely unbiased map of the larger cenotes here,
0:19:08 > 0:19:11which I'm going to overlay on the Yucatan.
0:19:15 > 0:19:19Look at that. They lie in a perfect arc,
0:19:19 > 0:19:23centred around a very particular village,
0:19:23 > 0:19:26which is...there,
0:19:26 > 0:19:29and it's called Chicxulub.
0:19:29 > 0:19:33Now, to a geologist, there are very few natural events
0:19:33 > 0:19:39that can create a structure, such a perfect arc as that.
0:19:42 > 0:19:46All the evidence points to just one explanation.
0:19:50 > 0:19:54You're looking at what's left of a gigantic asteroid strike.
0:19:57 > 0:20:01One that wiped out three-quarters of all plant and animal species
0:20:01 > 0:20:06when it hit the Earth 65 million years ago.
0:20:06 > 0:20:09You may think that impacts from space are a thing of the past.
0:20:09 > 0:20:13A thing that only happened to the dinosaurs, but that's not true.
0:20:13 > 0:20:18About 55 million kilograms of rock hits the Earth every year.
0:20:18 > 0:20:21And around 2% of that is water.
0:20:23 > 0:20:28This hints that at least some of Earth's water arrived from space.
0:20:32 > 0:20:36Late in 2010, these glimpses of comet Hartley 2
0:20:36 > 0:20:38arrived back on Earth.
0:20:39 > 0:20:43They were sent by NASA's deep-impact probe.
0:20:43 > 0:20:48From its surface, dust and ice spray into space.
0:20:49 > 0:20:54Analysis of this water found it had a very similar mixture of isotopes
0:20:54 > 0:20:56to the water in our own oceans.
0:21:00 > 0:21:02This was the first firm evidence
0:21:02 > 0:21:04that icy comets must have contributed
0:21:04 > 0:21:07to the formation of our world's oceans.
0:21:23 > 0:21:26Earth began life as a molten hell.
0:21:28 > 0:21:32Its internal heat drove off any trace of moisture.
0:21:34 > 0:21:39But soon, the planet cooled and the first clouds grew.
0:21:41 > 0:21:44Then, 4.2 billion years ago,
0:21:44 > 0:21:47a deluge, the like of which the solar system
0:21:47 > 0:21:52had never seen before or since, rained down.
0:21:52 > 0:21:53THUNDERCLAP
0:22:12 > 0:22:15And again, thanks to those hydrogen bonds,
0:22:15 > 0:22:19water's boiling point is high enough to have allowed it to remain
0:22:19 > 0:22:23on the surface of the Earth to the present day.
0:22:23 > 0:22:26So from quite early in its history,
0:22:26 > 0:22:31our home has been able to hang on to this most vital of ingredients.
0:22:31 > 0:22:33But to trace the origin of the next ingredients,
0:22:33 > 0:22:36you have to look beyond our planet...
0:22:38 > 0:22:40..to our nearest star.
0:22:43 > 0:22:45And the rays of light it sends our way.
0:22:47 > 0:22:49This is the train from Los Mochis to Chihuahua,
0:22:49 > 0:22:52which inexplicably leaves at 6:00am in the morning.
0:22:52 > 0:22:56Um...the local name for this area in all the guidebooks
0:22:56 > 0:22:58is the Land of Turtles.
0:22:58 > 0:23:02Beautifully romantic name for this place on the Sea of Cortez.
0:23:02 > 0:23:05But we just found out it's probably more likely to have been called
0:23:05 > 0:23:08the Land of Spinach-type Vegetables.
0:23:08 > 0:23:11So we're going from the Land of Spinach-type Vegetables
0:23:11 > 0:23:13to Chihuahua,
0:23:13 > 0:23:16which is the Land of Very Small Dogs.
0:23:17 > 0:23:20One of the great railway journeys of the world.
0:23:29 > 0:23:31TRAIN HOOTS
0:23:35 > 0:23:40Almost all life depends on the energy that the sun sends our way.
0:23:41 > 0:23:45But the sun is a far-from-benevolent companion
0:23:45 > 0:23:50because its radiant rain can be as dangerous as it is nourishing.
0:24:03 > 0:24:05We're still round about sea level now
0:24:05 > 0:24:07and the sun is quite low in the sky.
0:24:07 > 0:24:10It's about 7:00am, so it's not been up long.
0:24:10 > 0:24:12I'm going to measure the amount of UV radiation
0:24:12 > 0:24:15falling on every square centimetre with this,
0:24:15 > 0:24:18a digital, ultraviolet radiometer.
0:24:21 > 0:24:26At the moment, it says there's about 22 microwatts
0:24:26 > 0:24:30per square centimetre falling on my skin.
0:24:30 > 0:24:33But as we climb in altitude, then that UVB light
0:24:33 > 0:24:36is going to have to travel through less and less of the atmosphere,
0:24:36 > 0:24:39so less of it is going to be absorbed.
0:24:45 > 0:24:47And sure enough, as the miles pass by
0:24:47 > 0:24:50and we head into the mountainous interior,
0:24:50 > 0:24:53the meter readings start to go up.
0:25:18 > 0:25:21Now it's about 10:00am, so the sun's significantly higher in the sky.
0:25:21 > 0:25:25The train's also climbed quite a bit in altitude.
0:25:25 > 0:25:27Now...
0:25:29 > 0:25:32..we're getting nearly 250 microwatts per square centimetre.
0:25:32 > 0:25:34So that's about a factor of ten higher.
0:25:34 > 0:25:39And that's just because the UVB has had significantly less atmosphere
0:25:39 > 0:25:43to travel through, from the top of the Earth's atmosphere down to me.
0:25:49 > 0:25:52That's more than enough to burn unprotected skin
0:25:52 > 0:25:53in just a few minutes.
0:25:55 > 0:25:57And that's because what arrived from the sun
0:25:57 > 0:26:01is far more than just the stuff we can see.
0:26:05 > 0:26:08Beyond the visible, the higher energy part of the spectrum,
0:26:08 > 0:26:12there's ultraviolet light, particularly UVB,
0:26:12 > 0:26:16which does get through the Earth's atmosphere and gets to the surface.
0:26:16 > 0:26:19Now, UVB can be beneficial to life.
0:26:19 > 0:26:23We use it to produce vitamin D, for example.
0:26:23 > 0:26:26But because it's higher energy, it can also be extremely damaging.
0:26:26 > 0:26:30It can damage DNA, it can burn our skin as well as give us a suntan,
0:26:30 > 0:26:34and, of course, ultimately, it can give us skin cancer.
0:26:34 > 0:26:37WHISTLE HOOTS
0:26:37 > 0:26:39If ultraviolet light is a problem
0:26:39 > 0:26:41for life on Earth to deal with today,
0:26:41 > 0:26:43then the physicists might raise
0:26:43 > 0:26:45an interesting problem for the biologists.
0:26:45 > 0:26:48Because we know that 3.5 billion years ago,
0:26:48 > 0:26:50when life on Earth began,
0:26:50 > 0:26:54although the sun was much dimmer in the visible part of the spectrum,
0:26:54 > 0:26:58it was significantly brighter in the ultraviolet.
0:27:03 > 0:27:05The young sun seems like a paradox.
0:27:06 > 0:27:08It was fainter to the eye,
0:27:08 > 0:27:13perhaps 30% less bright than the sun we enjoy today,
0:27:13 > 0:27:15yet rich in deadly ultraviolet.
0:27:17 > 0:27:21Inside, the core was spinning much faster,
0:27:21 > 0:27:24which created more electromagnetic heating
0:27:24 > 0:27:26of the plasma on its surface.
0:27:28 > 0:27:31And this plasma emitted more energy,
0:27:31 > 0:27:33not in the lower visible frequencies,
0:27:33 > 0:27:36but in the higher frequencies.
0:27:37 > 0:27:39Like X-rays...
0:27:39 > 0:27:41and ultraviolet.
0:27:47 > 0:27:51It seems as if just as life was getting settled on its wet home,
0:27:51 > 0:27:57the faint young sun was making it tough to survive near the surface.
0:28:09 > 0:28:13This is the top of Copper Canyon, so the summit of the railway journey.
0:28:13 > 0:28:15It's about 2,200 metres, which is about...
0:28:15 > 0:28:18somewhere between 7,000 and 8,000 feet.
0:28:20 > 0:28:23So I'll take a UV reading of the sun.
0:28:23 > 0:28:26It's actually reading about 260 now.
0:28:26 > 0:28:29Now, if you remember, at midday, down at sea level,
0:28:29 > 0:28:32we were getting readings around 260.
0:28:32 > 0:28:34So although the sun has dropped in the sky,
0:28:34 > 0:28:38so the sunlight and the UV are coming through much more atmosphere,
0:28:38 > 0:28:42that's been compensated for by the thinness of the air up here.
0:28:42 > 0:28:44I'm getting more UV now than I would have been
0:28:44 > 0:28:46at the same time of day at sea level.
0:28:50 > 0:28:51It's hard to be sure,
0:28:51 > 0:28:55but we think that it's these kinds of radiation levels
0:28:55 > 0:28:58that early life had to deal with.
0:28:58 > 0:29:01Because back then, the sun's ultraviolet output
0:29:01 > 0:29:04was significantly stronger.
0:29:08 > 0:29:10So I think it is fair to say
0:29:10 > 0:29:13that that could have posed a significant threat
0:29:13 > 0:29:16to the development of early life on Earth.
0:29:16 > 0:29:18WHINNYING
0:29:18 > 0:29:21ANIMATED CHATTER
0:29:23 > 0:29:27Today, life has painted the surface of our home
0:29:27 > 0:29:29in all the colours of the rainbow.
0:29:31 > 0:29:34From greens to blues,
0:29:34 > 0:29:36reds to yellows,
0:29:36 > 0:29:38oranges and violets.
0:29:40 > 0:29:44And the origin of all life's hues can be traced back
0:29:44 > 0:29:47to the way it interacts with sunlight.
0:29:50 > 0:29:53I'm a particle physicist, so I'm allowed to think of everything
0:29:53 > 0:29:56in terms of the interactions of particles.
0:29:56 > 0:29:59So I would picture the light from the sun
0:29:59 > 0:30:02as being really a rain of particles.
0:30:02 > 0:30:05Photons, they're called, particles of light
0:30:05 > 0:30:09of different energies, raining down on the surface of the Earth.
0:30:09 > 0:30:12The blue ones are the highest-energy photons,
0:30:12 > 0:30:14the red ones are the lowest-energy photons
0:30:14 > 0:30:16and all the colours of the rainbow in the middle
0:30:16 > 0:30:19are just simply photons of different energies.
0:30:19 > 0:30:24- SHE SPEAKS IN NATIVE TONGUE - Oh, thank you.
0:30:24 > 0:30:25Wow.
0:30:26 > 0:30:31For this, the chilli salsa which I see as red, there are pigment
0:30:31 > 0:30:35molecules in there that are absorbing the blue photons,
0:30:35 > 0:30:36the blue light from the sun.
0:30:36 > 0:30:38The red ones, it doesn't interact with,
0:30:38 > 0:30:42so they bounce back into my eye, and that is why I see it as red.
0:30:42 > 0:30:44The same with the green chilli,
0:30:44 > 0:30:48but in this case the red photons are interacting, doing something,
0:30:48 > 0:30:50talking to pigments in here,
0:30:50 > 0:30:55and what I am seeing are the green photons and some of the blue photons
0:30:55 > 0:30:58coming into my eye, mixing up, allowing me to see that as green.
0:31:02 > 0:31:05Pigments bring colour to the world.
0:31:05 > 0:31:07The planet is painted by genes,
0:31:07 > 0:31:11honed by billions of years of evolution.
0:31:15 > 0:31:17'Some colours warn of danger...'
0:31:17 > 0:31:19This stuff is on fire, I tell you!
0:31:23 > 0:31:24'..or attract pollinators.'
0:31:41 > 0:31:44Pigments are one of the ways that life has evolved
0:31:44 > 0:31:47to take on the sun's powerful ultraviolet light.
0:32:17 > 0:32:20This little guy is called a bombardier beetle.
0:32:20 > 0:32:23If I just grab him...
0:32:28 > 0:32:31His name comes from his unique defence mechanism.
0:32:32 > 0:32:36He produces two chemicals. One of them you might have heard of -
0:32:36 > 0:32:40hydrogen peroxide. The other one is something called hydroquinone,
0:32:40 > 0:32:42and when you scare him,
0:32:42 > 0:32:46both those chemicals are injected into a little chamber in his body.
0:32:47 > 0:32:50It raises the temperature to the boiling point of water,
0:32:50 > 0:32:52and increases the pressure,
0:32:52 > 0:32:56squirting a hot and noxious chemical out of its rear.
0:32:59 > 0:33:01A clever way to defend yourself.
0:33:03 > 0:33:06But this is just one of the ways this character uses chemistry
0:33:06 > 0:33:09to increase the chance of survival.
0:33:12 > 0:33:14The bombardier beetle and me,
0:33:14 > 0:33:18and in fact every living thing you can see, are exposed to
0:33:18 > 0:33:22the same threat on the high plains of Mexico, the high-energy
0:33:22 > 0:33:26ultraviolet photons raining down on this landscape from the sun.
0:33:29 > 0:33:33If they hit DNA in my skin, for example, they damage the DNA.
0:33:33 > 0:33:35So that must be prevented.
0:33:37 > 0:33:41Me and my friend, the beetle, have both reached the same solution -
0:33:41 > 0:33:45you see that the beetle is brown and black.
0:33:45 > 0:33:48My skin, when it is exposed to the sun, is going brown.
0:33:48 > 0:33:54I am producing a pigment called melanin, and so is the beetle.
0:33:54 > 0:33:56Melanin is a very simple molecule,
0:33:56 > 0:33:59it's just a ring of carbon atoms with a few extra bits bolted on,
0:33:59 > 0:34:04but the sea of electrons behaves in a very specific way.
0:34:04 > 0:34:07When a high-energy ultraviolet photon from the sun
0:34:07 > 0:34:12hits one of those electrons, it very quickly dissipates that energy.
0:34:12 > 0:34:15That potentially threatening photon has been absorbed
0:34:15 > 0:34:20and all its energy has been dissipated away as heat.
0:34:23 > 0:34:24Melanin is so efficient,
0:34:24 > 0:34:30over 99.9% of the harmful ultraviolet radiation is absorbed.
0:34:32 > 0:34:35So melanin is protecting
0:34:35 > 0:34:40both my skin and my friend, the bombardier beetle,
0:34:40 > 0:34:42from the potentially harmful effects of the sun.
0:35:04 > 0:35:05From the start,
0:35:05 > 0:35:10life had to evolve strategies for coping with the energetic young sun.
0:35:14 > 0:35:16Life is nothing if not resourceful.
0:35:16 > 0:35:20Pigments are the way that living things interact with
0:35:20 > 0:35:26the radiation from the sun. So why just use them to dissipate energy,
0:35:26 > 0:35:27to protect?
0:35:27 > 0:35:31Why not use them to harness that energy for its own ends?
0:35:31 > 0:35:33That is exactly what life did.
0:35:37 > 0:35:43In doing so, it transformed our planet by introducing
0:35:43 > 0:35:45a wonderful new ingredient.
0:35:56 > 0:35:59Earth has an atmosphere unlike any other planet
0:35:59 > 0:36:01we know of in the universe.
0:36:06 > 0:36:11Only in the air on our world do fires burn.
0:36:15 > 0:36:19Only on our world has a gas been released which allowed
0:36:19 > 0:36:22complex life to evolve.
0:36:29 > 0:36:34What makes our home unique is its oxygen-rich atmosphere.
0:36:40 > 0:36:45Deep in a cave in the hills of Tabasco, you can find a hint
0:36:45 > 0:36:49of what living planet without oxygen might be like.
0:37:00 > 0:37:03This is one of the more unique environments on our planet.
0:37:05 > 0:37:10This cave is full of sulphur, you can see it in the water.
0:37:10 > 0:37:14You can see that milky colour flowing through the cave.
0:37:14 > 0:37:16That is dissolved sulphur.
0:37:16 > 0:37:18It is coming from hydrogen-sulphide gas,
0:37:18 > 0:37:22the source of which is actually not entirely known.
0:37:26 > 0:37:29The hydrogen sulphide is toxic to me.
0:37:29 > 0:37:33It has another rather alarming effect on this hellhole.
0:37:35 > 0:37:37It is a bad-smelling gas,
0:37:37 > 0:37:40but it is also a gas that drives the oxygen out,
0:37:40 > 0:37:44so as you go on into the cave, you get less and less oxygen.
0:37:48 > 0:37:51In a sense, some of the chemistry,
0:37:51 > 0:37:56the biochemistry that takes place in the dark of this cave system,
0:37:56 > 0:38:00could be very similar to the chemistry
0:38:00 > 0:38:04and biochemistry that occurred when our planet was very young.
0:38:06 > 0:38:09For the first half of its history,
0:38:09 > 0:38:11Earth was without oxygen in the atmosphere.
0:38:15 > 0:38:19But incredibly, in this echo of the past, which I can only visit
0:38:19 > 0:38:24for a few minutes, there are forms of life that are completely at home.
0:38:26 > 0:38:28Look at that!
0:38:30 > 0:38:33There they are, cities of sulphur-eating bacteria
0:38:33 > 0:38:35living off the hydrogen-sulphide gas.
0:38:42 > 0:38:44Colonies of extremophiles,
0:38:44 > 0:38:49organisms living off a very different environment of gases
0:38:49 > 0:38:52to the one that we are used to on the surface.
0:38:57 > 0:39:00They are a window on a much earlier time.
0:39:05 > 0:39:09Because without oxygen, the ancestors of these extremophiles
0:39:09 > 0:39:13were the only forms of life our planet could support.
0:39:29 > 0:39:31Understanding how Earth developed
0:39:31 > 0:39:35an atmosphere rich in oxygen has taken centuries.
0:39:37 > 0:39:40The secret lies with ancient bacteria.
0:39:53 > 0:39:58In 1676, a Dutchman called Antonie Leeuwenhoek
0:39:58 > 0:40:03was trying to find out why pepper is spicy.
0:40:03 > 0:40:06See, they thought that there were little spikes on peppercorns
0:40:06 > 0:40:09that dug into your tongue.
0:40:09 > 0:40:11He was using the microscope,
0:40:11 > 0:40:13which had been discovered about 60 years before,
0:40:13 > 0:40:17but inexplicably, had never been used for anything useful before.
0:40:17 > 0:40:20He put the peppercorns on there and looked down and he couldn't see anything,
0:40:20 > 0:40:22so he thought he would grind them up,
0:40:22 > 0:40:26dissolve them in water and have a look. When he did that,
0:40:26 > 0:40:28he didn't see anything interesting in the peppercorns,
0:40:28 > 0:40:33but he found that there were little animals swimming around.
0:40:33 > 0:40:35He said that he estimated
0:40:35 > 0:40:38you could line about 100 of the "wee little creatures" -
0:40:38 > 0:40:43those are his words - on the length of a single coarse sand grain.
0:40:45 > 0:40:48What Leeuwenhoek thought were animals were, in all probability,
0:40:48 > 0:40:50not animals at all.
0:40:52 > 0:40:54Although he didn't know it at the time,
0:40:54 > 0:40:58he had discovered a whole new domain of life.
0:41:02 > 0:41:04Bacteria.
0:41:12 > 0:41:16They are by far the most numerous organisms on the Earth.
0:41:17 > 0:41:21In fact, there are more bacteria on our planet than
0:41:21 > 0:41:24there are stars in the observable universe.
0:41:28 > 0:41:33And there is one kind of bacteria more numerous than all the rest.
0:41:37 > 0:41:40One of the most striking structures I can see on this slide is
0:41:40 > 0:41:45a kind of blue-green filament which is a little colony
0:41:45 > 0:41:49of a type of bacteria called cyanobacteria.
0:41:52 > 0:41:56These things are incredibly important organisms.
0:42:02 > 0:42:06Fossilised cyanobacteria had been found as far back
0:42:06 > 0:42:08as 3.5 billion years ago.
0:42:11 > 0:42:16And at some point, around 2.4 billion years ago,
0:42:16 > 0:42:20they became the first living things to use pigments
0:42:20 > 0:42:23to split water apart and use it to make food.
0:42:26 > 0:42:30This evolutionary invention was incredibly complex.
0:42:30 > 0:42:36Even its name is a mouthful - oxygenic photosynthesis.
0:42:39 > 0:42:42It starts with a photon from the sun
0:42:42 > 0:42:45hitting that green pigment, chlorophyll.
0:42:45 > 0:42:49Chlorophyll takes that energy and uses it
0:42:49 > 0:42:53to boost electrons up a hill, if you like.
0:42:53 > 0:42:58And when they get to the top, they cascade down a molecular waterfall,
0:42:58 > 0:43:01and the energy is used to make something called ATP,
0:43:01 > 0:43:06which is potentially the energy currency of life.
0:43:06 > 0:43:10This little molecular machine is called photosystem II,
0:43:10 > 0:43:14and it makes energy for the cell from sunlight.
0:43:14 > 0:43:17But when the electrons reach the bottom of that waterfall,
0:43:17 > 0:43:19they enter photosystem I.
0:43:19 > 0:43:21They meet some more chlorophyll,
0:43:21 > 0:43:24which is hit by another photon from the sun,
0:43:24 > 0:43:27and that energy raises the electrons up again,
0:43:27 > 0:43:30and forces them onto carbon dioxide,
0:43:30 > 0:43:34turning that carbon dioxide eventually into sugars,
0:43:34 > 0:43:36into food for the cell.
0:43:36 > 0:43:39Now, why all this complexity?
0:43:39 > 0:43:42Why do you need these two photosystems
0:43:42 > 0:43:44joined together in this way,
0:43:44 > 0:43:48just to get some electrons and make sugar and a bit of energy out of it?
0:43:52 > 0:43:54It's because
0:43:54 > 0:43:57only when life coupled these two biological machines together
0:43:57 > 0:44:01that it could split water apart and turn it into food.
0:44:02 > 0:44:04But it wasn't easy.
0:44:05 > 0:44:09The thing is that water is extremely difficult to split,
0:44:09 > 0:44:12so for a leaf to do it, for a blade of grass to do it,
0:44:12 > 0:44:16just using a trickle of light from the sun, is extremely difficult.
0:44:20 > 0:44:25In fact, the task is SO complex that, unlike flight or vision,
0:44:25 > 0:44:29which have evolved separately many times during our history,
0:44:29 > 0:44:34oxygenic photosynthesis has only evolved once.
0:44:37 > 0:44:42Every tree, every plant, every blade of grass on the planet,
0:44:42 > 0:44:47everything that carries out oxygenic photosynthesis today
0:44:47 > 0:44:49does it in EXACTLY the same way.
0:44:49 > 0:44:53And the structures inside every leaf that do that
0:44:53 > 0:44:57look remarkably similar to cyanobacteria.
0:45:01 > 0:45:05In other words, the descendants of one cyanobacterium
0:45:05 > 0:45:08that worked out, for some reason,
0:45:08 > 0:45:11how to couple those complex molecular machines together
0:45:11 > 0:45:15in some primordial ocean, billions of years ago,
0:45:15 > 0:45:18are still present on the Earth today.
0:45:36 > 0:45:39The cyanobacteria changed the world...
0:45:40 > 0:45:42..turning it green.
0:45:49 > 0:45:51And that had a wonderful consequence.
0:45:58 > 0:46:00With this new way of living,
0:46:00 > 0:46:04life released oxygen into the atmosphere of our planet
0:46:04 > 0:46:07for the first time. And in doing so,
0:46:07 > 0:46:11over hundreds of millions of years,
0:46:11 > 0:46:16it eventually completely transformed the face of our home.
0:46:20 > 0:46:22And as the oxygen levels grew
0:46:22 > 0:46:26the stage was set for the arrival of ever more complex creatures.
0:46:28 > 0:46:32But on Earth, the emergence of complex life required
0:46:32 > 0:46:35a rather more intangible ingredient.
0:46:39 > 0:46:43Something that you can't see, touch or smell,
0:46:43 > 0:46:46and yet you pass through every day.
0:46:54 > 0:46:56Late January,
0:46:56 > 0:47:00and the monarch butterflies have found their way home.
0:47:02 > 0:47:06They've entered a hibernation state, huddling together for warmth.
0:47:10 > 0:47:14But they're only here at all thanks to one of the most accurate
0:47:14 > 0:47:17biological clocks found in nature.
0:47:36 > 0:47:41These are the pine and oyamel forests, high altitude,
0:47:41 > 0:47:44about, what, three hours north-west of Mexico City,
0:47:44 > 0:47:48and one of the few wintering grounds of the monarch butterflies,
0:47:48 > 0:47:50as you can see.
0:47:50 > 0:47:53But there is a colony of millions of monarchs
0:47:53 > 0:47:55somewhere due north of here,
0:47:55 > 0:47:57so if I head off into the forest
0:47:57 > 0:48:02then hopefully this will just be a taster of what's to come.
0:48:05 > 0:48:10To find the butterflies, I need to get an accurate bearing on them.
0:48:10 > 0:48:13And to do this I need a timepiece.
0:48:15 > 0:48:17If you don't have a compass,
0:48:17 > 0:48:20how can you tell which direction is north and which direction is south?
0:48:20 > 0:48:22Well, you can use the sun.
0:48:22 > 0:48:25The sun rises in the east, sets in the west,
0:48:25 > 0:48:29and at midday, in the northern hemisphere, it's due south.
0:48:29 > 0:48:31But what if it ISN'T midday?
0:48:31 > 0:48:35Well, there's an old trick, which is to use a watch.
0:48:35 > 0:48:38See, it's about three in the afternoon now,
0:48:38 > 0:48:41and if you line the hour hand of your watch up with the sun,
0:48:41 > 0:48:43then, in the northern hemisphere,
0:48:43 > 0:48:48the line in between the hour hand and 12 o'clock
0:48:48 > 0:48:50will point due south.
0:48:50 > 0:48:54Which means north is that way.
0:48:59 > 0:49:02For thousands of miles on their way here,
0:49:02 > 0:49:05the monarchs have faced the same problem.
0:49:06 > 0:49:11To make their way south, it's no good simply following the sun.
0:49:12 > 0:49:14Because, as the day progresses,
0:49:14 > 0:49:17the sun's position drifts across the sky.
0:49:21 > 0:49:24Somehow they have to correct for this.
0:49:45 > 0:49:49They use what's called a time-compensated sun compass.
0:49:51 > 0:49:55They measure the position of the sun every day, using their eyes,
0:49:55 > 0:49:58but it's also thought they can measure the position
0:49:58 > 0:50:02even when it's cloudy, by using the polarisation of the light.
0:50:03 > 0:50:08Having locked onto the sun, their brain then corrects for its movement
0:50:08 > 0:50:13across the sky by using one of nature's most accurate timepieces.
0:50:13 > 0:50:17By combining the information from their precise clocks
0:50:17 > 0:50:21and their eyes, they can navigate due south.
0:50:23 > 0:50:27That ability to orientate themselves is, I think,
0:50:27 > 0:50:29one of the most remarkable things I've seen.
0:50:36 > 0:50:40The biological clocks that have brought the monarchs home
0:50:40 > 0:50:42are not unique to butterflies.
0:50:44 > 0:50:48Almost all life shares in these circadian rhythms.
0:50:50 > 0:50:54They're an evolutionary consequence of living on a spinning rock.
0:51:01 > 0:51:07Our world turns on its axis once every 24 hours, giving us a day.
0:51:12 > 0:51:15It's on a billion-kilometre journey around the sun,
0:51:15 > 0:51:18and each orbit gives us a year.
0:51:21 > 0:51:24We live inside a celestial clock,
0:51:24 > 0:51:29one that has been ticking away for over 4.5 billion years.
0:51:31 > 0:51:35And that's a full third of the age of the universe.
0:51:50 > 0:51:55This is the final ingredient that our home has provided.
0:51:55 > 0:51:56Time.
0:52:06 > 0:52:08Take the horse.
0:52:08 > 0:52:13Like all complex living things, it's here because our planet
0:52:13 > 0:52:16has been stable enough for long enough
0:52:16 > 0:52:18to allow evolution time to play.
0:52:33 > 0:52:36The horse is the animal whose family tree
0:52:36 > 0:52:38we know with the highest precision.
0:52:44 > 0:52:48So it's possible to lay out just one unbroken chain of life
0:52:48 > 0:52:51that stretches back nearly four billion years.
0:52:55 > 0:52:59Animals that are recognisably horselike have
0:52:59 > 0:53:01been around for a long time -
0:53:01 > 0:53:04something like 55 million years.
0:53:04 > 0:53:09You then have to jump quite a lot to something like 225 million years
0:53:09 > 0:53:13if you want to ask the question, where is the earliest mammal?
0:53:13 > 0:53:17And it's this thing, which looks something like a little shrew.
0:53:17 > 0:53:19535 million.
0:53:19 > 0:53:22This is the point when complex life really began to explode
0:53:22 > 0:53:24in the oceans.
0:53:24 > 0:53:28You then have to sweep back a long, long time to find the next
0:53:28 > 0:53:33evolutionary milestone, arguably the most important milestone -
0:53:33 > 0:53:37the emergence of the complex self, the eukaryote.
0:53:37 > 0:53:41And then, you have to step back a long way in time.
0:53:42 > 0:53:47You have to step back all the way to here,
0:53:47 > 0:53:51the emergence of the prokaryote, the first life form.
0:53:51 > 0:53:55And so, we have this beautiful long line.
0:53:55 > 0:53:59We can trace my friend, the horse, and his ancestry
0:53:59 > 0:54:06back to the events that happened 3.5, 3.6, 3.7 billion years ago
0:54:06 > 0:54:08on the primordial Earth.
0:54:15 > 0:54:18Looking back over that vast sweep of time,
0:54:18 > 0:54:24you could ask yourself the question, well, do you need 3.5 billion years
0:54:24 > 0:54:29to go from a simple form of life to something as complex as a horse?
0:54:31 > 0:54:35Well, the answer to that question is, we don't know for sure.
0:54:35 > 0:54:40It seems that you need vast expanses of time, but do you need
0:54:40 > 0:54:44those big gaps from the simple cell to the complex cell,
0:54:44 > 0:54:47do you need the gap from the complex cell
0:54:47 > 0:54:50to the evolution of multicellular life?
0:54:50 > 0:54:51We don't know.
0:54:53 > 0:54:55We only have one example.
0:54:55 > 0:54:58There is only one planet where we've been able to study
0:54:58 > 0:55:01the evolution of life, and it's this one.
0:55:02 > 0:55:07And Earth has been an interesting mixture of stability and upheaval.
0:55:07 > 0:55:09It's had an environment
0:55:09 > 0:55:13that's never completely conspired to wipe out life,
0:55:13 > 0:55:16but it's constantly thrown it challenges.
0:55:19 > 0:55:22The deep time that our planet has given life
0:55:22 > 0:55:27has allowed it to grow from a tiny seed of genetic possibility
0:55:27 > 0:55:32to the planet-wide web of complexity we are part of today.
0:55:42 > 0:55:46Only a few of us have ever stepped outside of this world.
0:55:47 > 0:55:51But those that have discovered something rather wonderful.
0:55:53 > 0:55:57'For all the people back on Earth,
0:55:57 > 0:56:01'the crew of Apollo 8 has a message that we would like to send to you.'
0:56:01 > 0:56:06On Christmas Eve 1968, my first Christmas Eve,
0:56:06 > 0:56:09the Apollo 8 spacecraft entered the darkness
0:56:09 > 0:56:11on the far side of the moon.
0:56:11 > 0:56:16'In the beginning, God created the heaven and the earth.
0:56:16 > 0:56:18'And the earth was without form.'
0:56:18 > 0:56:22The three astronauts, Borman, Lovell and Anders,
0:56:22 > 0:56:24became the first human beings in history
0:56:24 > 0:56:27to lose sight of the Earth.
0:56:27 > 0:56:30'And God said, let there be light.
0:56:30 > 0:56:36'And there was light. And God saw the light, that it was good.'
0:56:37 > 0:56:39When they emerged from the dark side of the moon,
0:56:39 > 0:56:43and the Earth rose into view, they chose to broadcast
0:56:43 > 0:56:47their culture's creation story back to the inhabitants of Earth.
0:56:47 > 0:56:50And, just like the Aztecs and the Mayans
0:56:50 > 0:56:53and every civilisation before them,
0:56:53 > 0:56:56it told of the origins of their home.
0:56:56 > 0:56:59'And God called the dry land Earth,
0:56:59 > 0:57:03'and the gathering together of the waters called He seas.
0:57:03 > 0:57:06'And God saw that it was good.'
0:57:06 > 0:57:13It must be innately human, the desire to understand how our home
0:57:13 > 0:57:15came to be the way that it is.
0:57:15 > 0:57:19And seen from lunar orbit against the blackness of space,
0:57:19 > 0:57:22the Earth is a fragile world,
0:57:22 > 0:57:24but seen by science, it's a world
0:57:24 > 0:57:29that's been crafted and shaped by life over almost four billion years.
0:57:31 > 0:57:33So we're on our way to understanding
0:57:33 > 0:57:37how we came to be here, but as the Apollo astronauts discovered,
0:57:37 > 0:57:40the journey of discovery has already delivered much more
0:57:40 > 0:57:42than just the facts, because it's given us
0:57:42 > 0:57:47a powerful perspective on the intricacy and beauty of our home.
0:57:49 > 0:57:54'From the crew of Apollo 8, we close with good night, good luck,
0:57:54 > 0:57:58'a merry Christmas, and God bless all of you,
0:57:58 > 0:58:01'all of you on the good Earth.'
0:58:08 > 0:58:12Subtitles by Red Bee Media Ltd