1 00:00:07,960 --> 00:00:10,200 This creature... 2 00:00:10,200 --> 00:00:12,800 is a wonder of nature. 3 00:00:12,800 --> 00:00:14,240 BIRDSONG 4 00:00:16,200 --> 00:00:21,360 Its biology is hard-wired to the heavens. 5 00:00:21,360 --> 00:00:24,000 BUZZING 6 00:00:25,200 --> 00:00:28,160 It has an exquisitely sensitive eye 7 00:00:28,160 --> 00:00:29,800 that locks onto the sun 8 00:00:29,800 --> 00:00:34,280 and allows it to navigate its way across the face of the planet. 9 00:00:38,400 --> 00:00:40,040 In a sense, 10 00:00:40,040 --> 00:00:43,960 it has an instinctive understanding of its place in the solar system. 11 00:00:45,120 --> 00:00:47,160 A tiny insect brain 12 00:00:47,160 --> 00:00:51,040 joined to the movements of the sun and the planets. 13 00:00:53,880 --> 00:00:58,520 This connection steers the monarch and millions of its brethren 14 00:00:58,520 --> 00:01:03,800 as they make one of the longest migrations of any butterfly species. 15 00:01:10,320 --> 00:01:13,240 They're heading for these trees known locally as the oyamel, 16 00:01:13,240 --> 00:01:15,000 or sacred firs. 17 00:01:15,000 --> 00:01:19,160 Some of the butterflies began their journey over 4,000 kilometres away, 18 00:01:19,160 --> 00:01:21,240 that's 2,500 miles, 19 00:01:21,240 --> 00:01:24,440 up here in the north-eastern United States and Canada. 20 00:01:24,440 --> 00:01:26,600 And over the autumn and the winter, 21 00:01:26,600 --> 00:01:30,640 they've migrated south across the United States 22 00:01:30,640 --> 00:01:34,600 and arrived here, in central Mexico. 23 00:01:34,600 --> 00:01:38,280 Incredibly, no butterfly has ever learned this route. 24 00:01:38,280 --> 00:01:39,840 It can't have, 25 00:01:39,840 --> 00:01:43,840 because it takes at least three generations to make the round trip. 26 00:01:43,840 --> 00:01:46,680 Instead, the homing instinct is carried 27 00:01:46,680 --> 00:01:51,320 on a river of genetic information that flows through each butterfly. 28 00:01:54,800 --> 00:01:57,440 The allure of this place to the butterflies, 29 00:01:57,440 --> 00:01:59,760 this sense of belonging, 30 00:01:59,760 --> 00:02:02,640 is a deep feeling we all share. 31 00:02:02,640 --> 00:02:06,120 We even have a word for it - home. 32 00:02:11,240 --> 00:02:16,200 Every living thing that we know to exist is found on this one rock. 33 00:02:19,000 --> 00:02:21,600 So, what is it about our planet 34 00:02:21,600 --> 00:02:25,800 that makes it such a rich, colourful, living world? 35 00:02:27,160 --> 00:02:29,160 I want to show you why our world 36 00:02:29,160 --> 00:02:33,800 is the only habitable planet we know of anywhere in the universe. 37 00:02:33,800 --> 00:02:36,800 Now, the answer depends on the presence of a handful 38 00:02:36,800 --> 00:02:40,960 of precious ingredients that make our world a home. 39 00:02:59,800 --> 00:03:01,440 SQUAWKING 40 00:03:10,840 --> 00:03:15,360 'In the beginning, God created the heaven and the earth. 41 00:03:15,360 --> 00:03:19,280 'And the earth was without form and void. 42 00:03:19,280 --> 00:03:22,760 'And darkness was upon the face of the deep.' 43 00:03:24,240 --> 00:03:26,800 SQUAWKING 44 00:03:26,800 --> 00:03:30,080 Home is such an evocative word. 45 00:03:30,080 --> 00:03:32,040 I mean, it will mean something to you. 46 00:03:32,040 --> 00:03:35,160 The place you went to school, the place you live, 47 00:03:35,160 --> 00:03:38,000 the place where your kids had their first Christmas. 48 00:03:38,000 --> 00:03:41,640 But in a scientific sense, what does it mean? 49 00:03:45,120 --> 00:03:49,760 It means...that the ingredients are there for you to live. 50 00:03:49,760 --> 00:03:51,880 An atmosphere, 51 00:03:51,880 --> 00:03:53,280 food, water. 52 00:03:53,280 --> 00:03:56,680 You need the temperature to be right. 53 00:03:58,880 --> 00:04:03,800 Home is the place that has the things you need for your biology 54 00:04:03,800 --> 00:04:06,400 and chemistry to work. 55 00:04:06,400 --> 00:04:08,560 And it's no less evocative for that. 56 00:04:12,320 --> 00:04:14,600 YELLING AND WHINNYING 57 00:04:22,080 --> 00:04:23,360 This is Mexico. 58 00:04:23,360 --> 00:04:27,760 A country rich in the ingredients that set our world apart. 59 00:04:29,200 --> 00:04:31,200 It's not a bad place to come 60 00:04:31,200 --> 00:04:35,440 because, with about 1% of the land surface area of our planet, 61 00:04:35,440 --> 00:04:38,640 it's home to 12% of the species. 62 00:04:38,640 --> 00:04:41,480 There are 26,000 plant species here, 63 00:04:41,480 --> 00:04:43,400 there are 700 species of reptiles 64 00:04:43,400 --> 00:04:45,840 and 400 species of mammals. 65 00:04:45,840 --> 00:04:48,680 It's also been home to some of the world's great civilisations. 66 00:04:48,680 --> 00:04:52,200 The Maya built their temples out there in the forest here 67 00:04:52,200 --> 00:04:55,160 for thousands and thousands of years. 68 00:04:55,160 --> 00:04:57,800 NATIVE SINGING 69 00:04:57,800 --> 00:05:01,080 Mexico is bursting with life. 70 00:05:01,080 --> 00:05:03,760 And if you know where to look, 71 00:05:03,760 --> 00:05:05,760 hidden inside these creatures 72 00:05:05,760 --> 00:05:09,920 are clues that tell how this planet became their home. 73 00:05:15,560 --> 00:05:19,040 First stop is in the southeast of the country. 74 00:05:19,040 --> 00:05:21,240 An area covered in thick jungle. 75 00:05:23,200 --> 00:05:27,040 The Yucatan's a strip of essentially pure limestone 76 00:05:27,040 --> 00:05:30,200 that separates the Caribbean from the Gulf of Mexico. 77 00:05:30,200 --> 00:05:32,960 And it's got all the ingredients you might think you need 78 00:05:32,960 --> 00:05:36,400 for a rich and diverse ecosystem. 79 00:05:37,760 --> 00:05:40,040 The tropical sun warms the forest, 80 00:05:40,040 --> 00:05:43,920 delivering precious energy to each and every leaf. 81 00:05:45,560 --> 00:05:49,120 Oxygen escapes from the plants and trees, 82 00:05:49,120 --> 00:05:52,800 which is breathed in by the forest animals. 83 00:05:56,400 --> 00:05:58,960 And where they can, each of them 84 00:05:58,960 --> 00:06:02,240 draws deeply from the region's hidden water supply. 85 00:06:04,080 --> 00:06:06,880 But there are some of the ingredients you need 86 00:06:06,880 --> 00:06:09,120 to grow this tropical forest 87 00:06:09,120 --> 00:06:11,640 that are far more important than others. 88 00:06:26,160 --> 00:06:29,400 You might think that this place would be awash with water. 89 00:06:29,400 --> 00:06:32,600 It does rain a lot and it's incredibly humid. 90 00:06:32,600 --> 00:06:35,920 But actually, there are no surface rivers at all 91 00:06:35,920 --> 00:06:37,520 on the Yucatan Peninsula 92 00:06:37,520 --> 00:06:41,200 because the water just seeps into the porous limestone. 93 00:06:41,200 --> 00:06:44,720 That's where these things come in. These are cenotes. 94 00:06:44,720 --> 00:06:48,520 They're caverns dissolved out of the limestone by the rain. 95 00:06:48,520 --> 00:06:50,760 And they collect water. 96 00:06:50,760 --> 00:06:53,000 And they play a vital role in the ecosystem. 97 00:06:53,000 --> 00:06:56,640 I mean, the forest changes when you get around a cenote. 98 00:06:56,640 --> 00:06:58,200 Just listen to that. 99 00:06:58,200 --> 00:07:00,920 RIBBITING 100 00:07:00,920 --> 00:07:02,320 Those are frogs. 101 00:07:02,320 --> 00:07:05,200 And you don't hear those frogs anywhere else in the forest, 102 00:07:05,200 --> 00:07:07,680 just around the cenotes. 103 00:07:20,200 --> 00:07:21,800 The cenotes are flooded caves 104 00:07:21,800 --> 00:07:24,680 that have been cut off from the outside world 105 00:07:24,680 --> 00:07:27,120 for thousands of years. 106 00:07:45,760 --> 00:07:49,560 Lilies, troglodytic fish, even the occasional turtle, 107 00:07:49,560 --> 00:07:54,440 all thrive around the openings of these freshwater wells. 108 00:08:01,240 --> 00:08:02,840 As I head deeper into the cave, 109 00:08:02,840 --> 00:08:07,320 the temperature drops and the light fades. 110 00:08:09,960 --> 00:08:15,240 One by one, the ingredients I depend upon begin to disappear. 111 00:08:19,320 --> 00:08:23,360 Yet even here, far from the soil and air, 112 00:08:23,360 --> 00:08:26,840 strangely-coloured algae still find a home in the water. 113 00:08:41,840 --> 00:08:45,520 If there's one thing that unites every form of life in the cenote, 114 00:08:45,520 --> 00:08:48,960 in fact, every form of life out there in the forests, 115 00:08:48,960 --> 00:08:52,200 in fact, every form of life we've ever discovered 116 00:08:52,200 --> 00:08:54,440 anywhere on planet Earth, 117 00:08:54,440 --> 00:08:57,280 it's that it has to be wet. 118 00:09:00,800 --> 00:09:05,560 Only on our home does water run freely between the skies, 119 00:09:05,560 --> 00:09:09,280 oceans, rivers and on, into every living thing. 120 00:09:09,280 --> 00:09:12,040 MARIACHI MUSIC PLAYS 121 00:09:17,400 --> 00:09:19,040 CAR HORN BEEPS 122 00:09:44,400 --> 00:09:46,400 SHE SPEAKS IN NATIVE TONGUE 123 00:09:49,200 --> 00:09:53,360 To understand why life and water are so intertwined, 124 00:09:53,360 --> 00:09:55,400 we need to look a little deeper 125 00:09:55,400 --> 00:09:58,640 into one of the strangest substances we know. 126 00:09:58,640 --> 00:10:01,200 ANIMATED CHATTER 127 00:10:03,120 --> 00:10:05,760 Now, I may be a bit of a middle-aged academic, 128 00:10:05,760 --> 00:10:08,280 but I can still do the odd experiment every now and again. 129 00:10:08,280 --> 00:10:11,240 So what I'm doing is I'm charging up this Perspex rod. 130 00:10:11,240 --> 00:10:14,480 So giving it an electric charge by rubbing it on the fleece. 131 00:10:14,480 --> 00:10:16,800 Now, watch what happens... 132 00:10:16,800 --> 00:10:21,240 when I put the rod next to a stream of water. 133 00:10:21,240 --> 00:10:23,520 You see that? 134 00:10:23,520 --> 00:10:25,040 Look at that. 135 00:10:25,040 --> 00:10:27,080 The electric field, the electric charge, 136 00:10:27,080 --> 00:10:29,600 is bending the water towards it. 137 00:10:29,600 --> 00:10:32,320 Now, the reason for that, 138 00:10:32,320 --> 00:10:34,960 the reason that water behaves in that way 139 00:10:34,960 --> 00:10:37,440 when it's passing through an electric field, 140 00:10:37,440 --> 00:10:41,360 is exactly the same reason that it is vital for all life on Earth. 141 00:10:51,480 --> 00:10:53,280 Water is a polar molecule, 142 00:10:53,280 --> 00:10:56,880 which means it responds to electric charge. 143 00:11:06,000 --> 00:11:07,560 Its polarity comes about 144 00:11:07,560 --> 00:11:11,480 because of the structure of water molecules themselves. 145 00:11:13,000 --> 00:11:15,280 Now, water is H2O, 146 00:11:15,280 --> 00:11:18,440 two hydrogens and one oxygen atom bound together. 147 00:11:18,440 --> 00:11:22,520 So two hydrogen atoms approach oxygen. 148 00:11:22,520 --> 00:11:26,400 Now, oxygen's got a cloud of eight electrons around it, 149 00:11:26,400 --> 00:11:29,000 so when the hydrogens come in, then what happens 150 00:11:29,000 --> 00:11:33,680 is the electrons get dragged over here, around the oxygen. 151 00:11:33,680 --> 00:11:37,560 So you end up with an electron cloud around here and, to some extent, 152 00:11:37,560 --> 00:11:42,240 pretty isolated, positively-charged protons out here. 153 00:11:42,240 --> 00:11:45,880 So you get a net positive charge over here 154 00:11:45,880 --> 00:11:49,840 and the electron cloud with its negative charge over here, 155 00:11:49,840 --> 00:11:53,200 so you get what's called a polar molecule. 156 00:11:53,200 --> 00:11:56,040 And that's why, when you bring a charged Perspex rod 157 00:11:56,040 --> 00:11:59,280 close to water molecules, they bend towards it. 158 00:12:02,960 --> 00:12:04,400 BIRDSONG 159 00:12:13,040 --> 00:12:17,360 Water's polar nature means that although its molecules are simple, 160 00:12:17,360 --> 00:12:21,960 together, they form a subtle, endlessly complex liquid. 161 00:12:23,880 --> 00:12:27,520 A home in which one tiny creature thrives. 162 00:12:56,360 --> 00:12:58,800 There he is. Look at that. 163 00:12:58,800 --> 00:13:01,880 That...is a pond skater. 164 00:13:01,880 --> 00:13:05,120 A predator that floats on the surface of the water 165 00:13:05,120 --> 00:13:09,360 and actually uses the surface of the water to sense its prey. 166 00:13:09,360 --> 00:13:12,840 Pond skaters are vicious predators 167 00:13:12,840 --> 00:13:16,880 that live for most of their lives on the surface. 168 00:13:18,040 --> 00:13:21,160 Tiny hairs on their legs provide a large area 169 00:13:21,160 --> 00:13:23,240 that spreads their weight. 170 00:13:25,800 --> 00:13:29,080 Their middle legs thrust them forward. 171 00:13:29,080 --> 00:13:32,120 Hind legs are employed to steer. 172 00:13:37,440 --> 00:13:40,240 They're so well adapted to life in this flat world 173 00:13:40,240 --> 00:13:42,880 that they even sense their sexual partners 174 00:13:42,880 --> 00:13:47,160 through tiny vibrations in the water's surface. 175 00:13:51,440 --> 00:13:55,280 The reason it can do that is the result of a complex interaction 176 00:13:55,280 --> 00:13:58,440 between adaptions in the animal itself 177 00:13:58,440 --> 00:14:01,360 and the physics and the chemistry 178 00:14:01,360 --> 00:14:03,120 of the surface of water. 179 00:14:06,040 --> 00:14:08,320 Water molecules are polar. 180 00:14:08,320 --> 00:14:13,760 And that means that water molecules themselves can bond together. 181 00:14:13,760 --> 00:14:17,440 So you can get a hydrogen with its slight positive charge 182 00:14:17,440 --> 00:14:21,080 getting close to the oxygen of another water molecule 183 00:14:21,080 --> 00:14:25,240 with its slight negative charge and bonding to it. 184 00:14:25,240 --> 00:14:27,280 You can build up quite large, 185 00:14:27,280 --> 00:14:30,600 in fact, VERY large structures in liquid water. 186 00:14:35,240 --> 00:14:38,200 This is what gives water its unique ability 187 00:14:38,200 --> 00:14:41,920 to form a surface habitat for the pond skaters. 188 00:14:43,240 --> 00:14:45,840 Clumps of H2O stick together, 189 00:14:45,840 --> 00:14:48,480 keeping the surface under tension. 190 00:14:50,240 --> 00:14:53,160 Forming a chorus of water molecules, 191 00:14:53,160 --> 00:14:56,120 all joined together by hydrogen bonds. 192 00:15:00,560 --> 00:15:05,920 Then a pond skater comes along and it puts its legs or its... 193 00:15:05,920 --> 00:15:09,960 dangly things into the water and pushes it down, 194 00:15:09,960 --> 00:15:12,280 bends the surface of the water. 195 00:15:12,280 --> 00:15:14,320 Now, the water doesn't like that 196 00:15:14,320 --> 00:15:17,960 because a bend in the water is increasing its surface area. 197 00:15:17,960 --> 00:15:19,760 It's increasing its energy. 198 00:15:19,760 --> 00:15:23,160 It's making it harder for all the molecules to bond together 199 00:15:23,160 --> 00:15:25,920 with the hydrogen bonds. So they try to push back. 200 00:15:25,920 --> 00:15:29,120 They exert a force on the pond skater's leg 201 00:15:29,120 --> 00:15:32,200 because they want to bond as much as they can. 202 00:15:32,200 --> 00:15:35,720 And that's how pond skaters stay on the surface of the water. 203 00:15:40,720 --> 00:15:42,520 Hydrogen bonds do far more 204 00:15:42,520 --> 00:15:45,320 than just give the pond skaters a place to live. 205 00:15:46,560 --> 00:15:49,960 They're fundamental to all life. 206 00:15:53,720 --> 00:15:57,600 I've heard it said that we won't truly understand biology 207 00:15:57,600 --> 00:16:00,320 until we understand water. 208 00:16:06,080 --> 00:16:11,960 These are...very thin tubes of glass. 209 00:16:11,960 --> 00:16:16,040 They're about a millimetre in diameter. 210 00:16:16,040 --> 00:16:20,520 And if I dip one into the surface of this river... 211 00:16:22,760 --> 00:16:27,040 ..can you see that the water just climbs up the tube? 212 00:16:27,040 --> 00:16:31,080 It pulls itself up, quite literally, against the force of gravity. 213 00:16:31,080 --> 00:16:35,760 Now, in trees, there are tubes which are about half the diameter of this, 214 00:16:35,760 --> 00:16:38,600 perhaps about half a millimetre or even less. 215 00:16:38,600 --> 00:16:41,040 And they are called xylem. 216 00:16:41,040 --> 00:16:45,480 And they allow the tree to lift water up through the root system 217 00:16:45,480 --> 00:16:48,680 because the water molecules strongly attract each other 218 00:16:48,680 --> 00:16:52,200 and are strongly attracted to the sides of the tubes. 219 00:16:52,200 --> 00:16:56,640 So when you look at trees like that, which are very high, 220 00:16:56,640 --> 00:16:58,480 and you ask yourself the question, 221 00:16:58,480 --> 00:17:02,000 "How do they get the water from the roots to the top of the tree?", 222 00:17:02,000 --> 00:17:04,600 a big part of that is capillary action, 223 00:17:04,600 --> 00:17:07,960 which is down to the polar nature of water. 224 00:17:13,160 --> 00:17:15,960 One of water's most important qualities 225 00:17:15,960 --> 00:17:18,600 is its ability to dissolve and carry 226 00:17:18,600 --> 00:17:21,680 all manner of substances around the living world. 227 00:17:24,880 --> 00:17:28,560 Because its molecules are very small and polar, 228 00:17:28,560 --> 00:17:32,280 water is a tremendously effective solvent. 229 00:17:32,280 --> 00:17:36,080 Those molecules can get in amongst other substances, 230 00:17:36,080 --> 00:17:40,280 salts and sugars, for example, and disperse them, if you like, 231 00:17:40,280 --> 00:17:42,960 in that sea of hydrogen bonds. 232 00:17:44,680 --> 00:17:46,520 Within every one of us, 233 00:17:46,520 --> 00:17:50,880 water is constantly flowing around each and every cell. 234 00:17:53,960 --> 00:17:57,000 Blood plasma is over 90% water. 235 00:17:57,000 --> 00:18:00,480 And in it are dissolved everything I need to live - 236 00:18:00,480 --> 00:18:04,320 oxygen, the nutrients from food, everything - 237 00:18:04,320 --> 00:18:09,520 distributed around my body in rivers of water. 238 00:18:13,040 --> 00:18:16,880 We live on a beautiful blue anomaly of a world. 239 00:18:16,880 --> 00:18:22,920 The only planet we know with a surface drenched in liquid water. 240 00:18:28,720 --> 00:18:33,040 The story of how each drop ended up here has been hard to fathom. 241 00:18:35,280 --> 00:18:37,600 Largely because it happened so long ago, 242 00:18:37,600 --> 00:18:39,680 there's very little direct evidence. 243 00:18:47,360 --> 00:18:49,600 But back in the Yucatan jungle, 244 00:18:49,600 --> 00:18:53,040 clues to how it turned up can still be found. 245 00:18:55,680 --> 00:18:57,880 Every civilisation on the Yucatan, 246 00:18:57,880 --> 00:19:00,760 be it the modern Mexicans or the Mayans, 247 00:19:00,760 --> 00:19:04,640 had to get their water from those deep wells, the cenotes. 248 00:19:04,640 --> 00:19:09,120 And I've got a completely unbiased map of the larger cenotes here, 249 00:19:09,120 --> 00:19:11,960 which I'm going to overlay on the Yucatan. 250 00:19:16,600 --> 00:19:20,720 Look at that. They lie in a perfect arc, 251 00:19:20,720 --> 00:19:24,080 centred around a very particular village, 252 00:19:24,080 --> 00:19:27,600 which is...there, 253 00:19:27,600 --> 00:19:30,440 and it's called Chicxulub. 254 00:19:30,440 --> 00:19:34,480 Now, to a geologist, there are very few natural events 255 00:19:34,480 --> 00:19:40,120 that can create a structure, such a perfect arc as that. 256 00:19:43,720 --> 00:19:47,600 All the evidence points to just one explanation. 257 00:19:51,200 --> 00:19:55,200 You're looking at what's left of a gigantic asteroid strike. 258 00:19:58,080 --> 00:20:02,920 One that wiped out three-quarters of all plant and animal species 259 00:20:02,920 --> 00:20:07,200 when it hit the Earth 65 million years ago. 260 00:20:07,200 --> 00:20:10,240 You may think that impacts from space are a thing of the past. 261 00:20:10,240 --> 00:20:14,680 A thing that only happened to the dinosaurs, but that's not true. 262 00:20:14,680 --> 00:20:19,400 About 55 million kilograms of rock hits the Earth every year. 263 00:20:19,400 --> 00:20:22,640 And around 2% of that is water. 264 00:20:24,080 --> 00:20:29,320 This hints that at least some of Earth's water arrived from space. 265 00:20:33,080 --> 00:20:37,520 Late in 2010, these glimpses of comet Hartley 2 266 00:20:37,520 --> 00:20:39,080 arrived back on Earth. 267 00:20:40,800 --> 00:20:44,760 They were sent by NASA's deep-impact probe. 268 00:20:44,760 --> 00:20:48,840 From its surface, dust and ice spray into space. 269 00:20:50,880 --> 00:20:55,400 Analysis of this water found it had a very similar mixture of isotopes 270 00:20:55,400 --> 00:20:57,640 to the water in our own oceans. 271 00:21:01,400 --> 00:21:03,240 This was the first firm evidence 272 00:21:03,240 --> 00:21:05,680 that icy comets must have contributed 273 00:21:05,680 --> 00:21:08,320 to the formation of our world's oceans. 274 00:21:24,800 --> 00:21:27,760 Earth began life as a molten hell. 275 00:21:28,880 --> 00:21:33,120 Its internal heat drove off any trace of moisture. 276 00:21:35,760 --> 00:21:40,320 But soon, the planet cooled and the first clouds grew. 277 00:21:42,560 --> 00:21:46,080 Then, 4.2 billion years ago, 278 00:21:46,080 --> 00:21:48,760 a deluge, the like of which the solar system 279 00:21:48,760 --> 00:21:52,880 had never seen before or since, rained down. 280 00:21:52,880 --> 00:21:54,440 THUNDERCLAP 281 00:22:13,120 --> 00:22:16,200 And again, thanks to those hydrogen bonds, 282 00:22:16,200 --> 00:22:20,680 water's boiling point is high enough to have allowed it to remain 283 00:22:20,680 --> 00:22:24,720 on the surface of the Earth to the present day. 284 00:22:24,720 --> 00:22:27,360 So from quite early in its history, 285 00:22:27,360 --> 00:22:32,040 our home has been able to hang on to this most vital of ingredients. 286 00:22:32,040 --> 00:22:34,880 But to trace the origin of the next ingredients, 287 00:22:34,880 --> 00:22:37,640 you have to look beyond our planet... 288 00:22:39,520 --> 00:22:41,560 ..to our nearest star. 289 00:22:43,920 --> 00:22:46,760 And the rays of light it sends our way. 290 00:22:48,440 --> 00:22:50,720 This is the train from Los Mochis to Chihuahua, 291 00:22:50,720 --> 00:22:53,560 which inexplicably leaves at 6:00am in the morning. 292 00:22:53,560 --> 00:22:57,360 Um...the local name for this area in all the guidebooks 293 00:22:57,360 --> 00:22:59,440 is the Land of Turtles. 294 00:22:59,440 --> 00:23:03,240 Beautifully romantic name for this place on the Sea of Cortez. 295 00:23:03,240 --> 00:23:06,360 But we just found out it's probably more likely to have been called 296 00:23:06,360 --> 00:23:09,200 the Land of Spinach-type Vegetables. 297 00:23:09,200 --> 00:23:12,320 So we're going from the Land of Spinach-type Vegetables 298 00:23:12,320 --> 00:23:14,680 to Chihuahua, 299 00:23:14,680 --> 00:23:17,520 which is the Land of Very Small Dogs. 300 00:23:18,840 --> 00:23:21,680 One of the great railway journeys of the world. 301 00:23:30,440 --> 00:23:32,000 TRAIN HOOTS 302 00:23:36,720 --> 00:23:41,000 Almost all life depends on the energy that the sun sends our way. 303 00:23:42,720 --> 00:23:46,320 But the sun is a far-from-benevolent companion 304 00:23:46,320 --> 00:23:51,760 because its radiant rain can be as dangerous as it is nourishing. 305 00:24:04,240 --> 00:24:06,240 We're still round about sea level now 306 00:24:06,240 --> 00:24:08,120 and the sun is quite low in the sky. 307 00:24:08,120 --> 00:24:10,960 It's about 7:00am, so it's not been up long. 308 00:24:10,960 --> 00:24:13,240 I'm going to measure the amount of UV radiation 309 00:24:13,240 --> 00:24:16,200 falling on every square centimetre with this, 310 00:24:16,200 --> 00:24:19,480 a digital, ultraviolet radiometer. 311 00:24:22,640 --> 00:24:27,120 At the moment, it says there's about 22 microwatts 312 00:24:27,120 --> 00:24:31,040 per square centimetre falling on my skin. 313 00:24:31,040 --> 00:24:34,520 But as we climb in altitude, then that UVB light 314 00:24:34,520 --> 00:24:37,760 is going to have to travel through less and less of the atmosphere, 315 00:24:37,760 --> 00:24:40,080 so less of it is going to be absorbed. 316 00:24:46,280 --> 00:24:48,520 And sure enough, as the miles pass by 317 00:24:48,520 --> 00:24:51,160 and we head into the mountainous interior, 318 00:24:51,160 --> 00:24:53,840 the meter readings start to go up. 319 00:25:19,080 --> 00:25:22,880 Now it's about 10:00am, so the sun's significantly higher in the sky. 320 00:25:22,880 --> 00:25:25,880 The train's also climbed quite a bit in altitude. 321 00:25:25,880 --> 00:25:28,040 Now... 322 00:25:29,960 --> 00:25:33,400 ..we're getting nearly 250 microwatts per square centimetre. 323 00:25:33,400 --> 00:25:35,680 So that's about a factor of ten higher. 324 00:25:35,680 --> 00:25:40,360 And that's just because the UVB has had significantly less atmosphere 325 00:25:40,360 --> 00:25:44,440 to travel through, from the top of the Earth's atmosphere down to me. 326 00:25:50,080 --> 00:25:53,120 That's more than enough to burn unprotected skin 327 00:25:53,120 --> 00:25:54,400 in just a few minutes. 328 00:25:55,960 --> 00:25:58,200 And that's because what arrived from the sun 329 00:25:58,200 --> 00:26:01,920 is far more than just the stuff we can see. 330 00:26:05,920 --> 00:26:09,760 Beyond the visible, the higher energy part of the spectrum, 331 00:26:09,760 --> 00:26:13,080 there's ultraviolet light, particularly UVB, 332 00:26:13,080 --> 00:26:17,000 which does get through the Earth's atmosphere and gets to the surface. 333 00:26:17,000 --> 00:26:20,520 Now, UVB can be beneficial to life. 334 00:26:20,520 --> 00:26:23,960 We use it to produce vitamin D, for example. 335 00:26:23,960 --> 00:26:27,360 But because it's higher energy, it can also be extremely damaging. 336 00:26:27,360 --> 00:26:31,880 It can damage DNA, it can burn our skin as well as give us a suntan, 337 00:26:31,880 --> 00:26:35,360 and, of course, ultimately, it can give us skin cancer. 338 00:26:35,360 --> 00:26:38,200 WHISTLE HOOTS 339 00:26:38,200 --> 00:26:40,040 If ultraviolet light is a problem 340 00:26:40,040 --> 00:26:42,000 for life on Earth to deal with today, 341 00:26:42,000 --> 00:26:43,920 then the physicists might raise 342 00:26:43,920 --> 00:26:46,520 an interesting problem for the biologists. 343 00:26:46,520 --> 00:26:49,560 Because we know that 3.5 billion years ago, 344 00:26:49,560 --> 00:26:51,280 when life on Earth began, 345 00:26:51,280 --> 00:26:55,680 although the sun was much dimmer in the visible part of the spectrum, 346 00:26:55,680 --> 00:26:59,280 it was significantly brighter in the ultraviolet. 347 00:27:04,040 --> 00:27:06,680 The young sun seems like a paradox. 348 00:27:07,840 --> 00:27:09,680 It was fainter to the eye, 349 00:27:09,680 --> 00:27:13,920 perhaps 30% less bright than the sun we enjoy today, 350 00:27:13,920 --> 00:27:16,600 yet rich in deadly ultraviolet. 351 00:27:18,240 --> 00:27:22,240 Inside, the core was spinning much faster, 352 00:27:22,240 --> 00:27:25,120 which created more electromagnetic heating 353 00:27:25,120 --> 00:27:26,960 of the plasma on its surface. 354 00:27:29,600 --> 00:27:32,440 And this plasma emitted more energy, 355 00:27:32,440 --> 00:27:34,640 not in the lower visible frequencies, 356 00:27:34,640 --> 00:27:37,080 but in the higher frequencies. 357 00:27:38,520 --> 00:27:40,400 Like X-rays... 358 00:27:40,400 --> 00:27:42,640 and ultraviolet. 359 00:27:48,160 --> 00:27:52,840 It seems as if just as life was getting settled on its wet home, 360 00:27:52,840 --> 00:27:58,600 the faint young sun was making it tough to survive near the surface. 361 00:28:10,040 --> 00:28:13,880 This is the top of Copper Canyon, so the summit of the railway journey. 362 00:28:13,880 --> 00:28:16,960 It's about 2,200 metres, which is about... 363 00:28:16,960 --> 00:28:19,600 somewhere between 7,000 and 8,000 feet. 364 00:28:20,960 --> 00:28:24,360 So I'll take a UV reading of the sun. 365 00:28:24,360 --> 00:28:27,280 It's actually reading about 260 now. 366 00:28:27,280 --> 00:28:30,280 Now, if you remember, at midday, down at sea level, 367 00:28:30,280 --> 00:28:32,960 we were getting readings around 260. 368 00:28:32,960 --> 00:28:35,800 So although the sun has dropped in the sky, 369 00:28:35,800 --> 00:28:39,280 so the sunlight and the UV are coming through much more atmosphere, 370 00:28:39,280 --> 00:28:43,160 that's been compensated for by the thinness of the air up here. 371 00:28:43,160 --> 00:28:45,440 I'm getting more UV now than I would have been 372 00:28:45,440 --> 00:28:47,720 at the same time of day at sea level. 373 00:28:51,240 --> 00:28:52,680 It's hard to be sure, 374 00:28:52,680 --> 00:28:56,520 but we think that it's these kinds of radiation levels 375 00:28:56,520 --> 00:28:59,000 that early life had to deal with. 376 00:28:59,000 --> 00:29:02,640 Because back then, the sun's ultraviolet output 377 00:29:02,640 --> 00:29:04,880 was significantly stronger. 378 00:29:09,120 --> 00:29:11,440 So I think it is fair to say 379 00:29:11,440 --> 00:29:14,400 that that could have posed a significant threat 380 00:29:14,400 --> 00:29:17,080 to the development of early life on Earth. 381 00:29:17,080 --> 00:29:19,480 WHINNYING 382 00:29:19,480 --> 00:29:22,120 ANIMATED CHATTER 383 00:29:24,640 --> 00:29:28,000 Today, life has painted the surface of our home 384 00:29:28,000 --> 00:29:30,600 in all the colours of the rainbow. 385 00:29:32,480 --> 00:29:35,240 From greens to blues, 386 00:29:35,240 --> 00:29:37,040 reds to yellows, 387 00:29:37,040 --> 00:29:39,320 oranges and violets. 388 00:29:41,760 --> 00:29:45,560 And the origin of all life's hues can be traced back 389 00:29:45,560 --> 00:29:48,680 to the way it interacts with sunlight. 390 00:29:51,120 --> 00:29:54,040 I'm a particle physicist, so I'm allowed to think of everything 391 00:29:54,040 --> 00:29:57,640 in terms of the interactions of particles. 392 00:29:57,640 --> 00:29:59,920 So I would picture the light from the sun 393 00:29:59,920 --> 00:30:03,600 as being really a rain of particles. 394 00:30:03,600 --> 00:30:06,440 Photons, they're called, particles of light 395 00:30:06,440 --> 00:30:10,080 of different energies, raining down on the surface of the Earth. 396 00:30:10,080 --> 00:30:13,200 The blue ones are the highest-energy photons, 397 00:30:13,200 --> 00:30:15,760 the red ones are the lowest-energy photons 398 00:30:15,760 --> 00:30:17,800 and all the colours of the rainbow in the middle 399 00:30:17,800 --> 00:30:20,840 are just simply photons of different energies. 400 00:30:20,840 --> 00:30:25,280 SHE SPEAKS IN NATIVE TONGUE Oh, thank you. 401 00:30:25,280 --> 00:30:26,520 Wow. 402 00:30:27,760 --> 00:30:32,720 For this, the chilli salsa which I see as red, there are pigment 403 00:30:32,720 --> 00:30:36,040 molecules in there that are absorbing the blue photons, 404 00:30:36,040 --> 00:30:37,280 the blue light from the sun. 405 00:30:37,280 --> 00:30:39,520 The red ones, it doesn't interact with, 406 00:30:39,520 --> 00:30:43,680 so they bounce back into my eye, and that is why I see it as red. 407 00:30:43,680 --> 00:30:45,280 The same with the green chilli, 408 00:30:45,280 --> 00:30:49,480 but in this case the red photons are interacting, doing something, 409 00:30:49,480 --> 00:30:51,480 talking to pigments in here, 410 00:30:51,480 --> 00:30:56,000 and what I am seeing are the green photons and some of the blue photons 411 00:30:56,000 --> 00:30:59,640 coming into my eye, mixing up, allowing me to see that as green. 412 00:31:03,280 --> 00:31:06,520 Pigments bring colour to the world. 413 00:31:06,520 --> 00:31:08,880 The planet is painted by genes, 414 00:31:08,880 --> 00:31:12,000 honed by billions of years of evolution. 415 00:31:16,480 --> 00:31:18,720 'Some colours warn of danger...' 416 00:31:18,720 --> 00:31:20,360 This stuff is on fire, I tell you! 417 00:31:24,000 --> 00:31:25,360 '..or attract pollinators.' 418 00:31:42,800 --> 00:31:45,720 Pigments are one of the ways that life has evolved 419 00:31:45,720 --> 00:31:48,600 to take on the sun's powerful ultraviolet light. 420 00:32:18,040 --> 00:32:21,680 This little guy is called a bombardier beetle. 421 00:32:21,680 --> 00:32:24,040 If I just grab him... 422 00:32:29,640 --> 00:32:32,440 His name comes from his unique defence mechanism. 423 00:32:33,680 --> 00:32:37,000 He produces two chemicals. One of them you might have heard of - 424 00:32:37,000 --> 00:32:41,000 hydrogen peroxide. The other one is something called hydroquinone, 425 00:32:41,000 --> 00:32:43,000 and when you scare him, 426 00:32:43,000 --> 00:32:46,920 both those chemicals are injected into a little chamber in his body. 427 00:32:48,320 --> 00:32:51,560 It raises the temperature to the boiling point of water, 428 00:32:51,560 --> 00:32:53,400 and increases the pressure, 429 00:32:53,400 --> 00:32:56,920 squirting a hot and noxious chemical out of its rear. 430 00:33:00,280 --> 00:33:02,120 A clever way to defend yourself. 431 00:33:04,320 --> 00:33:07,800 But this is just one of the ways this character uses chemistry 432 00:33:07,800 --> 00:33:10,800 to increase the chance of survival. 433 00:33:13,400 --> 00:33:15,120 The bombardier beetle and me, 434 00:33:15,120 --> 00:33:18,960 and in fact every living thing you can see, are exposed to 435 00:33:18,960 --> 00:33:22,960 the same threat on the high plains of Mexico, the high-energy 436 00:33:22,960 --> 00:33:26,960 ultraviolet photons raining down on this landscape from the sun. 437 00:33:30,320 --> 00:33:34,600 If they hit DNA in my skin, for example, they damage the DNA. 438 00:33:34,600 --> 00:33:36,040 So that must be prevented. 439 00:33:38,960 --> 00:33:42,560 Me and my friend, the beetle, have both reached the same solution - 440 00:33:42,560 --> 00:33:46,600 you see that the beetle is brown and black. 441 00:33:46,600 --> 00:33:49,640 My skin, when it is exposed to the sun, is going brown. 442 00:33:49,640 --> 00:33:54,960 I am producing a pigment called melanin, and so is the beetle. 443 00:33:54,960 --> 00:33:57,000 Melanin is a very simple molecule, 444 00:33:57,000 --> 00:34:00,600 it's just a ring of carbon atoms with a few extra bits bolted on, 445 00:34:00,600 --> 00:34:05,480 but the sea of electrons behaves in a very specific way. 446 00:34:05,480 --> 00:34:08,440 When a high-energy ultraviolet photon from the sun 447 00:34:08,440 --> 00:34:13,520 hits one of those electrons, it very quickly dissipates that energy. 448 00:34:13,520 --> 00:34:16,880 That potentially threatening photon has been absorbed 449 00:34:16,880 --> 00:34:20,920 and all its energy has been dissipated away as heat. 450 00:34:24,040 --> 00:34:25,800 Melanin is so efficient, 451 00:34:25,800 --> 00:34:31,320 over 99.9% of the harmful ultraviolet radiation is absorbed. 452 00:34:33,800 --> 00:34:36,280 So melanin is protecting 453 00:34:36,280 --> 00:34:40,920 both my skin and my friend, the bombardier beetle, 454 00:34:40,920 --> 00:34:43,640 from the potentially harmful effects of the sun. 455 00:35:05,000 --> 00:35:06,240 From the start, 456 00:35:06,240 --> 00:35:10,880 life had to evolve strategies for coping with the energetic young sun. 457 00:35:15,360 --> 00:35:17,440 Life is nothing if not resourceful. 458 00:35:17,440 --> 00:35:21,160 Pigments are the way that living things interact with 459 00:35:21,160 --> 00:35:27,040 the radiation from the sun. So why just use them to dissipate energy, 460 00:35:27,040 --> 00:35:28,400 to protect? 461 00:35:28,400 --> 00:35:32,000 Why not use them to harness that energy for its own ends? 462 00:35:32,000 --> 00:35:34,240 That is exactly what life did. 463 00:35:38,920 --> 00:35:44,000 In doing so, it transformed our planet by introducing 464 00:35:44,000 --> 00:35:46,560 a wonderful new ingredient. 465 00:35:57,000 --> 00:36:00,240 Earth has an atmosphere unlike any other planet 466 00:36:00,240 --> 00:36:02,000 we know of in the universe. 467 00:36:07,760 --> 00:36:12,240 Only in the air on our world do fires burn. 468 00:36:16,320 --> 00:36:20,720 Only on our world has a gas been released which allowed 469 00:36:20,720 --> 00:36:23,120 complex life to evolve. 470 00:36:30,480 --> 00:36:35,600 What makes our home unique is its oxygen-rich atmosphere. 471 00:36:41,880 --> 00:36:46,240 Deep in a cave in the hills of Tabasco, you can find a hint 472 00:36:46,240 --> 00:36:50,000 of what living planet without oxygen might be like. 473 00:37:01,320 --> 00:37:04,640 This is one of the more unique environments on our planet. 474 00:37:06,760 --> 00:37:11,440 This cave is full of sulphur, you can see it in the water. 475 00:37:11,440 --> 00:37:15,000 You can see that milky colour flowing through the cave. 476 00:37:15,000 --> 00:37:17,040 That is dissolved sulphur. 477 00:37:17,040 --> 00:37:19,680 It is coming from hydrogen-sulphide gas, 478 00:37:19,680 --> 00:37:23,080 the source of which is actually not entirely known. 479 00:37:27,200 --> 00:37:30,440 The hydrogen sulphide is toxic to me. 480 00:37:30,440 --> 00:37:34,480 It has another rather alarming effect on this hellhole. 481 00:37:36,840 --> 00:37:38,560 It is a bad-smelling gas, 482 00:37:38,560 --> 00:37:41,600 but it is also a gas that drives the oxygen out, 483 00:37:41,600 --> 00:37:45,240 so as you go on into the cave, you get less and less oxygen. 484 00:37:49,760 --> 00:37:52,800 In a sense, some of the chemistry, 485 00:37:52,800 --> 00:37:57,640 the biochemistry that takes place in the dark of this cave system, 486 00:37:57,640 --> 00:38:01,320 could be very similar to the chemistry 487 00:38:01,320 --> 00:38:04,960 and biochemistry that occurred when our planet was very young. 488 00:38:07,800 --> 00:38:09,880 For the first half of its history, 489 00:38:09,880 --> 00:38:12,440 Earth was without oxygen in the atmosphere. 490 00:38:16,920 --> 00:38:20,400 But incredibly, in this echo of the past, which I can only visit 491 00:38:20,400 --> 00:38:24,880 for a few minutes, there are forms of life that are completely at home. 492 00:38:27,080 --> 00:38:29,120 Look at that! 493 00:38:31,120 --> 00:38:34,040 There they are, cities of sulphur-eating bacteria 494 00:38:34,040 --> 00:38:36,720 living off the hydrogen-sulphide gas. 495 00:38:43,240 --> 00:38:45,760 Colonies of extremophiles, 496 00:38:45,760 --> 00:38:50,880 organisms living off a very different environment of gases 497 00:38:50,880 --> 00:38:53,680 to the one that we are used to on the surface. 498 00:38:58,640 --> 00:39:01,680 They are a window on a much earlier time. 499 00:39:06,280 --> 00:39:10,600 Because without oxygen, the ancestors of these extremophiles 500 00:39:10,600 --> 00:39:14,160 were the only forms of life our planet could support. 501 00:39:30,640 --> 00:39:32,720 Understanding how Earth developed 502 00:39:32,720 --> 00:39:36,360 an atmosphere rich in oxygen has taken centuries. 503 00:39:38,400 --> 00:39:41,440 The secret lies with ancient bacteria. 504 00:39:54,680 --> 00:39:59,640 In 1676, a Dutchman called Antonie Leeuwenhoek 505 00:39:59,640 --> 00:40:04,320 was trying to find out why pepper is spicy. 506 00:40:04,320 --> 00:40:07,800 See, they thought that there were little spikes on peppercorns 507 00:40:07,800 --> 00:40:10,080 that dug into your tongue. 508 00:40:10,080 --> 00:40:11,920 He was using the microscope, 509 00:40:11,920 --> 00:40:14,640 which had been discovered about 60 years before, 510 00:40:14,640 --> 00:40:18,440 but inexplicably, had never been used for anything useful before. 511 00:40:18,440 --> 00:40:21,680 He put the peppercorns on there and looked down and he couldn't see anything, 512 00:40:21,680 --> 00:40:23,640 so he thought he would grind them up, 513 00:40:23,640 --> 00:40:26,960 dissolve them in water and have a look. When he did that, 514 00:40:26,960 --> 00:40:29,800 he didn't see anything interesting in the peppercorns, 515 00:40:29,800 --> 00:40:34,200 but he found that there were little animals swimming around. 516 00:40:34,200 --> 00:40:35,960 He said that he estimated 517 00:40:35,960 --> 00:40:39,120 you could line about 100 of the "wee little creatures" - 518 00:40:39,120 --> 00:40:44,000 those are his words - on the length of a single coarse sand grain. 519 00:40:46,040 --> 00:40:49,480 What Leeuwenhoek thought were animals were, in all probability, 520 00:40:49,480 --> 00:40:51,320 not animals at all. 521 00:40:53,360 --> 00:40:55,800 Although he didn't know it at the time, 522 00:40:55,800 --> 00:40:59,160 he had discovered a whole new domain of life. 523 00:41:03,720 --> 00:41:05,760 Bacteria. 524 00:41:13,480 --> 00:41:17,120 They are by far the most numerous organisms on the Earth. 525 00:41:18,560 --> 00:41:22,000 In fact, there are more bacteria on our planet than 526 00:41:22,000 --> 00:41:25,800 there are stars in the observable universe. 527 00:41:28,920 --> 00:41:34,240 And there is one kind of bacteria more numerous than all the rest. 528 00:41:38,440 --> 00:41:41,080 One of the most striking structures I can see on this slide is 529 00:41:41,080 --> 00:41:45,960 a kind of blue-green filament which is a little colony 530 00:41:45,960 --> 00:41:49,840 of a type of bacteria called cyanobacteria. 531 00:41:53,880 --> 00:41:57,720 These things are incredibly important organisms. 532 00:42:03,320 --> 00:42:07,400 Fossilised cyanobacteria had been found as far back 533 00:42:07,400 --> 00:42:09,720 as 3.5 billion years ago. 534 00:42:12,800 --> 00:42:17,240 And at some point, around 2.4 billion years ago, 535 00:42:17,240 --> 00:42:21,040 they became the first living things to use pigments 536 00:42:21,040 --> 00:42:24,160 to split water apart and use it to make food. 537 00:42:27,400 --> 00:42:31,360 This evolutionary invention was incredibly complex. 538 00:42:31,360 --> 00:42:37,160 Even its name is a mouthful - oxygenic photosynthesis. 539 00:42:40,000 --> 00:42:43,680 It starts with a photon from the sun 540 00:42:43,680 --> 00:42:46,800 hitting that green pigment, chlorophyll. 541 00:42:46,800 --> 00:42:50,360 Chlorophyll takes that energy and uses it 542 00:42:50,360 --> 00:42:54,240 to boost electrons up a hill, if you like. 543 00:42:54,240 --> 00:42:59,000 And when they get to the top, they cascade down a molecular waterfall, 544 00:42:59,000 --> 00:43:02,760 and the energy is used to make something called ATP, 545 00:43:02,760 --> 00:43:06,920 which is potentially the energy currency of life. 546 00:43:06,920 --> 00:43:10,960 This little molecular machine is called photosystem II, 547 00:43:10,960 --> 00:43:14,960 and it makes energy for the cell from sunlight. 548 00:43:14,960 --> 00:43:18,000 But when the electrons reach the bottom of that waterfall, 549 00:43:18,000 --> 00:43:20,000 they enter photosystem I. 550 00:43:20,000 --> 00:43:22,360 They meet some more chlorophyll, 551 00:43:22,360 --> 00:43:25,280 which is hit by another photon from the sun, 552 00:43:25,280 --> 00:43:28,360 and that energy raises the electrons up again, 553 00:43:28,360 --> 00:43:31,360 and forces them onto carbon dioxide, 554 00:43:31,360 --> 00:43:35,640 turning that carbon dioxide eventually into sugars, 555 00:43:35,640 --> 00:43:37,480 into food for the cell. 556 00:43:37,480 --> 00:43:40,200 Now, why all this complexity? 557 00:43:40,200 --> 00:43:42,920 Why do you need these two photosystems 558 00:43:42,920 --> 00:43:45,000 joined together in this way, 559 00:43:45,000 --> 00:43:49,240 just to get some electrons and make sugar and a bit of energy out of it? 560 00:43:53,440 --> 00:43:54,960 It's because 561 00:43:54,960 --> 00:43:58,400 only when life coupled these two biological machines together 562 00:43:58,400 --> 00:44:02,000 that it could split water apart and turn it into food. 563 00:44:03,280 --> 00:44:04,840 But it wasn't easy. 564 00:44:06,440 --> 00:44:10,200 The thing is that water is extremely difficult to split, 565 00:44:10,200 --> 00:44:13,400 so for a leaf to do it, for a blade of grass to do it, 566 00:44:13,400 --> 00:44:17,280 just using a trickle of light from the sun, is extremely difficult. 567 00:44:21,280 --> 00:44:26,720 In fact, the task is SO complex that, unlike flight or vision, 568 00:44:26,720 --> 00:44:30,880 which have evolved separately many times during our history, 569 00:44:30,880 --> 00:44:35,560 oxygenic photosynthesis has only evolved once. 570 00:44:38,040 --> 00:44:43,480 Every tree, every plant, every blade of grass on the planet, 571 00:44:43,480 --> 00:44:48,120 everything that carries out oxygenic photosynthesis today 572 00:44:48,120 --> 00:44:50,320 does it in EXACTLY the same way. 573 00:44:50,320 --> 00:44:54,240 And the structures inside every leaf that do that 574 00:44:54,240 --> 00:44:58,080 look remarkably similar to cyanobacteria. 575 00:45:02,600 --> 00:45:06,440 In other words, the descendants of one cyanobacterium 576 00:45:06,440 --> 00:45:08,960 that worked out, for some reason, 577 00:45:08,960 --> 00:45:12,960 how to couple those complex molecular machines together 578 00:45:12,960 --> 00:45:16,200 in some primordial ocean, billions of years ago, 579 00:45:16,200 --> 00:45:19,240 are still present on the Earth today. 580 00:45:37,320 --> 00:45:40,200 The cyanobacteria changed the world... 581 00:45:41,600 --> 00:45:43,600 ..turning it green. 582 00:45:50,000 --> 00:45:52,720 And that had a wonderful consequence. 583 00:45:59,240 --> 00:46:01,120 With this new way of living, 584 00:46:01,120 --> 00:46:04,960 life released oxygen into the atmosphere of our planet 585 00:46:04,960 --> 00:46:08,600 for the first time. And in doing so, 586 00:46:08,600 --> 00:46:12,280 over hundreds of millions of years, 587 00:46:12,280 --> 00:46:17,320 it eventually completely transformed the face of our home. 588 00:46:21,000 --> 00:46:23,640 And as the oxygen levels grew 589 00:46:23,640 --> 00:46:27,760 the stage was set for the arrival of ever more complex creatures. 590 00:46:29,320 --> 00:46:34,000 But on Earth, the emergence of complex life required 591 00:46:34,000 --> 00:46:36,040 a rather more intangible ingredient. 592 00:46:40,040 --> 00:46:44,160 Something that you can't see, touch or smell, 593 00:46:44,160 --> 00:46:46,800 and yet you pass through every day. 594 00:46:55,360 --> 00:46:57,160 Late January, 595 00:46:57,160 --> 00:47:01,000 and the monarch butterflies have found their way home. 596 00:47:03,240 --> 00:47:07,520 They've entered a hibernation state, huddling together for warmth. 597 00:47:10,960 --> 00:47:14,920 But they're only here at all thanks to one of the most accurate 598 00:47:14,920 --> 00:47:17,880 biological clocks found in nature. 599 00:47:37,240 --> 00:47:42,040 These are the pine and oyamel forests, high altitude, 600 00:47:42,040 --> 00:47:45,240 about, what, three hours north-west of Mexico City, 601 00:47:45,240 --> 00:47:48,960 and one of the few wintering grounds of the monarch butterflies, 602 00:47:48,960 --> 00:47:51,160 as you can see. 603 00:47:51,160 --> 00:47:54,240 But there is a colony of millions of monarchs 604 00:47:54,240 --> 00:47:56,280 somewhere due north of here, 605 00:47:56,280 --> 00:47:58,560 so if I head off into the forest 606 00:47:58,560 --> 00:48:03,360 then hopefully this will just be a taster of what's to come. 607 00:48:06,400 --> 00:48:11,080 To find the butterflies, I need to get an accurate bearing on them. 608 00:48:11,080 --> 00:48:14,560 And to do this I need a timepiece. 609 00:48:16,160 --> 00:48:18,000 If you don't have a compass, 610 00:48:18,000 --> 00:48:21,800 how can you tell which direction is north and which direction is south? 611 00:48:21,800 --> 00:48:23,320 Well, you can use the sun. 612 00:48:23,320 --> 00:48:26,320 The sun rises in the east, sets in the west, 613 00:48:26,320 --> 00:48:30,680 and at midday, in the northern hemisphere, it's due south. 614 00:48:30,680 --> 00:48:32,600 But what if it ISN'T midday? 615 00:48:32,600 --> 00:48:36,280 Well, there's an old trick, which is to use a watch. 616 00:48:36,280 --> 00:48:39,320 See, it's about three in the afternoon now, 617 00:48:39,320 --> 00:48:42,520 and if you line the hour hand of your watch up with the sun, 618 00:48:42,520 --> 00:48:44,520 then, in the northern hemisphere, 619 00:48:44,520 --> 00:48:49,160 the line in between the hour hand and 12 o'clock 620 00:48:49,160 --> 00:48:51,800 will point due south. 621 00:48:51,800 --> 00:48:54,760 Which means north is that way. 622 00:49:00,840 --> 00:49:03,360 For thousands of miles on their way here, 623 00:49:03,360 --> 00:49:06,080 the monarchs have faced the same problem. 624 00:49:07,360 --> 00:49:12,040 To make their way south, it's no good simply following the sun. 625 00:49:13,480 --> 00:49:15,320 Because, as the day progresses, 626 00:49:15,320 --> 00:49:18,280 the sun's position drifts across the sky. 627 00:49:22,480 --> 00:49:25,240 Somehow they have to correct for this. 628 00:49:46,840 --> 00:49:50,600 They use what's called a time-compensated sun compass. 629 00:49:52,400 --> 00:49:56,120 They measure the position of the sun every day, using their eyes, 630 00:49:56,120 --> 00:49:58,960 but it's also thought they can measure the position 631 00:49:58,960 --> 00:50:03,000 even when it's cloudy, by using the polarisation of the light. 632 00:50:04,720 --> 00:50:09,320 Having locked onto the sun, their brain then corrects for its movement 633 00:50:09,320 --> 00:50:14,320 across the sky by using one of nature's most accurate timepieces. 634 00:50:14,320 --> 00:50:18,040 By combining the information from their precise clocks 635 00:50:18,040 --> 00:50:22,640 and their eyes, they can navigate due south. 636 00:50:24,240 --> 00:50:28,080 That ability to orientate themselves is, I think, 637 00:50:28,080 --> 00:50:30,840 one of the most remarkable things I've seen. 638 00:50:37,760 --> 00:50:41,400 The biological clocks that have brought the monarchs home 639 00:50:41,400 --> 00:50:43,640 are not unique to butterflies. 640 00:50:45,680 --> 00:50:49,680 Almost all life shares in these circadian rhythms. 641 00:50:51,480 --> 00:50:55,600 They're an evolutionary consequence of living on a spinning rock. 642 00:51:02,160 --> 00:51:08,800 Our world turns on its axis once every 24 hours, giving us a day. 643 00:51:13,360 --> 00:51:16,680 It's on a billion-kilometre journey around the sun, 644 00:51:16,680 --> 00:51:19,360 and each orbit gives us a year. 645 00:51:22,440 --> 00:51:25,280 We live inside a celestial clock, 646 00:51:25,280 --> 00:51:30,600 one that has been ticking away for over 4.5 billion years. 647 00:51:32,200 --> 00:51:36,040 And that's a full third of the age of the universe. 648 00:51:51,280 --> 00:51:56,160 This is the final ingredient that our home has provided. 649 00:51:56,160 --> 00:51:57,760 Time. 650 00:52:07,720 --> 00:52:09,680 Take the horse. 651 00:52:09,680 --> 00:52:14,840 Like all complex living things, it's here because our planet 652 00:52:14,840 --> 00:52:17,280 has been stable enough for long enough 653 00:52:17,280 --> 00:52:19,760 to allow evolution time to play. 654 00:52:34,120 --> 00:52:37,360 The horse is the animal whose family tree 655 00:52:37,360 --> 00:52:39,800 we know with the highest precision. 656 00:52:44,920 --> 00:52:49,560 So it's possible to lay out just one unbroken chain of life 657 00:52:49,560 --> 00:52:52,600 that stretches back nearly four billion years. 658 00:52:56,640 --> 00:52:59,960 Animals that are recognisably horselike have 659 00:52:59,960 --> 00:53:02,760 been around for a long time - 660 00:53:02,760 --> 00:53:05,240 something like 55 million years. 661 00:53:05,240 --> 00:53:09,920 You then have to jump quite a lot to something like 225 million years 662 00:53:09,920 --> 00:53:14,040 if you want to ask the question, where is the earliest mammal? 663 00:53:14,040 --> 00:53:18,000 And it's this thing, which looks something like a little shrew. 664 00:53:18,000 --> 00:53:20,080 535 million. 665 00:53:20,080 --> 00:53:23,720 This is the point when complex life really began to explode 666 00:53:23,720 --> 00:53:25,840 in the oceans. 667 00:53:25,840 --> 00:53:29,800 You then have to sweep back a long, long time to find the next 668 00:53:29,800 --> 00:53:34,280 evolutionary milestone, arguably the most important milestone - 669 00:53:34,280 --> 00:53:38,120 the emergence of the complex self, the eukaryote. 670 00:53:38,120 --> 00:53:42,160 And then, you have to step back a long way in time. 671 00:53:43,200 --> 00:53:48,680 You have to step back all the way to here, 672 00:53:48,680 --> 00:53:52,520 the emergence of the prokaryote, the first life form. 673 00:53:52,520 --> 00:53:56,760 And so, we have this beautiful long line. 674 00:53:56,760 --> 00:54:00,880 We can trace my friend, the horse, and his ancestry 675 00:54:00,880 --> 00:54:07,520 back to the events that happened 3.5, 3.6, 3.7 billion years ago 676 00:54:07,520 --> 00:54:09,640 on the primordial Earth. 677 00:54:15,960 --> 00:54:19,560 Looking back over that vast sweep of time, 678 00:54:19,560 --> 00:54:24,920 you could ask yourself the question, well, do you need 3.5 billion years 679 00:54:24,920 --> 00:54:29,960 to go from a simple form of life to something as complex as a horse? 680 00:54:32,120 --> 00:54:36,440 Well, the answer to that question is, we don't know for sure. 681 00:54:36,440 --> 00:54:41,720 It seems that you need vast expanses of time, but do you need 682 00:54:41,720 --> 00:54:45,480 those big gaps from the simple cell to the complex cell, 683 00:54:45,480 --> 00:54:48,200 do you need the gap from the complex cell 684 00:54:48,200 --> 00:54:51,040 to the evolution of multicellular life? 685 00:54:51,040 --> 00:54:52,240 We don't know. 686 00:54:54,880 --> 00:54:56,560 We only have one example. 687 00:54:56,560 --> 00:54:59,760 There is only one planet where we've been able to study 688 00:54:59,760 --> 00:55:02,200 the evolution of life, and it's this one. 689 00:55:03,640 --> 00:55:08,800 And Earth has been an interesting mixture of stability and upheaval. 690 00:55:08,800 --> 00:55:10,720 It's had an environment 691 00:55:10,720 --> 00:55:14,000 that's never completely conspired to wipe out life, 692 00:55:14,000 --> 00:55:16,960 but it's constantly thrown it challenges. 693 00:55:20,280 --> 00:55:23,960 The deep time that our planet has given life 694 00:55:23,960 --> 00:55:28,440 has allowed it to grow from a tiny seed of genetic possibility 695 00:55:28,440 --> 00:55:33,080 to the planet-wide web of complexity we are part of today. 696 00:55:42,920 --> 00:55:46,880 Only a few of us have ever stepped outside of this world. 697 00:55:48,320 --> 00:55:52,360 But those that have discovered something rather wonderful. 698 00:55:54,800 --> 00:55:57,920 'For all the people back on Earth, 699 00:55:57,920 --> 00:56:02,520 'the crew of Apollo 8 has a message that we would like to send to you.' 700 00:56:02,520 --> 00:56:07,640 On Christmas Eve 1968, my first Christmas Eve, 701 00:56:07,640 --> 00:56:10,240 the Apollo 8 spacecraft entered the darkness 702 00:56:10,240 --> 00:56:12,080 on the far side of the moon. 703 00:56:12,080 --> 00:56:17,040 'In the beginning, God created the heaven and the earth. 704 00:56:17,040 --> 00:56:19,600 'And the earth was without form.' 705 00:56:19,600 --> 00:56:23,040 The three astronauts, Borman, Lovell and Anders, 706 00:56:23,040 --> 00:56:25,760 became the first human beings in history 707 00:56:25,760 --> 00:56:28,520 to lose sight of the Earth. 708 00:56:28,520 --> 00:56:31,600 'And God said, let there be light. 709 00:56:31,600 --> 00:56:36,880 'And there was light. And God saw the light, that it was good.' 710 00:56:37,960 --> 00:56:40,920 When they emerged from the dark side of the moon, 711 00:56:40,920 --> 00:56:44,440 and the Earth rose into view, they chose to broadcast 712 00:56:44,440 --> 00:56:48,640 their culture's creation story back to the inhabitants of Earth. 713 00:56:48,640 --> 00:56:51,520 And, just like the Aztecs and the Mayans 714 00:56:51,520 --> 00:56:54,320 and every civilisation before them, 715 00:56:54,320 --> 00:56:57,560 it told of the origins of their home. 716 00:56:57,560 --> 00:57:00,640 'And God called the dry land Earth, 717 00:57:00,640 --> 00:57:04,840 'and the gathering together of the waters called He seas. 718 00:57:04,840 --> 00:57:07,960 'And God saw that it was good.' 719 00:57:07,960 --> 00:57:14,120 It must be innately human, the desire to understand how our home 720 00:57:14,120 --> 00:57:16,880 came to be the way that it is. 721 00:57:16,880 --> 00:57:20,720 And seen from lunar orbit against the blackness of space, 722 00:57:20,720 --> 00:57:22,960 the Earth is a fragile world, 723 00:57:22,960 --> 00:57:25,640 but seen by science, it's a world 724 00:57:25,640 --> 00:57:30,280 that's been crafted and shaped by life over almost four billion years. 725 00:57:32,320 --> 00:57:34,560 So we're on our way to understanding 726 00:57:34,560 --> 00:57:38,000 how we came to be here, but as the Apollo astronauts discovered, 727 00:57:38,000 --> 00:57:41,200 the journey of discovery has already delivered much more 728 00:57:41,200 --> 00:57:43,680 than just the facts, because it's given us 729 00:57:43,680 --> 00:57:48,760 a powerful perspective on the intricacy and beauty of our home. 730 00:57:50,360 --> 00:57:55,360 'From the crew of Apollo 8, we close with good night, good luck, 731 00:57:55,360 --> 00:57:59,200 'a merry Christmas, and God bless all of you, 732 00:57:59,200 --> 00:58:02,000 'all of you on the good Earth.' 733 00:58:09,680 --> 00:58:12,880 Subtitles by Red Bee Media Ltd