1 00:00:08,440 --> 00:00:14,120 Viewed from above, Planet Earth is a riot of colours. 2 00:00:17,600 --> 00:00:24,080 But there's one particular colour that marks Earth out as special. 3 00:00:24,080 --> 00:00:29,360 The colour that shows it's a living, breathing planet. 4 00:00:33,080 --> 00:00:34,440 Green. 5 00:00:51,400 --> 00:00:52,560 Here we go. 6 00:00:53,480 --> 00:00:57,640 Take a look at this little beauty. This is Lysimachia glutinosa. 7 00:00:57,640 --> 00:00:59,200 And I know it's not the most 8 00:00:59,200 --> 00:01:00,720 glamorous plant in the world, 9 00:01:00,720 --> 00:01:03,840 but its claim to fame is that it grows here 10 00:01:03,840 --> 00:01:05,200 and only here, 11 00:01:05,200 --> 00:01:09,200 on this one side of this one small island, 12 00:01:09,200 --> 00:01:13,640 which makes it sound very fragile, very vulnerable. 13 00:01:13,640 --> 00:01:16,840 That couldn't be further from the truth. 14 00:01:16,840 --> 00:01:20,800 Because in the story of this plant, indeed all plants, 15 00:01:20,800 --> 00:01:24,120 lies the story of our Earth. 16 00:01:27,040 --> 00:01:30,240 It's a story that begins billions of years ago... 17 00:01:32,040 --> 00:01:34,840 ..in the chaos of Earth's early years... 18 00:01:36,120 --> 00:01:38,480 ..before plant life transformed it... 19 00:01:40,200 --> 00:01:42,640 ..into a world of opportunity... 20 00:01:48,200 --> 00:01:50,640 ..as plants rose from the oceans... 21 00:01:52,720 --> 00:01:58,000 ..to conquer a hostile and alien land, 22 00:01:58,000 --> 00:02:02,440 fighting and evolving through triumph and disaster. 23 00:02:04,520 --> 00:02:10,400 And just as they finally built the perfect garden world, 24 00:02:10,400 --> 00:02:17,200 their global domination almost wiped out all life on the planet. 25 00:02:44,680 --> 00:02:49,120 The story of plants begins deep in Earth's ancient history. 26 00:02:52,800 --> 00:02:55,240 Four billion years ago. 27 00:02:57,320 --> 00:03:00,280 When the planet was an inhospitable world... 28 00:03:03,200 --> 00:03:07,480 ..shrouded in a noxious atmosphere of methane clouds... 29 00:03:08,560 --> 00:03:11,240 ..and covered by an endless ocean... 30 00:03:15,200 --> 00:03:19,720 ..broken only by a few remote volcanic islands... 31 00:03:22,960 --> 00:03:25,080 ..with no sign of life. 32 00:03:30,120 --> 00:03:33,080 But to find plants' ancestors, 33 00:03:33,080 --> 00:03:37,240 you'd have to go about as far from the surface as you can get. 34 00:03:46,200 --> 00:03:48,400 In the depths of the oceans... 35 00:03:50,520 --> 00:03:52,640 ..sheltered inside geothermal vents... 36 00:03:56,800 --> 00:03:59,040 ..are something miraculous. 37 00:04:03,360 --> 00:04:04,920 Extremophiles. 38 00:04:06,040 --> 00:04:09,920 An extraordinary form of single-celled life. 39 00:04:11,400 --> 00:04:16,800 The ancestors of every living organism on Earth, 40 00:04:16,800 --> 00:04:18,640 including plants. 41 00:04:22,680 --> 00:04:25,240 But they are stuck here. 42 00:04:30,880 --> 00:04:35,160 At this point, their chances of making the leap onto dry land... 43 00:04:38,160 --> 00:04:40,240 ..are virtually nil. 44 00:04:41,920 --> 00:04:43,280 CHRIS SIGHS 45 00:04:43,280 --> 00:04:44,920 Four billion years ago, 46 00:04:44,920 --> 00:04:48,200 any dry land on Earth would have looked like this - 47 00:04:48,200 --> 00:04:54,360 black, barren, volcanic islands peeping out of a vast ocean. 48 00:04:56,400 --> 00:05:00,200 If plants had any aspirations to leap out onto land, 49 00:05:00,200 --> 00:05:02,200 it was going to be very rapidly disappointed, 50 00:05:02,200 --> 00:05:05,480 because this land was very short-lived. 51 00:05:06,800 --> 00:05:11,120 The Earth's earliest islands were made up of basalt. 52 00:05:15,000 --> 00:05:16,600 Solidified lava... 53 00:05:20,600 --> 00:05:25,280 ..that was easily devastated by explosive eruptions... 54 00:05:30,360 --> 00:05:33,240 ..smashed by extreme tides. 55 00:05:35,120 --> 00:05:38,240 This was no place for life. 56 00:05:39,920 --> 00:05:41,400 So the question is, 57 00:05:41,400 --> 00:05:43,280 and it's a big question, 58 00:05:43,280 --> 00:05:49,080 how did plants forge a permanent base on the land? 59 00:05:49,080 --> 00:05:52,520 Because, if the Earth's only trick 60 00:05:52,520 --> 00:05:55,840 when it came to land-building was volcanism, 61 00:05:55,840 --> 00:05:57,800 it's very likely that that life 62 00:05:57,800 --> 00:06:00,320 would've never made it out of the ocean. 63 00:06:00,320 --> 00:06:05,480 What was needed was another land-creating force, 64 00:06:05,480 --> 00:06:09,240 and it came in the form of a celestial intervention. 65 00:06:21,960 --> 00:06:25,240 The culprits were giant asteroids. 66 00:06:30,920 --> 00:06:34,960 Some nearly 60km in diameter. 67 00:06:37,200 --> 00:06:40,200 More than four times the size of the one 68 00:06:40,200 --> 00:06:43,920 believed to have caused the extinction of the dinosaurs. 69 00:07:09,960 --> 00:07:13,400 The consequences were earth-shattering. 70 00:07:16,040 --> 00:07:18,240 Fracturing our planet's crust... 71 00:07:23,200 --> 00:07:27,480 ..and triggering a process that would re-write Earth's story. 72 00:07:29,160 --> 00:07:31,240 Plate tectonics. 73 00:07:33,160 --> 00:07:36,960 Vast subterranean plates were formed, 74 00:07:36,960 --> 00:07:39,760 and where they meet and collide, 75 00:07:39,760 --> 00:07:41,760 rocks like basalt, 76 00:07:41,760 --> 00:07:44,360 along with sea water and sediment, 77 00:07:44,360 --> 00:07:47,240 are pulled into Earth's fiery mantle... 78 00:07:49,000 --> 00:07:52,560 ..where they're transformed into a new type of rock. 79 00:07:54,600 --> 00:07:57,240 A rock with a superpower. 80 00:07:58,760 --> 00:08:00,240 Granite. 81 00:08:12,200 --> 00:08:14,600 So what is it about this hard, heavy, 82 00:08:14,600 --> 00:08:18,520 unforgiving rock that sets it apart from the crowd? 83 00:08:18,520 --> 00:08:22,480 Well, rather counterintuitively, it's its buoyancy. 84 00:08:22,480 --> 00:08:25,360 Now, we know that ice is less dense than water, 85 00:08:25,360 --> 00:08:27,040 therefore ice floats in water, 86 00:08:27,040 --> 00:08:29,280 therefore we have icebergs. 87 00:08:29,280 --> 00:08:35,560 But it turns out that granite, here, is 10% less dense than basalt. 88 00:08:35,560 --> 00:08:38,400 So, when it's formed deep down inside the Earth, 89 00:08:38,400 --> 00:08:41,120 it naturally rises to the surface. 90 00:08:41,120 --> 00:08:44,360 So you could say that the continents on which we are walking 91 00:08:44,360 --> 00:08:49,040 are vast floating granite icebergs. 92 00:08:49,040 --> 00:08:54,200 So, when the tectonic plates collide 93 00:08:54,200 --> 00:08:57,400 and the basaltic crust is forced down, 94 00:08:57,400 --> 00:08:59,080 granite isn't. 95 00:08:59,080 --> 00:09:01,400 Granite is pushed up, 96 00:09:01,400 --> 00:09:07,040 where it's crumpled into these giant mountain ranges that we see today. 97 00:09:31,160 --> 00:09:33,560 And over the course of geological time, 98 00:09:33,560 --> 00:09:37,120 more and more granite accumulates on the surface, 99 00:09:37,120 --> 00:09:39,600 perched on those tectonic plates, 100 00:09:39,600 --> 00:09:42,800 which are gyrating around the planet 101 00:09:42,800 --> 00:09:45,320 in a grand continental dance. 102 00:09:47,640 --> 00:09:51,080 Now, you may be wondering how we know this. 103 00:09:51,080 --> 00:09:53,800 - Ten, nine... 104 00:09:53,800 --> 00:09:55,760 Ignition sequence started. 105 00:09:55,760 --> 00:09:59,680 - Part of the answer is that, since the 1960s... 106 00:09:59,680 --> 00:10:02,200 - Zero, all engines running. 107 00:10:02,200 --> 00:10:06,440 - ..the space programme has provided a unique insight 108 00:10:06,440 --> 00:10:08,280 into the workings of our planet. 109 00:10:13,000 --> 00:10:15,520 For the last 42 years, 110 00:10:15,520 --> 00:10:20,200 we've been able observe the movement of the Earth's tectonic plates 111 00:10:20,200 --> 00:10:21,520 from orbit. 112 00:10:25,080 --> 00:10:31,200 In 1976, Nasa launched Lageos - 113 00:10:31,200 --> 00:10:34,720 the Laser Geodynamic Satellite - 114 00:10:34,720 --> 00:10:38,840 which used a high precision laser measuring system 115 00:10:38,840 --> 00:10:40,920 and thousands of reflectors... 116 00:10:44,120 --> 00:10:47,200 ..to confirm the theory that the continents 117 00:10:47,200 --> 00:10:49,040 are constantly moving. 118 00:10:52,320 --> 00:10:55,400 Nothing on Earth is staying still. 119 00:10:57,760 --> 00:11:00,920 And by combining this data with other measurements, 120 00:11:00,920 --> 00:11:04,200 we've been able to rewind the clock, 121 00:11:04,200 --> 00:11:07,200 to see how the continents have evolved 122 00:11:07,200 --> 00:11:09,400 over hundreds of millions of years. 123 00:11:12,080 --> 00:11:14,480 And what this tells us 124 00:11:14,480 --> 00:11:17,240 is that one billion years ago... 125 00:11:18,920 --> 00:11:21,800 ..the surface of the Earth was a place 126 00:11:21,800 --> 00:11:25,640 full of possibility and promise. 127 00:11:38,400 --> 00:11:42,600 Vast granite landmasses covered the planet. 128 00:11:44,800 --> 00:11:48,640 All a potential home for life. 129 00:11:55,200 --> 00:11:58,640 If only it could find its way there. 130 00:12:14,640 --> 00:12:16,200 And, fortunately, 131 00:12:16,200 --> 00:12:19,400 one life form was waiting in the wings, 132 00:12:19,400 --> 00:12:21,600 ready to seize its opportunity. 133 00:12:22,960 --> 00:12:24,240 Plants. 134 00:12:28,200 --> 00:12:33,240 Life had migrated from the depths to the shallows... 135 00:12:35,200 --> 00:12:37,720 ..in the form of marine algae. 136 00:12:42,600 --> 00:12:49,840 The first instantly recognisable plant-like organism on Earth. 137 00:12:52,200 --> 00:12:55,240 They had mastered a revolutionary new art. 138 00:12:57,080 --> 00:12:59,400 Harvesting energy from the sun... 139 00:13:01,960 --> 00:13:04,920 ..using photosynthesis. 140 00:13:08,720 --> 00:13:12,200 But before these plants could escape the ocean, 141 00:13:12,200 --> 00:13:14,640 they needed to overcome a hurdle... 142 00:13:17,960 --> 00:13:21,520 ..greater than anything they'd faced before. 143 00:13:36,320 --> 00:13:38,200 It's quite difficult to get your head around 144 00:13:38,200 --> 00:13:42,200 just what a challenge getting onto dry land was for plant life. 145 00:13:42,200 --> 00:13:46,640 The world that it would have to overcome was harsh, hostile, 146 00:13:46,640 --> 00:13:51,520 gravity bound, constantly battered by storms, wind, rain, 147 00:13:51,520 --> 00:13:55,880 UV light, pounded by hot sunlight. 148 00:13:55,880 --> 00:13:59,200 So, just like these contemporary relatives, 149 00:13:59,200 --> 00:14:02,880 green algae rapidly chose the easy pickings, 150 00:14:02,880 --> 00:14:05,920 living in those freshwater rivers and lakes that formed 151 00:14:05,920 --> 00:14:07,800 on the early landmasses. 152 00:14:09,400 --> 00:14:12,680 Cocooned in the safety of the water, 153 00:14:12,680 --> 00:14:15,720 where nutrients and minerals were abundant. 154 00:14:31,760 --> 00:14:37,280 Life stayed in the water for 500 million years... 155 00:14:41,480 --> 00:14:45,600 ..until a moment about half a billion years ago, 156 00:14:45,600 --> 00:14:48,800 when, for reasons we don't entirely understand, 157 00:14:48,800 --> 00:14:52,600 plants' ancestors set off into the unknown. 158 00:14:57,400 --> 00:15:02,240 Making base camp on rocky sediments at the water's edge. 159 00:15:04,680 --> 00:15:08,520 Having evolved a thick waxy coating 160 00:15:08,520 --> 00:15:12,680 to stop themselves drying out in their harsh new environment. 161 00:15:16,480 --> 00:15:21,120 But this brilliant adaptation proved to be a double-edged sword... 162 00:15:23,880 --> 00:15:28,240 ..making it much harder to absorb the nutrients they needed. 163 00:15:33,520 --> 00:15:36,440 So, despite their best efforts, 164 00:15:36,440 --> 00:15:39,680 they slowly dried out, 165 00:15:39,680 --> 00:15:41,720 dying on the rocks. 166 00:15:49,680 --> 00:15:52,680 But plants aren't the type to give up easily. 167 00:15:54,320 --> 00:15:57,400 They just needed to find something to help them. 168 00:16:03,040 --> 00:16:04,320 And they did, 169 00:16:04,320 --> 00:16:06,760 because, as it turned out... 170 00:16:09,200 --> 00:16:11,040 ..they were not alone. 171 00:16:24,080 --> 00:16:26,000 Half a billion years before plants 172 00:16:26,000 --> 00:16:28,200 successfully made it onto dry land, 173 00:16:28,200 --> 00:16:31,200 it's believed that another group of organisms were surviving 174 00:16:31,200 --> 00:16:34,840 on these hostile early landmasses. 175 00:16:34,840 --> 00:16:37,680 In this small rock are the fossilised remains 176 00:16:37,680 --> 00:16:39,960 of Tortotubus protuberans, 177 00:16:39,960 --> 00:16:43,680 a 440-million-year-old species. 178 00:16:43,680 --> 00:16:44,960 Now, you can't see it. 179 00:16:44,960 --> 00:16:48,600 It's 200 micrometres in length. 180 00:16:48,600 --> 00:16:54,080 But its earlier ancestors were those that were surviving on that land. 181 00:16:54,080 --> 00:16:56,920 Their ongoing success was down to their ability 182 00:16:56,920 --> 00:17:02,280 to chemically degrade that substrate to get nutrients. 183 00:17:02,280 --> 00:17:06,600 They were feasting on the bare rock itself. 184 00:17:06,600 --> 00:17:09,680 Now, if that sounds otherworldly, I've got to tell you, 185 00:17:09,680 --> 00:17:12,400 you probably know these organisms very well. 186 00:17:12,400 --> 00:17:14,400 You might have even had some on toast 187 00:17:14,400 --> 00:17:16,080 for breakfast this morning. 188 00:17:16,080 --> 00:17:18,880 Because they're fungi. 189 00:17:27,200 --> 00:17:31,200 The next waves of plant life making their way onto the land 190 00:17:31,200 --> 00:17:32,680 developed a new trick. 191 00:17:36,480 --> 00:17:41,400 They evolved specialised cells that could connect with fungi, 192 00:17:41,400 --> 00:17:43,960 allowing them to trade resources 193 00:17:43,960 --> 00:17:46,960 like nutrients and food between each other. 194 00:17:49,680 --> 00:17:53,840 And this new, mutually beneficial partnership 195 00:17:53,840 --> 00:17:56,800 turned out to be a match made in heaven. 196 00:17:58,440 --> 00:18:01,600 This was truly a pivotal moment. 197 00:18:01,600 --> 00:18:04,200 Fungi and plants had come together to produce 198 00:18:04,200 --> 00:18:08,960 the first complex terrestrial ecosystem on Earth. 199 00:18:08,960 --> 00:18:11,040 Now, the plants got from the fungi 200 00:18:11,040 --> 00:18:13,440 the nutrients they could extract from the rocks - 201 00:18:13,440 --> 00:18:16,520 and they repaid their fungal partners with glucose, 202 00:18:16,520 --> 00:18:19,000 the sugar product of photosynthesis, 203 00:18:19,000 --> 00:18:23,360 using energy from the sun and CO2 from the atmosphere. 204 00:18:23,360 --> 00:18:27,560 And this symbiosis meant that plants could survive 205 00:18:27,560 --> 00:18:31,240 permanently on these new landmasses. 206 00:18:33,200 --> 00:18:36,320 They'd finally made it out of the water, 207 00:18:36,320 --> 00:18:39,240 and they were ready to start conquering the world. 208 00:19:08,400 --> 00:19:11,200 It's incredible to think that that first collaboration 209 00:19:11,200 --> 00:19:13,400 between fungi and plants would lead to such 210 00:19:13,400 --> 00:19:15,200 an extraordinary relationship, 211 00:19:15,200 --> 00:19:18,680 and one which would endure till today. 212 00:19:18,680 --> 00:19:21,200 Look at this bracket fungus here, growing on this log. 213 00:19:21,200 --> 00:19:25,000 This species is all about decay and decomposition, 214 00:19:25,000 --> 00:19:28,720 but we mustn't think about fungi being about death. 215 00:19:28,720 --> 00:19:31,720 Here in the forest, they're very much about life. 216 00:19:31,720 --> 00:19:34,600 There's an extensive network of their hyphae, 217 00:19:34,600 --> 00:19:38,480 their roots if you like, stretching out into the woodland here, 218 00:19:38,480 --> 00:19:41,880 intrinsically linking with the roots of all of the trees. 219 00:19:47,200 --> 00:19:50,560 And they are allowing them to share nutrients, 220 00:19:50,560 --> 00:19:53,040 even communicate with one another. 221 00:19:59,520 --> 00:20:03,560 And the key thing is that none of these plants could survive 222 00:20:03,560 --> 00:20:04,960 without the fungi 223 00:20:04,960 --> 00:20:08,640 and the fungi couldn't survive without the plants. 224 00:20:08,640 --> 00:20:12,360 And yet we always think of them being down here in the damp, 225 00:20:12,360 --> 00:20:16,120 in the undergrowth, very much subservient to the plants 226 00:20:16,120 --> 00:20:18,440 which are up here, towering above them. 227 00:20:27,080 --> 00:20:29,920 But in the earliest days of terrestrial plants, 228 00:20:29,920 --> 00:20:32,480 the situation couldn't have been more different. 229 00:20:37,560 --> 00:20:41,400 The clues were strange circular fossils. 230 00:20:44,200 --> 00:20:47,520 At first, scientists thought they were ancient trees. 231 00:20:49,920 --> 00:20:55,680 But, looking closer, they found microscopic filaments, 232 00:20:55,680 --> 00:20:58,640 revealing them to be fungi, 233 00:20:58,640 --> 00:21:02,400 but on a scale bigger than anything we know today. 234 00:21:15,560 --> 00:21:21,200 70 million years after plants and fungi first teamed up, 235 00:21:21,200 --> 00:21:25,640 something utterly astonishing has happened to fungi. 236 00:21:28,200 --> 00:21:30,560 They are now giants. 237 00:21:32,680 --> 00:21:36,800 In the staggering form of Prototaxites. 238 00:21:43,920 --> 00:21:50,680 Gargantuan, leathery, spore-bearing fungal monsters 239 00:21:50,680 --> 00:21:54,640 standing an incredible eight metres tall. 240 00:21:58,720 --> 00:22:02,440 They towered over the tiny plants 241 00:22:02,440 --> 00:22:05,240 still clinging to the water's edge. 242 00:22:14,040 --> 00:22:18,480 Before plants could challenge the dominance of fungi, 243 00:22:18,480 --> 00:22:22,240 they needed to come up with yet another cunning plan. 244 00:22:24,760 --> 00:22:27,440 The problem was that most of the planet's surface 245 00:22:27,440 --> 00:22:29,480 was just rock. 246 00:22:29,480 --> 00:22:31,440 So, away from the water's edge, 247 00:22:31,440 --> 00:22:34,200 where plants had ready access to that water, 248 00:22:34,200 --> 00:22:36,160 they really had no hope. 249 00:22:37,200 --> 00:22:41,200 As soon as any moisture appeared on those impervious surfaces, 250 00:22:41,200 --> 00:22:45,920 it drained away into the streams, into the rivers, into the lakes, 251 00:22:45,920 --> 00:22:51,000 leaving the rock too exposed, too dry for plants to survive. 252 00:22:51,000 --> 00:22:54,200 So, at this point, it did appear as if the Earth would be 253 00:22:54,200 --> 00:22:58,000 a fungal paradise for all eternity. 254 00:22:59,200 --> 00:23:01,440 If the plants wanted to compete, 255 00:23:01,440 --> 00:23:04,200 if they wanted to stake their claim on the land, 256 00:23:04,200 --> 00:23:07,560 they would have to change their equation. 257 00:23:07,560 --> 00:23:10,360 They needed some magic. 258 00:23:23,040 --> 00:23:26,160 Today, plants are everywhere. 259 00:23:29,080 --> 00:23:32,800 In every niche and every environment. 260 00:23:39,920 --> 00:23:44,760 But almost all plants have one thing that allows them to thrive. 261 00:23:48,200 --> 00:23:49,360 Soil. 262 00:23:52,560 --> 00:23:55,840 In the modern world, there are many different types of soil. 263 00:23:55,840 --> 00:24:00,760 Most, like this crumbling brown wonder stuff here, 264 00:24:00,760 --> 00:24:03,720 are made through the activities of invertebrates, fungi, 265 00:24:03,720 --> 00:24:07,520 bacteria, enzymes, all breaking down organic matter - 266 00:24:07,520 --> 00:24:10,200 like leaf litter or animal excrement - 267 00:24:10,200 --> 00:24:12,920 and then mixing it with minerals that have eroded 268 00:24:12,920 --> 00:24:14,720 from the bedrock below. 269 00:24:14,720 --> 00:24:19,200 And the result is this magical substance, 270 00:24:19,200 --> 00:24:24,080 packed full of nutrients and, essentially, able to hold moisture, 271 00:24:24,080 --> 00:24:27,840 meaning that plants can get what they want from it all year round, 272 00:24:27,840 --> 00:24:31,360 those nutrients and that water. 273 00:24:31,360 --> 00:24:35,400 But 450 million years ago, 274 00:24:35,400 --> 00:24:37,640 there was no soil. 275 00:24:37,640 --> 00:24:42,600 No soil because there were few or no animals living on land 276 00:24:42,600 --> 00:24:46,480 and precious little organic matter for anything to work with. 277 00:24:46,480 --> 00:24:52,640 The very idea of making soil was seemingly impossible. 278 00:24:59,120 --> 00:25:03,240 But plants weren't going to let a little thing like that stop them. 279 00:25:12,720 --> 00:25:17,960 So they began scratching the rock with tiny root-like hairs... 280 00:25:19,960 --> 00:25:21,840 ..turning it to dust... 281 00:25:28,920 --> 00:25:32,680 ..which they mixed with enzymes secreted by fungi. 282 00:25:36,840 --> 00:25:39,720 But the truly transformative ingredient... 283 00:25:42,760 --> 00:25:44,680 ..was the plants themselves. 284 00:25:48,440 --> 00:25:50,920 Generation after generation... 285 00:25:53,800 --> 00:26:00,080 ..laying themselves down to form the magical substance 286 00:26:00,080 --> 00:26:02,440 that would set their descendants free. 287 00:26:23,840 --> 00:26:28,200 Fast forward 30 million years and, thanks to soil, 288 00:26:28,200 --> 00:26:30,600 plants have transformed. 289 00:26:38,280 --> 00:26:40,200 They are much bigger 290 00:26:40,200 --> 00:26:44,520 and in possession of brand-new evolutionary tricks. 291 00:26:45,920 --> 00:26:49,440 A vascular system that allows them to move water 292 00:26:49,440 --> 00:26:51,440 through their tissues. 293 00:26:51,440 --> 00:26:57,200 And the first true roots that draw nutrients from the soil 294 00:26:57,200 --> 00:26:59,840 and support taller stems. 295 00:27:05,120 --> 00:27:09,240 The colossal Prototaxites still tower over them. 296 00:27:17,040 --> 00:27:21,200 But now the soil offers fertile ground 297 00:27:21,200 --> 00:27:26,240 for countless wind-borne, seed-like spores released by plants. 298 00:27:31,800 --> 00:27:35,960 Thanks to this, plants can finally break free 299 00:27:35,960 --> 00:27:37,720 and move away from the water... 300 00:27:46,120 --> 00:27:49,760 ..riding the wind far and wide, 301 00:27:49,760 --> 00:27:52,720 spreading across plains and hillsides. 302 00:28:05,840 --> 00:28:09,200 For the first time in history, 303 00:28:09,200 --> 00:28:14,120 Planet Earth was turning green. 304 00:28:22,400 --> 00:28:24,920 Ironically, there was a very real danger 305 00:28:24,920 --> 00:28:27,680 that this new-found success could have instigated 306 00:28:27,680 --> 00:28:29,600 the beginning of the end. 307 00:28:29,600 --> 00:28:32,480 You see, no matter how big we think it is, 308 00:28:32,480 --> 00:28:35,200 the Earth is essentially a closed system. 309 00:28:35,200 --> 00:28:39,920 So any massive increase or decrease in the amount of plants 310 00:28:39,920 --> 00:28:41,200 wouldn't occur in isolation, 311 00:28:41,200 --> 00:28:44,200 it would have a profound effect both then and now. 312 00:28:44,200 --> 00:28:47,840 I mean, just imagine, if we were monumentally stupid enough 313 00:28:47,840 --> 00:28:51,920 to cut down all of the trees and poison all of the plants, 314 00:28:51,920 --> 00:28:54,520 then the amount of CO2 in the atmosphere 315 00:28:54,520 --> 00:28:57,200 would rocket up, along with the global temperature, 316 00:28:57,200 --> 00:28:59,880 and the amount of oxygen would decrease. 317 00:28:59,880 --> 00:29:02,880 So ultimately we wouldn't be able to breathe. 318 00:29:02,880 --> 00:29:06,200 But of course, we wouldn't be silly enough to do that. 319 00:29:06,200 --> 00:29:09,400 400 million years ago, however, 320 00:29:09,400 --> 00:29:11,760 the situation was the polar opposite, 321 00:29:11,760 --> 00:29:15,200 there was a massive increase in the amount 322 00:29:15,200 --> 00:29:16,960 of terrestrial plants. 323 00:29:16,960 --> 00:29:18,480 And as a consequence, 324 00:29:18,480 --> 00:29:20,880 the amount of carbon dioxide in the atmosphere 325 00:29:20,880 --> 00:29:22,200 began to plummet. 326 00:29:22,200 --> 00:29:26,200 In fact, in the first 30 million years of the Devonian period, 327 00:29:26,200 --> 00:29:29,680 it went down by 25%. 328 00:29:31,640 --> 00:29:34,640 Now, given that CO2 is one of the most important 329 00:29:34,640 --> 00:29:36,680 resources for terrestrial plants, 330 00:29:36,680 --> 00:29:41,400 this had the potential to develop into a very real problem. 331 00:29:46,640 --> 00:29:50,480 You see, if plants wanted to continue to grow bigger, 332 00:29:50,480 --> 00:29:54,560 they were going to need some new, well, inspiration. 333 00:30:00,400 --> 00:30:02,720 The funny thing about plants 334 00:30:02,720 --> 00:30:06,240 is that we generally think of them as fairly inanimate. 335 00:30:09,600 --> 00:30:13,800 But look closer and there's an awful lot going on. 336 00:30:16,640 --> 00:30:20,720 Such as the photosynthetic dance of green chloroplast, 337 00:30:20,720 --> 00:30:23,680 excited by the sunlight they capture. 338 00:30:30,080 --> 00:30:31,920 But most incredibly, 339 00:30:31,920 --> 00:30:33,840 looked at the right way, 340 00:30:33,840 --> 00:30:36,480 you might almost swear 341 00:30:36,480 --> 00:30:38,600 you can see them breathing. 342 00:30:42,360 --> 00:30:46,240 Stomata - like tiny green mouths... 343 00:30:49,280 --> 00:30:51,520 ..taking in carbon dioxide 344 00:30:51,520 --> 00:30:55,040 and exhaling oxygen and water vapour. 345 00:31:03,120 --> 00:31:05,600 Back in the Devonian period, 346 00:31:05,600 --> 00:31:09,320 most stomata existed only in plant stems. 347 00:31:13,040 --> 00:31:16,960 But falling atmospheric carbon dioxide meant they needed 348 00:31:16,960 --> 00:31:21,400 more stomata to absorb the same quantity to survive. 349 00:31:22,840 --> 00:31:25,560 The problem was where to put them. 350 00:31:27,360 --> 00:31:32,520 The answer was as elegant as it was revolutionary. 351 00:31:39,400 --> 00:31:40,560 Leaves. 352 00:31:40,560 --> 00:31:41,760 Just look at these beauties. 353 00:31:41,760 --> 00:31:44,760 These are called elephant's ear. 354 00:31:44,760 --> 00:31:48,200 Now, of course, the earliest leaves were nowhere near as big, 355 00:31:48,200 --> 00:31:52,360 but they were a similar triumph of botanical form and function. 356 00:31:52,360 --> 00:31:54,480 Waxy on top, keeping them waterproof. 357 00:31:54,480 --> 00:31:57,960 And underneath, these strong ribs to keep them flat. 358 00:31:57,960 --> 00:32:00,040 Also, a greater surface area, 359 00:32:00,040 --> 00:32:04,760 allowing many more stomata in here for better gas exchange. 360 00:32:04,760 --> 00:32:08,560 And on the top, that surface area provides more room 361 00:32:08,560 --> 00:32:12,680 for more chlorophyll to harvest more sunlight from the sun. 362 00:32:13,840 --> 00:32:15,760 But there is one problem with leaves - 363 00:32:15,760 --> 00:32:17,200 they generate shade, 364 00:32:17,200 --> 00:32:19,200 which promotes competition. 365 00:32:19,200 --> 00:32:23,200 And this fired the starting gun on a race for light 366 00:32:23,200 --> 00:32:26,200 that once again would completely transform 367 00:32:26,200 --> 00:32:28,120 the surface of the Earth. 368 00:32:37,200 --> 00:32:40,800 Leaves did far more than just allow plants to harvest 369 00:32:40,800 --> 00:32:43,360 more carbon dioxide. 370 00:32:43,360 --> 00:32:47,400 They made photosynthesis more efficient, 371 00:32:47,400 --> 00:32:49,240 which boosted energy... 372 00:32:52,320 --> 00:32:57,360 ..leading to the birth of a new magic ingredient 373 00:32:57,360 --> 00:32:59,960 in the form of wood. 374 00:33:01,560 --> 00:33:04,760 This wonder material led to the creation 375 00:33:04,760 --> 00:33:07,200 of biological machines, 376 00:33:07,200 --> 00:33:10,240 unlike anything Earth had ever seen. 377 00:33:12,360 --> 00:33:16,200 With strong, durable trunks that could push past 378 00:33:16,200 --> 00:33:18,960 the competition towards the sunlight. 379 00:33:21,680 --> 00:33:23,240 Trees. 380 00:33:49,200 --> 00:33:50,640 For terrestrial plants, 381 00:33:50,640 --> 00:33:54,200 trees represented a quantum leap forwards. 382 00:33:54,200 --> 00:33:55,520 I suppose we could say 383 00:33:55,520 --> 00:33:58,560 they were the epitome of everything that plants had learned 384 00:33:58,560 --> 00:34:00,040 up until this point. 385 00:34:00,040 --> 00:34:02,200 Deep-rooted, long-lived 386 00:34:02,200 --> 00:34:04,200 photosynthetic powerhouses, 387 00:34:04,200 --> 00:34:07,200 perfectly adapted to exploiting all of the resources 388 00:34:07,200 --> 00:34:08,760 that they required. 389 00:34:08,760 --> 00:34:12,600 And perhaps most ahead of its time was Archaeopteris, 390 00:34:12,600 --> 00:34:15,560 considered by many to be the first true tree - 391 00:34:15,560 --> 00:34:17,200 enormously successful, 392 00:34:17,200 --> 00:34:19,960 fossils found all over the world. 393 00:34:19,960 --> 00:34:22,680 And just like this contemporary Sitka spruce, 394 00:34:22,680 --> 00:34:27,000 it had a timber trunk, thick bark and lateral branches 395 00:34:27,000 --> 00:34:31,720 covered with masses of green photosynthetic leaves 396 00:34:31,720 --> 00:34:34,040 competing for light. 397 00:34:34,040 --> 00:34:38,760 And with that competition came the need to grow ever taller. 398 00:34:44,840 --> 00:34:46,600 And they did. 399 00:34:46,600 --> 00:34:49,640 Until they towered above everything else, 400 00:34:49,640 --> 00:34:52,240 reaching heights of 30 metres. 401 00:35:06,080 --> 00:35:10,240 Earth was now on its way to becoming a forest world. 402 00:35:15,680 --> 00:35:19,200 A home for countless new species of plants 403 00:35:19,200 --> 00:35:21,760 and insects at every level, 404 00:35:21,760 --> 00:35:24,360 from the canopy to the forest floor. 405 00:35:33,200 --> 00:35:36,960 The former masters of the land, Prototaxites, 406 00:35:36,960 --> 00:35:39,400 were gone, never to return. 407 00:35:46,560 --> 00:35:51,000 Fungi were reduced to life in the shadows, 408 00:35:51,000 --> 00:35:52,560 where they've remained, 409 00:35:52,560 --> 00:35:55,720 working their quiet magic, ever since. 410 00:36:06,440 --> 00:36:09,400 The meteoric rise of plant life, 411 00:36:09,400 --> 00:36:11,640 from uncertain pioneers 412 00:36:11,640 --> 00:36:14,480 to undisputed masters of the land, 413 00:36:14,480 --> 00:36:15,760 was complete. 414 00:36:22,200 --> 00:36:25,560 It was a new chapter in Earth's story. 415 00:36:32,200 --> 00:36:34,560 But this triumph brought with it 416 00:36:34,560 --> 00:36:36,640 the threat of global catastrophe. 417 00:36:49,320 --> 00:36:53,000 If you've never stood and gazed up into the high canopy of a forest, 418 00:36:53,000 --> 00:36:55,200 then it's something that I can thoroughly recommend. 419 00:36:55,200 --> 00:36:57,960 Because if you're in the right place at the right time, 420 00:36:57,960 --> 00:37:01,200 with the right species, you might see something special. 421 00:37:01,200 --> 00:37:03,200 So stand, stare and blink, 422 00:37:03,200 --> 00:37:06,280 and look for a unique pattern. 423 00:37:06,280 --> 00:37:08,560 You see, all of the branches and leaves 424 00:37:08,560 --> 00:37:11,040 from neighbouring trees don't quite meet, 425 00:37:11,040 --> 00:37:13,800 leaving a silvery line between them. 426 00:37:13,800 --> 00:37:17,640 It's almost as if they're being kind to their neighbours. 427 00:37:17,640 --> 00:37:21,800 It's a phenomenon called crown shyness, 428 00:37:21,800 --> 00:37:24,200 part of a peaceful process of evolution 429 00:37:24,200 --> 00:37:27,880 which has allowed all of the species in this ecosystem 430 00:37:27,880 --> 00:37:31,040 to come together and live harmoniously. 431 00:37:31,040 --> 00:37:32,280 And it works. 432 00:37:32,280 --> 00:37:33,960 It's beautiful. 433 00:37:33,960 --> 00:37:35,200 And when they're living, 434 00:37:35,200 --> 00:37:38,960 these magnificent trees are providing homes, 435 00:37:38,960 --> 00:37:42,560 shelter and food for a whole range of different animals, 436 00:37:42,560 --> 00:37:45,560 other species of plants and fungi. 437 00:37:45,560 --> 00:37:48,200 And you know, even when they're dead, 438 00:37:48,200 --> 00:37:49,720 even when they are dead, 439 00:37:49,720 --> 00:37:52,560 they just keep giving. 440 00:37:52,560 --> 00:37:54,920 Through this process of decomposition, 441 00:37:54,920 --> 00:37:59,280 again, they're feeding animals, other species of plants and fungi. 442 00:37:59,280 --> 00:38:01,600 But it hasn't always been like this. 443 00:38:01,600 --> 00:38:03,200 There was a time when it was different, 444 00:38:03,200 --> 00:38:07,200 when intense competition was driving an arms race 445 00:38:07,200 --> 00:38:11,040 that produced a very dangerous substance, 446 00:38:11,040 --> 00:38:15,680 a substance which could have led to the end of all life on Earth. 447 00:38:50,080 --> 00:38:54,240 The rapid spread of terrestrial plants has changed the Earth. 448 00:38:58,000 --> 00:39:02,760 Atmospheric carbon dioxide has fallen even further, 449 00:39:02,760 --> 00:39:04,800 causing global cooling. 450 00:39:10,200 --> 00:39:11,760 In the southern hemisphere, 451 00:39:11,760 --> 00:39:15,200 ice sheets have formed for the first time 452 00:39:15,200 --> 00:39:18,240 in more than a quarter of a billion years. 453 00:39:23,000 --> 00:39:25,000 But near the equator, 454 00:39:25,000 --> 00:39:29,240 the climate is still extremely hot and very wet. 455 00:39:48,520 --> 00:39:54,200 Fluctuating sea levels have caused huge deltas to form, 456 00:39:54,200 --> 00:39:58,520 where vast carbon-hungry swamp forests have sprung up... 457 00:40:00,200 --> 00:40:04,800 ..covering as much as 20 million square kilometres. 458 00:40:07,280 --> 00:40:10,960 A sweltering jungle paradise 459 00:40:10,960 --> 00:40:12,640 teeming with life... 460 00:40:15,120 --> 00:40:18,120 ..where intense competition for light 461 00:40:18,120 --> 00:40:22,800 has given rise to a whole host of new plant species... 462 00:40:24,200 --> 00:40:26,880 ..who would go on to threaten the future 463 00:40:26,880 --> 00:40:28,960 of terrestrial life on Earth. 464 00:40:34,840 --> 00:40:38,200 The largest amongst them were Lepidodendrons, 465 00:40:38,200 --> 00:40:41,120 known as "scale trees", 466 00:40:41,120 --> 00:40:45,240 towering up to an incredible 50 metres tall. 467 00:41:12,200 --> 00:41:15,200 In many ways, we can see this as the modern equivalent 468 00:41:15,200 --> 00:41:18,000 of a carboniferous swamp forest. 469 00:41:19,480 --> 00:41:21,760 It's certainly very swampy, soft underfoot. 470 00:41:21,760 --> 00:41:23,200 And in spring and summer, 471 00:41:23,200 --> 00:41:27,200 green, lush, very productive - as it was back then, 472 00:41:27,200 --> 00:41:30,920 when large scale trees proliferated 473 00:41:30,920 --> 00:41:33,640 because their roots had adapted to allow them to grow 474 00:41:33,640 --> 00:41:37,200 on the land and beneath the surface of the water. 475 00:41:37,200 --> 00:41:40,320 But unlike these modern day cypresses - 476 00:41:40,320 --> 00:41:42,640 and Archaeopteris, which preceded them - 477 00:41:42,640 --> 00:41:44,720 their trunks were very different. 478 00:41:44,720 --> 00:41:46,480 They weren't made of wood. 479 00:41:46,480 --> 00:41:50,040 The interior was a soft, corky material 480 00:41:50,040 --> 00:41:55,560 and the exterior, a very robust, tough structural shell, 481 00:41:55,560 --> 00:41:58,920 which allowed them to perhaps grow to 50 metres 482 00:41:58,920 --> 00:42:02,200 in as little as 15 years. 483 00:42:02,200 --> 00:42:06,200 But maybe that tough structural shell 484 00:42:06,200 --> 00:42:08,640 was just a little too indestructible. 485 00:42:08,640 --> 00:42:12,000 Because when they finally matured and died and toppled 486 00:42:12,000 --> 00:42:15,200 into this oxygen depleted ooze, 487 00:42:15,200 --> 00:42:18,800 they didn't decompose as modern trees do - 488 00:42:18,800 --> 00:42:23,040 breaking down slowly, giving their carbon back to the system. 489 00:42:23,040 --> 00:42:27,560 No, those scale trees hung on to it, they hoarded that carbon, 490 00:42:27,560 --> 00:42:30,080 and the consequences for planet Earth 491 00:42:30,080 --> 00:42:31,960 were astonishingly dire. 492 00:42:48,440 --> 00:42:52,200 The floor of the swamp forests became log-jammed 493 00:42:52,200 --> 00:42:54,960 with fallen trees and decaying plant matter. 494 00:43:05,200 --> 00:43:08,480 When this carbon-rich mixture was then buried 495 00:43:08,480 --> 00:43:11,360 under millions of tonnes of sediment, 496 00:43:11,360 --> 00:43:17,440 all the elements were in place for a remarkable alchemy. 497 00:43:37,640 --> 00:43:39,680 Under intense heat and pressure, 498 00:43:39,680 --> 00:43:41,640 and consumed by the passage of time, 499 00:43:41,640 --> 00:43:45,840 this vast swathe of plant material was transformed 500 00:43:45,840 --> 00:43:48,720 by the Earth into a new type of rock, 501 00:43:48,720 --> 00:43:51,520 a type of rock that would come back to haunt us. 502 00:43:53,800 --> 00:43:55,120 Here we are. 503 00:43:57,520 --> 00:43:59,480 It's coal. 504 00:43:59,480 --> 00:44:02,080 Yes, coal. 505 00:44:02,080 --> 00:44:06,640 And there's a seam of coal running through this cliff here - 506 00:44:06,640 --> 00:44:08,200 that black line - 507 00:44:08,200 --> 00:44:13,560 which is constantly being eroded by the wind, waves and rain. 508 00:44:16,000 --> 00:44:20,320 Now, throughout the 60 million years of the Carboniferous, 509 00:44:20,320 --> 00:44:23,200 plants fixed carbon in the form of coal 510 00:44:23,200 --> 00:44:29,520 to the tune of 100,000 million tonnes every single year, 511 00:44:29,520 --> 00:44:33,840 taking an enormous amount of free carbon out of the carbon cycle. 512 00:44:35,880 --> 00:44:40,680 And what this added up to was a deadly downward spiral. 513 00:45:02,760 --> 00:45:05,960 These carbon-hoarding swamp forests 514 00:45:05,960 --> 00:45:08,120 had pushed the Earth to the brink. 515 00:45:17,480 --> 00:45:19,200 In the frozen south, 516 00:45:19,200 --> 00:45:22,240 the Archaeopteris forests are long dead. 517 00:45:24,600 --> 00:45:26,720 And to make matters worse, 518 00:45:26,720 --> 00:45:31,440 atmospheric carbon dioxide is plummeting fast. 519 00:45:31,440 --> 00:45:34,200 Nearly a quarter of the world's land 520 00:45:34,200 --> 00:45:37,560 is now buried beneath a blanket of ice. 521 00:45:41,760 --> 00:45:45,560 Earth sits within a hair's breadth of descending 522 00:45:45,560 --> 00:45:47,400 into a snowball event... 523 00:45:49,520 --> 00:45:53,800 ..where reflection of the sun's rays by the frozen surface 524 00:45:53,800 --> 00:45:57,280 could lead to the total glaciation of the planet... 525 00:46:01,760 --> 00:46:05,080 ..threatening almost all life on Earth. 526 00:46:13,520 --> 00:46:16,760 This could've been the end of plants' journey... 527 00:46:18,960 --> 00:46:21,720 ..but for another timely intervention. 528 00:46:39,360 --> 00:46:41,600 Beneath the frozen surface, 529 00:46:41,600 --> 00:46:45,440 the giant tectonic plates that set all these events in motion 530 00:46:45,440 --> 00:46:47,080 in the first place 531 00:46:47,080 --> 00:46:50,240 had been continuing their perpetual dance. 532 00:46:56,880 --> 00:47:00,600 And over the 60 million years of the Carboniferous, 533 00:47:00,600 --> 00:47:03,600 they'd slowly been shifting the landmasses 534 00:47:03,600 --> 00:47:05,920 where the swamp forests thrived... 535 00:47:07,320 --> 00:47:12,040 ..raising huge granite mountains in their place, 536 00:47:12,040 --> 00:47:14,480 which changed weather patterns, 537 00:47:14,480 --> 00:47:17,360 denying water to the deltas below. 538 00:47:26,480 --> 00:47:29,000 And with this intervention 539 00:47:29,000 --> 00:47:33,040 about 280 million years ago, 540 00:47:33,040 --> 00:47:37,560 most of the coal-producing swamps dried up for good. 541 00:47:41,400 --> 00:47:45,800 Atmospheric carbon dioxide began to rebound. 542 00:47:45,800 --> 00:47:47,400 Temperatures rose... 543 00:47:51,040 --> 00:47:53,240 ..melting the southern glaciers... 544 00:48:00,200 --> 00:48:02,240 ..which eventually disappeared... 545 00:48:08,400 --> 00:48:12,200 ..setting the scene for a plant renaissance. 546 00:48:15,200 --> 00:48:19,080 Allowing plants to diversify, 547 00:48:19,080 --> 00:48:23,360 developing flowers and fruit, 548 00:48:23,360 --> 00:48:25,960 grasses and grains. 549 00:48:28,760 --> 00:48:34,240 Transforming their signature green into a kaleidoscope of colour. 550 00:48:39,400 --> 00:48:44,880 Evolving new species to exploit every niche on the planet. 551 00:48:47,200 --> 00:48:51,720 Right down to one side of one small island. 552 00:48:53,680 --> 00:48:57,640 Like our old friend, Lysimachia glutinosa. 553 00:49:07,200 --> 00:49:10,400 Plants' long journey has been an astonishing 554 00:49:10,400 --> 00:49:14,480 four-billion-year struggle from humble beginnings... 555 00:49:18,680 --> 00:49:21,520 ..through the deadly fight to escape the water... 556 00:49:22,560 --> 00:49:25,920 ..to the countless generations that have reshaped 557 00:49:25,920 --> 00:49:27,560 the surface of our planet. 558 00:49:29,720 --> 00:49:33,480 Transforming it from bare rock 559 00:49:33,480 --> 00:49:37,720 to a lush and verdant home for life. 560 00:49:43,320 --> 00:49:46,200 In the aftermath of all of these tumultuous events 561 00:49:46,200 --> 00:49:49,120 came a new world order. 562 00:49:49,120 --> 00:49:52,200 That partnership of forces which had been shaping the planet 563 00:49:52,200 --> 00:49:53,880 found a harmony. 564 00:49:53,880 --> 00:49:56,640 Even plants finally found a balance, 565 00:49:56,640 --> 00:50:00,200 instinctively aligning the amount of biomass on Earth 566 00:50:00,200 --> 00:50:01,640 with the carbon cycle 567 00:50:01,640 --> 00:50:04,800 and the composition of the atmosphere. 568 00:50:04,800 --> 00:50:11,880 And this equilibrium has lasted more than a quarter of a billion years. 569 00:50:11,880 --> 00:50:16,200 You see, plants had taken up that role as guardians 570 00:50:16,200 --> 00:50:17,800 of the Earth's climate, 571 00:50:17,800 --> 00:50:20,760 breathing in and out as and when required 572 00:50:20,760 --> 00:50:24,920 and paving the way for the world that we've inherited today. 573 00:50:24,920 --> 00:50:27,640 This bountiful, blooming miracle. 574 00:50:27,640 --> 00:50:31,040 This blue-green jewel. 575 00:50:31,040 --> 00:50:32,880 This Eden. 576 00:51:04,200 --> 00:51:09,000 How do scientists piece together what was happening on our Earth 577 00:51:09,000 --> 00:51:11,720 millions or even billions of years ago? 578 00:51:14,800 --> 00:51:19,200 - Our planet has a 4.5-billion-year history of change. 579 00:51:19,200 --> 00:51:22,840 And when I say change, I mean radical, dramatic, 580 00:51:22,840 --> 00:51:24,800 just astonishing change. 581 00:51:26,560 --> 00:51:31,480 - This episode featured the bizarre giant fungi, Prototaxites, 582 00:51:31,480 --> 00:51:35,480 that dominated land over 400 million years ago. 583 00:51:36,920 --> 00:51:40,760 - Prototaxites was a fossil fungal spike. 584 00:51:40,760 --> 00:51:44,520 They could grow up to be about 26 feet tall. 585 00:51:44,520 --> 00:51:46,880 So about the size of a two-storey house. 586 00:51:49,960 --> 00:51:53,320 - Not only are these one of the strangest organisms 587 00:51:53,320 --> 00:51:55,200 ever to grace the planet, 588 00:51:55,200 --> 00:51:58,200 the only clue to their existence were a series 589 00:51:58,200 --> 00:52:04,040 of mysterious fossils first discovered in 1843. 590 00:52:04,040 --> 00:52:05,800 - So here you have this great big thing. 591 00:52:05,800 --> 00:52:08,200 And when they started finding more of them, they were like, 592 00:52:08,200 --> 00:52:09,640 "Well, this is kind of like a trunk, 593 00:52:09,640 --> 00:52:11,560 "or it's shaped like a chunk of wood." 594 00:52:11,560 --> 00:52:12,840 But there's no wood. 595 00:52:12,840 --> 00:52:14,440 There's no trees. 596 00:52:14,440 --> 00:52:15,680 So what is this? 597 00:52:17,080 --> 00:52:19,200 - Until, in 2007, 598 00:52:19,200 --> 00:52:23,320 when microscope technology was able to take a closer look, 599 00:52:23,320 --> 00:52:27,960 revealing a cellular structure that was strangely familiar. 600 00:52:27,960 --> 00:52:31,280 - So what this is, is a very thin slice of Prototaxites. 601 00:52:35,760 --> 00:52:37,960 And we find that, unlike a log, 602 00:52:37,960 --> 00:52:39,760 which would be full of woody cells, 603 00:52:39,760 --> 00:52:42,720 instead we find a mass of these fungal filaments. 604 00:52:44,080 --> 00:52:45,880 - Looking at it more closely, 605 00:52:45,880 --> 00:52:49,280 they realised the structures were actually more similar to fungi. 606 00:52:51,520 --> 00:52:54,720 - These were gigantic tree-like fungi. 607 00:52:57,800 --> 00:53:01,520 - It creates, in my mind, one of the most bizarre 608 00:53:01,520 --> 00:53:03,960 prehistoric landscapes of all. 609 00:53:03,960 --> 00:53:08,480 And it's a great example of how ancient organisms 610 00:53:08,480 --> 00:53:12,760 sometimes look completely different from anything that's alive today. 611 00:53:16,400 --> 00:53:20,840 - Sometimes the challenge isn't identifying what a fossil is, 612 00:53:20,840 --> 00:53:24,240 it's figuring out how the parts fit together. 613 00:53:27,600 --> 00:53:30,200 As it was with Archaeopteris, 614 00:53:30,200 --> 00:53:32,840 one of the earliest trees on Earth. 615 00:53:34,600 --> 00:53:38,000 - Archaeopteris has a remarkable fossil history. 616 00:53:38,000 --> 00:53:40,200 So, first, the stem was discovered, 617 00:53:40,200 --> 00:53:43,240 and they recognised it because of its distinct type of wood. 618 00:53:44,960 --> 00:53:48,240 And then, at the same time, they found lots of fern-like foliage. 619 00:53:49,360 --> 00:53:51,480 However, we didn't think they were connected 620 00:53:51,480 --> 00:53:53,920 because they looked so drastically different. 621 00:53:57,520 --> 00:54:02,680 - Scientists initially thought they had two distinct plants. 622 00:54:02,680 --> 00:54:06,120 - Eventually, someone found a specimen that showed 623 00:54:06,120 --> 00:54:07,800 the two structures connected. 624 00:54:07,800 --> 00:54:11,200 And that's how we discovered that this was all 625 00:54:11,200 --> 00:54:12,440 part of the same plant. 626 00:54:14,640 --> 00:54:17,840 - And so Archaeopteris was discovered... 627 00:54:19,320 --> 00:54:23,600 ..and another chapter of the story of plants came into focus. 628 00:54:25,520 --> 00:54:28,600 - Archaeopteris fundamentally changed the Earth's landscape. 629 00:54:28,600 --> 00:54:29,760 For the first time, 630 00:54:29,760 --> 00:54:32,440 we had forests that we're so familiar with today. 631 00:54:33,440 --> 00:54:37,240 - Now, finding ancient fossilised plant life is one thing. 632 00:54:38,640 --> 00:54:41,840 How do we begin to learn about geological processes 633 00:54:41,840 --> 00:54:44,240 billions of years in the past? 634 00:54:45,720 --> 00:54:50,800 Like the origin of plate tectonics, an event still shrouded in mystery. 635 00:54:52,880 --> 00:54:54,960 - Studying the onset of plate tectonics 636 00:54:54,960 --> 00:54:56,880 is a hugely controversial area, 637 00:54:56,880 --> 00:54:59,480 and that's because the evidence is just so scant. 638 00:55:04,120 --> 00:55:08,200 - An eye-catching new idea leads some scientists to think 639 00:55:08,200 --> 00:55:10,920 plate tectonics started with a bang. 640 00:55:18,440 --> 00:55:21,200 - I would so love to have been there to watch 641 00:55:21,200 --> 00:55:24,240 a 30-mile asteroid smash into Earth. 642 00:55:34,200 --> 00:55:36,360 Now, I'd want to be out in space someplace, 643 00:55:36,360 --> 00:55:37,720 maybe on the moon. 644 00:55:40,440 --> 00:55:44,280 - But what's the evidence for such a cataclysmic event? 645 00:55:44,280 --> 00:55:46,560 - One thing that geologists can do is they can pick out 646 00:55:46,560 --> 00:55:51,560 small minerals from rocks and date them using isotopes. 647 00:55:51,560 --> 00:55:56,520 - For example, there are now evidence for very large asteroid impacts - 648 00:55:56,520 --> 00:55:59,200 in Australia, in South Africa - 649 00:55:59,200 --> 00:56:02,040 and that seems to correlate with some of the starting 650 00:56:02,040 --> 00:56:04,080 of plate tectonics. 651 00:56:04,080 --> 00:56:09,200 - Inside ancient rocks, geologists have discovered spherules, 652 00:56:09,200 --> 00:56:12,520 tiny droplets of melted material that form 653 00:56:12,520 --> 00:56:16,920 under the intense heat and pressure of asteroid impacts. 654 00:56:18,200 --> 00:56:22,560 Modelling of these impacts indicates that these massive bombardments 655 00:56:22,560 --> 00:56:25,800 played a role in triggering plate tectonics. 656 00:56:28,840 --> 00:56:31,520 - What happens if an asteroid hits that crust? 657 00:56:31,520 --> 00:56:34,760 Well, it smashes it like a plate falling on the floor, 658 00:56:34,760 --> 00:56:37,640 and those pieces get pushed down and moved around. 659 00:56:37,640 --> 00:56:39,640 And that's how the mantle and the crust 660 00:56:39,640 --> 00:56:42,520 could begin plate tectonics. 661 00:56:42,520 --> 00:56:44,800 - By hunting down clues today, 662 00:56:44,800 --> 00:56:49,720 scientists can unlock the secrets of the Earth's deep history, 663 00:56:49,720 --> 00:56:53,240 allowing us to tell our planet's story like never before. 664 00:57:00,760 --> 00:57:02,880 We journey back to where it all began... 665 00:57:04,920 --> 00:57:07,920 ..to tell the story of our atmosphere. 666 00:57:09,520 --> 00:57:13,680 How it emerged from a toxic orange hell 667 00:57:13,680 --> 00:57:17,880 and transformed a violent ball of rock 668 00:57:17,880 --> 00:57:22,200 into a beautiful, life-sustaining blue bubble, 669 00:57:22,200 --> 00:57:24,840 unique in the universe. 670 00:57:28,800 --> 00:57:31,600 If the Earth could talk, what would it tell us? 671 00:57:31,600 --> 00:57:34,200 Well, the Open University imagine how it might answer 672 00:57:34,200 --> 00:57:35,760 some of our questions. 673 00:57:35,760 --> 00:57:38,200 To experience this interactive presentation, 674 00:57:38,200 --> 00:57:39,960 go to the website on the screen 675 00:57:39,960 --> 00:57:42,200 and follow the links to the Open University.