1 00:00:01,500 --> 00:00:04,000 ♪ ♪ 2 00:00:04,000 --> 00:00:06,666 NARRATOR: 700 million years ago, 3 00:00:06,666 --> 00:00:11,800 ice threatened to make our planet uninhabitable... 4 00:00:13,700 --> 00:00:17,266 ...snuffing out new complex life forms. 5 00:00:19,633 --> 00:00:21,833 This is some of the deepest, most intense cold 6 00:00:21,833 --> 00:00:23,733 that Earth has ever experienced. 7 00:00:23,733 --> 00:00:25,766 Some people think the planet was completely 8 00:00:25,766 --> 00:00:30,166 encased in ice, a so-called Snowball Earth. 9 00:00:30,166 --> 00:00:34,700 ♪ ♪ 10 00:00:34,700 --> 00:00:36,200 MICHAEL WONG: You would have seen it 11 00:00:36,200 --> 00:00:38,666 as a glistening white marble hanging 12 00:00:38,666 --> 00:00:40,533 in the blackness of space. 13 00:00:40,533 --> 00:00:43,233 NARRATOR: A seemingly impossible fate 14 00:00:43,233 --> 00:00:46,700 for a planet so close to the sun. 15 00:00:46,700 --> 00:00:48,966 When Snowball Earth was first proposed, 16 00:00:48,966 --> 00:00:50,833 people thought it was an outrageous idea 17 00:00:50,833 --> 00:00:53,633 that this actually could have happened on Earth. 18 00:00:53,633 --> 00:00:55,833 NARRATOR: But now 19 00:00:55,833 --> 00:00:57,866 new techniques that are peering 20 00:00:57,866 --> 00:01:00,866 into the past say it was not only likely... 21 00:01:00,866 --> 00:01:03,833 CHARLOTTE SPRUZEN: Now we have geological evidence 22 00:01:03,833 --> 00:01:06,766 that nearly the whole world was plunged into a deep glaciation. 23 00:01:06,766 --> 00:01:10,633 NARRATOR: ...but also lucky for us. 24 00:01:10,633 --> 00:01:14,200 This snowball event most certainly shaped life on Earth. 25 00:01:14,200 --> 00:01:18,033 NARRATOR: From this climate catastrophe, 26 00:01:18,033 --> 00:01:20,866 new forms of life arose, 27 00:01:20,866 --> 00:01:24,666 eventually leading to the living world today. 28 00:01:25,700 --> 00:01:29,066 But it was a close call. 29 00:01:30,133 --> 00:01:34,400 How did life survive in this planetary deep freeze... 30 00:01:34,400 --> 00:01:36,133 (eruption roars) 31 00:01:36,133 --> 00:01:39,800 ...and thrive in its aftermath? 32 00:01:39,800 --> 00:01:43,266 "Ancient Earth: Frozen," 33 00:01:43,266 --> 00:01:45,633 right now on "NOVA." 34 00:01:45,633 --> 00:01:50,433 ♪ ♪ 35 00:02:02,200 --> 00:02:03,866 ("Never Close Enough" by SIPHO. playing) 36 00:02:03,866 --> 00:02:07,233 ♪ Oh, we won't ever hear the silence ♪ 37 00:02:07,233 --> 00:02:10,266 ♪ Or ever see the colors ♪ 38 00:02:10,266 --> 00:02:12,566 (exploding) 39 00:02:12,566 --> 00:02:15,566 ♪ That never lived in our minds ♪ 40 00:02:15,566 --> 00:02:18,266 ♪ ♪ 41 00:02:18,266 --> 00:02:19,600 ♪ Just a moment ♪ 42 00:02:19,600 --> 00:02:22,900 ♪ Never too far out ♪ 43 00:02:22,900 --> 00:02:26,566 ♪ Never close enough ♪ 44 00:02:37,066 --> 00:02:41,366 ♪ ♪ 45 00:02:41,366 --> 00:02:44,766 NARRATOR: Nearly a billion years ago, 46 00:02:44,766 --> 00:02:50,566 most of Earth's land forms a single giant continent. 47 00:02:53,533 --> 00:02:58,200 Rodinia. 48 00:02:58,200 --> 00:03:01,500 Thousands of miles coast to coast, 49 00:03:01,500 --> 00:03:05,633 this enormous landmass dominates the planet. 50 00:03:07,666 --> 00:03:09,266 ♪ ♪ 51 00:03:09,266 --> 00:03:13,833 But despite its size, from its jagged peaks 52 00:03:13,833 --> 00:03:16,233 across its desolate lowlands, 53 00:03:16,233 --> 00:03:19,133 there are no plants 54 00:03:19,133 --> 00:03:21,433 and no animals. 55 00:03:21,433 --> 00:03:24,866 ♪ ♪ 56 00:03:24,866 --> 00:03:30,566 Yet, in the shallow coastal waters, there is life. 57 00:03:30,566 --> 00:03:36,166 ♪ ♪ 58 00:03:36,166 --> 00:03:40,866 Microscopic bacteria gather to form living mats, 59 00:03:40,866 --> 00:03:44,666 clinging to the seafloor. 60 00:03:44,666 --> 00:03:48,000 For around three billion years, 61 00:03:48,000 --> 00:03:52,933 this has essentially been the extent of life on Earth. 62 00:03:52,933 --> 00:03:58,733 ♪ ♪ 63 00:03:58,733 --> 00:04:01,000 It's a planet dramatically different 64 00:04:01,000 --> 00:04:04,833 from the world we live in today. 65 00:04:07,200 --> 00:04:11,433 ♪ ♪ 66 00:04:15,000 --> 00:04:18,266 ♪ ♪ 67 00:04:26,733 --> 00:04:30,366 (insects chirping) 68 00:04:30,366 --> 00:04:32,500 Large, complex organisms that we have today, 69 00:04:32,500 --> 00:04:33,666 like plants and animals, 70 00:04:33,666 --> 00:04:36,966 wouldn't have existed. 71 00:04:36,966 --> 00:04:38,300 800 million years ago, 72 00:04:38,300 --> 00:04:40,366 the world would have looked pretty much alien 73 00:04:40,366 --> 00:04:42,533 for us today. 74 00:04:42,533 --> 00:04:43,800 SPRUZEN: If you were transported 75 00:04:43,800 --> 00:04:45,166 to the planet back then, 76 00:04:45,166 --> 00:04:46,966 you might not even think anything was living there. 77 00:04:49,766 --> 00:04:51,966 When in fact, Earth was teeming with life-- 78 00:04:51,966 --> 00:04:53,633 just microbial life. 79 00:04:53,633 --> 00:04:56,300 ♪ ♪ 80 00:04:56,300 --> 00:04:59,933 NARRATOR: As distant as this ancient world may seem... 81 00:05:01,900 --> 00:05:04,566 ...we are connected to it. 82 00:05:06,866 --> 00:05:10,200 Because the origins of animal life today can be traced 83 00:05:10,200 --> 00:05:14,966 back to those shallow coastal waters 800 million years ago. 84 00:05:23,200 --> 00:05:26,533 Amongst the bacteria... 85 00:05:30,933 --> 00:05:35,033 ...are giant cells that tower over the living mats. 86 00:05:35,033 --> 00:05:39,700 ♪ ♪ 87 00:05:39,700 --> 00:05:41,833 They are called eukaryotes 88 00:05:41,833 --> 00:05:46,100 and are the ancestors of all plants and animals living today, 89 00:05:46,100 --> 00:05:48,666 including us. 90 00:05:50,933 --> 00:05:55,533 Slowly evolving for over a billion years, 91 00:05:55,533 --> 00:06:00,200 larger and more complex than bacteria, 92 00:06:00,200 --> 00:06:02,066 they require more energy 93 00:06:02,066 --> 00:06:04,166 and nutrients from their surroundings. 94 00:06:08,100 --> 00:06:11,900 They are sustained by bacteria in the water and minerals 95 00:06:11,900 --> 00:06:14,333 washed in from the slowly eroding land. 96 00:06:14,333 --> 00:06:17,200 ♪ ♪ 97 00:06:17,200 --> 00:06:19,633 It's enough for them to survive, 98 00:06:19,633 --> 00:06:24,466 but probably too limited for them to get any more complex. 99 00:06:26,500 --> 00:06:29,033 All life on Earth is essentially made up 100 00:06:29,033 --> 00:06:30,400 of two types of cell, 101 00:06:30,400 --> 00:06:33,800 which we call prokaryotes and eukaryotes. 102 00:06:35,966 --> 00:06:39,533 NARRATOR: Prokaryotes are very simple, microscopic life 103 00:06:39,533 --> 00:06:42,366 and include many bacteria, 104 00:06:42,366 --> 00:06:47,233 with a loop of genetic material floating in the cell. 105 00:06:47,233 --> 00:06:52,033 A eukaryotic cell differs from a prokaryotic cell in that 106 00:06:52,033 --> 00:06:54,733 it contains all of its DNA 107 00:06:54,733 --> 00:06:59,333 inside of a central package that we call a nucleus. 108 00:07:00,733 --> 00:07:02,666 SUSANNAH PORTER: And these cells have a lot of other 109 00:07:02,666 --> 00:07:05,800 structures inside that we call organelles, 110 00:07:05,800 --> 00:07:08,700 and that have special, different functions 111 00:07:08,700 --> 00:07:09,733 inside the cell. 112 00:07:11,800 --> 00:07:15,400   NARRATOR: This is why we consider them more complex. 113 00:07:15,400 --> 00:07:19,566 It's a radical change from before. 114 00:07:19,566 --> 00:07:22,566 When they first appeared, they would have looked very simple. 115 00:07:24,000 --> 00:07:29,266 NARRATOR: But, over time, developed into more familiar forms. 116 00:07:29,266 --> 00:07:32,100 This is tiny fossil 117 00:07:32,100 --> 00:07:35,600 from North China, and it look very much like 118 00:07:35,600 --> 00:07:38,633 modern seaweed, but it just small. 119 00:07:38,633 --> 00:07:41,700 Orders of magnitude smaller than modern seaweed. 120 00:07:41,700 --> 00:07:45,266 NARRATOR: But for life to gain a foothold 121 00:07:45,266 --> 00:07:50,833 and further diversify, the Earth itself needed to change. 122 00:07:50,833 --> 00:07:56,800 ♪ ♪ 123 00:07:56,800 --> 00:08:00,366 In fact, the story of life is the story 124 00:08:00,366 --> 00:08:03,133 of our planet's changing geology. 125 00:08:05,500 --> 00:08:09,033 Life and rocks have co-evolved 126 00:08:09,033 --> 00:08:11,366 over billions of years. 127 00:08:11,366 --> 00:08:12,866 They're totally interconnected, 128 00:08:12,866 --> 00:08:17,033 interdependent on each other. 129 00:08:17,033 --> 00:08:19,333 Geology and biology 130 00:08:19,333 --> 00:08:23,366 are entwined together in the formation of the Earth. 131 00:08:23,366 --> 00:08:26,533 CHRISTOPHER JACKSON: When you think about geological timescales, 132 00:08:26,533 --> 00:08:27,966 it's actually quite easy 133 00:08:27,966 --> 00:08:30,166 to think about the really long-term, 134 00:08:30,166 --> 00:08:32,766 complex evolutions that can happen. 135 00:08:34,666 --> 00:08:36,533 RAYMOND PIERREHUMBERT: To understand these sweeps of time, 136 00:08:36,533 --> 00:08:40,166 you have to learn to think like a rock. 137 00:08:40,166 --> 00:08:42,233 Things happen very slowly if you're a rock. 138 00:08:45,200 --> 00:08:46,700 NARRATOR: And on Rodinia, 139 00:08:46,700 --> 00:08:52,833 an epic geological process will change life forever. 140 00:08:53,966 --> 00:08:59,800 ♪ ♪ 141 00:08:59,800 --> 00:09:02,200 It's on vast geologic time scales 142 00:09:02,200 --> 00:09:04,933 that profound and lasting change becomes apparent. 143 00:09:07,033 --> 00:09:09,166 COHEN: So we're in Iceland, 144 00:09:09,166 --> 00:09:11,300 and that kind of long-term geological change 145 00:09:11,300 --> 00:09:13,033 is happening here right now. 146 00:09:16,500 --> 00:09:17,700 It's really amazing knowing 147 00:09:17,700 --> 00:09:19,433 that I am standing on a place 148 00:09:19,433 --> 00:09:21,900 where two tectonic plates are pulling apart. 149 00:09:23,666 --> 00:09:27,166 NARRATOR: Tectonic plates are giant slabs of rock 150 00:09:27,166 --> 00:09:31,200 that lie beneath land and ocean and are constantly on the move. 151 00:09:32,833 --> 00:09:37,433 COHEN: That process happens about one to two inches a year. 152 00:09:37,433 --> 00:09:40,366 That's about the same speed that your fingernails grow. 153 00:09:42,500 --> 00:09:44,866 NARRATOR: This might not seem fast, 154 00:09:44,866 --> 00:09:47,600 but over time, movement like this has formed 155 00:09:47,600 --> 00:09:53,066 entire mountain ranges and torn continents apart. 156 00:09:53,066 --> 00:09:56,133 ♪ ♪ 157 00:09:56,133 --> 00:09:58,600 The rocks beneath my feet are being pulled apart 158 00:09:58,600 --> 00:10:01,966 along the Mid-Atlantic Ridge, 159 00:10:01,966 --> 00:10:04,633 which runs this way and this way for thousands of miles 160 00:10:04,633 --> 00:10:08,466 and has been opening for almost 200 million years. 161 00:10:11,166 --> 00:10:14,400 NARRATOR: These tectonic forces from deep within the Earth 162 00:10:14,400 --> 00:10:16,800 have been creating and then destroying 163 00:10:16,800 --> 00:10:19,333 Earth's most prominent features 164 00:10:19,333 --> 00:10:22,300 for billions of years. 165 00:10:22,300 --> 00:10:25,233 ♪ ♪ 166 00:10:25,233 --> 00:10:28,700 And around 800 million years ago, 167 00:10:28,700 --> 00:10:31,900 they began to divide Rodinia. 168 00:10:31,900 --> 00:10:37,900 ♪ ♪ 169 00:10:39,533 --> 00:10:42,233 Deep underground... 170 00:10:43,766 --> 00:10:46,500 ...over millions of years... 171 00:10:47,800 --> 00:10:53,500 ...swirling currents of heat cause tectonic plates 172 00:10:53,500 --> 00:10:57,433 to shift. 173 00:10:57,433 --> 00:11:01,400 Great rifts gape open across the landscape. 174 00:11:01,400 --> 00:11:03,833 (ground rumbling) 175 00:11:03,833 --> 00:11:08,533 This vast continent is slowly being ripped apart. 176 00:11:08,533 --> 00:11:13,066 ♪ ♪ 177 00:11:13,066 --> 00:11:17,433 Over millions of years, it creates deep valleys... 178 00:11:19,000 --> 00:11:24,033 ...and new seas, reshaping the continent. 179 00:11:28,700 --> 00:11:31,300 Minerals and nutrients wash 180 00:11:31,300 --> 00:11:33,500 from the land into the seas. 181 00:11:35,100 --> 00:11:39,733 ♪ ♪ 182 00:11:39,733 --> 00:11:43,433 Making the world around these eukaryotes 183 00:11:43,433 --> 00:11:46,266 a much more enriched place to live. 184 00:11:49,066 --> 00:11:51,166 This influx of resources 185 00:11:51,166 --> 00:11:54,666 radically increases their number and variety. 186 00:11:56,100 --> 00:12:00,100 And they are not alone. 187 00:12:00,100 --> 00:12:03,833 The whole ecosystem becomes filled 188 00:12:03,833 --> 00:12:07,400 with new and more complex life. 189 00:12:11,433 --> 00:12:13,533 The world is beginning to see 190 00:12:13,533 --> 00:12:17,300 more interconnectedness between organisms. 191 00:12:19,066 --> 00:12:23,200 Driven by Earth's dynamic geology. 192 00:12:23,200 --> 00:12:29,466 ♪ ♪ 193 00:12:31,733 --> 00:12:35,533 ♪ ♪ 194 00:12:35,533 --> 00:12:37,466 We can find some incredible evidence 195 00:12:37,466 --> 00:12:39,466 for this in the fossil record. 196 00:12:44,966 --> 00:12:46,400 COHEN: The fossils themselves 197 00:12:46,400 --> 00:12:48,200 are really small and they're hard to find. 198 00:12:48,200 --> 00:12:50,600 It's like trying to put together a puzzle 199 00:12:50,600 --> 00:12:53,800 when you only have half the pieces in the box. 200 00:12:53,800 --> 00:12:57,533 So this rock here, this has got the gray part 201 00:12:57,533 --> 00:12:59,233 and the dark part, 202 00:12:59,233 --> 00:13:02,300 and both of these contain microscopic fossils. 203 00:13:04,633 --> 00:13:07,433 They are structures that were made by early eukaryotes, 204 00:13:07,433 --> 00:13:09,900 and maybe 40 of them would fit 205 00:13:09,900 --> 00:13:12,800 in the period at an end of a sentence. 206 00:13:12,800 --> 00:13:15,300 NARRATOR: What these tiny fossil cells tell us 207 00:13:15,300 --> 00:13:17,433 is they were interacting 208 00:13:17,433 --> 00:13:21,233 with one another in an entirely new way. 209 00:13:23,100 --> 00:13:24,366 We can see spikes, 210 00:13:24,366 --> 00:13:27,366 spines, and other really amazing structures 211 00:13:27,366 --> 00:13:30,733 made from minerals that eroded off of Rodinia. 212 00:13:31,900 --> 00:13:34,100 It's energetically costly to make structures like this, 213 00:13:34,100 --> 00:13:36,566 and so there had to be a really good reason 214 00:13:36,566 --> 00:13:38,633 for organisms to spend all of this time 215 00:13:38,633 --> 00:13:40,200 and energy making them. 216 00:13:42,066 --> 00:13:46,633 NARRATOR: One idea to explain this evolutionary development 217 00:13:46,633 --> 00:13:51,033 suggests that early eukaryotes were facing a deadly new threat. 218 00:13:52,633 --> 00:13:55,633 So we think these organisms made these structures 219 00:13:55,633 --> 00:13:58,633 as defensive weapons to protect themselves from being eaten. 220 00:13:58,633 --> 00:14:04,466 ♪ ♪ 221 00:14:04,466 --> 00:14:05,766 They are some of the first evidence 222 00:14:05,766 --> 00:14:07,300 of predation in the fossil record. 223 00:14:09,233 --> 00:14:11,966 NARRATOR: So, what was eating them? 224 00:14:14,900 --> 00:14:18,466 I think the most exciting clues are that we see 225 00:14:18,466 --> 00:14:22,266 holes that are similar to bite marks. 226 00:14:22,266 --> 00:14:25,866 But in this case, they're microscopic. 227 00:14:25,866 --> 00:14:27,466 They're tinier than a, 228 00:14:27,466 --> 00:14:30,066 a thousandth of a millimeter in size. 229 00:14:33,466 --> 00:14:36,366 NARRATOR: More evidence is found in fossils 230 00:14:36,366 --> 00:14:40,000 from the Grand Canyon. 231 00:14:40,000 --> 00:14:42,600 So the fossil remains in this rock 232 00:14:42,600 --> 00:14:46,566 are little casings of organisms 233 00:14:46,566 --> 00:14:49,066 that look remarkably like 234 00:14:49,066 --> 00:14:51,833 a modern group of organisms that live today 235 00:14:51,833 --> 00:14:57,933 called testate amoebae, that are voracious predators. 236 00:14:57,933 --> 00:15:02,200 NARRATOR: They make holes in cells to suck out their prey, 237 00:15:02,200 --> 00:15:04,366 suggesting that something similar 238 00:15:04,366 --> 00:15:08,933 may have been eating ancient eukaryotes. 239 00:15:08,933 --> 00:15:13,033 If true, this would have changed everything. 240 00:15:14,200 --> 00:15:15,566 SANDERS: Predation, 241 00:15:15,566 --> 00:15:17,100 just like environmental pressures, 242 00:15:17,100 --> 00:15:22,300 can push life to experiment and try new things. 243 00:15:25,633 --> 00:15:28,233 GIRGUIS: There's a good chance that the seas around 244 00:15:28,233 --> 00:15:30,233 Rodinia may have been teeming 245 00:15:30,233 --> 00:15:32,500 with more diverse life than ever before. 246 00:15:34,266 --> 00:15:39,566 NARRATOR: And this life is slowly becoming more recognizable. 247 00:15:39,566 --> 00:15:43,900 But it's about to come under immense pressure. 248 00:15:43,900 --> 00:15:49,233 (ground cracking, rumbling) 249 00:15:49,233 --> 00:15:51,066 Although the supercontinent break-up 250 00:15:51,066 --> 00:15:52,800 could have helped fuel eukaryotic life, 251 00:15:52,800 --> 00:15:55,966 it also set into motion a series of events 252 00:15:55,966 --> 00:15:58,733 that could have led to catastrophe for them. 253 00:16:00,966 --> 00:16:04,400 ♪ ♪ 254 00:16:04,400 --> 00:16:06,933 NARRATOR: And that catastrophe... 255 00:16:06,933 --> 00:16:08,400 (erupts) 256 00:16:08,400 --> 00:16:10,766 ...begins with fire. 257 00:16:22,800 --> 00:16:28,866 On Rodinia, geological change accelerates rapidly. 258 00:16:34,400 --> 00:16:39,566 Superheated magma bursts through the surface as rifting 259 00:16:39,566 --> 00:16:42,633 thins Earth's crust in multiple places. 260 00:16:46,500 --> 00:16:50,800 Molten rock swamps over 800,000 square miles 261 00:16:50,800 --> 00:16:53,566 of the continent's surface, 262 00:16:53,566 --> 00:16:57,733 one of the largest outpourings in Earth's history. 263 00:16:58,833 --> 00:17:01,766 (hissing) 264 00:17:01,766 --> 00:17:03,466 As this lava cools, 265 00:17:03,466 --> 00:17:06,933 it becomes a dark rock called basalt. 266 00:17:06,933 --> 00:17:09,166 ♪ ♪ 267 00:17:09,166 --> 00:17:11,700 Creating a vast black stain 268 00:17:11,700 --> 00:17:14,500 across the heart of the continent. 269 00:17:17,666 --> 00:17:21,000 Which will have dire consequences. 270 00:17:26,266 --> 00:17:30,033 ♪ ♪ 271 00:17:30,033 --> 00:17:31,800 WONG: Basalt is incredibly important 272 00:17:31,800 --> 00:17:35,133 at impacting the atmosphere of the planet. 273 00:17:37,100 --> 00:17:38,466 NARRATOR: Basalt breaks down 274 00:17:38,466 --> 00:17:41,433 more readily than other rocks, like granite, 275 00:17:41,433 --> 00:17:44,366 in a process called silicate weathering. 276 00:17:47,233 --> 00:17:49,266 So when rain mixes with carbon dioxide 277 00:17:49,266 --> 00:17:52,233 in that atmosphere, it forms this weak acid 278 00:17:52,233 --> 00:17:55,700   that falls onto Earth's surface, weathers these rocks, 279 00:17:55,700 --> 00:17:58,800 weathers these silicates, and creates these compounds 280 00:17:58,800 --> 00:18:02,333 that wash into the oceans and eventually form carbonates. 281 00:18:02,333 --> 00:18:04,433 So, trapping carbon dioxide in the form 282 00:18:04,433 --> 00:18:06,600 of rocks on the seafloor. 283 00:18:06,600 --> 00:18:08,866 SPRUZEN: This can control our climate on a geological 284 00:18:08,866 --> 00:18:11,033 timescale, because it can affect the amount 285 00:18:11,033 --> 00:18:12,600 of carbon dioxide that can be drawn down 286 00:18:12,600 --> 00:18:13,900 from the atmosphere. 287 00:18:13,900 --> 00:18:15,733 So, when millions of tons of this rock 288 00:18:15,733 --> 00:18:17,533 poured out over the surface of Earth 289 00:18:17,533 --> 00:18:19,000 in the heart of Rodinia, 290 00:18:19,000 --> 00:18:22,366 it could have had a profound effect on Earth's climate. 291 00:18:23,500 --> 00:18:29,066 NARRATOR: The amount of basalt created only tells part of the story. 292 00:18:30,666 --> 00:18:34,233 Where it formed also plays a role. 293 00:18:35,700 --> 00:18:38,366 BENJAMIN MILLS: We think that it formed around the tropics, 294 00:18:38,366 --> 00:18:39,633 and this means they would have been subject 295 00:18:39,633 --> 00:18:40,633 to huge amounts of rainfall. 296 00:18:40,633 --> 00:18:43,533 (thunder claps, rain falling) 297 00:18:43,533 --> 00:18:46,033 JACKSON: So, the combination of large amounts of basalt 298 00:18:46,033 --> 00:18:48,300 being exposed at the Earth's surface 299 00:18:48,300 --> 00:18:50,166 and heavy, intense rainfall 300 00:18:50,166 --> 00:18:52,733 meant that a large amount of carbon dioxide 301 00:18:52,733 --> 00:18:55,133 was drawn out of the atmosphere. 302 00:18:57,100 --> 00:19:00,500 NARRATOR: This cools the Earth. 303 00:19:00,500 --> 00:19:02,066 WORDSWORTH: And this is because carbon dioxide 304 00:19:02,066 --> 00:19:03,700 is a greenhouse gas. 305 00:19:03,700 --> 00:19:06,433 It traps heat trying to escape from Earth, 306 00:19:06,433 --> 00:19:08,933 kind of like a warm blanket on a winter's day. 307 00:19:08,933 --> 00:19:11,666 So, you make that blanket thinner, 308 00:19:11,666 --> 00:19:14,000 the Earth has to get colder as a result. 309 00:19:24,033 --> 00:19:25,733 NARRATOR: Falling levels of carbon dioxide 310 00:19:25,733 --> 00:19:30,600 cause temperatures to plummet over millions of years. 311 00:19:33,400 --> 00:19:37,166 Eventually, ice forms. 312 00:19:42,233 --> 00:19:46,333 At first, it's confined to the poles. 313 00:19:46,333 --> 00:19:49,233 But it spreads. 314 00:19:49,233 --> 00:19:51,300 And for life on the planet, 315 00:19:51,300 --> 00:19:54,700 that's a potentially deadly challenge. 316 00:19:54,700 --> 00:20:00,666 ♪ ♪ 317 00:20:00,666 --> 00:20:03,633 Today, in certain places on Earth, 318 00:20:03,633 --> 00:20:07,766 you can vividly see the impacts that ice 319 00:20:07,766 --> 00:20:11,633 and falling temperatures have on ecosystems. 320 00:20:11,633 --> 00:20:17,400 ♪ ♪ 321 00:20:17,400 --> 00:20:19,333 SANDERS: In the valleys surrounding 322 00:20:19,333 --> 00:20:21,133 high mountains in a mountain range, 323 00:20:21,133 --> 00:20:23,333 you might expect to see 324 00:20:23,333 --> 00:20:25,266 a lot of different types of life. 325 00:20:25,266 --> 00:20:29,466 It's a very lush and hospitable environment. 326 00:20:29,466 --> 00:20:31,400 But as you go up the mountainside 327 00:20:31,400 --> 00:20:33,800 and temperatures get a little cooler, 328 00:20:33,800 --> 00:20:37,566   we start to see fewer and fewer kinds of life flourishing. 329 00:20:39,600 --> 00:20:42,633 NARRATOR: Cold and ice can destroy cells 330 00:20:42,633 --> 00:20:45,266 and slow down chemical reactions, 331 00:20:45,266 --> 00:20:47,266 impacting life. 332 00:20:49,200 --> 00:20:53,300 It's likely that 717 million years ago, 333 00:20:53,300 --> 00:20:57,600 as the Snowball Earth glaciation began, 334 00:20:57,600 --> 00:21:00,333 microscopic webs of life 335 00:21:00,333 --> 00:21:02,233 were also threatened. 336 00:21:10,366 --> 00:21:14,400 NARRATOR: On ancient Earth, 337 00:21:14,400 --> 00:21:17,266 the cold advances on early life. 338 00:21:17,266 --> 00:21:21,033 ♪ ♪ 339 00:21:21,033 --> 00:21:26,533 Sheets of sea ice thicken year by year, 340 00:21:26,533 --> 00:21:29,633 as they creep towards the shores of Rodinia 341 00:21:29,633 --> 00:21:32,066 from the North and South poles. 342 00:21:34,200 --> 00:21:36,433 And on land, 343 00:21:36,433 --> 00:21:40,100 huge ice caps expand down from the mountains. 344 00:21:41,766 --> 00:21:47,333 The places life can hold on are rapidly shrinking... 345 00:21:50,400 --> 00:21:54,866 ...as the ice tightens its grip across the planet. 346 00:21:56,566 --> 00:22:00,233 The big question for scientists is, 347 00:22:00,233 --> 00:22:06,533 why did this cooling trend turn into a runaway deep freeze? 348 00:22:06,533 --> 00:22:09,200 It's not like there's never been supercontinent break-up 349 00:22:09,200 --> 00:22:11,333 or vast outpourings of basalt before. 350 00:22:11,333 --> 00:22:13,766 So what was so different this time? 351 00:22:13,766 --> 00:22:15,966 It's really difficult to find out 352 00:22:15,966 --> 00:22:18,300 exactly what happened millions of years ago, 353 00:22:18,300 --> 00:22:21,000 because all we have left is this incomplete rock record, 354 00:22:21,000 --> 00:22:22,733 and translating that 355 00:22:22,733 --> 00:22:28,133 into its climate impacts is a very complicated process. 356 00:22:28,133 --> 00:22:29,566 It could have been that because 357 00:22:29,566 --> 00:22:31,366 Rodinia was breaking up around the Equator, 358 00:22:31,366 --> 00:22:33,666 that caused a lot more weathering 359 00:22:33,666 --> 00:22:36,100 than other supercontinent break-ups in the past. 360 00:22:37,800 --> 00:22:40,500 NARRATOR: One theory suggests new complex life 361 00:22:40,500 --> 00:22:43,666 may have changed ocean chemistry, 362 00:22:43,666 --> 00:22:47,666 which drew down carbon dioxide, trapping it. 363 00:22:47,666 --> 00:22:50,500 (eruption explodes) 364 00:22:50,500 --> 00:22:54,966 Another involves volcanic gases. 365 00:22:54,966 --> 00:22:59,400 PIERREHUMBERT: These large volcanic eruptions put a lot of sulfur dioxide, 366 00:22:59,400 --> 00:23:00,733 which turns into sulfuric acid, 367 00:23:00,733 --> 00:23:01,933 into the atmosphere, 368 00:23:01,933 --> 00:23:03,566 which reflects sunlight back to space. 369 00:23:05,066 --> 00:23:08,333   NARRATOR: Cooling the Earth. 370 00:23:08,333 --> 00:23:09,766 WORDSWORTH: Once this cooling began, 371 00:23:09,766 --> 00:23:12,433 it would have likely been incredibly difficult to stop. 372 00:23:20,100 --> 00:23:22,900 NARRATOR: The ice is bearing down. 373 00:23:22,900 --> 00:23:27,700 But a web of life can still thrive at lower latitudes. 374 00:23:30,566 --> 00:23:34,566 As the white expanse spreads, 375 00:23:34,566 --> 00:23:38,300 it reflects more of the sun's energy back into space. 376 00:23:41,000 --> 00:23:42,733 Driving down temperatures 377 00:23:42,733 --> 00:23:45,800 and expanding the reach of the ice, 378 00:23:45,800 --> 00:23:50,333 until it becomes a runaway process. 379 00:23:51,833 --> 00:23:56,733 This is known as the ice-albedo feedback effect. 380 00:23:56,733 --> 00:23:58,733 WONG: Once you cover 381 00:23:58,733 --> 00:24:01,300 a little patch of Earth with ice, 382 00:24:01,300 --> 00:24:03,533 the albedo of the planet has risen, 383 00:24:03,533 --> 00:24:06,133 which means it reflects more light to outer space 384 00:24:06,133 --> 00:24:09,033 and cools down, 385 00:24:09,033 --> 00:24:10,900 causing more water to freeze into ice, 386 00:24:10,900 --> 00:24:13,466 which reflects yet more light into space, 387 00:24:13,466 --> 00:24:16,866 cooling the planet, on and on and on. 388 00:24:18,200 --> 00:24:22,500 NARRATOR: Driving a continuous loop of self-reinforcing change 389 00:24:22,500 --> 00:24:26,000 that remakes the surface of the planet. 390 00:24:27,566 --> 00:24:30,500 Sea ice surges over the oceans, 391 00:24:30,500 --> 00:24:33,966 reaching a crushing half-mile thick in places. 392 00:24:37,033 --> 00:24:41,666 And near the Equator, ice closes in... 393 00:24:41,666 --> 00:24:46,766 ♪ ♪ 394 00:24:46,766 --> 00:24:50,633 ...creating a frozen planet. 395 00:24:57,333 --> 00:25:01,400 We suspect Earth became a snowball, 396 00:25:01,400 --> 00:25:03,400 because we know how far the ice reached, 397 00:25:03,400 --> 00:25:08,566 thanks to clues that were left behind. 398 00:25:08,566 --> 00:25:09,733 SPRUZEN: So, today, if you look 399 00:25:09,733 --> 00:25:13,033 underneath glaciers, you can see rocks 400 00:25:13,033 --> 00:25:16,666 that are trapped at the bottom of the ice. 401 00:25:16,666 --> 00:25:18,233 And then as it advances out to sea, 402 00:25:18,233 --> 00:25:20,833 it will carry these rocks along with it. 403 00:25:20,833 --> 00:25:22,866 And then as it melts, these big rocks at the bottom 404 00:25:22,866 --> 00:25:26,633 will drop into the deep ocean. 405 00:25:26,633 --> 00:25:28,933 NARRATOR: Scientists call these dropstones, 406 00:25:28,933 --> 00:25:31,566 because once they drop to the bottom of the ocean, 407 00:25:31,566 --> 00:25:35,100 they become embedded in the seafloor and look different 408 00:25:35,100 --> 00:25:37,566 from the surrounding layers. 409 00:25:37,566 --> 00:25:41,066 Over millions of years, that ocean floor turns to rock 410 00:25:41,066 --> 00:25:44,766 and can be exposed as dry land. 411 00:25:44,766 --> 00:25:47,866 This rock, funnily enough, is from the desert in Namibia, 412 00:25:47,866 --> 00:25:49,933 uh, but you can see 413 00:25:49,933 --> 00:25:55,366 there's this big other rock embedded in this brown mass. 414 00:25:55,366 --> 00:25:58,266 The only thing that could have dropped this stone 415 00:25:58,266 --> 00:26:00,000 into the ocean would have been ice. 416 00:26:00,000 --> 00:26:02,700 These dropstones from the Snowball Earth period 417 00:26:02,700 --> 00:26:05,566 have been documented literally around the globe. 418 00:26:05,566 --> 00:26:08,066 And that's how we know that Snowball Earth 419 00:26:08,066 --> 00:26:12,766 was an entire global event. 420 00:26:12,766 --> 00:26:14,533 There's still a lot of debate, however, 421 00:26:14,533 --> 00:26:16,133 on what exactly the Snowball Earth might have looked like. 422 00:26:18,466 --> 00:26:22,366 WONG: We rely on our knowledge of the geological record, 423 00:26:22,366 --> 00:26:24,800 as well as computer models, to tell us 424 00:26:24,800 --> 00:26:27,766 what Snowball Earth would have been like. 425 00:26:27,766 --> 00:26:30,700   One of the ideas is something we call a hard snowball. 426 00:26:30,700 --> 00:26:35,433 ♪ ♪ 427 00:26:35,433 --> 00:26:37,733 A complete entombment of the planet in ice, 428 00:26:37,733 --> 00:26:39,566 with not even any communication 429 00:26:39,566 --> 00:26:41,500 between the sea and the atmosphere. 430 00:26:43,333 --> 00:26:48,066 NARRATOR: Another theory is called Slushball, or Waterbelt Earth. 431 00:26:49,333 --> 00:26:51,333 WORDSWORTH: So, Slushball Earth would have 432 00:26:51,333 --> 00:26:54,233 kind of looked something halfway between our Earth today 433 00:26:54,233 --> 00:26:56,433 and a full global snowball. 434 00:26:56,433 --> 00:26:58,433 There would have been ice extending down, 435 00:26:58,433 --> 00:27:02,566 but probably some open water still around the Equator. 436 00:27:02,566 --> 00:27:04,166 COHEN: Other models suggest that 437 00:27:04,166 --> 00:27:05,466 while the seas would have been 438 00:27:05,466 --> 00:27:07,100 mainly covered by ice, 439 00:27:07,100 --> 00:27:09,000 on land, there would have been things 440 00:27:09,000 --> 00:27:11,200 like dry valleys and exposed mountains, 441 00:27:11,200 --> 00:27:13,866 like Antarctica today. 442 00:27:13,866 --> 00:27:17,000 NARRATOR: Whatever the scenario, this frozen world 443 00:27:17,000 --> 00:27:21,133 produced incredibly harsh conditions. 444 00:27:21,133 --> 00:27:23,133 BRENHIN KELLER: We're talking about temperatures 445 00:27:23,133 --> 00:27:25,566 near or below freezing 446 00:27:25,566 --> 00:27:28,533 even at the Equator. 447 00:27:30,033 --> 00:27:33,466 SPRUZEN: Climate models predict that the ice covering the oceans 448 00:27:33,466 --> 00:27:36,066 would have been several hundred feet to half a mile thick. 449 00:27:36,066 --> 00:27:39,466 ♪ ♪ 450 00:27:39,466 --> 00:27:43,833 NARRATOR: This would have had a huge impact on life. 451 00:27:43,833 --> 00:27:47,166 ♪ ♪ 452 00:27:54,566 --> 00:27:57,966 As the ice chokes our planet, 453 00:27:57,966 --> 00:28:02,733 it is a disaster for the majority of living things. 454 00:28:05,033 --> 00:28:09,900 Under ice, the ocean is mostly cut off from the atmosphere. 455 00:28:09,900 --> 00:28:13,333 Darkness closes in. 456 00:28:13,333 --> 00:28:18,100 And the flow of nutrients from the land slows to a trickle. 457 00:28:20,233 --> 00:28:23,333 Under this pressure, 458 00:28:23,333 --> 00:28:28,166 ecosystems begin to fall apart. 459 00:28:28,166 --> 00:28:30,400 Imprisoned by the ice, 460 00:28:30,400 --> 00:28:35,766 the predecessors of all animal life falter. 461 00:28:39,166 --> 00:28:42,266 And many die. 462 00:28:42,266 --> 00:28:47,400 ♪ ♪ 463 00:28:52,866 --> 00:28:58,000 ♪ ♪ 464 00:29:01,133 --> 00:29:03,733 It's easy to imagine how this could have ended 465 00:29:03,733 --> 00:29:05,700 all complex life on Earth. 466 00:29:09,400 --> 00:29:10,766 JAZ MILLAR: We know that life 467 00:29:10,766 --> 00:29:12,266 must have survived through Snowball Earth 468 00:29:12,266 --> 00:29:13,333 because we still have a lot of life 469 00:29:13,333 --> 00:29:14,633 in the Earth today. 470 00:29:14,633 --> 00:29:16,666 We had life before and after, 471 00:29:16,666 --> 00:29:18,000 so, somehow, it must have survived. 472 00:29:20,433 --> 00:29:22,000   NARRATOR: But it's hard to prove where, 473 00:29:22,000 --> 00:29:26,366 as all the ice from back then has melted. 474 00:29:28,500 --> 00:29:30,866 MILLAR: To understand how life survived in ice in the past, 475 00:29:30,866 --> 00:29:33,833 then we look to how life survives in ice today. 476 00:29:33,833 --> 00:29:35,833 And there is one habitat 477 00:29:35,833 --> 00:29:38,433 that Snowball Earth scientists are particularly interested in. 478 00:29:38,433 --> 00:29:43,433 So, we're gonna see if we can try and find that today. 479 00:29:43,433 --> 00:29:47,666 ♪ ♪ 480 00:29:47,666 --> 00:29:51,033 NARRATOR: There is more to this frigid landscape than meets the eye. 481 00:29:51,033 --> 00:29:55,033 ♪ ♪ 482 00:29:58,866 --> 00:30:02,300 What I found here is a hole in the glacier surface. 483 00:30:02,300 --> 00:30:05,600 We refer to this as a cryoconite hole. 484 00:30:05,600 --> 00:30:08,266 Sediment lands on the glacier surface, 485 00:30:08,266 --> 00:30:11,333 swept in by water and by wind. 486 00:30:11,333 --> 00:30:14,200 Then the radiation from the sun 487 00:30:14,200 --> 00:30:16,600 warms it, and it melts the underlying ice. 488 00:30:16,600 --> 00:30:19,566 So it melts almost vertically down 489 00:30:19,566 --> 00:30:21,900 to make this pocket that we see here, 490 00:30:21,900 --> 00:30:26,033 with meltwater at the bottom and open at the top. 491 00:30:27,066 --> 00:30:30,100 NARRATOR: Look hard enough in the right places, 492 00:30:30,100 --> 00:30:34,466 and you'll find these on glacial surfaces everywhere. 493 00:30:37,000 --> 00:30:39,666 MILLAR: The cryoconite holes are perfect 494 00:30:39,666 --> 00:30:43,166 for microorganisms because they have access to the sun, 495 00:30:43,166 --> 00:30:45,133 and they have access to meltwater. 496 00:30:46,700 --> 00:30:49,300 NARRATOR: That makes these holes refuges 497 00:30:49,300 --> 00:30:52,133 for a variety of life. (device beeps) 498 00:30:52,133 --> 00:30:55,133 I'm gonna try and sample this cryoconite hole. 499 00:30:55,133 --> 00:30:59,166 ♪ ♪ 500 00:31:06,400 --> 00:31:08,333 I'm seeing some bits are a little bit green, 501 00:31:08,333 --> 00:31:11,100 so they might be bits of 502 00:31:11,100 --> 00:31:14,966 photosynthetic microorganisms or they might be bits of plants. 503 00:31:14,966 --> 00:31:18,900 NARRATOR: Other samples have revealed even more life. 504 00:31:18,900 --> 00:31:21,300 A huge range of organisms, from fungi 505 00:31:21,300 --> 00:31:22,766 to micro-animals. 506 00:31:24,700 --> 00:31:28,000 This captures the diversity of the organisms 507 00:31:28,000 --> 00:31:30,133 living in cryoconite holes, 508 00:31:30,133 --> 00:31:32,466 not just in species, but in size 509 00:31:32,466 --> 00:31:34,733 and shape and function. 510 00:31:34,733 --> 00:31:37,866 ♪ ♪ 511 00:31:37,866 --> 00:31:41,033 It's surprising and it's quite magnificent 512 00:31:41,033 --> 00:31:43,333 that these quite empty-looking spots 513 00:31:43,333 --> 00:31:45,733 can have this much diversity. 514 00:31:47,366 --> 00:31:50,066 NARRATOR: Even when life like this is subjected 515 00:31:50,066 --> 00:31:52,666 to harsh Snowball Earth conditions, 516 00:31:52,666 --> 00:31:55,200 it survives. 517 00:31:55,200 --> 00:31:57,600 MILLAR: For us, as humans, Snowball Earth will always seem 518 00:31:57,600 --> 00:32:00,400 like a harsh, almost impossible planet to live on. 519 00:32:00,400 --> 00:32:04,466 However, when we look at the depths of Antarctica 520 00:32:04,466 --> 00:32:06,666 and the middle of ice sheets in the Arctic, 521 00:32:06,666 --> 00:32:09,866 there is life wherever you look for it. 522 00:32:12,900 --> 00:32:15,400 NARRATOR: And it's not just cryoconites. 523 00:32:15,400 --> 00:32:18,233 There's various different places eukaryotic life 524 00:32:18,233 --> 00:32:21,866 may have survived on Snowball Earth. 525 00:32:21,866 --> 00:32:23,300 GIRGUIS: It could have been 526 00:32:23,300 --> 00:32:27,733 in little glacial ponds or lakes. 527 00:32:27,733 --> 00:32:30,033   Uh, hot springs, which surely would have been around then 528 00:32:30,033 --> 00:32:32,033 as they are today. 529 00:32:32,033 --> 00:32:33,633 SANDERS: Some may have survived 530 00:32:33,633 --> 00:32:37,233 deep underground in subterranean caves, 531 00:32:37,233 --> 00:32:38,800 where there's water 532 00:32:38,800 --> 00:32:39,900 and temperatures are higher. 533 00:32:39,900 --> 00:32:41,666 And then, of course, the oceans. 534 00:32:41,666 --> 00:32:44,400 ♪ ♪ 535 00:32:44,400 --> 00:32:48,800 NARRATOR: But this is no easy life. 536 00:32:48,800 --> 00:32:50,566 For life to flourish again, 537 00:32:50,566 --> 00:32:52,833 we would need some kind of drastic change 538 00:32:52,833 --> 00:32:56,633   for life to escape this icy prison. 539 00:32:56,633 --> 00:32:59,133 NARRATOR: Ironically, this change 540 00:32:59,133 --> 00:33:02,233 would be driven by the same powerful tectonic process 541 00:33:02,233 --> 00:33:05,133 thought to have triggered Snowball Earth 542 00:33:05,133 --> 00:33:07,466 in the first place-- 543 00:33:07,466 --> 00:33:11,200 a process that had never stopped. 544 00:33:15,600 --> 00:33:20,600 ♪ ♪ 545 00:33:20,600 --> 00:33:23,666 Deep below Earth's surface, 546 00:33:23,666 --> 00:33:27,900 the fire still burns. 547 00:33:27,900 --> 00:33:31,933 ♪ ♪ 548 00:33:35,666 --> 00:33:37,166 (eruption roars) 549 00:33:42,033 --> 00:33:44,933 The ice can't stop volcanoes 550 00:33:44,933 --> 00:33:49,933 from punching through the planet's frozen shell. 551 00:33:49,933 --> 00:33:53,166 (volcano erupts) 552 00:33:57,100 --> 00:33:58,633 WORDSWORTH: Even in the depths of the Snowball Earth period, 553 00:33:58,633 --> 00:34:01,700 volcanism would never stop completely, 554 00:34:01,700 --> 00:34:03,033 because there would still be 555 00:34:03,033 --> 00:34:04,566 Earth's internal heat driving it. 556 00:34:04,566 --> 00:34:08,966 (eruption roars) 557 00:34:15,400 --> 00:34:17,666 NARRATOR: But it is not heat from the lava 558 00:34:17,666 --> 00:34:22,300 that impacts Earth's climate. 559 00:34:22,300 --> 00:34:23,800 Lava is not what melts the snowball. 560 00:34:23,800 --> 00:34:25,966 It's actually an invisible force, 561 00:34:25,966 --> 00:34:27,566 which is the carbon dioxide 562 00:34:27,566 --> 00:34:30,966 being produced by volcanoes and building up in the atmosphere. 563 00:34:30,966 --> 00:34:34,900 ♪ ♪ 564 00:34:34,900 --> 00:34:36,100 This would have served 565 00:34:36,100 --> 00:34:37,233 to trap heat against the Earth, 566 00:34:37,233 --> 00:34:39,500 causing a rise in global temperatures. 567 00:34:41,300 --> 00:34:43,700 NARRATOR: Before the Snowball Earth period, 568 00:34:43,700 --> 00:34:47,166 carbon dioxide was removed from the atmosphere by reacting 569 00:34:47,166 --> 00:34:49,566 with sea water or with rocks on land 570 00:34:49,566 --> 00:34:51,466 via silicate weathering. 571 00:34:53,233 --> 00:34:54,700 But if you cover the entire planet in ice, 572 00:34:54,700 --> 00:34:56,066 as was the case during Snowball Earth, 573 00:34:56,066 --> 00:34:59,166 this is effectively shut off. 574 00:34:59,166 --> 00:35:00,500 NARRATOR: The ice layer 575 00:35:00,500 --> 00:35:02,233 would stop the weathering process 576 00:35:02,233 --> 00:35:04,133 that had been pulling carbon dioxide 577 00:35:04,133 --> 00:35:07,366 from the atmosphere. 578 00:35:07,366 --> 00:35:08,733 And so the carbon dioxide 579 00:35:08,733 --> 00:35:09,933 builds up into the atmosphere, 580 00:35:09,933 --> 00:35:12,466 until it gets warm enough to melt the ice. 581 00:35:17,100 --> 00:35:20,933 (volcano erupts) 582 00:35:23,200 --> 00:35:27,500 NARRATOR: As each eruption tears through the ice-bound plains... 583 00:35:27,500 --> 00:35:29,933 ♪ ♪ 584 00:35:29,933 --> 00:35:34,966 ...it releases a mix of volcanic gases, 585 00:35:34,966 --> 00:35:39,333 including carbon dioxide, 586 00:35:39,333 --> 00:35:43,266 each time adding to the atmosphere, 587 00:35:43,266 --> 00:35:46,733 leading to tiny increases in global temperature. 588 00:35:48,333 --> 00:35:53,600 As concentrations rise and build over millions of years, 589 00:35:53,600 --> 00:35:57,766 the gas traps ever more heat. 590 00:35:57,766 --> 00:36:01,133 The frozen planet 591 00:36:01,133 --> 00:36:05,400 begins to thaw. 592 00:36:05,400 --> 00:36:09,000 ♪ ♪ 593 00:36:09,000 --> 00:36:13,600 How long it took is still debated. 594 00:36:13,600 --> 00:36:15,800 PIERREHUMBERT: Almost everybody agrees that, 595 00:36:15,800 --> 00:36:18,133 in geological terms, the melting, 596 00:36:18,133 --> 00:36:21,266 the deglaciation of Snowball Earth, was really fast. 597 00:36:21,266 --> 00:36:23,266 Where the disputes are is whether fast 598 00:36:23,266 --> 00:36:25,466 means a matter of hundreds of years 599 00:36:25,466 --> 00:36:27,166 or maybe up to a million years. 600 00:36:27,166 --> 00:36:29,066 It is incredible to imagine 601 00:36:29,066 --> 00:36:30,766 that a global glaciation 602 00:36:30,766 --> 00:36:32,066 could have been undone in just 603 00:36:32,066 --> 00:36:33,900 a couple thousands of years. 604 00:36:33,900 --> 00:36:36,966 This is a geologically instantaneous amount of time. 605 00:36:36,966 --> 00:36:40,133 ♪ ♪ 606 00:36:47,633 --> 00:36:49,200 NARRATOR: The rapid, thousand-year 607 00:36:49,200 --> 00:36:53,200 melting speed is a contentious theory, 608 00:36:53,200 --> 00:36:57,900 but there are geological clues left that support it. 609 00:36:57,900 --> 00:37:01,566 ♪ ♪ 610 00:37:01,566 --> 00:37:03,166 JACKSON: We don't know precisely what happened 611 00:37:03,166 --> 00:37:05,766 millions of years ago because we weren't there. 612 00:37:05,766 --> 00:37:07,100 But one theory is that, 613 00:37:07,100 --> 00:37:09,400 by looking at these types of rocks, 614 00:37:09,400 --> 00:37:10,966 called cap carbonates, 615 00:37:10,966 --> 00:37:14,900 we can understand how Snowball Earth melted. 616 00:37:14,900 --> 00:37:18,466 NARRATOR: These are found in vast quantities. 617 00:37:18,466 --> 00:37:19,900 JACKSON: This was actually found 618 00:37:19,900 --> 00:37:23,066 in a rock sequence that was 72 feet thick, 619 00:37:23,066 --> 00:37:24,433 so that's a whole cliffside. 620 00:37:26,100 --> 00:37:28,566 One idea is that they represent the products 621 00:37:28,566 --> 00:37:30,000 of the intense chemical weathering 622 00:37:30,000 --> 00:37:31,833 that happened at the end of Snowball Earth, 623 00:37:31,833 --> 00:37:33,600 as the snowball was melting. 624 00:37:34,633 --> 00:37:39,100 NARRATOR: But this rock can also suggest how fast it happened. 625 00:37:39,100 --> 00:37:40,833 If the melt had been slow, 626 00:37:40,833 --> 00:37:43,466 impurities would have had time to accumulate in here, 627 00:37:43,466 --> 00:37:44,733 and that's not what we see. 628 00:37:44,733 --> 00:37:46,633 There's no impurities within it. 629 00:37:46,633 --> 00:37:49,500 This tells us that the melting was actually very rapid. 630 00:37:50,766 --> 00:37:53,200 NARRATOR: Further evidence that this could have happened 631 00:37:53,200 --> 00:37:56,366 in around 1,000 years. 632 00:37:59,633 --> 00:38:05,033 (water trickling) 633 00:38:05,033 --> 00:38:10,000 635 million years ago, 634 00:38:10,000 --> 00:38:11,900 as the planet warms... 635 00:38:11,900 --> 00:38:16,066 ♪ ♪ 636 00:38:16,066 --> 00:38:18,533 ...the melting of gigantic glaciers 637 00:38:18,533 --> 00:38:23,333 drives intense evaporation of water into the atmosphere. 638 00:38:23,333 --> 00:38:27,833 ♪ ♪ 639 00:38:27,833 --> 00:38:29,633 Water vapor is a strong greenhouse gas. 640 00:38:29,633 --> 00:38:30,933 So this would have kind of acted 641 00:38:30,933 --> 00:38:32,966 like an accelerator pedal on the whole process. 642 00:38:32,966 --> 00:38:34,700 Once the thawing begins, 643 00:38:34,700 --> 00:38:36,466 and the ice caps start to retreat, 644 00:38:36,466 --> 00:38:38,000 the planet becomes darker, 645 00:38:38,000 --> 00:38:41,766 enabling it to absorb more energy from the sun. 646 00:38:41,766 --> 00:38:45,600 PU: As the dark oceans and continents absorb more heat 647 00:38:45,600 --> 00:38:47,300 from the sun, this would have helped 648 00:38:47,300 --> 00:38:50,066 warm the planet and decrease the amount of ice left on it. 649 00:38:50,066 --> 00:38:51,966 This can be considered 650 00:38:51,966 --> 00:38:53,466 the exact reverse process 651 00:38:53,466 --> 00:38:54,833 of how the snowball was formed. 652 00:38:58,000 --> 00:39:02,066 NARRATOR: But this is not a tranquil time. 653 00:39:07,600 --> 00:39:10,233 When the Snowball Earth ended, 654 00:39:10,233 --> 00:39:13,200 it ended in a catastrophic way. 655 00:39:13,200 --> 00:39:18,866 ♪ ♪ 656 00:39:18,866 --> 00:39:21,533 NARRATOR: What starts as a trickle 657 00:39:21,533 --> 00:39:25,166 becomes a torrent. 658 00:39:35,600 --> 00:39:41,600 And the melt becomes unstoppable. 659 00:39:44,533 --> 00:39:48,266 A runaway event 660 00:39:48,266 --> 00:39:51,633 that releases a deluge of meltwater 661 00:39:51,633 --> 00:39:54,266 on an almost unimaginable scale. 662 00:39:57,733 --> 00:39:59,666 You can imagine these huge waterfalls 663 00:39:59,666 --> 00:40:01,700 coming off the edge of glaciers. 664 00:40:01,700 --> 00:40:03,700 PIERREHUMBERT: You would have 665 00:40:03,700 --> 00:40:07,500 almost certainly had these really massive tropical storms 666 00:40:07,500 --> 00:40:11,833 unlike anything that we see today. 667 00:40:11,833 --> 00:40:12,833 We call them hypercanes. 668 00:40:12,833 --> 00:40:14,633 PU: There must have been 669 00:40:14,633 --> 00:40:18,466 acid rain coming down from the skies above... 670 00:40:21,100 --> 00:40:23,333 ...just completely obliterating the surface of the Earth. 671 00:40:23,333 --> 00:40:26,200 ♪ ♪ 672 00:40:26,200 --> 00:40:27,666 NARRATOR: At the peak of the melt, 673 00:40:27,666 --> 00:40:33,133 sea levels rise over six feet every decade, 674 00:40:33,133 --> 00:40:37,633 swallowing great chunks of coastline. 675 00:40:39,066 --> 00:40:43,833 After a total of over 50 million years in the deep freeze, 676 00:40:43,833 --> 00:40:46,166 the world beneath the ice 677 00:40:46,166 --> 00:40:50,133 finally emerges, 678 00:40:50,133 --> 00:40:55,100 its warm shores a stark contrast. 679 00:40:56,633 --> 00:41:02,133 Perhaps over 120 degrees Fahrenheit at the Equator. 680 00:41:05,333 --> 00:41:06,733 COHEN: If you were transported back, 681 00:41:06,733 --> 00:41:09,100 the oceans would have been very tropical. 682 00:41:12,333 --> 00:41:16,233 XIAO: That change from freeze to fry is a big challenge 683 00:41:16,233 --> 00:41:17,966 to all kinds of life. 684 00:41:17,966 --> 00:41:21,733 GIRGUIS: Eukaryotes and all life on Earth of course 685 00:41:21,733 --> 00:41:22,733 would have evolved 686 00:41:22,733 --> 00:41:23,733 to the warming temperatures. 687 00:41:23,733 --> 00:41:25,066 But evolution is slow. 688 00:41:25,066 --> 00:41:29,066 Doesn't happen overnight. 689 00:41:29,066 --> 00:41:31,333 NARRATOR: Many would have died. 690 00:41:31,333 --> 00:41:32,900 And those that survived 691 00:41:32,900 --> 00:41:36,933 would remain reliant on nutrient streams from the land. 692 00:41:36,933 --> 00:41:42,033 ♪ ♪ 693 00:41:44,600 --> 00:41:48,533 This is a changed world, 694 00:41:48,533 --> 00:41:50,966 the land utterly transformed 695 00:41:50,966 --> 00:41:55,300 by the incredible power of glaciers, 696 00:41:55,300 --> 00:41:58,933 a force that can still be seen today. 697 00:41:58,933 --> 00:42:01,633 ♪ ♪ 698 00:42:04,233 --> 00:42:05,700 Well, I'm lucky enough to be standing 699 00:42:05,700 --> 00:42:07,366 at the foot of a glacier, 700 00:42:07,366 --> 00:42:10,033 and this big face here next to me, 701 00:42:10,033 --> 00:42:12,566 it's really dark, really black. 702 00:42:12,566 --> 00:42:15,766 But it's not rock-- it's actually ice. 703 00:42:15,766 --> 00:42:19,033 And although it looks pretty stable and static now, 704 00:42:19,033 --> 00:42:22,266 it's actually moving towards me at three feet per day. 705 00:42:23,700 --> 00:42:28,166 I mean, it's a really kind of eerie place to be standing. 706 00:42:33,600 --> 00:42:35,266 If we look up into the mountains, 707 00:42:35,266 --> 00:42:38,100 we can see that this glacier has carved its way down 708 00:42:38,100 --> 00:42:40,866 into this deep valley right to where I'm standing. 709 00:42:40,866 --> 00:42:43,833 This has broken up all of this 710 00:42:43,833 --> 00:42:46,566 material around us and these boulders, 711 00:42:46,566 --> 00:42:51,466 and then this really fine clay-like material, as well. 712 00:42:51,466 --> 00:42:53,933 But imagine this happening during Snowball Earth, 713 00:42:53,933 --> 00:42:56,066 across the whole planet. 714 00:42:58,200 --> 00:43:00,600 NARRATOR: It's thought that the glaciers back then 715 00:43:00,600 --> 00:43:05,000 were just as scouring as today's. 716 00:43:05,000 --> 00:43:07,300 JACKSON: These glaciers were able to remove 717 00:43:07,300 --> 00:43:09,900 about two miles of rock from below them. 718 00:43:09,900 --> 00:43:12,800 That's about twice the depth of the Grand Canyon. 719 00:43:12,800 --> 00:43:15,700 So, the power of these glaciers was enough to remove 720 00:43:15,700 --> 00:43:18,533 vast swathes of the rock record, 721 00:43:18,533 --> 00:43:23,100 leaving behind gaps. 722 00:43:23,100 --> 00:43:25,366 ♪ ♪ 723 00:43:25,366 --> 00:43:28,000 NARRATOR: Many scientists believe that this could explain 724 00:43:28,000 --> 00:43:33,300 a geologic mystery known as the Great Unconformity. 725 00:43:34,900 --> 00:43:39,066 So, this rock is about 520 million years old, 726 00:43:39,066 --> 00:43:41,133 but it sits just above 727 00:43:41,133 --> 00:43:43,200 this three-billion-year-old granite. 728 00:43:43,200 --> 00:43:44,533 So, the amount of time missing 729 00:43:44,533 --> 00:43:46,400 is about 2.5 billion years. 730 00:43:46,400 --> 00:43:49,533 And we think that the rocks may have been partly eroded away 731 00:43:49,533 --> 00:43:52,066 by glaciers during the Snowball Earth period. 732 00:43:53,033 --> 00:43:55,033 NARRATOR: Another theory is, this gap 733 00:43:55,033 --> 00:44:00,266 may be due to the movement of tectonic plates. 734 00:44:00,266 --> 00:44:03,333 KELLER: But if it's true that glacial erosion is responsible, 735 00:44:03,333 --> 00:44:05,933 we're talking about more than a vertical mile 736 00:44:05,933 --> 00:44:08,066 of erosion on average globally. 737 00:44:08,066 --> 00:44:10,166 That's a billion billion tons. 738 00:44:10,166 --> 00:44:16,933 ♪ ♪ 739 00:44:16,933 --> 00:44:20,033 NARRATOR: This incredible amount of ground-up rock 740 00:44:20,033 --> 00:44:24,866 is deposited in the world's rivers and oceans... 741 00:44:26,300 --> 00:44:30,866 ...helping single-celled algae and bacteria to flourish... 742 00:44:32,666 --> 00:44:36,366 ...growing in massive blooms... 743 00:44:37,433 --> 00:44:43,166 ...likely visible from space. 744 00:44:43,166 --> 00:44:47,333 And these may have impacted the entire ocean systems. 745 00:44:50,933 --> 00:44:53,000 The oxygen levels in the water 746 00:44:53,000 --> 00:44:57,100 greatly increase, 747 00:44:57,100 --> 00:45:01,266 generated by high levels of photosynthesis. 748 00:45:01,266 --> 00:45:06,266 ♪ ♪ 749 00:45:07,866 --> 00:45:10,900 In the aftermath of the great thaw, 750 00:45:10,900 --> 00:45:14,166 conditions seem possible for eukaryotic life 751 00:45:14,166 --> 00:45:16,566 to take another leap forward. 752 00:45:16,566 --> 00:45:19,066 COHEN: The oceans would have been a really great place for life, 753 00:45:19,066 --> 00:45:22,933 with sunlight, oxygen, warmth, and nutrients. 754 00:45:25,333 --> 00:45:26,666 NARRATOR: And the melting of the ice 755 00:45:26,666 --> 00:45:29,433 would have created more shallow marine habitats 756 00:45:29,433 --> 00:45:31,500 to expand into. 757 00:45:33,566 --> 00:45:37,366 SANDERS: It's a time when life really has a lot of opportunities 758 00:45:37,366 --> 00:45:40,533 to experiment and diversify. 759 00:45:40,533 --> 00:45:44,000 Once more stable and productive interaction 760 00:45:44,000 --> 00:45:46,266 starts to happen between the Earth 761 00:45:46,266 --> 00:45:48,033 and the life that existed on it, 762 00:45:48,033 --> 00:45:49,833 it didn't actually take that long 763 00:45:49,833 --> 00:45:53,000 for much more complex life forms to evolve. 764 00:45:54,800 --> 00:45:56,933 NARRATOR: This is a major turning point, 765 00:45:56,933 --> 00:46:00,500 unlike anything that came before. 766 00:46:00,500 --> 00:46:05,200 Finally, individual organisms are becoming complex 767 00:46:05,200 --> 00:46:10,900 and big enough that we could see them with our own eyes. 768 00:46:13,166 --> 00:46:14,933 XIAO: Some scientists think that 769 00:46:14,933 --> 00:46:18,033 the increase in the size of eukaryotic life 770 00:46:18,033 --> 00:46:21,600 has to do with the availability of oxygen. 771 00:46:21,600 --> 00:46:23,466 The increased availability of oxygen 772 00:46:23,466 --> 00:46:26,966 may have allowed organisms to access 773 00:46:26,966 --> 00:46:28,733 a greater amount of energy 774 00:46:28,733 --> 00:46:32,400 than, perhaps, they were previously doing. 775 00:46:32,400 --> 00:46:35,633 It's also possible that phosphorus played a key role. 776 00:46:35,633 --> 00:46:37,466 Photosynthesis requires phosphorus, 777 00:46:37,466 --> 00:46:39,000 and that forms the basis of the food chain. 778 00:46:39,000 --> 00:46:43,366 This could have fueled a very productive ecosystem. 779 00:46:43,366 --> 00:46:45,833 And it might not be oxygen or phosphorus 780 00:46:45,833 --> 00:46:47,900 or any of the other theories we have, 781 00:46:47,900 --> 00:46:50,166 and it may just be they needed the time 782 00:46:50,166 --> 00:46:52,333 and the space to evolve. 783 00:46:52,333 --> 00:46:57,366 ♪ ♪ 784 00:46:59,366 --> 00:47:03,000 NARRATOR: In shallow coastal waters 785 00:47:03,000 --> 00:47:06,166 across the planet, 786 00:47:06,166 --> 00:47:11,566 life blossoms. 787 00:47:11,566 --> 00:47:16,100 This myriad of strange forms may all look like plants... 788 00:47:17,466 --> 00:47:20,833 ...but some are animals-- 789 00:47:20,833 --> 00:47:23,933 grazers, predators, 790 00:47:23,933 --> 00:47:26,733 and prey. 791 00:47:26,733 --> 00:47:30,300 Descendants of those first tiny pioneers, 792 00:47:30,300 --> 00:47:35,466 but more complex. 793 00:47:35,466 --> 00:47:39,333 Now collections of these cells work together, 794 00:47:39,333 --> 00:47:44,000 finally broken free of the microscopic world, 795 00:47:44,000 --> 00:47:49,066 all living within and supported by a web of life 796 00:47:49,066 --> 00:47:55,366 just as intricate and productive as any on Earth today. 797 00:47:55,366 --> 00:47:59,166 These first truly complex living creatures 798 00:47:59,166 --> 00:48:04,900 are Ediacaran organisms. 799 00:48:04,900 --> 00:48:07,833 SPRUZEN: Life has been on this incredible journey. 800 00:48:07,833 --> 00:48:11,400 We've gone from these tiny microscopic little fossils 801 00:48:11,400 --> 00:48:13,700 that we can only see with microscopes 802 00:48:13,700 --> 00:48:16,666 all the way to these large, complex organisms, 803 00:48:16,666 --> 00:48:20,700 which show signs of having animal features. 804 00:48:23,533 --> 00:48:24,600 XIAO: This fossil, 805 00:48:24,600 --> 00:48:28,633 Yilingia, was found in South China. 806 00:48:28,633 --> 00:48:31,466 About 550 million years old. 807 00:48:31,466 --> 00:48:36,266 What are you looking at is a trail made by this animal, 808 00:48:36,266 --> 00:48:39,366 perhaps in the last day of its life, 809 00:48:39,366 --> 00:48:42,100 and then it died. 810 00:48:42,100 --> 00:48:45,066 And then the animal and the trail it made 811 00:48:45,066 --> 00:48:48,166 were preserved together. 812 00:48:48,166 --> 00:48:51,333 NARRATOR: Striking proof these animals moved 813 00:48:51,333 --> 00:48:56,200 and made choices over where to go. 814 00:48:56,200 --> 00:48:59,966 But these creatures are so odd-looking, 815 00:48:59,966 --> 00:49:03,900 they are hard to study and classify. 816 00:49:03,900 --> 00:49:07,466 XIAO: So this is something called Dickinsonia. 817 00:49:07,466 --> 00:49:09,733 It is probably an animal. 818 00:49:09,733 --> 00:49:12,300 But scientists still debating 819 00:49:12,300 --> 00:49:16,066 what kind of animal exactly it is. 820 00:49:16,066 --> 00:49:18,266 ♪ ♪ 821 00:49:18,266 --> 00:49:20,633 NARRATOR: Trying to understand how they interacted 822 00:49:20,633 --> 00:49:23,100 with each other is tricky. 823 00:49:24,433 --> 00:49:25,966 MITCHELL: I'm gonna press the red button. 824 00:49:25,966 --> 00:49:28,900 NARRATOR: But thanks to advanced laser technology, 825 00:49:28,900 --> 00:49:30,733 scientists can now learn more about them 826 00:49:30,733 --> 00:49:34,000 than ever before. 827 00:49:34,000 --> 00:49:37,933 (device tapping rapidly) 828 00:49:37,933 --> 00:49:39,300 MITCHELL: So, by laser-scanning 829 00:49:39,300 --> 00:49:42,066 these fossils, we've got a three-dimensional surface 830 00:49:42,066 --> 00:49:45,333 covered in the fossils. 831 00:49:45,333 --> 00:49:46,566 And what this means is, 832 00:49:46,566 --> 00:49:47,566 you have, essentially, 833 00:49:47,566 --> 00:49:49,600 a snapshot of Ediacaran life 834 00:49:49,600 --> 00:49:51,666 captured on the rock surfaces. 835 00:49:51,666 --> 00:49:55,733 ♪ ♪ 836 00:49:55,733 --> 00:49:58,666 To date, we have over 20,000 fossils 837 00:49:58,666 --> 00:50:00,333 that we've laser-scanned. 838 00:50:00,333 --> 00:50:03,800 And using different sorts of statistics 839 00:50:03,800 --> 00:50:06,333 and mathematical approaches, we can then work out 840 00:50:06,333 --> 00:50:07,433 what they were doing and how 841 00:50:07,433 --> 00:50:08,700 they're interacting with each other. 842 00:50:11,533 --> 00:50:14,733 ♪ ♪ 843 00:50:14,733 --> 00:50:17,000 NARRATOR: Looking at the whole ecosystem 844 00:50:17,000 --> 00:50:19,633 is helping scientists gain precious knowledge 845 00:50:19,633 --> 00:50:21,600 about these animals 846 00:50:21,600 --> 00:50:24,866 and how they interacted with each other. 847 00:50:24,866 --> 00:50:30,833 ♪ ♪ 848 00:50:30,833 --> 00:50:33,500 It doesn't seem that we have any of the creatures 849 00:50:33,500 --> 00:50:36,200 from the Ediacaran period still alive today, 850 00:50:36,200 --> 00:50:38,633 except for sponges. 851 00:50:43,400 --> 00:50:45,266 COHEN: The geologically short time 852 00:50:45,266 --> 00:50:47,366 that the Ediacaran fauna were around on our planet 853 00:50:47,366 --> 00:50:49,333 indicates that, after Snowball Earth, 854 00:50:49,333 --> 00:50:51,333 the evolution of complex life 855 00:50:51,333 --> 00:50:53,533 occurred at a rate never seen before. 856 00:50:53,533 --> 00:50:58,833 ♪ ♪ 857 00:50:58,833 --> 00:51:03,900 NARRATOR: Complexity increased and ecosystems diversified, 858 00:51:03,900 --> 00:51:07,733 leading to the ancestors of large animals and plants 859 00:51:07,733 --> 00:51:12,633 in our world today. 860 00:51:12,633 --> 00:51:15,733 Great climate and geological change 861 00:51:15,733 --> 00:51:18,466 took eukaryotic life to the edge. 862 00:51:18,466 --> 00:51:20,466 But it seems that eukaryotic life 863 00:51:20,466 --> 00:51:23,300 always found a way to persist. 864 00:51:23,300 --> 00:51:28,533 ♪ ♪ 865 00:51:28,533 --> 00:51:30,633 Eukaryotic life has survived events 866 00:51:30,633 --> 00:51:31,833 that scientists previously thought 867 00:51:31,833 --> 00:51:33,400 were unsurvivable. 868 00:51:33,400 --> 00:51:36,500 Re-emerging, diversifying, and exploring 869 00:51:36,500 --> 00:51:39,166 every possible niche that the world has to offer. 870 00:51:42,933 --> 00:51:44,266 GIRGUIS: Our Earth 871 00:51:44,266 --> 00:51:46,966 finally became the stage that ushered in a new age. 872 00:51:49,566 --> 00:51:52,500 One of complex life that would eventually reach 873 00:51:52,500 --> 00:51:56,466 the scale, beauty, and complexity that we see today. 874 00:51:56,466 --> 00:52:01,566 ♪ ♪ 875 00:52:07,166 --> 00:52:08,433 ANNOUNCER: On "NOVA: Ancient Earth"... 876 00:52:08,433 --> 00:52:10,833 Our green planet wasn't preordained. 877 00:52:10,833 --> 00:52:12,566 SUSANNAH LYDON: It's the result of an immense struggle 878 00:52:12,566 --> 00:52:14,566 over millions of years. 879 00:52:14,566 --> 00:52:17,633   Plants transformed bare rock into life-giving soil. 880 00:52:17,633 --> 00:52:19,500 KIRK JOHNSON: It creates 881 00:52:19,500 --> 00:52:22,066 one of the most bizarre prehistoric landscapes of all. 882 00:52:22,066 --> 00:52:23,766 DORI CONTRERAS: An alien world. 883 00:52:23,766 --> 00:52:25,866 ROBERT HAZEN: Life and Earth-- they're not separate entities, 884 00:52:25,866 --> 00:52:28,433 they're totally interconnected. 885 00:52:28,433 --> 00:52:31,766 ANNOUNCER: "Life Rising" on "NOVA." 886 00:52:31,766 --> 00:52:33,633 Next time. 887 00:52:33,633 --> 00:52:35,666 SINGER: ♪ Wish I could go back in time ♪ 888 00:52:46,866 --> 00:52:54,400 ♪ ♪ 889 00:52:58,233 --> 00:53:05,833 ♪ ♪ 890 00:53:07,466 --> 00:53:15,000 ♪ ♪ 891 00:53:16,633 --> 00:53:24,166 ♪ ♪ 892 00:53:29,900 --> 00:53:37,066 ♪ ♪