1 00:00:01,166 --> 00:00:02,366 ♪ ♪ 2 00:00:02,366 --> 00:00:05,033 NARRATOR: Earth, our home. 3 00:00:06,933 --> 00:00:09,066 A rocky world 4 00:00:09,066 --> 00:00:12,733 enveloped by a thin blue layer 5 00:00:12,733 --> 00:00:16,066 clinging to the surface. 6 00:00:16,066 --> 00:00:17,766 Our atmosphere. 7 00:00:17,766 --> 00:00:21,500 ROBERT HAZEN: The atmosphere is the only thing 8 00:00:21,500 --> 00:00:25,800 that stands between us and that cold, dark void of space. 9 00:00:27,200 --> 00:00:30,433 (rumbling) 10 00:00:30,433 --> 00:00:33,666 NARRATOR: The story of the air that surrounds us 11 00:00:33,666 --> 00:00:36,033 is the story of Earth itself. 12 00:00:36,033 --> 00:00:37,566 ♪ ♪ 13 00:00:37,566 --> 00:00:38,900 SONIA TIKOO: It is incredible to think that 14 00:00:38,900 --> 00:00:42,733 this 4.5 billion-year-old rock could hold clues 15 00:00:42,733 --> 00:00:45,100 as to how our early atmosphere formed. 16 00:00:45,100 --> 00:00:47,700 (lightning crackling, rain falling) 17 00:00:47,700 --> 00:00:50,066 NARRATOR: Clues that reveal colossal transformations. 18 00:00:51,266 --> 00:00:55,300 MIKE WONG: It is absolutely mind-blowing that a tiny bacteria 19 00:00:55,300 --> 00:00:57,733 could affect a planetary-scale phenomenon 20 00:00:57,733 --> 00:00:58,933 like our atmosphere. 21 00:00:58,933 --> 00:01:00,766 ♪ ♪ 22 00:01:00,766 --> 00:01:06,433 NARRATOR: How did our atmosphere make Earth a unique place, 23 00:01:06,433 --> 00:01:09,666 the only home to life as we know it? 24 00:01:09,666 --> 00:01:12,533 We have never found another planet that has 25 00:01:12,533 --> 00:01:15,066 an atmosphere like ours. 26 00:01:15,066 --> 00:01:17,233 PETER GIRGUIS: And understanding how Earth got its atmosphere 27 00:01:17,233 --> 00:01:21,533 is the first step in understanding how to protect it. 28 00:01:21,533 --> 00:01:25,700 NARRATOR: "Ancient Earth: Birth of the Sky," 29 00:01:25,700 --> 00:01:28,966 right now, on "NOVA." 30 00:01:28,966 --> 00:01:33,633 ♪ ♪ 31 00:01:46,800 --> 00:01:49,533 (rumbling) 32 00:01:52,266 --> 00:01:55,066 NARRATOR: In the depths of space 33 00:01:55,066 --> 00:01:58,966 lies a young, unrecognizable planet. 34 00:01:58,966 --> 00:02:03,966 ♪ ♪ 35 00:02:03,966 --> 00:02:08,633 Bombarded by asteroids, 36 00:02:08,633 --> 00:02:13,633 this battered world is our Earth. 37 00:02:13,633 --> 00:02:19,633 ♪ ♪ 38 00:02:19,633 --> 00:02:23,800 A desolate and volatile globe 39 00:02:23,800 --> 00:02:29,333 overlooked by a new-born moon that dominates the sky. 40 00:02:30,633 --> 00:02:35,300 With no recognizable atmosphere, the void of space 41 00:02:35,300 --> 00:02:40,100 nearly touches the surface of slowly cooling lava. 42 00:02:40,100 --> 00:02:44,466 ♪ ♪ 43 00:02:44,466 --> 00:02:48,866 There's little protection from the young sun's harsh radiation. 44 00:02:51,900 --> 00:02:56,533 This world looks nothing like the Earth we know today. 45 00:02:59,900 --> 00:03:02,133 Around four and a half billion years ago, 46 00:03:02,133 --> 00:03:05,100 Earth was inhospitable for life. 47 00:03:05,100 --> 00:03:08,133 (rumbling) 48 00:03:08,133 --> 00:03:11,500 HAZEN: There were meteors and asteroids bombarding the surface. 49 00:03:11,500 --> 00:03:12,533 It was too hot. 50 00:03:12,533 --> 00:03:13,766 It was a terrible place to be. 51 00:03:13,766 --> 00:03:16,566 ♪ ♪ 52 00:03:16,566 --> 00:03:19,200 LINGAPPA: The question is, 53 00:03:19,200 --> 00:03:22,066 how did our planet transform from this barren rock, 54 00:03:22,066 --> 00:03:24,900 into somewhere capable of supporting life? 55 00:03:24,900 --> 00:03:26,966 ♪ ♪ 56 00:03:26,966 --> 00:03:30,533 WONG: This is the story of how our planet became a living world, 57 00:03:30,533 --> 00:03:34,000 and the dance, the co-evolution 58 00:03:34,000 --> 00:03:35,800 between life and our atmosphere. 59 00:03:35,800 --> 00:03:41,833 ♪ ♪ 60 00:03:44,700 --> 00:03:46,266 ("Never Close Enough" by SIPHO. playing) 61 00:03:46,266 --> 00:03:51,166 ♪ Oh, we won't ever hear the silence ♪ 62 00:03:51,166 --> 00:03:54,200 ♪ Or ever see the colors ♪ 63 00:03:54,200 --> 00:03:56,500 (exploding) 64 00:03:56,500 --> 00:03:59,500 ♪ That never lived in our minds ♪ 65 00:03:59,500 --> 00:04:02,266 ♪ ♪ 66 00:04:02,266 --> 00:04:03,600 ♪ Just a moment ♪ 67 00:04:03,600 --> 00:04:06,900 ♪ Never too far out ♪ 68 00:04:06,900 --> 00:04:10,566 ♪ Never close enough ♪ 69 00:04:14,733 --> 00:04:18,633 ♪ ♪ 70 00:04:21,966 --> 00:04:26,300 NARRATOR: Today, our planet is an oasis for life, 71 00:04:26,300 --> 00:04:28,400 enveloped by a layer of 72 00:04:28,400 --> 00:04:32,300 nourishing and protective gases-- 73 00:04:32,300 --> 00:04:35,700 the atmosphere. 74 00:04:35,700 --> 00:04:37,733 It's extremely thin, it's almost 75 00:04:37,733 --> 00:04:39,600 like the skin on apple, 76 00:04:39,600 --> 00:04:42,066 if you pretended the apple was planet Earth. 77 00:04:42,066 --> 00:04:43,733 TIKOO: The diameter of the earth 78 00:04:43,733 --> 00:04:47,233 is about 8,000 miles across, 79 00:04:47,233 --> 00:04:48,800 and the atmosphere adds 80 00:04:48,800 --> 00:04:52,233 just another 60 or so miles on top of that. 81 00:04:52,233 --> 00:04:54,433 (laughing): We wouldn't... we wouldn't be 82 00:04:54,433 --> 00:04:56,000 on planet Earth if it weren't for the atmosphere. 83 00:04:56,000 --> 00:04:57,366 And when I say we, 84 00:04:57,366 --> 00:05:01,500 not just you and me sitting here, but any life. 85 00:05:01,500 --> 00:05:03,800 GIRGUIS: Everything from fungi to bacteria, 86 00:05:03,800 --> 00:05:06,000 to tiny insects, to giant reptiles, 87 00:05:06,000 --> 00:05:07,700 are entirely dependent 88 00:05:07,700 --> 00:05:09,233 on this atmosphere of ours. 89 00:05:09,233 --> 00:05:12,433 ♪ ♪ 90 00:05:12,433 --> 00:05:13,966 It does many things for us, 91 00:05:13,966 --> 00:05:16,066 and we can think of the simple answer that we, 92 00:05:16,066 --> 00:05:19,700 as human beings, use the oxygen to live. 93 00:05:19,700 --> 00:05:23,000 NARRATOR: Besides oxygen, other important gases 94 00:05:23,000 --> 00:05:26,366 make up the atmosphere. 95 00:05:26,366 --> 00:05:29,766 LYONS: Perhaps the single most critical aspect 96 00:05:29,766 --> 00:05:32,766 of our atmosphere is the fact that carbon dioxide 97 00:05:32,766 --> 00:05:35,966 is one of the greenhouse gases that keeps our planet habitable. 98 00:05:36,966 --> 00:05:38,733 PIERREHUMBERT: Nitrogen is an essential 99 00:05:38,733 --> 00:05:41,166 component of the atmosphere, 100 00:05:41,166 --> 00:05:42,300 because all life as we know it-- 101 00:05:42,300 --> 00:05:44,600 amino acids, DNA, 102 00:05:44,600 --> 00:05:46,133 all of the things that are essential to us-- 103 00:05:46,133 --> 00:05:49,733 rely on nitrogen. 104 00:05:49,733 --> 00:05:51,166 The atmosphere protects us 105 00:05:51,166 --> 00:05:52,300 from ultraviolet radiation 106 00:05:52,300 --> 00:05:53,700 because it has a small amount 107 00:05:53,700 --> 00:05:55,533 of a molecule called ozone. 108 00:05:58,233 --> 00:06:00,266 LINGAPPA: It controls our climate, 109 00:06:00,266 --> 00:06:02,633 which makes this planet, like, 110 00:06:02,633 --> 00:06:04,200 a livable temperature for us at all. 111 00:06:04,200 --> 00:06:08,966 It sort of insulates us from the cold of space. 112 00:06:10,866 --> 00:06:13,200 NARRATOR: But the journey to today's atmosphere 113 00:06:13,200 --> 00:06:17,366 has been full of drama. 114 00:06:17,366 --> 00:06:19,966 WONG: It's a long, winding road 115 00:06:19,966 --> 00:06:22,433 between the initial atmosphere that Earth had 116 00:06:22,433 --> 00:06:25,700 through all of the various different kinds of atmospheres 117 00:06:25,700 --> 00:06:27,666 in the middle of its history, 118 00:06:27,666 --> 00:06:30,066 and finally arriving at the atmosphere that we have now. 119 00:06:40,966 --> 00:06:47,000 ♪ ♪ 120 00:06:52,766 --> 00:06:56,833 NARRATOR: Millions of years after the birth of the planet, 121 00:06:56,833 --> 00:07:01,933 there's still no atmosphere as we know it. 122 00:07:06,300 --> 00:07:10,133 But that is going to change. 123 00:07:11,233 --> 00:07:12,733 Four and a half billion years ago, 124 00:07:12,733 --> 00:07:14,366 the Earth would have been a very violent place. 125 00:07:14,366 --> 00:07:16,100 (rumbling) 126 00:07:16,100 --> 00:07:19,600 LYONS: Such that it's called the Hadean, which comes from 127 00:07:19,600 --> 00:07:22,633 the god Hades, the Greek god of the underworld, 128 00:07:22,633 --> 00:07:24,500 because it was imagined to be 129 00:07:24,500 --> 00:07:26,800 a hellish kind of place. 130 00:07:26,800 --> 00:07:32,700 (thundering, rumbling) 131 00:07:32,700 --> 00:07:34,866 (bursting) 132 00:07:34,866 --> 00:07:39,300 (magma rushing) 133 00:07:41,066 --> 00:07:42,666 NARRATOR: Across the globe, 134 00:07:42,666 --> 00:07:45,333 molten magma surges up from within. 135 00:07:45,333 --> 00:07:47,666 (splashing) 136 00:07:47,666 --> 00:07:52,833 Rivers of fire unleash a cocktail of gases. 137 00:07:52,833 --> 00:07:56,033 (booming) 138 00:07:56,033 --> 00:07:59,166 ♪ ♪ 139 00:07:59,166 --> 00:08:02,233 Carbon dioxide, nitrogen, and methane 140 00:08:02,233 --> 00:08:06,466 explode out of volcanoes in thick plumes. 141 00:08:06,466 --> 00:08:09,900 ♪ ♪ 142 00:08:09,900 --> 00:08:12,166 These are the building blocks 143 00:08:12,166 --> 00:08:13,900 of our early atmosphere. 144 00:08:13,900 --> 00:08:16,833 ♪ ♪ 145 00:08:24,333 --> 00:08:28,933 And we can see echoes of this young planet 146 00:08:28,933 --> 00:08:31,233 still on Earth today. 147 00:08:31,233 --> 00:08:35,866 ♪ ♪ 148 00:08:47,033 --> 00:08:48,633 The volcanoes that we have here in Iceland look 149 00:08:48,633 --> 00:08:50,066 very similar to those that we had 150 00:08:50,066 --> 00:08:51,500 four and a half billion years ago. 151 00:08:51,500 --> 00:08:56,133 The early Earth was dominated by volcanic activity, 152 00:08:56,133 --> 00:08:57,766 and this lasted for millions of years. 153 00:08:57,766 --> 00:09:01,366 ♪ ♪ 154 00:09:01,366 --> 00:09:03,800 Those volcanoes acted as pathways 155 00:09:03,800 --> 00:09:06,500 for the gases that are trapped inside the magma 156 00:09:06,500 --> 00:09:10,066 to escape into our atmosphere. 157 00:09:10,066 --> 00:09:12,166 NARRATOR: But how do scientists know 158 00:09:12,166 --> 00:09:16,900 what the atmosphere was like in the past? 159 00:09:16,900 --> 00:09:18,833 Surprisingly, clues are hidden 160 00:09:18,833 --> 00:09:23,533 in some of the rocks we find today. 161 00:09:23,533 --> 00:09:25,800 Throughout Earth's history and evolution, 162 00:09:25,800 --> 00:09:29,266 there's been a dynamic interplay between the Earth's surface 163 00:09:29,266 --> 00:09:30,700 and the Earth's atmosphere. 164 00:09:30,700 --> 00:09:32,266 And because of this interplay, 165 00:09:32,266 --> 00:09:36,466 there are chemical fingerprints that are trapped in rocks. 166 00:09:38,900 --> 00:09:40,400 The rock I'm holding in my hand 167 00:09:40,400 --> 00:09:43,433 is 3.4 billion years old. 168 00:09:43,433 --> 00:09:45,633 And there are tiny bubbles 169 00:09:45,633 --> 00:09:47,333 trapped in this rock 170 00:09:47,333 --> 00:09:49,533 that are samples 171 00:09:49,533 --> 00:09:51,133 of Earth's ancient atmosphere. 172 00:09:51,133 --> 00:09:53,900 So we can measure the gases 173 00:09:53,900 --> 00:09:55,933 and piece together evidence 174 00:09:55,933 --> 00:09:58,900 of what the atmosphere was like in the past. 175 00:09:58,900 --> 00:10:00,900 ♪ ♪ 176 00:10:00,900 --> 00:10:02,333 NARRATOR: However, 177 00:10:02,333 --> 00:10:05,166 to understand the very earliest atmosphere, 178 00:10:05,166 --> 00:10:07,600 scientists have to go back to a time 179 00:10:07,600 --> 00:10:10,433 before Earth was formed. 180 00:10:12,566 --> 00:10:16,800 The planet is made up of rock that started as debris 181 00:10:16,800 --> 00:10:19,966 floating in the early solar system. 182 00:10:20,966 --> 00:10:26,766 But this debris is also falling to Earth today as meteorites. 183 00:10:26,766 --> 00:10:28,200 WILLIAMS: In my pocket here, 184 00:10:28,200 --> 00:10:29,500 I've got the most incredible rock. 185 00:10:29,500 --> 00:10:31,533 This is a chondrite meteorite. 186 00:10:31,533 --> 00:10:34,766 These chondrite meteorites 187 00:10:34,766 --> 00:10:36,866 formed when rock dust came together 188 00:10:36,866 --> 00:10:39,466 to form small chunks of rock like this. 189 00:10:39,466 --> 00:10:41,233 They eventually smashed together, 190 00:10:41,233 --> 00:10:43,033 forming our planet Earth. 191 00:10:43,033 --> 00:10:46,033 And so these are really the building blocks of our planet. 192 00:10:48,500 --> 00:10:50,100 And so it's so humbling, 193 00:10:50,100 --> 00:10:51,800 because it's not only older than all of the life 194 00:10:51,800 --> 00:10:53,366 that we know on Earth, 195 00:10:53,366 --> 00:10:55,033 but even the planet itself, 196 00:10:55,033 --> 00:10:57,200 right back to 4.6 billion years ago. 197 00:10:58,533 --> 00:11:02,366 NARRATOR: These rocks are not just the building blocks of our planet, 198 00:11:02,366 --> 00:11:05,400 but of our early atmosphere as well. 199 00:11:05,400 --> 00:11:06,900 WILLIAMS: Thanks to these space rocks, 200 00:11:06,900 --> 00:11:10,266 the ingredients for our early atmosphere were there. 201 00:11:10,266 --> 00:11:12,833 It's just that they were locked inside the planet. 202 00:11:15,266 --> 00:11:19,400 NARRATOR: Chondrite meteorites are like time machines, 203 00:11:19,400 --> 00:11:22,600 allowing scientists to travel back billions of years 204 00:11:22,600 --> 00:11:25,466 to when the atmosphere first developed. 205 00:11:27,133 --> 00:11:30,133 WILLIAMS: Experts can study these chondrites in great detail. 206 00:11:30,133 --> 00:11:33,000 They can analyze them in the laboratory 207 00:11:33,000 --> 00:11:34,600 to understand their chemical signatures, 208 00:11:34,600 --> 00:11:36,400 and that's going to tell them about the gases 209 00:11:36,400 --> 00:11:39,166 that they would have contributed to 210 00:11:39,166 --> 00:11:41,333 our early Earth's atmosphere, 211 00:11:41,333 --> 00:11:43,600 such as water vapor, carbon dioxide, 212 00:11:43,600 --> 00:11:47,500 sulfur dioxide, hydrogen, methane, and nitrogen. 213 00:11:49,533 --> 00:11:51,500 HAZEN: It's remarkable that you can learn 214 00:11:51,500 --> 00:11:54,000 about the history of the atmosphere 215 00:11:54,000 --> 00:11:56,300 going back billions and billions of years 216 00:11:56,300 --> 00:11:58,133 just by studying rocks. 217 00:12:01,033 --> 00:12:04,033 WONG: When I learn about this, I feel simultaneously 218 00:12:04,033 --> 00:12:06,966 like a time traveler and a detective, 219 00:12:06,966 --> 00:12:10,100 being able to piece together the story of our ancient Earth. 220 00:12:11,666 --> 00:12:14,900 LYONS: Each year, techniques become more and more powerful 221 00:12:14,900 --> 00:12:18,400 at deciphering bits of information of what the Earth 222 00:12:18,400 --> 00:12:19,600 may have been like 223 00:12:19,600 --> 00:12:21,233 four and a half billion years ago, 224 00:12:21,233 --> 00:12:24,833 that even a short time ago would have been completely a mystery. 225 00:12:24,833 --> 00:12:29,966 (rumbling) 226 00:12:29,966 --> 00:12:32,400 (steam hissing) 227 00:12:35,000 --> 00:12:38,866 NARRATOR: While the scorching Earth slowly cools, 228 00:12:38,866 --> 00:12:43,500 the sky is undergoing a dramatic transformation. 229 00:12:49,066 --> 00:12:52,900 The vast sea of stars becomes obscured. 230 00:12:52,900 --> 00:12:55,933 ♪ ♪ 231 00:12:55,933 --> 00:13:00,933 And as the young sun rises, 232 00:13:00,933 --> 00:13:04,633 color bursts across Earth's sky. 233 00:13:08,033 --> 00:13:10,800 These dense gases are the beginnings 234 00:13:10,800 --> 00:13:14,133 of the atmosphere as we know it. 235 00:13:16,566 --> 00:13:22,333 What this fledgling atmosphere looked like is debated. 236 00:13:22,333 --> 00:13:25,300 Because it's hard to know the precise amounts 237 00:13:25,300 --> 00:13:28,300 of the various gases it contained. 238 00:13:29,400 --> 00:13:32,233 Gases like methane. 239 00:13:32,233 --> 00:13:34,433 PIERREHUMBERT: If there's enough methane, 240 00:13:34,433 --> 00:13:37,400 it can reach a concentration where it starts breaking up 241 00:13:37,400 --> 00:13:39,800 under the action of ultraviolet light 242 00:13:39,800 --> 00:13:42,100 and then recombining into longer molecules, 243 00:13:42,100 --> 00:13:44,833 making these smoggy substances. 244 00:13:44,833 --> 00:13:46,566 That can actually make these hazes 245 00:13:46,566 --> 00:13:48,300 which cause an orangey glow. 246 00:13:48,300 --> 00:13:51,333 ♪ ♪ 247 00:13:51,333 --> 00:13:53,566 NARRATOR: With enough methane, 248 00:13:53,566 --> 00:13:59,233 thick, intermittent orange hazes would have covered the planet. 249 00:13:59,233 --> 00:14:02,700 TIKOO: There was probably carbon dioxide. 250 00:14:02,700 --> 00:14:06,466 There might have been carbon monoxide, water vapor. 251 00:14:06,466 --> 00:14:07,833 There might have been 252 00:14:07,833 --> 00:14:09,533 some nitrogen or cyanide. 253 00:14:09,533 --> 00:14:11,200 All in all, it was a pretty 254 00:14:11,200 --> 00:14:13,300 noxious combination of stuff. 255 00:14:14,333 --> 00:14:16,566 HAZEN: If you were dropped onto early Earth 256 00:14:16,566 --> 00:14:18,333 and experienced that early atmosphere, 257 00:14:18,333 --> 00:14:20,500 you'd die very quickly because there was no oxygen. 258 00:14:21,500 --> 00:14:23,800 Carbon dioxide, some of the other molecules 259 00:14:23,800 --> 00:14:25,633 that might have been present in the atmosphere, 260 00:14:25,633 --> 00:14:27,700 you just simply couldn't live on those. 261 00:14:31,633 --> 00:14:34,733 ♪ ♪ 262 00:14:34,733 --> 00:14:40,133 NARRATOR: And the planet is inhospitable for another reason-- 263 00:14:40,133 --> 00:14:44,966 the landscape has never seen a drop of liquid water. 264 00:14:49,966 --> 00:14:52,700 But Earth does have water. 265 00:14:52,700 --> 00:14:54,233 (thunder crackling) 266 00:14:54,233 --> 00:14:59,000 It is suspended high above the searing surface, 267 00:14:59,000 --> 00:15:01,800 pumped into the atmosphere by volcanoes 268 00:15:01,800 --> 00:15:05,566 as water vapor with the other early gases. 269 00:15:05,566 --> 00:15:09,166 ♪ ♪ 270 00:15:09,166 --> 00:15:11,000 (woman vocalizing, thunder crackling) 271 00:15:11,000 --> 00:15:15,033 If this ocean in the sky could reach the ground, 272 00:15:15,033 --> 00:15:19,466 it would have the power to utterly transform the planet. 273 00:15:20,433 --> 00:15:23,300 (water rushing) 274 00:15:23,300 --> 00:15:26,066 (roaring) 275 00:15:29,766 --> 00:15:34,600 ♪ ♪ 276 00:15:34,600 --> 00:15:37,133 Today, over 70% of our world 277 00:15:37,133 --> 00:15:39,233 is covered in water. 278 00:15:42,300 --> 00:15:44,400 A planet of abundant blue 279 00:15:44,400 --> 00:15:47,433 with endless rivers, 280 00:15:47,433 --> 00:15:50,933 freezing icebergs, 281 00:15:50,933 --> 00:15:55,866 and vast oceans. 282 00:15:55,866 --> 00:15:58,100 ♪ ♪ 283 00:15:58,100 --> 00:16:01,466 GIRGUIS: Water that we get out of our tap 284 00:16:01,466 --> 00:16:03,900 has been through a long journey. 285 00:16:03,900 --> 00:16:06,566 ♪ ♪ 286 00:16:06,566 --> 00:16:08,033 DOTTIN: And the water on our planet 287 00:16:08,033 --> 00:16:12,933 gets incorporated into various aspects of our life, 288 00:16:12,933 --> 00:16:15,033   including weather, into the food that we eat. 289 00:16:15,033 --> 00:16:16,666 And because of this, the water is recycled 290 00:16:16,666 --> 00:16:22,566 over and over and over again throughout Earth's history. 291 00:16:22,566 --> 00:16:24,666 WONG: So the water that we drink 292 00:16:24,666 --> 00:16:26,733 was consumed by dinosaurs 293 00:16:26,733 --> 00:16:28,100 and had the first microbes 294 00:16:28,100 --> 00:16:29,700 swimming around in it. (laughs) 295 00:16:29,700 --> 00:16:32,533 (distant rumbling) 296 00:16:32,533 --> 00:16:34,300 NARRATOR: On the early Earth, 297 00:16:34,300 --> 00:16:37,066 water is stuck in the atmosphere. 298 00:16:37,066 --> 00:16:38,933 (wind howling, thunder crackling) 299 00:16:38,933 --> 00:16:43,066 But that is about to change. 300 00:16:43,066 --> 00:16:48,466 ♪ ♪ 301 00:16:48,466 --> 00:16:50,033 High up in the sky, 302 00:16:50,033 --> 00:16:52,766 droplets of water, 303 00:16:52,766 --> 00:16:58,033 just a fraction of a millimeter across, are condensing. 304 00:16:58,033 --> 00:17:03,466 So light and small, they just float, 305 00:17:03,466 --> 00:17:05,333 drifting in the air. 306 00:17:08,133 --> 00:17:11,333 Colliding and merging with others... 307 00:17:11,333 --> 00:17:13,100 (squishing) 308 00:17:13,100 --> 00:17:17,033 ...they slowly grow until a tipping point is reached. 309 00:17:17,033 --> 00:17:22,066 ♪ ♪ 310 00:17:29,633 --> 00:17:32,766 Earth's gravity draws them downwards... 311 00:17:35,066 --> 00:17:37,833 but the journey is short-lived. 312 00:17:40,766 --> 00:17:43,533 The planet is still hot. 313 00:17:43,533 --> 00:17:45,600 (sizzling) 314 00:17:45,600 --> 00:17:50,000 So any drops nearing the ground... 315 00:17:50,000 --> 00:17:53,866 (hissing) 316 00:17:53,866 --> 00:17:56,033 ...simply evaporate. 317 00:17:57,633 --> 00:18:02,733 ♪ ♪ (wind whipping) 318 00:18:04,666 --> 00:18:06,533 But over time, 319 00:18:06,533 --> 00:18:09,566 that heat is radiating out into space. 320 00:18:13,000 --> 00:18:16,266 Earth is cooling down. 321 00:18:16,266 --> 00:18:19,833 (wind whistling) 322 00:18:19,833 --> 00:18:21,600 (fizzing) 323 00:18:30,200 --> 00:18:36,066 ♪ ♪ 324 00:18:38,200 --> 00:18:40,433 (thunder booming) 325 00:18:40,433 --> 00:18:45,733 (rain falling) 326 00:18:45,733 --> 00:18:46,800 (lightning crackling) 327 00:18:46,800 --> 00:18:47,966 A deluge of rain 328 00:18:47,966 --> 00:18:50,466 begins to pour down on the surface. 329 00:18:50,466 --> 00:18:52,366 (booming) 330 00:18:52,366 --> 00:18:56,533 (rain pelting down) 331 00:18:59,866 --> 00:19:03,466 Huge storms sweep through the atmosphere. 332 00:19:03,466 --> 00:19:07,900 (thunder rumbling) 333 00:19:07,900 --> 00:19:10,866 Oceans of water are dumped onto the land. 334 00:19:12,533 --> 00:19:17,800 (rain pouring) 335 00:19:17,800 --> 00:19:21,066 (thunder rumbling) 336 00:19:21,066 --> 00:19:24,766 ♪ ♪ 337 00:19:24,766 --> 00:19:26,400 (lightning striking) 338 00:19:27,866 --> 00:19:33,166 The atmosphere has unleashed a key molecule for life. 339 00:19:37,166 --> 00:19:38,333 Water changes the surface 340 00:19:38,333 --> 00:19:39,566 of a planet. 341 00:19:39,566 --> 00:19:40,800 It evaporates, 342 00:19:40,800 --> 00:19:41,800 it allows clouds to form 343 00:19:41,800 --> 00:19:43,166 and rain to fall, 344 00:19:43,166 --> 00:19:45,000 it erodes things, 345 00:19:45,000 --> 00:19:47,200 and it also became a medium 346 00:19:47,200 --> 00:19:49,933 that would eventually harbor life. 347 00:19:49,933 --> 00:19:54,633 (wildlife chittering) 348 00:19:54,633 --> 00:19:55,700 Without liquid water, 349 00:19:55,700 --> 00:19:56,733 we certainly would not 350 00:19:56,733 --> 00:19:58,300 get life as we know it. 351 00:19:58,300 --> 00:20:00,733 ♪ ♪ 352 00:20:00,733 --> 00:20:02,400 LANE: Things that life needs it to do 353 00:20:02,400 --> 00:20:03,533 here on Earth, 354 00:20:03,533 --> 00:20:04,700 to have structures like cells, 355 00:20:04,700 --> 00:20:05,766 for example, 356 00:20:05,766 --> 00:20:06,933 and all kinds of machinery 357 00:20:06,933 --> 00:20:08,166 that operate in water, 358 00:20:08,166 --> 00:20:09,333 that works really well. 359 00:20:09,333 --> 00:20:13,133 ♪ ♪ 360 00:20:13,133 --> 00:20:16,366 NARRATOR: The early atmosphere had started a transformation 361 00:20:16,366 --> 00:20:19,566 that would set our planet apart 362 00:20:19,566 --> 00:20:23,333 from every other in the solar system. 363 00:20:31,966 --> 00:20:37,700 ♪ ♪ 364 00:20:39,833 --> 00:20:44,833 The young planet is only a few hundred million years old. 365 00:20:44,833 --> 00:20:48,766 The atmosphere has made Earth a water world. 366 00:20:48,766 --> 00:20:51,433 (waves crashing) 367 00:20:51,433 --> 00:20:54,600 ♪ ♪ 368 00:20:54,600 --> 00:20:59,266 And in that water, something remarkable is happening... 369 00:20:59,266 --> 00:21:02,866 ♪ ♪ 370 00:21:02,866 --> 00:21:07,866 Arguably, the most pivotal moment in Earth's history. 371 00:21:09,233 --> 00:21:14,966 It would lead to the reshaping of the entire atmosphere. 372 00:21:14,966 --> 00:21:17,733 ♪ ♪ 373 00:21:17,733 --> 00:21:21,233 So much of this moment is still a mystery. 374 00:21:23,266 --> 00:21:27,900 But we do know that one day on Earth, 375 00:21:27,900 --> 00:21:31,666 complex molecules come together 376 00:21:31,666 --> 00:21:36,166 to form something never seen on the planet before... 377 00:21:37,933 --> 00:21:40,300 Life. 378 00:21:42,166 --> 00:21:43,900 WONG: The origin of life 379 00:21:43,900 --> 00:21:46,066 is one of the greatest questions in science, 380 00:21:46,066 --> 00:21:48,500 and it's fair to say that we don't know when, 381 00:21:48,500 --> 00:21:50,800 where, or how life started. 382 00:21:50,800 --> 00:21:53,733 ♪ ♪ 383 00:21:53,733 --> 00:21:57,000 (waves crashing) 384 00:21:57,000 --> 00:22:01,900 NARRATOR: A shallow rock pool is one of the leading theories. 385 00:22:01,900 --> 00:22:03,433 ANDERSON: People think that shallow pools 386 00:22:03,433 --> 00:22:04,733 would have been 387 00:22:04,733 --> 00:22:05,900 a potentially important site 388 00:22:05,900 --> 00:22:07,133 for the origin of life, 389 00:22:07,133 --> 00:22:08,266 because they can get wet and dry 390 00:22:08,266 --> 00:22:09,400 over and over again. 391 00:22:09,400 --> 00:22:12,300 (waves lapping) 392 00:22:12,300 --> 00:22:15,700 LYONS: Through this repeated cycling of wetting and drying, 393 00:22:15,700 --> 00:22:17,233 reflooding and evaporating, 394 00:22:17,233 --> 00:22:18,800 maybe through a tide, 395 00:22:18,800 --> 00:22:20,366 maybe through seasonal variation, 396 00:22:20,366 --> 00:22:23,133 more and more complex molecules can form. 397 00:22:25,200 --> 00:22:27,433 ANDERSON: And that process could've been the precursors 398 00:22:27,433 --> 00:22:29,900 for things like DNA, which is what makes up 399 00:22:29,900 --> 00:22:31,600 the information in our cells today. 400 00:22:33,533 --> 00:22:35,766 NARRATOR: But there are many other theories 401 00:22:35,766 --> 00:22:39,100 about how life could have started. 402 00:22:39,100 --> 00:22:43,100 ♪ ♪ 403 00:22:43,100 --> 00:22:44,766 LINGAPPA: Some scientists think 404 00:22:44,766 --> 00:22:48,233 life began in a deep sea hydrothermal vent. 405 00:22:50,266 --> 00:22:53,266 LANE: Hydrothermal vents are sources of gases-- 406 00:22:53,266 --> 00:22:55,466 like hydrogen sulfide, for example-- 407 00:22:55,466 --> 00:22:57,800 and provide the kind of reactive conditions 408 00:22:57,800 --> 00:23:00,666 between hydrogen and CO2 to make the building blocks of life. 409 00:23:00,666 --> 00:23:04,933 ♪ ♪ 410 00:23:04,933 --> 00:23:07,533 LINGAPPA: Others think that 411 00:23:07,533 --> 00:23:08,633 life originated somewhere 412 00:23:08,633 --> 00:23:09,633 completely else, 413 00:23:09,633 --> 00:23:11,266 not on the Earth at all, 414 00:23:11,266 --> 00:23:15,266 and landed here on a meteorite. 415 00:23:15,266 --> 00:23:19,533 (crashing, booming) 416 00:23:19,533 --> 00:23:21,066 All of these different theories 417 00:23:21,066 --> 00:23:22,400 have sort of different details, 418 00:23:22,400 --> 00:23:24,000 but the punchline is 419 00:23:24,000 --> 00:23:25,900 that life needed water, 420 00:23:25,900 --> 00:23:29,233 and it needed a way to harness energy. 421 00:23:29,233 --> 00:23:31,633 ♪ ♪ 422 00:23:31,633 --> 00:23:33,866 NARRATOR: Scientists have some idea 423 00:23:33,866 --> 00:23:36,033 what early life might have looked like. 424 00:23:39,033 --> 00:23:40,666 The very first lifeform 425 00:23:40,666 --> 00:23:41,933 that existed was probably 426 00:23:41,933 --> 00:23:43,733 a little bit of genetic material 427 00:23:43,733 --> 00:23:46,000 enclosed in a bubble 428 00:23:46,000 --> 00:23:48,000 of fatty material. 429 00:23:48,000 --> 00:23:50,433 ♪ ♪ 430 00:23:50,433 --> 00:23:52,266 NARRATOR: And while simple, 431 00:23:52,266 --> 00:23:54,333 some of these early organisms 432 00:23:54,333 --> 00:23:58,133 would have a powerful effect on the atmosphere. 433 00:24:00,133 --> 00:24:02,933 We think that one of the earliest organisms 434 00:24:02,933 --> 00:24:05,333 were organisms that produce methane 435 00:24:05,333 --> 00:24:07,700 as part of the way they eat, and breathe, and live. 436 00:24:07,700 --> 00:24:10,666 ♪ ♪ 437 00:24:10,666 --> 00:24:14,200 NARRATOR: They take in carbon dioxide and hydrogen, 438 00:24:14,200 --> 00:24:17,466 producing water and methane as waste products. 439 00:24:17,466 --> 00:24:20,300 LYONS: So it is reasonable to estimate 440 00:24:20,300 --> 00:24:22,233 that a vast portion of the early atmosphere 441 00:24:22,233 --> 00:24:24,000 contains large amounts of methane. 442 00:24:24,000 --> 00:24:26,833 ♪ ♪ 443 00:24:26,833 --> 00:24:28,466 NARRATOR: This increased methane 444 00:24:28,466 --> 00:24:31,433 could have made the orange hazes thicker. 445 00:24:34,566 --> 00:24:36,300 The evolution of our atmosphere is, in many respects, 446 00:24:36,300 --> 00:24:38,566 the story of the evolution of life on our planet. 447 00:24:38,566 --> 00:24:40,833 (wind howling) 448 00:24:40,833 --> 00:24:43,766 Life can change a planet fundamentally. 449 00:24:45,633 --> 00:24:49,300 But it's always this cause and effect kind of dance 450 00:24:49,300 --> 00:24:51,466 between the environment changing life 451 00:24:51,466 --> 00:24:53,266 and life changing the environment. 452 00:24:53,266 --> 00:24:57,133 ♪ ♪ 453 00:24:57,133 --> 00:25:00,633 NARRATOR: And it's the story of life that reveals how we got from 454 00:25:00,633 --> 00:25:04,133 this orange hazy atmosphere 455 00:25:04,133 --> 00:25:07,966 to the blue, oxygen-rich one we know today. 456 00:25:10,600 --> 00:25:16,633 ♪ ♪ 457 00:25:18,133 --> 00:25:21,666 This young world is not one we could survive in. 458 00:25:23,833 --> 00:25:26,900 Earth's early oceans are blue-green, 459 00:25:26,900 --> 00:25:30,800 full of dissolved iron, 460 00:25:30,800 --> 00:25:34,133 and they've absorbed large amounts of carbon dioxide, 461 00:25:34,133 --> 00:25:37,433 making them acidic. 462 00:25:37,433 --> 00:25:41,000 (waves crashing) 463 00:25:41,000 --> 00:25:42,800 ANDERSON: Early life would have been 464 00:25:42,800 --> 00:25:44,733 adapted specifically to living in high temperatures 465 00:25:44,733 --> 00:25:47,233 or fairly acidic conditions. 466 00:25:49,533 --> 00:25:51,866 ROTHSCHILD: The ultraviolet radiation was incredibly harsh 467 00:25:51,866 --> 00:25:54,200 because there was no ozone layer. 468 00:25:55,966 --> 00:25:57,900 Furthermore, there was no oxygen in the atmosphere; 469 00:25:57,900 --> 00:25:59,433 it was a very different composition. 470 00:26:01,633 --> 00:26:04,866 NARRATOR: And some organisms today offer clues 471 00:26:04,866 --> 00:26:07,333 to how life survived in this early environment. 472 00:26:07,333 --> 00:26:11,333 ♪ ♪ 473 00:26:11,333 --> 00:26:13,900 High in the Andes is one of 474 00:26:13,900 --> 00:26:16,200 the largest geyser fields in the world. 475 00:26:16,200 --> 00:26:21,233 ♪ ♪ 476 00:26:24,266 --> 00:26:27,133 LINGAPPA: This environment is extreme for a lot of reasons. 477 00:26:27,133 --> 00:26:29,600 First of all, the water that's coming 478 00:26:29,600 --> 00:26:31,766 out of this pool is boilingly hot. 479 00:26:31,766 --> 00:26:33,600 And the altitude means 480 00:26:33,600 --> 00:26:35,766 that this environment is subject 481 00:26:35,766 --> 00:26:36,966 to a lot of ultraviolet 482 00:26:36,966 --> 00:26:38,333 radiation from the sun. 483 00:26:40,466 --> 00:26:45,133 NARRATOR: These are conditions most animals would find toxic. 484 00:26:46,966 --> 00:26:50,800 But amongst the boiling water and steam, 485 00:26:50,800 --> 00:26:54,466 life is flourishing. 486 00:26:54,466 --> 00:26:56,166 LINGAPPA: So the colors that we see 487 00:26:56,166 --> 00:26:57,966 in these mats around this pool 488 00:26:57,966 --> 00:27:00,266 are billions of thriving bacteria 489 00:27:00,266 --> 00:27:02,800 that have sort of carved out a niche 490 00:27:02,800 --> 00:27:04,700 in this extreme environment. 491 00:27:05,866 --> 00:27:09,666 It's incredible to find so much life, 492 00:27:09,666 --> 00:27:14,800 not just surviving, but thriving in these conditions. 493 00:27:14,800 --> 00:27:18,633 NARRATOR: Organisms like these are called extremophiles. 494 00:27:20,400 --> 00:27:23,166 They can survive in environments most complex life 495 00:27:23,166 --> 00:27:27,233 would struggle to live in. 496 00:27:27,233 --> 00:27:28,666 LANE: We call them extremophiles 497 00:27:28,666 --> 00:27:32,066 because we didn't really think that life could exist 498 00:27:32,066 --> 00:27:33,433 in such extreme conditions. 499 00:27:36,100 --> 00:27:39,100 ROTHSCHILD: In fact, we know of organisms 500 00:27:39,100 --> 00:27:43,366 that can live up to 122 centigrade. 501 00:27:43,366 --> 00:27:46,400 So this is well over the boiling temperature of water, 502 00:27:46,400 --> 00:27:48,433 and, conversely, we know of ones 503 00:27:48,433 --> 00:27:50,800 that can live well below freezing. 504 00:27:50,800 --> 00:27:52,700 Bacteria could survive in outer space 505 00:27:52,700 --> 00:27:54,100 quite comfortably as well. 506 00:27:54,100 --> 00:27:57,066 These are behaviors that we consider to be extreme 507 00:27:57,066 --> 00:27:58,600 because we would die in them. 508 00:28:01,366 --> 00:28:05,200 NARRATOR: Today, life on Earth is prolific, 509 00:28:05,200 --> 00:28:08,466 thriving in the most unlikely of places. 510 00:28:09,700 --> 00:28:12,300 From under ice sheets, 511 00:28:12,300 --> 00:28:16,166 to acid-filled pools, 512 00:28:16,166 --> 00:28:21,333 and the dark, high-pressure depths of the oceans. 513 00:28:21,333 --> 00:28:25,200 And researching these modern extremophiles 514 00:28:25,200 --> 00:28:28,533 can hint at what was happening 515 00:28:28,533 --> 00:28:31,833 in the early atmosphere. 516 00:28:31,833 --> 00:28:34,333 LINGAPPA: There's been extremophiles 517 00:28:34,333 --> 00:28:36,166 as long as there's been life on Earth. 518 00:28:37,200 --> 00:28:40,133 Studying today's extremophiles 519 00:28:40,133 --> 00:28:42,700 expands our understanding of what life can do 520 00:28:42,700 --> 00:28:43,866 and what life can look like, 521 00:28:43,866 --> 00:28:45,800 including trying to understand 522 00:28:45,800 --> 00:28:50,266 the earliest life and our most ancient ancestors. 523 00:28:50,266 --> 00:28:54,300 GIRGUIS: Life originated on an earth without oxygen. 524 00:28:54,300 --> 00:28:56,766 So looking at the microbes that live 525 00:28:56,766 --> 00:28:59,033 in areas without oxygen 526 00:28:59,033 --> 00:29:01,700 helps us understand how they evolved 527 00:29:01,700 --> 00:29:04,400 to do well in those environments. 528 00:29:04,400 --> 00:29:06,366 (water fizzing) 529 00:29:06,366 --> 00:29:10,933 NARRATOR: But evolving to live in one very specialized habitat 530 00:29:10,933 --> 00:29:13,933 can have its drawbacks. 531 00:29:13,933 --> 00:29:16,133 At this period of time in Earth's history, 532 00:29:16,133 --> 00:29:20,366 life would have been limited in its energy sources. 533 00:29:21,666 --> 00:29:23,166 LINGAPPA: If you're dependent 534 00:29:23,166 --> 00:29:26,366 on something that's being released from a vent in order 535 00:29:26,366 --> 00:29:30,300 to generate energy to be alive, you're going to be 536 00:29:30,300 --> 00:29:35,033 extremely limited in how fast you can grow. 537 00:29:35,033 --> 00:29:38,666 NARRATOR: Because the energy sources early life used 538 00:29:38,666 --> 00:29:42,333 to survive in the oceans were comparatively rare, 539 00:29:42,333 --> 00:29:46,433 organisms struggled to proliferate. 540 00:29:46,433 --> 00:29:48,666 PIERREHUMBERT: They had to take various chemicals 541 00:29:48,666 --> 00:29:50,500 that were produced by geology 542 00:29:50,500 --> 00:29:52,833 and then combine them to release energy. 543 00:29:55,700 --> 00:29:57,966 WONG: But there's about to be 544 00:29:57,966 --> 00:30:02,700 a grand innovation by life that will open up 545 00:30:02,700 --> 00:30:05,400 a nearly limitless source of energy. 546 00:30:07,533 --> 00:30:09,866 NARRATOR: An innovation that would also change 547 00:30:09,866 --> 00:30:14,600 the course of the evolution of the atmosphere. 548 00:30:14,600 --> 00:30:18,633 ♪ ♪ 549 00:30:22,166 --> 00:30:23,866 Today, life can be found 550 00:30:23,866 --> 00:30:27,033 in puddles, lakes, and oceans across the planet. 551 00:30:30,033 --> 00:30:31,766 (waves crashing) 552 00:30:31,766 --> 00:30:32,900 LINGAPPA: Our world today 553 00:30:32,900 --> 00:30:35,233 is absolutely teeming with life. 554 00:30:35,233 --> 00:30:39,300 Just a single drop of water from the ocean 555 00:30:39,300 --> 00:30:43,300 would contain tens of thousands of cells, 556 00:30:43,300 --> 00:30:47,000 and among them, you would find organisms called cyanobacteria. 557 00:30:48,933 --> 00:30:52,700 NARRATOR: There are over 2,000 known types of cyanobacteria, 558 00:30:52,700 --> 00:30:55,366 and they come in all shapes and sizes. 559 00:30:58,233 --> 00:30:59,400 From long filaments 560 00:30:59,400 --> 00:31:03,666 to geometric shapes. 561 00:31:04,666 --> 00:31:06,233 LYONS: Cyanobacteria evolved, 562 00:31:06,233 --> 00:31:07,800 as best as we understand, 563 00:31:07,800 --> 00:31:09,966 around three billion years ago. 564 00:31:09,966 --> 00:31:12,000 They are astounding, in that 565 00:31:12,000 --> 00:31:14,566 they have the ability to adapt to environmental change, 566 00:31:14,566 --> 00:31:16,800 and to change the environment 567 00:31:16,800 --> 00:31:19,866 because they can be so abundant. 568 00:31:19,866 --> 00:31:22,733 NARRATOR: Cyanobacteria sometimes changed 569 00:31:22,733 --> 00:31:25,266 their local environments by creating structures, 570 00:31:25,266 --> 00:31:30,233 which they used as habitats to live in. 571 00:31:30,233 --> 00:31:35,100 This is one of the clear-cut examples that life was living 572 00:31:35,100 --> 00:31:36,366 even three billion years ago. 573 00:31:36,366 --> 00:31:38,466 This is a formation called a stromatolite. 574 00:31:38,466 --> 00:31:43,333 It's really inspiring that you can pick up a rock like this 575 00:31:43,333 --> 00:31:45,500 and read the history of what was happening 576 00:31:45,500 --> 00:31:46,733 billions of years ago. 577 00:31:46,733 --> 00:31:49,000 That's an amazing book to read. 578 00:31:49,000 --> 00:31:52,400 ♪ ♪ 579 00:31:52,400 --> 00:31:54,533 LINGAPPA: Cyanobacteria are some of the most 580 00:31:54,533 --> 00:31:58,533 successful organisms ever to live on our planet. 581 00:31:58,533 --> 00:32:02,666 They completely transformed the world. 582 00:32:05,366 --> 00:32:07,500 NARRATOR: This transformation was due 583 00:32:07,500 --> 00:32:09,400 to a revolutionary process 584 00:32:09,400 --> 00:32:15,000 that changed the fundamental composition of the atmosphere. 585 00:32:17,800 --> 00:32:20,333 Around three billion years ago, 586 00:32:20,333 --> 00:32:23,833 Cyanobacteria are developing a unique way 587 00:32:23,833 --> 00:32:27,066 to produce energy. 588 00:32:27,066 --> 00:32:30,300 They capture sunlight 589 00:32:30,300 --> 00:32:35,300 and use it to combine hydrogen from nearby water molecules... 590 00:32:37,000 --> 00:32:41,800 with the carbon dioxide dissolved in the oceans. 591 00:32:41,800 --> 00:32:44,733 With this, they make carbohydrates... 592 00:32:48,800 --> 00:32:54,400 And a very reactive waste product-- 593 00:32:54,400 --> 00:32:58,033 oxygen. 594 00:32:58,033 --> 00:33:00,833 LINGAPPA: So this vial contains a culture 595 00:33:00,833 --> 00:33:03,533 of cyanobacteria that I grew. 596 00:33:03,533 --> 00:33:05,266 And these are cyanobacteria of today, 597 00:33:05,266 --> 00:33:09,433 but they are descended from the organisms 598 00:33:09,433 --> 00:33:12,166 that first evolved the ability 599 00:33:12,166 --> 00:33:15,366 to do oxygen-producing photosynthesis. 600 00:33:18,066 --> 00:33:19,133 LANE: There are these 601 00:33:19,133 --> 00:33:20,966 moments in the history of life 602 00:33:20,966 --> 00:33:22,133 that seem to have 603 00:33:22,133 --> 00:33:23,433 only happened once. 604 00:33:23,433 --> 00:33:27,333 Oxygen-producing photosynthesis is one of them. 605 00:33:27,333 --> 00:33:28,800 Was it a freak accident? 606 00:33:28,800 --> 00:33:31,333 We just don't know. 607 00:33:31,333 --> 00:33:34,833 Suddenly, the oceans became the fuel. 608 00:33:34,833 --> 00:33:37,000 It allowed life to scale up at least tenfold. 609 00:33:39,000 --> 00:33:41,500 NARRATOR: Lifeforms can now make energy 610 00:33:41,500 --> 00:33:44,700 using one of the most abundant raw materials 611 00:33:44,700 --> 00:33:49,966 on the planet's surface: water. 612 00:33:49,966 --> 00:33:53,466 This kind of photosynthesis now enables organisms 613 00:33:53,466 --> 00:33:57,133 to get energy from virtually anywhere, 614 00:33:57,133 --> 00:34:01,600 allowing them to spread throughout the oceans. 615 00:34:04,400 --> 00:34:08,166 One cyanobacteria cell becomes two; 616 00:34:08,166 --> 00:34:10,233 two become four. 617 00:34:12,066 --> 00:34:17,133 Multiplying again and again... 618 00:34:18,666 --> 00:34:23,800 until there are trillions. 619 00:34:30,666 --> 00:34:34,666 And we can still find evidence of them everywhere we look, 620 00:34:34,666 --> 00:34:39,233 including in the harshest environments. 621 00:34:42,733 --> 00:34:43,900 LINGAPPA: If you look really closely 622 00:34:43,900 --> 00:34:46,600 in the mats in this pool, 623 00:34:46,600 --> 00:34:48,633 you can see in some of these greener areas 624 00:34:48,633 --> 00:34:52,933 little bubbles that are literally the oxygen 625 00:34:52,933 --> 00:34:54,700 being produced by them 626 00:34:54,700 --> 00:34:57,266 doing photosynthesis right now. 627 00:34:59,933 --> 00:35:02,900 Cyanobacteria demonstrate the power of life on Earth 628 00:35:02,900 --> 00:35:05,500 like nothing else does. 629 00:35:05,500 --> 00:35:06,933 Over two billion years ago, 630 00:35:06,933 --> 00:35:09,900 the ancestors of these extraordinary microbes 631 00:35:09,900 --> 00:35:12,333 unlocked this spectacular ability. 632 00:35:14,433 --> 00:35:16,833 LANE: Cyanobacteria were the first, 633 00:35:16,833 --> 00:35:18,366 and the only, organisms 634 00:35:18,366 --> 00:35:24,066 that ever evolved oxygen-producing photosynthesis. 635 00:35:24,066 --> 00:35:25,700 As soon as they cracked the problem 636 00:35:25,700 --> 00:35:27,433 of how to use water as a fuel, 637 00:35:27,433 --> 00:35:29,566 the oceans became the fuel, and they could 638 00:35:29,566 --> 00:35:33,466 live almost anywhere. 639 00:35:35,733 --> 00:35:39,066 These are seaweeds from the ocean-- 640 00:35:39,066 --> 00:35:42,000 ooh, right off the West Coast of Scotland-- 641 00:35:42,000 --> 00:35:45,333 and oxygen-producing photosynthesis 642 00:35:45,333 --> 00:35:48,166 was such a brilliant innovation, 643 00:35:48,166 --> 00:35:50,733 that some organisms at that time 644 00:35:50,733 --> 00:35:53,633 actually swallowed the cyanobacteria, 645 00:35:53,633 --> 00:35:57,900 and that's how we got algae like this. 646 00:35:57,900 --> 00:36:00,200 And then the algae ultimately gave rise 647 00:36:00,200 --> 00:36:01,800 to the plants, as well. 648 00:36:01,800 --> 00:36:04,666 (thunder rumbling) 649 00:36:04,666 --> 00:36:07,900 NARRATOR: While life was spreading through the oceans, 650 00:36:07,900 --> 00:36:10,733 above the waves, 651 00:36:10,733 --> 00:36:12,966 the atmosphere was also undergoing 652 00:36:12,966 --> 00:36:14,666 a dramatic transformation. 653 00:36:24,733 --> 00:36:28,300 ♪ ♪ 654 00:36:30,100 --> 00:36:32,800 Over millions of years, 655 00:36:32,800 --> 00:36:34,733 tectonic movements in the earth 656 00:36:34,733 --> 00:36:38,733 are forging the first large-scale landmass. 657 00:36:44,266 --> 00:36:46,633 And this directly affects 658 00:36:46,633 --> 00:36:49,300 the dynamics of the atmosphere. 659 00:36:49,300 --> 00:36:54,366 ♪ ♪ 660 00:36:54,366 --> 00:36:56,466 (wind howling) 661 00:36:56,466 --> 00:36:58,833 Air warmed by the new land 662 00:36:58,833 --> 00:37:02,466 drives novel weather patterns across the globe. 663 00:37:02,466 --> 00:37:04,233 (wind blowing) 664 00:37:04,233 --> 00:37:06,766 (thunder rumbling) 665 00:37:06,766 --> 00:37:12,600 Winds sweep dust high up into the air, 666 00:37:12,600 --> 00:37:16,600 where tiny particles seed vast pillars of clouds. 667 00:37:16,600 --> 00:37:19,433 ♪ ♪ 668 00:37:19,433 --> 00:37:22,800 Something resembling our modern-day weather system 669 00:37:22,800 --> 00:37:27,333 is, at last, taking shape. 670 00:37:27,333 --> 00:37:29,700 HAZEN: The appearance of large continental masses 671 00:37:29,700 --> 00:37:32,100 had a huge impact on the atmosphere. 672 00:37:32,100 --> 00:37:33,400 We would have seen 673 00:37:33,400 --> 00:37:34,400 high-pressure systems, 674 00:37:34,400 --> 00:37:35,500 low-pressure systems, 675 00:37:35,500 --> 00:37:37,233 prevailing winds, rains, 676 00:37:37,233 --> 00:37:39,966 very much like the modern world. 677 00:37:39,966 --> 00:37:44,000 ♪ ♪ 678 00:37:47,033 --> 00:37:48,966 NARRATOR: Meanwhile, in the oceans, 679 00:37:48,966 --> 00:37:51,033 trillions of cyanobacteria 680 00:37:51,033 --> 00:37:55,400 are photosynthesizing and producing oxygen. 681 00:37:55,400 --> 00:37:58,766 The first time it has ever been made 682 00:37:58,766 --> 00:38:02,200 in significant amounts on Earth. 683 00:38:04,366 --> 00:38:07,733 Oxygen bubbles race upwards. 684 00:38:07,733 --> 00:38:11,333 ♪ ♪ 685 00:38:14,433 --> 00:38:20,333 But most vanish before they can reach the surface. 686 00:38:20,333 --> 00:38:22,600 (waves crashing) 687 00:38:22,600 --> 00:38:25,500 Earth's early atmosphere seems destined 688 00:38:25,500 --> 00:38:29,800 to stay as thick orange hazes. 689 00:38:32,366 --> 00:38:37,966 So what is making the oxygen disappear? 690 00:38:37,966 --> 00:38:43,033 The answer lies in a unique property oxygen has. 691 00:38:43,033 --> 00:38:45,366 Oxygen is an extremely reactive element. 692 00:38:47,433 --> 00:38:49,833 DOTTIN: What's unique about oxygen 693 00:38:49,833 --> 00:38:52,100 is that it has a chemical makeup 694 00:38:52,100 --> 00:38:54,266 that allows it to bond with 695 00:38:54,266 --> 00:38:56,433 so many different elements; 696 00:38:56,433 --> 00:38:59,900   almost all of the elements that exist. 697 00:38:59,900 --> 00:39:02,700 NARRATOR: And this incredible reactive power 698 00:39:02,700 --> 00:39:05,000 is the reason many scientists think 699 00:39:05,000 --> 00:39:09,166 oxygen struggled to escape the water. 700 00:39:12,833 --> 00:39:20,166 ♪ ♪ 701 00:39:20,166 --> 00:39:23,233 Around three billion years ago, 702 00:39:23,233 --> 00:39:25,800 Earth's oceans were packed 703 00:39:25,800 --> 00:39:29,333 full of dissolved metals. 704 00:39:29,333 --> 00:39:32,333 Metals like iron. 705 00:39:33,366 --> 00:39:35,166 This key that we see here 706 00:39:35,166 --> 00:39:36,200 is rusty, 707 00:39:36,200 --> 00:39:37,966 and in order to form rust 708 00:39:37,966 --> 00:39:39,300 we need three ingredients. 709 00:39:39,300 --> 00:39:41,000 The first is iron, which is 710 00:39:41,000 --> 00:39:42,766 what this key is made of, 711 00:39:42,766 --> 00:39:43,966 the second is water, 712 00:39:43,966 --> 00:39:45,533 or water vapor in the air, 713 00:39:45,533 --> 00:39:47,800 and the third crucial ingredient 714 00:39:47,800 --> 00:39:50,200 is oxygen. 715 00:39:50,200 --> 00:39:52,000 NARRATOR: And in Earth's early oceans, 716 00:39:52,000 --> 00:39:57,566 oxygen began to react more and more with iron and other metals, 717 00:39:57,566 --> 00:40:01,666 as cyanobacteria spread across the oceans. 718 00:40:01,666 --> 00:40:05,500 ♪ ♪ 719 00:40:09,200 --> 00:40:11,766 Below the surface... 720 00:40:13,566 --> 00:40:16,533 it's raining. 721 00:40:16,533 --> 00:40:20,800 As oxygen and iron interact, 722 00:40:20,800 --> 00:40:24,066 rust particles form in the water, 723 00:40:24,066 --> 00:40:25,900 and sink down to the seabed... 724 00:40:28,333 --> 00:40:30,933 trapping the oxygen and iron 725 00:40:30,933 --> 00:40:35,333 to form solid rock at the bottom of the ocean. 726 00:40:36,466 --> 00:40:41,366 Vast swaths of the water turn red. 727 00:40:41,366 --> 00:40:42,666 LYONS: There were many, 728 00:40:42,666 --> 00:40:44,166 many ingredients in the ocean 729 00:40:44,166 --> 00:40:45,733 at that time that would 730 00:40:45,733 --> 00:40:47,333 readily react with oxygen. 731 00:40:48,966 --> 00:40:51,500 HAZEN: So there was no way for atmospheric oxygen 732 00:40:51,500 --> 00:40:53,600 to build up to any significant level. 733 00:40:56,066 --> 00:40:58,500 (seagulls calling) 734 00:40:58,500 --> 00:41:00,333 NARRATOR: A record of the reactions 735 00:41:00,333 --> 00:41:03,233 that happened long ago can be seen 736 00:41:03,233 --> 00:41:07,366 in formations today. 737 00:41:07,366 --> 00:41:08,900 This is a sedimentary rock, 738 00:41:08,900 --> 00:41:11,066 meaning it's made up of many different layers 739 00:41:11,066 --> 00:41:12,333 that were all deposited 740 00:41:12,333 --> 00:41:13,433 on the sea floor 741 00:41:13,433 --> 00:41:14,733 over millions of years. 742 00:41:17,700 --> 00:41:19,600 NARRATOR: And this sample is evidence 743 00:41:19,600 --> 00:41:24,433 of how iron can react to form solid rock. 744 00:41:24,433 --> 00:41:26,433 This particular sedimentary rock 745 00:41:26,433 --> 00:41:28,166 is a banded iron formation, 746 00:41:28,166 --> 00:41:29,900 and we can see red color 747 00:41:29,900 --> 00:41:31,066 in the rock here, 748 00:41:31,066 --> 00:41:32,800 and that red is evidence 749 00:41:32,800 --> 00:41:35,000 of iron oxidation or rust. 750 00:41:36,333 --> 00:41:38,533 NARRATOR: As these reactions continued, 751 00:41:38,533 --> 00:41:40,666 the metals in the ocean water 752 00:41:40,666 --> 00:41:45,400 started to diminish, locked away in the seafloor. 753 00:41:45,400 --> 00:41:46,866 LINGAPPA: Eventually, 754 00:41:46,866 --> 00:41:48,700 the oceans would have 755 00:41:48,700 --> 00:41:50,333 been depleted of the things 756 00:41:50,333 --> 00:41:51,500 that would readily react 757 00:41:51,500 --> 00:41:56,000 with the oxygen that was being released. 758 00:41:56,000 --> 00:42:00,100 NARRATOR: But cyanobacteria continued to produce oxygen. 759 00:42:01,400 --> 00:42:06,433 ♪ ♪ 760 00:42:10,866 --> 00:42:13,333 For up to half a billion years, 761 00:42:13,333 --> 00:42:15,033 this oxygen has been trapped 762 00:42:15,033 --> 00:42:17,966 in the oceans. 763 00:42:25,300 --> 00:42:28,300 But now, increasing amounts are free to escape. 764 00:42:28,300 --> 00:42:33,300 (water bubbling) 765 00:42:33,300 --> 00:42:37,733 ♪ ♪ 766 00:42:37,733 --> 00:42:40,900 Over just a few million years, 767 00:42:40,900 --> 00:42:44,700 oxygen floods into the atmosphere. 768 00:42:44,700 --> 00:42:46,900 (bubbling intensifies) 769 00:42:46,900 --> 00:42:50,933 ♪ ♪ 770 00:42:59,933 --> 00:43:05,266 The very composition of the atmosphere itself is changing. 771 00:43:08,500 --> 00:43:15,266 ♪ ♪ 772 00:43:20,566 --> 00:43:25,800 This is known as the Great Oxidation Event. 773 00:43:27,000 --> 00:43:29,000 DOTTIN: The Great Oxidation Event 774 00:43:29,000 --> 00:43:32,166 was one of the most significant periods in Earth's history. 775 00:43:32,166 --> 00:43:36,466 ♪ ♪ 776 00:43:36,466 --> 00:43:38,133 It was a period of time 777 00:43:38,133 --> 00:43:39,700 over which oxygen built up 778 00:43:39,700 --> 00:43:42,500 in the atmosphere. 779 00:43:42,500 --> 00:43:44,000 LINGAPPA: Not only did it 780 00:43:44,000 --> 00:43:46,000 completely change the environment, 781 00:43:46,000 --> 00:43:49,566 it also completely changed the possibilities 782 00:43:49,566 --> 00:43:51,600 available for life. 783 00:43:51,600 --> 00:43:52,866 ROTHSCHILD: It is almost 784 00:43:52,866 --> 00:43:56,366 impossible to overstate how important 785 00:43:56,366 --> 00:43:59,700 the production of oxygen on the Earth was 786 00:43:59,700 --> 00:44:03,400 to the history of planet Earth. 787 00:44:03,400 --> 00:44:06,966 All of a sudden, new chemical possibilities arose 788 00:44:06,966 --> 00:44:08,733 all over the surface of Earth, 789 00:44:08,733 --> 00:44:11,066 and amazing changes were to follow. 790 00:44:12,200 --> 00:44:15,100 NARRATOR: Oxygen is used by organisms today 791 00:44:15,100 --> 00:44:16,700 to release energy 792 00:44:16,700 --> 00:44:19,466 from the food they eat, so their bodies can use it. 793 00:44:19,466 --> 00:44:22,066 And since oxygen is so reactive, 794 00:44:22,066 --> 00:44:26,166 this energy release is immense. 795 00:44:26,166 --> 00:44:29,700 ROTHSCHILD: If you use oxygen, 796 00:44:29,700 --> 00:44:32,566 you are 19 times more efficient 797 00:44:32,566 --> 00:44:36,833 than an organism that doesn't use oxygen. 798 00:44:39,900 --> 00:44:45,166 ♪ ♪ 799 00:44:45,166 --> 00:44:47,500 NARRATOR: Because of its reactive power, 800 00:44:47,500 --> 00:44:51,900 one of the first consequences of oxygen in the atmosphere 801 00:44:51,900 --> 00:44:56,200 is on the rocks themselves. 802 00:44:56,200 --> 00:44:57,833 Now, depending on your perspective, 803 00:44:57,833 --> 00:45:01,433 oxygen can be a creative or a destructive force. 804 00:45:01,433 --> 00:45:07,600 ♪ ♪ 805 00:45:07,600 --> 00:45:09,733 NARRATOR: As it spreads through the atmosphere, 806 00:45:09,733 --> 00:45:12,366 oxygen begins to react vigorously 807 00:45:12,366 --> 00:45:16,300 with the surface of the Earth. 808 00:45:16,300 --> 00:45:19,666 Rocks with metals like iron react 809 00:45:19,666 --> 00:45:23,633 with the oxygen and crumble. 810 00:45:23,633 --> 00:45:29,033 All the elements are in place for vast dust storms to form. 811 00:45:31,366 --> 00:45:36,533 The young world is being ripped up by its own atmosphere. 812 00:45:38,266 --> 00:45:42,766 Earth's surface is changing color 813 00:45:42,766 --> 00:45:46,266 as new minerals are created. 814 00:45:49,800 --> 00:45:52,000 TIKOO: We had the formation of all these minerals 815 00:45:52,000 --> 00:45:53,733 that did not exist prior 816 00:45:53,733 --> 00:45:55,366 to the Great Oxidation Event. 817 00:45:55,366 --> 00:45:56,500 Copper minerals like these, 818 00:45:56,500 --> 00:45:58,200 blue and green minerals, 819 00:45:58,200 --> 00:46:01,066 yellow and orange, uranium minerals, 820 00:46:01,066 --> 00:46:03,166 minerals of molybdenum, minerals of nickel, 821 00:46:03,166 --> 00:46:06,333 minerals of cobalt, color of the rainbow, 822 00:46:06,333 --> 00:46:08,166 all in new minerals 823 00:46:08,166 --> 00:46:11,433 form because of the Great Oxidation Event. 824 00:46:13,300 --> 00:46:17,266 NARRATOR: It isn't just minerals. 825 00:46:17,266 --> 00:46:20,800 Oxygen also makes key nutrients vital for life 826 00:46:20,800 --> 00:46:23,100 far more available 827 00:46:23,100 --> 00:46:28,800 by reacting with a number of elements to form acids. 828 00:46:28,800 --> 00:46:30,466 Things like sulfuric acid 829 00:46:30,466 --> 00:46:31,800 and nitric acid, 830 00:46:31,800 --> 00:46:34,333 and acid has this property 831 00:46:34,333 --> 00:46:36,366 that when it rains out onto the surface of the earth, 832 00:46:36,366 --> 00:46:38,900 it can dissolve rocks, cause weathering 833 00:46:38,900 --> 00:46:41,766 and runoff of nutrients and minerals 834 00:46:41,766 --> 00:46:43,733 into lakes and oceans. 835 00:46:45,833 --> 00:46:47,500 HAZEN: Those nutrients flowed 836 00:46:47,500 --> 00:46:49,900 into the ocean, there were new opportunities for life. 837 00:46:49,900 --> 00:46:51,533 It really was a game changer. 838 00:46:56,800 --> 00:46:59,933 LYONS: And as oxygen began to increase in the atmosphere, 839 00:46:59,933 --> 00:47:02,433 the relatively minor elements 840 00:47:02,433 --> 00:47:05,466 that are essential for all the processes of life 841 00:47:05,466 --> 00:47:07,166 became more abundant. 842 00:47:07,166 --> 00:47:09,566 In many cases, those are metals. 843 00:47:11,733 --> 00:47:15,100 ♪ ♪ 844 00:47:18,400 --> 00:47:21,466 LINGAPPA: Life on Earth evolved 845 00:47:21,466 --> 00:47:24,600 because of the conditions that the Earth provided. 846 00:47:24,600 --> 00:47:28,366 But, in turn, life has completely remodeled 847 00:47:28,366 --> 00:47:30,966 the planet, and the world that we know today 848 00:47:30,966 --> 00:47:36,133 is, in so many ways, been created by life. 849 00:47:36,133 --> 00:47:39,500 (flamingos squawking) 850 00:47:42,566 --> 00:47:46,466 ♪ ♪ 851 00:47:46,466 --> 00:47:48,366 NARRATOR: This co-evolution 852 00:47:48,366 --> 00:47:53,600 of earth and the atmosphere is apparent everywhere we look. 853 00:47:53,600 --> 00:47:55,133 (waves crashing) 854 00:47:55,133 --> 00:47:57,100 ANDERSON: These particular cliffs were formed 855 00:47:57,100 --> 00:47:59,100 when dinosaurs walked the earth. 856 00:47:59,100 --> 00:48:00,800 But the beautiful red that you can see 857 00:48:00,800 --> 00:48:02,433 in these bands here is formed 858 00:48:02,433 --> 00:48:04,266 from the iron in these cliffs 859 00:48:04,266 --> 00:48:07,866 combined with water and oxygen to create iron oxides. 860 00:48:07,866 --> 00:48:11,666 So we see an example of oxygen changing the very landscape. 861 00:48:13,600 --> 00:48:19,133 NARRATOR: Oxygen transformed our oceans, reshaped our land, 862 00:48:19,133 --> 00:48:22,000 and became a vital ingredient for life today. 863 00:48:23,900 --> 00:48:25,633 But this volatile gas 864 00:48:25,633 --> 00:48:30,433 was to have one more dramatic impact on the world. 865 00:48:30,433 --> 00:48:38,066 ♪ ♪ 866 00:48:38,066 --> 00:48:43,533 Across the globe, oxygen levels are rising, 867 00:48:43,533 --> 00:48:46,966 slowly enriching the air, reacting with the methane 868 00:48:46,966 --> 00:48:49,300 to form carbon dioxide and water. 869 00:48:49,300 --> 00:48:53,566 (waves crashing) 870 00:48:53,566 --> 00:48:55,233 Over millions of years, 871 00:48:55,233 --> 00:48:58,500 the orange hazes are fading away. 872 00:49:02,933 --> 00:49:08,100 Sunlight is scattered by the nitrogen and oxygen, 873 00:49:08,100 --> 00:49:11,066 producing Earth's first oxygen-rich 874 00:49:11,066 --> 00:49:14,166 blue skies. 875 00:49:14,166 --> 00:49:21,366 ♪ ♪ 876 00:49:21,366 --> 00:49:23,733 In the upper atmosphere, 877 00:49:23,733 --> 00:49:25,966 ultraviolet light from the sun 878 00:49:25,966 --> 00:49:29,600 causes the oxygen to form a new protective molecule... 879 00:49:31,233 --> 00:49:33,266 ozone. 880 00:49:33,266 --> 00:49:34,400 HAZEN: Ozone is made 881 00:49:34,400 --> 00:49:37,500 of three oxygen atoms bonded together, 882 00:49:37,500 --> 00:49:40,233 and they absorb ultraviolet radiation really well. 883 00:49:40,233 --> 00:49:41,866 So, if you put a little bit of ozone 884 00:49:41,866 --> 00:49:44,066   in the atmosphere, it's like sunblock. 885 00:49:47,633 --> 00:49:49,933 NARRATOR: The level of oxygen in the atmosphere 886 00:49:49,933 --> 00:49:54,866 will fluctuate over the next two billion years, 887 00:49:54,866 --> 00:49:58,900 but eventually, reach the 21% we have today. 888 00:50:03,766 --> 00:50:07,033 ♪ ♪ 889 00:50:07,033 --> 00:50:10,633 But the story of our atmosphere is not over. 890 00:50:10,633 --> 00:50:12,566 It will continue to evolve, 891 00:50:12,566 --> 00:50:16,733 both naturally, and under the influence of human activity. 892 00:50:18,600 --> 00:50:19,800 ROTHSCHILD: If we don't understand 893 00:50:19,800 --> 00:50:21,366 the history of the atmosphere, 894 00:50:21,366 --> 00:50:24,133 how can we possibly be the stewards 895 00:50:24,133 --> 00:50:25,900 of the atmosphere moving forward? 896 00:50:29,000 --> 00:50:31,066 PARAI: Because our atmosphere is totally unique, 897 00:50:31,066 --> 00:50:32,700 there's nothing like it that we found 898 00:50:32,700 --> 00:50:36,133 on any other planetary body. 899 00:50:36,133 --> 00:50:39,766 WONG: Our atmospheric composition may actually be 900 00:50:39,766 --> 00:50:42,566 the thing that is screaming out to the cosmos, 901 00:50:42,566 --> 00:50:45,633 a signal to the universe that we're here, 902 00:50:45,633 --> 00:50:48,066 that life exists on our world. 903 00:50:48,066 --> 00:50:49,233 DOTTIN: But then, 904 00:50:49,233 --> 00:50:50,800 in many ways, we're going back in time, 905 00:50:50,800 --> 00:50:52,600 where we're putting all of 906 00:50:52,600 --> 00:50:56,933 the chemicals that are toxic to life into the atmosphere. 907 00:50:59,166 --> 00:51:01,766 WONG: Over the past couple hundreds of years, 908 00:51:01,766 --> 00:51:03,500 humanity has made a difference 909 00:51:03,500 --> 00:51:05,566 in the chemical composition of our atmosphere, 910 00:51:05,566 --> 00:51:08,600 most predominantly through the release of 911 00:51:08,600 --> 00:51:13,166 carbon dioxide, and this causes myriad effects on our world. 912 00:51:14,800 --> 00:51:17,400 HAZEN: The atmosphere isn't fragile. 913 00:51:17,400 --> 00:51:18,733 But we are. 914 00:51:18,733 --> 00:51:19,966 We can make changes 915 00:51:19,966 --> 00:51:22,333 to the atmosphere that will make Earth 916 00:51:22,333 --> 00:51:24,900 a very unpleasant place to live. 917 00:51:24,900 --> 00:51:27,000 And so we need to look to ourselves 918 00:51:27,000 --> 00:51:30,466 when we think about changes to the atmosphere. 919 00:51:32,433 --> 00:51:34,600 LINGAPPA: We have immense power 920 00:51:34,600 --> 00:51:35,833 over our planet. 921 00:51:35,833 --> 00:51:39,666 Our atmosphere is precious. 922 00:51:39,666 --> 00:51:43,233 And, if we change it too much, 923 00:51:43,233 --> 00:51:45,366 the whole world will pay the price. 924 00:51:45,366 --> 00:51:47,233 (magma rushing) 925 00:51:47,233 --> 00:51:50,266 GIRGUIS: But by understanding the huge and complex steps 926 00:51:50,266 --> 00:51:53,800 it took to develop our atmosphere, hopefully, 927 00:51:53,800 --> 00:51:55,000 we can develop approaches 928 00:51:55,000 --> 00:51:57,000 to take care of it for generations to come. 929 00:51:57,000 --> 00:52:00,800 ♪ ♪ 930 00:52:08,533 --> 00:52:09,666 ANNOUNCER: On "NOVA: Ancient Earth"... 931 00:52:09,666 --> 00:52:12,233 People thought it was an outrageous idea. 932 00:52:12,233 --> 00:52:14,733 The planet completely encased in ice. 933 00:52:14,733 --> 00:52:17,666 CECILIA SANDERS: Some of the deepest, most intense cold 934 00:52:17,666 --> 00:52:20,266 Earth has ever experienced. 935 00:52:20,266 --> 00:52:21,800 JAZ MILLAR: Life survived events 936 00:52:21,800 --> 00:52:23,966 that scientists previously thought were unsurvivable. 937 00:52:23,966 --> 00:52:25,566 SUSANNAH PORTER: But, for life to flourish again, 938 00:52:25,566 --> 00:52:28,166 we would need some kind of drastic change. 939 00:52:28,166 --> 00:52:29,633 (explosion roars) 940 00:52:29,633 --> 00:52:32,700 ANNOUNCER: "Frozen," on "NOVA." 941 00:52:32,700 --> 00:52:34,933 Next time. 942 00:52:34,933 --> 00:52:36,733 SINGER: ♪ Wish I could go back in time ♪ 943 00:52:48,233 --> 00:52:55,766 ♪ ♪ 944 00:52:59,600 --> 00:53:07,200 ♪ ♪ 945 00:53:08,833 --> 00:53:16,366 ♪ ♪ 946 00:53:18,000 --> 00:53:25,533 ♪ ♪ 947 00:53:31,266 --> 00:53:38,433 ♪ ♪