Trascrizione completa di Mass Extinctions: The Five Times Earth Nearly Died

now, and 70 00:04:06.539 --> 00:04:10.180 that sudden worldwide disasters were a relic of biblical thinking. 71 00:04:10.919 --> 00:04:14.020 Darwin took the same view. In The Origin of Species, 72 00:04:14.360 --> 00:04:16.730 he spent an entire chapter on what he called the 73 00:04:16.870 --> 00:04:20.550 imperfection of the geological record, and his argument was was 74 00:04:20.589 --> 00:04:23.449 that the sharp breaks in the fossil sequence were gaps 75 00:04:23.490 --> 00:04:26.670 in the record, rather than real events. The pages were 76 00:04:26.750 --> 00:04:31.870 missing in his reading. The story was continuous. The rocks 77 00:04:31.949 --> 00:04:33.670 kept disagreeing with them in. 78 00:04:33.610 --> 00:04:34.379 A quiet way. 79 00:04:35.389 --> 00:04:39.600 In 1841, an English geologist named John Phillips divided the 80 00:04:39.660 --> 00:04:42.839 history of life into three great eras, which he named 81 00:04:42.879 --> 00:04:47.160 the Paleozoic, the Mesozoic, and what we now call the Cenozoic. 82 00:04:47.889 --> 00:04:50.790 He drew the lines where the fossils changed most completely, 83 00:04:51.189 --> 00:04:53.790 where one set of animals vanished from the rock and 84 00:04:53.850 --> 00:04:57.029 a different set took over above them. Phillips didn't frame 85 00:04:57.069 --> 00:05:00.449 those lines as disasters. He was just describing what he saw. 86 00:05:01.029 --> 00:05:03.600 But the two boundaries he picked are the two biggest 87 00:05:03.680 --> 00:05:07.500 mass extinctions that have ever happened, and every geology student 88 00:05:07.579 --> 00:05:11.079 since has learned the eras without necessarily learning that the 89 00:05:11.120 --> 00:05:16.120 dividing lines are graveyards. For the next hundred years, the 90 00:05:16.180 --> 00:05:20.810 subject mostly sat there, paleontologists knew the dinosaurs had disappeared 91 00:05:21.069 --> 00:05:23.680 and that the ammonites had disappeared with them. And the 92 00:05:23.759 --> 00:05:27.500 explanations on offer ran from climate to disease to the 93 00:05:27.600 --> 00:05:31.540 eggs being eaten by early mammals. In the 1960s, a 94 00:05:31.560 --> 00:05:35.740 paleontologist at the American Museum of Natural History named Norman 95 00:05:35.819 --> 00:05:38.970 Newell wrote a series of papers arguing that the fossil 96 00:05:39.050 --> 00:05:43.589 record showed several genuine crises, times when extinction ran far 97 00:05:43.670 --> 00:05:48.199 above normal across many unrelated groups at once. He counted six. 98 00:05:48.899 --> 00:05:51.759 He thought changes in sea level were the most likely cause. 99 00:05:52.459 --> 00:05:55.560 His work was respected, and it didn't change much, because 100 00:05:55.639 --> 00:05:58.199 nobody had a mechanism that could kill on that scale, 101 00:05:58.620 --> 00:06:01.089 and nobody had a way to measure the timing closely 102 00:06:01.149 --> 00:06:04.750 enough to argue about it. What changed that was a 103 00:06:04.810 --> 00:06:07.870 layer of clay in Italy, about as thick as a finger. 104 00:06:09.550 --> 00:06:12.629 The town of Gubbio sits in the Apennines in central Italy, 105 00:06:12.949 --> 00:06:14.269 and just outside. 106 00:06:13.870 --> 00:06:15.769 It there's a gorge called the Bottaccione. 107 00:06:16.209 --> 00:06:19.329 Where the road cuts through a long sequence of pink limestone. 108 00:06:19.810 --> 00:06:22.649 The limestone is made almost entirely of the shells of 109 00:06:22.750 --> 00:06:27.160 microscopic plankton called foraminifera, laid down on a deep seafloor 110 00:06:27.259 --> 00:06:31.139 over tens of millions of years. In the 1970s, a 111 00:06:31.199 --> 00:06:34.519 young geologist named Walter Alvarez was working there on the 112 00:06:34.560 --> 00:06:37.500 magnetic record preserved in the rock, and he kept coming 113 00:06:37.560 --> 00:06:41.759 back to one spot in the sequence. Below a certain level, 114 00:06:42.040 --> 00:06:45.689 the limestone was full of large foraminifera, species that had 115 00:06:45.709 --> 00:06:49.490 been thriving for millions of years. Right above it, the 116 00:06:49.529 --> 00:06:52.790 shells were tiny and belonged to different species. And in 117 00:06:52.810 --> 00:06:55.389 between the two was a thin bed of clay with 118 00:06:55.490 --> 00:06:58.810 almost no fossils in it at all. That clay marked 119 00:06:58.829 --> 00:07:01.480 the end of the Cretaceous period and the beginning of 120 00:07:01.500 --> 00:07:05.120 the Paleogene. The same break shows up all over the world. 121 00:07:05.879 --> 00:07:08.180 Walter wanted to know how long the clay had taken 122 00:07:08.220 --> 00:07:10.879 to form, because that would tell him whether the turnover 123 00:07:10.920 --> 00:07:15.189 in the plankton was gradual or fast. His father was 124 00:07:15.250 --> 00:07:18.769 Luis Alvarez, a physicist at Berkeley who'd won the Nobel 125 00:07:18.810 --> 00:07:21.810 Prize in 1968 and who had spent the war on 126 00:07:21.910 --> 00:07:26.209 radar and on the Manhattan Project. Luis suggested a clock. 127 00:07:26.939 --> 00:07:29.560 Dust from space falls onto the Earth at a slow 128 00:07:29.660 --> 00:07:33.240 and more or less steady rate. And that dust carries iridium, 129 00:07:33.600 --> 00:07:36.800 an element that's extremely rare in the Earth's crust because 130 00:07:36.860 --> 00:07:39.079 most of our iridium sank into the core when the 131 00:07:39.120 --> 00:07:41.920 planet was molten. If the clay had taken a long 132 00:07:41.980 --> 00:07:44.279 time to form, it would have soaked up a lot 133 00:07:44.300 --> 00:07:47.480 of cosmic iridium. If it formed quickly, it would have 134 00:07:47.560 --> 00:07:51.420 very little. Two nuclear chemists at the Lawrence Berkeley Laboratory, 135 00:07:51.740 --> 00:07:54.990 Frank Asaro and Helen Michelle, ran the samples using a 136 00:07:55.050 --> 00:07:59.230 technique called neutron activation analysis, which can pick up

iridium 137 00:07:59.310 --> 00:08:02.910 at the level of parts per billion. The number they 138 00:08:02.949 --> 00:08:06.850 got back didn't fit either expectation. The clay at Gubbio 139 00:08:06.910 --> 00:08:10.410 held roughly 30 times the iridium of the limestone above 140 00:08:10.509 --> 00:08:12.819 and below it. That was far too much to be 141 00:08:12.860 --> 00:08:16.620 explained by normal dust falling over any reasonable length of time. 142 00:08:17.220 --> 00:08:20.000 They checked a second site, a sea cliff in Denmark 143 00:08:20.040 --> 00:08:23.339 called Stevins Klint, where the same boundary is exposed in 144 00:08:23.360 --> 00:08:26.279 a dark layer the locals call the fish clay, and 145 00:08:26.319 --> 00:08:31.439 the spike there was about 160 times background. For a while, 146 00:08:31.519 --> 00:08:34.519 the Berkeley group thought about a nearby supernova, which could 147 00:08:34.539 --> 00:08:37.769 have showered the planet with heavy elements. They tested for 148 00:08:37.820 --> 00:08:40.940 a particular isotope of plutonium that a supernova would have 149 00:08:40.960 --> 00:08:44.700 left behind and didn't find it. What was left was 150 00:08:44.759 --> 00:08:47.879 the explanation they published in the journal Science in June 151 00:08:47.899 --> 00:08:52.279 of 1980. An asteroid, by their estimate somewhere around 10 152 00:08:52.279 --> 00:08:56.340 kilometers across, had hit the Earth 65 million years ago, 153 00:08:56.779 --> 00:08:59.940 thrown enough pulverized rock into the atmosphere to darken the 154 00:08:59.980 --> 00:09:04.480 sky worldwide for months, shut down photosynthesis, and collapsed the 155 00:09:04.519 --> 00:09:07.500 food chains from the plankton up. The iridium in the 156 00:09:07.539 --> 00:09:11.639 clay was the asteroid itself, spread thin across the whole planet. 157 00:09:12.840 --> 00:09:14.860 The paper landed in a field that had spent a 158 00:09:14.960 --> 00:09:18.730 century and a half being taught that catastrophes were unscientific, 159 00:09:19.289 --> 00:09:21.889 and the reaction was about what anyone familiar with the 160 00:09:21.950 --> 00:09:25.690 field would have predicted. A lot of paleontologists pointed out 161 00:09:25.730 --> 00:09:28.529 that the dinosaurs seemed to have been declining for millions 162 00:09:28.570 --> 00:09:31.409 of years before the boundary, and that a physicist and 163 00:09:31.450 --> 00:09:34.330 some chemists were telling them what their own fossils meant. 164 00:09:35.230 --> 00:09:38.370 Others pointed out that there was no crater. A 10-kilometer 165 00:09:38.509 --> 00:09:41.929 asteroid would leave a hole well over 100 kilometers wide, 166 00:09:42.360 --> 00:09:44.480 and nobody knew of one that was the right age. 167 00:09:45.799 --> 00:09:49.639 The evidence kept accumulating anyway. The Iridium spike turned up 168 00:09:49.700 --> 00:09:54.350 at more than 100 boundary sites around the world. In 1984, 169 00:09:54.350 --> 00:09:57.889 a geologist with the United States Geological Survey named Bruce 170 00:09:57.970 --> 00:10:01.100 Bohor found grains of quartz in the boundary clay that 171 00:10:01.129 --> 00:10:05.259 carried microscopic parallel fractures, the kind of damage that only 172 00:10:05.340 --> 00:10:08.059 forms under the shock pressures of an impact or a 173 00:10:08.120 --> 00:10:11.879 nuclear test. Tiny glass spheres showed up in boundary layers 174 00:10:11.960 --> 00:10:14.740 in Haiti, the kind that form when molten rock is 175 00:10:14.779 --> 00:10:18.220 thrown into the air and cools in flight. Soot turned 176 00:10:18.279 --> 00:10:21.039 up in the clay. which some researchers read as evidence 177 00:10:21.059 --> 00:10:25.799 of continent-scale fires. The crater, as it turned out, had 178 00:10:25.870 --> 00:10:31.090 already been found, but nobody had connected it. In the 179 00:10:31.129 --> 00:10:35.549 late 1970s, a geophysicist named Glenn Penfield was working for 180 00:10:35.570 --> 00:10:39.970 the Mexican national oil company Pemex, flying magnetic surveys over 181 00:10:40.009 --> 00:10:42.120 the northern coast of the Yucatan Peninsula. 182 00:10:42.700 --> 00:10:45.700 The data showed a huge arc buried under the limestone. 183 00:10:46.360 --> 00:10:49.460 When he matched it against older gravity surveys, the arc 184 00:10:49.519 --> 00:10:53.980 closed into a circle roughly 180 kilometers across, centered near 185 00:10:54.000 --> 00:10:58.179 a coastal town called Chicxulub. Penfield and a Pemex colleague 186 00:10:58.250 --> 00:11:01.950 named Antonio Camargo thought it was an impact crater. They 187 00:11:01.990 --> 00:11:05.970 presented it at a geophysics meeting in Los Angeles in 1981, 188 00:11:05.970 --> 00:11:08.220 and a reporter for the Houston Chronicle wrote it up, 189 00:11:08.789 --> 00:11:12.649 and almost nobody in the impact debate noticed. Pemex had 190 00:11:12.690 --> 00:11:16.029 drilled wells into the structure decades earlier, and the cores 191 00:11:16.070 --> 00:11:19.490 were company property. Penfield later said much of that core 192 00:11:19.529 --> 00:11:23.409 material had been lost in a warehouse fire. The connection 193 00:11:23.460 --> 00:11:26.200 got made 10 years later by a graduate student at 194 00:11:26.240 --> 00:11:30.000 the University of Arizona named Alan Hildebrand, who was tracing 195 00:11:30.039 --> 00:11:33.200 the Haitian glass spheres and some thick boundary deposits in 196 00:11:33.259 --> 00:11:35.659 Texas that looked like they'd been laid down by an 197 00:11:35.750 --> 00:11:39.269 enormous wave, and who reasoned that the impact had to 198 00:11:39.309 --> 00:11:42.370 be somewhere around the Gulf of Mexico. He found the 199 00:11:42.389 --> 00:11:45.830 Houston Chronicle story, called Penfield, and the two of them 200 00:11:45.870 --> 00:11:49.509 tracked down some surviving Pemex core samples that contained shocked 201 00:11:49.570 --> 00:11:54.370 quartz and impact

melt. Their paper, identifying Chicxulub as the crater, 202 00:11:54.570 --> 00:11:58.840 came out in 1991, with Penfield and Camargo as co-authors. 203 00:12:00.220 --> 00:12:04.320 The dating has only gotten tighter since. In 2013, a 204 00:12:04.360 --> 00:12:07.720 team led by Paul Rennie at the Berkeley Geochronology Center 205 00:12:08.100 --> 00:12:11.279 used argon dating on the impact glass and on volcanic 206 00:12:11.500 --> 00:12:14.629 ash just above and below the boundary in Lantana, and 207 00:12:14.669 --> 00:12:18.539 found the impact and the extinction fell within about 32,000 208 00:12:18.539 --> 00:12:21.820 years of each other, which in rocks 66 million years 209 00:12:21.980 --> 00:12:25.379 old is as close to simultaneous as the method can resolve. 210 00:12:26.019 --> 00:12:30.509 In 2016, an international drilling expedition parked a jack-up platform 211 00:12:30.610 --> 00:12:33.649 off the Yucatan coast and cored straight down into the 212 00:12:33.710 --> 00:12:36.549 crater's central ring and brought up granite that had been 213 00:12:36.610 --> 00:12:40.029 lifted from something like 10 kilometers deep and shattered in 214 00:12:40.070 --> 00:12:43.870 the first minutes after the hit. The impact is not 215 00:12:43.929 --> 00:12:46.230 the only candidate on the table, and I don't want 216 00:12:46.269 --> 00:12:49.330 to pretend otherwise. On the other side of the world, 217 00:12:49.649 --> 00:12:52.950 in western India, a stack of basalt lava flows called 218 00:12:52.970 --> 00:12:56.379 the Deccan Traps erupted across the same window of time, 219 00:12:56.860 --> 00:12:59.039 and those flows cover an area the size of a 220 00:12:59.120 --> 00:13:02.159 large country and run more than a kilometer. 221 00:13:01.659 --> 00:13:02.460 Thick in places. 222 00:13:03.200 --> 00:13:07.620 A French geophysicist named Vincent Courtelot and an American paleontologist 223 00:13:07.659 --> 00:13:11.159 named Gerda Keller have argued for decades that the volcanic 224 00:13:11.240 --> 00:13:15.399 gases from the Deccan especially sulfur and carbon dioxide, were 225 00:13:15.519 --> 00:13:19.080 already wrecking the climate before the asteroid arrived, and that 226 00:13:19.100 --> 00:13:22.820 the impact finished something the volcanoes had started, or was 227 00:13:22.860 --> 00:13:26.590 a sideshow to it. In 2010, a group of 41 228 00:13:26.590 --> 00:13:30.509 researchers published a review in Science, concluding the impact alone 229 00:13:30.570 --> 00:13:35.970 was sufficient. In 2019, two separate dating teams published side-by-side 230 00:13:36.009 --> 00:13:38.570 in the same journal and came to different readings of 231 00:13:38.610 --> 00:13:41.850 when the biggest Deccan pulses happened relative to the boundary. 232 00:13:42.500 --> 00:13:45.279 one placing much of the lava before the impact and 233 00:13:45.299 --> 00:13:48.179 the other placing most of it after. That's where the 234 00:13:48.279 --> 00:13:51.570 argument currently sits, and the honest summary is that the 235 00:13:51.710 --> 00:13:54.789 asteroid is the established cause of the extinction, and the 236 00:13:54.850 --> 00:13:57.889 Deccan's contribution is still being fought over by people who 237 00:13:57.929 --> 00:13:59.250 have spent their careers on it. 238 00:14:00.509 --> 00:14:01.950 What the Alvarez paper did. 239 00:14:02.350 --> 00:14:06.450 Beyond settling anything about the dinosaurs, was make mass extinction 240 00:14:06.549 --> 00:14:09.419 a respectable thing to study. And the people who took 241 00:14:09.460 --> 00:14:12.549 it up first were a pair of paleontologists in Chicago 242 00:14:12.870 --> 00:14:18.049 who'd been counting fossils by hand for years. Jack Sebkoski 243 00:14:18.110 --> 00:14:21.870 spent most of the 1970s in libraries. He was building 244 00:14:21.909 --> 00:14:24.889 a list of every family of marine animal known from 245 00:14:24.929 --> 00:14:27.840 the fossil record, with the first and last appearance of 246 00:14:27.899 --> 00:14:30.240 each one. And he did it by reading through the 247 00:14:30.299 --> 00:14:33.840 published literature and writing the ranges down. By the early 1980s, 248 00:14:34.029 --> 00:14:38.370 the list ran to several thousand families spanning the whole 249 00:14:38.370 --> 00:14:41.750 600 million year history of animal life. And it let 250 00:14:41.789 --> 00:14:44.950 him do something nobody had done before, which was plot 251 00:14:44.990 --> 00:14:48.169 the number of families alive against time and look at 252 00:14:48.190 --> 00:14:51.669 the shape of the curve. His colleague at the University 253 00:14:51.690 --> 00:14:55.870 of Chicago was David Raup. a paleontologist with a statistician's 254 00:14:55.929 --> 00:14:59.190 temperament who had spent years arguing that his field needed 255 00:14:59.230 --> 00:15:03.269 to test its ideas with numbers instead of narratives. Stay 256 00:15:03.289 --> 00:15:11.129 tuned for more disturbing history. We'll be back after these messages. 257 00:15:11.240 --> 00:15:13.919 In 1982, the two of them published a short paper 258 00:15:13.960 --> 00:15:17.879 in Science called Mass Extinctions in the Marine Fossil Record. 259 00:15:18.440 --> 00:15:22.159 They took Sapkoski's data, calculated the extinction rate for each 260 00:15:22.220 --> 00:15:25.950 slice of geologic time, and asked which intervals stood so 261 00:15:26.009 --> 00:15:29.230 far above the normal background that they couldn't be explained 262 00:15:29.289 --> 00:15:30.470 as ordinary turnover. 263 00:15:31.789 --> 00:15:33.330 Five intervals cleared the bar. 264 00:15:33.929 --> 00:15:38.399 The end of the Ordovician period, about 445 million years ago. 265 00:15:39.000 --> 00:15:41.799 The late Devonian, around 372 million. The end of the Permian, 266 00:15:41.799 --> 00:15:44.340 252 million. The end of the Triassic, 201 million. And 267 00:15:44.360 --> 00:15:53.929 the end of the Cretaceous, 66 million.

Those became the 268 00:15:53.970 --> 00:15:57.230 Big Five, and the phrase stuck so completely that it's 269 00:15:57.330 --> 00:16:00.009 easy to forget it was a statistical finding from one 270 00:16:00.049 --> 00:16:05.350 dataset rather than something handed down. Ropp and Sapkowski kept going, 271 00:16:05.750 --> 00:16:08.240 and two years later, they published something that got them 272 00:16:08.269 --> 00:16:11.519 into trouble. When they looked at the smaller extinction peaks 273 00:16:11.639 --> 00:16:14.220 as well as the big ones over the last 250 274 00:16:14.220 --> 00:16:17.679 million years, the peaks seemed to fall at regular intervals, 275 00:16:18.220 --> 00:16:22.889 roughly every 26 million years. A periodic signal in extinction 276 00:16:23.000 --> 00:16:26.480 suggested a periodic cause, and the only things in nature 277 00:16:26.500 --> 00:16:30.600 that keep time on that scale are astronomical. Within months, 278 00:16:31.019 --> 00:16:34.659 two groups of astronomers independently proposed that the Sun had 279 00:16:34.700 --> 00:16:38.759 an undiscovered companion star on a long, stretched orbit that 280 00:16:38.820 --> 00:16:40.779 swung through the cloud of comets at the edge of 281 00:16:40.820 --> 00:16:44.419 the solar system every 26 million years and knocked a 282 00:16:44.500 --> 00:16:47.700 shower of them inward. One of the groups, which included 283 00:16:47.740 --> 00:16:52.929 the Berkeley physicist Richard Muller, named the star Nemesis. The 284 00:16:52.970 --> 00:16:56.980 search for Nemesis went on for years, using infrared sky surveys, 285 00:16:57.399 --> 00:17:00.580 and nothing was found. Later sky surveys that would have 286 00:17:00.600 --> 00:17:03.740 picked up a companion star of almost any kind also 287 00:17:03.799 --> 00:17:08.380 found nothing. The periodicity itself has been reanalyzed many times since, 288 00:17:08.740 --> 00:17:10.880 and the current state of that argument is that some 289 00:17:10.940 --> 00:17:14.180 statisticians still see a weak signal in the data, and 290 00:17:14.240 --> 00:17:17.470 most don't think it survives careful treatment of the dating errors. 291 00:17:18.119 --> 00:17:19.339 Ropp wrote a book about. 292 00:17:19.160 --> 00:17:21.910 The whole episode and seemed to enjoy how far out 293 00:17:21.970 --> 00:17:25.849 on a limb he'd gone. Sapkowski died in 1999 at 294 00:17:25.869 --> 00:17:30.529 the age of 50, still refining the database. What survived 295 00:17:30.549 --> 00:17:34.250 from their work was the framework. Extinction in the fossil 296 00:17:34.309 --> 00:17:37.329 record is not a steady drip. Most of the time, 297 00:17:37.390 --> 00:17:40.430 it runs at a low background rate, with species lasting 298 00:17:40.470 --> 00:17:43.009 on the order of a few million years each before 299 00:17:43.029 --> 00:17:46.390 they blink out and are replaced. And a handful of times, 300 00:17:46.789 --> 00:17:48.789 the rate has jumped by a factor of 10 or 301 00:17:48.809 --> 00:17:52.250 more across the whole planet, across groups of animals that 302 00:17:52.289 --> 00:17:55.029 had nothing to do with each other, inside windows of 303 00:17:55.089 --> 00:17:58.609 time that are short by geologic standards. Those are the 304 00:17:58.670 --> 00:18:01.930 events Cuvier was reaching for with his revolutions of the globe. 305 00:18:02.490 --> 00:18:05.130 They've now been studied for 40 years with tools he 306 00:18:05.190 --> 00:18:07.930 couldn't have imagined, and each one turns out to be 307 00:18:07.970 --> 00:18:12.720 its own story with its own suspects. The oldest of 308 00:18:12.759 --> 00:18:15.640 the five happened in a world with almost nothing living 309 00:18:15.720 --> 00:18:20.680 on land. 445 million years ago, there were no trees, 310 00:18:21.019 --> 00:18:24.809 no insects, no vertebrates out of the water. The continents 311 00:18:24.849 --> 00:18:27.630 were mostly bare rock with maybe a crust of early 312 00:18:27.690 --> 00:18:31.029 mosses near the wet places. Everything that mattered was in 313 00:18:31.049 --> 00:18:35.269 the ocean, and the ocean was crowded. Trilobites had been 314 00:18:35.289 --> 00:18:40.430 diversifying for 100 million years. The seafloor was covered in brachiopods, 315 00:18:40.789 --> 00:18:45.069 clam-shaped animals on stalks that filtered the water. Floating colonies 316 00:18:45.130 --> 00:18:49.190 of graptolites drifted in the currents, and tiny eel-shaped animals 317 00:18:49.259 --> 00:18:52.680 called conodonts, which left behind almost nothing but their teeth, 318 00:18:53.140 --> 00:18:56.319 swarmed in the open water. Reefs were being built by 319 00:18:56.400 --> 00:19:00.579 sponges and early corals. Most of the land was gathered 320 00:19:00.619 --> 00:19:05.329 into a single southern supercontinent called Gondwana, and Gondwana was 321 00:19:05.390 --> 00:19:09.970 drifting over the South Pole. In the 1960s, French geologists 322 00:19:10.029 --> 00:19:14.420 mapping the Sahara in southern Algeria found the unmistakable signatures 323 00:19:14.460 --> 00:19:19.130 of glaciation in rock of this age. Scratched and polished bedrock, 324 00:19:19.750 --> 00:19:23.450 boulder clays, the remains of ice sheets, in the middle 325 00:19:23.470 --> 00:19:26.269 of what's now the hottest desert on Earth, because in 326 00:19:26.289 --> 00:19:29.259 the Ordovician, that part of Africa sat near the pole. 327 00:19:30.000 --> 00:19:32.519 The ice sheet that grew there was comparable in scale 328 00:19:32.559 --> 00:19:35.259 to the ones that covered Canada and Scandinavia in the 329 00:19:35.299 --> 00:19:40.000 last ice age, and it grew fast. The extinction came 330 00:19:40.099 --> 00:19:43.440 in two pulses. When the ice built up, it pulled 331 00:19:43.519 --> 00:19:45.940 enough water out of the ocean to drop sea level 332 00:19:46.000 --> 00:19:49.470 by something on the order of 100 meters. The wide, 333 00:19:49.569 --> 00:19:51.640

shallow surface seas that had covered the edges of the 334 00:19:51.700 --> 00:19:55.420 continents drained, and those shallow seas were where most marine 335 00:19:55.480 --> 00:19:59.390 life lived. The animals that were adapted to warm, stable, 336 00:19:59.529 --> 00:20:03.109 shallow water lost their habitat, and the water that remained 337 00:20:03.430 --> 00:20:07.450 got colder. That was the first pulse. The second came 338 00:20:07.490 --> 00:20:10.529 maybe a million years later, when the ice melted again, 339 00:20:10.970 --> 00:20:14.230 sea level rose back, and oxygen-poor water from the deep 340 00:20:14.410 --> 00:20:17.869 ocean flooded up onto the shelves. Whatever had survived the 341 00:20:17.910 --> 00:20:20.730 cold and the draining got hit again by the return 342 00:20:20.750 --> 00:20:24.839 of the water. Between the two pulses, the estimates run 343 00:20:24.880 --> 00:20:30.200 around 85% of marine species gone. Trilobites lost a large 344 00:20:30.259 --> 00:20:33.599 share of their families and never fully recovered their earlier range. 345 00:20:34.200 --> 00:20:38.450 The brachiopods were devastated. many of the conodont lineages ended. 346 00:20:38.970 --> 00:20:44.000 Graptolites nearly disappeared. As mass extinctions go, it ranks second 347 00:20:44.079 --> 00:20:47.200 in the raw count of what died, behind only the Permian, 348 00:20:47.680 --> 00:20:50.099 and it's the one that gets the least attention because 349 00:20:50.240 --> 00:20:53.549 nothing in it is familiar. There's no charismatic victim. 350 00:20:53.970 --> 00:20:54.990 There's no asteroid. 351 00:20:56.190 --> 00:20:59.869 The glaciation is the accepted immediate cause, and the harder 352 00:20:59.910 --> 00:21:03.930 question is what caused the glaciation. Carbon dioxide in the 353 00:21:04.009 --> 00:21:07.660 Ordovician atmosphere was many times higher than today, which should 354 00:21:07.680 --> 00:21:10.279 have kept the planet warm, and the ice age appears 355 00:21:10.299 --> 00:21:13.099 to have been short, perhaps a million years or so. 356 00:21:13.819 --> 00:21:16.740 Several ideas are on the table. A group led by 357 00:21:16.819 --> 00:21:19.890 Tim Linton at the University of Exeter proposed in 2012 358 00:21:19.890 --> 00:21:23.930 that the first land plants, the mosses and liverworts that 359 00:21:23.970 --> 00:21:27.589 were just then spreading across the rocks, accelerated the chemical 360 00:21:27.650 --> 00:21:30.970 weathering of the continents enough to pull carbon dioxide out 361 00:21:31.019 --> 00:21:33.599 of the air and and tipped the climate into glaciation. 362 00:21:34.319 --> 00:21:38.480 In 2004, a physicist named Adrian Mellott suggested a gamma 363 00:21:38.500 --> 00:21:40.960 ray burst from an exploding star in our part of 364 00:21:41.000 --> 00:21:44.440 the galaxy could have stripped the ozone layer and triggered cooling. 365 00:21:45.019 --> 00:21:47.680 And that idea gets repeated a lot in popular accounts, 366 00:21:48.140 --> 00:21:50.480 but there is no physical evidence for it in The Rock, 367 00:21:50.859 --> 00:21:53.640 and Mellott himself framed it as a hypothesis that fit 368 00:21:53.680 --> 00:21:57.660 the pattern, rather than something he could demonstrate. More recently, 369 00:21:58.009 --> 00:22:01.190 Geochemists have found spikes of mercury in boundary rocks of 370 00:22:01.230 --> 00:22:03.960 this age, and since mercury in the rock record is 371 00:22:04.119 --> 00:22:08.119 often a fingerprint of large volcanic eruptions, some researchers now 372 00:22:08.180 --> 00:22:11.859 think volcanism was involved here too, though no large volcanic 373 00:22:11.920 --> 00:22:16.119 province of exactly the right age has been identified. The 374 00:22:16.180 --> 00:22:20.849 record ends there. The ice retreated, sea level stabilized, and 375 00:22:20.910 --> 00:22:24.329 over the next several million years, the survivors refilled the 376 00:22:24.470 --> 00:22:27.890 oceans with new species. the reefs came back and then 377 00:22:27.930 --> 00:22:31.650 grew far larger than anything that had existed before, which 378 00:22:31.710 --> 00:22:36.730 is what set up the next one. By 375 million 379 00:22:36.769 --> 00:22:40.380 years ago, the Devonian seas held the largest reef systems 380 00:22:40.440 --> 00:22:44.279 the planet has ever produced. They weren't built mainly by corals. 381 00:22:44.920 --> 00:22:49.539 The architects were sponges called stromatoporoids, which laid down layered 382 00:22:49.579 --> 00:22:53.960 skeletons of calcium carbonate, along with tabulate corals and together, 383 00:22:54.329 --> 00:22:57.609 they built structures that ran for hundreds of kilometers along 384 00:22:57.650 --> 00:23:01.839 the continental margins. The Canning Basin Reef in Western Australia 385 00:23:02.130 --> 00:23:05.190 is one of the surviving examples, now exposed as a 386 00:23:05.259 --> 00:23:08.900 line of limestone ranges in the desert, and it's about 387 00:23:08.900 --> 00:23:13.759 350 kilometers long. The land had changed too. This was 388 00:23:13.799 --> 00:23:18.180 the period when plants became trees. A genus called Archaeopterus, 389 00:23:18.480 --> 00:23:20.759 which had a trunk like a conifer and leaves like 390 00:23:20.779 --> 00:23:24.240 a fern, formed the first real forests. and it had 391 00:23:24.299 --> 00:23:26.079 something earlier plants hadn't had. 392 00:23:26.500 --> 00:23:28.000 Which was a deep root system. 393 00:23:28.779 --> 00:23:32.789 Fish had diversified into everything from sharks to the armored placoderms, 394 00:23:33.220 --> 00:23:36.269 some of which, like Dunkleosteus, grew to the size of 395 00:23:36.329 --> 00:23:39.609 a modern great white and bit with bony blades instead 396 00:23:39.630 --> 00:23:42.329 of teeth. And near the very end of the period, 397 00:23:42.670 --> 00:23:46.490 a few lobe-finned fish were hauling themselves onto mudflats with 398 00:23:46.569 -->

00:23:50.930 limbs that had fingers. The Devonian extinction is the hardest 399 00:23:50.970 --> 00:23:54.410 of the five to describe cleanly. because it wasn't one event. 400 00:23:55.049 --> 00:23:58.970 There were two main crises separated by about 13 million years. 401 00:23:59.650 --> 00:24:02.849 The first, at the boundary between the Frasnian and Feminian 402 00:24:02.910 --> 00:24:06.920 stages around 372 million years ago, is recorded in a 403 00:24:06.930 --> 00:24:09.640 pair of black shale layers that were first described in 404 00:24:09.660 --> 00:24:13.440 the Kelwasser Valley in Germany's Harz Mountains, and they're called 405 00:24:13.460 --> 00:24:17.359 the Kelwasser events after that valley. Black shale forms when 406 00:24:17.380 --> 00:24:20.880 the seafloor has no oxygen and dead organic matter piles 407 00:24:20.980 --> 00:24:24.009 up without decaying. The same black layers show up at 408 00:24:24.049 --> 00:24:28.470 the same horizon in North America, Morocco, China, and Australia. 409 00:24:29.049 --> 00:24:31.869 The ocean floor over much of the world went anoxic 410 00:24:31.950 --> 00:24:35.930 twice in fairly quick succession, and the reefs died. The 411 00:24:35.950 --> 00:24:40.829 stromatoporoids collapsed and never recovered their dominance. Tabulate corals were 412 00:24:40.869 --> 00:24:45.029 cut down. Reef ecosystems on the Devonian scale wouldn't exist 413 00:24:45.089 --> 00:24:49.640 again for more than 100 million years. The second crisis, 414 00:24:49.980 --> 00:24:53.940 called the Hangenberg event after a locality in Germany's Rhenish mountains, 415 00:24:54.319 --> 00:24:57.970 came at the very end of the Devonian, about 359 416 00:24:57.970 --> 00:25:01.990 million years ago, and it hit the vertebrates. The placoderms, 417 00:25:02.150 --> 00:25:05.430 which had been the dominant large fish for 70 million years, 418 00:25:05.869 --> 00:25:11.089 disappeared completely. Most of the lobe-finned fish lineages ended. Aminoids 419 00:25:11.130 --> 00:25:14.779 were cut down to a handful of survivors. A paleontologist 420 00:25:14.819 --> 00:25:17.960 named Lauren Salen, working with Michael Coates at the University 421 00:25:17.980 --> 00:25:21.799 of Chicago, showed in 2010 that the whole vertebrate world 422 00:25:21.859 --> 00:25:25.420 was reorganized across this boundary, with the big armored fish 423 00:25:25.480 --> 00:25:28.759 gone and the ray-finned fish and sharks taking over the 424 00:25:28.819 --> 00:25:32.200 open water. The early tetrapods went through a bottleneck too, 425 00:25:32.559 --> 00:25:34.500 which is part of why there's a long stretch of 426 00:25:34.599 --> 00:25:38.279 rock after the Devonian with almost no land vertebrate fossils 427 00:25:38.359 --> 00:25:44.569 in it, a stretch paleontologists call Romer's Gap. Some paleontologists, 428 00:25:44.930 --> 00:25:48.569 notably George McGee and Richard Bambach, have argued that the 429 00:25:48.609 --> 00:25:51.650 Devonian doesn't really belong on the Big Five list at all, 430 00:25:52.049 --> 00:25:54.690 because on a close look, the problem was less an 431 00:25:54.769 --> 00:25:58.759 unusually high death rate than an unusually low birth rate. 432 00:25:59.420 --> 00:26:03.400 New species stopped appearing for millions of years. Whether that 433 00:26:03.460 --> 00:26:05.980 counts as a mass extinction or a different kind of 434 00:26:06.039 --> 00:26:10.960 crisis is partly a matter of definition. The leading explanation 435 00:26:11.019 --> 00:26:14.920 for the anoxia comes back to the trees. In 1998, 436 00:26:14.920 --> 00:26:19.440 two geologists, Thomas Algeo and Stephen Sheckler, proposed that the 437 00:26:19.500 --> 00:26:22.829 spread of deep-rooted forests broke up the continental rock into 438 00:26:22.900 --> 00:26:25.690 soil for the first time on a large scale, and 439 00:26:25.710 --> 00:26:28.410 that the flood of nutrients washing off that new soil 440 00:26:28.470 --> 00:26:32.829 into the oceans fertilized enormous blooms of algae, which died 441 00:26:32.930 --> 00:26:35.670 and sank and used up the oxygen as they rotted. 442 00:26:36.390 --> 00:26:39.369 The same thing happens today, on a small scale. when 443 00:26:39.430 --> 00:26:42.589 fertilizer runoff creates dead zones in the Gulf of Mexico. 444 00:26:43.210 --> 00:26:47.450 The Devonian version would have been planetary. The forests also 445 00:26:47.509 --> 00:26:51.380 pulled down atmospheric carbon dioxide, which could have triggered cooling 446 00:26:51.460 --> 00:26:54.200 and glaciation near the end of the period. And there 447 00:26:54.299 --> 00:26:56.859 is evidence of ice in the late Devonian rocks of 448 00:26:56.920 --> 00:27:02.480 South America. Impacts have been proposed and mostly dismissed. There's 449 00:27:02.529 --> 00:27:05.569 a large crater in Sweden called the Siljan Ring that's 450 00:27:05.609 --> 00:27:08.369 a few million years too old. There's a layer of 451 00:27:08.450 --> 00:27:11.890 shattered rock in Nevada called the Alamo Breccia that records 452 00:27:11.930 --> 00:27:14.730 an impact in the right general period, but not at 453 00:27:14.750 --> 00:27:18.880 the extinction horizon. A large volcanic province in Siberia called 454 00:27:18.920 --> 00:27:21.640 the Vilui Traps was erupting around the time of the 455 00:27:21.680 --> 00:27:25.200 Kelwasser events, and mercury spikes have been found at the boundary, 456 00:27:25.519 --> 00:27:28.440 so volcanism is in the mix here too. The current 457 00:27:28.480 --> 00:27:32.140 picture most Devonian specialists work with is a combination, with 458 00:27:32.180 --> 00:27:36.009 the forests and the volcanoes both pushing the oceans toward anoxia, 459 00:27:36.349 --> 00:27:41.180 and the exact weighting still unresolved. Of all of them, 460 00:27:41.640 --> 00:27:43.839 the end of the Permian is the one that nearly 461 00:27:43.900 --> 00:27:48.589 took everything. Paleontologists have taken to

calling it the Great Dying, 462 00:27:49.109 --> 00:27:53.150 and for once, the nickname undersells it. The date has 463 00:27:53.190 --> 00:27:56.430 been nailed down more tightly than any other extinction except 464 00:27:56.470 --> 00:27:59.759 the Cretaceous. The reference section for the boundary is a 465 00:27:59.799 --> 00:28:02.779 set of quarry exposures near the town of Meishan in 466 00:28:02.819 --> 00:28:06.049 Zhejiang Province, China. where the layers are numbered and a 467 00:28:06.089 --> 00:28:08.329 bronze marker has been set into the rock at the 468 00:28:08.410 --> 00:28:12.609 exact bed the international geological community agreed to use as 469 00:28:12.650 --> 00:28:16.900 the boundary. In 2014, a team led by Seth Burgess 470 00:28:16.980 --> 00:28:20.920 and Samuel Bowring at the Massachusetts Institute of Technology, working 471 00:28:20.960 --> 00:28:25.539 with the Chinese paleontologist Xu Zhongshan, dated volcanic ash beds 472 00:28:25.680 --> 00:28:29.339 above and below the extinction at Machon using uranium and 473 00:28:29.400 --> 00:28:34.779 lead isotopes and found the main die-off took place about 251.9 474 00:28:34.779 --> 00:28:39.309 million years ago and lasted something like 60,000 years. The 475 00:28:39.369 --> 00:28:43.170 uncertainty on that duration is large, and 60,000 years is 476 00:28:43.210 --> 00:28:46.130 an eternity on a human scale. But against the slow 477 00:28:46.170 --> 00:28:49.779 turnover of the fossil record, it's an instant. most of 478 00:28:49.819 --> 00:28:52.619 the species on Earth were gone inside a window that's 479 00:28:52.680 --> 00:28:56.839 a rounding error on the geologic clock. How many is 480 00:28:56.900 --> 00:29:01.569 a question with a history. In 1979, David Ropp published 481 00:29:01.630 --> 00:29:06.369 a calculation suggesting 96% of marine species had died, and 482 00:29:06.390 --> 00:29:09.650 that number went into textbooks and has stayed there. But 483 00:29:09.730 --> 00:29:13.390 Ropp's method was an extrapolation from family-level data, and it 484 00:29:13.430 --> 00:29:17.049 was meant as an upper bound. A reanalysis by Stephen 485 00:29:17.089 --> 00:29:21.150 Stanley in 2016 that corrected for the ways incomplete sampling 486 00:29:21.230 --> 00:29:25.859 inflates extinction counts, came out closer to 81% of marine species. 487 00:29:26.500 --> 00:29:29.500 That's still the worst loss in the history of animal life. 488 00:29:30.279 --> 00:29:34.829 On land, roughly 70% of the vertebrate species disappeared, and 489 00:29:34.869 --> 00:29:36.990 this is the only one of the five that reached 490 00:29:37.029 --> 00:29:40.589 the insects, which otherwise ride out these events with little trouble. 491 00:29:41.769 --> 00:29:44.809 What went extinct reads like an inventory. 492 00:29:44.250 --> 00:29:45.190 Of the Paleozoic. 493 00:29:45.809 --> 00:29:49.190 The trilobites, which had already been reduced to a few lineages, 494 00:29:49.630 --> 00:29:54.009 ended here for good after nearly 300 million years. The 495 00:29:54.049 --> 00:29:56.950 tabulate and rugose corals that had built reefs since the 496 00:29:57.039 --> 00:30:03.690 Ordovician vanished entirely. The eurypterids, the sea scorpions, ended. The blastoids, 497 00:30:04.009 --> 00:30:07.230 a group of stalked animals related to sea lilies, ended. 498 00:30:07.869 --> 00:30:10.529 Most of the brachiopods were wiped out, and the group 499 00:30:10.650 --> 00:30:13.309 never regained its dominance, which is part of why the 500 00:30:13.349 --> 00:30:17.240 seafloor today is covered in clams instead of brachiopods. The 501 00:30:17.359 --> 00:30:20.640 aminoids were cut down to a few genera. On land, 502 00:30:20.940 --> 00:30:24.359 the synapsids, the group that includes the ancestors of mammals 503 00:30:24.640 --> 00:30:27.259 and that had dominated the late Permian with animals like 504 00:30:27.279 --> 00:30:32.420 the saber-toothed gorgonopsians, were devastated. Stay tuned for more disturbing history. 505 00:30:32.900 --> 00:30:40.109 We'll be back after these messages. The cause sits in Siberia, 506 00:30:40.549 --> 00:30:44.430 in plain sight. The Siberian traps are a stack of 507 00:30:44.490 --> 00:30:48.190 basalt lava flows that today cover about 2 million square 508 00:30:48.230 --> 00:30:52.359 kilometers of the Siberian platform, an area larger than Alaska. 509 00:30:52.839 --> 00:30:56.460 And the original extent was likely several times that before erosion. 510 00:30:57.180 --> 00:31:00.240 The total volume of magma is estimated at somewhere between 511 00:31:00.240 --> 00:31:04.480 1 and 4 million cubic kilometers. That's not a volcano 512 00:31:04.539 --> 00:31:08.359 in any ordinary sense. It's a province, fed from deep 513 00:31:08.400 --> 00:31:10.920 in the mantle, that erupted on and off for something 514 00:31:10.960 --> 00:31:14.759 like 2 million years through fissures tens of kilometers long. 515 00:31:15.400 --> 00:31:18.779 Dating by the same Burgess and Bowring group in 2015 516 00:31:18.779 --> 00:31:20.549 showed the bulk of the lava came up in a 517 00:31:20.619 --> 00:31:24.109 pulse that bracketed the extinction, with roughly two-thirds of the 518 00:31:24.150 --> 00:31:27.609 volume erupting before the main die-off and the rest after. 519 00:31:28.890 --> 00:31:32.710 Basalt by itself doesn't kill a planet. The problem was 520 00:31:32.769 --> 00:31:35.829 what the magma came up through. The Tunguska Basin in 521 00:31:35.869 --> 00:31:41.099 central Siberia held enormous deposits of coal, oil shale, limestone, 522 00:31:41.359 --> 00:31:45.279 and evaporite salts. And the Siberian magma didn't just erupt 523 00:31:45.480 --> 00:31:49.160 onto that basin. It intruded into it as horizontal sheets 524 00:31:49.220 --> 00:31:53.519 called sills, baking the sediments from the inside. A Norwegian 525 00:31:53.539 --> 00:31:57.059 geologist named Henrik Svensson and his colleagues showed in

2009 526 00:31:57.059 --> 00:32:01.759 that the basin is riddled with pipe-like structures, hundreds of them, 527 00:32:02.180 --> 00:32:05.640 where superheated gas blasted up through the sediments to the surface. 528 00:32:06.329 --> 00:32:10.930 Cooking coal and limestone and salt releases carbon dioxide, methane, 529 00:32:11.250 --> 00:32:15.450 sulfur dioxide, and halogen gases like chlorine and bromine. 530 00:32:15.309 --> 00:32:16.329 That destroy ozone. 531 00:32:16.990 --> 00:32:20.700 Burgess published a follow-up in 2017 showing the sills were 532 00:32:20.759 --> 00:32:24.359 emplaced right at the moment the extinction began and argued 533 00:32:24.380 --> 00:32:27.819 that the sill intrusion, not the surface lava, was the trigger. 534 00:32:28.359 --> 00:32:30.839 The lava had been erupting for hundreds of thousands of 535 00:32:30.920 --> 00:32:34.339 years without ending the world. When the magma started cooking 536 00:32:34.380 --> 00:32:38.839 the coal, the world ended. The chain of consequences has 537 00:32:38.880 --> 00:32:42.420 been reconstructed from the chemistry of the rocks. The carbon 538 00:32:42.559 --> 00:32:46.789 isotopes shift sharply at the boundaries. recording an enormous injection 539 00:32:46.829 --> 00:32:50.970 of carbon into the atmosphere and oceans. The oceans acidified, 540 00:32:51.230 --> 00:32:54.450 which is recorded in the boron isotopes of surviving shells 541 00:32:54.750 --> 00:32:57.630 and in the disappearance of animals that built thick calcium 542 00:32:57.690 --> 00:33:01.519 carbonate skeletons. The planet warmed by something on the order 543 00:33:01.579 --> 00:33:05.519 of 10 degrees Celsius. In 2012, a team led by 544 00:33:05.599 --> 00:33:09.759 Yadong Sun at the China University of Geosciences used oxygen 545 00:33:09.900 --> 00:33:14.079 isotopes in conodont teeth to reconstruct sea surface temperatures across 546 00:33:14.119 --> 00:33:18.079 the early Triassic and found tropical surface waters reaching about 547 00:33:18.079 --> 00:33:21.579 40 degrees Celsius, which is above the lethal limit for 548 00:33:21.640 --> 00:33:25.799 most marine animals and most plants. The equator, in other words, 549 00:33:26.099 --> 00:33:29.359 became a dead zone. And for several million years, the 550 00:33:29.519 --> 00:33:33.109 only abundant fossils in the tropics are of small, simple, 551 00:33:33.400 --> 00:33:39.200 heat-tolerant animals. The oceans also lost their oxygen. Black shales 552 00:33:39.240 --> 00:33:42.579 and the chemical signatures of anoxia spread across deep and 553 00:33:42.660 --> 00:33:46.319 shallow water alike, and in places, the chemistry points to 554 00:33:46.380 --> 00:33:50.549 something worse than anoxia. When seawater loses all its oxygen, 555 00:33:50.920 --> 00:33:54.650 bacteria that breathe sulfate instead take over, and their waste 556 00:33:54.690 --> 00:33:59.950 product is hydrogen sulfide. Biomarkers from green sulfur bacteria, which 557 00:34:00.009

--> 00:34:03.369 need both light and sulfide to live, and therefore only 558 00:34:03.410 --> 00:34:06.710 grow where poisoned water has reached the sunlit surface, have 559 00:34:06.730 --> 00:34:10.099 been found in boundary rocks in Western Australia and elsewhere. 560 00:34:10.739 --> 00:34:14.559 The paleontologist Peter Ward built a book around this, arguing 561 00:34:14.599 --> 00:34:17.360 that hydrogen sulfide welling up out of the ocean and 562 00:34:17.420 --> 00:34:21.139 poisoning the air was a major killer on land. That 563 00:34:21.239 --> 00:34:24.639 is a minority position, and most researchers see the sulfide 564 00:34:24.719 --> 00:34:27.420 as a regional problem, and the heat and the acid 565 00:34:27.480 --> 00:34:30.659 and the collapse of oxygen as the main engines. But 566 00:34:30.679 --> 00:34:33.559 the chemistry of a sulfitic ocean reaching the surface is 567 00:34:33.679 --> 00:34:37.539 real and is in the rock. On land, the best 568 00:34:37.619 --> 00:34:40.440 record is the Karoo Basin of South Africa, where a 569 00:34:40.460 --> 00:34:44.329 continuous sequence of river and floodplain sediments crosses the boundary 570 00:34:44.610 --> 00:34:47.989 and is packed with vertebrate fossils. Roger Smith of the 571 00:34:48.030 --> 00:34:51.349 South African Museum and Peter Ward published a study of 572 00:34:51.389 --> 00:34:55.329 that sequence in 2001, walking the boundary interval bed by 573 00:34:55.389 --> 00:34:59.550 bed and recording what disappeared where. The diverse community of 574 00:34:59.630 --> 00:35:02.730 late Permian synapsids thins out across a few tens of 575 00:35:02.789 --> 00:35:05.539 meters of rock. and the rivers change character at the 576 00:35:05.579 --> 00:35:10.079 same horizon, from meandering channels with vegetated banks to braided 577 00:35:10.159 --> 00:35:13.599 streams running over bare ground, which Smith and Ward read 578 00:35:13.719 --> 00:35:17.010 as the collapse of the plant cover. What comes out 579 00:35:17.050 --> 00:35:21.150 the other side is a single animal. A pig-sized herbivorous 580 00:35:21.190 --> 00:35:25.329 synapsid called Lystrosaurus, with two tusks and a beak and 581 00:35:25.349 --> 00:35:28.329 a build that suggests it burrowed, shows up in early 582 00:35:28.369 --> 00:35:32.070 Triassic rocks on every southern continent, and in some beds, 583 00:35:32.389 --> 00:35:35.730 makes up something like 9 out of 10 vertebrate fossils recovered. 584 00:35:36.329 --> 00:35:38.630 There is no other time in the history of land 585 00:35:38.670 --> 00:35:43.139 vertebrates when one genus so completely owned the planet. Nobody 586 00:35:43.179 --> 00:35:46.239 knows exactly why it made it through. Burrowing is a 587 00:35:46.280 --> 00:35:50.460 reasonable guess. A tolerance for low oxygen is another. It 588 00:35:50.519 --> 00:35:54.119 may simply have been in the right place. The recovery 589 00:35:54.159 --> 00:35:57.179 took longer than any other. There's a gap

of about 590 00:35:57.179 --> 00:36:00.599 10 million years in which no coal formed anywhere on 591 00:36:00.699 --> 00:36:03.800 Earth because the swamp forests that make coal were gone. 592 00:36:04.420 --> 00:36:07.960 There's a reef gap of similar length. The conodonts, which 593 00:36:08.019 --> 00:36:11.719 recovered quickly, show by their diversity that the marine ecosystem 594 00:36:11.820 --> 00:36:15.469 didn't rebuild to its former complexity until the Middle Triassic. 595 00:36:15.929 --> 00:36:19.630 And some paleontologists, Michael Benton among them, have argued that 596 00:36:19.690 --> 00:36:23.769 repeated aftershocks of volcanism and heat kept knocking the recovery 597 00:36:23.829 --> 00:36:26.710 back down in the first few million years. When the 598 00:36:26.789 --> 00:36:30.969 ecosystem did come back, it came back different. The Paleozoic 599 00:36:31.030 --> 00:36:34.500 seafloor had been a place of stalked, sessile filter feeders. 600 00:36:35.119 --> 00:36:40.360 The Mesozoic seafloor was a place of burrowing clams, active snails, crabs, 601 00:36:40.659 --> 00:36:44.559 and fast predators. The modern ocean and its basic cast 602 00:36:44.599 --> 00:36:49.489 of characters starts here. The animals that filled the land 603 00:36:49.530 --> 00:36:53.489 in the Triassic weren't the dinosaurs. For most of that period, 604 00:36:53.840 --> 00:36:57.199 The dominant large reptiles were a group of archosaurs on 605 00:36:57.239 --> 00:37:00.659 the crocodile side of the family tree. The phytosaurs looked 606 00:37:00.699 --> 00:37:05.099 like long-snouted crocodiles and lived the same way. The atosaurs 607 00:37:05.139 --> 00:37:08.400 were armored herbivores built like a cross between a crocodile 608 00:37:08.559 --> 00:37:13.219 and an armadillo. The Rawasukians were big, upright predators, some 609 00:37:13.260 --> 00:37:15.949 of them more than six meters long, that walked with 610 00:37:15.969 --> 00:37:18.190 their legs under their bodies and would have been the 611 00:37:18.250 --> 00:37:21.909 top of the food chain wherever they lived. Dinosaurs existed 612 00:37:21.989 --> 00:37:25.269 alongside them for 20 or 30 million years and were 613 00:37:25.329 --> 00:37:30.449 mostly small and not especially common. Then the supercontinent began 614 00:37:30.489 --> 00:37:34.510 to break. Pangea had held all the Earth's land in 615 00:37:34.719 --> 00:37:38.420 one block through the Permian and Triassic, and around 200 616 00:37:38.420 --> 00:37:41.480 million years ago, it started to rift along a line 617 00:37:41.500 --> 00:37:44.820 that would become the Atlantic Ocean. The rifting was accompanied 618 00:37:44.860 --> 00:37:48.440 by a flood basalt event on a scale comparable to Siberia. 619 00:37:48.960 --> 00:37:52.190 The lavas are now scattered across four continents because the 620 00:37:52.329 --> 00:37:55.349 ocean that opened between them has been widening ever since. 621 00:37:55.989 --> 00:37:59.409 They form the

Palisades Cliffs across the Hudson River from Manhattan. 622 00:37:59.909 --> 00:38:02.349 They're in the Bay of Fundy in Nova Scotia, in 623 00:38:02.389 --> 00:38:05.889 the High Atlas of Morocco, in the Amazon Basin of Brazil, 624 00:38:06.230 --> 00:38:09.710 and in Spain and Portugal. Geologists call the whole thing 625 00:38:09.769 --> 00:38:13.949 the Central Atlantic Magmatic Province. and the original area covered 626 00:38:13.989 --> 00:38:17.409 by its lavas is estimated at something like 10 million 627 00:38:17.469 --> 00:38:22.719 square kilometers. In 2013, a team led by Terence Blackburn, 628 00:38:22.969 --> 00:38:26.860 working again with Sam Bowring's dating lab at MIT, published 629 00:38:26.980 --> 00:38:30.780 uranium lead ages on zircon crystals from the lavas in Morocco, 630 00:38:31.179 --> 00:38:35.260 Nova Scotia, and the Eastern United States. The eruptions came 631 00:38:35.300 --> 00:38:39.159 in at least four pulses over about 600,000 years, and 632 00:38:39.199 --> 00:38:42.079 the oldest lavas in Morocco dated to within a few 633 00:38:42.119 --> 00:38:45.199 thousand years of the extinction horizon in the sediments just 634 00:38:45.239 --> 00:38:49.039 beneath them. The volcanism and the die-off are, within the 635 00:38:49.079 --> 00:38:53.510 resolution of the method, the same event. The mechanism looks 636 00:38:53.550 --> 00:38:57.389 like a smaller version of the Permian. In 1999, a 637 00:38:57.409 --> 00:39:01.230 paleobotanist named Jennifer McElwain published a study in Science of 638 00:39:01.309 --> 00:39:05.289 fossil leaves from Greenland that cross the boundary. Plants regulate 639 00:39:05.349 --> 00:39:07.760 how many pores they open on their leaves according to 640 00:39:07.820 --> 00:39:10.559 how much carbon dioxide is in the air. so the 641 00:39:10.619 --> 00:39:13.139 density of those pores on a fossil leaf is a 642 00:39:13.239 --> 00:39:17.800 record of the ancient atmosphere. McIlwain's leaves showed carbon dioxide 643 00:39:17.909 --> 00:39:21.690 roughly quadrupling across the boundary, and the plant communities above 644 00:39:21.730 --> 00:39:25.550 it were dominated by species with small, dissected leaves that 645 00:39:25.570 --> 00:39:29.309 could shed heat. Mercury Spikes at the Boundary, published by 646 00:39:29.429 --> 00:39:33.699 Allison Thibodeau, in 2016, tie the carbon injection to the volcanism. 647 00:39:34.380 --> 00:39:38.019 Ocean acidification shows up in the collapse of calcifying organisms 648 00:39:38.340 --> 00:39:42.000 and in a gap in carbonate deposition. In the sea, 649 00:39:42.469 --> 00:39:46.110 The conodonts, which had survived every crisis since the Cambrian, 650 00:39:46.369 --> 00:39:49.130 and whose teeth are so useful for dating rock that 651 00:39:49.170 --> 00:39:53.449 geologists have built whole timescales on them, went extinct completely. 652 00:39:54.110 --> 00:39:56.849 The aminoids were cut down to a handful of lineages 653 00:39:57.010 -->

00:39:57.510 once again. 654 00:39:58.219 --> 00:39:59.639 Reef building shut down. 655 00:40:00.480 --> 00:40:04.579 On land, the crocodile-line archosaurs that had dominated the Triassic 656 00:40:04.619 --> 00:40:09.239 were nearly wiped out, with the phytosaurs, atosaurs, and rawasuchians 657 00:40:09.460 --> 00:40:13.679 all gone. and only the small ancestors of true crocodiles surviving. 658 00:40:14.900 --> 00:40:17.929 The dinosaurs came through. The best record of what that 659 00:40:18.010 --> 00:40:21.230 looked like comes from the rift basins of eastern North America, 660 00:40:21.570 --> 00:40:26.010 where Paul Olson of Columbia University's Lamont-Doherty Earth Observatory has 661 00:40:26.050 --> 00:40:29.309 spent his career reading the lake sediments and the footprints. 662 00:40:30.070 --> 00:40:33.159 Below the boundary, the trackways are a mixed community with 663 00:40:33.199 --> 00:40:37.780 a lot of crocodile-line animals and modest dinosaur prints. Right 664 00:40:37.840 --> 00:40:40.760 above it, within tens of thousands of years of the lava, 665 00:40:41.199 --> 00:40:46.760 the footprints of large, three-toed predatory dinosaurs appear and get big, fast, 666 00:40:47.139 --> 00:40:51.010 with no transitional interval. Olsen's reading is that the dinosaurs 667 00:40:51.070 --> 00:40:54.670 didn't win a long contest. Their competitors died, and they 668 00:40:54.730 --> 00:40:57.769 filled the space. And within a few million years, the 669 00:40:57.809 --> 00:41:01.110 body size of predatory dinosaurs had jumped to levels that 670 00:41:01.139 --> 00:41:05.139 would hold for the rest of the Mesozoic. The N-Triassic 671 00:41:05.300 --> 00:41:09.260 estimate runs around 75% of species, which puts it in 672 00:41:09.300 --> 00:41:12.000 the same range as the Cretaceous event that would end 673 00:41:12.039 --> 00:41:16.619 the reign it began. The Cretaceous impact happened at a 674 00:41:16.659 --> 00:41:20.090 particular spot on the planet, and the target rock mattered. 675 00:41:20.769 --> 00:41:23.650 The Yucatan platform where the asteroid hit was a thick 676 00:41:23.730 --> 00:41:28.789 stack of limestone and anhydrite, which is calcium sulfate. Vaporizing 677 00:41:28.849 --> 00:41:34.840 limestone releases carbon dioxide. Vaporizing anhydrite releases sulfur. And sulfur 678 00:41:34.880 --> 00:41:38.300 in the stratosphere forms fine droplets of sulfuric acid that 679 00:41:38.340 --> 00:41:41.880 reflect sunlight. The mechanisms of the kill have been argued 680 00:41:41.940 --> 00:41:45.349 over since 1980, and the consensus has shifted away from 681 00:41:45.389 --> 00:41:48.510 the original picture of a dust cloud and toward the sulfur. 682 00:41:49.130 --> 00:41:52.130 A modeling study led by Julia Bruegger at the Potsdam 683 00:41:52.190 --> 00:41:56.860 Institute in 2017 estimated that the sulfate aerosols alone could 684 00:41:56.900 --> 00:42:00.320 have dropped global average surface temperatures by more than 20 685 00:42:00.320 --> 00:42:04.420 degrees Celsius within a few years. with sub-freezing conditions lasting 686 00:42:04.539 --> 00:42:09.179 several years and the ocean staying cold for decades. The 687 00:42:09.219 --> 00:42:12.699 immediate effects near the crater have been reconstructed from the deposits. 688 00:42:13.360 --> 00:42:16.960 The impact dug a transient hole about 100 kilometers wide 689 00:42:17.039 --> 00:42:19.800 and 30 deep in a matter of seconds, and the 690 00:42:19.860 --> 00:42:23.309 rock rebounded into a central peak that collapsed outward into 691 00:42:23.349 --> 00:42:27.719 the ring the 2016 drilling expedition sampled. Material thrown out 692 00:42:27.760 --> 00:42:30.420 at the highest speeds went into orbit and came back 693 00:42:30.460 --> 00:42:33.699 down around the world as the glass spherules and the iridium. 694 00:42:34.280 --> 00:42:37.159 Ejecta falling through the atmosphere would have heated the sky 695 00:42:37.260 --> 00:42:39.780 over much of the planet to the point where exposed 696 00:42:39.940 --> 00:42:43.760 animals cooked and vegetation ignited, which is one reading of 697 00:42:43.820 --> 00:42:46.769 the soot layer, though other researchers attribute much of the 698 00:42:46.829 --> 00:42:49.710 soot to burning oil and organic matter from the target 699 00:42:49.750 --> 00:42:53.730 rock itself. Seismic shaking and tsunamis on a scale with 700 00:42:53.849 --> 00:42:57.869 no modern comparison hit the Gulf Coast. The boundary deposits 701 00:42:57.909 --> 00:43:01.070 in Texas and Mexico are tens of meters of churned 702 00:43:01.110 --> 00:43:06.130 sediment in places. Then it got dark and cold. The 703 00:43:06.170 --> 00:43:10.110 plankton record is the clearest. Among the foraminifera that Walter 704 00:43:10.250 --> 00:43:13.349 Alvarez had been looking at in the Gubbio limestone, something 705 00:43:13.389 --> 00:43:17.510 like 90% of the open ocean species disappeared at the boundary. 706 00:43:18.170 --> 00:43:21.699 The coccolithophores, which are single-celled algae that make plates of 707 00:43:21.760 --> 00:43:24.219 calcite and are the base of much of the marine 708 00:43:24.260 --> 00:43:28.039 food chain, were devastated in the same way. On land, 709 00:43:28.300 --> 00:43:31.360 the pollen record across the boundary in North America shows 710 00:43:31.420 --> 00:43:35.380 the flowering plant and conifer forests collapsing and being replaced 711 00:43:35.659 --> 00:43:38.480 for a span of years to decades by an almost 712 00:43:38.579 --> 00:43:42.110 pure cover of ferns, which is what colonizes burned and 713 00:43:42.139 --> 00:43:47.449 barren ground today. Paleobotanists call it the fern spike. The 714 00:43:47.489 --> 00:43:50.929 most argued oversight from the day itself is in southwestern 715 00:43:50.989 --> 00:43:54.269 North Dakota, on a private ranch in the Hell Creek Formation, 716 00:43:54.659 --> 00:43:58.590 at a place a paleontologist named Robert De

Palma calls Tannis. 717 00:43:59.409 --> 00:44:02.969 De Palma began excavating there in 2012 and published a 718 00:44:03.010 --> 00:44:06.260 description in the Proceedings of the National Academy of Sciences 719 00:44:06.489 --> 00:44:11.199 in 2019 with Walter Alvarez among the co-authors. The deposit 720 00:44:11.280 --> 00:44:14.760 is a jumbled bed of freshwater fish, tree trunks, and 721 00:44:14.900 --> 00:44:18.579 ammonite shells washed inland together. and the fish have tiny 722 00:44:18.639 --> 00:44:22.119 glass spherules packed into their gills, which De Palma reads 723 00:44:22.199 --> 00:44:25.300 as the animals breathing in impact debris as it rained 724 00:44:25.340 --> 00:44:27.739 down in the first hour or two after the strike, 725 00:44:28.099 --> 00:44:30.900 before a surge of water driven by the seismic shaking 726 00:44:31.179 --> 00:44:36.110 buried them. Spherules are also preserved in amber on the site. 727 00:44:36.260 --> 00:44:39.829 In 2022, a separate team led by Melanie During published 728 00:44:39.889 --> 00:44:42.309 an analysis of the growth lines in the fish bones 729 00:44:42.630 --> 00:44:45.960 and argued the animals died in the northern spring. The 730 00:44:46.000 --> 00:44:50.119 site has been contested almost from the start. Several paleontologists 731 00:44:50.139 --> 00:44:53.219 have raised concerns about access to the specimens and the 732 00:44:53.260 --> 00:44:56.179 pace at which claims have been made before data was published. 733 00:44:56.679 --> 00:45:00.039 Enduring later filed a formal complaint about a competing paper 734 00:45:00.079 --> 00:45:03.400 by De Palma on the same seasonal question. None of 735 00:45:03.440 --> 00:45:07.079 that has overturned the basic finding that Tanis preserves the boundary, 736 00:45:07.440 --> 00:45:10.280 but the details are still being checked by people outside 737 00:45:10.320 --> 00:45:14.110 the original team. Stay tuned for more disturbing history. We'll 738 00:45:14.130 --> 00:45:21.389 be back after these messages. What died is the famous list. 739 00:45:22.150 --> 00:45:26.409 Every non-avian dinosaur. The pterosaurs, which had been flying for 740 00:45:26.409 --> 00:45:32.000 150 million years. The mosasaurs and plesiosaurs that ruled the seas. 741 00:45:32.699 --> 00:45:35.860 The ammonites, which had survived three of the four previous 742 00:45:35.940 --> 00:45:40.719 mass extinctions in reduced form, and this time didn't. The rudists, 743 00:45:41.139 --> 00:45:44.800 strange reef-building clans that had displaced corals across much of 744 00:45:44.840 --> 00:45:49.349 the tropics. The total estimate is around three-quarters of all species. 745 00:45:50.590 --> 00:45:54.090 The survivors say something about the mechanism. A study by 746 00:45:54.130 --> 00:45:58.030 Peter Sheehan and Thor Hansen in 1986 found that animals 747 00:45:58.110 --> 00:46:01.230 living in freshwater, which could feed on detritus washing in 748 00:46:01.269 --> 00:46:04.329 from the dead land, came through far better than animals 749 00:46:04.369 --> 00:46:07.860 in the sea or on dry ground. Small animals did 750 00:46:07.889 --> 00:46:13.380 better than large. Burrowers did better than anything exposed. Birds survived, 751 00:46:13.659 --> 00:46:16.539 though only a few lineages did. and the mammals that 752 00:46:16.599 --> 00:46:21.320 survived were mostly small insect eaters and seed eaters. Crocodilians 753 00:46:21.360 --> 00:46:24.130 and turtles, which can go for long periods without eating 754 00:46:24.369 --> 00:46:27.150 and live in the freshwater systems that held up, came 755 00:46:27.190 --> 00:46:30.909 through almost untouched. Everything that needed a lot of food 756 00:46:31.070 --> 00:46:35.070 every day from a functioning photosynthetic food chain died, and 757 00:46:35.159 --> 00:46:38.019 everything that could wait, or eat the dead, or live 758 00:46:38.099 --> 00:46:42.219 on stored energy had a chance. The question of whether 759 00:46:42.280 --> 00:46:45.059 the dinosaurs were already on their way out has never 760 00:46:45.119 --> 00:46:48.619 been settled, Some analyses of the diversity curves find a 761 00:46:48.699 --> 00:46:52.420 long decline starting tens of millions of years before the boundary, 762 00:46:52.900 --> 00:46:55.679 and others find that the apparent decline is an artifact 763 00:46:55.699 --> 00:46:57.960 of how much rock of each age happens to be 764 00:46:58.000 --> 00:47:01.920 exposed and collected. The Montana record, which is the best studied, 765 00:47:02.280 --> 00:47:05.679 has dinosaurs present and apparently healthy within a few meters 766 00:47:05.739 --> 00:47:08.699 of the boundary clay. I don't think the decline argument 767 00:47:08.760 --> 00:47:12.000 changes much either way. An ecosystem that had been running 768 00:47:12.039 --> 00:47:15.969 for 150 million years ended within the resolution of the 769 00:47:16.030 --> 00:47:19.210 dating at the moment the asteroid arrived, and the mammals 770 00:47:19.250 --> 00:47:22.489 that inherited it were, within a few hundred thousand years, 771 00:47:22.949 --> 00:47:27.809 already starting to grow. The sixth one is the argument 772 00:47:27.889 --> 00:47:31.739 about now. The animals that were missing from the world 773 00:47:31.800 --> 00:47:34.659 when people started writing things down had been gone for 774 00:47:34.699 --> 00:47:39.380 a geologic instant. Between about 50,000 and 10,000 years ago, 775 00:47:39.699 --> 00:47:43.880 across every continent except Africa and Antarctica, the largest land 776 00:47:43.960 --> 00:47:47.659 animals disappeared in a pattern that followed, continent by continent, 777 00:47:48.019 --> 00:47:52.400 the arrival of modern humans. Australia lost the Diprotodon, a 778 00:47:52.440 --> 00:47:55.860 marsupial herbivore the size of a rhinoceros, along with a 779 00:47:55.920 --> 00:48:00.059 giant flightless bird called Gineornis, a marsupial lion, and a 780

00:48:00.119 --> 00:48:03.300 monitor lizard several times the size of a Komodo dragon, 781 00:48:03.639 --> 00:48:07.210 with most of the losses dated to roughly 45,000 years ago, 782 00:48:07.570 --> 00:48:10.030 which is close to the earliest secure human dates on 783 00:48:10.050 --> 00:48:17.099 that continent. North America lost mammoths, mastodons, giant ground sloths, horses, camels, 784 00:48:17.440 --> 00:48:21.119 the saber-toothed cat, the dire wolf, and a short-faced bear 785 00:48:21.159 --> 00:48:23.610 that stood taller than a man at the shoulder, with 786 00:48:23.639 --> 00:48:27.610 the main pulse of extinctions falling between about 13,000 and 11,000 787 00:48:27.610 --> 00:48:30.630 years ago, which is close to the first widespread human 788 00:48:30.670 --> 00:48:34.250 culture in the Americas. South America lost even more of 789 00:48:34.309 --> 00:48:38.400 its large animals. Madagascar lost its giant lemurs and elephant 790 00:48:38.440 --> 00:48:43.489 birds within the last 2,000 years, after people reached the island. 791 00:48:43.610 --> 00:48:47.489 In 1967, an ecologist at the University of Arizona named 792 00:48:47.590 --> 00:48:51.190 Paul Martin published a paper arguing that the correlation was 793 00:48:51.230 --> 00:48:55.309 the explanation, and that newly arrived human hunters, meeting large 794 00:48:55.449 --> 00:48:58.130 animals that had never evolved a fear of people, had 795 00:48:58.170 --> 00:49:01.710 killed them off. He called it overkill, and his critics 796 00:49:01.769 --> 00:49:05.019 called it the Blitzkrieg hypothesis. and the fight over it 797 00:49:05.059 --> 00:49:09.300 has gone on for nearly 60 years. The alternative explanation 798 00:49:09.579 --> 00:49:12.590 is climate. The end of the last ice age brought 799 00:49:12.699 --> 00:49:16.050 rapid warming, the breakup of the grassland steppes the mammoths 800 00:49:16.090 --> 00:49:19.550 depended on, and the spread of forests. And in Eurasia, 801 00:49:19.869 --> 00:49:22.719 the big animals declined over a long period in a 802 00:49:22.760 --> 00:49:25.380 way that tracks the climate swings better than it tracks 803 00:49:25.420 --> 00:49:29.719 human arrival. Most researchers now work with some combination, with 804 00:49:29.739 --> 00:49:33.550 the weighting varying by continent. In Australia and the Americas, 805 00:49:33.829 --> 00:49:38.210 the timing favors humans. In Eurasia, it favors climate. The 806 00:49:38.250 --> 00:49:42.239 debate is still alive in the literature. One detail from 807 00:49:42.280 --> 00:49:44.840 that record is harder to argue around than the rest. 808 00:49:45.579 --> 00:49:49.039 Woolly mammoths survived on Wrangel Island in the Arctic Ocean 809 00:49:49.139 --> 00:49:54.880 off Siberia until about 4,000 years ago, isolated, inbred, and shrinking, 810 00:49:55.380 --> 00:49:57.860 and were still alive while the pyramids at Giza were 811 00:49:57.880 --> 00:50:01.139 being built. They disappeared within a few centuries of the 812 00:50:01.199 --> 00:50:03.949 first archaeological evidence of people on the island. 813 00:50:05.210 --> 00:50:07.260 Closer to the present... The record. 814 00:50:07.030 --> 00:50:11.929 Stops being bones and starts being ship's logs. Dutch sailors 815 00:50:12.030 --> 00:50:15.070 reached the island of Mauritius in the Indian Ocean in 816 00:50:15.070 --> 00:50:18.909 1598 and described a large, flightless bird that didn't run 817 00:50:18.969 --> 00:50:22.380 from them. The last broadly accepted sighting of a dodo 818 00:50:22.440 --> 00:50:26.179 was recorded by a shipwrecked sailor named Volkert Everts in 819 00:50:26.179 --> 00:50:29.960 1662 on a small islet off the coast. The bird 820 00:50:29.980 --> 00:50:34.000 had been known to Europeans for 64 years. The hunting 821 00:50:34.039 --> 00:50:37.150 was part of it, but the pigs, rats, and monkeys 822 00:50:37.210 --> 00:50:40.639 the ships released, which ate the eggs of a ground-nesting bird, 823 00:50:40.889 --> 00:50:46.780 were probably more. In 1741, a German naturalist named Georg 824 00:50:46.820 --> 00:50:50.760 Wilhelm Steller, shipwrecked on an uninhabited island in the Bering 825 00:50:50.860 --> 00:50:54.380 Sea with the remnants of Vitus Bering's expedition, described a 826 00:50:54.440 --> 00:50:57.679 sea mammal that nobody in Europe had seen before. It 827 00:50:57.719 --> 00:51:01.090 was related to the manatee and the dugong, but was enormous, 828 00:51:01.670 --> 00:51:05.360 eight to nine meters long and weighing several tons. and 829 00:51:05.400 --> 00:51:08.239 it lived in the cold, shallow kelp beds around the island, 830 00:51:08.659 --> 00:51:13.130 floating at the surface and grazing. Steller measured one, dissected it, 831 00:51:13.440 --> 00:51:16.789 and recorded that the animals were slow, trusting, and would 832 00:51:16.829 --> 00:51:19.110 come to the aid of a wounded member of the herd. 833 00:51:19.730 --> 00:51:23.610 His account was published after his death. Fur hunters working 834 00:51:23.650 --> 00:51:26.570 the sea otter trade out of Kamchatka used the island 835 00:51:26.630 --> 00:51:29.429 as a provisioning stop and killed the sea cows for meat. 836 00:51:30.099 --> 00:51:34.780 The last one was reported killed in 1768. 27 years 837 00:51:34.929 --> 00:51:38.860 after the species had been described. The great auk was 838 00:51:38.900 --> 00:51:42.380 a large, flightless seabird of the North Atlantic, standing about 839 00:51:42.380 --> 00:51:45.659 80 centimeters tall, that nested on a handful of rocky 840 00:51:45.840 --> 00:51:49.750 islands from Newfoundland to Iceland and had been hunted for meat, oil, 841 00:51:49.829 --> 00:51:53.849 and feathers for centuries. By the early 1800s, it was rare, 842 00:51:54.210 --> 00:51:58.130 and because it was rare, museums and private collectors wanted specimens, 843 00:51:58.530 --> 00:52:01.260 and the price for a skin or an egg climbed. 844 00:52:01.940 -->

00:52:05.780 The last well-documented pair was killed on the 3rd of June, 1844, 845 00:52:05.780 --> 00:52:09.469 on a sea stack called Eldi, off the southwest coast 846 00:52:09.510 --> 00:52:12.670 of Iceland, by three fishermen hired to collect birds for 847 00:52:12.710 --> 00:52:15.849 a dealer. Two of the men caught and strangled the adults, 848 00:52:16.210 --> 00:52:18.389 while the third found a single egg on the rock, 849 00:52:18.789 --> 00:52:21.869 saw that it was cracked, and crushed it. The account 850 00:52:21.929 --> 00:52:25.329 comes from the men themselves, interviewed 14 years later by 851 00:52:25.409 --> 00:52:29.340 two English naturalists, John Wally and Alfred Newton. who went 852 00:52:29.380 --> 00:52:32.039 to Iceland specifically to find out what had happened to 853 00:52:32.079 --> 00:52:35.409 the species. The ox eggs now sell at auction for 854 00:52:35.469 --> 00:52:38.050 sums that would have been unimaginable to the men who 855 00:52:38.110 --> 00:52:39.170 smashed the last one. 856 00:52:40.550 --> 00:52:41.610 Then there's the pigeon. 857 00:52:43.150 --> 00:52:46.900 In the autumn of 1813, John James Audubon was riding 858 00:52:46.940 --> 00:52:50.619 from Hardinsburg to Louisville, a distance of about 55 miles, 859 00:52:51.019 --> 00:52:54.239 when a flock of passenger pigeons passed overhead and blocked 860 00:52:54.260 --> 00:52:57.219 the sun. He wrote that the birds were still passing 861 00:52:57.260 --> 00:53:00.449 when he reached Louisville. and that they continued for three days. 862 00:53:01.170 --> 00:53:03.590 He tried to estimate the number by timing the flock 863 00:53:03.630 --> 00:53:06.050 against its width and got a figure of more than 864 00:53:06.090 --> 00:53:09.429 a billion birds, which he presented as a conservative count. 865 00:53:10.070 --> 00:53:12.570 Other observers in the first half of the 19th century 866 00:53:12.650 --> 00:53:15.869 described flocks that took hours to pass and broke tree 867 00:53:15.929 --> 00:53:19.159 limbs where they roosted. The best modern estimates put the 868 00:53:19.219 --> 00:53:22.739 total population of the species at somewhere between three and 869 00:53:22.840 --> 00:53:25.800 five billion birds. which would have made it the most 870 00:53:25.860 --> 00:53:29.000 abundant bird in North America and possibly in the world, 871 00:53:29.519 --> 00:53:32.300 accounting for perhaps a quarter or more of all the 872 00:53:32.340 --> 00:53:36.719 land birds on the continent. The species nested in enormous 873 00:53:36.800 --> 00:53:39.780 colonies that moved from year to year following the massed 874 00:53:39.820 --> 00:53:44.360 crops of beech and oak. In 1871, a single nesting 875 00:53:44.400 --> 00:53:49.300 in central Wisconsin covered an estimated 850 square miles, with 876 00:53:49.460 --> 00:53:53.800 every tree carrying dozens of nests. And the ornithologist A.W. 877 00:53:53.880 --> 00:53:58.780 Shorger later calculated that colony at more

than 130 million birds. 878 00:53:59.599 --> 00:54:04.369 In 1878, the last of the great nestings formed near Petoskey, Michigan. 879 00:54:05.510 --> 00:54:08.460 What killed them was commerce with a railroad behind it. 880 00:54:09.070 --> 00:54:12.320 Passenger pigeon was cheap meat. And in the decades after 881 00:54:12.360 --> 00:54:15.320 the Civil War, the Telegraph could report where a nesting 882 00:54:15.360 --> 00:54:18.380 had formed... and the rail network could get professional hunters 883 00:54:18.440 --> 00:54:21.090 there within days and ship the dead birds back to 884 00:54:21.119 --> 00:54:25.550 Chicago and New York in refrigerated cars. The hunters netted them, 885 00:54:25.949 --> 00:54:29.269 shot them, cut down the nesting trees, and burned sulfur 886 00:54:29.369 --> 00:54:34.010 under the roosts to suffocate them. At Petoskey in 1878, 887 00:54:34.010 --> 00:54:38.170 by the estimates compiled afterward, something like 50,000 birds a 888 00:54:38.230 --> 00:54:41.190 day were being killed for the better part of five months. 889 00:54:41.889 --> 00:54:44.719 The birds appear to have needed enormous colonies to breed 890 00:54:44.760 --> 00:54:47.800 successfully at all, So once the colonies were broken up, 891 00:54:48.199 --> 00:54:51.780 the population didn't decline gradually. It fell off a cliff. 892 00:54:52.500 --> 00:54:56.519 By the 1890s, the flocks were gone. The last confirmed 893 00:54:56.599 --> 00:54:59.949 wild passenger pigeon was shot on the 24th of March, 1900, 894 00:54:59.949 --> 00:55:04.610 in Pike County, Ohio, by a 14-year-old boy named Press 895 00:55:04.710 --> 00:55:08.510 Clay Southworth, whose family had the bird mounted. It still 896 00:55:08.550 --> 00:55:12.349 exists in the collection of the Ohio History Connection in Columbus, 897 00:55:12.829 --> 00:55:15.559 with black shoe buttons where its eyes should be. because 898 00:55:15.599 --> 00:55:19.869 the taxidermist had nothing else on hand. A few captive 899 00:55:19.909 --> 00:55:23.409 flocks lingered. The Cincinnati Zoo had a group that dwindled 900 00:55:23.429 --> 00:55:26.090 through the first decade of the new century until one 901 00:55:26.150 --> 00:55:30.750 bird remained, a female the keepers had named Martha. The 902 00:55:30.789 --> 00:55:33.309 zoo offered a $ 1, 000 reward for a mate. 903 00:55:34.110 --> 00:55:34.869 None was found. 904 00:55:35.690 --> 00:55:39.030 She died on the 1st of September, 1914, at about 905 00:55:39.030 --> 00:55:42.769 1 in the afternoon, at an estimated age of 29. 906 00:55:42.769 --> 00:55:45.590 and the zoo froze her body in a 300-pound block 907 00:55:45.630 --> 00:55:48.429 of ice and shipped her by train to the Smithsonian, 908 00:55:48.710 --> 00:55:52.409 where she was skinned and mounted and her internal organs preserved. 909 00:55:53.659 --> 00:55:54.119 In May of. 910 00:55:54.079 --> 00:55:58.579 1947, the Wisconsin Society for Ornithology put up a bronze 911 00:55:58.659 --> 00:56:02.340 plaque on a limestone marker in Wyalusing State Park on 912 00:56:02.360 --> 00:56:05.000 the bluffs above the spot where the Wisconsin River meets 913 00:56:05.059 --> 00:56:08.710 the Mississippi to mark the species. It reads that the 914 00:56:08.750 --> 00:56:12.920 passenger pigeon became extinct through the avarice and thoughtlessness of man, 915 00:56:13.460 --> 00:56:15.699 and it stands a short drive from the ground where 916 00:56:15.719 --> 00:56:19.980 the 1871 nesting had been. Aldo Leopold, who taught at 917 00:56:20.000 --> 00:56:23.219 the University in Madison, wrote the address for the dedication, 918 00:56:23.619 --> 00:56:25.730 and he pointed out that it was the first monument 919 00:56:25.869 --> 00:56:28.769 anyone had ever raised to a species rather than to 920 00:56:28.809 --> 00:56:31.570 a person. He also noted that the men who had 921 00:56:31.610 --> 00:56:35.210 done the netting and shooting in the 1870s were, some 922 00:56:35.250 --> 00:56:38.929 of them, still alive to read it. The last of 923 00:56:38.969 --> 00:56:43.289 the thylacines, the striped marsupial carnivore of Tasmania that settlers 924 00:56:43.349 --> 00:56:47.010 called the Tasmanian tiger, died at the Bomaris Zoo in 925 00:56:47.050 --> 00:56:52.159 Hobart on the 7th of September, 1936. The Tasmanian government 926 00:56:52.199 --> 00:56:54.880 had paid a bounty of one pound per adult animal 927 00:56:54.920 --> 00:56:58.860 from 1888 to 1909 on the claim that the thylacine 928 00:56:58.900 --> 00:57:02.360 was killing sheep, and the records show more than 2,000 929 00:57:02.360 --> 00:57:05.309 bounties paid out before the payments stopped because there were 930 00:57:05.389 --> 00:57:08.519 so few animals left to claim them for. The species 931 00:57:08.559 --> 00:57:11.579 was granted legal protection by the Tasmanian government on the 932 00:57:11.619 --> 00:57:16.000 10th of July, 1936, 59 days before the last known 933 00:57:16.119 --> 00:57:18.860 animal died. The zoo had a short film of that 934 00:57:19.010 --> 00:57:23.010 animal pacing its enclosure, shot in 1933 by a naturalist 935 00:57:23.050 --> 00:57:25.949 named David Flay, who was bitten on the backside while 936 00:57:25.989 --> 00:57:29.130 filming it. That footage is the only moving picture of 937 00:57:29.190 --> 00:57:33.469 a living thylacine, and in 2021, Australia's National Film and 938 00:57:33.510 --> 00:57:37.210 Sound Archive released a colorized version of it. The animals' 939 00:57:37.269 --> 00:57:41.110 remains were thought lost for decades until 2022, when two 940 00:57:41.170 --> 00:57:44.869 researchers going through the Tasmanian Museum and Art Gallery's collection 941 00:57:45.230 --> 00:57:48.469 found the skin and skeleton in an unlabeled cupboard where 942 00:57:48.489 --> 00:57:52.889 they'd been sitting since the 1930s. Whether what has happened 943 00:57:52.989 --> 00:57:55.739 since adds up to a sixth mass extinction is a 944 00:57:55.840

--> 00:57:59.639 real scientific argument, and the people on both sides are serious. 945 00:58:00.440 --> 00:58:03.880 In 2011, a group led by Anthony Barnoski at Berkeley 946 00:58:04.230 --> 00:58:06.909 published a paper in Nature that tried to compare modern 947 00:58:06.969 --> 00:58:10.489 rates with the fossil record on equal terms, and concluded 948 00:58:10.530 --> 00:58:13.420 that the current rate of loss is far above background, 949 00:58:13.820 --> 00:58:17.059 but that the total body count is nowhere near the 950 00:58:17.059 --> 00:58:20.079 75% threshold of the Big Five, and that reaching it 951 00:58:20.199 --> 00:58:24.920 would take centuries to millennia at present rates. In 2015, 952 00:58:24.920 --> 00:58:28.900 Gerardo Ceballos and Paul Ehrlich published an estimate that vertebrate 953 00:58:28.960 --> 00:58:31.860 extinction rates over the last century have run as much 954 00:58:31.900 --> 00:58:35.289 as 100 times the background rate and frame that as 955 00:58:35.349 --> 00:58:39.670 the sixth extinction already underway. Stuart Pim's group at Duke 956 00:58:39.969 --> 00:58:42.710 put the figure for all species at around a thousand 957 00:58:42.769 --> 00:58:47.590 times background. A report by an international panel on biodiversity 958 00:58:47.730 --> 00:58:51.050 in 2019 estimated that about a million species of plants 959 00:58:51.090 --> 00:58:55.519 and animals are currently at risk. The objection doesn't come 960 00:58:55.559 --> 00:58:58.420 from people who think the losses aren't happening. It comes 961 00:58:58.460 --> 00:59:01.420 from paleontologists who work on the Big Five and who 962 00:59:01.460 --> 00:59:05.199 think the comparison is being made badly. Doug Irwin, the 963 00:59:05.219 --> 00:59:08.869 Smithsonian paleontologist who has spent his career on the End Permian, 964 00:59:09.230 --> 00:59:11.820 has said in interviews, that a true mass extinction on 965 00:59:11.840 --> 00:59:14.099 the scale of the ones in the rock record would 966 00:59:14.119 --> 00:59:18.920 be unmistakable, with ecosystems collapsing globally within a human lifetime, 967 00:59:19.380 --> 00:59:22.199 and that we are not in one now. His point 968 00:59:22.260 --> 00:59:24.530 is that the events in the fossil record aren't just 969 00:59:24.590 --> 00:59:27.289 a lot of species dying. They're the collapse of the 970 00:59:27.349 --> 00:59:31.239 entire planetary system that supports life. with the oceans turning 971 00:59:31.280 --> 00:59:34.380 acidic and anoxic and the climate going off the rails. 972 00:59:34.980 --> 00:59:38.000 And that calling the present a sixth mass extinction before 973 00:59:38.039 --> 00:59:43.190 that threshold is reached confuses two different things. Barnosky's own position, 974 00:59:43.469 --> 00:59:46.010 as he wrote it in that 2011 paper, was that 975 00:59:46.050 --> 00:59:49.610 the sixth extinction hasn't arrived, but that it's within reach. 976 00:59:50.789 --> 00:59:54.030 Where the comparison does hold is in the chemistry. The 977 00:59:54.070 --> 00:59:56.949 Permian and the Triassic were both set off by large 978 00:59:57.010 --> 01:00:00.429 volumes of carbon being put into the atmosphere quickly. over 979 01:00:00.510 --> 01:00:03.679 tens of thousands of years, with the oceans warming and 980 01:00:03.719 --> 01:00:07.960 acidifying and losing oxygen in response. The carbon now being 981 01:00:08.000 --> 01:00:11.320 released from fossil fuels is being released faster than either 982 01:00:11.360 --> 01:00:14.119 of those events by a wide margin, and the ocean 983 01:00:14.159 --> 01:00:17.860 chemistry is responding in the same direction. Whether the biology 984 01:00:17.929 --> 01:00:20.809 follows is the open question, and it's not one the 985 01:00:20.849 --> 01:00:24.369 fossil record can answer, because every previous time, the answer 986 01:00:24.409 --> 01:00:28.989 came in after the fact. Martha is still at the Smithsonian, 987 01:00:29.710 --> 01:00:32.050 She's on the mounted skin's third or fourth set of 988 01:00:32.070 --> 01:00:35.429 glass eyes by now, and she spent decades off display 989 01:00:35.449 --> 01:00:38.170 in a storage cabinet before being brought out again in 990 01:00:38.170 --> 01:00:42.559 2014 for the 100th anniversary of her death. The passenger 991 01:00:42.599 --> 01:00:46.280 pigeon exists today as a few thousand skins in museum drawers, 992 01:00:46.780 --> 01:00:50.079 a handful of skeletons, some eggs, and the sequence of 993 01:00:50.119 --> 01:00:54.139 its genome, which was recovered from those skins in 2017 994 01:00:54.139 --> 01:00:57.260 and showed a species that had been enormously abundant and 995 01:00:57.280 --> 01:01:01.269 genetically uniform for tens of thousands of years, built for 996 01:01:01.309 --> 01:01:04.789 a world of billions, with no reserve for a world 997 01:01:04.829 --> 01:01:09.469 of hundreds. Cuvier's mastodon jaw from the Ohio River is 998 01:01:09.550 --> 01:01:12.340 still in Paris, in the museum where he first laid 999 01:01:12.360 --> 01:01:15.460 it beside the elephants. The clay at Gubbio is still 1000 01:01:15.500 --> 01:01:17.880 in the road cut, with a small plaque now set 1001 01:01:17.920 --> 01:01:21.159 beside it. The bronze marker at Machon is still in 1002 01:01:21.179 --> 01:01:25.050 the quarry wall. Those are the physical things, a jaw, 1003 01:01:25.409 --> 01:01:27.969 a layer of clay, a spike in a numbered bed 1004 01:01:27.989 --> 01:01:30.929 of limestone. And all of them mark the same kind 1005 01:01:30.969 --> 01:01:34.050 of line, where one world stops in the rock and 1006 01:01:34.090 --> 01:01:36.730 a different one starts an inch above it. What the 1007 01:01:36.769 --> 01:01:39.239 people who found them share is that none of them 1008 01:01:39.280 --> 01:01:43.360 went looking for an ending. Cuvier was comparing teeth. Walter 1009 01:01:43.500 --> 01:01:45.880 Alvarez wanted to know how long a layer of clay 1010 01:01:45.920 --> 01:01:49.840 had taken to form. Sepkoski was making a list. The 1011 01:01:49.940 --> 01:01:53.530 endings were already there, in the ground, waiting for someone 1012 01:01:53.570 --> 01:01:53.969 to count. 1013 01:01:59.519 --> 01:02:16.630 Your skin, I've got a taste for you. Your heart, 1014 01:02:16.949 --> 01:02:38.329 I'm seeking. Watch out. The moon is out now. 1015 01:02:38.329 --> 01:02:52.239 You're gonna hear my howl Howl, howl, howl, howl Blood skies, 1016 01:02:52.800 --> 01:03:01.949 red eyes Can't get enough of it My dream is 1017 01:03:02.469 --> 01:03:13.389 your nightmare for you. 1018 01:03:48.070 --> 01:03:57.869 For the morning's out now You're gonna hear my air