1 00:00:01,068 --> 00:00:03,269 Viewers like you make this program possible. 2 00:00:03,270 --> 00:00:05,339 Support your local PBS station. 3 00:00:14,382 --> 00:00:16,049 Mammals. 4 00:00:16,050 --> 00:00:18,585 They live in almost every corner of the planet. 5 00:00:18,586 --> 00:00:20,754 Thousands of different species. 6 00:00:22,690 --> 00:00:23,990 From tiny desert creatures 7 00:00:23,991 --> 00:00:26,092 to giant ocean beasts 8 00:00:26,093 --> 00:00:27,694 to us. 9 00:00:27,695 --> 00:00:30,231 But when did our shared history begin? 10 00:00:33,167 --> 00:00:35,268 You might think the start of the mammal story 11 00:00:35,269 --> 00:00:37,404 was the extinction of the dinosaurs 12 00:00:37,405 --> 00:00:39,072 some 66 million years ago... 13 00:00:40,641 --> 00:00:45,278 {\an8}...but the story actually begins much, much earlier... 14 00:00:45,279 --> 00:00:46,713 We have the very earliest ancestors of mammals. 15 00:00:46,714 --> 00:00:49,682 Those animals are four times as old 16 00:00:49,683 --> 00:00:50,918 as the oldest T. rex. 17 00:00:52,920 --> 00:00:55,789 ...in a time when mysterious animals ruled. 18 00:00:55,790 --> 00:00:57,657 They looked very different from anything alive today. 19 00:00:57,658 --> 00:01:00,460 In some ways, 20 00:01:00,461 --> 00:01:01,861 they looked like reptiles. 21 00:01:01,862 --> 00:01:03,897 But it's from here that mammals took shape... 22 00:01:03,898 --> 00:01:05,098 They were our ancestors. 23 00:01:05,099 --> 00:01:08,101 ...surviving a tumultuous period 24 00:01:08,102 --> 00:01:09,836 in Earth's history. 25 00:01:09,837 --> 00:01:11,271 It became apparent 26 00:01:11,272 --> 00:01:12,472 that there were two extinction events. 27 00:01:12,473 --> 00:01:14,207 Our ancestors made it through 28 00:01:14,208 --> 00:01:15,875 for very specific reasons. 29 00:01:15,876 --> 00:01:18,011 We had very specific adaptations as a group, 30 00:01:18,012 --> 00:01:21,414 and that allowed us to make it past these extinction events. 31 00:01:21,415 --> 00:01:24,050 {\an8} What was the secret to our ancestors' survival 32 00:01:24,051 --> 00:01:25,952 in the face of deadly catastrophes? 33 00:01:27,455 --> 00:01:29,956 "Mammal Origins." 34 00:01:29,957 --> 00:01:33,026 Right now, on "NOVA." 35 00:01:50,177 --> 00:01:51,511 Mammals are 36 00:01:51,512 --> 00:01:53,746 spectacularly diverse. 37 00:01:53,747 --> 00:01:55,348 They not only survive, 38 00:01:55,349 --> 00:02:00,019 but thrive in all kinds of environments. 39 00:02:00,020 --> 00:02:01,821 Mammals have basically taken over the world. 40 00:02:01,822 --> 00:02:04,057 {\an8}There's mammals all over the land. 41 00:02:04,058 --> 00:02:06,192 {\an8}There's mammals that fly in the air. 42 00:02:06,193 --> 00:02:07,594 {\an8}There's mammals that swim in the oceans. 43 00:02:10,197 --> 00:02:12,098 They range in size 44 00:02:12,099 --> 00:02:14,634 from tiny elephant shrews 45 00:02:14,635 --> 00:02:17,036 to gargantuan 200-ton whales. 46 00:02:17,037 --> 00:02:20,840 Yet all mammal species-- more than 6,000 of them-- 47 00:02:20,841 --> 00:02:23,877 share many of the same basic traits. 48 00:02:25,579 --> 00:02:28,214 {\an8}They have hair or fur 49 00:02:28,215 --> 00:02:30,251 {\an8}and four chambers in their hearts. 50 00:02:31,619 --> 00:02:34,187 {\an8}Mammals produce milk for their young. 51 00:02:34,188 --> 00:02:37,190 {\an8}And they are all warm-blooded, 52 00:02:37,191 --> 00:02:41,060 {\an8}a trait that has long puzzled scientists. 53 00:02:41,061 --> 00:02:42,862 The creatures that mammals evolved from 54 00:02:42,863 --> 00:02:44,664 were cold-blooded, 55 00:02:44,665 --> 00:02:46,566 and so it's a really key thing 56 00:02:46,567 --> 00:02:47,667 in the history of mammals, 57 00:02:47,668 --> 00:02:49,702 figuring out how and when and why 58 00:02:49,703 --> 00:02:53,072 our ancestors changed their metabolism 59 00:02:53,073 --> 00:02:55,208 and started warming up their bodies. 60 00:02:56,977 --> 00:02:59,279 {\an8} So, where does the mammal story begin? 61 00:02:59,280 --> 00:03:01,581 Where do we come from? 62 00:03:09,156 --> 00:03:13,326 {\an8}Rewind the clock 270 million years, 63 00:03:13,327 --> 00:03:17,463 {\an8}to a time even before the dinosaurs. 64 00:03:17,464 --> 00:03:21,034 {\an8}Back then, our planet was home not only 65 00:03:21,035 --> 00:03:24,203 {\an8}to the ancestors of the dinosaurs and other reptiles, 66 00:03:24,204 --> 00:03:28,775 {\an8}but to another group, also now long gone. 67 00:03:28,776 --> 00:03:32,612 Among them, creatures that would one day give rise to mammals: 68 00:03:32,613 --> 00:03:35,248 the therapsids. 69 00:03:35,249 --> 00:03:38,718 Paleontologists have known about them for decades, 70 00:03:38,719 --> 00:03:40,887 but in 2023, 71 00:03:40,888 --> 00:03:44,390 some of the oldest known remains of these enigmatic animals 72 00:03:44,391 --> 00:03:45,692 were discovered in South Africa. 73 00:03:50,931 --> 00:03:53,266 {\an8} This skull is 266 million years old, 74 00:03:53,267 --> 00:03:55,101 {\an8}which is really interesting, 75 00:03:55,102 --> 00:03:57,170 because it's one of the oldest fossils we have. 76 00:03:57,171 --> 00:03:59,373 The snout is here, in this direction. 77 00:04:00,708 --> 00:04:04,744 You can also clearly see its teeth. 78 00:04:04,745 --> 00:04:07,914 This creature would have been the size of a large dog, 79 00:04:07,915 --> 00:04:10,583 but with sprawling limbs like a reptile 80 00:04:10,584 --> 00:04:13,486 and a cold-blooded metabolism. 81 00:04:13,487 --> 00:04:15,655 So what is it that has scientists convinced 82 00:04:15,656 --> 00:04:17,457 it's not a reptile? 83 00:04:19,560 --> 00:04:21,127 Here is the eye. 84 00:04:21,128 --> 00:04:23,162 And here, behind the eye, 85 00:04:23,163 --> 00:04:25,064 there's a pit called the temporal fossa, 86 00:04:25,065 --> 00:04:26,199 which is an attachment point 87 00:04:26,200 --> 00:04:29,435 for the muscle that closes the jaw. 88 00:04:29,436 --> 00:04:32,673 It's a defining characteristic of therapsids. 89 00:04:33,874 --> 00:04:36,409 Dinosaurs, crocodiles, and many reptiles 90 00:04:36,410 --> 00:04:39,979 have two of these pits, called temporal fossae, 91 00:04:39,980 --> 00:04:41,615 on each side of their skulls. 92 00:04:42,583 --> 00:04:46,020 But therapsids have just one pit behind each eye. 93 00:04:47,287 --> 00:04:48,554 It's the place where the jaw muscles 94 00:04:48,555 --> 00:04:50,957 that control biting and chewing are anchored. 95 00:04:52,760 --> 00:04:55,328 And the only other group that has one pit behind each eye 96 00:04:55,329 --> 00:04:59,799 is our group: mammals. 97 00:04:59,800 --> 00:05:02,969 This anatomical similarity was one of the first clues 98 00:05:02,970 --> 00:05:04,637 that led scientists to suspect 99 00:05:04,638 --> 00:05:06,873 that mammals and therapsids 100 00:05:06,874 --> 00:05:08,709 are part of the same evolutionary line. 101 00:05:10,244 --> 00:05:12,712 But that's not the only evidence. 102 00:05:12,713 --> 00:05:15,181 BENOÎT You can see another characteristic 103 00:05:15,182 --> 00:05:17,450 that distinguishes therapsids from other animals, 104 00:05:17,451 --> 00:05:19,318 and that's their teeth. 105 00:05:19,319 --> 00:05:21,421 Therapsids have specialized teeth, 106 00:05:21,422 --> 00:05:23,289 incisors for nipping 107 00:05:23,290 --> 00:05:25,191 and back teeth known as "cheek teeth." 108 00:05:25,192 --> 00:05:27,026 The two groups of teeth 109 00:05:27,027 --> 00:05:29,162 are separated by rather impressive canines. 110 00:05:30,397 --> 00:05:32,965 Just like mammals today, 111 00:05:32,966 --> 00:05:35,268 including us humans. 112 00:05:35,269 --> 00:05:38,037 These shared features helped convince scientists 113 00:05:38,038 --> 00:05:40,441 that therapsids gave rise to mammals. 114 00:05:41,608 --> 00:05:43,276 BENOÎT However, 115 00:05:43,277 --> 00:05:45,578 if you came across any of them here today, 116 00:05:45,579 --> 00:05:46,679 you wouldn't realize 117 00:05:46,680 --> 00:05:48,047 that they were the ancestors of mammals, 118 00:05:48,048 --> 00:05:50,183 because they looked a lot like reptiles. 119 00:05:50,184 --> 00:05:52,085 Some were as big as rhinoceroses 120 00:05:52,086 --> 00:05:53,853 and others as small as mice. 121 00:05:53,854 --> 00:05:56,255 Some were carnivores, others herbivores. 122 00:05:56,256 --> 00:05:58,792 There wasn't just one type of therapsid, but lots. 123 00:06:01,462 --> 00:06:04,964 In fact, six main groups of therapsids roamed ancient Earth, 124 00:06:04,965 --> 00:06:09,368 and each group included many different species, 125 00:06:09,369 --> 00:06:13,106 including the 2023 fossil, 126 00:06:13,107 --> 00:06:15,875 which may be from a group called the biarmosuchians, 127 00:06:15,876 --> 00:06:20,246 the most ancient and primitive therapsids found so far. 128 00:06:20,247 --> 00:06:23,316 Many of the other groups have more specialized traits, 129 00:06:23,317 --> 00:06:25,051 like dinocephalians, 130 00:06:25,052 --> 00:06:26,853 recognizable by their heads-- 131 00:06:26,854 --> 00:06:28,689 thick skulls with bumps. 132 00:06:30,758 --> 00:06:32,125 And dicynodonts, 133 00:06:32,126 --> 00:06:34,828 which are distinguished by a horny beak and tusks. 134 00:06:37,397 --> 00:06:41,434 Gorgonopsians are characterized by their oversized canines, 135 00:06:41,435 --> 00:06:44,438 some even larger than the teeth of a T. Rex. 136 00:06:47,474 --> 00:06:50,343 Therocephalians had smaller jaws and teeth, 137 00:06:50,344 --> 00:06:52,579 but were more upright and agile. 138 00:06:54,548 --> 00:06:57,150 Finally, the cynodonts, 139 00:06:57,151 --> 00:06:59,453 many of which were small, burrowing creatures. 140 00:07:00,788 --> 00:07:03,022 Altogether, there were perhaps hundreds, 141 00:07:03,023 --> 00:07:04,924 if not thousands of therapsid species 142 00:07:04,925 --> 00:07:06,893 among these groups. 143 00:07:06,894 --> 00:07:08,661 They were hugely varied 144 00:07:08,662 --> 00:07:10,129 in their appearance and size, 145 00:07:10,130 --> 00:07:12,899 much like mammals are today. 146 00:07:12,900 --> 00:07:14,500 One group would lead 147 00:07:14,501 --> 00:07:15,902 to the emergence of mammals on Earth 148 00:07:15,903 --> 00:07:19,005 225 million years ago, 149 00:07:19,006 --> 00:07:22,475 {\an8}eventually evolving and then likely passing down 150 00:07:22,476 --> 00:07:25,344 {\an8}the most iconic mammal features: 151 00:07:25,345 --> 00:07:27,380 {\an8}hair, 152 00:07:27,381 --> 00:07:30,383 {\an8}warm blood, 153 00:07:30,384 --> 00:07:34,320 {\an8}and lactation. 154 00:07:34,321 --> 00:07:37,957 {\an8}But which group was it? 155 00:07:41,795 --> 00:07:46,465 Therapsid fossils have been found on every continent, 156 00:07:46,466 --> 00:07:50,437 but one part of South Africa is especially rich in clues. 157 00:07:54,074 --> 00:07:56,142 The Karoo is one of the most 158 00:07:56,143 --> 00:07:57,877 important fossil sites in the world. 159 00:07:57,878 --> 00:08:00,613 90% of fossils found in these rocks 160 00:08:00,614 --> 00:08:04,083 are therapsids, the ancestors of mammals. 161 00:08:04,084 --> 00:08:06,452 Many thousands of fossils 162 00:08:06,453 --> 00:08:08,788 have been discovered in the Karoo, 163 00:08:08,789 --> 00:08:14,027 {\an8}and every field trip delivers more fossils and new species. 164 00:08:16,330 --> 00:08:17,597 {\an8} This has been the case 165 00:08:17,598 --> 00:08:20,166 {\an8}since the first therapsid fossil was found here 166 00:08:20,167 --> 00:08:21,935 in the 1830s. 167 00:08:24,004 --> 00:08:26,739 In the 1900s, during apartheid, 168 00:08:26,740 --> 00:08:30,844 access to dig sites was mostly limited to white scientists. 169 00:08:32,179 --> 00:08:33,279 But today, 170 00:08:33,280 --> 00:08:35,181 the government has opened up access 171 00:08:35,182 --> 00:08:36,783 and is supporting new projects. 172 00:08:37,918 --> 00:08:41,021 Now discoveries are on the rise. 173 00:08:43,624 --> 00:08:46,960 But why have so many therapsid fossils been found here? 174 00:08:48,095 --> 00:08:49,161 What was this place like 175 00:08:49,162 --> 00:08:51,097 hundreds of millions of years ago 176 00:08:51,098 --> 00:08:54,567 that allowed therapsids not only to thrive, 177 00:08:54,568 --> 00:08:58,004 but to be preserved as stone? 178 00:08:58,005 --> 00:09:00,439 Well, over here, you can see these rocks 179 00:09:00,440 --> 00:09:02,441 and these ball structures, 180 00:09:02,442 --> 00:09:05,278 they form underwater. 181 00:09:05,279 --> 00:09:06,746 And accompanying with these things, 182 00:09:06,747 --> 00:09:08,748 we find fossilized fishes 183 00:09:08,749 --> 00:09:11,183 and also fossilized wood, 184 00:09:11,184 --> 00:09:12,952 which didn't grow here, 185 00:09:12,953 --> 00:09:14,654 but it was transported in by the rivers. 186 00:09:14,655 --> 00:09:18,325 We also find lots of ripple marks. 187 00:09:19,626 --> 00:09:22,495 Fossil evidence suggests a watery past. 188 00:09:22,496 --> 00:09:24,030 But to fully understand 189 00:09:24,031 --> 00:09:26,332 what the Karoo of the therapsids' time looked like, 190 00:09:26,333 --> 00:09:29,702 we need to go back 300 million years, 191 00:09:29,703 --> 00:09:32,672 to a time when all of today's continents 192 00:09:32,673 --> 00:09:35,041 existed as one landmass: 193 00:09:35,042 --> 00:09:37,476 Pangaea. 194 00:09:37,477 --> 00:09:39,445 What was to become South Africa 195 00:09:39,446 --> 00:09:41,914 was much closer to the South Pole 196 00:09:41,915 --> 00:09:43,983 and under an ice cap. 197 00:09:43,984 --> 00:09:46,686 By 265 million years ago, 198 00:09:46,687 --> 00:09:49,021 the Earth was warming up. 199 00:09:49,022 --> 00:09:51,757 The ice that covered the Karoo had melted, 200 00:09:51,758 --> 00:09:54,360 forming an inland sea. 201 00:09:54,361 --> 00:09:56,329 Over the next few million years, 202 00:09:56,330 --> 00:09:59,332 the Karoo Sea filled with sediments. 203 00:09:59,333 --> 00:10:02,936 It nourished lush vegetation that grew in the new wetlands. 204 00:10:04,504 --> 00:10:06,038 This was the environment 205 00:10:06,039 --> 00:10:09,142 where therapsids were first preserved in the Karoo. 206 00:10:11,311 --> 00:10:12,845 The remains of the animals that we are finding there, 207 00:10:12,846 --> 00:10:14,246 the fossils, 208 00:10:14,247 --> 00:10:17,317 were once living on the banks of these rivers. 209 00:10:19,186 --> 00:10:22,555 But what was life like for these ancient therapsids? 210 00:10:22,556 --> 00:10:26,225 Were any mammal-like traits beginning to appear? 211 00:10:28,195 --> 00:10:30,196 One of the most exciting discoveries 212 00:10:30,197 --> 00:10:32,365 was made in the mid-1980s 213 00:10:32,366 --> 00:10:36,236 by South African fossil collector John Nyaphuli. 214 00:10:37,204 --> 00:10:40,272 John was my preparator 215 00:10:40,273 --> 00:10:43,275 at the National Museum in Bloemfontein, where I worked. 216 00:10:43,276 --> 00:10:46,312 {\an8}He and I spent many, many years 217 00:10:46,313 --> 00:10:47,780 {\an8}out living in a tent, 218 00:10:47,781 --> 00:10:49,315 {\an8}collecting fossils. 219 00:10:49,316 --> 00:10:51,484 {\an8}It's not easy to find fossil. 220 00:10:51,485 --> 00:10:53,085 {\an8}But your eyes must be trained. 221 00:10:53,086 --> 00:10:55,855 {\an8}You can't find fossil anywhere. 222 00:10:55,856 --> 00:10:58,090 {\an8}You must go to the right areas, 223 00:10:58,091 --> 00:11:01,127 {\an8}because geology must first tell you 224 00:11:01,128 --> 00:11:03,596 {\an8}that there was, there's good place to trace. 225 00:11:03,597 --> 00:11:06,233 {\an8}And you must love the thing, and then you can find them. 226 00:11:07,868 --> 00:11:11,237 {\an8} In 1984, he said to me, 227 00:11:11,238 --> 00:11:13,239 {\an8}"I found some big bone in the rock." 228 00:11:13,240 --> 00:11:16,742 {\an8}So I went to look at it, and it was big bone. 229 00:11:16,743 --> 00:11:18,111 {\an8}It was sort of this size. 230 00:11:19,146 --> 00:11:21,514 {\an8}John started opening it up, 231 00:11:21,515 --> 00:11:23,682 {\an8}and we found that by the middle of the day, 232 00:11:23,683 --> 00:11:25,918 we had a complete skull of an animal. 233 00:11:25,919 --> 00:11:27,319 There were some teeth sticking out. 234 00:11:27,320 --> 00:11:30,523 It went on into the ground, and for several months, 235 00:11:30,524 --> 00:11:32,425 we started excavating it. 236 00:11:35,996 --> 00:11:40,166 It took 23 years to expose that skeleton fully. 237 00:11:40,167 --> 00:11:42,635 But it was a completely new species of animal, 238 00:11:42,636 --> 00:11:44,403 it had never been found, 239 00:11:44,404 --> 00:11:46,907 and it was a very primitive dinocephalian. 240 00:11:48,442 --> 00:11:51,744 This is the most complete dinocephalian skeleton 241 00:11:51,745 --> 00:11:53,914 that's yet been found in the world. 242 00:11:56,583 --> 00:12:00,619 Dinocephalians were a group of early therapsids. 243 00:12:00,620 --> 00:12:04,423 This remarkable specimen was more than nine feet long. 244 00:12:04,424 --> 00:12:06,392 The name "dinocephalian" 245 00:12:06,393 --> 00:12:10,029 comes from the Greek for "terrible heads," 246 00:12:10,030 --> 00:12:12,698 because they had especially thick skulls-- 247 00:12:12,699 --> 00:12:17,203 in some areas, more than four inches of solid bone. 248 00:12:17,204 --> 00:12:21,240 But what was all that extra skull thickness used for? 249 00:12:25,045 --> 00:12:28,013 In the French Alps, scientists are looking for clues 250 00:12:28,014 --> 00:12:31,284 with a powerful X-ray source called a synchrotron. 251 00:12:32,686 --> 00:12:36,322 Here, in a ring larger than seven soccer fields, 252 00:12:36,323 --> 00:12:39,459 electrons circulate at almost the speed of light. 253 00:12:42,929 --> 00:12:44,830 VINCENT FERNANDEZ The synchrotron radiation is so intense 254 00:12:44,831 --> 00:12:48,467 that it produces 10,000 billion times more intense X-rays 255 00:12:48,468 --> 00:12:50,936 than a hospital or laboratory scanner. 256 00:12:50,937 --> 00:12:53,339 With this very intense signal, we're able to see 257 00:12:53,340 --> 00:12:54,874 through the thickness of the fossil 258 00:12:54,875 --> 00:12:56,976 and produce beautiful images. 259 00:13:00,180 --> 00:13:02,715 Paleontologist Vincent Fernandez 260 00:13:02,716 --> 00:13:04,984 is scanning another dinocephalian skull 261 00:13:04,985 --> 00:13:06,486 found in the Karoo. 262 00:13:08,421 --> 00:13:10,756 His colleague, paleontologist Julien Benoît, 263 00:13:10,757 --> 00:13:13,592 is doing the analysis. 264 00:13:13,593 --> 00:13:16,128 BENOÎT: Here you can see, that's the brain cavity. 265 00:13:16,129 --> 00:13:20,132 And it's very small compared to the large size of the skull, 266 00:13:20,133 --> 00:13:23,335 and in particular, you have that thickness of bone. 267 00:13:23,336 --> 00:13:25,070 {\an8}This is all bone. Wow. 268 00:13:25,071 --> 00:13:26,539 This is not rock. 269 00:13:26,540 --> 00:13:28,675 Why is the bone so thick here? 270 00:13:30,343 --> 00:13:32,912 Back at the University of the Witwatersrand, 271 00:13:32,913 --> 00:13:35,514 Julien uses the synchrotron scans 272 00:13:35,515 --> 00:13:37,983 to reconstruct a 3-D model 273 00:13:37,984 --> 00:13:40,253 of the organs that were once inside the skull. 274 00:13:42,055 --> 00:13:44,256 You can see the brain in green. 275 00:13:44,257 --> 00:13:45,824 What's really interesting about this scan 276 00:13:45,825 --> 00:13:48,427 is that we can also very clearly see 277 00:13:48,428 --> 00:13:50,729 this animal's inner ear. 278 00:13:50,730 --> 00:13:52,598 It's shown here in purple. 279 00:13:54,668 --> 00:13:55,968 This organ is used for hearing, 280 00:13:55,969 --> 00:13:58,337 but also for balance, 281 00:13:58,338 --> 00:14:01,941 {\an8}particularly these three semicircular canals. 282 00:14:01,942 --> 00:14:04,443 {\an8}One here, a second here, 283 00:14:04,444 --> 00:14:05,611 {\an8}and the third. 284 00:14:05,612 --> 00:14:09,081 {\an8}Let me move the image-- it's here. 285 00:14:09,082 --> 00:14:12,418 The orientation of these ear canals in the skull 286 00:14:12,419 --> 00:14:15,054 suggests that this dinocephalian's head 287 00:14:15,055 --> 00:14:16,722 would usually point down, 288 00:14:16,723 --> 00:14:20,359 a bit like that of a goat or a sheep today. 289 00:14:21,728 --> 00:14:23,529 And if mammals like these are any guide, 290 00:14:23,530 --> 00:14:26,932 that might mean that the dinocephalians also engaged 291 00:14:26,933 --> 00:14:29,002 in social behaviors like ramming. 292 00:14:37,577 --> 00:14:40,446 {\an8}As the sun rose on ancestral Earth, 293 00:14:40,447 --> 00:14:43,315 {\an8}dinocephalians awoke. 294 00:14:50,223 --> 00:14:53,459 {\an8}Living in a swampy landscape where food was abundant, 295 00:14:53,460 --> 00:14:56,195 {\an8}they thrived, 296 00:14:56,196 --> 00:14:58,097 {\an8}great herds of them perhaps living together 297 00:14:58,098 --> 00:14:59,398 {\an8}and interacting as a group. 298 00:15:01,668 --> 00:15:04,103 {\an8}With behaviors similar to some of today's mammals, 299 00:15:04,104 --> 00:15:06,538 {\an8}could these dinocephalians 300 00:15:06,539 --> 00:15:08,440 {\an8}have been the mammal ancestors? 301 00:15:14,247 --> 00:15:16,115 {\an8}Fossils found in the Karoo reveal 302 00:15:16,116 --> 00:15:18,484 {\an8}that 265 million years ago-- 303 00:15:18,485 --> 00:15:20,552 {\an8}nearly 40 million years before 304 00:15:20,553 --> 00:15:21,987 {\an8}the appearance of mammals-- 305 00:15:21,988 --> 00:15:23,589 {\an8}five therapsid groups 306 00:15:23,590 --> 00:15:25,457 {\an8}were living here. 307 00:15:25,458 --> 00:15:27,026 {\an8}And the dinocephalians 308 00:15:27,027 --> 00:15:29,595 {\an8}were likely the most dominant. 309 00:15:33,199 --> 00:15:36,902 This is the skull of Anteosaurus, 310 00:15:36,903 --> 00:15:40,739 a particularly menacing type of dinocephalian. 311 00:15:40,740 --> 00:15:42,574 BENOÎT: The top of the snout is there, 312 00:15:42,575 --> 00:15:44,376 and the back of the skull is there. 313 00:15:44,377 --> 00:15:45,611 And this was a therapsid, 314 00:15:45,612 --> 00:15:49,381 because you can see this temporal fenestra here. 315 00:15:49,382 --> 00:15:51,216 It's very big, 316 00:15:51,217 --> 00:15:54,053 so the jaw muscle must have been incredibly powerful. 317 00:15:54,054 --> 00:15:56,455 And it is a dinocephalian 318 00:15:56,456 --> 00:15:58,991 because of the thickness of the bones 319 00:15:58,992 --> 00:16:01,894 that make up the whole skull. 320 00:16:01,895 --> 00:16:03,295 But it's also very different 321 00:16:03,296 --> 00:16:06,465 from the other species of thick-skulled dinocephalians 322 00:16:06,466 --> 00:16:07,767 Julien studies. 323 00:16:09,369 --> 00:16:12,604 {\an8}BENOÎT: That canine here is beautifully recurved, 324 00:16:12,605 --> 00:16:15,074 {\an8}which is really a signature of carnivorous animals. 325 00:16:15,075 --> 00:16:18,877 These Anteosaurus were large creatures, 326 00:16:18,878 --> 00:16:20,179 likely predators, 327 00:16:20,180 --> 00:16:24,149 some 20 feet long when fully grown. 328 00:16:24,150 --> 00:16:25,984 But as successful as they might have been, 329 00:16:25,985 --> 00:16:29,055 the dinocephalians weren't the only therapsid group. 330 00:16:31,091 --> 00:16:34,360 They were just one part of a complex ecosystem 331 00:16:34,361 --> 00:16:37,062 that included other therapsid groups. 332 00:16:39,099 --> 00:16:41,500 Based on fossils found in the Karoo, 333 00:16:41,501 --> 00:16:43,502 we know that many species 334 00:16:43,503 --> 00:16:45,237 from all five groups of therapsids 335 00:16:45,238 --> 00:16:47,206 existed alongside each other. 336 00:16:50,310 --> 00:16:51,744 Like dicynodonts. 337 00:16:51,745 --> 00:16:53,612 Equipped with unique beaks, 338 00:16:53,613 --> 00:16:57,049 these therapsids had their own specialized feeding habits, 339 00:16:57,050 --> 00:17:01,020 able to efficiently tear and clip plant material. 340 00:17:04,791 --> 00:17:06,492 {\an8}Carnivorous therapsids, 341 00:17:06,493 --> 00:17:08,928 {\an8}such as gorgonopsians, would have been a threat. 342 00:17:14,601 --> 00:17:18,504 But the most fearsome was still the Anteosaurus. 343 00:17:18,505 --> 00:17:22,274 This dinocephalian beast was a true apex predator 344 00:17:22,275 --> 00:17:24,843 in the ancient Middle Permian Karoo. 345 00:17:31,317 --> 00:17:34,753 Therapsids lived here about 265 million 346 00:17:34,754 --> 00:17:38,157 to 125 million years ago. 347 00:17:38,158 --> 00:17:40,659 Within that time, they came up 348 00:17:40,660 --> 00:17:42,361 against more than one global catastrophe 349 00:17:42,362 --> 00:17:45,664 that threatened to wipe out nearly all life on Earth. 350 00:17:45,665 --> 00:17:49,168 And today, the Karoo still bears the scars. 351 00:17:53,673 --> 00:17:55,340 For more than 15 years, 352 00:17:55,341 --> 00:17:57,943 paleontologist Mike Day and his team have been 353 00:17:57,944 --> 00:18:01,281 accumulating evidence of these large-scale extinctions. 354 00:18:03,483 --> 00:18:04,683 {\an8} If you were to go a little further south, 355 00:18:04,684 --> 00:18:06,418 {\an8}into slightly older rocks, 356 00:18:06,419 --> 00:18:08,787 {\an8}you'd find all those typical Middle Permian faunas: 357 00:18:08,788 --> 00:18:10,722 the large dinocephalians, 358 00:18:10,723 --> 00:18:13,091 the big therocephalians, and lots of dicynodonts. 359 00:18:13,092 --> 00:18:16,895 But by the time you get to this sandstone behind us here, 360 00:18:16,896 --> 00:18:19,865 they've all gone extinct. 361 00:18:19,866 --> 00:18:22,534 Victims of a mass extinction event 362 00:18:22,535 --> 00:18:24,771 called the Permian Extinction. 363 00:18:26,973 --> 00:18:29,341 Fossil evidence suggests 364 00:18:29,342 --> 00:18:30,876 that for as long as eight million years, 365 00:18:30,877 --> 00:18:35,215 life on Earth was under tremendous pressure. 366 00:18:38,318 --> 00:18:40,652 And as more and more fossils have been found 367 00:18:40,653 --> 00:18:42,855 and plotted along the rock record, 368 00:18:42,856 --> 00:18:44,356 scientists now believe 369 00:18:44,357 --> 00:18:46,859 that within these eight million years, 370 00:18:46,860 --> 00:18:49,963 there were actually two distinct extinction events. 371 00:18:54,100 --> 00:18:55,734 Now, both of these mass extinctions 372 00:18:55,735 --> 00:18:57,903 affected therapsids badly, 373 00:18:57,904 --> 00:18:59,238 but there were survivors. 374 00:18:59,239 --> 00:19:01,940 And those survivors include the ancestors of mammals. 375 00:19:01,941 --> 00:19:05,310 But what caused the first of these huge events, 376 00:19:05,311 --> 00:19:09,882 a global catastrophe known as the Capitanian crisis? 377 00:19:09,883 --> 00:19:12,918 In terms of a smoking gun here in the Karoo, 378 00:19:12,919 --> 00:19:14,486 we haven't really found any good evidence 379 00:19:14,487 --> 00:19:16,321 for what caused this mass extinction here. 380 00:19:16,322 --> 00:19:18,390 We do know that in Southeast China, 381 00:19:18,391 --> 00:19:20,359 there was huge outpourings of lava 382 00:19:20,360 --> 00:19:21,960 at about the same time, 383 00:19:21,961 --> 00:19:24,930 so it's very likely that that had something to do 384 00:19:24,931 --> 00:19:26,865 with the Capitanian extinction here in the Karoo. 385 00:19:29,736 --> 00:19:32,204 Around 260 million years ago, 386 00:19:32,205 --> 00:19:34,239 far away from the Karoo, 387 00:19:34,240 --> 00:19:36,074 at the other end of Pangaea, 388 00:19:36,075 --> 00:19:39,678 Earth's surface was volatile. 389 00:19:39,679 --> 00:19:43,749 In what was to become China, immense molten lava flows 390 00:19:43,750 --> 00:19:47,419 spread across around 100,000 square miles, 391 00:19:47,420 --> 00:19:49,087 releasing vast quantities 392 00:19:49,088 --> 00:19:51,456 of carbon dioxide, methane, and sulfur 393 00:19:51,457 --> 00:19:52,725 into the atmosphere. 394 00:19:54,727 --> 00:19:57,963 The oceans became acidified and depleted of oxygen, 395 00:19:57,964 --> 00:20:00,265 and the climate changed. 396 00:20:00,266 --> 00:20:01,900 This is most likely the event 397 00:20:01,901 --> 00:20:04,470 that caused the Capitanian crisis. 398 00:20:08,942 --> 00:20:11,476 It was disastrous for life, 399 00:20:11,477 --> 00:20:13,979 particularly for large animals. 400 00:20:13,980 --> 00:20:15,447 But we know that at least 401 00:20:15,448 --> 00:20:16,982 some therapsids survived. 402 00:20:16,983 --> 00:20:19,451 Otherwise, we wouldn't be here. 403 00:20:19,452 --> 00:20:21,119 But which ones? 404 00:20:21,120 --> 00:20:22,655 And how? 405 00:20:25,525 --> 00:20:26,892 During the crisis, 406 00:20:26,893 --> 00:20:29,094 environmental disturbances like acid rain 407 00:20:29,095 --> 00:20:30,996 and a rapidly changing climate 408 00:20:30,997 --> 00:20:34,366 likely ravaged the vegetation, 409 00:20:34,367 --> 00:20:35,867 causing the extinction 410 00:20:35,868 --> 00:20:37,971 of the large plant-eating dinocephalians. 411 00:20:40,173 --> 00:20:41,406 Their carnivorous cousins 412 00:20:41,407 --> 00:20:43,442 eventually suffered the same fate. 413 00:20:44,644 --> 00:20:47,779 Earth's first giants, the dinocephalians, 414 00:20:47,780 --> 00:20:50,950 did not survive to become our direct ancestors. 415 00:20:52,719 --> 00:20:54,486 But what about the other four groups 416 00:20:54,487 --> 00:20:56,955 alive in the Karoo at this time, 417 00:20:56,956 --> 00:21:00,225 the gorgonopsians, with their huge teeth, 418 00:21:00,226 --> 00:21:02,494 the upright and agile therocephalians, 419 00:21:02,495 --> 00:21:05,530 the beaked dicynodonts, 420 00:21:05,531 --> 00:21:08,166 and the primitive biarmosuchians? 421 00:21:08,167 --> 00:21:10,203 Did the rest of them make it? 422 00:21:14,974 --> 00:21:18,177 The Karoo fossil record reveals the survivors. 423 00:21:20,580 --> 00:21:22,948 This is the Rubidge Collection of Fossils. 424 00:21:22,949 --> 00:21:25,183 It was amassed largely through the enthusiasm 425 00:21:25,184 --> 00:21:29,054 of my grandfather from the 1930s. 426 00:21:29,055 --> 00:21:31,890 These fossils were collected by Sidney Rubidge 427 00:21:31,891 --> 00:21:34,593 from sediment above a volcanic ash layer 428 00:21:34,594 --> 00:21:37,362 dated to the end of the Capitanian crisis. 429 00:21:37,363 --> 00:21:40,165 They show that in the Karoo, 430 00:21:40,166 --> 00:21:43,835 all therapsid groups other than the dinocephalians made it, 431 00:21:43,836 --> 00:21:47,139 but only some of the smallest species of each. 432 00:21:47,140 --> 00:21:50,043 The largest were lost. 433 00:21:51,611 --> 00:21:54,913 The question then becomes, which of these remaining groups 434 00:21:54,914 --> 00:21:57,582 would give rise to the mammal line? 435 00:21:57,583 --> 00:21:59,618 One of the most interesting fossils 436 00:21:59,619 --> 00:22:01,119 in the Rubidge Collection 437 00:22:01,120 --> 00:22:02,889 is a member of the gorgonopsian group. 438 00:22:04,724 --> 00:22:08,627 Gorgonopsians were the apex predators of the time. 439 00:22:08,628 --> 00:22:11,196 With the dinocephalians gone, 440 00:22:11,197 --> 00:22:13,565 they became top of the food chain, 441 00:22:13,566 --> 00:22:18,036 and the new species that evolved were much larger in size. 442 00:22:18,037 --> 00:22:19,972 And this one is called Rubidgea atrox. 443 00:22:21,240 --> 00:22:23,008 These large gorgonopsians 444 00:22:23,009 --> 00:22:25,077 might have been three-and-a-half meters long. 445 00:22:25,078 --> 00:22:27,612 They probably had a more leathery skin, 446 00:22:27,613 --> 00:22:29,314 something like a rhino or an elephant. 447 00:22:29,315 --> 00:22:33,919 They're characterized by having very large canine teeth 448 00:22:33,920 --> 00:22:36,923 {\an8}and large incisors for flesh eating. 449 00:22:38,257 --> 00:22:40,959 {\an8} Only small species of gorgonopsians 450 00:22:40,960 --> 00:22:43,729 survived the Capitanian crisis, 451 00:22:43,730 --> 00:22:45,764 but the Rubidge Collection shows 452 00:22:45,765 --> 00:22:48,600 that as the ecosystem recovered and food returned, 453 00:22:48,601 --> 00:22:53,872 these small gorgonopsians not only survived, but thrived. 454 00:22:53,873 --> 00:22:58,010 Over time, they evolved to be the size of modern-day bears. 455 00:22:59,445 --> 00:23:01,580 And in the fossil record, there is evidence 456 00:23:01,581 --> 00:23:04,349 that some new traits might have been developing. 457 00:23:07,453 --> 00:23:11,356 A gorgonopsian fossil originally found in the 1940s 458 00:23:11,357 --> 00:23:12,691 recently provided new clues 459 00:23:12,692 --> 00:23:14,761 to how they might have lived. 460 00:23:16,262 --> 00:23:18,597 {\an8}Inside this skull are the remains 461 00:23:18,598 --> 00:23:20,532 {\an8}of an encounter with another animal. 462 00:23:20,533 --> 00:23:23,835 BENOÎT Here is a tooth stuck in its snout, 463 00:23:23,836 --> 00:23:25,971 and this tooth isn't just any tooth, 464 00:23:25,972 --> 00:23:28,039 {\an8}it's the tooth of another gorgonopsian. 465 00:23:28,040 --> 00:23:29,241 {\an8}The same species. 466 00:23:29,242 --> 00:23:30,275 {\an8}And you can even see 467 00:23:30,276 --> 00:23:31,910 {\an8}that it survived the fight, 468 00:23:31,911 --> 00:23:33,279 {\an8}because the wound around the tooth has healed. 469 00:23:35,181 --> 00:23:36,648 For Julien, 470 00:23:36,649 --> 00:23:40,118 this is a sign of how these animals may have interacted. 471 00:23:40,119 --> 00:23:42,354 These animals fought among themselves, 472 00:23:42,355 --> 00:23:45,290 biting each other on the snout, 473 00:23:45,291 --> 00:23:46,725 exactly as mammals do today 474 00:23:46,726 --> 00:23:49,928 to determine dominance within groups 475 00:23:49,929 --> 00:23:52,298 and also to find mates and claim territories. 476 00:23:54,200 --> 00:23:55,700 Until this discovery, 477 00:23:55,701 --> 00:23:57,869 the teeth of carnivorous therapsids 478 00:23:57,870 --> 00:24:00,072 had been considered solely hunting weapons. 479 00:24:01,941 --> 00:24:04,976 And although not direct evidence of mammal ancestry, 480 00:24:04,977 --> 00:24:07,045 using them for more social interactions, 481 00:24:07,046 --> 00:24:08,847 like competition, 482 00:24:08,848 --> 00:24:10,982 is certainly a characteristic seen 483 00:24:10,983 --> 00:24:12,984 {\an8}in many mammals alive today. 484 00:24:14,187 --> 00:24:15,754 But the gorgonopsians 485 00:24:15,755 --> 00:24:18,957 weren't the only ones developing new behaviors. 486 00:24:18,958 --> 00:24:21,026 The dicynodonts were adapting 487 00:24:21,027 --> 00:24:23,162 in the post-extinction world, as well. 488 00:24:24,897 --> 00:24:28,033 Perhaps the most varied of the therapsids, 489 00:24:28,034 --> 00:24:29,968 they came in a wide range of sizes, 490 00:24:29,969 --> 00:24:32,405 some as small as guinea pigs. 491 00:24:35,041 --> 00:24:38,176 The only known therapsid group with beaks, 492 00:24:38,177 --> 00:24:40,378 some also had tusks. 493 00:24:42,548 --> 00:24:46,451 What was it about the dicynodont group that helped it survive? 494 00:24:51,090 --> 00:24:54,460 A fossil in South Africa might hold the answer. 495 00:25:01,200 --> 00:25:03,802 {\an8}Here we see a new behavior in the late Permian, 496 00:25:03,803 --> 00:25:06,671 {\an8}where animals were starting to burrow underground 497 00:25:06,672 --> 00:25:09,441 {\an8}and create these really interesting spirals 498 00:25:09,442 --> 00:25:11,543 with a chamber underneath, where they would live. 499 00:25:11,544 --> 00:25:14,480 We can see it here because it's actually preserved as a cast. 500 00:25:15,948 --> 00:25:18,149 This cast is natural. 501 00:25:18,150 --> 00:25:20,785 A flood poured sediment into a hollow burrow, 502 00:25:20,786 --> 00:25:22,854 which hardened into rock over time. 503 00:25:22,855 --> 00:25:25,490 But what were these burrows used for? 504 00:25:25,491 --> 00:25:27,860 How were they an advantage? 505 00:25:30,496 --> 00:25:32,764 {\an8}To see through this mass of rock, 506 00:25:32,765 --> 00:25:34,032 {\an8}a dicynodont burrow cast 507 00:25:34,033 --> 00:25:36,067 was sent to the synchrotron. 508 00:25:36,068 --> 00:25:38,203 And hidden inside, 509 00:25:38,204 --> 00:25:39,871 fossilized remains. 510 00:25:39,872 --> 00:25:41,773 This is the burrow that was discovered 511 00:25:41,774 --> 00:25:43,441 with the dicynodont. 512 00:25:43,442 --> 00:25:47,546 Removing the rock reveals a spectacular find. 513 00:25:48,614 --> 00:25:51,316 We have the adult dicynodont. 514 00:25:51,317 --> 00:25:53,385 We can see, like, 515 00:25:53,386 --> 00:25:56,321 right in the middle, this big eye socket 516 00:25:56,322 --> 00:25:57,555 and teeth in front. 517 00:25:57,556 --> 00:25:59,124 The beak would be there, as well. 518 00:25:59,125 --> 00:26:02,193 That's not all this ancient burrow was hiding. 519 00:26:02,194 --> 00:26:05,864 There was another, more unexpected find. 520 00:26:05,865 --> 00:26:07,198 It's like it's a, 521 00:26:07,199 --> 00:26:08,801 it's a baby version of the same species. 522 00:26:10,303 --> 00:26:12,370 So basically, it's telling us 523 00:26:12,371 --> 00:26:14,539 that dicynodont, they were using burrows 524 00:26:14,540 --> 00:26:17,475 kind of as nurseries to take care of their babies. 525 00:26:19,011 --> 00:26:20,378 {\an8} It appears that a semblance 526 00:26:20,379 --> 00:26:22,781 {\an8}of today's mammalian parent-child bond 527 00:26:22,782 --> 00:26:26,885 {\an8}may have existed 260 million years ago, 528 00:26:26,886 --> 00:26:29,889 but it's an idea that scientists are still exploring. 529 00:26:31,090 --> 00:26:32,757 What we do know is that these burrows 530 00:26:32,758 --> 00:26:38,263 allowed dicynodonts to survive tough post-crisis conditions. 531 00:26:38,264 --> 00:26:39,831 That kind of group sharing behavior 532 00:26:39,832 --> 00:26:42,434 has a lot of different advantages. 533 00:26:42,435 --> 00:26:44,569 If you're in a burrow, you're protected from predators. 534 00:26:44,570 --> 00:26:46,671 You're protected from the sun. 535 00:26:46,672 --> 00:26:48,807 It's, it's pretty good to be in a burrow 536 00:26:48,808 --> 00:26:50,810 if it's hot outside-- hot and dry. 537 00:26:52,278 --> 00:26:55,414 But how well did the other therapsid groups fare? 538 00:27:00,152 --> 00:27:02,120 {\an8}After the Capitanian crisis, 539 00:27:02,121 --> 00:27:05,290 {\an8}not all therapsid groups had the same fate. 540 00:27:05,291 --> 00:27:08,260 {\an8}The dinocephalians disappeared completely. 541 00:27:10,596 --> 00:27:13,399 {\an8}The gorgonopsians emerged as the apex predators. 542 00:27:16,502 --> 00:27:18,236 {\an8}As for the therocephalians, 543 00:27:18,237 --> 00:27:21,706 {\an8}they took second place to their larger cousins. 544 00:27:21,707 --> 00:27:23,541 The biarmosuchians 545 00:27:23,542 --> 00:27:24,676 were still holding on, 546 00:27:24,677 --> 00:27:27,345 but also remained the most primitive. 547 00:27:27,346 --> 00:27:29,080 It was the dicynodonts 548 00:27:29,081 --> 00:27:30,115 who adapted best 549 00:27:30,116 --> 00:27:31,549 to the changing environments. 550 00:27:31,550 --> 00:27:33,385 But as some groups struggled, 551 00:27:33,386 --> 00:27:35,554 the picture grew more complicated. 552 00:27:36,889 --> 00:27:38,456 A new group emerged 553 00:27:38,457 --> 00:27:40,458 in a post-crisis therapsid boom, 554 00:27:40,459 --> 00:27:43,361 taking the number of existing therapsid groups 555 00:27:43,362 --> 00:27:45,997 back up to five: 556 00:27:45,998 --> 00:27:47,166 the cynodonts. 557 00:27:48,768 --> 00:27:51,870 They shared a common ancestor with the therocephalians, 558 00:27:51,871 --> 00:27:53,371 and it's possible that they were 559 00:27:53,372 --> 00:27:55,975 actually just a sub-group of therocephalians themselves. 560 00:27:57,209 --> 00:27:59,010 {\an8}But one thing is certain: 561 00:27:59,011 --> 00:28:00,311 {\an8}one of the secrets 562 00:28:00,312 --> 00:28:01,646 {\an8}to their success 563 00:28:01,647 --> 00:28:03,015 {\an8}was their size. 564 00:28:04,483 --> 00:28:06,885 {\an8}Cynodonts generally were quite small. 565 00:28:06,886 --> 00:28:09,255 {\an8}You could hold most of them in your arms. 566 00:28:11,757 --> 00:28:14,092 {\an8} They also had relatively larger brains 567 00:28:14,093 --> 00:28:15,361 {\an8}and more specialized teeth. 568 00:28:17,263 --> 00:28:19,664 {\an8}With the Karoo ecosystem having recovered 569 00:28:19,665 --> 00:28:21,032 {\an8}from the Capitanian crisis, 570 00:28:21,033 --> 00:28:23,736 {\an8}the five groups of therapsids stabilized. 571 00:28:25,371 --> 00:28:29,508 {\an8}Cynodonts thrived, feeding off insects and other small animals. 572 00:28:30,476 --> 00:28:33,778 {\an8}Meanwhile, dicynodonts continued with their plant-eating diets 573 00:28:33,779 --> 00:28:37,348 {\an8}and still used burrows as their homes. 574 00:28:37,349 --> 00:28:39,885 {\an8}But this wasn't always an advantage. 575 00:28:41,620 --> 00:28:45,190 Dicynodonts were also the prey of choice for therocephalians. 576 00:28:45,191 --> 00:28:48,326 These larger predators were perhaps able to hunt down 577 00:28:48,327 --> 00:28:50,562 entire dicynodont families. 578 00:28:50,563 --> 00:28:53,932 Even so, gorgonopsians remained 579 00:28:53,933 --> 00:28:55,500 the apex predators. 580 00:28:56,836 --> 00:28:58,403 But, at the end of the Permian period, 581 00:28:58,404 --> 00:29:00,572 another crisis. 582 00:29:00,573 --> 00:29:04,375 In the rock layers of the Karoo dating to the period 583 00:29:04,376 --> 00:29:06,544 after 252 million years ago, 584 00:29:06,545 --> 00:29:10,281 most big dicynodonts and gorgonopsians disappear, 585 00:29:10,282 --> 00:29:13,384 signaling another extinction crisis. 586 00:29:15,654 --> 00:29:18,490 Was it something local to the Karoo 587 00:29:18,491 --> 00:29:20,359 or more global? 588 00:29:21,727 --> 00:29:23,495 Since the 1990s, 589 00:29:23,496 --> 00:29:26,197 scientists have suspected that the Siberian Traps, 590 00:29:26,198 --> 00:29:29,968 enormous volcanic eruptions in what is today's Russia, 591 00:29:29,969 --> 00:29:32,204 were responsible. 592 00:29:33,839 --> 00:29:36,808 In 2023, an international team of scientists 593 00:29:36,809 --> 00:29:39,078 published a project testing the idea. 594 00:29:41,514 --> 00:29:44,482 When scientists studied the rocks in Siberia, 595 00:29:44,483 --> 00:29:45,783 they found that there was 596 00:29:45,784 --> 00:29:47,118 an increase in mercury 597 00:29:47,119 --> 00:29:48,820 in the rocks that they sampled. 598 00:29:48,821 --> 00:29:50,555 So there was a mercury spike, 599 00:29:50,556 --> 00:29:52,357 and they found this associated 600 00:29:52,358 --> 00:29:54,226 with those volcanic eruptions. 601 00:29:56,028 --> 00:29:59,731 Mercury is often associated with volcanoes. 602 00:29:59,732 --> 00:30:02,400 As a volcano erupts, spewing out lava, 603 00:30:02,401 --> 00:30:06,371 it also releases mercury vapor into the atmosphere. 604 00:30:06,372 --> 00:30:09,107 This vapor can be transported across continents 605 00:30:09,108 --> 00:30:10,742 by currents and winds. 606 00:30:14,313 --> 00:30:15,847 But we did not know 607 00:30:15,848 --> 00:30:19,151 whether we had such a spike in the Karoo Basin of South Africa. 608 00:30:20,753 --> 00:30:23,421 If found, that spike could be evidence 609 00:30:23,422 --> 00:30:25,356 that these Siberian eruptions 610 00:30:25,357 --> 00:30:26,891 {\an8}were responsible for the climate changes 611 00:30:26,892 --> 00:30:30,361 {\an8}that caused the extinction event seen in the Karoo. 612 00:30:30,362 --> 00:30:33,998 So our team went to the field and we collected samples, 613 00:30:33,999 --> 00:30:35,867 and we ran the analyses, 614 00:30:35,868 --> 00:30:37,402 and we found that indeed, 615 00:30:37,403 --> 00:30:40,439 we are finding a mercury spike in South Africa. 616 00:30:42,608 --> 00:30:45,076 The therapsid extinctions in the Karoo 617 00:30:45,077 --> 00:30:47,245 252 million years ago 618 00:30:47,246 --> 00:30:49,815 were indeed part of a global crisis. 619 00:30:51,450 --> 00:30:53,818 In the northeastern corner of Pangaea, 620 00:30:53,819 --> 00:30:55,687 in what is now Siberia, 621 00:30:55,688 --> 00:30:57,922 one of the biggest volcanic eruptions 622 00:30:57,923 --> 00:30:59,757 life on Earth has ever seen 623 00:30:59,758 --> 00:31:00,959 began. 624 00:31:04,663 --> 00:31:08,433 The scale of this event would dwarf the Capitanian crisis. 625 00:31:08,434 --> 00:31:12,670 The lava spread across nearly three million square miles, 626 00:31:12,671 --> 00:31:15,908 an area almost as large as the U.S. Lower 48. 627 00:31:17,977 --> 00:31:22,047 The eruptions lasted for at least 200,000 years. 628 00:31:23,682 --> 00:31:26,251 Consequences for life were terrible. 629 00:31:26,252 --> 00:31:30,555 On land, around 70% of species disappeared. 630 00:31:30,556 --> 00:31:33,258 In the seas, the toll was even heavier. 631 00:31:33,259 --> 00:31:36,127 Changes in acidity and oxygen levels 632 00:31:36,128 --> 00:31:39,998 wiped out as many as 95% of species. 633 00:31:42,368 --> 00:31:45,203 The greenhouse effect also went into overdrive, 634 00:31:45,204 --> 00:31:48,340 causing temperatures to rise dramatically. 635 00:31:49,441 --> 00:31:53,244 Paleontologists have dubbed this event "the Great Dying," 636 00:31:53,245 --> 00:31:55,848 and it marked the end of the Permian period. 637 00:31:57,583 --> 00:31:59,951 This was the closest life has ever come 638 00:31:59,952 --> 00:32:01,519 to completely dying out. 639 00:32:01,520 --> 00:32:03,421 Our ancestors, 640 00:32:03,422 --> 00:32:07,492 they were there when it happened and they made it through. 641 00:32:07,493 --> 00:32:11,663 And if they didn't, then we wouldn't be here today. 642 00:32:11,664 --> 00:32:16,768 {\an8} This crisis heralded the start of a new geological era. 643 00:32:16,769 --> 00:32:19,637 {\an8}It transformed the planet's climate and landscape, 644 00:32:19,638 --> 00:32:22,073 {\an8}including in the Karoo. 645 00:32:22,074 --> 00:32:24,242 {\an8}But what was it like to be there? 646 00:32:24,243 --> 00:32:26,144 {\an8}And what did this mean for the many therapsids 647 00:32:26,145 --> 00:32:29,081 {\an8}that called this part of the world home? 648 00:32:33,185 --> 00:32:35,253 At the Iziko South African Museum, 649 00:32:35,254 --> 00:32:39,824 a dicynodont fossil sheds light on the Karoo's climate. 650 00:32:39,825 --> 00:32:41,726 So here we have the skull of the animal. 651 00:32:41,727 --> 00:32:43,194 The head. 652 00:32:43,195 --> 00:32:46,197 There's the outline of one of its eyes, front of its nose. 653 00:32:46,198 --> 00:32:48,833 And then coming around, we have the front foot. 654 00:32:48,834 --> 00:32:51,302 The fingers are missing. 655 00:32:51,303 --> 00:32:52,670 Then, if you follow it along, 656 00:32:52,671 --> 00:32:53,738 here's the beautifully articulated 657 00:32:53,739 --> 00:32:56,174 complete spine of the animal. 658 00:32:56,175 --> 00:32:58,309 And the animal's kind of spread-eagle. 659 00:32:58,310 --> 00:33:01,346 This unusual position can tell us 660 00:33:01,347 --> 00:33:03,081 how this dicynodont died. 661 00:33:03,082 --> 00:33:04,615 One of the ways in which 662 00:33:04,616 --> 00:33:06,517 an animal dies of exhaustion 663 00:33:06,518 --> 00:33:09,287 is actually by collapsing, 664 00:33:09,288 --> 00:33:10,988 and collapsing with its feet spread out 665 00:33:10,989 --> 00:33:14,125 in this very distinctive spread-eagle pose. 666 00:33:14,126 --> 00:33:17,929 In the Triassic Karoo, therapsids struggled. 667 00:33:17,930 --> 00:33:21,999 Hunger, thirst, and exhaustion were all common threats. 668 00:33:22,000 --> 00:33:23,801 What adaptations made it possible 669 00:33:23,802 --> 00:33:25,137 for any of them to survive? 670 00:33:31,243 --> 00:33:33,478 In Bethulie, east of the Karoo, 671 00:33:33,479 --> 00:33:37,648 the remains of a species of dicynodont known as Lystrosaurus 672 00:33:37,649 --> 00:33:39,618 have given us clues. 673 00:33:40,819 --> 00:33:42,620 This is an area that I like to call 674 00:33:42,621 --> 00:33:44,489 the Lystrosaurus killing fields. 675 00:33:44,490 --> 00:33:47,892 I've been able to map the actual embedded skeletons 676 00:33:47,893 --> 00:33:50,395 throughout this entire area. 677 00:33:50,396 --> 00:33:52,630 And I have 600 in situ skeletons already. 678 00:33:52,631 --> 00:33:54,699 So originally, there must have been 679 00:33:54,700 --> 00:33:56,135 literally many thousands. 680 00:33:58,404 --> 00:34:01,939 {\an8} These dicynodonts represent 95% of the therapsid fossils 681 00:34:01,940 --> 00:34:05,777 {\an8}discovered in this area from this time period. 682 00:34:05,778 --> 00:34:07,413 {\an8}But why were there so many here? 683 00:34:10,749 --> 00:34:14,085 Paleontologist Jennifer Botha has discovered a clue hidden 684 00:34:14,086 --> 00:34:16,422 in the microstructure of their bones. 685 00:34:18,891 --> 00:34:20,291 What you are looking at here 686 00:34:20,292 --> 00:34:22,894 is a cross-section through the bone. 687 00:34:22,895 --> 00:34:26,097 The organic components of the bone have disappeared, 688 00:34:26,098 --> 00:34:28,299 but the position of all the bone fibers 689 00:34:28,300 --> 00:34:30,201 have remained in place. 690 00:34:30,202 --> 00:34:33,304 So we can tell certain things about how the animal grew. 691 00:34:33,305 --> 00:34:35,373 I can see these lines 692 00:34:35,374 --> 00:34:38,142 running down through here through the bone. 693 00:34:38,143 --> 00:34:41,345 And they represent yearly growth marks. 694 00:34:41,346 --> 00:34:43,781 So if I count the number of growth marks, 695 00:34:43,782 --> 00:34:46,651 I can tell how old the animal was when it died. 696 00:34:46,652 --> 00:34:50,455 But how can the age at death shed light 697 00:34:50,456 --> 00:34:52,423 on why Lystrosaurus was so prolific in the Karoo 698 00:34:52,424 --> 00:34:55,693 during the Triassic after the Permian crisis? 699 00:34:59,298 --> 00:35:02,066 Here I have a Lystrosaurus species from the Permian 700 00:35:02,067 --> 00:35:03,234 on the left 701 00:35:03,235 --> 00:35:05,903 and a Lystrosaurus species from the Triassic 702 00:35:05,904 --> 00:35:07,972 on the right. 703 00:35:07,973 --> 00:35:10,541 And highlighted in green are the number of growth marks 704 00:35:10,542 --> 00:35:12,343 I have been able to count. 705 00:35:12,344 --> 00:35:14,245 So I can tell that this animal 706 00:35:14,246 --> 00:35:16,914 was at least eight years old when it died. 707 00:35:16,915 --> 00:35:20,718 In comparison, this Triassic specimen 708 00:35:20,719 --> 00:35:24,355 was only at least two years old when it died. 709 00:35:24,356 --> 00:35:26,457 And very importantly, 710 00:35:26,458 --> 00:35:30,461 this is a large Lystrosaurus Permian species, 711 00:35:30,462 --> 00:35:33,297 and this is the largest known Triassic specimen. 712 00:35:33,298 --> 00:35:35,800 So even the largest Triassic specimens 713 00:35:35,801 --> 00:35:37,502 that we have ever discovered 714 00:35:37,503 --> 00:35:38,936 are not fully grown. 715 00:35:38,937 --> 00:35:41,772 They are at most two or three years old. 716 00:35:43,609 --> 00:35:47,011 After the crisis, Lystrosauruses were dying young. 717 00:35:47,012 --> 00:35:49,981 So what explains their large population? 718 00:35:49,982 --> 00:35:51,449 In order to be so abundant, 719 00:35:51,450 --> 00:35:54,252 they had to be breeding quite abundantly. 720 00:35:54,253 --> 00:35:58,523 This suggests that the birth rate increased rapidly. 721 00:36:04,563 --> 00:36:05,630 During periods of drought, 722 00:36:05,631 --> 00:36:08,267 times were tough for all the animals here. 723 00:36:10,569 --> 00:36:13,371 {\an8}Predators, who depended on the herbivores for food, 724 00:36:13,372 --> 00:36:14,840 {\an8}went hungry. 725 00:36:17,042 --> 00:36:18,609 {\an8}The carnivorous gorgonopsians 726 00:36:18,610 --> 00:36:21,647 {\an8}succumbed to the hostile conditions and vanished. 727 00:36:23,749 --> 00:36:25,884 {\an8}And so did the biarmosuchians. 728 00:36:30,856 --> 00:36:33,491 {\an8}The dicynodonts were dying young, 729 00:36:33,492 --> 00:36:37,295 {\an8}but their rapid reproduction rates and their burrowing habits 730 00:36:37,296 --> 00:36:39,631 {\an8}helped them survive this period's climate chaos. 731 00:36:43,201 --> 00:36:44,869 {\an8}The therocephalians initially made it, 732 00:36:44,870 --> 00:36:48,941 {\an8}but as time went on, they, too, eventually died out. 733 00:36:50,142 --> 00:36:53,712 {\an8}The cynodonts survived this tumultuous period, barely. 734 00:36:54,713 --> 00:36:58,015 Only two groups remained: 735 00:36:58,016 --> 00:37:00,084 the dicynodonts 736 00:37:00,085 --> 00:37:01,653 and the cynodonts. 737 00:37:07,059 --> 00:37:10,629 The fossil record shows just how successful the dicynodonts were. 738 00:37:12,164 --> 00:37:14,932 {\an8} This is what this animal looked like. 739 00:37:14,933 --> 00:37:17,301 {\an8}It was about 50 to 70 centimeters long, 740 00:37:17,302 --> 00:37:18,937 {\an8}so about the size of a small pig. 741 00:37:21,340 --> 00:37:23,774 {\an8}Dicynodonts literally swarmed 742 00:37:23,775 --> 00:37:26,711 {\an8}during the first five million years of the Triassic period. 743 00:37:26,712 --> 00:37:28,846 {\an8}So much so that they rediversified 744 00:37:28,847 --> 00:37:31,415 {\an8}and gave rise to large herbivores, 745 00:37:31,416 --> 00:37:34,452 as illustrated here by this skull, 746 00:37:34,453 --> 00:37:36,520 which belonged to an animal that must have been roughly 747 00:37:36,521 --> 00:37:40,491 the size of a modern wildebeest, weighing several hundred kilos. 748 00:37:42,494 --> 00:37:45,997 And these giant dicynodonts repopulated the world. 749 00:37:45,998 --> 00:37:47,732 They spread across the whole of Pangaea. 750 00:37:49,968 --> 00:37:52,169 Fossil remains of dicynodonts have been found 751 00:37:52,170 --> 00:37:53,371 on every continent. 752 00:37:53,372 --> 00:37:57,141 But the dicynodonts weren't the only group 753 00:37:57,142 --> 00:37:59,176 that found a way to adapt to their new environment. 754 00:37:59,177 --> 00:38:01,946 Paleontologist Roger Smith has found a fossil 755 00:38:01,947 --> 00:38:05,282 that reveals that some of the cynodonts 756 00:38:05,283 --> 00:38:07,251 had discovered some of the same tricks. 757 00:38:10,622 --> 00:38:13,891 Here we have an underground burrow cast 758 00:38:13,892 --> 00:38:16,861 made into the ancient floodplain by a cynodont. 759 00:38:16,862 --> 00:38:20,765 The architecture of the cynodont burrow 760 00:38:20,766 --> 00:38:23,534 is a tube going into the ground, and when it gets to the end, 761 00:38:23,535 --> 00:38:26,270 it forms this round terminal chamber. 762 00:38:26,271 --> 00:38:28,305 This doesn't have a skeleton in it. 763 00:38:28,306 --> 00:38:29,640 If it did have a skeleton-- 764 00:38:29,641 --> 00:38:31,409 and we do have skeletons in these-- 765 00:38:31,410 --> 00:38:33,044 they're curled up generally 766 00:38:33,045 --> 00:38:34,845 with their back to the outside, 767 00:38:34,846 --> 00:38:39,150 with the tail curled around against the wall. 768 00:38:39,151 --> 00:38:42,153 This burrow cast could well be 769 00:38:42,154 --> 00:38:46,524 a behavioral adaptation of the cynodonts to go underground 770 00:38:46,525 --> 00:38:48,826 to escape the extremes of heat during the day and night, 771 00:38:48,827 --> 00:38:51,696 and perhaps to even breed. 772 00:38:51,697 --> 00:38:53,898 {\an8}Being able to burrow, 773 00:38:53,899 --> 00:38:56,233 {\an8}that was a major survival strategy. 774 00:38:56,234 --> 00:38:57,635 Being able to hide away. 775 00:38:57,636 --> 00:38:59,170 And that is probably one of the things 776 00:38:59,171 --> 00:39:02,907 that enabled our ancestors to endure. 777 00:39:02,908 --> 00:39:06,410 The same adaptations that helped the dicynodonts 778 00:39:06,411 --> 00:39:09,914 emerge from the first crisis eight million years before 779 00:39:09,915 --> 00:39:13,017 also helped the cynodonts survive. 780 00:39:16,154 --> 00:39:19,590 As the ecosystem recovered from the crisis, 781 00:39:19,591 --> 00:39:22,326 the cynodonts led a relatively stable existence. 782 00:39:25,063 --> 00:39:28,065 But according to the fossil record, 783 00:39:28,066 --> 00:39:30,367 the dicynodonts really thrived. 784 00:39:33,038 --> 00:39:35,539 Freed from the gorgonopsian threat, 785 00:39:35,540 --> 00:39:38,909 they took full advantage of available ecological niches 786 00:39:38,910 --> 00:39:40,879 and grew many times larger. 787 00:39:44,082 --> 00:39:45,449 Meanwhile, the therapsids 788 00:39:45,450 --> 00:39:46,951 weren't the only creatures doing well. 789 00:39:46,952 --> 00:39:50,021 {\an8}Others were starting to appear. 790 00:39:53,225 --> 00:39:55,259 {\an8} The top predators of the late Triassic 791 00:39:55,260 --> 00:39:57,027 are things called pseudosuchians. 792 00:39:57,028 --> 00:39:59,196 They're sort of the cousins of dinosaurs. 793 00:39:59,197 --> 00:40:01,532 They look a bit like modern crocs. 794 00:40:01,533 --> 00:40:03,368 They're covered in body armor along their back. 795 00:40:05,070 --> 00:40:07,872 {\an8} Soon to be found in almost every corner of the globe, 796 00:40:07,873 --> 00:40:09,173 {\an8}they are evidence 797 00:40:09,174 --> 00:40:12,143 {\an8}of yet another planet-wide environmental change: 798 00:40:12,144 --> 00:40:13,978 {\an8}one that would greatly impact 799 00:40:13,979 --> 00:40:16,615 {\an8}the therapsids and their mammal descendants. 800 00:40:19,918 --> 00:40:22,553 232 million-year-old plant fossils 801 00:40:22,554 --> 00:40:25,089 show new flora was developing: 802 00:40:25,090 --> 00:40:26,724 the type of plants that only exist 803 00:40:26,725 --> 00:40:28,426 in extremely rainy climates. 804 00:40:30,662 --> 00:40:33,397 Heavy rainfall was a planet-wide phenomenon, 805 00:40:33,398 --> 00:40:35,966 and paleontologists have dubbed this period 806 00:40:35,967 --> 00:40:38,536 "the Carnian pluvial episode." 807 00:40:38,537 --> 00:40:40,137 Over about two million years, 808 00:40:40,138 --> 00:40:43,007 the climate changed from extreme to extreme: 809 00:40:43,008 --> 00:40:44,642 cold to hot, 810 00:40:44,643 --> 00:40:48,112 dry to wet. 811 00:40:48,113 --> 00:40:51,682 In the Karoo, the desert turned green. 812 00:40:51,683 --> 00:40:56,487 But only one therapsid lineage would survive this time. 813 00:40:56,488 --> 00:40:59,690 Would it be the thriving and vastly varied dicynodonts 814 00:40:59,691 --> 00:41:03,527 or the newly emerged cynodonts? 815 00:41:03,528 --> 00:41:07,031 Which would give rise to all of mammal-kind? 816 00:41:07,032 --> 00:41:09,333 To us? 817 00:41:09,334 --> 00:41:12,570 The remaining therapsids had to contend 818 00:41:12,571 --> 00:41:14,873 with new creatures dominating the Karoo. 819 00:41:15,974 --> 00:41:18,008 {\an8}Dinosaurs, like the ones in front of me, 820 00:41:18,009 --> 00:41:20,010 soon become the most dominant large-bodied animals 821 00:41:20,011 --> 00:41:21,312 on the landscape. 822 00:41:21,313 --> 00:41:23,080 The therapsids and the other animals 823 00:41:23,081 --> 00:41:24,615 that were once so common on landscapes 824 00:41:24,616 --> 00:41:26,617 become much more rare. 825 00:41:26,618 --> 00:41:28,986 And instead of seeing therapsids eating plants, 826 00:41:28,987 --> 00:41:31,255 we'd be seeing dinosaurs like this one 827 00:41:31,256 --> 00:41:34,158 browsing on the vegetation. 828 00:41:34,159 --> 00:41:36,060 It was also a time when new characteristics 829 00:41:36,061 --> 00:41:37,795 that would prove helpful for survival 830 00:41:37,796 --> 00:41:39,964 were appearing. 831 00:41:39,965 --> 00:41:43,133 And one in particular, a very mammal-like quality, 832 00:41:43,134 --> 00:41:47,037 may have been the biggest advantage of all: 833 00:41:47,038 --> 00:41:50,040 a metabolism that kept the body warm. 834 00:41:50,041 --> 00:41:53,644 Being warm-blooded is really important. 835 00:41:53,645 --> 00:41:57,281 It means that we can go outside when it's wintertime. 836 00:41:57,282 --> 00:42:00,084 It means that we can be active in the morning 837 00:42:00,085 --> 00:42:01,852 or when the sun's not out. 838 00:42:01,853 --> 00:42:03,587 You're not like a cold-blooded animal 839 00:42:03,588 --> 00:42:05,522 that's at the mercy of the sun. 840 00:42:05,523 --> 00:42:07,391 And that's really unusual. 841 00:42:07,392 --> 00:42:08,759 It's a superpower. 842 00:42:08,760 --> 00:42:12,763 Modern-day mammals are warm-blooded. 843 00:42:12,764 --> 00:42:14,298 When did this trait emerge? 844 00:42:14,299 --> 00:42:16,667 And how? 845 00:42:16,668 --> 00:42:18,302 It is really challenging 846 00:42:18,303 --> 00:42:20,771 to figure out if a fossil species 847 00:42:20,772 --> 00:42:22,139 was warm-blooded or cold-blooded. 848 00:42:22,140 --> 00:42:25,009 You can't just stick a thermometer into them 849 00:42:25,010 --> 00:42:26,477 and see how their body temperature changes 850 00:42:26,478 --> 00:42:27,745 over the day. 851 00:42:27,746 --> 00:42:29,480 So you have to look for clues in the fossils. 852 00:42:29,481 --> 00:42:32,149 There's one very interesting clue, 853 00:42:32,150 --> 00:42:34,752 this opening in the top of the head 854 00:42:34,753 --> 00:42:36,787 for what's called a third eye. 855 00:42:36,788 --> 00:42:38,989 It's, it's a bunch of cells that sense light. 856 00:42:38,990 --> 00:42:41,358 It's thought that the third eye 857 00:42:41,359 --> 00:42:43,360 helped ancient cold-blooded therapsids 858 00:42:43,361 --> 00:42:46,830 regulate their daily activities. 859 00:42:46,831 --> 00:42:49,533 In modern species, the third eye only exists 860 00:42:49,534 --> 00:42:52,369 among cold-blooded animals. 861 00:42:52,370 --> 00:42:54,705 Therefore, the disappearance of the third eye may be linked 862 00:42:54,706 --> 00:42:57,875 to the appearance of warm-blooded metabolism. 863 00:42:57,876 --> 00:42:59,910 My colleagues and I decided to look 864 00:42:59,911 --> 00:43:03,747 for when the therapsids lost this third eye. 865 00:43:06,151 --> 00:43:08,585 Julien and his team have studied fossils 866 00:43:08,586 --> 00:43:10,955 from across the reign of the therapsids, 867 00:43:10,956 --> 00:43:14,892 all the way back to almost 270 million years ago, 868 00:43:14,893 --> 00:43:18,862 in the Permian period, before the Great Dying. 869 00:43:18,863 --> 00:43:22,367 In the Permian, almost all therapsids had a third eye. 870 00:43:24,202 --> 00:43:26,971 Then, when we enter the Triassic, 871 00:43:26,972 --> 00:43:31,075 the first Triassic species still had a third eye. 872 00:43:31,076 --> 00:43:32,409 But a little later, 873 00:43:32,410 --> 00:43:34,745 we begin to encounter species that no longer have 874 00:43:34,746 --> 00:43:36,780 a hole at the top of their skull. 875 00:43:36,781 --> 00:43:40,184 They no longer had a third eye. 876 00:43:40,185 --> 00:43:43,253 By 240 million years ago, the third eye 877 00:43:43,254 --> 00:43:45,589 had all but vanished from the fossil record 878 00:43:45,590 --> 00:43:49,593 of one therapsid group, the cynodonts. 879 00:43:49,594 --> 00:43:52,463 This isn't definitive proof of warm blood, 880 00:43:52,464 --> 00:43:56,367 but scientists have found another clue. 881 00:43:56,368 --> 00:43:59,804 BENOÎT The most decisive argument was the study of the inner ear. 882 00:44:01,106 --> 00:44:04,508 The warmer the internal body temperature of an animal, 883 00:44:04,509 --> 00:44:06,877 {\an8}the runnier their ear canal fluid becomes 884 00:44:06,878 --> 00:44:09,980 {\an8}and the thinner the tubes can be. 885 00:44:09,981 --> 00:44:12,016 We were able to measure changes 886 00:44:12,017 --> 00:44:13,951 in the size of the canals in cynodonts 887 00:44:13,952 --> 00:44:16,487 and determine precisely that the body temperature 888 00:44:16,488 --> 00:44:18,322 {\an8}had increased by ten degrees 889 00:44:18,323 --> 00:44:19,323 {\an8}exactly here, 890 00:44:19,324 --> 00:44:21,025 {\an8}between this Triassic animal 891 00:44:21,026 --> 00:44:22,026 {\an8}and this animal 892 00:44:22,027 --> 00:44:25,430 {\an8}233 million years ago. 893 00:44:27,665 --> 00:44:29,800 Evidence suggests that the cynodonts 894 00:44:29,801 --> 00:44:33,670 were warm-blooded during the Carnian pluvial episode. 895 00:44:33,671 --> 00:44:36,040 There was a time of, of fairly rapid change 896 00:44:36,041 --> 00:44:38,342 in climate and in precipitation, 897 00:44:38,343 --> 00:44:39,543 and being warm-blooded 898 00:44:39,544 --> 00:44:41,712 would have been beneficial at that time, 899 00:44:41,713 --> 00:44:44,982 because if you can control your own body temperature, 900 00:44:44,983 --> 00:44:47,918 that can protect you from the whims of the weather. 901 00:44:47,919 --> 00:44:50,888 What did this mean for therapsids 902 00:44:50,889 --> 00:44:52,890 that were not warm-blooded? 903 00:44:52,891 --> 00:44:55,392 This Carnian rainfall event had 904 00:44:55,393 --> 00:44:56,828 a huge impact on the therapsids. 905 00:44:59,030 --> 00:45:01,198 {\an8} By about 200 million years ago, 906 00:45:01,199 --> 00:45:04,201 {\an8}the dicynodonts disappear from the fossil record. 907 00:45:06,971 --> 00:45:09,006 There was one small group that came out on top, 908 00:45:09,007 --> 00:45:10,374 the cynodonts. 909 00:45:10,375 --> 00:45:13,444 And this group is the one that gave rise to mammals. 910 00:45:15,914 --> 00:45:18,582 It was the small size of these cynodonts, 911 00:45:18,583 --> 00:45:22,086 their ability to dig burrows, being able to grow fast, 912 00:45:22,087 --> 00:45:23,854 being able to eat lots of different foods. 913 00:45:23,855 --> 00:45:25,355 That's what helped make them through. 914 00:45:25,356 --> 00:45:27,558 And then on the other side, they would have found 915 00:45:27,559 --> 00:45:29,426 this wide-open world, 916 00:45:29,427 --> 00:45:31,929 a land of new frontiers and opportunities. 917 00:45:33,631 --> 00:45:36,366 230 million years ago, the cynodonts were just starting 918 00:45:36,367 --> 00:45:39,336 to adapt to this new world. 919 00:45:39,337 --> 00:45:42,206 How did they give rise to mammals 920 00:45:42,207 --> 00:45:43,740 just five million years later? 921 00:45:43,741 --> 00:45:46,176 {\an8}When did the other key mammal traits, 922 00:45:46,177 --> 00:45:48,345 {\an8}lactation and hair, 923 00:45:48,346 --> 00:45:50,348 {\an8}appear in their evolution? 924 00:45:52,617 --> 00:45:54,384 Hair is the insulating layer 925 00:45:54,385 --> 00:45:56,887 that allows mammals to retain heat. 926 00:45:58,389 --> 00:46:01,391 The rarity of hair or fur in the fossil record 927 00:46:01,392 --> 00:46:03,093 has made it difficult for scientists to find exactly 928 00:46:03,094 --> 00:46:07,064 when this mammal trait evolved. 929 00:46:07,065 --> 00:46:09,900 But there is one type of hair that leaves a mark: 930 00:46:09,901 --> 00:46:12,136 whiskers. 931 00:46:12,137 --> 00:46:14,404 And although mammals are not the only animals 932 00:46:14,405 --> 00:46:15,606 with whisker-like features today, 933 00:46:15,607 --> 00:46:20,377 they're a sure sign that hair has evolved. 934 00:46:20,378 --> 00:46:21,545 We can track the evolution of whiskers 935 00:46:21,546 --> 00:46:24,181 thanks to traces left behind 936 00:46:24,182 --> 00:46:25,582 by the nerve which makes them sensitive. 937 00:46:29,053 --> 00:46:30,954 Whiskers help mammals detect small changes 938 00:46:30,955 --> 00:46:32,856 in the world around them, 939 00:46:32,857 --> 00:46:37,094 find prey, and even communicate. 940 00:46:37,095 --> 00:46:39,196 {\an8}They're incredibly sensitive to external surroundings. 941 00:46:39,197 --> 00:46:41,131 {\an8}This is because 942 00:46:41,132 --> 00:46:43,233 {\an8}they are connected directly to the brain 943 00:46:43,234 --> 00:46:44,369 {\an8}by the trigeminal nerve. 944 00:46:46,137 --> 00:46:48,939 {\an8}It's evidence of how this nerve emerged from cynodont skulls 945 00:46:48,940 --> 00:46:52,309 {\an8}that has revealed when whiskers first appeared in this line. 946 00:46:57,081 --> 00:47:00,284 In animals that lived around 230 million years ago, 947 00:47:00,285 --> 00:47:01,585 a single hole appears, 948 00:47:01,586 --> 00:47:05,789 which is exactly what we find in modern mammals. 949 00:47:05,790 --> 00:47:07,658 This single hole is the passageway 950 00:47:07,659 --> 00:47:08,892 for the trigeminal nerve, 951 00:47:08,893 --> 00:47:11,428 which innervates the whiskers, located here, 952 00:47:11,429 --> 00:47:13,565 in this cavity. 953 00:47:15,200 --> 00:47:17,034 This creature had whiskers 954 00:47:17,035 --> 00:47:19,203 just after the Carnian pluvial episode, 955 00:47:19,204 --> 00:47:22,039 around the same time warm-bloodedness appeared. 956 00:47:22,040 --> 00:47:23,707 Coincidence? 957 00:47:23,708 --> 00:47:25,509 Or could they be linked? 958 00:47:25,510 --> 00:47:28,212 {\an8}Typically, traits evolve at different rates 959 00:47:28,213 --> 00:47:30,113 over millions of years 960 00:47:30,114 --> 00:47:33,217 unless they are somehow connected. 961 00:47:33,218 --> 00:47:35,986 The final missing piece in the puzzle 962 00:47:35,987 --> 00:47:38,322 is the appearance of lactation, 963 00:47:38,323 --> 00:47:41,058 mothers producing milk to feed their young. 964 00:47:41,059 --> 00:47:42,093 When did that appear? 965 00:47:45,196 --> 00:47:48,665 Lactation is tough to spot in the fossil record, 966 00:47:48,666 --> 00:47:52,469 so scientists have instead turned to DNA for answers, 967 00:47:52,470 --> 00:47:54,605 with tantalizing results. 968 00:47:54,606 --> 00:47:57,574 The evolution of a single gene seems to be related 969 00:47:57,575 --> 00:48:01,111 to all three mammalian traits. 970 00:48:01,112 --> 00:48:03,347 The MSX2 gene controlled 971 00:48:03,348 --> 00:48:04,881 {\an8}not only the disappearance of the third eye 972 00:48:04,882 --> 00:48:06,483 {\an8}and the appearance of hair, 973 00:48:06,484 --> 00:48:08,752 {\an8}but also the appearance of mammary glands, 974 00:48:08,753 --> 00:48:10,787 {\an8}and, with that, lactation. 975 00:48:10,788 --> 00:48:13,790 {\an8}This is the most characteristic trait of mammals. 976 00:48:16,327 --> 00:48:21,365 {\an8} The evolution of MSX2 contributed to the emergence 977 00:48:21,366 --> 00:48:23,800 {\an8}of these mammalian features in cynodonts, 978 00:48:23,801 --> 00:48:27,971 {\an8}which they then passed on to their mammal descendants, 979 00:48:27,972 --> 00:48:29,606 giving them the defining characteristics 980 00:48:29,607 --> 00:48:31,575 of all mammal-kind. 981 00:48:31,576 --> 00:48:32,976 But it's not the only gene responsible 982 00:48:32,977 --> 00:48:34,611 for big evolutionary changes. 983 00:48:34,612 --> 00:48:38,382 Whether it's turning fish fins into limbs 984 00:48:38,383 --> 00:48:40,817 or developing a spoken language, 985 00:48:40,818 --> 00:48:42,519 there is a set of key regulator genes 986 00:48:42,520 --> 00:48:43,920 that have a profound impact 987 00:48:43,921 --> 00:48:46,758 on development and evolutionary change. 988 00:48:48,459 --> 00:48:52,095 {\an8}At the boundary between the Triassic and Jurassic periods, 989 00:48:52,096 --> 00:48:55,132 {\an8}the dicynodont group had gone extinct 990 00:48:55,133 --> 00:48:57,902 {\an8}and the cynodonts were the last surviving therapsids. 991 00:48:59,871 --> 00:49:02,439 {\an8}So at last, we have an answer. 992 00:49:02,440 --> 00:49:05,309 {\an8}This is the group that gave rise to the mammal line 993 00:49:05,310 --> 00:49:09,913 {\an8}around 225 million years ago, 994 00:49:09,914 --> 00:49:11,815 {\an8}not long before the other therapsid lineages ended. 995 00:49:14,619 --> 00:49:17,354 {\an8}The very first true mammals 996 00:49:17,355 --> 00:49:18,855 {\an8}were tiny. 997 00:49:18,856 --> 00:49:20,691 {\an8}And if you saw one, it would have looked 998 00:49:20,692 --> 00:49:22,326 {\an8}like a little mouse or a little shrew. 999 00:49:22,327 --> 00:49:23,927 {\an8}They would have been covered in hair. 1000 00:49:23,928 --> 00:49:25,228 {\an8}They would have had molar teeth. 1001 00:49:25,229 --> 00:49:26,997 {\an8}They would have fed their babies milk. 1002 00:49:26,998 --> 00:49:29,299 {\an8}You would recognize them as mammals, 1003 00:49:29,300 --> 00:49:31,134 but you would probably just think they were 1004 00:49:31,135 --> 00:49:33,271 some kind of little rodent scurrying around. 1005 00:49:35,039 --> 00:49:37,808 {\an8} It wasn't easy for the newly evolved mammals, 1006 00:49:37,809 --> 00:49:39,576 {\an8}as the dinosaurs reigned 1007 00:49:39,577 --> 00:49:43,547 {\an8}for roughly the next 160 million years. 1008 00:49:43,548 --> 00:49:44,948 {\an8}The fates of dinosaurs and mammals 1009 00:49:44,949 --> 00:49:46,249 {\an8}have always been intertwined. 1010 00:49:46,250 --> 00:49:48,452 {\an8}And while dinosaurs took over the world, 1011 00:49:48,453 --> 00:49:49,920 mammals went the other way. 1012 00:49:49,921 --> 00:49:51,655 They went small. 1013 00:49:51,656 --> 00:49:53,523 They evolved stronger jaw muscles 1014 00:49:53,524 --> 00:49:55,325 so they could bite harder. 1015 00:49:55,326 --> 00:49:57,494 They evolved a keener sense of hearing. 1016 00:49:57,495 --> 00:49:59,229 All these things would have helped them 1017 00:49:59,230 --> 00:50:04,501 survive in a world controlled by massive, enormous dinosaurs 1018 00:50:04,502 --> 00:50:07,104 that could have smashed one of these little mammals 1019 00:50:07,105 --> 00:50:09,539 with a single footstep. 1020 00:50:09,540 --> 00:50:11,208 As had been the case for the therapsids 1021 00:50:11,209 --> 00:50:13,710 in the extinction events they faced, 1022 00:50:13,711 --> 00:50:16,480 it would be primarily the smallest of the mammals 1023 00:50:16,481 --> 00:50:19,816 that would survive the next major crisis: 1024 00:50:19,817 --> 00:50:22,686 the asteroid strike that wiped out the dinosaurs 1025 00:50:22,687 --> 00:50:25,622 66 million years ago. 1026 00:50:25,623 --> 00:50:27,023 Our ancestors made it through that boundary 1027 00:50:27,024 --> 00:50:29,192 {\an8}for very specific reasons. 1028 00:50:29,193 --> 00:50:32,028 {\an8}We had very specific adaptations as a group 1029 00:50:32,029 --> 00:50:34,632 {\an8}that allowed us to make it past these extinction events. 1030 00:50:36,234 --> 00:50:38,368 Over millions of years of evolution, 1031 00:50:38,369 --> 00:50:42,372 cynodonts, these small burrowing animals, 1032 00:50:42,373 --> 00:50:44,374 survived multiple crises, 1033 00:50:44,375 --> 00:50:47,177 adapting and changing to fit each new environment. 1034 00:50:48,813 --> 00:50:52,482 They gave rise to the mammals some 200 million years ago, 1035 00:50:52,483 --> 00:50:53,884 at a time when the dinosaurs 1036 00:50:53,885 --> 00:50:56,186 were just beginning their reign on Earth. 1037 00:50:56,187 --> 00:50:59,890 Mammals were not born 1038 00:50:59,891 --> 00:51:02,125 as a result of the extinction of the dinosaurs, 1039 00:51:02,126 --> 00:51:05,395 but had appeared millions of years before. 1040 00:51:05,396 --> 00:51:08,231 {\an8}For 160 million years, 1041 00:51:08,232 --> 00:51:12,135 {\an8}they lived alongside these giants. 1042 00:51:12,136 --> 00:51:14,237 {\an8}And only once the dinosaurs were extinct 1043 00:51:14,238 --> 00:51:16,940 {\an8}could the surviving mammals rise, 1044 00:51:16,941 --> 00:51:20,143 {\an8}diversifying into the spectacular variety 1045 00:51:20,144 --> 00:51:21,211 {\an8}we see today. 1046 00:51:23,247 --> 00:51:25,015 We and all mammals alive today, 1047 00:51:25,016 --> 00:51:28,351 we are the descendants of survivors. 1048 00:51:28,352 --> 00:51:29,553 So ultimately, 1049 00:51:29,554 --> 00:51:32,656 we come from this line 1050 00:51:32,657 --> 00:51:34,825 that's been small, adaptable, 1051 00:51:34,826 --> 00:51:38,128 a line that's been resilient, that could handle everything 1052 00:51:38,129 --> 00:51:40,197 that nature's been able to throw at us 1053 00:51:40,198 --> 00:51:42,299 over hundreds of millions of years. 1054 00:51:45,536 --> 00:51:47,971 With more than 270 million years 1055 00:51:47,972 --> 00:51:50,507 of shared evolution running through our veins... 1056 00:51:52,510 --> 00:51:54,978 ...we are therapsids... 1057 00:51:58,182 --> 00:52:02,452 ...just one of the thousands of modern mammal species 1058 00:52:02,453 --> 00:52:05,522 that are their descendants. 1059 00:52:05,523 --> 00:52:08,225 Long live the therapsids.