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These are the user uploaded subtitles that are being translated: 1 00:00:04,710 --> 00:00:07,245 This is a story about you 2 00:00:07,279 --> 00:00:10,448 and me and your dog. 3 00:00:14,219 --> 00:00:19,390 There was a time not long ago... before dogs. 4 00:00:19,425 --> 00:00:21,058 They didn't exist. 5 00:00:21,093 --> 00:00:24,529 Now there are big ones, small ones, snugglers, 6 00:00:24,563 --> 00:00:26,097 guardians, hunters. 7 00:00:26,131 --> 00:00:28,699 Every kind of dog you could possibly want. 8 00:00:28,734 --> 00:00:31,269 How did that happen? 9 00:00:31,303 --> 00:00:32,570 It's not just dogs. 10 00:00:32,604 --> 00:00:33,805 Where did all the different kinds 11 00:00:33,839 --> 00:00:36,574 of living creatures come from? 12 00:00:36,608 --> 00:00:40,945 The answer is a transforming power that sounds like something 13 00:00:40,979 --> 00:00:43,581 straight out of a fairy tale or myth, 14 00:00:43,615 --> 00:00:45,716 but it's no such thing. 15 00:02:10,795 --> 00:02:14,238 "Some Of The Things That Molecules Do" 16 00:02:17,843 --> 00:02:22,713 Let's go back across 30,000 years to a time before dogs, 17 00:02:22,748 --> 00:02:25,883 when our ancestors lived in the endless winter 18 00:02:25,918 --> 00:02:27,885 of the last ice age. 19 00:02:27,920 --> 00:02:31,422 Our ancestors were wanderers living in small bands. 20 00:02:31,457 --> 00:02:33,725 They slept beneath the stars. 21 00:02:33,759 --> 00:02:37,028 The sky was their storybook, calendar, 22 00:02:37,062 --> 00:02:40,031 an instruction manual for living. 23 00:02:40,065 --> 00:02:42,734 It told them when the bitter colds would come, 24 00:02:42,768 --> 00:02:44,736 when the wild grains would ripen, 25 00:02:44,770 --> 00:02:48,339 when the herds of caribou and bison would be on the move. 26 00:02:48,373 --> 00:02:52,343 Their idea of home was Earth itself. 27 00:02:52,377 --> 00:02:56,080 But they lived in fear of other hungry creatures... 28 00:02:56,115 --> 00:02:58,216 the mountain lions and the bears 29 00:02:58,250 --> 00:03:00,852 that competed with them for the same prey 30 00:03:00,886 --> 00:03:03,621 and the wolves that threatened to carry off and devour 31 00:03:03,655 --> 00:03:05,823 the most vulnerable among them. 32 00:03:23,008 --> 00:03:25,943 All the wolves want to get at the bone, 33 00:03:25,978 --> 00:03:28,546 but most of them are too frightened to come close enough. 34 00:03:28,580 --> 00:03:31,048 Their fear is due to high levels 35 00:03:31,083 --> 00:03:32,884 of stress hormones in their blood. 36 00:03:32,918 --> 00:03:34,786 It's a matter of survival. 37 00:03:34,820 --> 00:03:37,488 Because coming too close to humans can be fatal. 38 00:03:37,523 --> 00:03:40,792 But a few wolves-- due to natural variations-- 39 00:03:40,826 --> 00:03:42,593 have lower levels of those hormones. 40 00:03:42,628 --> 00:03:45,530 This makes them less afraid of humans. 41 00:03:49,968 --> 00:03:51,402 This wolf has discovered 42 00:03:51,437 --> 00:03:53,070 what a branch of his ancestors figured out 43 00:03:53,105 --> 00:03:55,073 some 15,000 years ago... 44 00:03:55,107 --> 00:03:58,075 an excellent survival strategy; 45 00:03:58,110 --> 00:04:01,345 the domestication of humans. 46 00:04:01,380 --> 00:04:04,916 Let the humans do the hunting, don't threaten them, 47 00:04:04,950 --> 00:04:06,918 and they'll let you scavenge their garbage. 48 00:04:06,952 --> 00:04:10,121 You'll eat more regularly, you'll leave more offspring, 49 00:04:10,155 --> 00:04:12,957 and those offspring will inherit your disposition. 50 00:04:12,991 --> 00:04:16,360 This selection for tameness 51 00:04:16,395 --> 00:04:18,496 would be reinforced with each generation 52 00:04:18,530 --> 00:04:23,334 until that line of wild wolves evolves into... 53 00:04:23,368 --> 00:04:25,436 dogs. 54 00:04:25,471 --> 00:04:28,473 You might call this "survival of the friendliest." 55 00:04:32,845 --> 00:04:36,114 Then as now, this was a good deal for the humans, too. 56 00:04:36,148 --> 00:04:39,450 The scavenging dogs weren't just a sanitation squad. 57 00:04:39,485 --> 00:04:41,285 They worked security. 58 00:04:56,702 --> 00:05:00,037 As this interspecies partnership continued over time, 59 00:05:00,072 --> 00:05:03,741 the dogs' appearance changed also. 60 00:05:03,776 --> 00:05:06,010 Cuteness became a selective advantage. 61 00:05:06,044 --> 00:05:08,546 The more adorable you were, the better chance you had 62 00:05:08,580 --> 00:05:11,983 to live and pass on your genes to another generation. 63 00:05:12,017 --> 00:05:13,985 What began as an alliance of convenience 64 00:05:14,019 --> 00:05:18,256 became a friendship that deepened over time. 65 00:05:18,290 --> 00:05:19,891 To see what happens next, 66 00:05:19,925 --> 00:05:22,360 let's leave our distant ancestors 67 00:05:22,394 --> 00:05:26,664 of some 20,000 years ago to visit the more recent past 68 00:05:26,698 --> 00:05:29,534 during an intermission in the Ice Age. 69 00:05:29,568 --> 00:05:33,337 This break in the climate starts a revolution. 70 00:05:33,372 --> 00:05:37,008 Instead of wandering, people are settling down. 71 00:05:37,042 --> 00:05:40,945 There's something new in the world... villages. 72 00:05:40,979 --> 00:05:42,680 People still hunt and gather, 73 00:05:42,714 --> 00:05:46,350 but now they also produce food and clothing... 74 00:05:46,385 --> 00:05:47,885 agriculture. 75 00:05:52,591 --> 00:05:55,860 The wolves have traded their freedom 76 00:05:55,894 --> 00:05:58,296 in exchange for a steady meal. 77 00:05:58,330 --> 00:06:01,065 They've given up their right to choose a mate. 78 00:06:01,099 --> 00:06:04,101 Now the humans choose for them. 79 00:06:04,136 --> 00:06:08,206 They consistently kill off the dogs that can't be trained; 80 00:06:08,240 --> 00:06:09,907 the ones that bite the feeding hand. 81 00:06:09,942 --> 00:06:14,145 And they breed the dogs that please them. 82 00:06:17,182 --> 00:06:20,284 They nurture those dogs that do their bidding... 83 00:06:20,319 --> 00:06:23,488 hunting, herding, guarding, hauling, 84 00:06:23,522 --> 00:06:24,589 and keeping them company. 85 00:06:24,623 --> 00:06:26,224 From every litter, 86 00:06:26,258 --> 00:06:29,227 the humans select the puppies they like best. 87 00:06:29,261 --> 00:06:31,829 Over the generations, the dogs evolve. 88 00:06:31,864 --> 00:06:36,134 This kind of evolution is called "artificial selection" 89 00:06:36,168 --> 00:06:37,668 or "breeding." 90 00:06:37,703 --> 00:06:39,937 Turning wolves into dogs was the first time 91 00:06:39,972 --> 00:06:43,107 we humans took evolution into our own hands. 92 00:06:43,142 --> 00:06:45,243 And we've been doing it ever since 93 00:06:45,277 --> 00:06:49,347 to shape all the plants and animals that we depend on. 94 00:06:49,381 --> 00:06:53,518 In a blink of cosmic time, just 15,000 or 20,000 years, 95 00:06:53,552 --> 00:06:56,954 we turned gray wolves into all the kinds of dogs 96 00:06:56,989 --> 00:06:58,322 we love today. 97 00:06:58,357 --> 00:06:59,857 Think of it. 98 00:06:59,892 --> 00:07:01,859 Every breed of dog you've ever seen 99 00:07:01,894 --> 00:07:04,662 was sculpted by human hands. 100 00:07:04,696 --> 00:07:08,166 Many of our best friends-- the most popular breeds-- 101 00:07:08,200 --> 00:07:10,802 were created in only the last few centuries. 102 00:07:13,705 --> 00:07:15,773 The awesome power of evolution 103 00:07:15,808 --> 00:07:17,375 transformed the ravenous wolf 104 00:07:17,409 --> 00:07:20,011 into the faithful shepherd... 105 00:07:20,045 --> 00:07:23,881 ...who protects the herd and drives the wolf away. 106 00:07:37,729 --> 00:07:41,032 Artificial selection turned the wolf into the shepherd 107 00:07:41,066 --> 00:07:43,968 and the wild grasses into wheat and corn. 108 00:07:44,002 --> 00:07:47,505 In fact, almost every plant and animal that we eat today 109 00:07:47,539 --> 00:07:51,209 was bred from a wild, less-edible ancestor. 110 00:07:51,243 --> 00:07:54,846 If artificial selection can work such profound changes 111 00:07:54,880 --> 00:07:57,582 in only 10,000 or 15,000 years, 112 00:07:57,616 --> 00:07:59,750 what can natural selection do 113 00:07:59,785 --> 00:08:02,520 operating over billions of years? 114 00:08:02,554 --> 00:08:07,825 The answer is all the beauty and diversity of life. 115 00:08:07,860 --> 00:08:09,760 How does it work? 116 00:08:09,795 --> 00:08:14,866 Our Ship of the Imagination can take us anywhere in space 117 00:08:14,900 --> 00:08:18,536 and time, even to the hidden microcosmos, 118 00:08:18,570 --> 00:08:22,573 where one kind of life can be transformed into another. 119 00:08:22,608 --> 00:08:24,542 Come with me. 120 00:08:33,424 --> 00:08:35,358 May not seem like it, 121 00:08:35,393 --> 00:08:37,260 but we've been living in an ice age 122 00:08:37,295 --> 00:08:39,029 for the last two million years. 123 00:08:39,063 --> 00:08:41,598 This just happens to be one of the long intermissions. 124 00:08:41,632 --> 00:08:43,400 For most of those two million years, 125 00:08:43,434 --> 00:08:45,902 the climate has been cold and dry. 126 00:08:45,937 --> 00:08:48,638 The North Polar ice cap extended much farther south 127 00:08:48,673 --> 00:08:50,807 than it does today. 128 00:08:50,842 --> 00:08:54,211 In one of those long, cold glacial periods 129 00:08:54,245 --> 00:08:57,114 when the winter sea ice stretched from the North Pole 130 00:08:57,148 --> 00:08:59,616 all the way down to what is now Los Angeles, 131 00:08:59,650 --> 00:09:04,354 great bears roamed the frozen wastes of Ireland. 132 00:09:09,961 --> 00:09:12,729 This might look like an ordinary bear, 133 00:09:12,763 --> 00:09:16,066 but something extraordinary is happening inside her. 134 00:09:16,100 --> 00:09:19,569 Something that will give rise to a new species. 135 00:09:19,604 --> 00:09:21,238 In order to see it, 136 00:09:21,272 --> 00:09:24,608 we'll need to descend down to a much smaller scale, 137 00:09:24,642 --> 00:09:27,577 to the cellular level, 138 00:09:27,612 --> 00:09:30,847 so that we can explore the bear's reproductive system. 139 00:09:34,352 --> 00:09:38,555 We'll take the subclavian artery through the heart. 140 00:10:00,745 --> 00:10:02,879 Almost there. 141 00:10:09,353 --> 00:10:10,887 Those are some of her eggs. 142 00:10:10,922 --> 00:10:13,457 To see what's going on in one of them, 143 00:10:13,491 --> 00:10:15,358 we'll have to get even smaller. 144 00:10:15,393 --> 00:10:18,762 We'll have to shrink down to the molecular level. 145 00:10:21,365 --> 00:10:24,401 Our Ship of the Imagination is now so small, 146 00:10:24,435 --> 00:10:28,338 you could fit a million of them into a grain of sand. 147 00:10:28,372 --> 00:10:32,309 See those guys over there strutting along those girders? 148 00:10:32,343 --> 00:10:35,579 They are proteins called kinesin. 149 00:10:35,613 --> 00:10:38,315 These kinesin are part of the transport crew 150 00:10:38,349 --> 00:10:41,151 that's busy moving cargo around the cell. 151 00:10:41,185 --> 00:10:43,019 How alien they seem. 152 00:10:43,054 --> 00:10:46,823 And yet these tiny creatures-- and beings like them-- 153 00:10:46,858 --> 00:10:51,561 are a part of every living cell, including the ones inside you. 154 00:10:53,531 --> 00:10:56,199 If life has a sanctuary, 155 00:10:56,234 --> 00:11:00,070 it's here in the nucleus which contains our DNA... 156 00:11:00,104 --> 00:11:03,774 the ancient scripture of our genetic code. 157 00:11:03,808 --> 00:11:08,912 And it's written in a language that all life can read. 158 00:11:15,386 --> 00:11:19,089 DNA is a molecule shaped like a long twisted ladder 159 00:11:19,123 --> 00:11:21,258 or double helix. 160 00:11:21,292 --> 00:11:23,059 The rungs of the ladder are made 161 00:11:23,094 --> 00:11:25,395 of four different kinds of smaller molecules. 162 00:11:25,429 --> 00:11:28,398 These are the letters of the genetic alphabet. 163 00:11:28,432 --> 00:11:31,368 Particular arrangements of those letters spell out 164 00:11:31,402 --> 00:11:33,804 the instructions for all living things, 165 00:11:33,838 --> 00:11:37,240 telling them how to grow, move, digest, 166 00:11:37,275 --> 00:11:40,744 sense the environment, heal, reproduce. 167 00:11:40,778 --> 00:11:43,914 The DNA double helix is a molecular machine 168 00:11:43,948 --> 00:11:47,984 with about 100 billion parts called "atoms." 169 00:11:48,019 --> 00:11:51,588 There are as many atoms in a single molecule of your DNA 170 00:11:51,622 --> 00:11:55,392 as there are stars in a typical galaxy. 171 00:11:55,426 --> 00:11:57,494 The same is true for dogs 172 00:11:57,528 --> 00:12:00,096 and bears 173 00:12:00,131 --> 00:12:03,567 and every living thing. 174 00:12:03,601 --> 00:12:07,871 We are, each of us, a little universe. 175 00:12:16,380 --> 00:12:19,282 The DNA message handed down from cell to cell 176 00:12:19,317 --> 00:12:21,418 and from generation to generation is copied 177 00:12:21,452 --> 00:12:23,153 with extreme care. 178 00:12:23,187 --> 00:12:27,757 The birth of a new DNA molecule begins when an unwinding protein 179 00:12:27,792 --> 00:12:30,260 separates the two strands of the double helix, 180 00:12:30,294 --> 00:12:32,162 breaking the rungs apart. 181 00:12:32,196 --> 00:12:34,498 Inside the liquid of the nucleus, 182 00:12:34,532 --> 00:12:38,435 the molecular letters of the genetic code float freely. 183 00:12:38,469 --> 00:12:41,838 Each strand of the helix copies its lost partner, 184 00:12:41,873 --> 00:12:44,841 resulting in two identical DNA molecules. 185 00:12:44,876 --> 00:12:47,878 That's how life reproduces genes and transmits them 186 00:12:47,912 --> 00:12:49,946 from one generation to the next. 187 00:12:49,981 --> 00:12:52,549 When a living cell divides in two, 188 00:12:52,583 --> 00:12:57,053 each one takes away with it a complete copy of the DNA. 189 00:12:57,088 --> 00:13:00,824 A specialized protein proofreads to make sure 190 00:13:00,858 --> 00:13:02,659 that only the right letters are accepted 191 00:13:02,693 --> 00:13:05,529 so that the DNA is accurately copied. 192 00:13:05,563 --> 00:13:07,297 But nobody's perfect. 193 00:13:07,331 --> 00:13:10,500 Occasionally, a proofreading error slips through, 194 00:13:10,535 --> 00:13:14,304 making a small, random change in the genetic instructions. 195 00:13:14,338 --> 00:13:17,541 A mutation has occurred in the bear's egg cell. 196 00:13:17,575 --> 00:13:21,478 A random event as tiny as this one can have consequences 197 00:13:21,512 --> 00:13:23,747 on a far grander scale. 198 00:13:25,783 --> 00:13:30,587 That mutation altered the gene that controls fur color. 199 00:13:30,621 --> 00:13:33,590 It will affect the production of dark pigment in the fur 200 00:13:33,624 --> 00:13:35,592 of the bear's offspring. 201 00:13:35,626 --> 00:13:37,894 Most mutations are harmless. 202 00:13:37,929 --> 00:13:39,663 Some are deadly. 203 00:13:39,697 --> 00:13:42,566 But a few, purely by chance, can give an organism 204 00:13:42,600 --> 00:13:46,269 a critical advantage over the competition. 205 00:13:46,304 --> 00:13:48,271 A year has passed. 206 00:13:48,306 --> 00:13:50,106 Our bear is now a mother. 207 00:13:50,141 --> 00:13:51,842 And as a result of that mutation, 208 00:13:51,876 --> 00:13:55,512 one of her two cubs was born with a white coat. 209 00:13:55,546 --> 00:13:58,248 When the cubs get old enough to venture out on their own, 210 00:13:58,282 --> 00:14:00,550 which bear is more likely 211 00:14:00,585 --> 00:14:04,754 to be able to sneak up on unsuspecting prey? 212 00:14:04,789 --> 00:14:08,024 The brown bear can be seen against the snow a mile away. 213 00:14:08,059 --> 00:14:10,927 The white bear prospers and passes on 214 00:14:10,962 --> 00:14:12,896 its own particular set of genes. 215 00:14:12,930 --> 00:14:15,765 This happens repeatedly. 216 00:14:15,800 --> 00:14:17,934 Over succeeding generations, 217 00:14:17,969 --> 00:14:20,537 the gene for white fur spreads through 218 00:14:20,571 --> 00:14:23,206 the entire population of Arctic bears. 219 00:14:23,241 --> 00:14:25,942 The gene for dark fur loses out 220 00:14:25,977 --> 00:14:28,745 in the competition for survival. 221 00:14:30,748 --> 00:14:34,618 Mutations are entirely random and happen all the time. 222 00:14:34,652 --> 00:14:36,586 But the environment rewards those 223 00:14:36,621 --> 00:14:38,889 that increase the chance for survival. 224 00:14:38,923 --> 00:14:41,291 It naturally selects the living things 225 00:14:41,325 --> 00:14:43,960 that are better suited to survive. 226 00:14:43,995 --> 00:14:46,796 And that selection is the opposite of random. 227 00:14:49,634 --> 00:14:52,102 The two populations of bears separated, 228 00:14:52,136 --> 00:14:53,970 and over thousands of years, 229 00:14:54,005 --> 00:14:57,140 evolved other characteristics that set them apart. 230 00:14:57,175 --> 00:15:00,377 They became different species. 231 00:15:02,780 --> 00:15:04,848 That's what Charles Darwin meant 232 00:15:04,882 --> 00:15:08,518 by "the origin of species." 233 00:15:08,553 --> 00:15:11,087 An individual bear doesn't evolve; 234 00:15:11,122 --> 00:15:15,025 the population of bears evolves over generations. 235 00:15:16,727 --> 00:15:18,795 If the Arctic ice continues to dwindle 236 00:15:18,830 --> 00:15:22,499 due to global warming, the polar bears may go extinct. 237 00:15:22,533 --> 00:15:24,534 They'll be replaced by brown bears, 238 00:15:24,569 --> 00:15:27,704 better adapted to the now defrosted environment. 239 00:15:27,738 --> 00:15:32,209 This is a different story from the one about the dogs. 240 00:15:32,243 --> 00:15:34,711 No breeder guided these changes. 241 00:15:34,745 --> 00:15:38,215 Instead, the environment itself selects them. 242 00:15:38,249 --> 00:15:40,884 This is evolution by natural selection, 243 00:15:40,918 --> 00:15:44,688 the most revolutionary concept in the history of science. 244 00:15:44,722 --> 00:15:48,859 Darwin first presented the evidence for this idea in 1859. 245 00:15:48,893 --> 00:15:52,896 The uproar it caused has never subsided. 246 00:15:52,930 --> 00:15:54,931 Why? 247 00:16:04,812 --> 00:16:06,913 We all understand the twinge of discomfort 248 00:16:06,948 --> 00:16:10,183 at the thought that we share a common ancestor with the apes. 249 00:16:10,218 --> 00:16:13,553 No one can embarrass you like a relative. 250 00:16:13,588 --> 00:16:15,756 Our closest ones, the chimpanzees, 251 00:16:15,790 --> 00:16:18,425 they frequently behave inappropriately in public. 252 00:16:18,459 --> 00:16:20,694 There's an understandable human need 253 00:16:20,728 --> 00:16:22,929 to distance ourselves from them. 254 00:16:22,964 --> 00:16:25,198 A central premise of traditional belief 255 00:16:25,233 --> 00:16:27,200 is that we were created separately 256 00:16:27,235 --> 00:16:29,202 from all the other animals. 257 00:16:29,237 --> 00:16:31,972 It's easy to see why this idea has taken hold. 258 00:16:32,006 --> 00:16:34,808 It makes us feel... special. 259 00:16:34,842 --> 00:16:38,912 But what about our kinship with the trees? 260 00:16:38,946 --> 00:16:41,348 How does that make you feel? 261 00:16:49,023 --> 00:16:52,259 Okay, here's a segment of the oak tree's DNA. 262 00:16:52,293 --> 00:16:54,428 Think of it like a bar code. 263 00:16:54,462 --> 00:16:57,064 The instructions written in the code of life 264 00:16:57,098 --> 00:17:00,067 tell the tree how to metabolize sugar. 265 00:17:00,101 --> 00:17:03,370 Now let's compare it with the same section of my own DNA. 266 00:17:08,609 --> 00:17:10,510 The DNA doesn't lie. 267 00:17:10,545 --> 00:17:13,847 This tree and me-- we're long-lost cousins. 268 00:17:13,881 --> 00:17:15,849 And it's not just the trees. 269 00:17:15,883 --> 00:17:18,752 If you go back far enough, you'll find that we share 270 00:17:18,786 --> 00:17:20,787 a common ancestor with... 271 00:17:20,822 --> 00:17:22,122 the butterfly... 272 00:17:22,156 --> 00:17:23,757 gray wolf... 273 00:17:23,791 --> 00:17:25,158 mushroom... 274 00:17:25,193 --> 00:17:26,793 shark... 275 00:17:26,828 --> 00:17:28,261 bacterium... 276 00:17:28,296 --> 00:17:29,596 sparrow. 277 00:17:29,630 --> 00:17:31,164 What a family! 278 00:17:31,199 --> 00:17:34,701 Other parts of the bar code vary from species to species. 279 00:17:34,736 --> 00:17:37,938 That's what makes the difference between an owl and an octopus. 280 00:17:37,972 --> 00:17:40,173 Unless you have an identical twin, 281 00:17:40,208 --> 00:17:42,476 there's no one else in the universe 282 00:17:42,510 --> 00:17:45,145 with the exact same DNA as you. 283 00:17:45,179 --> 00:17:47,381 Within other species, the genetic differences 284 00:17:47,415 --> 00:17:50,784 provide the raw material for natural selection. 285 00:17:50,818 --> 00:17:54,721 The environment selects which genes survive and multiply. 286 00:17:54,756 --> 00:17:56,890 When it comes to the genetic instructions 287 00:17:56,924 --> 00:17:58,892 for life's most basic functions-- 288 00:17:58,926 --> 00:18:01,695 say, digesting sugars-- we and other species 289 00:18:01,729 --> 00:18:03,730 are almost identical. 290 00:18:03,765 --> 00:18:06,867 That's because those functions are so basic to life, 291 00:18:06,901 --> 00:18:09,803 they evolved before the various life-forms 292 00:18:09,837 --> 00:18:11,805 branched off from each other. 293 00:18:11,839 --> 00:18:14,274 This is our Tree of Life. 294 00:18:14,308 --> 00:18:15,809 Science has made it possible 295 00:18:15,843 --> 00:18:17,978 for us to construct this family tree 296 00:18:18,012 --> 00:18:19,980 for all the species of life on Earth. 297 00:18:20,014 --> 00:18:23,817 Close genetic relatives occupy the same branch of the tree, 298 00:18:23,851 --> 00:18:27,454 while more distant cousins are farther away. 299 00:18:27,488 --> 00:18:29,656 Each twig is a living species. 300 00:18:31,659 --> 00:18:34,828 And the trunk of the tree represents the common ancestors 301 00:18:34,862 --> 00:18:37,664 of all life on Earth. 302 00:18:37,698 --> 00:18:40,067 The stuff of life is so malleable 303 00:18:40,101 --> 00:18:43,036 that once it got started, the environment molded it 304 00:18:43,071 --> 00:18:46,339 into a staggering variety of forms-- 305 00:18:46,374 --> 00:18:50,177 10,000 times more than we can possibly show here. 306 00:18:52,180 --> 00:18:54,314 Biologists have catalogued 307 00:18:54,348 --> 00:18:57,851 a half a million different kinds of beetles alone. 308 00:19:00,555 --> 00:19:03,156 Not to mention the numberless varieties 309 00:19:03,191 --> 00:19:04,825 of bacteria. 310 00:19:04,859 --> 00:19:07,194 There are many millions of living species 311 00:19:07,228 --> 00:19:09,596 of animals and plants, most of them 312 00:19:09,630 --> 00:19:11,998 still unknown to science. 313 00:19:12,033 --> 00:19:15,102 Think of that-- we have yet to make contact 314 00:19:15,136 --> 00:19:18,438 with most of the forms of terrestrial life. 315 00:19:18,473 --> 00:19:20,874 That's how many kinds of life there are 316 00:19:20,908 --> 00:19:22,642 on this tiny planet alone. 317 00:19:24,645 --> 00:19:27,180 The Tree of Life extends its feelers in all directions, 318 00:19:27,215 --> 00:19:28,782 finding and exploiting what works, 319 00:19:28,816 --> 00:19:30,517 creating new environments 320 00:19:30,551 --> 00:19:32,786 and opportunities for new forms. 321 00:19:34,956 --> 00:19:39,693 The Tree of Life is three and a half billion years old. 322 00:19:39,727 --> 00:19:41,895 That's plenty of time to develop 323 00:19:41,929 --> 00:19:45,098 an impressive repertoire of tricks. 324 00:19:47,502 --> 00:19:50,137 Evolution can disguise an animal 325 00:19:50,171 --> 00:19:52,606 as a plant... 326 00:19:57,111 --> 00:19:59,079 ...taking thousands of generations 327 00:19:59,113 --> 00:20:02,249 to contrive an elaborate costume 328 00:20:02,283 --> 00:20:04,151 that fools predators into looking elsewhere 329 00:20:04,185 --> 00:20:06,653 for someone to eat. 330 00:20:06,687 --> 00:20:10,657 Or it can disguise a plant as an animal, 331 00:20:10,691 --> 00:20:13,794 evolving blossoms that take on the appearance of a wasp-- 332 00:20:13,828 --> 00:20:17,831 the orchid's way of fooling real wasps into pollinating it. 333 00:20:18,866 --> 00:20:20,934 This is the awesome 334 00:20:20,968 --> 00:20:24,371 shape-shifting power of natural selection. 335 00:20:31,112 --> 00:20:35,782 Among the dense, tangled limbs of the vast Tree of Life... 336 00:20:35,817 --> 00:20:37,951 you are here. 337 00:20:37,985 --> 00:20:42,255 One tiny branch among countless millions. 338 00:20:42,290 --> 00:20:47,294 Science reveals that all life on Earth is one. 339 00:20:49,964 --> 00:20:53,133 Darwin discovered the actual mechanism of evolution. 340 00:20:53,167 --> 00:20:56,169 The prevailing belief was that the complexity 341 00:20:56,204 --> 00:20:58,839 and variety of life must be the work 342 00:20:58,873 --> 00:21:01,808 of an intelligent designer, who created each 343 00:21:01,843 --> 00:21:04,978 of these millions of different species separately. 344 00:21:05,012 --> 00:21:08,482 Living things are just too intricate, it was said, 345 00:21:08,516 --> 00:21:12,052 to be the result of unguided evolution. 346 00:21:12,086 --> 00:21:14,521 Consider the human eye, 347 00:21:14,555 --> 00:21:16,790 a masterpiece of complexity. 348 00:21:20,461 --> 00:21:22,562 It requires a cornea, 349 00:21:22,597 --> 00:21:26,032 iris, lens, retina, 350 00:21:26,067 --> 00:21:28,435 optic nerves, muscles, 351 00:21:28,469 --> 00:21:31,171 let alone the brain's elaborate neural network 352 00:21:31,205 --> 00:21:34,107 to interpret images. 353 00:21:34,142 --> 00:21:37,010 It's more complicated than any device 354 00:21:37,044 --> 00:21:40,113 ever crafted by human intelligence. 355 00:21:40,148 --> 00:21:41,782 Therefore, it was argued, 356 00:21:41,816 --> 00:21:43,850 the human eye can't be the result 357 00:21:43,885 --> 00:21:46,186 of mindless evolution. 358 00:21:46,220 --> 00:21:50,223 To know if that's true, we need to travel across time 359 00:21:50,258 --> 00:21:53,894 to a world before there were eyes to see. 360 00:22:13,307 --> 00:22:15,875 In the beginning, 361 00:22:15,909 --> 00:22:17,510 life was blind. 362 00:22:19,580 --> 00:22:21,714 This is what our world looked like 363 00:22:21,748 --> 00:22:23,716 four billion years ago, 364 00:22:23,750 --> 00:22:27,386 before there were any eyes to see. 365 00:22:27,421 --> 00:22:30,456 Until a few hundred million years passed, 366 00:22:30,490 --> 00:22:32,091 and then, one day, 367 00:22:32,125 --> 00:22:34,160 there was a microscopic copying error 368 00:22:34,194 --> 00:22:35,728 in the DNA of a bacterium. 369 00:22:35,762 --> 00:22:38,464 This random mutation gave that microbe 370 00:22:38,498 --> 00:22:41,267 a protein molecule that absorbed sunlight. 371 00:22:41,301 --> 00:22:43,102 Want to know what the world looked like 372 00:22:43,136 --> 00:22:44,804 to a light-sensitive bacterium? 373 00:22:44,838 --> 00:22:48,107 Take a look at the right side of the screen. 374 00:22:48,141 --> 00:22:51,744 Mutations continued to occur at random, 375 00:22:51,778 --> 00:22:55,014 as they always do in any population of living things. 376 00:22:57,017 --> 00:22:59,118 Another mutation 377 00:22:59,152 --> 00:23:03,089 caused a dark bacterium to flee intense light. 378 00:23:03,123 --> 00:23:05,491 What is going on here? 379 00:23:05,525 --> 00:23:07,260 Night and day. 380 00:23:07,294 --> 00:23:09,295 Those bacteria that could tell light from dark 381 00:23:09,329 --> 00:23:11,964 had a decisive advantage over the ones that couldn't. 382 00:23:11,999 --> 00:23:13,799 Why? 383 00:23:13,834 --> 00:23:16,969 Because the daytime brought harsh, ultraviolet light 384 00:23:17,004 --> 00:23:18,938 that damages DNA. 385 00:23:18,972 --> 00:23:22,508 The sensitive bacteria fled the intense light 386 00:23:22,543 --> 00:23:25,044 to safely exchange their DNA in the dark. 387 00:23:25,078 --> 00:23:26,979 They survived in greater numbers 388 00:23:27,014 --> 00:23:29,448 than the bacteria that stayed at the surface. 389 00:23:29,483 --> 00:23:32,552 Over time, those light-sensitive proteins 390 00:23:32,586 --> 00:23:35,187 became concentrated in a pigment spot 391 00:23:35,222 --> 00:23:38,491 on the more advanced, one-celled organism. 392 00:23:38,525 --> 00:23:41,694 This made it possible to find the light, 393 00:23:41,728 --> 00:23:43,829 an overwhelming advantage 394 00:23:43,864 --> 00:23:47,166 for an organism that harvests sunlight to make food. 395 00:23:54,274 --> 00:23:57,176 Here's a flatworm's-eye view of the world. 396 00:23:57,210 --> 00:23:59,345 This multi-celled organism 397 00:23:59,379 --> 00:24:01,681 evolved a dimple in the pigment spot. 398 00:24:01,715 --> 00:24:03,983 The bowl-shaped depression 399 00:24:04,017 --> 00:24:07,353 allowed the animal to distinguish light from shadow 400 00:24:07,387 --> 00:24:10,723 to crudely make out objects in its vicinity, 401 00:24:10,757 --> 00:24:14,760 including those to eat and those that might eat it... 402 00:24:14,795 --> 00:24:17,330 a tremendous advantage. 403 00:24:17,364 --> 00:24:19,999 Later, things became a little clearer. 404 00:24:20,033 --> 00:24:21,734 The dimple deepened 405 00:24:21,768 --> 00:24:24,503 and evolved into a socket with a small opening. 406 00:24:24,538 --> 00:24:26,872 Over thousands of generations, 407 00:24:26,907 --> 00:24:30,109 natural selection was slowly sculpting the eye. 408 00:24:31,912 --> 00:24:35,047 The opening contracted to a pinhole covered 409 00:24:35,082 --> 00:24:37,383 by a protective transparent membrane. 410 00:24:37,417 --> 00:24:39,885 Only a little light could enter the tiny hole, 411 00:24:39,920 --> 00:24:42,288 but it was enough to paint a dim image 412 00:24:42,322 --> 00:24:44,857 on the sensitive inner surface of the eye. 413 00:24:44,892 --> 00:24:46,759 This sharpened the focus. 414 00:24:46,793 --> 00:24:49,095 A larger opening would have let in more light 415 00:24:49,129 --> 00:24:52,698 to make a brighter image but one that was out of focus. 416 00:24:52,733 --> 00:24:55,601 This development launched the visual equivalent 417 00:24:55,636 --> 00:24:58,271 of an arms race. 418 00:25:08,682 --> 00:25:13,252 The competition needed to keep up to survive. 419 00:25:13,287 --> 00:25:16,989 But then a splendid new feature of the eye evolved, 420 00:25:17,024 --> 00:25:21,994 a lens that provided both brightness and sharp focus. 421 00:25:22,029 --> 00:25:24,430 In the eyes of primitive fish, 422 00:25:24,464 --> 00:25:28,167 the transparent gel near the pinhole formed into a lens. 423 00:25:28,201 --> 00:25:30,937 At the same time, the pinhole enlarged 424 00:25:30,971 --> 00:25:33,005 to let in more and more light. 425 00:25:33,040 --> 00:25:35,441 Fish could now see in high-def, 426 00:25:35,475 --> 00:25:39,178 both close up and far away. 427 00:25:39,212 --> 00:25:42,815 And then something terrible happened. 428 00:25:42,849 --> 00:25:46,319 Have you ever noticed that a straw in a glass of water 429 00:25:46,353 --> 00:25:48,821 looks bent at the surface of the water? 430 00:25:48,855 --> 00:25:51,591 That's because light bends when it goes from one medium 431 00:25:51,625 --> 00:25:54,493 to another, say from water to air. 432 00:25:54,528 --> 00:25:58,264 Our eyes originally evolved to see in water. 433 00:25:58,298 --> 00:26:01,167 The watery fluid in those eyes neatly 434 00:26:01,201 --> 00:26:04,971 eliminated the distortion of that bending effect. 435 00:26:05,973 --> 00:26:08,140 But for land animals, 436 00:26:08,175 --> 00:26:10,543 the light carries images from dry air 437 00:26:10,577 --> 00:26:13,646 into their still-watery eyes. 438 00:26:13,680 --> 00:26:15,214 That bends the light rays, 439 00:26:15,249 --> 00:26:19,218 causing all kinds of distortions. 440 00:26:19,253 --> 00:26:21,988 When our amphibious ancestors left the water for the land, 441 00:26:22,022 --> 00:26:25,224 their eyes, exquisitely evolved to see in water, 442 00:26:25,259 --> 00:26:27,560 were lousy for seeing in the air. 443 00:26:27,594 --> 00:26:30,229 Our vision has never been as good since. 444 00:26:30,264 --> 00:26:33,165 We like to think of our eyes as state-of-the-art, 445 00:26:33,200 --> 00:26:36,068 but 375 million years later, 446 00:26:36,103 --> 00:26:38,504 we still can't see things right in front of our noses 447 00:26:38,538 --> 00:26:41,173 or discern fine details in near darkness 448 00:26:41,208 --> 00:26:43,075 the way fish can. 449 00:26:43,110 --> 00:26:44,577 When we left the water, 450 00:26:44,611 --> 00:26:46,746 why didn't nature just start over again 451 00:26:46,780 --> 00:26:48,381 and evolve us a new set of eyes 452 00:26:48,415 --> 00:26:50,716 that were optimal for seeing in the air? 453 00:26:50,751 --> 00:26:52,818 Nature doesn't work that way. 454 00:26:52,853 --> 00:26:56,222 Evolution reshapes existing structures over generations, 455 00:26:56,256 --> 00:26:58,424 adapting them with small changes. 456 00:26:58,458 --> 00:27:00,092 It can't just go back to the drawing board 457 00:27:00,127 --> 00:27:01,928 and start from scratch. 458 00:27:01,962 --> 00:27:05,498 At every stage of its development, the evolving eye 459 00:27:05,532 --> 00:27:07,233 functioned well enough 460 00:27:07,267 --> 00:27:10,403 to provide a selective advantage for survival. 461 00:27:10,437 --> 00:27:13,272 And among animals alive today, we find eyes 462 00:27:13,307 --> 00:27:16,475 at all these stages of development. 463 00:27:19,746 --> 00:27:23,049 And all of them function. 464 00:27:23,083 --> 00:27:25,217 The complexity of the human eye poses 465 00:27:25,252 --> 00:27:27,353 no challenge to evolution by natural selection. 466 00:27:27,387 --> 00:27:31,857 In fact, the eye and all of biology makes no sense 467 00:27:31,892 --> 00:27:33,526 without evolution. 468 00:27:33,560 --> 00:27:36,195 Some claim that evolution is just a theory, 469 00:27:36,229 --> 00:27:39,398 as if it were merely an opinion. 470 00:27:39,433 --> 00:27:42,134 The theory of evolution, like the theory of gravity, 471 00:27:42,169 --> 00:27:44,237 is a scientific fact. 472 00:27:44,271 --> 00:27:46,973 Evolution really happened. 473 00:27:47,007 --> 00:27:49,408 Accepting our kinship with all life on Earth 474 00:27:49,443 --> 00:27:51,477 is not only solid science. 475 00:27:51,511 --> 00:27:56,682 In my view, it's also a soaring spiritual experience. 476 00:28:03,457 --> 00:28:06,292 Because evolution is blind, 477 00:28:06,326 --> 00:28:10,563 it cannot anticipate or adapt to catastrophic events. 478 00:28:10,597 --> 00:28:14,400 The Tree of Life has some broken branches. 479 00:28:14,434 --> 00:28:17,903 Many of them were severed in the five greatest catastrophes 480 00:28:17,938 --> 00:28:19,472 that life has ever known. 481 00:28:19,806 --> 00:28:21,774 Somewhere, there's a memorial 482 00:28:21,808 --> 00:28:24,777 to the multitude of lost species, 483 00:28:24,811 --> 00:28:26,946 the Halls of Extinction. 484 00:28:26,980 --> 00:28:29,415 Come with me. 485 00:28:43,798 --> 00:28:47,038 Welcome to the Halls of Extinction. 486 00:28:47,825 --> 00:28:51,722 A monument to the broken branches of the Tree of Life. 487 00:29:01,631 --> 00:29:05,334 For every single one of the millions of species 488 00:29:05,368 --> 00:29:09,605 alive today, perhaps a thousand others have perished. 489 00:29:09,639 --> 00:29:12,107 Most of them died out in the everyday competition 490 00:29:12,142 --> 00:29:13,676 with other life-forms. 491 00:29:13,710 --> 00:29:17,279 But many of them were swept away in vast cataclysms 492 00:29:17,314 --> 00:29:19,515 that overwhelmed the planet. 493 00:29:19,549 --> 00:29:21,950 In the last 500 million years, 494 00:29:21,985 --> 00:29:24,653 this has happened five times. 495 00:29:24,688 --> 00:29:29,858 Five mass extinctions that devastated life on Earth. 496 00:29:29,893 --> 00:29:35,130 The worst one of all happened some 250 million years ago, 497 00:29:35,165 --> 00:29:37,700 at the end of an era 498 00:29:37,734 --> 00:29:40,169 known as the Permian. 499 00:29:56,553 --> 00:29:59,288 Trilobites were armored animals that hunted 500 00:29:59,322 --> 00:30:01,790 in great herds across the seafloor. 501 00:30:01,825 --> 00:30:04,493 They were among the first animals to evolve 502 00:30:04,527 --> 00:30:06,929 image-forming eyes. 503 00:30:09,499 --> 00:30:13,502 Trilobites had a good long run, some 270 million years. 504 00:30:13,536 --> 00:30:17,072 Earth was once the planet of the trilobites. 505 00:30:17,107 --> 00:30:20,509 But now they're all gone, extinct. 506 00:30:20,543 --> 00:30:23,245 The last of them were swept from life's stage 507 00:30:23,279 --> 00:30:25,347 along with countless other species 508 00:30:25,382 --> 00:30:29,585 in an unparalleled environmental disaster. 509 00:30:38,128 --> 00:30:41,730 The apocalypse began in what is now Siberia, 510 00:30:41,765 --> 00:30:46,402 with volcanic eruptions on a scale unlike anything 511 00:30:46,436 --> 00:30:48,604 in human experience. 512 00:31:01,051 --> 00:31:03,585 Earth was very different then, 513 00:31:03,620 --> 00:31:07,923 with one single supercontinent and one great ocean. 514 00:31:07,957 --> 00:31:10,426 Relentless floods of fiery lava 515 00:31:10,460 --> 00:31:13,228 engulfed an area larger than Western Europe. 516 00:31:13,263 --> 00:31:15,931 The pulsing eruptions went on for hundreds 517 00:31:15,965 --> 00:31:18,100 of thousands of years. 518 00:31:18,134 --> 00:31:19,702 The molten rock 519 00:31:19,736 --> 00:31:22,438 ignited coal deposits and polluted the air 520 00:31:22,472 --> 00:31:25,441 with carbon dioxide and other greenhouse gases. 521 00:31:25,475 --> 00:31:27,376 This heated the Earth 522 00:31:27,410 --> 00:31:30,512 and stopped the ocean currents from circulating. 523 00:31:45,528 --> 00:31:47,830 Noxious bacteria bloomed, 524 00:31:47,864 --> 00:31:52,000 but nearly everything else in the seas died. 525 00:31:52,035 --> 00:31:55,604 The stagnant waters belched deadly hydrogen sulfide gas 526 00:31:55,638 --> 00:31:57,072 into the air, 527 00:31:57,107 --> 00:31:59,641 which suffocated most of the land animals. 528 00:32:12,655 --> 00:32:16,992 Nine in ten of all species on the planet went extinct. 529 00:32:17,027 --> 00:32:19,094 We call it... 530 00:32:19,129 --> 00:32:21,130 The Great Dying. 531 00:32:33,743 --> 00:32:36,378 Life on Earth came so near to being wiped out 532 00:32:36,413 --> 00:32:40,215 that it took more than ten million years to recover. 533 00:32:40,250 --> 00:32:42,551 But new life-forms slowly evolved 534 00:32:42,585 --> 00:32:46,255 to fill the openings left by the Permian holocaust. 535 00:32:54,030 --> 00:32:56,899 Among the biggest winners were the dinosaurs. 536 00:32:56,933 --> 00:32:59,735 Now the Earth was their planet. 537 00:32:59,769 --> 00:33:04,006 Their reign continued for over 150 million years. 538 00:33:04,040 --> 00:33:06,642 Until it, too, came crashing down 539 00:33:06,676 --> 00:33:09,578 in another mass extinction. 540 00:33:09,612 --> 00:33:12,214 Life on Earth has taken quite a beating over the eons. 541 00:33:12,248 --> 00:33:15,050 And yet it's still there. 542 00:33:15,085 --> 00:33:17,886 The tenacity of life is mind-boggling. 543 00:33:17,921 --> 00:33:20,723 We keep finding it where no one thought it could be. 544 00:33:25,195 --> 00:33:28,497 That nameless corridor? 545 00:33:28,531 --> 00:33:30,532 That's for another day. 546 00:33:36,039 --> 00:33:39,608 I know an animal that can live in boiling water 547 00:33:39,642 --> 00:33:41,777 or in solid ice. 548 00:33:41,811 --> 00:33:44,179 It can go ten years without a drop of water. 549 00:33:44,214 --> 00:33:46,849 It can travel naked in the cold vacuum 550 00:33:46,883 --> 00:33:51,387 and intense radiation of space and will return unscathed. 551 00:33:51,421 --> 00:33:53,856 The tardigrade, or water bear. 552 00:33:53,890 --> 00:33:56,959 It's equally at home atop the tallest mountains 553 00:33:56,993 --> 00:33:58,794 and in the deepest trenches of the sea. 554 00:33:58,828 --> 00:34:01,864 And in our own backyards, where they live among the moss 555 00:34:01,898 --> 00:34:04,366 in countless numbers. 556 00:34:04,401 --> 00:34:05,768 You've probably never noticed them 557 00:34:05,802 --> 00:34:07,269 because they're so small. 558 00:34:07,303 --> 00:34:08,904 About the size of a pinpoint. 559 00:34:08,938 --> 00:34:10,639 But they're tough. 560 00:34:10,673 --> 00:34:14,943 The tardigrades have survived all five mass extinctions. 561 00:34:14,978 --> 00:34:17,713 They've been in business for a half a billion years. 562 00:34:17,747 --> 00:34:20,082 We used to think that life was finicky, 563 00:34:20,116 --> 00:34:22,051 that it would only take hold where it was 564 00:34:22,085 --> 00:34:24,053 not too hot, not too cold, 565 00:34:24,087 --> 00:34:27,823 not too dark or salty or acidic or radioactive. 566 00:34:27,857 --> 00:34:30,626 And whatever you do, don't forget to add water. 567 00:34:30,660 --> 00:34:32,394 We were wrong. 568 00:34:32,429 --> 00:34:33,729 As the hardy tardigrade demonstrates, 569 00:34:33,763 --> 00:34:35,931 life can endure conditions 570 00:34:35,965 --> 00:34:39,168 that would mean certain death for us humans. 571 00:34:39,202 --> 00:34:41,003 But differences between us and life found 572 00:34:41,037 --> 00:34:43,572 in even the most extreme environments on our planet 573 00:34:43,606 --> 00:34:46,575 are only variations on a single theme, 574 00:34:46,609 --> 00:34:49,144 dialects of a single language. 575 00:34:49,179 --> 00:34:51,947 The genetic code of Earth life. 576 00:34:56,886 --> 00:34:59,421 But what would life be like on other worlds? 577 00:34:59,456 --> 00:35:02,458 Worlds with a completely different history, 578 00:35:02,492 --> 00:35:05,494 chemistry and evolution from our planet? 579 00:35:07,964 --> 00:35:11,500 There's a distant world I want to take you to-- 580 00:35:11,534 --> 00:35:13,936 a world far different from our own, 581 00:35:13,970 --> 00:35:17,306 but one that may harbor life. 582 00:35:17,340 --> 00:35:21,210 If it does, it promises to be unlike anything 583 00:35:21,244 --> 00:35:23,679 we've ever seen before. 584 00:35:42,957 --> 00:35:46,326 Clouds and haze completely hide the surface of Titan, 585 00:35:46,360 --> 00:35:48,328 Saturn's giant moon. 586 00:35:48,362 --> 00:35:50,930 Titan reminds me a little bit of home. 587 00:35:50,965 --> 00:35:54,434 Like Earth, it has an atmosphere that's mostly nitrogen. 588 00:35:54,468 --> 00:35:56,602 But it's four times denser. 589 00:35:56,637 --> 00:35:59,505 Titan's air has no oxygen at all. 590 00:35:59,540 --> 00:36:02,008 And it's far colder than anywhere on Earth. 591 00:36:02,042 --> 00:36:05,645 But still... I want to go there. 592 00:36:06,814 --> 00:36:09,315 We have to descend 593 00:36:09,350 --> 00:36:11,784 through a couple hundred kilometers of smog 594 00:36:11,819 --> 00:36:14,787 before we can even see the surface. 595 00:36:14,822 --> 00:36:18,358 But hidden beneath lies a weirdly familiar landscape. 596 00:36:26,066 --> 00:36:29,502 Titan is the only other world in the solar system 597 00:36:29,536 --> 00:36:30,970 where it ever rains. 598 00:36:31,005 --> 00:36:34,407 It has rivers and coastlines. 599 00:36:39,346 --> 00:36:41,414 Titan has hundreds of lakes. 600 00:36:41,448 --> 00:36:44,984 One of them larger than Lake Superior in North America. 601 00:36:45,019 --> 00:36:46,986 Vapor rising from the lakes 602 00:36:47,021 --> 00:36:49,222 condenses and falls again as rain. 603 00:36:49,256 --> 00:36:53,159 The rain feeds rivers, 604 00:36:53,193 --> 00:36:56,496 which carve valleys into the landscape, 605 00:36:56,530 --> 00:36:58,531 just like on Earth. 606 00:37:00,801 --> 00:37:02,869 But with one big difference. 607 00:37:02,903 --> 00:37:05,605 On Titan, the seas and the rain 608 00:37:05,639 --> 00:37:09,542 are made not of water but of methane and ethane. 609 00:37:09,576 --> 00:37:12,512 On Earth, those molecules form natural gas. 610 00:37:12,546 --> 00:37:15,381 On frigid Titan, 611 00:37:15,416 --> 00:37:17,417 they're liquid. 612 00:37:22,723 --> 00:37:24,724 Titan has lots of water, 613 00:37:24,758 --> 00:37:27,460 but all of it is frozen hard as rock. 614 00:37:27,494 --> 00:37:29,862 In fact, the landscape and mountains 615 00:37:29,897 --> 00:37:32,365 are made mainly of water ice. 616 00:37:32,399 --> 00:37:34,600 At hundreds of degrees below zero, 617 00:37:34,635 --> 00:37:38,538 Titan is far too cold for water to ever be liquid. 618 00:37:40,774 --> 00:37:43,977 Astrobiologists since Carl Sagan have wondered 619 00:37:44,011 --> 00:37:47,447 if life might swim in Titan's hydrocarbon lakes. 620 00:37:49,783 --> 00:37:51,718 The chemical basis for such life 621 00:37:51,752 --> 00:37:55,154 would have to be entirely different from anything we know. 622 00:37:55,189 --> 00:37:57,991 All life on Earth depends on liquid water. 623 00:37:58,025 --> 00:38:00,326 And Titan's surface has none of that. 624 00:38:00,361 --> 00:38:02,895 But we can imagine other kinds of life. 625 00:38:02,930 --> 00:38:05,498 There might be creatures that inhale hydrogen 626 00:38:05,532 --> 00:38:07,000 instead of oxygen. 627 00:38:07,034 --> 00:38:10,503 And exhale methane instead of carbon dioxide. 628 00:38:10,537 --> 00:38:13,973 They might use acetylene instead of sugar as an energy source. 629 00:38:14,008 --> 00:38:16,442 How could we find out if such creatures 630 00:38:16,477 --> 00:38:20,980 rule a hidden empire beneath the oil-dark waves? 631 00:38:36,230 --> 00:38:39,499 We're diving down deep into the Kraken Sea, 632 00:38:39,533 --> 00:38:42,535 named for the mythic Norse sea monster. 633 00:38:45,906 --> 00:38:48,107 Even if there is one of those down there, 634 00:38:48,142 --> 00:38:49,809 we probably couldn't see it. 635 00:38:49,843 --> 00:38:52,378 It's so dark. 636 00:38:52,413 --> 00:38:56,816 If you took all the oil and natural gas on Earth, 637 00:38:56,850 --> 00:39:00,586 it would amount to but a tiny fraction of Titan's reserves. 638 00:39:04,091 --> 00:39:06,025 Let's turn on some lights. 639 00:39:10,331 --> 00:39:13,766 We're now 200 meters beneath the surface. 640 00:39:17,037 --> 00:39:19,572 Did you see something? 641 00:39:19,606 --> 00:39:22,809 Over there, by that vent. 642 00:39:22,843 --> 00:39:25,311 Maybe it was just my imagination. 643 00:39:25,346 --> 00:39:27,313 I guess we'll have to come back 644 00:39:27,348 --> 00:39:30,350 if we want to find out for sure. 645 00:39:32,586 --> 00:39:36,222 There's one last story I want to tell you. 646 00:39:36,256 --> 00:39:40,426 And it's the greatest story science has ever told. 647 00:39:44,765 --> 00:39:48,201 It's the story of life on our world. 648 00:40:10,658 --> 00:40:13,760 Welcome to the Earth of four billion years ago. 649 00:40:13,794 --> 00:40:17,463 This was our planet before life. 650 00:40:17,498 --> 00:40:20,566 Nobody knows how life got started. 651 00:40:20,601 --> 00:40:22,969 Most of the evidence from that time was destroyed 652 00:40:23,003 --> 00:40:25,605 by impact and erosion. 653 00:40:25,639 --> 00:40:29,409 Science works on the frontier between knowledge and ignorance. 654 00:40:29,443 --> 00:40:31,377 We're not afraid to admit what we don't know. 655 00:40:31,412 --> 00:40:33,713 There's no shame in that. 656 00:40:33,747 --> 00:40:37,583 The only shame is to pretend that we have all the answers. 657 00:40:37,618 --> 00:40:39,285 Maybe someone watching this 658 00:40:39,320 --> 00:40:41,821 will be the first to solve the mystery 659 00:40:41,855 --> 00:40:44,257 of how life on Earth began. 660 00:40:51,832 --> 00:40:53,666 The evidence from living microbes 661 00:40:53,701 --> 00:40:55,601 suggest that their earliest ancestors 662 00:40:55,636 --> 00:40:57,337 preferred high temperatures. 663 00:40:57,371 --> 00:41:00,273 Life on Earth may have arisen in hot water 664 00:41:00,307 --> 00:41:02,608 around submerged volcanic vents. 665 00:41:07,381 --> 00:41:09,849 In Carl Sagan's original Cosmos series, 666 00:41:09,883 --> 00:41:12,652 he traced the unbroken thread that stretches 667 00:41:12,686 --> 00:41:14,754 directly from the one-celled organisms 668 00:41:14,788 --> 00:41:17,323 of nearly four billion years ago... 669 00:41:17,358 --> 00:41:19,625 to you. 670 00:41:19,660 --> 00:41:22,795 Four billion years in 40 seconds. 671 00:41:22,830 --> 00:41:26,366 From creatures who had yet to discern day from night 672 00:41:26,400 --> 00:41:30,203 to beings who are exploring the cosmos. 673 00:42:19,353 --> 00:42:22,355 Those are some of the things that molecules do 674 00:42:22,389 --> 00:42:25,858 given four billion years of evolution. 54975

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