1 00:00:04,588 --> 00:00:10,266 TYSON: This is a story about you and me and your dog. 2 00:00:11,094 --> 00:00:14,268 [ANIMAL HOWLING] 3 00:00:14,431 --> 00:00:19,028 There was a time not long ago, before dogs. 4 00:00:19,186 --> 00:00:21,029 They didn't exist. 5 00:00:21,188 --> 00:00:25,694 Now there are big ones, small ones, snugglers, guardians, hunters. 6 00:00:25,859 --> 00:00:28,783 Every kind of dog you could possibly want. 7 00:00:28,946 --> 00:00:30,914 How did that happen? 8 00:00:31,073 --> 00:00:32,245 It's not just dogs. 9 00:00:32,407 --> 00:00:35,786 Where did all the different kinds of living creatures come from? 10 00:00:36,328 --> 00:00:40,299 The answer is a transforming power that sounds like something... 11 00:00:40,457 --> 00:00:45,304 ...straight out of a fairy tale or myth, but it's no such thing. 12 00:02:17,387 --> 00:02:22,109 Let's go back across 30,000 years to a time before dogs... 13 00:02:22,851 --> 00:02:26,526 ...when our ancestors lived in the endless winter of the last ice age. 14 00:02:27,397 --> 00:02:31,072 Our ancestors were wanderers living in small bands. 15 00:02:31,234 --> 00:02:33,578 They slept beneath the stars. 16 00:02:33,737 --> 00:02:39,665 The sky was their storybook, calendar, an instruction manual for living. 17 00:02:39,826 --> 00:02:42,204 It told them when the bitter colds would come... 18 00:02:42,371 --> 00:02:44,499 ...when the wild grains would ripen... 19 00:02:44,665 --> 00:02:47,919 ...when the herds of caribou and bison would be on the move. 20 00:02:48,085 --> 00:02:51,931 Their idea of home was Earth itself. 21 00:02:52,506 --> 00:02:55,726 But they lived in fear of other hungry creatures. 22 00:02:55,884 --> 00:02:59,934 The mountain lions and the bears that competed with them for the same prey. 23 00:03:00,097 --> 00:03:03,226 And the wolves that threatened to carry off and devour... 24 00:03:03,392 --> 00:03:05,440 ...the most vulnerable among them. 25 00:03:05,686 --> 00:03:09,862 [WOLF GROWLING] 26 00:03:12,275 --> 00:03:13,902 [SNARLING] 27 00:03:23,662 --> 00:03:25,585 All the wolves want to get at the bone... 28 00:03:25,747 --> 00:03:28,170 ...but most are too frightened to come close enough. 29 00:03:28,959 --> 00:03:33,180 Their fear is due to high levels of stress hormones in their blood. 30 00:03:33,338 --> 00:03:34,760 It's a matter of survival. 31 00:03:34,923 --> 00:03:37,972 Because coming too close to humans can be fatal. 32 00:03:38,135 --> 00:03:40,479 But a few wolves, due to natural variations... 33 00:03:40,637 --> 00:03:42,856 ...have lower levels of those hormones. 34 00:03:43,014 --> 00:03:46,188 This makes them less afraid of humans. 35 00:03:49,730 --> 00:03:52,984 This wolf has discovered what a branch of his ancestors figured out... 36 00:03:53,150 --> 00:03:57,701 ...some 15,000 years ago. An excellent survival strategy. 37 00:03:57,863 --> 00:04:01,037 Domestication, humans. 38 00:04:01,199 --> 00:04:04,578 Let the humans do the hunting, don't threaten them... 39 00:04:04,745 --> 00:04:06,998 ...and they'll let you scavenge their garbage. 40 00:04:07,164 --> 00:04:09,792 You'll eat more regularly, you'll leave more offspring... 41 00:04:09,958 --> 00:04:13,508 ...and those offspring will inherit your disposition. 42 00:04:13,962 --> 00:04:18,138 This selection for tameness would be reinforced with each generation... 43 00:04:18,300 --> 00:04:20,553 ...until that line of wild wolves... 44 00:04:20,719 --> 00:04:25,691 ...evolves into dogs. 45 00:04:25,849 --> 00:04:28,102 You might call this "survival of the friendliest." 46 00:04:29,478 --> 00:04:30,650 [CHUCKLES] 47 00:04:30,812 --> 00:04:32,439 [DOG WHIMPERS] 48 00:04:32,606 --> 00:04:36,031 Then as now, this was a good deal for the humans too. 49 00:04:36,193 --> 00:04:39,037 The scavenging dogs weren't just a sanitation squad. 50 00:04:39,196 --> 00:04:40,914 They worked security. 51 00:04:41,865 --> 00:04:44,038 [DOG BARKING] 52 00:04:46,953 --> 00:04:49,081 [WOLF GROWLING] 53 00:04:51,041 --> 00:04:52,714 [DOG BARKING] 54 00:04:56,880 --> 00:05:00,259 As this interspecies partnership continued over time... 55 00:05:00,425 --> 00:05:02,974 ...the dogs' appearance changed also. 56 00:05:03,136 --> 00:05:05,559 Cuteness became a selective advantage. 57 00:05:05,722 --> 00:05:08,271 The more adorable you were, the better chance you had... 58 00:05:08,433 --> 00:05:11,687 ...to live and pass on your genes to another generation. 59 00:05:11,853 --> 00:05:14,151 What began as an alliance of convenience... 60 00:05:14,314 --> 00:05:18,035 ...became a friendship that deepened over time. 61 00:05:18,193 --> 00:05:19,445 To see what happens next... 62 00:05:19,778 --> 00:05:22,031 ...let's leave our distant ancestors... 63 00:05:22,197 --> 00:05:26,623 ...of some 20,000 years ago to visit the more recent past... 64 00:05:26,785 --> 00:05:29,629 ...during an intermission in the Ice Age. 65 00:05:29,788 --> 00:05:33,258 This break in the climate starts a revolution. 66 00:05:33,416 --> 00:05:36,636 Instead of wandering, people are settling down. 67 00:05:36,795 --> 00:05:40,720 There's something new in the world: villages. 68 00:05:40,882 --> 00:05:46,264 People still hunt and gather, but now they also produce food and clothing. 69 00:05:46,429 --> 00:05:48,431 Agriculture. 70 00:05:53,603 --> 00:05:57,949 The wolves have traded their freedom in exchange for a steady meal. 71 00:05:58,692 --> 00:06:01,036 They've given up their right to choose a mate. 72 00:06:01,194 --> 00:06:03,743 Now the humans choose for them. 73 00:06:04,406 --> 00:06:07,660 They consistently kill off the dogs that can't be trained... 74 00:06:07,826 --> 00:06:10,204 ...the ones that bite the feeding hand. 75 00:06:10,370 --> 00:06:13,795 And they breed the dogs that please them. 76 00:06:14,124 --> 00:06:16,798 [DOGS BARKING] 77 00:06:17,252 --> 00:06:19,971 They nurture those dogs that do their bidding... 78 00:06:20,130 --> 00:06:24,476 ...hunting, herding, guarding, hauling, and keeping them company. 79 00:06:24,634 --> 00:06:25,851 From every litter... 80 00:06:26,011 --> 00:06:28,764 ...the humans select the puppies they like best. 81 00:06:28,930 --> 00:06:32,355 Over the generations, the dogs evolve. 82 00:06:32,517 --> 00:06:35,987 This kind of evolution is called artificial selection... 83 00:06:36,146 --> 00:06:37,238 ...or breeding. 84 00:06:37,397 --> 00:06:39,866 Turning wolves into dogs was the first time... 85 00:06:40,025 --> 00:06:42,995 ...we humans took evolution into our own hands. 86 00:06:43,153 --> 00:06:44,951 And we've been doing it ever since... 87 00:06:45,113 --> 00:06:48,993 ...to shape all the plants and animals that we depend on. 88 00:06:49,159 --> 00:06:53,460 In a blink of cosmic time, just 15- or 20,000 years... 89 00:06:53,622 --> 00:06:58,344 ...we turned gray wolves into all the kinds of dogs we love today. 90 00:06:58,501 --> 00:07:02,051 Think of it. Every breed of dog you've ever seen... 91 00:07:02,213 --> 00:07:04,557 ...was sculpted by human hands. 92 00:07:04,716 --> 00:07:08,141 Many of our best friends, the most popular breeds... 93 00:07:08,303 --> 00:07:11,773 ...were created in only the last few centuries. 94 00:07:13,475 --> 00:07:18,026 The awesome power of evolution transformed the ravenous wolf... 95 00:07:18,188 --> 00:07:19,656 ...into the faithful shepherd... 96 00:07:19,814 --> 00:07:23,535 ...who protects the herd and drives the wolf away. 97 00:07:23,693 --> 00:07:25,616 [GROWLING] 98 00:07:37,791 --> 00:07:41,045 Artificial selection turned the wolf into the shepherd... 99 00:07:41,211 --> 00:07:43,885 ...and the wild grasses into wheat and corn. 100 00:07:44,047 --> 00:07:47,597 In fact, almost every plant and animal that we eat today... 101 00:07:47,759 --> 00:07:51,480 ...was bred from a wild, less-edible ancestor. 102 00:07:51,638 --> 00:07:54,892 If artificial selection can work such profound changes... 103 00:07:55,058 --> 00:07:59,985 ...in only 10 or 15,000 years, what can natural selection do... 104 00:08:00,146 --> 00:08:02,649 ...operating over billions of years? 105 00:08:03,608 --> 00:08:08,409 The answer is all the beauty and diversity of life. 106 00:08:08,571 --> 00:08:10,244 How does it work? 107 00:08:10,407 --> 00:08:15,789 Our ship of the imagination can take us anywhere in space and time... 108 00:08:15,954 --> 00:08:18,707 ...even to the hidden microcosmos... 109 00:08:18,873 --> 00:08:22,798 ...where one kind of life can be transformed into another. 110 00:08:22,961 --> 00:08:24,804 Come with me. 111 00:08:34,097 --> 00:08:35,144 May not seem like it... 112 00:08:35,306 --> 00:08:39,152 ...but we've been living in an ice age for the last two million years. 113 00:08:39,310 --> 00:08:41,938 This just happens to be one of the long intermissions. 114 00:08:42,105 --> 00:08:46,110 For most of those two million years, the climate has been cold and dry. 115 00:08:46,276 --> 00:08:48,745 The North Polar ice cap extended much farther south... 116 00:08:48,903 --> 00:08:51,031 ...than it does today. 117 00:08:51,197 --> 00:08:53,620 In one of those long, cold glacial periods... 118 00:08:53,783 --> 00:08:56,457 ...when the winter sea ice stretched from the North Pole... 119 00:08:56,619 --> 00:08:59,418 ...all the way down to what is now Los Angeles... 120 00:09:00,373 --> 00:09:04,469 ...great bears roamed the frozen wastes of Ireland. 121 00:09:10,967 --> 00:09:12,935 This might look like an ordinary bear... 122 00:09:13,094 --> 00:09:15,813 ...but something extraordinary is happening inside her. 123 00:09:16,556 --> 00:09:19,935 Something that will give rise to a new species. 124 00:09:20,101 --> 00:09:24,982 In order to see it, we'll need to descend down to a much smaller scale... 125 00:09:25,148 --> 00:09:26,821 ...to the cellular level... 126 00:09:26,983 --> 00:09:30,613 ...so that we can explore the bear's reproductive system. 127 00:09:35,325 --> 00:09:38,875 We'll take the subclavian artery through the heart. 128 00:09:43,917 --> 00:09:46,136 [HEART BEATING] 129 00:10:01,226 --> 00:10:02,694 Almost there. 130 00:10:09,025 --> 00:10:11,153 Those are some of her eggs. 131 00:10:11,319 --> 00:10:15,449 To see what's going on in one of them, we'll have to get even smaller. 132 00:10:15,615 --> 00:10:18,585 We'll have to shrink down to the molecular level. 133 00:10:21,663 --> 00:10:24,587 Our ship of the imagination is now so small... 134 00:10:24,749 --> 00:10:28,879 ...you could fit a million of them into a grain of sand. 135 00:10:29,629 --> 00:10:32,883 See those guys over there strutting along those girders? 136 00:10:33,049 --> 00:10:35,598 They are proteins called kinesin. 137 00:10:35,760 --> 00:10:38,388 These kinesin are part of the transport crew... 138 00:10:38,555 --> 00:10:41,229 ...that's busy moving cargo around the cell. 139 00:10:41,391 --> 00:10:43,234 How alien they seem. 140 00:10:43,393 --> 00:10:46,522 And yet these tiny creatures, and beings like them... 141 00:10:46,688 --> 00:10:52,366 ...are a part of every living cell, including the ones inside you. 142 00:10:53,361 --> 00:10:56,080 If life has a sanctuary... 143 00:10:56,239 --> 00:11:00,585 ...it's here in the nucleus which contains our DNA. 144 00:11:01,286 --> 00:11:03,914 The ancient scripture of our genetic code. 145 00:11:04,080 --> 00:11:09,587 And it's written in a language that all life can read. 146 00:11:14,924 --> 00:11:18,849 DNA is a molecule shaped like a long twisted ladder... 147 00:11:19,012 --> 00:11:21,060 ...or double helix. 148 00:11:21,222 --> 00:11:25,443 The rungs of the ladder are made of four different kinds of smaller molecules. 149 00:11:25,602 --> 00:11:28,526 These are the letters of the genetic alphabet. 150 00:11:28,688 --> 00:11:30,656 Particular arrangements of those letters... 151 00:11:30,815 --> 00:11:33,534 ...spell out the instructions for all living things... 152 00:11:33,693 --> 00:11:36,788 ...telling them how to grow, move, digest... 153 00:11:36,946 --> 00:11:40,541 ...sense the environment, heal, and reproduce. 154 00:11:40,700 --> 00:11:43,544 The DNA double helix is a molecular machine... 155 00:11:43,703 --> 00:11:48,174 ...with about 100 billion parts called atoms. 156 00:11:48,333 --> 00:11:51,963 There are as many atoms in a single molecule of your DNA... 157 00:11:52,128 --> 00:11:55,098 ...as there are stars in a typical galaxy. 158 00:11:55,256 --> 00:12:00,057 The same is true for dogs and bears... 159 00:12:00,220 --> 00:12:02,598 ...and every living thing. 160 00:12:03,306 --> 00:12:08,654 We are, each of us, a little universe. 161 00:12:16,319 --> 00:12:19,163 The DNA message handed down from cell to cell... 162 00:12:19,322 --> 00:12:21,745 ...and from generation to generation is copied... 163 00:12:21,908 --> 00:12:22,955 ...with extreme care. 164 00:12:23,117 --> 00:12:27,497 The birth of a new DNA molecule begins when an unwinding protein... 165 00:12:27,664 --> 00:12:30,042 ...separates the two strands of the double helix... 166 00:12:30,208 --> 00:12:32,176 ...breaking the rungs apart. 167 00:12:32,335 --> 00:12:34,463 Inside the liquid of the nucleus... 168 00:12:34,629 --> 00:12:38,179 ...the molecular letters of the genetic code float freely. 169 00:12:38,341 --> 00:12:41,845 Each strand of the helix copies its lost partner... 170 00:12:42,011 --> 00:12:44,935 ...resulting in two identical DNA molecules. 171 00:12:45,098 --> 00:12:47,817 That's how life reproduces genes and transmits them... 172 00:12:47,976 --> 00:12:50,195 ...from one generation to the next. 173 00:12:50,353 --> 00:12:52,606 When a living cell divides in two... 174 00:12:52,772 --> 00:12:57,118 ...each one takes away with it a complete copy of the DNA. 175 00:12:57,277 --> 00:13:00,030 A specialized protein proofreads to make sure... 176 00:13:00,196 --> 00:13:05,418 ...that only the right letters are accepted so that the DNA is accurately copied. 177 00:13:05,576 --> 00:13:07,249 But nobody's perfect. 178 00:13:07,412 --> 00:13:10,461 Occasionally, a proofreading error slips through... 179 00:13:10,623 --> 00:13:14,253 ...making a small, random change in the genetic instructions. 180 00:13:14,419 --> 00:13:17,889 A mutation has occurred in the bear's egg cell. 181 00:13:18,047 --> 00:13:21,426 A random event as tiny as this one can have consequences... 182 00:13:21,592 --> 00:13:24,562 ...on a far grander scale. 183 00:13:26,055 --> 00:13:29,901 That mutation altered the gene that controls fur color. 184 00:13:30,059 --> 00:13:33,233 It will affect the production of dark pigment in the fur... 185 00:13:33,396 --> 00:13:35,444 ...of the bear's offspring. 186 00:13:35,606 --> 00:13:37,734 Most mutations are harmless. 187 00:13:37,900 --> 00:13:39,447 Some are deadly. 188 00:13:39,610 --> 00:13:42,329 But a few, purely by chance, can give an organism... 189 00:13:42,488 --> 00:13:46,083 ...a critical advantage over the competition. 190 00:13:46,242 --> 00:13:49,416 A year has passed. Our bear is now a mother. 191 00:13:49,579 --> 00:13:51,832 And as a result of that mutation... 192 00:13:51,998 --> 00:13:55,252 ...one of her two cubs was born with a white coat. 193 00:13:55,418 --> 00:13:59,013 When the cubs get old enough to venture out on their own... 194 00:13:59,172 --> 00:14:00,845 ...which bear is more likely... 195 00:14:01,007 --> 00:14:04,432 ...to be able to sneak up on unsuspecting prey? 196 00:14:04,594 --> 00:14:07,894 The brown bear can be seen against the snow a mile away. 197 00:14:08,056 --> 00:14:10,730 The white bear prospers and passes on... 198 00:14:10,892 --> 00:14:13,611 ...its own particular set of genes. 199 00:14:13,770 --> 00:14:16,193 This happens repeatedly. 200 00:14:16,356 --> 00:14:17,983 Over succeeding generations... 201 00:14:18,149 --> 00:14:23,451 ...the gene for white fur spreads through the entire population of Arctic bears. 202 00:14:23,613 --> 00:14:28,961 The gene for dark fur loses out in the competition for survival. 203 00:14:30,620 --> 00:14:34,750 Mutations are entirely random and happen all the time. 204 00:14:34,916 --> 00:14:36,634 But the environment rewards those... 205 00:14:36,793 --> 00:14:38,966 ...that increase the chance for survival. 206 00:14:39,128 --> 00:14:43,804 It naturally selects the living things that are better suited to survive. 207 00:14:43,966 --> 00:14:46,640 And that selection is the opposite of random. 208 00:14:50,139 --> 00:14:54,189 The two populations of bears separated and over thousands of years... 209 00:14:54,352 --> 00:14:57,777 ...evolved other characteristics that set them apart. 210 00:14:57,939 --> 00:15:01,364 They became different species. 211 00:15:02,693 --> 00:15:07,665 That's what Charles Darwin meant by "the origin of species. " 212 00:15:08,449 --> 00:15:10,827 An individual bear doesn't evolve. 213 00:15:10,993 --> 00:15:15,840 The population of bears evolves over generations. 214 00:15:16,666 --> 00:15:18,714 If the Arctic ice continues to dwindle... 215 00:15:18,876 --> 00:15:22,631 ...due to global warming, the polar bears may go extinct. 216 00:15:22,797 --> 00:15:24,720 They'll be replaced by brown bears... 217 00:15:24,882 --> 00:15:27,556 ...better adapted to the now defrosted environment. 218 00:15:29,178 --> 00:15:31,977 This is a different story from the one about the dogs. 219 00:15:32,140 --> 00:15:34,643 No breeder guided these changes. 220 00:15:34,809 --> 00:15:38,063 Instead, the environment itself selects them. 221 00:15:38,229 --> 00:15:40,857 This is evolution by natural selection... 222 00:15:41,023 --> 00:15:44,618 ...the most revolutionary concept in the history of science. 223 00:15:44,777 --> 00:15:48,873 Darwin first presented the evidence for this idea in 1859. 224 00:15:49,031 --> 00:15:52,706 The uproar it caused has never subsided. 225 00:15:52,869 --> 00:15:54,997 Why? 226 00:16:02,712 --> 00:16:04,589 [BIRDS CHIRPING] 227 00:16:04,755 --> 00:16:06,849 We all understand the twinge of discomfort... 228 00:16:07,008 --> 00:16:09,932 ...at the thought that we share a common ancestor with the apes. 229 00:16:10,845 --> 00:16:13,473 No one can embarrass you like a relative. 230 00:16:13,639 --> 00:16:15,562 Our closest ones, the chimpanzees... 231 00:16:15,725 --> 00:16:18,604 ...they frequently behave inappropriately in public. 232 00:16:18,769 --> 00:16:22,694 There's an understandable human need to distance ourselves from them. 233 00:16:22,857 --> 00:16:25,406 A central premise of traditional belief... 234 00:16:25,568 --> 00:16:29,198 ...is that we were created separately from all the other animals. 235 00:16:29,363 --> 00:16:34,585 It's easy to see why this idea has taken hold. It makes us feel special. 236 00:16:35,077 --> 00:16:41,130 But what about our kinship with the trees? How does that make you feel? 237 00:16:49,592 --> 00:16:54,644 Okay, here's a segment of the oak tree's DNA. Think of it like a barcode. 238 00:16:54,805 --> 00:17:00,107 The instructions written in the code of life tell the tree how to metabolize sugar. 239 00:17:00,269 --> 00:17:03,148 Now let's compare it with the same section of my own DNA. 240 00:17:08,986 --> 00:17:10,283 The DNA doesn't lie. 241 00:17:10,446 --> 00:17:14,246 This tree and me, we're long-lost cousins. 242 00:17:14,408 --> 00:17:15,910 And it's not just the trees. 243 00:17:16,077 --> 00:17:18,830 If you go back far enough, you'll find that we share... 244 00:17:18,996 --> 00:17:21,966 ...a common ancestor with the butterfly... 245 00:17:22,124 --> 00:17:23,501 ...gray wolf... 246 00:17:23,668 --> 00:17:25,011 ...mushroom... 247 00:17:25,169 --> 00:17:26,546 ...shark... 248 00:17:26,712 --> 00:17:28,214 ...bacterium... 249 00:17:28,381 --> 00:17:29,473 ...sparrow. 250 00:17:29,632 --> 00:17:31,009 What a family. 251 00:17:31,175 --> 00:17:34,395 Other parts of the barcode vary from species to species. 252 00:17:34,554 --> 00:17:38,149 That's what makes the difference between an owl and an octopus. 253 00:17:38,307 --> 00:17:40,355 Unless you have an identical twin... 254 00:17:40,518 --> 00:17:45,024 ...there's no one else in the universe with the exact same DNA as you. 255 00:17:45,189 --> 00:17:47,408 Within other species, the genetic differences... 256 00:17:47,567 --> 00:17:50,741 ...provide the raw material for natural selection. 257 00:17:50,903 --> 00:17:54,999 The environment selects which genes survive and multiply. 258 00:17:55,157 --> 00:17:58,661 When it comes to the genetic instructions for life's most basic functions... 259 00:17:58,828 --> 00:18:03,504 ...say, digesting sugars, we and other species are almost identical. 260 00:18:03,666 --> 00:18:06,966 That's because those functions are so basic to life... 261 00:18:07,128 --> 00:18:11,053 ...they evolved before the various life-forms branched off from each other. 262 00:18:11,215 --> 00:18:14,014 This is our tree of life. 263 00:18:14,176 --> 00:18:17,726 Science has made it possible for us to construct this family tree... 264 00:18:17,888 --> 00:18:20,232 ...for all the species of life on Earth. 265 00:18:20,391 --> 00:18:23,520 Close genetic relatives occupy the same branch of the tree... 266 00:18:23,686 --> 00:18:27,065 ...while more distant cousins are farther away. 267 00:18:27,231 --> 00:18:30,360 Each twig is a living species. 268 00:18:31,611 --> 00:18:34,785 And the trunk of the tree represents the common ancestors... 269 00:18:34,947 --> 00:18:37,666 ...of all life on Earth. 270 00:18:37,825 --> 00:18:40,044 The stuff of life is so malleable... 271 00:18:40,202 --> 00:18:42,921 ...that once it got started, the environment molded it... 272 00:18:43,080 --> 00:18:46,129 ...into a staggering variety of forms... 273 00:18:46,292 --> 00:18:50,968 ...10,000 times more than we can possibly show here. 274 00:18:52,965 --> 00:18:54,387 Biologists have cataloged... 275 00:18:54,550 --> 00:18:57,645 ...a half a million different kinds of beetles alone. 276 00:19:01,182 --> 00:19:04,652 Not to mention the numberless varieties of bacteria. 277 00:19:05,186 --> 00:19:07,564 There are many millions of living species... 278 00:19:07,730 --> 00:19:11,325 ...of animals and plants, most of them still unknown to science. 279 00:19:11,484 --> 00:19:14,533 Think of that. We have yet to make contact... 280 00:19:14,695 --> 00:19:18,245 ...with most of the forms of terrestrial life. 281 00:19:18,407 --> 00:19:23,379 That's how many kinds of life there are on this tiny planet alone. 282 00:19:24,288 --> 00:19:27,041 The tree of life extends its feelers in all directions... 283 00:19:27,208 --> 00:19:30,257 ...finding and exploiting what works, creating new environments... 284 00:19:30,419 --> 00:19:33,093 ...and opportunities for new forms. 285 00:19:36,300 --> 00:19:39,725 The tree of life is three and a half billion years old. 286 00:19:39,887 --> 00:19:44,893 That's plenty of time to develop an impressive repertoire of tricks. 287 00:19:48,187 --> 00:19:52,112 Evolution can disguise an animal as a plant... 288 00:19:57,697 --> 00:20:01,543 ...taking thousands of generations to contrive an elaborate costume... 289 00:20:01,701 --> 00:20:06,582 ...that fools predators into looking elsewhere for someone to eat. 290 00:20:06,747 --> 00:20:10,468 Or it can disguise a plant as an animal... 291 00:20:10,626 --> 00:20:13,630 ...evolving blossoms that take on the appearance of a wasp... 292 00:20:13,796 --> 00:20:17,642 ...the orchid's way of fooling real wasps into pollinating it. 293 00:20:19,677 --> 00:20:24,934 This is the awesome shape-shifting power of natural selection. 294 00:20:31,147 --> 00:20:35,653 Among the dense, tangled limbs of the vast tree of life... 295 00:20:35,818 --> 00:20:37,866 ...you are here. 296 00:20:38,028 --> 00:20:42,078 One tiny branch among countless millions. 297 00:20:42,241 --> 00:20:47,998 Science reveals that all life on Earth is one. 298 00:20:50,291 --> 00:20:53,761 Darwin discovered the actual mechanism of evolution. 299 00:20:53,919 --> 00:20:56,513 The prevailing belief was that the complexity... 300 00:20:56,672 --> 00:21:00,518 ...and variety of life must be the work of an intelligent designer... 301 00:21:00,676 --> 00:21:04,726 ...who created each of these millions of different species separately. 302 00:21:04,889 --> 00:21:08,359 Living things are just too intricate, it was said... 303 00:21:08,517 --> 00:21:12,067 ...to be the result of unguided evolution. 304 00:21:12,229 --> 00:21:17,611 Consider the human eye, a masterpiece of complexity. 305 00:21:20,529 --> 00:21:26,127 It requires a cornea, iris, lens, retina... 306 00:21:26,285 --> 00:21:28,413 ...optic nerves, muscles... 307 00:21:28,579 --> 00:21:33,585 ...let alone the brain's elaborate neural network to interpret images. 308 00:21:34,126 --> 00:21:39,599 It's more complicated than any device ever crafted by human intelligence. 309 00:21:39,757 --> 00:21:43,853 Therefore, it was argued, the human eye can't be the result... 310 00:21:44,011 --> 00:21:46,389 ...of mindless evolution. 311 00:21:46,555 --> 00:21:50,230 To know if that's true, we need to travel across time... 312 00:21:50,392 --> 00:21:54,863 ...to a world before there were eyes to see. 313 00:22:14,416 --> 00:22:17,295 In the beginning, life was blind. 314 00:22:19,755 --> 00:22:23,510 This is what our world looked like four billion years ago... 315 00:22:23,676 --> 00:22:26,555 ...before there were any eyes to see. 316 00:22:26,720 --> 00:22:31,601 Until a few hundred million years passed, and then, one day... 317 00:22:31,767 --> 00:22:35,692 ...there was a microscopic copying error in the DNA of a bacterium. 318 00:22:35,855 --> 00:22:41,112 This random mutation gave that microbe a protein molecule that absorbed sunlight. 319 00:22:41,277 --> 00:22:44,907 Want to know what the world looked like to a light-sensitive bacterium? 320 00:22:45,072 --> 00:22:48,372 Take a look at the right side of the screen. 321 00:22:49,368 --> 00:22:51,462 Mutations continued to occur at random... 322 00:22:51,620 --> 00:22:55,716 ...as they always do in any population of living things. 323 00:22:57,418 --> 00:23:02,640 Another mutation caused a dark bacterium to flee intense light. 324 00:23:03,299 --> 00:23:05,301 What is going on here? 325 00:23:05,467 --> 00:23:06,844 Night and day. 326 00:23:07,011 --> 00:23:09,639 Those bacteria that could tell light from dark... 327 00:23:09,805 --> 00:23:12,479 ...had a decisive advantage over the ones that couldn't. 328 00:23:12,641 --> 00:23:16,441 Why? Because the daytime brought harsh, ultraviolet light... 329 00:23:16,604 --> 00:23:18,698 ...that damages DNA. 330 00:23:19,273 --> 00:23:22,026 The sensitive bacteria fled the intense light... 331 00:23:22,192 --> 00:23:24,866 ...to safely exchange their DNA in the dark. 332 00:23:25,029 --> 00:23:26,656 They survived in greater numbers... 333 00:23:26,822 --> 00:23:29,245 ...than the bacteria that stayed at the surface. 334 00:23:30,117 --> 00:23:34,588 Over time, those light-sensitive proteins became concentrated in a pigment spot... 335 00:23:34,747 --> 00:23:37,876 ...on the more advanced, one-celled organism. 336 00:23:38,334 --> 00:23:41,554 This made it possible to find the light... 337 00:23:41,712 --> 00:23:43,180 ...an overwhelming advantage... 338 00:23:43,339 --> 00:23:46,969 ...for an organism that harvests sunlight to make food. 339 00:23:53,724 --> 00:23:57,319 Here's a flatworm's-eye view of the world. 340 00:23:57,478 --> 00:24:01,949 This multi-celled organism evolved a dimple in the pigment spot. 341 00:24:02,107 --> 00:24:03,609 The bowl-shaped depression... 342 00:24:03,776 --> 00:24:07,451 ...allowed the animal to distinguish light from shadow... 343 00:24:07,613 --> 00:24:10,412 ...to crudely make out objects in its vicinity... 344 00:24:10,574 --> 00:24:14,750 ...including those to eat and those that might eat it... 345 00:24:14,912 --> 00:24:17,131 ...a tremendous advantage. 346 00:24:17,790 --> 00:24:21,465 Later, things became a little clearer. The dimple deepened... 347 00:24:21,627 --> 00:24:24,801 ...and evolved into a socket with a small opening. 348 00:24:24,964 --> 00:24:26,637 Over thousands of generations... 349 00:24:26,799 --> 00:24:30,849 ...natural selection was slowly sculpting the eye. 350 00:24:32,054 --> 00:24:37,231 The opening contracted to a pinhole covered by a protective transparent membrane. 351 00:24:37,393 --> 00:24:41,990 Only a little light could enter the tiny hole but it was enough to paint a dim image... 352 00:24:42,147 --> 00:24:46,744 ...on the sensitive inner surface of the eye. This sharpened the focus. 353 00:24:46,902 --> 00:24:49,121 A larger opening would have let in more light... 354 00:24:49,279 --> 00:24:52,874 ...to make a brighter image but one that was out of focus. 355 00:24:53,033 --> 00:24:57,880 This development launched the visual equivalent of an arms race. 356 00:25:09,425 --> 00:25:13,180 The competition needed to keep up to survive. 357 00:25:13,345 --> 00:25:17,270 But then a splendid new feature of the eye evolved... 358 00:25:17,433 --> 00:25:22,610 ...a lens that provided both brightness and sharp focus. 359 00:25:23,272 --> 00:25:24,774 In the eyes of primitive fish... 360 00:25:24,940 --> 00:25:28,615 ...the transparent gel near the pinhole formed into a lens. 361 00:25:28,777 --> 00:25:32,782 At the same time, the pinhole enlarged to let in more and more light. 362 00:25:32,948 --> 00:25:35,827 Fish could now see in high-def... 363 00:25:35,993 --> 00:25:39,839 ...both close up and far away. 364 00:25:39,997 --> 00:25:42,591 And then something terrible happened. 365 00:25:43,751 --> 00:25:46,379 Have you ever noticed that a straw in a glass of water... 366 00:25:46,545 --> 00:25:48,513 ...looks bent at the surface of the water? 367 00:25:48,672 --> 00:25:51,346 That's because light bends when it goes from one medium... 368 00:25:51,508 --> 00:25:54,261 ...to another, say from water to air. 369 00:25:54,428 --> 00:25:58,478 Our eyes originally evolved to see in water. 370 00:25:58,640 --> 00:26:00,938 The watery fluid in those eyes... 371 00:26:01,101 --> 00:26:03,820 ...neatly eliminated the distortion of that bending effect. 372 00:26:06,482 --> 00:26:10,658 But for land animals, the light carries images from dry air... 373 00:26:10,819 --> 00:26:13,743 ...into their still-watery eyes. 374 00:26:13,906 --> 00:26:18,503 That bends the light rays causing all kinds of distortions. 375 00:26:19,161 --> 00:26:22,131 When our amphibious ancestors left the water for the land... 376 00:26:22,289 --> 00:26:25,293 ...their eyes, exquisitely evolved to see in water... 377 00:26:25,459 --> 00:26:27,553 ...were lousy for seeing in the air. 378 00:26:27,711 --> 00:26:30,385 Our vision has never been as good since. 379 00:26:30,547 --> 00:26:33,266 We like to think of our eyes as state-of-the-art... 380 00:26:33,425 --> 00:26:35,723 ...but 375 million years later... 381 00:26:35,886 --> 00:26:38,605 ...we still can't see things right in front of our noses... 382 00:26:38,764 --> 00:26:42,985 ...or discern fine details in near darkness the way fish can. 383 00:26:43,143 --> 00:26:46,522 When we left the water, why didn't nature just start over again... 384 00:26:46,688 --> 00:26:50,864 ...and evolve us a new set of eyes that were optimal for seeing in the air? 385 00:26:51,026 --> 00:26:52,778 Nature doesn't work that way. 386 00:26:52,945 --> 00:26:56,199 Evolution reshapes existing structures over generations... 387 00:26:56,365 --> 00:26:58,413 ...adapting them with small changes. 388 00:26:58,575 --> 00:27:02,751 It can't just go back to the drawing board and start from scratch. 389 00:27:02,913 --> 00:27:06,087 At every stage of its development, the evolving eye... 390 00:27:06,250 --> 00:27:10,175 ...functioned well enough to provide a selective advantage for survival. 391 00:27:10,337 --> 00:27:13,716 And among animals alive today, we find eyes... 392 00:27:13,882 --> 00:27:16,260 ...at all these stages of development. 393 00:27:19,680 --> 00:27:21,682 And all of them function. 394 00:27:23,475 --> 00:27:25,398 The complexity of the human eye... 395 00:27:25,561 --> 00:27:28,110 ...poses no challenge to evolution by natural selection. 396 00:27:28,272 --> 00:27:33,745 In fact, the eye and all of biology makes no sense without evolution. 397 00:27:33,902 --> 00:27:39,204 Some claim that evolution is just a theory as if it were merely an opinion. 398 00:27:39,366 --> 00:27:42,245 The theory of evolution, like the theory of gravity... 399 00:27:42,411 --> 00:27:44,413 ...is a scientific fact. 400 00:27:44,580 --> 00:27:47,003 Evolution really happened. 401 00:27:47,166 --> 00:27:51,717 Accepting our kinship with all life on Earth is not only solid science. 402 00:27:51,879 --> 00:27:56,476 In my view, it's also a soaring spiritual experience. 403 00:28:04,057 --> 00:28:06,059 Because evolution is blind... 404 00:28:06,226 --> 00:28:10,356 ...it cannot anticipate or adapt to catastrophic events. 405 00:28:11,440 --> 00:28:13,909 The tree of life has some broken branches. 406 00:28:14,067 --> 00:28:17,867 Many of them were severed in the five greatest catastrophes... 407 00:28:18,030 --> 00:28:20,283 ...that life has ever known. 408 00:28:20,449 --> 00:28:22,292 Somewhere, there's a memorial... 409 00:28:22,451 --> 00:28:27,298 ...to the multitude of lost species, the Halls of Extinction. 410 00:28:27,456 --> 00:28:29,129 Come with me. 411 00:28:43,639 --> 00:28:46,483 Welcome to the Halls of Extinction. 412 00:28:47,643 --> 00:28:52,399 A monument to the broken branches on the tree of life. 413 00:29:02,616 --> 00:29:06,541 For every single one of the millions of species alive today... 414 00:29:06,703 --> 00:29:09,832 ...perhaps a thousand others have perished. 415 00:29:09,998 --> 00:29:13,844 Most of them died out in everyday competition with other life-forms. 416 00:29:14,002 --> 00:29:17,472 But many of them were swept away in vast cataclysms... 417 00:29:17,631 --> 00:29:19,383 ...that overwhelmed the planet. 418 00:29:20,092 --> 00:29:24,518 In the last 500 million years, this has happened five times. 419 00:29:26,431 --> 00:29:30,436 Five extinctions devastated life on Earth. 420 00:29:31,144 --> 00:29:35,524 The worst happened 250 million years ago... 421 00:29:35,691 --> 00:29:40,367 ...at the end of an era known as the Permian. 422 00:29:57,004 --> 00:29:59,098 Trilobites were armored animals that hunted... 423 00:29:59,256 --> 00:30:02,135 ...in great herds across the seafloor. 424 00:30:02,301 --> 00:30:06,556 They were among the first animals to evolve image-forming eyes. 425 00:30:09,433 --> 00:30:13,654 Trilobites had a good long run, some 270 million years. 426 00:30:13,812 --> 00:30:20,366 Earth was once the planet of the trilobites. But now they're all gone. Extinct. 427 00:30:20,527 --> 00:30:22,996 The last of them were swept from life's stage... 428 00:30:23,155 --> 00:30:29,413 ...along with countless other species in an unparalleled environmental disaster. 429 00:30:33,999 --> 00:30:37,629 [RUMBLING AND EXPLOSIONS] 430 00:30:38,211 --> 00:30:42,091 The apocalypse began in what is now Siberia... 431 00:30:42,257 --> 00:30:48,765 ...with volcanic eruptions on a scale unlike anything in human experience. 432 00:31:01,151 --> 00:31:03,404 Earth was very different then... 433 00:31:03,570 --> 00:31:07,746 ...with one single supercontinent and one great ocean. 434 00:31:07,908 --> 00:31:10,331 Relentless floods of fiery lava... 435 00:31:10,494 --> 00:31:13,794 ...engulfed an area larger than Western Europe. 436 00:31:13,955 --> 00:31:18,426 The pulsing eruptions went on for hundreds of thousands of years. 437 00:31:18,585 --> 00:31:22,089 The molten rock ignited coal deposits and polluted the air... 438 00:31:22,255 --> 00:31:25,805 ...with carbon dioxide and other greenhouse gases. 439 00:31:25,967 --> 00:31:27,219 This heated the Earth... 440 00:31:27,386 --> 00:31:31,357 ...and stopped the ocean currents from circulating. 441 00:31:45,946 --> 00:31:47,948 Noxious bacteria bloomed... 442 00:31:48,115 --> 00:31:51,836 ...but nearly everything else in the seas died. 443 00:31:51,993 --> 00:31:56,669 The stagnant waters belched deadly hydrogen sulfide gas into the air... 444 00:31:56,832 --> 00:32:00,382 ...which suffocated most of the land animals. 445 00:32:13,181 --> 00:32:16,811 Nine in 10 of all species on the planet went extinct. 446 00:32:17,853 --> 00:32:20,948 We call it the Great Dying. 447 00:32:33,702 --> 00:32:36,626 Life on Earth came so near to being wiped out... 448 00:32:36,788 --> 00:32:40,088 ...that it took more than 10 million years to recover. 449 00:32:40,250 --> 00:32:42,298 But new life-forms slowly evolved... 450 00:32:42,461 --> 00:32:46,261 ...to fill the openings left by the Permian holocaust. 451 00:32:53,972 --> 00:32:57,226 Among the biggest winners were the dinosaurs. 452 00:32:57,392 --> 00:33:00,111 Now the Earth was their planet. 453 00:33:00,270 --> 00:33:04,320 Their reign continued for over 150 million years. 454 00:33:04,483 --> 00:33:09,364 Until it too came crashing down in another mass extinction. 455 00:33:09,821 --> 00:33:12,415 Life on Earth has taken quite a beating over the eons. 456 00:33:12,574 --> 00:33:17,831 And yet it's still there. The tenacity of life is mind-boggling. 457 00:33:17,996 --> 00:33:20,590 We keep finding it where no one thought it could be. 458 00:33:25,420 --> 00:33:27,798 That nameless corridor? 459 00:33:28,465 --> 00:33:31,059 That's for another day. 460 00:33:36,348 --> 00:33:41,149 I know an animal that can live in boiling water or in solid ice. 461 00:33:41,311 --> 00:33:44,485 It can go 10 years without a drop of water. 462 00:33:44,648 --> 00:33:46,650 It can travel naked in the cold vacuum... 463 00:33:46,816 --> 00:33:51,196 ...and intense radiation of space and will return unscathed. 464 00:33:51,363 --> 00:33:53,786 The tardigrade, or water bear. 465 00:33:53,949 --> 00:33:56,202 It's equally at home atop the tallest mountains... 466 00:33:56,368 --> 00:33:58,837 ...and in the deepest trenches of the sea. 467 00:33:58,995 --> 00:34:01,714 And in our own backyards, where they live among the moss... 468 00:34:01,873 --> 00:34:04,171 ...in countless numbers. 469 00:34:04,334 --> 00:34:07,304 You've probably never noticed them because they're so small. 470 00:34:07,462 --> 00:34:09,464 About the size of a pinpoint. 471 00:34:09,631 --> 00:34:10,974 But they're tough. 472 00:34:11,132 --> 00:34:14,978 The tardigrades have survived all five mass extinctions. 473 00:34:15,136 --> 00:34:17,810 They've been in business for a half a billion years. 474 00:34:17,973 --> 00:34:21,273 We used to think that life was finicky, that it would only take hold... 475 00:34:21,434 --> 00:34:24,233 ...where it was not too hot, not too cold... 476 00:34:24,396 --> 00:34:27,650 ...not too dark or salty or acidic or radioactive. 477 00:34:27,816 --> 00:34:30,945 And whatever you do, don't forget to add water. 478 00:34:31,111 --> 00:34:34,490 We were wrong. As the hardy tardigrade demonstrates... 479 00:34:34,656 --> 00:34:39,036 ...life can endure conditions that would mean certain death for us humans. 480 00:34:39,202 --> 00:34:41,204 But differences between us and life found... 481 00:34:41,371 --> 00:34:43,999 ...in even the most extreme environments on our planet... 482 00:34:44,165 --> 00:34:49,547 ...are only variations on a single theme, dialects of a single language. 483 00:34:49,713 --> 00:34:51,807 The genetic code of Earth life. 484 00:34:56,886 --> 00:35:02,234 But what would life be like on other worlds? Worlds with a completely different history... 485 00:35:02,392 --> 00:35:05,612 ...chemistry and evolution from our planet? 486 00:35:07,856 --> 00:35:11,326 There's a distant world I wanna take you to. 487 00:35:11,901 --> 00:35:17,158 A world far different from our own, but one that may harbor life. 488 00:35:17,324 --> 00:35:21,295 If it does, it promises to be unlike anything... 489 00:35:21,453 --> 00:35:24,127 ...we've ever seen before. 490 00:35:43,224 --> 00:35:46,444 Clouds and haze completely hide the surface of Titan... 491 00:35:46,603 --> 00:35:48,731 ...Saturn's giant moon. 492 00:35:48,897 --> 00:35:51,491 Titan reminds me a little bit of home. 493 00:35:51,650 --> 00:35:54,529 Like Earth, it has an atmosphere that's mostly nitrogen. 494 00:35:54,694 --> 00:35:56,742 But it's four times denser. 495 00:35:56,905 --> 00:35:59,533 Titan's air has no oxygen at all. 496 00:35:59,699 --> 00:36:06,048 And it's far colder than anywhere on Earth. But still, I wanna go there. 497 00:36:08,375 --> 00:36:11,845 We have to descend through a couple hundred kilometers of smog... 498 00:36:12,003 --> 00:36:14,597 ...before we can even see the surface. 499 00:36:14,756 --> 00:36:19,603 But hidden beneath lies a weirdly familiar landscape. 500 00:36:27,394 --> 00:36:31,649 Titan is the only other world in the solar system where it ever rains. 501 00:36:31,815 --> 00:36:35,285 It has rivers and coastlines. 502 00:36:39,656 --> 00:36:45,288 Titan has hundreds of lakes. One of them larger than Lake Superior in North America. 503 00:36:45,453 --> 00:36:49,253 Vapor rising from the lakes condenses and falls again as rain. 504 00:36:50,792 --> 00:36:53,011 The rain feeds rivers... 505 00:36:53,795 --> 00:36:59,393 ...which carve valleys into the landscape, just like on Earth. 506 00:37:00,969 --> 00:37:02,937 But with one big difference. 507 00:37:03,096 --> 00:37:05,474 On Titan, the seas and the rain... 508 00:37:05,640 --> 00:37:09,690 ...are made not of water, but of methane and ethane. 509 00:37:09,853 --> 00:37:12,527 On Earth, those molecules form natural gas. 510 00:37:14,107 --> 00:37:17,532 On frigid Titan, they're liquid. 511 00:37:22,991 --> 00:37:27,622 Titan has lots of water but all of it is frozen hard as rock. 512 00:37:27,787 --> 00:37:32,543 In fact, the landscape and mountains are made mainly of water ice. 513 00:37:32,709 --> 00:37:34,882 At hundreds of degrees below zero... 514 00:37:35,044 --> 00:37:38,548 ...Titan is far too cold for water to ever be liquid. 515 00:37:40,759 --> 00:37:44,104 Astrobiologists since Carl Sagan have wondered... 516 00:37:44,262 --> 00:37:47,983 ...if life might swim in Titan's hydrocarbon lakes. 517 00:37:49,893 --> 00:37:51,645 The chemical basis for such life... 518 00:37:51,811 --> 00:37:55,441 ...would have to be entirely different from anything we know. 519 00:37:55,607 --> 00:38:00,488 All life on Earth depends on liquid water, and Titan's surface has none of that. 520 00:38:00,653 --> 00:38:02,951 But we can imagine other kinds of life. 521 00:38:03,114 --> 00:38:07,164 There might be creatures that inhale hydrogen instead of oxygen. 522 00:38:07,327 --> 00:38:10,422 And exhale methane instead of carbon dioxide. 523 00:38:10,580 --> 00:38:14,050 They might use acetylene instead of sugar as an energy source. 524 00:38:14,209 --> 00:38:16,553 How could we find out if such creatures... 525 00:38:16,711 --> 00:38:21,012 ...rule a hidden empire beneath the oil-dark waves? 526 00:38:36,523 --> 00:38:39,868 We're diving down deep into the Kraken Sea... 527 00:38:40,026 --> 00:38:43,246 ...named for the mythic Norse sea monster. 528 00:38:46,074 --> 00:38:49,749 Even if there is one of those down there, we probably couldn't see it. 529 00:38:49,911 --> 00:38:52,209 It's so dark. 530 00:38:53,331 --> 00:38:56,756 If you took all the oil and natural gas on Earth... 531 00:38:56,918 --> 00:39:00,764 ...it would amount to but a tiny fraction of Titan's reserves. 532 00:39:04,259 --> 00:39:06,603 Let's turn on some lights. 533 00:39:10,682 --> 00:39:14,607 We're now 200 meters beneath the surface. 534 00:39:18,356 --> 00:39:22,987 Did you see something? Over there, by that vent. 535 00:39:23,152 --> 00:39:27,077 Maybe it was just my imagination. I guess we'll have to come back... 536 00:39:27,240 --> 00:39:29,493 ...if we want to find out for sure. 537 00:39:32,954 --> 00:39:36,333 There's one last story I want to tell you. 538 00:39:36,499 --> 00:39:40,549 And it's the greatest story science has ever told. 539 00:39:44,924 --> 00:39:49,100 It's the story of life on our world. 540 00:40:11,034 --> 00:40:13,878 Welcome to the Earth of four billion years ago. 541 00:40:14,829 --> 00:40:17,833 This was our planet before life. 542 00:40:17,999 --> 00:40:20,548 Nobody knows how life got started. 543 00:40:20,710 --> 00:40:23,133 Most of the evidence from that time was destroyed... 544 00:40:23,296 --> 00:40:25,674 ...by impact and erosion. 545 00:40:25,840 --> 00:40:29,515 Science works on the frontier between knowledge and ignorance. 546 00:40:29,677 --> 00:40:31,896 We're not afraid to admit what we don't know. 547 00:40:32,055 --> 00:40:33,728 There's no shame in that. 548 00:40:33,890 --> 00:40:37,861 The only shame is to pretend that we have all the answers. 549 00:40:38,019 --> 00:40:39,441 Maybe someone watching this... 550 00:40:39,604 --> 00:40:44,826 ...will be the first to solve the mystery of how life on Earth began. 551 00:40:52,075 --> 00:40:53,793 The evidence from living microbes... 552 00:40:53,952 --> 00:40:57,422 ...suggest that their earliest ancestors preferred high temperatures. 553 00:40:58,081 --> 00:41:02,712 Life on Earth may have arisen in hot water around submerged volcanic vents. 554 00:41:07,548 --> 00:41:09,926 In Carl Sagan's original Cosmos series... 555 00:41:10,093 --> 00:41:12,767 ...he traced the unbroken thread that stretches... 556 00:41:12,929 --> 00:41:15,182 ...directly from the one-celled organisms... 557 00:41:15,348 --> 00:41:19,649 ...of nearly four billion years ago to you. 558 00:41:19,811 --> 00:41:23,111 Four billion years in 40 seconds. 559 00:41:23,272 --> 00:41:26,617 From creatures who had yet to discern day from night... 560 00:41:26,776 --> 00:41:31,498 ...to beings who are exploring the cosmos. 561 00:42:19,412 --> 00:42:22,791 SAGAN: Those are some of the things that molecules do... 562 00:42:22,957 --> 00:42:26,461 ...given four billion years of evolution.