1 00:01:02,669 --> 00:01:07,333 All my life I've wondered about life beyond the Earth. 2 00:01:07,541 --> 00:01:11,534 On those countless other planets that we think circle other suns... 3 00:01:11,745 --> 00:01:13,610 ...is there also life? 4 00:01:13,814 --> 00:01:16,374 Might the beings of other worlds resemble us... 5 00:01:16,583 --> 00:01:19,245 ...or would they be astonishingly different? 6 00:01:19,453 --> 00:01:21,819 What would they be made of? 7 00:01:22,022 --> 00:01:24,183 In the vast Milky Way galaxy... 8 00:01:24,391 --> 00:01:27,792 ...how common is what we call life? 9 00:01:29,863 --> 00:01:31,296 The nature of life on Earth... 10 00:01:31,498 --> 00:01:33,261 ...and the quest for life elsewhere... 11 00:01:33,467 --> 00:01:36,595 ...are the two sides of the same question. 12 00:01:36,804 --> 00:01:39,568 The search for who we are. 13 00:01:44,645 --> 00:01:48,137 All living things on Earth are made of organic molecules... 14 00:01:48,348 --> 00:01:50,873 ...a complex microscopic architecture... 15 00:01:51,085 --> 00:01:53,815 ...built around atoms of carbon. 16 00:01:54,021 --> 00:01:56,080 In the great dark between the stars... 17 00:01:56,290 --> 00:01:59,225 ...there also are organic molecules... 18 00:01:59,426 --> 00:02:03,260 ...in immense clouds of gas and dust. 19 00:02:06,633 --> 00:02:08,260 Inside such clouds... 20 00:02:08,469 --> 00:02:11,961 ...there are batches of new worlds just forming. 21 00:02:12,172 --> 00:02:16,108 Their surfaces are very likely covered with organic molecules. 22 00:02:16,310 --> 00:02:19,404 These molecules almost certainly are not made by life... 23 00:02:19,613 --> 00:02:22,207 ...although they are the stuff of life. 24 00:02:22,416 --> 00:02:25,510 On suitable worlds, they may lead to life. 25 00:02:25,719 --> 00:02:29,246 Organic matter is abundant throughout the cosmos... 26 00:02:29,456 --> 00:02:33,222 ...produced by the same chemistry everywhere. 27 00:02:47,774 --> 00:02:50,265 Perhaps, given enough time... 28 00:02:50,477 --> 00:02:55,278 ...the origin and evolution of life is inevitable on every clement world. 29 00:02:55,482 --> 00:02:59,213 There will surely be some planets too hostile for life. 30 00:02:59,419 --> 00:03:02,115 On others, it may arise and die out... 31 00:03:02,322 --> 00:03:05,348 ...or never evolve beyond its simplest forms. 32 00:03:05,559 --> 00:03:07,959 And on some small fraction of worlds... 33 00:03:08,162 --> 00:03:11,461 ...there may develop intelligences and civilizations... 34 00:03:11,665 --> 00:03:13,997 ...more advanced than ours. 35 00:03:17,237 --> 00:03:20,206 All life on our planet is closely related. 36 00:03:20,407 --> 00:03:24,776 We have a common organic chemistry and a common evolutionary heritage. 37 00:03:24,978 --> 00:03:28,675 And so our biologists are profoundly limited. 38 00:03:28,882 --> 00:03:31,146 They study a single biology... 39 00:03:31,351 --> 00:03:35,515 ...one lonely theme in the music of life. 40 00:03:35,722 --> 00:03:39,419 Is it the only voice for thousands of light years... 41 00:03:39,626 --> 00:03:44,325 ...or is there a cosmic fugue, a billion different voices... 42 00:03:44,531 --> 00:03:48,467 ...playing the life music of the galaxy? 43 00:03:52,940 --> 00:03:55,841 This blue world is where we grew up. 44 00:03:56,043 --> 00:03:58,136 There was once a time before life. 45 00:03:58,345 --> 00:04:01,508 Our planet is now burgeoning with life. 46 00:04:01,715 --> 00:04:03,580 How did it come about? 47 00:04:03,784 --> 00:04:07,242 How were organic molecules originally made? 48 00:04:07,454 --> 00:04:09,718 How did life evolve to produce beings... 49 00:04:09,923 --> 00:04:12,824 ...as elaborate and complex as we... 50 00:04:13,026 --> 00:04:17,258 ...able to explore the mystery of our own origins? 51 00:04:20,500 --> 00:04:24,027 Let me tell you a story about one little phrase... 52 00:04:24,238 --> 00:04:26,172 ...in the music of life on Earth. 53 00:04:43,657 --> 00:04:45,522 In the history of humans... 54 00:04:45,726 --> 00:04:47,591 ...in the 12th century... 55 00:04:47,794 --> 00:04:52,390 ...Japan was ruled by a clan of warriors called the Heike. 56 00:05:02,042 --> 00:05:05,637 The nominal leader of the Heike, the emperor of Japan... 57 00:05:05,846 --> 00:05:09,043 ...was a 7-year-old boy named Antoku. 58 00:05:09,249 --> 00:05:13,549 His guardian was his grandmother, the Lady Nii. 59 00:05:22,629 --> 00:05:25,860 The Heike were engaged in a long and bloody war... 60 00:05:26,066 --> 00:05:29,433 ...with another Samurai clan, the Genji. 61 00:05:34,675 --> 00:05:38,008 Each asserted a superior ancestral claim... 62 00:05:38,211 --> 00:05:40,076 ...to the imperial throne. 63 00:05:42,783 --> 00:05:46,583 Their decisive encounter occurred at Dannoura... 64 00:05:46,787 --> 00:05:50,348 ...in the Japanese Inland Sea on April 24... 65 00:05:50,557 --> 00:05:53,492 ...in the year 1185. 66 00:05:55,962 --> 00:05:58,988 The Heike were badly outnumbered and outmaneuvered. 67 00:05:59,433 --> 00:06:01,367 With their cause clearly lost... 68 00:06:01,568 --> 00:06:06,505 ...the surviving Heike warriors threw themselves into the sea and drowned. 69 00:06:12,746 --> 00:06:15,180 The emperor's grandmother, the Lady Nii... 70 00:06:15,382 --> 00:06:18,112 ...resolved that they would not be captured by the enemy. 71 00:06:18,318 --> 00:06:22,721 What happened next is related in "The Tale of the Heike": 72 00:06:23,557 --> 00:06:27,015 "The young emperor asked the Lady Nii, 'Where are you to take me?' 73 00:06:27,894 --> 00:06:32,593 She turned to the youthful sovereign with tears streaming down her cheeks... 74 00:06:32,799 --> 00:06:34,198 ...and comforted him. 75 00:06:46,580 --> 00:06:48,207 Blinded with tears... 76 00:06:48,415 --> 00:06:52,784 ...the child sovereign put his beautiful small hands together. 77 00:06:55,122 --> 00:06:57,022 He turned first to the east... 78 00:06:57,224 --> 00:07:00,387 ...to say farewell to the god of Ise... 79 00:07:00,594 --> 00:07:02,118 ...and then to the west... 80 00:07:02,329 --> 00:07:05,992 ...to recite a prayer to the Amida Buddha. 81 00:07:07,267 --> 00:07:08,928 The Lady Nii... 82 00:07:09,136 --> 00:07:11,661 ...took him in her arms, and with the words: 83 00:07:11,872 --> 00:07:14,841 'In the depths of the ocean is our capital'... 84 00:07:15,041 --> 00:07:18,272 ...sank with him at last beneath the waves." 85 00:07:39,699 --> 00:07:43,499 The destruction of the Heike battle fleet at Dannoura... 86 00:07:43,703 --> 00:07:46,501 ...marked the end of the clan's 30-year rule. 87 00:07:46,706 --> 00:07:49,971 The Heike all but vanished from history. 88 00:07:54,981 --> 00:07:58,348 Only 43 Heike survived, all women. 89 00:07:58,552 --> 00:08:01,885 These former ladies-in-waiting of the Imperial Court... 90 00:08:02,088 --> 00:08:05,546 ...were reduced to selling flowers and other favors... 91 00:08:05,759 --> 00:08:08,819 ...to the fishermen near the scene of the battle. 92 00:08:15,302 --> 00:08:18,499 These women and their offspring by the fishermen... 93 00:08:18,705 --> 00:08:22,038 ...established a festival to commemorate the battle. 94 00:08:28,682 --> 00:08:32,345 To this day, every year, on the 24th of April... 95 00:08:32,552 --> 00:08:35,453 ...their descendants proceed to the Akama shrine... 96 00:08:35,655 --> 00:08:37,350 ...which contains the mausoleum... 97 00:08:37,557 --> 00:08:41,618 ...of the drowned 7-year-old emperor, Antoku. 98 00:08:45,232 --> 00:08:48,360 There, they conduct a ceremony of remembrance... 99 00:08:48,568 --> 00:08:52,527 ...for the life and death of the Heike warriors. 100 00:09:00,914 --> 00:09:03,883 But there is a strange postscript to this story: 101 00:09:04,084 --> 00:09:05,642 The fishermen say... 102 00:09:05,852 --> 00:09:10,585 ...that the Heike samurai wander the bottom of the Inland Sea... 103 00:09:10,790 --> 00:09:13,520 ...in the form of crabs. 104 00:09:14,160 --> 00:09:18,620 There are crabs here which have curious markings on their backs. 105 00:09:18,899 --> 00:09:21,891 Patterns which resemble a human face... 106 00:09:22,102 --> 00:09:25,071 ...with the aggressive scowl of a samurai warrior... 107 00:09:25,272 --> 00:09:27,365 ...from medieval Japan. 108 00:09:35,048 --> 00:09:38,142 These Heike crabs, when caught, are not eaten. 109 00:09:38,351 --> 00:09:40,819 They are thrown back into the sea... 110 00:09:41,021 --> 00:09:43,751 ...in commemoration of the doleful events... 111 00:09:43,957 --> 00:09:46,926 ...of the battle of Dannoura. 112 00:09:58,071 --> 00:10:01,268 This legend raises a lovely problem: 113 00:10:01,474 --> 00:10:05,308 How does it come about that the face of a warrior... 114 00:10:05,512 --> 00:10:10,279 ...is cut on the carapace of a Japanese crab? How could it be? 115 00:10:10,483 --> 00:10:15,045 The answer seems to be that humans made this face. 116 00:10:15,388 --> 00:10:16,855 But how? 117 00:10:17,057 --> 00:10:20,322 Like many other features, the patterns on the back... 118 00:10:20,527 --> 00:10:23,894 ...or carapace of this crab are inherited. 119 00:10:24,097 --> 00:10:28,500 But among crabs, as among humans, there are different hereditary lines. 120 00:10:28,935 --> 00:10:31,961 Now, suppose purely by chance... 121 00:10:32,172 --> 00:10:35,403 ...among the distant ancestors of this crab... 122 00:10:35,609 --> 00:10:40,342 ...there came to be one which looked just a little bit like a human face. 123 00:10:40,547 --> 00:10:44,039 Long before the battle, fishermen may have been reluctant... 124 00:10:44,250 --> 00:10:46,275 ...to eat a crab with a human face. 125 00:10:46,620 --> 00:10:48,986 In throwing it back into the sea... 126 00:10:49,189 --> 00:10:53,319 ...they were setting into motion a process of selection. 127 00:10:53,526 --> 00:10:58,054 If you're a crab and your carapace is just ordinary... 128 00:10:58,264 --> 00:11:00,562 ...the humans are gonna eat you. 129 00:11:00,767 --> 00:11:03,327 But if it looks a little bit like a face... 130 00:11:03,536 --> 00:11:07,028 ...they'll throw you back and you can have lots of baby crabs... 131 00:11:07,240 --> 00:11:09,003 ...that all look just like you. 132 00:11:09,209 --> 00:11:11,302 As many generations passed... 133 00:11:11,511 --> 00:11:14,378 ...of crabs and fisher-folk alike... 134 00:11:14,581 --> 00:11:18,449 ...the crabs with patterns that looked most like a samurai face... 135 00:11:18,652 --> 00:11:20,847 ...preferentially survived. 136 00:11:21,054 --> 00:11:24,615 Until eventually, there was produced not just a human face... 137 00:11:24,824 --> 00:11:27,292 ...not just a Japanese face... 138 00:11:27,494 --> 00:11:30,429 ...but the face of a samurai warrior. 139 00:11:30,630 --> 00:11:34,589 All this has nothing to do with what the crabs might want. 140 00:11:34,801 --> 00:11:38,237 Selection is imposed from the outside. 141 00:11:38,438 --> 00:11:42,374 The more you look like a samurai, the better your chances of survival. 142 00:11:42,575 --> 00:11:47,239 Eventually, there are a lot of crabs that look like samurai warriors. 143 00:12:08,568 --> 00:12:12,129 This process is called artificial selection. 144 00:12:12,872 --> 00:12:15,170 In the case of the Heike crab, it was effected... 145 00:12:15,375 --> 00:12:18,242 ...more or less unconsciously by the fishermen... 146 00:12:18,445 --> 00:12:22,711 ...and certainly without any serious contemplation by the crabs. 147 00:12:22,916 --> 00:12:25,476 Humans, for thousands of years... 148 00:12:25,685 --> 00:12:27,550 ...have deliberately selected... 149 00:12:27,754 --> 00:12:30,416 ...which plants and animals shall live. 150 00:12:30,657 --> 00:12:34,115 We're surrounded by farm and domestic animals... 151 00:12:34,327 --> 00:12:35,624 ...fruits, vegetables. 152 00:12:36,496 --> 00:12:40,125 Where do they come from? Were they once free-living in the wild... 153 00:12:40,333 --> 00:12:44,167 ...and then induced to adopt some less strenuous life on the farm? 154 00:12:44,637 --> 00:12:45,626 No. 155 00:12:45,839 --> 00:12:49,900 They are, almost all of them, made by us. 156 00:12:53,613 --> 00:12:58,141 The essence of artificial selection for a horse or a cow... 157 00:12:58,351 --> 00:13:01,684 ...a grain of rice or a Heike crab, is this: 158 00:13:01,888 --> 00:13:05,187 Many characteristics are inherited. They breed true. 159 00:13:07,160 --> 00:13:10,755 Humans encourage the reproduction of some varieties... 160 00:13:10,964 --> 00:13:12,898 ...and discourage the reproduction of others. 161 00:13:13,099 --> 00:13:16,535 The variety selected for, eventually becomes abundant. 162 00:13:16,736 --> 00:13:20,695 The variety selected against, becomes rare, maybe extinct. 163 00:13:21,908 --> 00:13:25,071 But if artificial selection makes such changes... 164 00:13:25,278 --> 00:13:27,337 ...in only a few thousand years... 165 00:13:27,547 --> 00:13:29,572 ...what must natural selection... 166 00:13:29,783 --> 00:13:33,776 ...working for billions of years, be capable of? 167 00:13:34,020 --> 00:13:35,078 The answer... 168 00:13:35,288 --> 00:13:39,088 ...is all the beauty and diversity in the biological world. 169 00:13:43,830 --> 00:13:47,891 That life evolved over the ages is clear... 170 00:13:48,101 --> 00:13:51,730 ...from the changes we've made in the beasts and vegetables... 171 00:13:52,071 --> 00:13:55,632 ...but also from the record in the rocks. 172 00:13:55,942 --> 00:13:58,706 The fossil evidence speaks to us unambiguously... 173 00:13:58,912 --> 00:14:03,406 ...of creatures that were once present in enormous numbers... 174 00:14:03,616 --> 00:14:05,641 ...and that have now vanished utterly. 175 00:14:05,852 --> 00:14:09,344 There are more species that have become extinct than exist today. 176 00:14:09,556 --> 00:14:13,652 They are the terminated experiments in evolution. 177 00:14:14,727 --> 00:14:19,323 These guys, the trilobites, appeared 600 million years ago. 178 00:14:19,532 --> 00:14:22,160 They were around for 300 million years. 179 00:14:22,368 --> 00:14:25,633 They're all gone. There's none left. 180 00:14:25,939 --> 00:14:30,569 But in those old rocks, there are no fossils of people or cattle. 181 00:14:30,777 --> 00:14:32,608 We've evolved only recently. 182 00:14:33,813 --> 00:14:37,044 Evolution is a fact, not a theory. 183 00:14:37,250 --> 00:14:39,184 It really happened. 184 00:14:45,124 --> 00:14:49,356 That the mechanism of evolution is natural selection was the discovery... 185 00:14:49,562 --> 00:14:52,998 ...of Charles Darwin and Alfred Russel Wallace. 186 00:14:53,666 --> 00:14:54,894 Here's how it works: 187 00:14:55,401 --> 00:14:56,925 Nature is prolific. 188 00:14:57,136 --> 00:15:01,004 There are many more creatures that are born than can possibly survive. 189 00:15:01,207 --> 00:15:06,144 So those varieties which are, by accident, less well adapted... 190 00:15:06,379 --> 00:15:09,871 ...don't survive, or at least they leave fewer offspring. 191 00:15:10,083 --> 00:15:14,543 Now, mutations, sudden changes in heredity... 192 00:15:14,754 --> 00:15:16,813 ...are passed on. They breed true. 193 00:15:17,023 --> 00:15:21,960 The environment selects the occasional mutations which enhance survival. 194 00:15:22,195 --> 00:15:26,632 The resulting series of slow changes in the nature of living beings... 195 00:15:26,833 --> 00:15:28,960 ...is the origin of new species. 196 00:15:30,536 --> 00:15:33,903 Many people were scandalized... 197 00:15:34,107 --> 00:15:36,302 ...by the ideas of evolution and natural selection. 198 00:15:36,509 --> 00:15:38,636 Our ancestors looked at... 199 00:15:38,845 --> 00:15:41,643 ...the intricacy and the beauty of life... 200 00:15:41,848 --> 00:15:45,648 ...and saw evidence for a great designer. 201 00:15:48,554 --> 00:15:52,046 The simplest organism is a far more complex machine... 202 00:15:52,258 --> 00:15:54,419 ...than the finest pocket watch. 203 00:15:54,627 --> 00:15:58,927 And yet, pocket watches don't spontaneously self-assemble... 204 00:15:59,132 --> 00:16:02,431 ...or evolve in slow stages on their own... 205 00:16:02,635 --> 00:16:05,433 ...from say, grandfather clocks. 206 00:16:05,738 --> 00:16:08,673 A watch implies a watchmaker. 207 00:16:10,310 --> 00:16:14,804 There seemed to be no way atoms could spontaneously fall together... 208 00:16:15,014 --> 00:16:17,448 ...and create, say... 209 00:16:19,152 --> 00:16:20,312 ...a dandelion. 210 00:16:20,987 --> 00:16:23,285 The idea of a designer... 211 00:16:23,489 --> 00:16:28,153 ...is an appealing and altogether human explanation of the biological world. 212 00:16:28,361 --> 00:16:31,592 But as Darwin and Wallace showed... 213 00:16:31,798 --> 00:16:33,459 ...there's another way... 214 00:16:33,666 --> 00:16:37,397 ...equally human and far more compelling. 215 00:16:37,804 --> 00:16:41,934 Natural selection, which makes the music of life more beautiful... 216 00:16:42,141 --> 00:16:44,336 ...as the eons pass. 217 00:16:50,383 --> 00:16:52,578 To understand the passage of the eons... 218 00:16:52,785 --> 00:16:56,118 ...we have compressed all of time into a single cosmic year... 219 00:16:56,322 --> 00:16:59,655 ...with the big bang on January first. 220 00:16:59,859 --> 00:17:04,296 Every month here represents a little over a billion years. 221 00:17:04,497 --> 00:17:08,365 The Earth didn't form until the cosmic year was two-thirds over. 222 00:17:08,568 --> 00:17:12,732 Our understanding of the history of life is very recent... 223 00:17:12,939 --> 00:17:16,375 ...occupying only the last few seconds of December 31... 224 00:17:16,576 --> 00:17:20,706 ...that small white spot at bottom right in the cosmic calendar. 225 00:17:20,913 --> 00:17:23,313 What happened on Earth may be more or less typical... 226 00:17:23,516 --> 00:17:25,984 ...of the evolution of life on many worlds. 227 00:17:26,185 --> 00:17:28,176 But in its details... 228 00:17:28,388 --> 00:17:30,583 ...the story of life on Earth... 229 00:17:30,790 --> 00:17:33,588 ...is probably unique in all the Milky Way galaxy. 230 00:17:34,260 --> 00:17:38,697 The secrets of evolution are time and death. 231 00:17:38,898 --> 00:17:42,732 Time for the slow accumulation of favorable mutations... 232 00:17:42,935 --> 00:17:46,530 ...and death to make room for new species. 233 00:17:47,240 --> 00:17:50,505 Life on Earth arose in September of the cosmic calendar... 234 00:17:50,710 --> 00:17:55,374 ...when our world, still battered and cratered from its violent origin... 235 00:17:55,581 --> 00:17:57,742 ...may have looked a little like the moon. 236 00:17:59,919 --> 00:18:03,446 The Earth is about four and a half billion years old. 237 00:18:03,656 --> 00:18:04,987 In the cosmic calendar... 238 00:18:05,191 --> 00:18:09,184 ...it condensed out of interstellar gas and dust... 239 00:18:09,395 --> 00:18:12,023 ...around September 14. 240 00:18:12,231 --> 00:18:15,894 We know from the fossil record that life originated soon after... 241 00:18:16,102 --> 00:18:19,367 ...maybe around September 25, something like that... 242 00:18:19,572 --> 00:18:23,440 ...probably in the ponds and oceans of the primitive Earth. 243 00:18:23,676 --> 00:18:28,272 The first living things were not as complex as a one-celled organism... 244 00:18:28,481 --> 00:18:32,110 ...which is already a highly sophisticated form of life. 245 00:18:32,318 --> 00:18:36,345 No, the first stirrings of life were much more humble... 246 00:18:36,556 --> 00:18:39,389 ...and happened on the molecular level. 247 00:18:39,592 --> 00:18:43,790 In those early days, lightning and ultraviolet light from the sun... 248 00:18:43,996 --> 00:18:48,194 ...were breaking apart hydrogen-rich molecules in the atmosphere. 249 00:18:48,401 --> 00:18:52,895 The fragments of the molecules were spontaneously recombining... 250 00:18:53,106 --> 00:18:56,906 ...into more and more complex molecules. 251 00:18:59,045 --> 00:19:03,004 The products of this early chemistry dissolved in the oceans... 252 00:19:03,216 --> 00:19:06,515 ...forming a kind of organic soup... 253 00:19:06,719 --> 00:19:08,846 ...of gradually increasing complexity. 254 00:19:09,055 --> 00:19:12,855 Until one day, quite by accident... 255 00:19:13,059 --> 00:19:17,155 ...a molecule arose that was able to make crude copies of itself... 256 00:19:17,363 --> 00:19:20,799 ...using as building blocks the other molecules in the soup. 257 00:19:21,000 --> 00:19:24,458 This was the ancestor of DNA... 258 00:19:24,670 --> 00:19:28,037 ...the master molecule of life on Earth. 259 00:19:28,241 --> 00:19:31,904 It's made of four different parts, called nucleotides... 260 00:19:32,111 --> 00:19:35,808 ...which constitute the four letters of the genetic code... 261 00:19:36,015 --> 00:19:37,573 ...the language of heredity. 262 00:19:37,783 --> 00:19:42,652 Each of the nucleotides, the rungs on the DNA ladder... 263 00:19:42,855 --> 00:19:45,153 ...are a different color in this model. 264 00:19:45,358 --> 00:19:48,623 The instructions are different for different organisms. 265 00:19:48,828 --> 00:19:51,194 That's why organisms are different. 266 00:19:51,397 --> 00:19:55,231 Now, a mutation is a change of a nucleotide... 267 00:19:55,434 --> 00:19:58,460 ...a misspelling of the genetic instructions. 268 00:19:58,671 --> 00:20:03,540 Most mutations spell genetic nonsense since they're random. 269 00:20:03,743 --> 00:20:05,938 They harm the next generation. 270 00:20:06,145 --> 00:20:08,443 But a very few, by accident... 271 00:20:08,648 --> 00:20:13,142 ...make better sense than the original codes, and aid evolution. 272 00:20:13,853 --> 00:20:17,152 DNA is about a billion times smaller... 273 00:20:17,356 --> 00:20:19,085 ...than we see it here. 274 00:20:19,292 --> 00:20:24,229 Each of those things that looks like a piece of fruit is an atom. 275 00:20:24,430 --> 00:20:26,022 Without the tools of science... 276 00:20:26,232 --> 00:20:29,531 ...the machinery of life would be invisible. 277 00:20:33,773 --> 00:20:35,400 Four billion years ago... 278 00:20:35,608 --> 00:20:39,635 ...the ancestors of DNA competed for molecular building blocks... 279 00:20:39,845 --> 00:20:42,814 ...and left crude copies of themselves. 280 00:20:43,015 --> 00:20:46,280 There were no predators; the stuff of life was everywhere. 281 00:20:46,485 --> 00:20:49,852 The oceans and murky pools that filled the craters... 282 00:20:50,056 --> 00:20:53,355 ...were, for these molecules, a Garden of Eden. 283 00:20:53,693 --> 00:20:56,685 With reproduction, mutation and natural selection... 284 00:20:56,896 --> 00:21:00,923 ...the evolution of living molecules was well underway. 285 00:21:01,467 --> 00:21:05,369 Varieties with specialized functions joined together... 286 00:21:05,571 --> 00:21:08,597 ...making a collective. The first cell. 287 00:21:08,808 --> 00:21:11,868 The organic soup eventually ate itself up. 288 00:21:12,078 --> 00:21:15,445 But by this time, plants had evolved, able to use sunlight... 289 00:21:15,648 --> 00:21:20,381 ...to make their own building blocks. They turned the waters green. 290 00:21:20,586 --> 00:21:22,781 One-celled plants joined together: 291 00:21:22,989 --> 00:21:26,015 The first multi-cellular organisms. 292 00:21:27,526 --> 00:21:31,963 Equally important was the invention, not made until early November... 293 00:21:32,164 --> 00:21:36,658 ...of sex. It was stumbled upon by the microbes. 294 00:21:39,338 --> 00:21:43,001 By December 1, green plants had released copious amounts... 295 00:21:43,209 --> 00:21:46,440 ...of oxygen and nitrogen into the atmosphere. 296 00:21:46,646 --> 00:21:49,945 The sky is made by life. 297 00:21:51,250 --> 00:21:53,980 Then, suddenly, on December 15... 298 00:21:54,186 --> 00:21:57,178 ...there was an enormous proliferation of new life forms... 299 00:21:57,390 --> 00:22:00,826 ...an event called the "Cambrian Explosion." 300 00:22:05,765 --> 00:22:10,259 We know from fossils that life arose shortly after the Earth formed... 301 00:22:10,469 --> 00:22:14,200 ...suggesting that the origin of life might be... 302 00:22:14,407 --> 00:22:18,605 ...an inevitable chemical process on countless Earth-like planets... 303 00:22:18,811 --> 00:22:20,403 ...throughout the cosmos. 304 00:22:20,613 --> 00:22:25,414 But on the Earth, in nearly 4 billion years, life advanced no further... 305 00:22:25,618 --> 00:22:26,585 ...than algae. 306 00:22:26,786 --> 00:22:31,587 So maybe more complex forms of life are harder to evolve... 307 00:22:31,791 --> 00:22:34,351 ...harder even than the origin of life itself. 308 00:22:34,560 --> 00:22:37,222 If this is right, the planets of the galaxy... 309 00:22:37,430 --> 00:22:40,092 ...might be filled with microorganisms... 310 00:22:40,299 --> 00:22:44,167 ...but big beasts and vegetables and thinking beings... 311 00:22:44,370 --> 00:22:47,168 ...might be comparatively rare. 312 00:22:50,643 --> 00:22:54,704 By December 18, there were vast herds of trilobites... 313 00:22:54,914 --> 00:22:57,041 ...foraging on the ocean bottom... 314 00:22:57,249 --> 00:23:00,685 ...and squid-like creatures with multicolored shells... 315 00:23:00,886 --> 00:23:02,353 ...were everywhere. 316 00:23:05,291 --> 00:23:08,886 We know enough to sketch in a few of the subsequent details. 317 00:23:09,095 --> 00:23:12,792 The first fish and the first vertebrates appeared on December 19. 318 00:23:12,998 --> 00:23:16,559 Plants began to colonize the land on December 20. 319 00:23:16,769 --> 00:23:20,865 The first winged insects fluttered by on December 22. 320 00:23:21,073 --> 00:23:24,201 On this date also, there were the first amphibians... 321 00:23:24,410 --> 00:23:26,844 ...creatures something like the lungfish... 322 00:23:27,046 --> 00:23:30,311 ...able to survive both on land and in water. 323 00:23:30,516 --> 00:23:34,748 Our direct ancestors were now leaving the oceans behind. 324 00:23:37,623 --> 00:23:42,560 The first trees and the first reptiles evolved on December 23: 325 00:23:42,795 --> 00:23:46,060 Two amazing evolutionary developments. 326 00:23:48,401 --> 00:23:51,837 We are descended from some of those reptiles. 327 00:23:56,242 --> 00:23:59,575 The dinosaurs appeared on Christmas Eve. 328 00:23:59,779 --> 00:24:01,838 There were many different kinds of dinosaurs. 329 00:24:02,047 --> 00:24:05,016 The Earth was once their planet. 330 00:24:08,154 --> 00:24:11,590 Many stood upright and had some fair intelligence. 331 00:24:11,791 --> 00:24:16,626 Great lizards crashed and thundered through the steaming jungles. 332 00:24:21,734 --> 00:24:24,362 Unnoticed by the dinosaurs, a new creature... 333 00:24:24,570 --> 00:24:26,731 ...whose young were born live and helpless... 334 00:24:26,939 --> 00:24:29,271 ...was making its timid debut. 335 00:24:29,475 --> 00:24:32,603 The first mammals emerged on December 26... 336 00:24:32,812 --> 00:24:36,077 ...the first birds on the following day. 337 00:24:39,385 --> 00:24:42,786 But the dinosaurs still dominated the planet. 338 00:24:42,988 --> 00:24:46,924 Then suddenly, without warning, all over the planet at once... 339 00:24:47,126 --> 00:24:48,889 ...the dinosaurs died. 340 00:24:49,094 --> 00:24:52,461 The cause is unknown, but the lesson is clear: 341 00:24:52,665 --> 00:24:57,364 Even 160 million years on a planet is no guarantee of survival. 342 00:24:57,570 --> 00:25:02,064 The dinosaurs perished around the time of the first flower. 343 00:25:03,909 --> 00:25:06,036 On December 30, the first creatures... 344 00:25:06,245 --> 00:25:09,078 ...who looked even a little bit human, evolved... 345 00:25:09,281 --> 00:25:13,445 ...accompanied by a spectacular increase in the size of their brains. 346 00:25:13,652 --> 00:25:17,520 And then, on the evening of the last day of the last month... 347 00:25:17,723 --> 00:25:19,350 ...only a few million years ago... 348 00:25:19,558 --> 00:25:24,393 ...the first true humans took their place on the cosmic calendar. 349 00:25:25,698 --> 00:25:27,495 The written record of history... 350 00:25:27,700 --> 00:25:31,363 ...occupies only the last 10 seconds of the cosmic year. 351 00:25:33,873 --> 00:25:37,775 Now, let's take a closer look at who our ancestors were. 352 00:25:37,977 --> 00:25:41,037 A simple chemical circumstance led to a great moment... 353 00:25:41,247 --> 00:25:43,272 ...in the history of our planet. 354 00:25:43,482 --> 00:25:46,417 There were many molecules in the primordial soup. 355 00:25:46,619 --> 00:25:51,454 Some were attracted to water on one side and repelled by it on the other. 356 00:25:51,657 --> 00:25:54,251 This drove them together... 357 00:25:54,460 --> 00:25:57,258 ...into a tiny enclosed spherical shell... 358 00:25:57,463 --> 00:26:00,261 ...like a soap bubble, which protected the interior. 359 00:26:00,466 --> 00:26:04,027 Within the bubble, the ancestors of DNA found a home... 360 00:26:04,236 --> 00:26:05,931 ...and the first cell arose. 361 00:26:06,138 --> 00:26:10,131 It took hundreds of millions of years for tiny plants to evolve... 362 00:26:10,342 --> 00:26:11,809 ...giving off oxygen. 363 00:26:12,011 --> 00:26:15,242 But that branch didn't lead to us. 364 00:26:15,748 --> 00:26:20,583 Bacteria that could breathe oxygen took over a billion years to evolve. 365 00:26:21,954 --> 00:26:26,288 From a naked nucleus, a cell developed with a nucleus inside. 366 00:26:27,693 --> 00:26:31,993 Some of these amoeba-like forms led eventually to plants. 367 00:26:36,068 --> 00:26:38,002 Others produced colonies... 368 00:26:38,203 --> 00:26:42,469 ...with inside and outside cells performing different functions. 369 00:26:44,310 --> 00:26:45,641 Becoming... 370 00:26:45,844 --> 00:26:49,507 ...a polyp attached to the ocean floor... 371 00:26:49,715 --> 00:26:52,081 ...filtering food from the water... 372 00:26:52,518 --> 00:26:55,043 ...and evolving little tentacles... 373 00:26:55,254 --> 00:26:58,587 ...to direct food into a primitive mouth. 374 00:26:59,825 --> 00:27:01,520 This humble ancestor of ours also led... 375 00:27:01,727 --> 00:27:05,891 ...to spiny-skinned armored animals with internal organs... 376 00:27:06,098 --> 00:27:09,966 ...including our cousin, the starfish. 377 00:27:10,169 --> 00:27:12,603 But we don't come from starfish. 378 00:27:13,539 --> 00:27:15,803 About 550 million years ago... 379 00:27:16,008 --> 00:27:19,000 ...filter feeders evolved gill slits... 380 00:27:19,211 --> 00:27:22,510 ...which were more efficient at straining food particles. 381 00:27:22,715 --> 00:27:26,446 One evolutionary branch led to acorn worms. 382 00:27:26,719 --> 00:27:31,247 Another led to a creature which swam freely in the larval stage... 383 00:27:31,457 --> 00:27:34,790 ...but, as an adult, was still firmly anchored to the ocean floor. 384 00:27:34,994 --> 00:27:37,656 Some became living hollow tubes. 385 00:27:38,197 --> 00:27:42,531 But others retained the larval forms throughout the life cycle... 386 00:27:42,735 --> 00:27:46,762 ...and became free-swimming adults with something like a backbone. 387 00:27:51,744 --> 00:27:53,302 Our ancestors now... 388 00:27:53,512 --> 00:27:57,846 ...500 million years ago, were jawless filter-feeding fish... 389 00:27:58,050 --> 00:28:00,484 ...a little like lampreys. 390 00:28:02,287 --> 00:28:04,517 Gradually, those tiny fish... 391 00:28:04,723 --> 00:28:07,385 ...evolved eyes and jaws. 392 00:28:07,893 --> 00:28:10,054 Fish then began to eat one another... 393 00:28:10,262 --> 00:28:13,322 ...if you could swim fast, you survived. 394 00:28:15,067 --> 00:28:19,936 If you had jaws to eat with, you could use your gills to breathe in the water. 395 00:28:20,239 --> 00:28:23,140 This is the way modern fish arose. 396 00:28:27,413 --> 00:28:29,973 During the summer, swamps and lakes dried up. 397 00:28:30,182 --> 00:28:34,983 Some fish evolved a primitive lung to breathe air until the rains came. 398 00:28:35,187 --> 00:28:37,485 Their brains were getting bigger. 399 00:28:37,690 --> 00:28:41,217 If the rains didn't come, it was handy to be able to pull yourself... 400 00:28:41,427 --> 00:28:42,689 ...to the next swamp. 401 00:28:42,895 --> 00:28:45,796 That was a very important adaptation. 402 00:28:49,334 --> 00:28:52,770 The first amphibians evolved, still with a fish-like tail. 403 00:28:52,971 --> 00:28:57,340 Amphibians, like fish, laid their eggs in water where they were easily eaten. 404 00:28:57,543 --> 00:29:00,171 But then a splendid new invention came along: 405 00:29:00,379 --> 00:29:04,975 The hard-shelled egg, laid on land where there were as yet no predators. 406 00:29:05,184 --> 00:29:09,746 Reptiles and turtles go back to those days. 407 00:29:11,724 --> 00:29:15,057 Many of the reptiles hatched on land never returned to the waters. 408 00:29:15,260 --> 00:29:17,785 Some became the dinosaurs. 409 00:29:18,163 --> 00:29:22,395 One line of dinosaurs developed feathers, useful for short flights. 410 00:29:22,601 --> 00:29:27,470 Today, the only living descendants of the dinosaurs are the birds. 411 00:29:29,174 --> 00:29:31,608 The great dinosaurs evolved along another branch. 412 00:29:31,810 --> 00:29:34,836 Some were the largest flesh-eaters ever to walk the land. 413 00:29:35,047 --> 00:29:39,814 But 65 million years ago they all mysteriously perished. 414 00:29:41,053 --> 00:29:43,453 Meanwhile, the forerunners of the dinosaurs... 415 00:29:43,655 --> 00:29:46,283 ...were also evolving in a different direction. 416 00:29:46,492 --> 00:29:49,222 Small, scurrying creatures... 417 00:29:49,428 --> 00:29:52,329 ...with the young growing inside the mother's body. 418 00:29:52,531 --> 00:29:56,934 After the extinction of the dinosaurs, many different forms developed. 419 00:30:01,240 --> 00:30:03,936 The young were very immature at birth. 420 00:30:04,143 --> 00:30:07,203 In the marsupials, the wombat, for example... 421 00:30:07,412 --> 00:30:11,007 ...and in the mammals, the young had to be taught how to survive. 422 00:30:11,216 --> 00:30:13,411 The brain grew larger still. 423 00:30:13,619 --> 00:30:17,953 Something like a shrew was the ancestor of all the mammals. 424 00:30:22,661 --> 00:30:26,495 One line took to the trees, developing dexterity... 425 00:30:26,698 --> 00:30:28,666 ...stereo vision, larger brains... 426 00:30:28,867 --> 00:30:31,358 ...and a curiosity about their environment. 427 00:30:31,570 --> 00:30:36,405 Some became baboons, but that's not the line to us. 428 00:30:37,776 --> 00:30:40,768 Apes and humans have a recent common ancestor. 429 00:30:40,979 --> 00:30:44,938 Bone for bone, muscle for muscle, molecule for molecule. 430 00:30:45,150 --> 00:30:50,087 There are almost no important differences between apes and humans. 431 00:30:53,058 --> 00:30:57,358 Unlike the chimpanzee, our ancestors walked upright... 432 00:30:57,563 --> 00:31:01,590 ...freeing their hands to poke and fix and experiment. 433 00:31:01,800 --> 00:31:05,258 We got smarter. We began to talk. 434 00:31:11,777 --> 00:31:14,439 Many collateral branches of the human family... 435 00:31:14,646 --> 00:31:18,104 ...became extinct in the last few million years. 436 00:31:18,317 --> 00:31:22,720 We, with our brains and our hands, are the survivors. 437 00:31:22,921 --> 00:31:27,858 There's an unbroken thread that stretches from those first cells to us. 438 00:31:28,193 --> 00:31:29,490 Let's look at it again... 439 00:31:29,695 --> 00:31:34,496 ...compressing 4 billion years of human evolution into 40 seconds. 440 00:32:20,579 --> 00:32:23,742 Those are some of the things that molecules do... 441 00:32:23,949 --> 00:32:27,009 ...given 4 billion years of evolution. 442 00:32:28,220 --> 00:32:32,520 We sometimes represent evolution as the ever-branching ramifications... 443 00:32:32,724 --> 00:32:34,089 ...of some original trunk... 444 00:32:34,293 --> 00:32:38,559 ...each branch pruned and clipped by natural selection. 445 00:32:39,398 --> 00:32:41,366 Every plant and animal alive today... 446 00:32:41,566 --> 00:32:45,969 ...has a history as ancient and illustrious as ours. 447 00:32:46,171 --> 00:32:49,538 Humans stand on one branch. 448 00:32:49,975 --> 00:32:52,876 But now we affect the future of every branch... 449 00:32:53,078 --> 00:32:56,138 ...of this 4-billion-year-old tree. 450 00:32:58,750 --> 00:33:01,810 How lovely trees are. 451 00:33:02,020 --> 00:33:05,251 The human species grew up in and around them. 452 00:33:05,457 --> 00:33:08,392 We have a natural affinity for trees. 453 00:33:08,593 --> 00:33:10,652 Trees photosynthesize... 454 00:33:10,862 --> 00:33:13,592 ...they harvest sunlight... 455 00:33:13,799 --> 00:33:17,701 ...they compete for the sun's favors. 456 00:33:18,570 --> 00:33:20,299 Look at those two trees there... 457 00:33:20,505 --> 00:33:23,702 ...pushing and shoving for sunlight... 458 00:33:23,909 --> 00:33:28,505 ...but with grace and astonishing slowness. 459 00:33:36,188 --> 00:33:38,418 There are so many plants on the Earth... 460 00:33:38,623 --> 00:33:41,183 ...that there's a danger of thinking them trivial... 461 00:33:41,393 --> 00:33:45,523 ...of losing sight of the subtlety and efficiency of their design. 462 00:33:45,731 --> 00:33:50,293 They are great and beautiful machines, powered by sunlight... 463 00:33:50,502 --> 00:33:54,336 ...taking in water from the ground and carbon dioxide from the air... 464 00:33:54,539 --> 00:33:59,135 ...and converting them into food for their use and ours. 465 00:34:07,486 --> 00:34:11,047 This is a museum of living plants. 466 00:34:11,256 --> 00:34:16,125 The Royal Botanic Gardens at Kew in London. 467 00:34:21,833 --> 00:34:24,802 Every plant uses the carbohydrates it makes... 468 00:34:25,003 --> 00:34:28,700 ...as an energy source to go about its planty business. 469 00:34:28,907 --> 00:34:32,741 And we animals, who are ultimately parasites on the plants... 470 00:34:32,944 --> 00:34:36,778 ...we steal the carbohydrates so we can go about our business. 471 00:34:42,120 --> 00:34:44,782 In eating the plants and their fruits... 472 00:34:44,990 --> 00:34:47,959 ...we combine the carbohydrates with oxygen... 473 00:34:48,160 --> 00:34:51,391 ...which as a result of breathing, we've dissolved in our blood. 474 00:34:51,596 --> 00:34:56,533 From this chemical reaction, we extract the energy which makes us go. 475 00:34:56,968 --> 00:34:59,835 In the process, we exhale carbon dioxide... 476 00:35:00,038 --> 00:35:03,439 ...which the plants then use to make more carbohydrates. 477 00:35:05,544 --> 00:35:08,240 What a marvelous cooperative arrangement. 478 00:35:08,447 --> 00:35:11,575 Plants and animals each using the other's waste gases... 479 00:35:11,783 --> 00:35:16,379 ...the whole cycle powered by abundant sunlight. 480 00:35:16,788 --> 00:35:20,554 But there would be carbon dioxide in the air even if there were no animals. 481 00:35:20,759 --> 00:35:24,354 We need the plants much more than they need us. 482 00:35:28,500 --> 00:35:32,459 There are family resemblances among the organisms of the Earth. 483 00:35:32,671 --> 00:35:37,074 Some are very apparent, such as the use of the number five. 484 00:35:37,275 --> 00:35:40,142 Humans have five major bodily projections: 485 00:35:40,345 --> 00:35:43,644 One head, two arms, two legs. 486 00:35:43,849 --> 00:35:45,316 So do ducks... 487 00:35:45,517 --> 00:35:49,920 ...although the functions of their projections are not quite the same. 488 00:35:50,122 --> 00:35:53,614 An octopus or a centipede has a different plan. 489 00:35:53,825 --> 00:35:58,524 And a being from another planet might be much stranger still. 490 00:35:58,730 --> 00:36:03,599 These family resemblances continue and on a much deeper level... 491 00:36:03,802 --> 00:36:06,737 ...when we go to the molecular basis of life. 492 00:36:06,938 --> 00:36:09,634 There are tens of billions... 493 00:36:09,841 --> 00:36:12,742 ...of different kinds of organic molecules. 494 00:36:12,944 --> 00:36:15,139 Yet only about 50 of them... 495 00:36:15,347 --> 00:36:18,441 ...are used in the essential machinery of life. 496 00:36:18,650 --> 00:36:21,084 The same 50 employed over and over again... 497 00:36:21,286 --> 00:36:25,882 ...ingenious, for different functions in every living thing. 498 00:36:26,091 --> 00:36:28,855 And when we go to the very kernel of life on Earth... 499 00:36:29,060 --> 00:36:32,518 ...to the proteins that control cell chemistry... 500 00:36:32,731 --> 00:36:36,633 ...to the spiral or helix of nucleic acids... 501 00:36:36,835 --> 00:36:39,429 ...which carry the hereditary information... 502 00:36:39,638 --> 00:36:43,665 ...we find these molecules to be identical... 503 00:36:43,875 --> 00:36:47,470 ...in all plants and animals of our planet. 504 00:37:12,270 --> 00:37:16,866 This oak tree and me, we're made of the same stuff. 505 00:37:17,075 --> 00:37:19,976 If you go back, you'll find that we have a common ancestor. 506 00:37:20,178 --> 00:37:23,409 That's why our chemistry is so alike. 507 00:37:26,451 --> 00:37:30,444 Let's take a trip to examine this common basis of life. 508 00:37:30,655 --> 00:37:34,056 A voyage to investigate the molecular machinery... 509 00:37:34,259 --> 00:37:36,386 ...at the heart of life on Earth. 510 00:37:36,595 --> 00:37:40,156 A journey to the nucleus of the cell. 511 00:37:40,398 --> 00:37:41,922 First we need a cell. 512 00:37:42,500 --> 00:37:46,766 I have trillions. I can afford to donate a few. 513 00:37:52,811 --> 00:37:55,541 The casual act of pricking a finger... 514 00:37:55,747 --> 00:37:59,376 ...is an event of some magnitude on the scale of the very small. 515 00:37:59,584 --> 00:38:04,283 Millions of red blood cells are detoured from their usual routes. 516 00:38:06,258 --> 00:38:09,091 But most continue to cruise about the body... 517 00:38:09,294 --> 00:38:13,253 ...carrying their cargoes of oxygen to the remotest freckle. 518 00:38:14,366 --> 00:38:16,561 We're about to enter the living cell... 519 00:38:16,768 --> 00:38:20,295 ...a realm, in its own way, as complex and beautiful... 520 00:38:20,505 --> 00:38:23,474 ...as the realm of galaxies and stars. 521 00:38:23,875 --> 00:38:26,867 Among the red blood cells, we encounter a white blood cell... 522 00:38:27,078 --> 00:38:28,670 ...a lymphocyte... 523 00:38:28,880 --> 00:38:31,849 ...whose job it is to protect me against invading microbes. 524 00:38:32,050 --> 00:38:34,985 It makes antibodies on its furrowed surface... 525 00:38:35,186 --> 00:38:38,713 ...but its interior is like that of many cells. 526 00:38:40,659 --> 00:38:44,652 Plunging through the membrane, we find ourselves inside the cell. 527 00:38:44,863 --> 00:38:48,526 Here, every structure has its function. 528 00:38:51,369 --> 00:38:54,270 These dark green blobs are factories... 529 00:38:54,472 --> 00:38:57,441 ...where messenger molecules are busy building the enzymes... 530 00:38:57,642 --> 00:38:59,940 ...which control the chemistry of the cell. 531 00:39:00,145 --> 00:39:03,239 The messengers were instructed and dispatched... 532 00:39:03,448 --> 00:39:07,544 ...from within the nucleus, the heart and brain of the cell. 533 00:39:07,886 --> 00:39:10,411 All the instructions on how to get a cell to work... 534 00:39:10,622 --> 00:39:13,523 ...and how to make another are hidden away in there. 535 00:39:13,725 --> 00:39:16,193 We find a tunnel, a nuclear pore... 536 00:39:16,394 --> 00:39:20,125 ...an approach to the biological holy of holies. 537 00:39:22,334 --> 00:39:27,271 These necklaces, these intricately looped and coiled strands... 538 00:39:27,472 --> 00:39:30,032 ...are nucleic acids, DNA. 539 00:39:31,576 --> 00:39:34,136 Everything you need to know on how to make a human being... 540 00:39:34,346 --> 00:39:39,283 ...is encoded in the language of life in the DNA molecule. 541 00:39:46,825 --> 00:39:50,022 This is the DNA double helix... 542 00:39:50,228 --> 00:39:55,165 ...a machine with about 100 billion moving parts, called atoms. 543 00:39:56,101 --> 00:39:59,195 There are as many atoms in one molecule of DNA... 544 00:39:59,404 --> 00:40:03,101 ...as there are stars in a typical galaxy. 545 00:40:07,445 --> 00:40:10,778 The sequence of nucleotides, here brightly colored... 546 00:40:10,982 --> 00:40:13,974 ...is all that's passed on from generation to generation. 547 00:40:14,185 --> 00:40:16,415 Change the order of the nucleotides... 548 00:40:16,621 --> 00:40:19,681 ...and you change the genetic instructions. 549 00:40:27,165 --> 00:40:31,192 DNA must replicate itself with extreme fidelity. 550 00:40:31,403 --> 00:40:35,999 The reproduction of a DNA molecule begins by separating the two helices. 551 00:40:36,207 --> 00:40:40,166 This is accomplished by an unwinding enzyme. 552 00:40:40,378 --> 00:40:44,371 Like some precision tool, this enzyme, shown in blue... 553 00:40:44,582 --> 00:40:49,519 ...breaks the chemical bonds that bind the two helices of DNA. 554 00:40:50,155 --> 00:40:52,749 The enzyme works its way down the molecule... 555 00:40:52,957 --> 00:40:56,222 ...unzipping DNA as it goes. 556 00:40:58,830 --> 00:41:00,923 Each helix copies the other... 557 00:41:01,132 --> 00:41:04,329 ...supervised by special enzymes. 558 00:41:04,536 --> 00:41:08,370 The organic soup inside the nucleus contains many free nucleotides. 559 00:41:08,640 --> 00:41:13,577 The enzyme recognizes an approaching nucleotide and clicks it into place... 560 00:41:13,878 --> 00:41:17,143 ...reproducing another rung in the double helix. 561 00:41:21,419 --> 00:41:24,047 When the DNA is replicating in one of your cells... 562 00:41:24,255 --> 00:41:27,122 ...a few dozen nucleotides are added every second. 563 00:41:27,459 --> 00:41:31,862 Thousands of these enzymes may be working on a given DNA molecule. 564 00:41:41,706 --> 00:41:44,470 When an arriving nucleotide doesn't fit... 565 00:41:44,676 --> 00:41:46,735 ...the enzyme throws it away. 566 00:41:46,945 --> 00:41:48,572 We call this proofreading. 567 00:41:48,780 --> 00:41:51,271 On the rare occasions of a proofreading error... 568 00:41:51,483 --> 00:41:53,417 ...the wrong nucleotide is attached... 569 00:41:53,618 --> 00:41:57,247 ...and a small random change has been made in the genetic instructions. 570 00:41:57,455 --> 00:41:59,821 A mutation has occurred. 571 00:42:01,359 --> 00:42:04,123 This enzyme is a pretty small molecule... 572 00:42:04,329 --> 00:42:07,321 ...but it catches nucleotides, assembles them in the right order... 573 00:42:07,532 --> 00:42:08,965 ...it knows how to proofread... 574 00:42:09,167 --> 00:42:12,034 ...it's responsible in the most fundamental way... 575 00:42:12,237 --> 00:42:16,731 ...for the reproduction of every cell and every being on Earth. 576 00:42:22,180 --> 00:42:24,671 That enzyme and DNA itself... 577 00:42:24,883 --> 00:42:28,819 ...are molecular machines with awesome powers. 578 00:42:31,523 --> 00:42:35,084 Within every living thing, the molecular machines are busy... 579 00:42:35,293 --> 00:42:39,889 ...making sure that nucleic acids will continue to reproduce. 580 00:43:00,818 --> 00:43:04,276 A minor cut in my skin sounds a local alarm... 581 00:43:04,489 --> 00:43:08,357 ...and the blood spins a complex net of strong fibers... 582 00:43:08,560 --> 00:43:11,723 ...to form a clot and staunch the flow of blood. 583 00:43:11,930 --> 00:43:13,625 There's a very delicate balance here: 584 00:43:13,831 --> 00:43:16,959 Too much clotting and your blood stream will solidify. 585 00:43:17,168 --> 00:43:21,298 Too little clotting and you'll bleed to death from the merest scratch. 586 00:43:21,606 --> 00:43:26,543 The balance is controlled by enzymes instructed by DNA. 587 00:43:29,180 --> 00:43:32,343 Down here, there's also a kind of sanitation squad... 588 00:43:32,550 --> 00:43:35,815 ...comprised of white blood cells, that swings into action... 589 00:43:36,020 --> 00:43:40,650 ...surrounds invading bacteria and ravenously consumes them. 590 00:43:40,858 --> 00:43:44,021 This mopping-up operation is a part of the healing process... 591 00:43:44,228 --> 00:43:47,220 ...again controlled by DNA. 592 00:43:48,800 --> 00:43:52,930 These cells are parts of us, but how alien they seem. 593 00:43:53,137 --> 00:43:55,765 Within each of them, within every cell... 594 00:43:55,974 --> 00:43:59,000 ...there are exquisitely evolved molecular machines. 595 00:43:59,210 --> 00:44:02,771 Nucleic acids, enzymes, the cell architecture... 596 00:44:02,981 --> 00:44:06,849 ...every cell is a triumph of natural selection. 597 00:44:07,051 --> 00:44:10,248 And we're made of trillions of cells. 598 00:44:10,455 --> 00:44:13,447 We are, each of us, a multitude. 599 00:44:15,059 --> 00:44:18,119 Within us is a little universe. 600 00:44:34,379 --> 00:44:39,316 Human DNA is a coiled ladder... 601 00:44:39,550 --> 00:44:41,814 ...a billion nucleotides long. 602 00:44:42,353 --> 00:44:46,687 Many possible combinations of nucleotides are nonsense. That is... 603 00:44:46,891 --> 00:44:51,260 ...they translate into proteins which serve no useful function whatever. 604 00:44:51,462 --> 00:44:54,693 Only a comparatively few nucleic acid molecules... 605 00:44:54,899 --> 00:44:59,563 ...are any good for life forms as complicated as we are. 606 00:45:00,338 --> 00:45:04,536 But even so, the number of useful ways of assembling nucleic acids... 607 00:45:04,742 --> 00:45:06,903 ...is stupefyingly large. 608 00:45:07,111 --> 00:45:11,980 It's probably larger than the total number of atoms in the universe. 609 00:45:12,183 --> 00:45:17,120 This means that the number of possible kinds of human beings... 610 00:45:17,422 --> 00:45:22,052 ...is vastly greater than the number of human beings that has ever lived. 611 00:45:22,293 --> 00:45:26,525 This untapped potential of the human species is immense. 612 00:45:26,731 --> 00:45:29,393 There are ways of putting nucleic acids together... 613 00:45:29,600 --> 00:45:33,866 ...which will function far better by any criterion you wish to choose... 614 00:45:34,072 --> 00:45:39,009 ...than the hereditary instructions of any human being who has ever lived. 615 00:45:39,444 --> 00:45:43,904 Fortunately, we do not know, or at least do not yet know... 616 00:45:44,115 --> 00:45:48,643 ...how to assemble alternative sequences of nucleotides... 617 00:45:48,853 --> 00:45:52,220 ...to make alternative kinds of human beings. 618 00:45:52,423 --> 00:45:55,950 In the future, we might be able to put nucleotides together... 619 00:45:56,160 --> 00:45:57,889 ...in any desired sequence... 620 00:45:58,096 --> 00:46:01,497 ...to produce human characteristics we think desirable. 621 00:46:02,500 --> 00:46:06,459 A disquieting and awesome prospect. 622 00:46:17,248 --> 00:46:21,412 We human beings don't look very much like a tree. 623 00:46:21,619 --> 00:46:25,646 We certainly view the world differently than a tree does. 624 00:46:25,857 --> 00:46:29,315 But down deep, at the molecular heart of life... 625 00:46:29,527 --> 00:46:32,690 ...we're essentially identical to trees. 626 00:46:33,264 --> 00:46:38,133 We both use nucleic acids as the hereditary material. 627 00:46:38,336 --> 00:46:43,239 We both use proteins as enzymes to control the chemistry of the cell. 628 00:46:44,609 --> 00:46:49,137 And most significantly, we both use the identical code book... 629 00:46:49,347 --> 00:46:54,250 ...to translate nucleic acid information into protein information. 630 00:46:54,452 --> 00:46:58,821 Any tree could read my genetic code. 631 00:46:59,757 --> 00:47:02,282 How did such astonishing similarities come about? 632 00:47:02,493 --> 00:47:06,623 Why are we cousins to the trees? 633 00:47:06,831 --> 00:47:09,959 Would life on some other planet use proteins? 634 00:47:10,168 --> 00:47:15,105 The same proteins? The same nucleic acids? The same genetic code? 635 00:47:15,807 --> 00:47:18,640 The usual explanation is that we are... 636 00:47:18,843 --> 00:47:22,404 ...all of us, trees and people... 637 00:47:22,613 --> 00:47:27,448 ...anglerfish, slime molds, bacteria... 638 00:47:27,652 --> 00:47:31,281 ...all descended from a single and common instance... 639 00:47:31,489 --> 00:47:34,356 ...of the origin of life 4 billion years ago... 640 00:47:34,559 --> 00:47:36,720 ...in the early days of our planet. 641 00:47:37,562 --> 00:47:39,359 Now, how... 642 00:47:39,564 --> 00:47:42,965 ...did the molecules of life arise? 643 00:48:01,319 --> 00:48:03,617 In a laboratory at Cornell University... 644 00:48:04,055 --> 00:48:07,684 ...we mix together the gases and waters of the primitive Earth... 645 00:48:07,892 --> 00:48:09,325 ...supply some energy... 646 00:48:09,527 --> 00:48:13,122 ...and see if we can make the stuff of life. 647 00:48:25,743 --> 00:48:29,975 But what was the early atmosphere made of, ordinary air? 648 00:48:30,181 --> 00:48:32,206 If we start with our present atmosphere... 649 00:48:32,416 --> 00:48:34,816 ...the experiment is a dismal failure. 650 00:48:35,019 --> 00:48:37,817 Instead of making proteins and nucleic acids... 651 00:48:38,022 --> 00:48:41,423 ...all we make is smog, a backwards step. 652 00:48:41,626 --> 00:48:43,685 Why doesn't such an experiment work? 653 00:48:43,895 --> 00:48:47,456 Because the air of today contains molecular oxygen. 654 00:48:47,665 --> 00:48:49,963 But oxygen is made by plants. 655 00:48:50,167 --> 00:48:53,796 It's obvious that there were no plants before the origin of life. 656 00:48:54,005 --> 00:48:56,235 We mustn't use oxygen in our experiments... 657 00:48:56,440 --> 00:48:59,307 ...because there wasn't any in the early atmosphere. 658 00:49:04,615 --> 00:49:08,847 This is reasonable because the cosmos is made mostly of hydrogen... 659 00:49:09,053 --> 00:49:10,520 ...which gobbles oxygen up. 660 00:49:10,721 --> 00:49:14,817 The Earth's low gravity has allowed most of our hydrogen gas... 661 00:49:15,026 --> 00:49:18,723 ...to trickle away to space. There's almost none left. 662 00:49:19,697 --> 00:49:21,130 But 4 billion years ago... 663 00:49:21,332 --> 00:49:24,392 ...our atmosphere was full of hydrogen-rich gases: 664 00:49:24,602 --> 00:49:27,093 Methane, ammonia, water vapor. 665 00:49:27,305 --> 00:49:29,773 These are the gases we should use. 666 00:49:34,745 --> 00:49:37,714 Taking great care to ensure the purity of these gases... 667 00:49:37,915 --> 00:49:42,045 ...my colleague, Bishun Khare, pumps them from their holding flasks. 668 00:49:55,933 --> 00:49:58,629 An experiment like this was first performed... 669 00:49:58,836 --> 00:50:02,863 ...by Stanley Miller and Harold Urey in the 1950s. 670 00:50:18,889 --> 00:50:23,485 The starting gases are now introduced into a large reaction vessel. 671 00:50:23,694 --> 00:50:27,892 We could shine ultraviolet light, simulating the early sun. 672 00:50:28,099 --> 00:50:29,464 But in this experiment... 673 00:50:29,700 --> 00:50:31,964 ...the gases will be sparked... 674 00:50:32,169 --> 00:50:36,105 ...as the primitive atmosphere was by early lightning. 675 00:50:56,794 --> 00:51:01,254 After only a few hours, the interior of the reaction vessel... 676 00:51:01,465 --> 00:51:04,593 ...becomes streaked with a strange brown pigment... 677 00:51:04,802 --> 00:51:08,329 ...a rich collection of complex organic molecules... 678 00:51:08,539 --> 00:51:13,101 ...including the building blocks of the proteins and the nucleic acids. 679 00:51:15,312 --> 00:51:18,941 Under the right conditions, these building blocks assemble themselves... 680 00:51:19,150 --> 00:51:23,610 ...into molecules resembling little proteins and little nucleic acids. 681 00:51:23,821 --> 00:51:28,349 These nucleic acids can even make identical copies of themselves. 682 00:51:33,798 --> 00:51:38,201 In this vessel are the notes of the music of life... 683 00:51:38,402 --> 00:51:41,269 ...although not yet the music itself. 684 00:51:43,808 --> 00:51:46,242 Now, no one, so far... 685 00:51:46,444 --> 00:51:50,813 ...has mixed together the gases and waters of the primitive Earth... 686 00:51:51,015 --> 00:51:55,952 ...and at the end of the experiment had something crawl out of the flask. 687 00:51:56,187 --> 00:51:59,816 There's still much to be understood about the origin of life... 688 00:52:00,024 --> 00:52:02,356 ...including the origin of the genetic code. 689 00:52:02,560 --> 00:52:06,018 But we've only been at such experiments for 30 years. 690 00:52:06,230 --> 00:52:09,529 Nature's had a 4-billion-year head start. 691 00:52:09,733 --> 00:52:14,329 Incidentally, there's nothing in such experiments that's unique to the Earth. 692 00:52:14,538 --> 00:52:18,133 The gases we start with, the energy sources we use... 693 00:52:18,342 --> 00:52:21,072 ...are entirely common through the cosmos. 694 00:52:21,278 --> 00:52:25,806 So chemical reactions something like these must be responsible for... 695 00:52:26,016 --> 00:52:28,177 ...the organic matter in interstellar space... 696 00:52:28,385 --> 00:52:30,751 ...and the amino acids in the meteorites. 697 00:52:30,955 --> 00:52:33,788 Similar chemical reactions must have occurred... 698 00:52:33,991 --> 00:52:37,427 ...on a billion other worlds in the Milky Way galaxy. 699 00:52:37,628 --> 00:52:41,587 Look how easy it is to make great globs of this stuff. 700 00:52:41,799 --> 00:52:46,065 The molecules of life fill the cosmos. 701 00:52:46,670 --> 00:52:47,728 Now... 702 00:52:48,272 --> 00:52:51,105 What would life elsewhere look like? 703 00:52:51,308 --> 00:52:55,301 Even if it had an identical molecular chemistry to life on Earth... 704 00:52:55,513 --> 00:52:57,344 ...which I very much doubt... 705 00:52:57,548 --> 00:53:01,109 ...it could not be very similar in form... 706 00:53:01,318 --> 00:53:03,582 ...to familiar organisms on the Earth. 707 00:53:03,787 --> 00:53:06,950 The random character of the evolutionary process... 708 00:53:07,158 --> 00:53:11,993 ...must create elsewhere creatures very different from any that we know. 709 00:53:13,764 --> 00:53:16,392 Think of a world something like Jupiter... 710 00:53:16,600 --> 00:53:21,162 ...with an atmosphere rich in hydrogen, helium, methane, water and ammonia... 711 00:53:21,372 --> 00:53:23,033 ...in which organic molecules might be... 712 00:53:23,240 --> 00:53:25,970 ...falling from the skies like manna from heaven... 713 00:53:26,177 --> 00:53:28,737 ...like the products of the Miller-Urey experiment. 714 00:53:28,946 --> 00:53:31,437 Could there be life on such a world? 715 00:53:32,383 --> 00:53:35,944 There's a special problem. The atmosphere is turbulent... 716 00:53:36,153 --> 00:53:39,782 ...and down deep, before we ever come to a surface, it's very hot. 717 00:53:39,990 --> 00:53:43,653 If you're not careful, you'll be carried down and fried. 718 00:53:43,861 --> 00:53:46,694 If you reproduce before you're fried... 719 00:53:46,897 --> 00:53:51,596 ...turbulence will carry your offspring into the higher and cooler layers. 720 00:53:51,802 --> 00:53:56,501 Such organisms could be very little. We call them sinkers. 721 00:53:57,942 --> 00:54:01,070 The physicist E.E. Salpeter and I at Cornell... 722 00:54:01,278 --> 00:54:03,872 ...have calculated something about the life... 723 00:54:04,081 --> 00:54:06,572 ...that might exist on such a world. 724 00:54:08,619 --> 00:54:11,884 Vast living balloons could stay buoyant... 725 00:54:12,089 --> 00:54:15,252 ...by pumping heavy gases from their interiors... 726 00:54:15,459 --> 00:54:17,450 ...or by keeping their insides warm. 727 00:54:17,661 --> 00:54:20,391 They might eat the organic molecules in the air... 728 00:54:20,598 --> 00:54:22,463 ...or make their own with sunlight. 729 00:54:22,666 --> 00:54:25,999 We call these creatures floaters. 730 00:54:27,671 --> 00:54:30,037 We imagine floaters kilometers across... 731 00:54:30,241 --> 00:54:33,677 ...enormously larger than the greatest whale that ever was... 732 00:54:33,877 --> 00:54:37,278 ...beings the size of cities. 733 00:54:37,481 --> 00:54:41,383 We conceive of them arrayed in great, lazy herds... 734 00:54:41,585 --> 00:54:43,416 ...as far as the eye can see... 735 00:54:43,621 --> 00:54:48,320 ...concentrated in the updrafts in the enormous sea of clouds. 736 00:54:48,525 --> 00:54:52,791 But there can be other creatures in this alien environment: hunters. 737 00:54:54,665 --> 00:54:56,826 Hunters are fast and maneuverable. 738 00:54:57,034 --> 00:55:00,333 They eat the floaters, both for their organic molecules... 739 00:55:00,537 --> 00:55:02,630 ...and for their store of pure hydrogen. 740 00:55:02,840 --> 00:55:04,535 But there can't be many hunters... 741 00:55:04,742 --> 00:55:09,406 ...because if they destroy all the floaters, they themselves will perish. 742 00:55:12,950 --> 00:55:16,078 Physics and chemistry permit such life forms. 743 00:55:16,287 --> 00:55:18,755 Art presents them with a certain reality... 744 00:55:18,956 --> 00:55:22,619 ...but nature is not obliged to follow our speculations. 745 00:55:22,826 --> 00:55:27,422 If there are billions of inhabited worlds in the Milky Way galaxy... 746 00:55:27,865 --> 00:55:32,359 ...then I think it's likely there are a few places which might have... 747 00:55:32,569 --> 00:55:33,968 ...hunters... 748 00:55:34,171 --> 00:55:36,969 ...and floaters and sinkers. 749 00:55:38,208 --> 00:55:42,440 Biology is more like history than it is like physics. 750 00:55:42,646 --> 00:55:45,706 You have to know the past to understand the present. 751 00:55:45,916 --> 00:55:50,080 There is no predictive theory of biology, nor is there for history. 752 00:55:50,287 --> 00:55:51,652 The reason is the same: 753 00:55:51,855 --> 00:55:55,188 Both subjects are still too complicated for us. 754 00:55:55,392 --> 00:55:58,020 But we can understand ourselves much better... 755 00:55:58,228 --> 00:56:00,458 ...by understanding other cases. 756 00:56:02,132 --> 00:56:05,590 The study of a single instance of extraterrestrial life... 757 00:56:05,803 --> 00:56:09,295 No matter how humble, a microbe would be just fine. 758 00:56:09,506 --> 00:56:12,441 ...will de-provincialize biology. 759 00:56:12,643 --> 00:56:16,238 It will show us what else is possible. 760 00:56:17,715 --> 00:56:22,652 We've heard so far the voice of life on only a single world... 761 00:56:22,886 --> 00:56:25,252 ...but for the first time, as we shall see... 762 00:56:25,456 --> 00:56:28,914 ...we've begun a serious scientific search... 763 00:56:29,126 --> 00:56:31,356 ...for the cosmic fugue. 764 00:56:41,572 --> 00:56:44,837 Recently, we've learned more about the origin of life. 765 00:56:45,042 --> 00:56:47,169 Do you remember RNA... 766 00:56:47,378 --> 00:56:50,745 ...that nucleic acid that our cells use as messengers... 767 00:56:50,948 --> 00:56:54,076 ...carrying the genetic information out of the cell nucleus? 768 00:56:54,752 --> 00:56:58,984 Well, it's been found that RNA, like protein... 769 00:56:59,189 --> 00:57:01,453 ...can control chemical reactions... 770 00:57:01,658 --> 00:57:05,321 ...as well as reproduce itself, which proteins can't do. 771 00:57:05,529 --> 00:57:08,760 Many scientists now wonder if the first life on Earth... 772 00:57:08,966 --> 00:57:11,127 ...was an RNA molecule. 773 00:57:11,335 --> 00:57:14,793 It now seems feasible that key molecular building blocks... 774 00:57:15,005 --> 00:57:18,941 ...for the origin of life, fell out of the skies 4 billion years ago. 775 00:57:19,710 --> 00:57:24,113 Comets have been found to have a lot of organic molecules in them... 776 00:57:24,314 --> 00:57:28,648 ...and they fell in huge numbers on the primitive Earth. 777 00:57:29,753 --> 00:57:32,586 We also mention the extinction of the dinosaurs... 778 00:57:32,790 --> 00:57:36,726 ...and most of the other species on Earth about 65 million years ago. 779 00:57:36,927 --> 00:57:41,091 We now know that a large comet hit the Earth at just that time. 780 00:57:41,298 --> 00:57:46,235 The dust pall from that collision must've cooled and darkened the Earth... 781 00:57:46,570 --> 00:57:49,505 ...perhaps killing all the dinosaurs, but sparing... 782 00:57:49,706 --> 00:57:54,166 ...the small, furry mammals who were our ancestors. 783 00:57:54,378 --> 00:57:58,246 Other cometary mass extinctions in other epochs seem likely. 784 00:57:58,449 --> 00:58:02,783 If true, this would mean that comets have been the bringers... 785 00:58:02,986 --> 00:58:05,454 ...both of life and death.