1 00:01:05,169 --> 00:01:09,833 SAGAN: All my life I've wondered about life beyond the Earth. 2 00:01:10,041 --> 00:01:14,034 On those countless other planets that we think circle other suns... 3 00:01:14,245 --> 00:01:16,110 ...is there also life? 4 00:01:16,314 --> 00:01:18,874 Might the beings of other worlds resemble us... 5 00:01:19,083 --> 00:01:21,745 ...or would they be astonishingly different? 6 00:01:21,953 --> 00:01:24,319 What would they be made of? 7 00:01:24,522 --> 00:01:26,683 In the vast Milky Way galaxy... 8 00:01:26,891 --> 00:01:30,292 ...how common is what we call life? 9 00:01:32,363 --> 00:01:33,796 The nature of life on Earth... 10 00:01:33,998 --> 00:01:35,761 ...and the quest for life elsewhere... 11 00:01:35,967 --> 00:01:39,095 ...are the two sides of the same question. 12 00:01:39,304 --> 00:01:42,068 The search for who we are. 13 00:01:47,145 --> 00:01:50,637 All living things on Earth are made of organic molecules... 14 00:01:50,848 --> 00:01:53,373 ...a complex microscopic architecture... 15 00:01:53,585 --> 00:01:56,315 ...built around atoms of carbon. 16 00:01:56,521 --> 00:01:58,580 In the great dark between the stars... 17 00:01:58,790 --> 00:02:01,725 ...there also are organic molecules... 18 00:02:01,926 --> 00:02:05,760 ...in immense clouds of gas and dust. 19 00:02:09,133 --> 00:02:10,760 Inside such clouds... 20 00:02:10,969 --> 00:02:14,461 ...there are batches of new worlds just forming. 21 00:02:14,672 --> 00:02:18,608 Their surfaces are very likely covered with organic molecules. 22 00:02:18,810 --> 00:02:21,904 These molecules almost certainly are not made by life... 23 00:02:22,113 --> 00:02:24,707 ...although they are the stuff of life. 24 00:02:24,916 --> 00:02:28,010 On suitable worlds, they may lead to life. 25 00:02:28,219 --> 00:02:31,746 Organic matter is abundant throughout the cosmos... 26 00:02:31,956 --> 00:02:35,722 ...produced by the same chemistry everywhere. 27 00:02:50,274 --> 00:02:52,765 Perhaps, given enough time... 28 00:02:52,977 --> 00:02:57,778 ...the origin and evolution of life is inevitable on every clement world. 29 00:02:57,982 --> 00:03:01,713 There will surely be some planets too hostile for life. 30 00:03:01,919 --> 00:03:04,615 On others, it may arise and die out... 31 00:03:04,822 --> 00:03:07,848 ...or never evolve beyond its simplest forms. 32 00:03:08,059 --> 00:03:10,459 And on some small fraction of worlds... 33 00:03:10,662 --> 00:03:13,961 ...there may develop intelligences and civilizations... 34 00:03:14,165 --> 00:03:16,497 ...more advanced than ours. 35 00:03:19,737 --> 00:03:22,706 All life on our planet is closely related. 36 00:03:22,907 --> 00:03:27,276 We have a common organic chemistry and a common evolutionary heritage. 37 00:03:27,478 --> 00:03:31,175 And so our biologists are profoundly limited. 38 00:03:31,382 --> 00:03:33,646 They study a single biology... 39 00:03:33,851 --> 00:03:38,015 ...one lonely theme in the music of life. 40 00:03:38,222 --> 00:03:41,919 Is it the only voice for thousands of light years... 41 00:03:42,126 --> 00:03:46,825 ...or is there a cosmic fugue, a billion different voices... 42 00:03:47,031 --> 00:03:50,967 ...playing the life music of the galaxy? 43 00:03:55,440 --> 00:03:58,341 This blue world is where we grew up. 44 00:03:58,543 --> 00:04:00,636 There was once a time before life. 45 00:04:00,845 --> 00:04:04,008 Our planet is now burgeoning with life. 46 00:04:04,215 --> 00:04:06,080 How did it come about? 47 00:04:06,284 --> 00:04:09,742 How were organic molecules originally made? 48 00:04:09,954 --> 00:04:12,218 How did life evolve to produce beings... 49 00:04:12,423 --> 00:04:15,324 ...as elaborate and complex as we... 50 00:04:15,526 --> 00:04:19,758 ...able to explore the mystery of our own origins? 51 00:04:23,000 --> 00:04:26,527 Let me tell you a story about one little phrase... 52 00:04:26,738 --> 00:04:28,672 ...in the music of life on Earth. 53 00:04:32,110 --> 00:04:33,702 (WIND BLOWS) 54 00:04:46,157 --> 00:04:48,022 In the history of humans... 55 00:04:48,226 --> 00:04:50,091 ...in the 12th century... 56 00:04:50,294 --> 00:04:54,890 ...Japan was ruled by a clan of warriors called the Heike. 57 00:05:04,542 --> 00:05:08,137 The nominal leader of the Heike, the emperor of Japan... 58 00:05:08,346 --> 00:05:11,543 ...was a 7-year-old boy named Antoku. 59 00:05:11,749 --> 00:05:16,049 His guardian was his grandmother, the Lady Nii. 60 00:05:18,756 --> 00:05:21,224 (DRUM BEATS) 61 00:05:25,129 --> 00:05:28,360 The Heike were engaged in a long and bloody war... 62 00:05:28,566 --> 00:05:31,933 ...with another Samurai clan, the Genji. 63 00:05:37,175 --> 00:05:40,508 Each asserted a superior ancestral claim... 64 00:05:40,711 --> 00:05:42,576 ...to the imperial throne. 65 00:05:42,780 --> 00:05:44,577 (BATTLE CRIES) 66 00:05:45,283 --> 00:05:49,083 Their decisive encounter occurred at Dannoura... 67 00:05:49,287 --> 00:05:52,848 ...in the Japanese Inland Sea on April 24... 68 00:05:53,057 --> 00:05:55,992 ...in the year 1185. 69 00:05:58,462 --> 00:06:01,488 The Heike were badly outnumbered and outmaneuvered. 70 00:06:01,933 --> 00:06:03,867 With their cause clearly lost... 71 00:06:04,068 --> 00:06:09,005 ...the surviving Heike warriors threw themselves into the sea and drowned. 72 00:06:15,246 --> 00:06:17,680 The emperor's grandmother, the Lady Nii... 73 00:06:17,882 --> 00:06:20,612 ...resolved that they would not be captured by the enemy. 74 00:06:20,818 --> 00:06:25,221 What happened next is related in "The Tale of the Heike": 75 00:06:26,057 --> 00:06:29,515 "The young emperor asked the Lady Nii, 'Where are you to take me?' 76 00:06:30,394 --> 00:06:35,093 She turned to the youthful sovereign with tears streaming down her cheeks... 77 00:06:35,299 --> 00:06:36,698 ...and comforted him. 78 00:06:39,704 --> 00:06:41,763 (WATER LAPS) 79 00:06:49,080 --> 00:06:50,707 Blinded with tears... 80 00:06:50,915 --> 00:06:55,284 ...the child sovereign put his beautiful small hands together. 81 00:06:57,622 --> 00:06:59,522 He turned first to the east... 82 00:06:59,724 --> 00:07:02,887 ...to say farewell to the god of Ise... 83 00:07:03,094 --> 00:07:04,618 ...and then to the west... 84 00:07:04,829 --> 00:07:08,492 ...to recite a prayer to the Amida Buddha. 85 00:07:09,767 --> 00:07:11,428 The Lady Nii... 86 00:07:11,636 --> 00:07:14,161 ...took him in her arms, and with the words: 87 00:07:14,372 --> 00:07:17,341 'In the depths of the ocean is our capital'... 88 00:07:17,541 --> 00:07:20,772 ...sank with him at last beneath the waves." 89 00:07:42,199 --> 00:07:45,999 The destruction of the Heike battle fleet at Dannoura... 90 00:07:46,203 --> 00:07:49,001 ...marked the end of the clan's 30-year rule. 91 00:07:49,206 --> 00:07:52,471 The Heike all but vanished from history. 92 00:07:57,481 --> 00:08:00,848 Only 43 Heike survived, all women. 93 00:08:01,052 --> 00:08:04,385 These former ladies-in-waiting of the Imperial Court... 94 00:08:04,588 --> 00:08:08,046 ...were reduced to selling flowers and other favors... 95 00:08:08,259 --> 00:08:11,319 ...to the fishermen near the scene of the battle. 96 00:08:17,802 --> 00:08:20,999 These women and their offspring by the fishermen... 97 00:08:21,205 --> 00:08:24,538 ...established a festival to commemorate the battle. 98 00:08:31,182 --> 00:08:34,845 To this day, every year, on the 24th of April... 99 00:08:35,052 --> 00:08:37,953 ...their descendants proceed to the Akama shrine... 100 00:08:38,155 --> 00:08:39,850 ...which contains the mausoleum... 101 00:08:40,057 --> 00:08:44,118 ...of the drowned 7-year-old emperor, Antoku. 102 00:08:47,732 --> 00:08:50,860 There, they conduct a ceremony of remembrance... 103 00:08:51,068 --> 00:08:55,027 ...for the life and death of the Heike warriors. 104 00:09:03,414 --> 00:09:06,383 But there is a strange postscript to this story: 105 00:09:06,584 --> 00:09:08,142 The fishermen say... 106 00:09:08,352 --> 00:09:13,085 ...that the Heike samurai wander the bottom of the Inland Sea... 107 00:09:13,290 --> 00:09:16,020 ...in the form of crabs. 108 00:09:16,660 --> 00:09:21,120 There are crabs here which have curious markings on their backs. 109 00:09:21,399 --> 00:09:24,391 Patterns which resemble a human face... 110 00:09:24,602 --> 00:09:27,571 ...with the aggressive scowl of a samurai warrior... 111 00:09:27,772 --> 00:09:29,865 ...from medieval Japan. 112 00:09:30,074 --> 00:09:32,736 (DRUM BEATS) 113 00:09:37,548 --> 00:09:40,642 These Heike crabs, when caught, are not eaten. 114 00:09:40,851 --> 00:09:43,319 They are thrown back into the sea... 115 00:09:43,521 --> 00:09:46,251 ...in commemoration of the doleful events... 116 00:09:46,457 --> 00:09:49,426 ...of the battle of Dannoura. 117 00:10:00,571 --> 00:10:03,768 This legend raises a lovely problem: 118 00:10:03,974 --> 00:10:07,808 How does it come about that the face of a warrior... 119 00:10:08,012 --> 00:10:12,779 ...is cut on the carapace of a Japanese crab? How could it be? 120 00:10:12,983 --> 00:10:17,545 The answer seems to be that humans made this face. 121 00:10:17,888 --> 00:10:19,355 But how? 122 00:10:19,557 --> 00:10:22,822 Like many other features, the patterns on the back... 123 00:10:23,027 --> 00:10:26,394 ...or carapace of this crab are inherited. 124 00:10:26,597 --> 00:10:31,000 But among crabs, as among humans, there are different hereditary lines. 125 00:10:31,435 --> 00:10:34,461 Now, suppose purely by chance... 126 00:10:34,672 --> 00:10:37,903 ...among the distant ancestors of this crab... 127 00:10:38,109 --> 00:10:42,842 ...there came to be one which looked just a little bit like a human face. 128 00:10:43,047 --> 00:10:46,539 Long before the battle, fishermen may have been reluctant... 129 00:10:46,750 --> 00:10:48,775 ...to eat a crab with a human face. 130 00:10:49,120 --> 00:10:51,486 In throwing it back into the sea... 131 00:10:51,689 --> 00:10:55,819 ...they were setting into motion a process of selection. 132 00:10:56,026 --> 00:11:00,554 If you're a crab and your carapace is just ordinary... 133 00:11:00,764 --> 00:11:03,062 ...the humans are gonna eat you. 134 00:11:03,267 --> 00:11:05,827 But if it looks a little bit like a face... 135 00:11:06,036 --> 00:11:09,528 ...they'll throw you back and you can have lots of baby crabs... 136 00:11:09,740 --> 00:11:11,503 ...that all look just like you. 137 00:11:11,709 --> 00:11:13,802 As many generations passed... 138 00:11:14,011 --> 00:11:16,878 ...of crabs and fisher-folk alike... 139 00:11:17,081 --> 00:11:20,949 ...the crabs with patterns that looked most like a samurai face... 140 00:11:21,152 --> 00:11:23,347 ...preferentially survived. 141 00:11:23,554 --> 00:11:27,115 Until eventually, there was produced not just a human face... 142 00:11:27,324 --> 00:11:29,792 ...not just a Japanese face... 143 00:11:29,994 --> 00:11:32,929 ...but the face of a samurai warrior. 144 00:11:33,130 --> 00:11:37,089 All this has nothing to do with what the crabs might want. 145 00:11:37,301 --> 00:11:40,737 Selection is imposed from the outside. 146 00:11:40,938 --> 00:11:44,874 The more you look like a samurai, the better your chances of survival. 147 00:11:45,075 --> 00:11:49,739 Eventually, there are a lot of crabs that look like samurai warriors. 148 00:11:49,947 --> 00:11:52,973 (DRUM BEATS) 149 00:12:11,068 --> 00:12:14,629 This process is called artificial selection. 150 00:12:15,372 --> 00:12:17,670 In the case of the Heike crab, it was effected... 151 00:12:17,875 --> 00:12:20,742 ...more or less unconsciously by the fishermen... 152 00:12:20,945 --> 00:12:25,211 ...and certainly without any serious contemplation by the crabs. 153 00:12:25,416 --> 00:12:27,976 Humans, for thousands of years... 154 00:12:28,185 --> 00:12:30,050 ...have deliberately selected... 155 00:12:30,254 --> 00:12:32,916 ...which plants and animals shall live. 156 00:12:33,157 --> 00:12:36,615 We're surrounded by farm and domestic animals... 157 00:12:36,827 --> 00:12:38,124 ...fruits, vegetables. 158 00:12:38,996 --> 00:12:42,625 Where do they come from? Were they once free-living in the wild... 159 00:12:42,833 --> 00:12:46,667 ...and then induced to adopt some less strenuous life on the farm? 160 00:12:47,137 --> 00:12:48,126 No. 161 00:12:48,339 --> 00:12:52,400 They are, almost all of them, made by us. 162 00:12:56,113 --> 00:13:00,641 The essence of artificial selection for a horse or a cow... 163 00:13:00,851 --> 00:13:04,184 ...a grain of rice or a Heike crab, is this: 164 00:13:04,388 --> 00:13:07,687 Many characteristics are inherited. They breed true. 165 00:13:09,660 --> 00:13:13,255 Humans encourage the reproduction of some varieties... 166 00:13:13,464 --> 00:13:15,398 ...and discourage the reproduction of others. 167 00:13:15,599 --> 00:13:19,035 The variety selected for, eventually becomes abundant. 168 00:13:19,236 --> 00:13:23,195 The variety selected against, becomes rare, maybe extinct. 169 00:13:24,408 --> 00:13:27,571 But if artificial selection makes such changes... 170 00:13:27,778 --> 00:13:29,837 ...in only a few thousand years... 171 00:13:30,047 --> 00:13:32,072 ...what must natural selection... 172 00:13:32,283 --> 00:13:36,276 ...working for billions of years, be capable of? 173 00:13:36,520 --> 00:13:37,578 The answer... 174 00:13:37,788 --> 00:13:41,588 ...is all the beauty and diversity in the biological world. 175 00:13:46,330 --> 00:13:50,391 That life evolved over the ages is clear... 176 00:13:50,601 --> 00:13:54,230 ...from the changes we've made in the beasts and vegetables... 177 00:13:54,571 --> 00:13:58,132 ...but also from the record in the rocks. 178 00:13:58,442 --> 00:14:01,206 The fossil evidence speaks to us unambiguously... 179 00:14:01,412 --> 00:14:05,906 ...of creatures that were once present in enormous numbers... 180 00:14:06,116 --> 00:14:08,141 ...and that have now vanished utterly. 181 00:14:08,352 --> 00:14:11,844 There are more species that have become extinct than exist today. 182 00:14:12,056 --> 00:14:16,152 They are the terminated experiments in evolution. 183 00:14:17,227 --> 00:14:21,823 These guys, the trilobites, appeared 600 million years ago. 184 00:14:22,032 --> 00:14:24,660 They were around for 300 million years. 185 00:14:24,868 --> 00:14:28,133 They're all gone. There's none left. 186 00:14:28,439 --> 00:14:33,069 But in those old rocks, there are no fossils of people or cattle. 187 00:14:33,277 --> 00:14:35,108 We've evolved only recently. 188 00:14:36,313 --> 00:14:39,544 Evolution is a fact, not a theory. 189 00:14:39,750 --> 00:14:41,684 It really happened. 190 00:14:43,253 --> 00:14:45,221 (BEE BUZZES) 191 00:14:47,624 --> 00:14:51,856 That the mechanism of evolution is natural selection was the discovery... 192 00:14:52,062 --> 00:14:55,498 ...of Charles Darwin and Alfred Russel Wallace. 193 00:14:56,166 --> 00:14:57,394 Here's how it works: 194 00:14:57,901 --> 00:14:59,425 Nature is prolific. 195 00:14:59,636 --> 00:15:03,504 There are many more creatures that are born than can possibly survive. 196 00:15:03,707 --> 00:15:08,644 So those varieties which are, by accident, less well adapted... 197 00:15:08,879 --> 00:15:12,371 ...don't survive, or at least they leave fewer offspring. 198 00:15:12,583 --> 00:15:17,043 Now, mutations, sudden changes in heredity... 199 00:15:17,254 --> 00:15:19,313 ...are passed on. They breed true. 200 00:15:19,523 --> 00:15:24,460 The environment selects the occasional mutations which enhance survival. 201 00:15:24,695 --> 00:15:29,132 The resulting series of slow changes in the nature of living beings... 202 00:15:29,333 --> 00:15:31,460 ...is the origin of new species. 203 00:15:33,036 --> 00:15:36,403 Many people were scandalized... 204 00:15:36,607 --> 00:15:38,802 ...by the ideas of evolution and natural selection. 205 00:15:39,009 --> 00:15:41,136 Our ancestors looked at... 206 00:15:41,345 --> 00:15:44,143 ...the intricacy and the beauty of life... 207 00:15:44,348 --> 00:15:48,148 ...and saw evidence for a great designer. 208 00:15:51,054 --> 00:15:54,546 The simplest organism is a far more complex machine... 209 00:15:54,758 --> 00:15:56,919 ...than the finest pocket watch. 210 00:15:57,127 --> 00:16:01,427 And yet, pocket watches don't spontaneously self-assemble... 211 00:16:01,632 --> 00:16:04,931 ...or evolve in slow stages on their own... 212 00:16:05,135 --> 00:16:07,933 ...from say, grandfather clocks. 213 00:16:08,238 --> 00:16:11,173 A watch implies a watchmaker. 214 00:16:12,810 --> 00:16:17,304 There seemed to be no way atoms could spontaneously fall together... 215 00:16:17,514 --> 00:16:19,948 ...and create, say... 216 00:16:21,652 --> 00:16:22,812 ...a dandelion. 217 00:16:23,487 --> 00:16:25,785 The idea of a designer... 218 00:16:25,989 --> 00:16:30,653 ...is an appealing and altogether human explanation of the biological world. 219 00:16:30,861 --> 00:16:34,092 But as Darwin and Wallace showed... 220 00:16:34,298 --> 00:16:35,959 ...there's another way... 221 00:16:36,166 --> 00:16:39,897 ...equally human and far more compelling. 222 00:16:40,304 --> 00:16:44,434 Natural selection, which makes the music of life more beautiful... 223 00:16:44,641 --> 00:16:46,836 ...as the eons pass. 224 00:16:50,514 --> 00:16:52,641 (LOUD RUMBLE) 225 00:16:52,883 --> 00:16:55,078 To understand the passage of the eons... 226 00:16:55,285 --> 00:16:58,618 ...we have compressed all of time into a single cosmic year... 227 00:16:58,822 --> 00:17:02,155 ...with the big bang on January first. 228 00:17:02,359 --> 00:17:06,796 Every month here represents a little over a billion years. 229 00:17:06,997 --> 00:17:10,865 The Earth didn't form until the cosmic year was two-thirds over. 230 00:17:11,068 --> 00:17:15,232 Our understanding of the history of life is very recent... 231 00:17:15,439 --> 00:17:18,875 ...occupying only the last few seconds of December 31... 232 00:17:19,076 --> 00:17:23,206 ...that small white spot at bottom right in the cosmic calendar. 233 00:17:23,413 --> 00:17:25,813 What happened on Earth may be more or less typical... 234 00:17:26,016 --> 00:17:28,484 ...of the evolution of life on many worlds. 235 00:17:28,685 --> 00:17:30,676 But in its details... 236 00:17:30,888 --> 00:17:33,083 ...the story of life on Earth... 237 00:17:33,290 --> 00:17:36,088 ...is probably unique in all the Milky Way galaxy. 238 00:17:36,760 --> 00:17:41,197 The secrets of evolution are time and death. 239 00:17:41,398 --> 00:17:45,232 Time for the slow accumulation of favorable mutations... 240 00:17:45,435 --> 00:17:49,030 ...and death to make room for new species. 241 00:17:49,740 --> 00:17:53,005 Life on Earth arose in September of the cosmic calendar... 242 00:17:53,210 --> 00:17:57,874 ...when our world, still battered and cratered from its violent origin... 243 00:17:58,081 --> 00:18:00,242 ...may have looked a little like the moon. 244 00:18:02,419 --> 00:18:05,946 The Earth is about four and a half billion years old. 245 00:18:06,156 --> 00:18:07,487 In the cosmic calendar... 246 00:18:07,691 --> 00:18:11,684 ...it condensed out of interstellar gas and dust... 247 00:18:11,895 --> 00:18:14,523 ...around September 14. 248 00:18:14,731 --> 00:18:18,394 We know from the fossil record that life originated soon after... 249 00:18:18,602 --> 00:18:21,867 ...maybe around September 25, something like that... 250 00:18:22,072 --> 00:18:25,940 ...probably in the ponds and oceans of the primitive Earth. 251 00:18:26,176 --> 00:18:30,772 The first living things were not as complex as a one-celled organism... 252 00:18:30,981 --> 00:18:34,610 ...which is already a highly sophisticated form of life. 253 00:18:34,818 --> 00:18:38,845 No, the first stirrings of life were much more humble... 254 00:18:39,056 --> 00:18:41,889 ...and happened on the molecular level. 255 00:18:42,092 --> 00:18:46,290 In those early days, lightning and ultraviolet light from the sun... 256 00:18:46,496 --> 00:18:50,694 ...were breaking apart hydrogen-rich molecules in the atmosphere. 257 00:18:50,901 --> 00:18:55,395 The fragments of the molecules were spontaneously recombining... 258 00:18:55,606 --> 00:18:59,406 ...into more and more complex molecules. 259 00:19:01,545 --> 00:19:05,504 The products of this early chemistry dissolved in the oceans... 260 00:19:05,716 --> 00:19:09,015 ...forming a kind of organic soup... 261 00:19:09,219 --> 00:19:11,346 ...of gradually increasing complexity. 262 00:19:11,555 --> 00:19:15,355 Until one day, quite by accident... 263 00:19:15,559 --> 00:19:19,655 ...a molecule arose that was able to make crude copies of itself... 264 00:19:19,863 --> 00:19:23,299 ...using as building blocks the other molecules in the soup. 265 00:19:23,500 --> 00:19:26,958 This was the ancestor of DNA... 266 00:19:27,170 --> 00:19:30,537 ...the master molecule of life on Earth. 267 00:19:30,741 --> 00:19:34,404 It's made of four different parts, called nucleotides... 268 00:19:34,611 --> 00:19:38,308 ...which constitute the four letters of the genetic code... 269 00:19:38,515 --> 00:19:40,073 ...the language of heredity. 270 00:19:40,283 --> 00:19:45,152 Each of the nucleotides, the rungs on the DNA ladder... 271 00:19:45,355 --> 00:19:47,653 ...are a different color in this model. 272 00:19:47,858 --> 00:19:51,123 The instructions are different for different organisms. 273 00:19:51,328 --> 00:19:53,694 That's why organisms are different. 274 00:19:53,897 --> 00:19:57,731 Now, a mutation is a change of a nucleotide... 275 00:19:57,934 --> 00:20:00,960 ...a misspelling of the genetic instructions. 276 00:20:01,171 --> 00:20:06,040 Most mutations spell genetic nonsense since they're random. 277 00:20:06,243 --> 00:20:08,438 They harm the next generation. 278 00:20:08,645 --> 00:20:10,943 But a very few, by accident... 279 00:20:11,148 --> 00:20:15,642 ...make better sense than the original codes, and aid evolution. 280 00:20:16,353 --> 00:20:19,652 DNA is about a billion times smaller... 281 00:20:19,856 --> 00:20:21,585 ...than we see it here. 282 00:20:21,792 --> 00:20:26,729 Each of those things that looks like a piece of fruit is an atom. 283 00:20:26,930 --> 00:20:28,522 Without the tools of science... 284 00:20:28,732 --> 00:20:32,031 ...the machinery of life would be invisible. 285 00:20:36,273 --> 00:20:37,900 Four billion years ago... 286 00:20:38,108 --> 00:20:42,135 ...the ancestors of DNA competed for molecular building blocks... 287 00:20:42,345 --> 00:20:45,314 ...and left crude copies of themselves. 288 00:20:45,515 --> 00:20:48,780 There were no predators; the stuff of life was everywhere. 289 00:20:48,985 --> 00:20:52,352 The oceans and murky pools that filled the craters... 290 00:20:52,556 --> 00:20:55,855 ...were, for these molecules, a Garden of Eden. 291 00:20:56,193 --> 00:20:59,185 With reproduction, mutation and natural selection... 292 00:20:59,396 --> 00:21:03,423 ...the evolution of living molecules was well underway. 293 00:21:03,967 --> 00:21:07,869 Varieties with specialized functions joined together... 294 00:21:08,071 --> 00:21:11,097 ...making a collective. The first cell. 295 00:21:11,308 --> 00:21:14,368 The organic soup eventually ate itself up. 296 00:21:14,578 --> 00:21:17,945 But by this time, plants had evolved, able to use sunlight... 297 00:21:18,148 --> 00:21:22,881 ...to make their own building blocks. They turned the waters green. 298 00:21:23,086 --> 00:21:25,281 One-celled plants joined together: 299 00:21:25,489 --> 00:21:28,515 The first multi-cellular organisms. 300 00:21:30,026 --> 00:21:34,463 Equally important was the invention, not made until early November... 301 00:21:34,664 --> 00:21:39,158 ...of sex. It was stumbled upon by the microbes. 302 00:21:41,838 --> 00:21:45,501 By December 1, green plants had released copious amounts... 303 00:21:45,709 --> 00:21:48,940 ...of oxygen and nitrogen into the atmosphere. 304 00:21:49,146 --> 00:21:52,445 The sky is made by life. 305 00:21:53,750 --> 00:21:56,480 Then, suddenly, on December 15... 306 00:21:56,686 --> 00:21:59,678 ...there was an enormous proliferation of new life forms... 307 00:21:59,890 --> 00:22:03,326 ...an event called the "Cambrian Explosion." 308 00:22:08,265 --> 00:22:12,759 We know from fossils that life arose shortly after the Earth formed... 309 00:22:12,969 --> 00:22:16,700 ...suggesting that the origin of life might be... 310 00:22:16,907 --> 00:22:21,105 ...an inevitable chemical process on countless Earth-like planets... 311 00:22:21,311 --> 00:22:22,903 ...throughout the cosmos. 312 00:22:23,113 --> 00:22:27,914 But on the Earth, in nearly 4 billion years, life advanced no further... 313 00:22:28,118 --> 00:22:29,085 ...than algae. 314 00:22:29,286 --> 00:22:34,087 So maybe more complex forms of life are harder to evolve... 315 00:22:34,291 --> 00:22:36,851 ...harder even than the origin of life itself. 316 00:22:37,060 --> 00:22:39,722 If this is right, the planets of the galaxy... 317 00:22:39,930 --> 00:22:42,592 ...might be filled with microorganisms... 318 00:22:42,799 --> 00:22:46,667 ...but big beasts and vegetables and thinking beings... 319 00:22:46,870 --> 00:22:49,668 ...might be comparatively rare. 320 00:22:53,143 --> 00:22:57,204 By December 18, there were vast herds of trilobites... 321 00:22:57,414 --> 00:22:59,541 ...foraging on the ocean bottom... 322 00:22:59,749 --> 00:23:03,185 ...and squid-like creatures with multicolored shells... 323 00:23:03,386 --> 00:23:04,853 ...were everywhere. 324 00:23:07,791 --> 00:23:11,386 We know enough to sketch in a few of the subsequent details. 325 00:23:11,595 --> 00:23:15,292 The first fish and the first vertebrates appeared on December 19. 326 00:23:15,498 --> 00:23:19,059 Plants began to colonize the land on December 20. 327 00:23:19,269 --> 00:23:23,365 The first winged insects fluttered by on December 22. 328 00:23:23,573 --> 00:23:26,701 On this date also, there were the first amphibians... 329 00:23:26,910 --> 00:23:29,344 ...creatures something like the lungfish... 330 00:23:29,546 --> 00:23:32,811 ...able to survive both on land and in water. 331 00:23:33,016 --> 00:23:37,248 Our direct ancestors were now leaving the oceans behind. 332 00:23:40,123 --> 00:23:45,060 The first trees and the first reptiles evolved on December 23: 333 00:23:45,295 --> 00:23:48,560 Two amazing evolutionary developments. 334 00:23:50,901 --> 00:23:54,337 We are descended from some of those reptiles. 335 00:23:58,742 --> 00:24:02,075 The dinosaurs appeared on Christmas Eve. 336 00:24:02,279 --> 00:24:04,338 There were many different kinds of dinosaurs. 337 00:24:04,547 --> 00:24:07,516 The Earth was once their planet. 338 00:24:10,654 --> 00:24:14,090 Many stood upright and had some fair intelligence. 339 00:24:14,291 --> 00:24:19,126 Great lizards crashed and thundered through the steaming jungles. 340 00:24:24,234 --> 00:24:26,862 Unnoticed by the dinosaurs, a new creature... 341 00:24:27,070 --> 00:24:29,231 ...whose young were born live and helpless... 342 00:24:29,439 --> 00:24:31,771 ...was making its timid debut. 343 00:24:31,975 --> 00:24:35,103 The first mammals emerged on December 26... 344 00:24:35,312 --> 00:24:38,577 ...the first birds on the following day. 345 00:24:41,885 --> 00:24:45,286 But the dinosaurs still dominated the planet. 346 00:24:45,488 --> 00:24:49,424 Then suddenly, without warning, all over the planet at once... 347 00:24:49,626 --> 00:24:51,389 ...the dinosaurs died. 348 00:24:51,594 --> 00:24:54,961 The cause is unknown, but the lesson is clear: 349 00:24:55,165 --> 00:24:59,864 Even 160 million years on a planet is no guarantee of survival. 350 00:25:00,070 --> 00:25:04,564 The dinosaurs perished around the time of the first flower. 351 00:25:06,409 --> 00:25:08,536 On December 30, the first creatures... 352 00:25:08,745 --> 00:25:11,578 ...who looked even a little bit human, evolved... 353 00:25:11,781 --> 00:25:15,945 ...accompanied by a spectacular increase in the size of their brains. 354 00:25:16,152 --> 00:25:20,020 And then, on the evening of the last day of the last month... 355 00:25:20,223 --> 00:25:21,850 ...only a few million years ago... 356 00:25:22,058 --> 00:25:26,893 ...the first true humans took their place on the cosmic calendar. 357 00:25:28,198 --> 00:25:29,995 The written record of history... 358 00:25:30,200 --> 00:25:33,863 ...occupies only the last 10 seconds of the cosmic year. 359 00:25:36,373 --> 00:25:40,275 Now, let's take a closer look at who our ancestors were. 360 00:25:40,477 --> 00:25:43,537 A simple chemical circumstance led to a great moment... 361 00:25:43,747 --> 00:25:45,772 ...in the history of our planet. 362 00:25:45,982 --> 00:25:48,917 There were many molecules in the primordial soup. 363 00:25:49,119 --> 00:25:53,954 Some were attracted to water on one side and repelled by it on the other. 364 00:25:54,157 --> 00:25:56,751 This drove them together... 365 00:25:56,960 --> 00:25:59,758 ...into a tiny enclosed spherical shell... 366 00:25:59,963 --> 00:26:02,761 ...like a soap bubble, which protected the interior. 367 00:26:02,966 --> 00:26:06,527 Within the bubble, the ancestors of DNA found a home... 368 00:26:06,736 --> 00:26:08,431 ...and the first cell arose. 369 00:26:08,638 --> 00:26:12,631 It took hundreds of millions of years for tiny plants to evolve... 370 00:26:12,842 --> 00:26:14,309 ...giving off oxygen. 371 00:26:14,511 --> 00:26:17,742 But that branch didn't lead to us. 372 00:26:18,248 --> 00:26:23,083 Bacteria that could breathe oxygen took over a billion years to evolve. 373 00:26:24,454 --> 00:26:28,788 From a naked nucleus, a cell developed with a nucleus inside. 374 00:26:30,193 --> 00:26:34,493 Some of these amoeba-like forms led eventually to plants. 375 00:26:38,568 --> 00:26:40,502 Others produced colonies... 376 00:26:40,703 --> 00:26:44,969 ...with inside and outside cells performing different functions. 377 00:26:46,810 --> 00:26:48,141 Becoming... 378 00:26:48,344 --> 00:26:52,007 ...a polyp attached to the ocean floor... 379 00:26:52,215 --> 00:26:54,581 ...filtering food from the water... 380 00:26:55,018 --> 00:26:57,543 ...and evolving little tentacles... 381 00:26:57,754 --> 00:27:01,087 ...to direct food into a primitive mouth. 382 00:27:02,325 --> 00:27:04,020 This humble ancestor of ours also led... 383 00:27:04,227 --> 00:27:08,391 ...to spiny-skinned armored animals with internal organs... 384 00:27:08,598 --> 00:27:12,466 ...including our cousin, the starfish. 385 00:27:12,669 --> 00:27:15,103 But we don't come from starfish. 386 00:27:16,039 --> 00:27:18,303 About 550 million years ago... 387 00:27:18,508 --> 00:27:21,500 ...filter feeders evolved gill slits... 388 00:27:21,711 --> 00:27:25,010 ...which were more efficient at straining food particles. 389 00:27:25,215 --> 00:27:28,946 One evolutionary branch led to acorn worms. 390 00:27:29,219 --> 00:27:33,747 Another led to a creature which swam freely in the larval stage... 391 00:27:33,957 --> 00:27:37,290 ...but, as an adult, was still firmly anchored to the ocean floor. 392 00:27:37,494 --> 00:27:40,156 Some became living hollow tubes. 393 00:27:40,697 --> 00:27:45,031 But others retained the larval forms throughout the life cycle... 394 00:27:45,235 --> 00:27:49,262 ...and became free-swimming adults with something like a backbone. 395 00:27:54,244 --> 00:27:55,802 Our ancestors now... 396 00:27:56,012 --> 00:28:00,346 ...500 million years ago, were jawless filter-feeding fish... 397 00:28:00,550 --> 00:28:02,984 ...a little like lampreys. 398 00:28:04,787 --> 00:28:07,017 Gradually, those tiny fish... 399 00:28:07,223 --> 00:28:09,885 ...evolved eyes and jaws. 400 00:28:10,393 --> 00:28:12,554 Fish then began to eat one another... 401 00:28:12,762 --> 00:28:15,822 ...if you could swim fast, you survived. 402 00:28:17,567 --> 00:28:22,436 If you had jaws to eat with, you could use your gills to breathe in the water. 403 00:28:22,739 --> 00:28:25,640 This is the way modern fish arose. 404 00:28:29,913 --> 00:28:32,473 During the summer, swamps and lakes dried up. 405 00:28:32,682 --> 00:28:37,483 Some fish evolved a primitive lung to breathe air until the rains came. 406 00:28:37,687 --> 00:28:39,985 Their brains were getting bigger. 407 00:28:40,190 --> 00:28:43,717 If the rains didn't come, it was handy to be able to pull yourself... 408 00:28:43,927 --> 00:28:45,189 ...to the next swamp. 409 00:28:45,395 --> 00:28:48,296 That was a very important adaptation. 410 00:28:51,834 --> 00:28:55,270 The first amphibians evolved, still with a fish-like tail. 411 00:28:55,471 --> 00:28:59,840 Amphibians, like fish, laid their eggs in water where they were easily eaten. 412 00:29:00,043 --> 00:29:02,671 But then a splendid new invention came along: 413 00:29:02,879 --> 00:29:07,475 The hard-shelled egg, laid on land where there were as yet no predators. 414 00:29:07,684 --> 00:29:12,246 Reptiles and turtles go back to those days. 415 00:29:14,224 --> 00:29:17,557 Many of the reptiles hatched on land never returned to the waters. 416 00:29:17,760 --> 00:29:20,285 Some became the dinosaurs. 417 00:29:20,663 --> 00:29:24,895 One line of dinosaurs developed feathers, useful for short flights. 418 00:29:25,101 --> 00:29:29,970 Today, the only living descendants of the dinosaurs are the birds. 419 00:29:31,674 --> 00:29:34,108 The great dinosaurs evolved along another branch. 420 00:29:34,310 --> 00:29:37,336 Some were the largest flesh-eaters ever to walk the land. 421 00:29:37,547 --> 00:29:42,314 But 65 million years ago they all mysteriously perished. 422 00:29:43,553 --> 00:29:45,953 Meanwhile, the forerunners of the dinosaurs... 423 00:29:46,155 --> 00:29:48,783 ...were also evolving in a different direction. 424 00:29:48,992 --> 00:29:51,722 Small, scurrying creatures... 425 00:29:51,928 --> 00:29:54,829 ...with the young growing inside the mother's body. 426 00:29:55,031 --> 00:29:59,434 After the extinction of the dinosaurs, many different forms developed. 427 00:30:03,740 --> 00:30:06,436 The young were very immature at birth. 428 00:30:06,643 --> 00:30:09,703 In the marsupials, the wombat, for example... 429 00:30:09,912 --> 00:30:13,507 ...and in the mammals, the young had to be taught how to survive. 430 00:30:13,716 --> 00:30:15,911 The brain grew larger still. 431 00:30:16,119 --> 00:30:20,453 Something like a shrew was the ancestor of all the mammals. 432 00:30:25,161 --> 00:30:28,995 One line took to the trees, developing dexterity... 433 00:30:29,198 --> 00:30:31,166 ...stereo vision, larger brains... 434 00:30:31,367 --> 00:30:33,858 ...and a curiosity about their environment. 435 00:30:34,070 --> 00:30:38,905 Some became baboons, but that's not the line to us. 436 00:30:40,276 --> 00:30:43,268 Apes and humans have a recent common ancestor. 437 00:30:43,479 --> 00:30:47,438 Bone for bone, muscle for muscle, molecule for molecule. 438 00:30:47,650 --> 00:30:52,587 There are almost no important differences between apes and humans. 439 00:30:55,558 --> 00:30:59,858 Unlike the chimpanzee, our ancestors walked upright... 440 00:31:00,063 --> 00:31:04,090 ...freeing their hands to poke and fix and experiment. 441 00:31:04,300 --> 00:31:07,758 We got smarter. We began to talk. 442 00:31:14,277 --> 00:31:16,939 Many collateral branches of the human family... 443 00:31:17,146 --> 00:31:20,604 ...became extinct in the last few million years. 444 00:31:20,817 --> 00:31:25,220 We, with our brains and our hands, are the survivors. 445 00:31:25,421 --> 00:31:30,358 There's an unbroken thread that stretches from those first cells to us. 446 00:31:30,693 --> 00:31:31,990 Let's look at it again... 447 00:31:32,195 --> 00:31:36,996 ...compressing 4 billion years of human evolution into 40 seconds. 448 00:32:23,079 --> 00:32:26,242 Those are some of the things that molecules do... 449 00:32:26,449 --> 00:32:29,509 ...given 4 billion years of evolution. 450 00:32:30,720 --> 00:32:35,020 We sometimes represent evolution as the ever-branching ramifications... 451 00:32:35,224 --> 00:32:36,589 ...of some original trunk... 452 00:32:36,793 --> 00:32:41,059 ...each branch pruned and clipped by natural selection. 453 00:32:41,898 --> 00:32:43,866 Every plant and animal alive today... 454 00:32:44,066 --> 00:32:48,469 ...has a history as ancient and illustrious as ours. 455 00:32:48,671 --> 00:32:52,038 Humans stand on one branch. 456 00:32:52,475 --> 00:32:55,376 But now we affect the future of every branch... 457 00:32:55,578 --> 00:32:58,638 ...of this 4-billion-year-old tree. 458 00:33:01,250 --> 00:33:04,310 How lovely trees are. 459 00:33:04,520 --> 00:33:07,751 The human species grew up in and around them. 460 00:33:07,957 --> 00:33:10,892 We have a natural affinity for trees. 461 00:33:11,093 --> 00:33:13,152 Trees photosynthesize... 462 00:33:13,362 --> 00:33:16,092 ...they harvest sunlight... 463 00:33:16,299 --> 00:33:20,201 ...they compete for the sun's favors. 464 00:33:21,070 --> 00:33:22,799 Look at those two trees there... 465 00:33:23,005 --> 00:33:26,202 ...pushing and shoving for sunlight... 466 00:33:26,409 --> 00:33:31,005 ...but with grace and astonishing slowness. 467 00:33:38,688 --> 00:33:40,918 There are so many plants on the Earth... 468 00:33:41,123 --> 00:33:43,683 ...that there's a danger of thinking them trivial... 469 00:33:43,893 --> 00:33:48,023 ...of losing sight of the subtlety and efficiency of their design. 470 00:33:48,231 --> 00:33:52,793 They are great and beautiful machines, powered by sunlight... 471 00:33:53,002 --> 00:33:56,836 ...taking in water from the ground and carbon dioxide from the air... 472 00:33:57,039 --> 00:34:01,635 ...and converting them into food for their use and ours. 473 00:34:09,986 --> 00:34:13,547 This is a museum of living plants. 474 00:34:13,756 --> 00:34:18,625 The Royal Botanic Gardens at Kew in London. 475 00:34:24,333 --> 00:34:27,302 Every plant uses the carbohydrates it makes... 476 00:34:27,503 --> 00:34:31,200 ...as an energy source to go about its planty business. 477 00:34:31,407 --> 00:34:35,241 And we animals, who are ultimately parasites on the plants... 478 00:34:35,444 --> 00:34:39,278 ...we steal the carbohydrates so we can go about our business. 479 00:34:44,620 --> 00:34:47,282 In eating the plants and their fruits... 480 00:34:47,490 --> 00:34:50,459 ...we combine the carbohydrates with oxygen... 481 00:34:50,660 --> 00:34:53,891 ...which as a result of breathing, we've dissolved in our blood. 482 00:34:54,096 --> 00:34:59,033 From this chemical reaction, we extract the energy which makes us go. 483 00:34:59,468 --> 00:35:02,335 In the process, we exhale carbon dioxide... 484 00:35:02,538 --> 00:35:05,939 ...which the plants then use to make more carbohydrates. 485 00:35:08,044 --> 00:35:10,740 What a marvelous cooperative arrangement. 486 00:35:10,947 --> 00:35:14,075 Plants and animals each using the other's waste gases... 487 00:35:14,283 --> 00:35:18,879 ...the whole cycle powered by abundant sunlight. 488 00:35:19,288 --> 00:35:23,054 But there would be carbon dioxide in the air even if there were no animals. 489 00:35:23,259 --> 00:35:26,854 We need the plants much more than they need us. 490 00:35:31,000 --> 00:35:34,959 There are family resemblances among the organisms of the Earth. 491 00:35:35,171 --> 00:35:39,574 Some are very apparent, such as the use of the number five. 492 00:35:39,775 --> 00:35:42,642 Humans have five major bodily projections: 493 00:35:42,845 --> 00:35:46,144 One head, two arms, two legs. 494 00:35:46,349 --> 00:35:47,816 So do ducks... 495 00:35:48,017 --> 00:35:52,420 ...although the functions of their projections are not quite the same. 496 00:35:52,622 --> 00:35:56,114 An octopus or a centipede has a different plan. 497 00:35:56,325 --> 00:36:01,024 And a being from another planet might be much stranger still. 498 00:36:01,230 --> 00:36:06,099 These family resemblances continue and on a much deeper level... 499 00:36:06,302 --> 00:36:09,237 ...when we go to the molecular basis of life. 500 00:36:09,438 --> 00:36:12,134 There are tens of billions... 501 00:36:12,341 --> 00:36:15,242 ...of different kinds of organic molecules. 502 00:36:15,444 --> 00:36:17,639 Yet only about 50 of them... 503 00:36:17,847 --> 00:36:20,941 ...are used in the essential machinery of life. 504 00:36:21,150 --> 00:36:23,584 The same 50 employed over and over again... 505 00:36:23,786 --> 00:36:28,382 ...ingenious, for different functions in every living thing. 506 00:36:28,591 --> 00:36:31,355 And when we go to the very kernel of life on Earth... 507 00:36:31,560 --> 00:36:35,018 ...to the proteins that control cell chemistry... 508 00:36:35,231 --> 00:36:39,133 ...to the spiral or helix of nucleic acids... 509 00:36:39,335 --> 00:36:41,929 ...which carry the hereditary information... 510 00:36:42,138 --> 00:36:46,165 ...we find these molecules to be identical... 511 00:36:46,375 --> 00:36:49,970 ...in all plants and animals of our planet. 512 00:37:14,770 --> 00:37:19,366 This oak tree and me, we're made of the same stuff. 513 00:37:19,575 --> 00:37:22,476 If you go back, you'll find that we have a common ancestor. 514 00:37:22,678 --> 00:37:25,909 That's why our chemistry is so alike. 515 00:37:28,951 --> 00:37:32,944 Let's take a trip to examine this common basis of life. 516 00:37:33,155 --> 00:37:36,556 A voyage to investigate the molecular machinery... 517 00:37:36,759 --> 00:37:38,886 ...at the heart of life on Earth. 518 00:37:39,095 --> 00:37:42,656 A journey to the nucleus of the cell. 519 00:37:42,898 --> 00:37:44,422 First we need a cell. 520 00:37:45,000 --> 00:37:49,266 I have trillions. I can afford to donate a few. 521 00:37:55,311 --> 00:37:58,041 The casual act of pricking a finger... 522 00:37:58,247 --> 00:38:01,876 ...is an event of some magnitude on the scale of the very small. 523 00:38:02,084 --> 00:38:06,783 Millions of red blood cells are detoured from their usual routes. 524 00:38:08,758 --> 00:38:11,591 But most continue to cruise about the body... 525 00:38:11,794 --> 00:38:15,753 ...carrying their cargoes of oxygen to the remotest freckle. 526 00:38:16,866 --> 00:38:19,061 We're about to enter the living cell... 527 00:38:19,268 --> 00:38:22,795 ...a realm, in its own way, as complex and beautiful... 528 00:38:23,005 --> 00:38:25,974 ...as the realm of galaxies and stars. 529 00:38:26,375 --> 00:38:29,367 Among the red blood cells, we encounter a white blood cell... 530 00:38:29,578 --> 00:38:31,170 ...a lymphocyte... 531 00:38:31,380 --> 00:38:34,349 ...whose job it is to protect me against invading microbes. 532 00:38:34,550 --> 00:38:37,485 It makes antibodies on its furrowed surface... 533 00:38:37,686 --> 00:38:41,213 ...but its interior is like that of many cells. 534 00:38:43,159 --> 00:38:47,152 Plunging through the membrane, we find ourselves inside the cell. 535 00:38:47,363 --> 00:38:51,026 Here, every structure has its function. 536 00:38:53,869 --> 00:38:56,770 These dark green blobs are factories... 537 00:38:56,972 --> 00:38:59,941 ...where messenger molecules are busy building the enzymes... 538 00:39:00,142 --> 00:39:02,440 ...which control the chemistry of the cell. 539 00:39:02,645 --> 00:39:05,739 The messengers were instructed and dispatched... 540 00:39:05,948 --> 00:39:10,044 ...from within the nucleus, the heart and brain of the cell. 541 00:39:10,386 --> 00:39:12,911 All the instructions on how to get a cell to work... 542 00:39:13,122 --> 00:39:16,023 ...and how to make another are hidden away in there. 543 00:39:16,225 --> 00:39:18,693 We find a tunnel, a nuclear pore... 544 00:39:18,894 --> 00:39:22,625 ...an approach to the biological holy of holies. 545 00:39:24,834 --> 00:39:29,771 These necklaces, these intricately looped and coiled strands... 546 00:39:29,972 --> 00:39:32,532 ...are nucleic acids, DNA. 547 00:39:34,076 --> 00:39:36,636 Everything you need to know on how to make a human being... 548 00:39:36,846 --> 00:39:41,783 ...is encoded in the language of life in the DNA molecule. 549 00:39:49,325 --> 00:39:52,522 This is the DNA double helix... 550 00:39:52,728 --> 00:39:57,665 ...a machine with about 100 billion moving parts, called atoms. 551 00:39:58,601 --> 00:40:01,695 There are as many atoms in one molecule of DNA... 552 00:40:01,904 --> 00:40:05,601 ...as there are stars in a typical galaxy. 553 00:40:09,945 --> 00:40:13,278 The sequence of nucleotides, here brightly colored... 554 00:40:13,482 --> 00:40:16,474 ...is all that's passed on from generation to generation. 555 00:40:16,685 --> 00:40:18,915 Change the order of the nucleotides... 556 00:40:19,121 --> 00:40:22,181 ...and you change the genetic instructions. 557 00:40:29,665 --> 00:40:33,692 DNA must replicate itself with extreme fidelity. 558 00:40:33,903 --> 00:40:38,499 The reproduction of a DNA molecule begins by separating the two helices. 559 00:40:38,707 --> 00:40:42,666 This is accomplished by an unwinding enzyme. 560 00:40:42,878 --> 00:40:46,871 Like some precision tool, this enzyme, shown in blue... 561 00:40:47,082 --> 00:40:52,019 ...breaks the chemical bonds that bind the two helices of DNA. 562 00:40:52,655 --> 00:40:55,249 The enzyme works its way down the molecule... 563 00:40:55,457 --> 00:40:58,722 ...unzipping DNA as it goes. 564 00:41:01,330 --> 00:41:03,423 Each helix copies the other... 565 00:41:03,632 --> 00:41:06,829 ...supervised by special enzymes. 566 00:41:07,036 --> 00:41:10,870 The organic soup inside the nucleus contains many free nucleotides. 567 00:41:11,140 --> 00:41:16,077 The enzyme recognizes an approaching nucleotide and clicks it into place... 568 00:41:16,378 --> 00:41:19,643 ...reproducing another rung in the double helix. 569 00:41:23,919 --> 00:41:26,547 When the DNA is replicating in one of your cells... 570 00:41:26,755 --> 00:41:29,622 ...a few dozen nucleotides are added every second. 571 00:41:29,959 --> 00:41:34,362 Thousands of these enzymes may be working on a given DNA molecule. 572 00:41:44,206 --> 00:41:46,970 When an arriving nucleotide doesn't fit... 573 00:41:47,176 --> 00:41:49,235 ...the enzyme throws it away. 574 00:41:49,445 --> 00:41:51,072 We call this proofreading. 575 00:41:51,280 --> 00:41:53,771 On the rare occasions of a proofreading error... 576 00:41:53,983 --> 00:41:55,917 ...the wrong nucleotide is attached... 577 00:41:56,118 --> 00:41:59,747 ...and a small random change has been made in the genetic instructions. 578 00:41:59,955 --> 00:42:02,321 A mutation has occurred. 579 00:42:03,859 --> 00:42:06,623 This enzyme is a pretty small molecule... 580 00:42:06,829 --> 00:42:09,821 ...but it catches nucleotides, assembles them in the right order... 581 00:42:10,032 --> 00:42:11,465 ...it knows how to proofread... 582 00:42:11,667 --> 00:42:14,534 ...it's responsible in the most fundamental way... 583 00:42:14,737 --> 00:42:19,231 ...for the reproduction of every cell and every being on Earth. 584 00:42:24,680 --> 00:42:27,171 That enzyme and DNA itself... 585 00:42:27,383 --> 00:42:31,319 ...are molecular machines with awesome powers. 586 00:42:34,023 --> 00:42:37,584 Within every living thing, the molecular machines are busy... 587 00:42:37,793 --> 00:42:42,389 ...making sure that nucleic acids will continue to reproduce. 588 00:43:03,318 --> 00:43:06,776 A minor cut in my skin sounds a local alarm... 589 00:43:06,989 --> 00:43:10,857 ...and the blood spins a complex net of strong fibers... 590 00:43:11,060 --> 00:43:14,223 ...to form a clot and staunch the flow of blood. 591 00:43:14,430 --> 00:43:16,125 There's a very delicate balance here: 592 00:43:16,331 --> 00:43:19,459 Too much clotting and your blood stream will solidify. 593 00:43:19,668 --> 00:43:23,798 Too little clotting and you'll bleed to death from the merest scratch. 594 00:43:24,106 --> 00:43:29,043 The balance is controlled by enzymes instructed by DNA. 595 00:43:31,680 --> 00:43:34,843 Down here, there's also a kind of sanitation squad... 596 00:43:35,050 --> 00:43:38,315 ...comprised of white blood cells, that swings into action... 597 00:43:38,520 --> 00:43:43,150 ...surrounds invading bacteria and ravenously consumes them. 598 00:43:43,358 --> 00:43:46,521 This mopping-up operation is a part of the healing process... 599 00:43:46,728 --> 00:43:49,720 ...again controlled by DNA. 600 00:43:51,300 --> 00:43:55,430 These cells are parts of us, but how alien they seem. 601 00:43:55,637 --> 00:43:58,265 Within each of them, within every cell... 602 00:43:58,474 --> 00:44:01,500 ...there are exquisitely evolved molecular machines. 603 00:44:01,710 --> 00:44:05,271 Nucleic acids, enzymes, the cell architecture... 604 00:44:05,481 --> 00:44:09,349 ...every cell is a triumph of natural selection. 605 00:44:09,551 --> 00:44:12,748 And we're made of trillions of cells. 606 00:44:12,955 --> 00:44:15,947 We are, each of us, a multitude. 607 00:44:17,559 --> 00:44:20,619 Within us is a little universe. 608 00:44:36,879 --> 00:44:41,816 Human DNA is a coiled ladder... 609 00:44:42,050 --> 00:44:44,314 ...a billion nucleotides long. 610 00:44:44,853 --> 00:44:49,187 Many possible combinations of nucleotides are nonsense. That is... 611 00:44:49,391 --> 00:44:53,760 ...they translate into proteins which serve no useful function whatever. 612 00:44:53,962 --> 00:44:57,193 Only a comparatively few nucleic acid molecules... 613 00:44:57,399 --> 00:45:02,063 ...are any good for life forms as complicated as we are. 614 00:45:02,838 --> 00:45:07,036 But even so, the number of useful ways of assembling nucleic acids... 615 00:45:07,242 --> 00:45:09,403 ...is stupefyingly large. 616 00:45:09,611 --> 00:45:14,480 It's probably larger than the total number of atoms in the universe. 617 00:45:14,683 --> 00:45:19,620 This means that the number of possible kinds of human beings... 618 00:45:19,922 --> 00:45:24,552 ...is vastly greater than the number of human beings that has ever lived. 619 00:45:24,793 --> 00:45:29,025 This untapped potential of the human species is immense. 620 00:45:29,231 --> 00:45:31,893 There are ways of putting nucleic acids together... 621 00:45:32,100 --> 00:45:36,366 ...which will function far better by any criterion you wish to choose... 622 00:45:36,572 --> 00:45:41,509 ...than the hereditary instructions of any human being who has ever lived. 623 00:45:41,944 --> 00:45:46,404 Fortunately, we do not know, or at least do not yet know... 624 00:45:46,615 --> 00:45:51,143 ...how to assemble alternative sequences of nucleotides... 625 00:45:51,353 --> 00:45:54,720 ...to make alternative kinds of human beings. 626 00:45:54,923 --> 00:45:58,450 In the future, we might be able to put nucleotides together... 627 00:45:58,660 --> 00:46:00,389 ...in any desired sequence... 628 00:46:00,596 --> 00:46:03,997 ...to produce human characteristics we think desirable. 629 00:46:05,000 --> 00:46:08,959 A disquieting and awesome prospect. 630 00:46:19,748 --> 00:46:23,912 We human beings don't look very much like a tree. 631 00:46:24,119 --> 00:46:28,146 We certainly view the world differently than a tree does. 632 00:46:28,357 --> 00:46:31,815 But down deep, at the molecular heart of life... 633 00:46:32,027 --> 00:46:35,190 ...we're essentially identical to trees. 634 00:46:35,764 --> 00:46:40,633 We both use nucleic acids as the hereditary material. 635 00:46:40,836 --> 00:46:45,739 We both use proteins as enzymes to control the chemistry of the cell. 636 00:46:47,109 --> 00:46:51,637 And most significantly, we both use the identical code book... 637 00:46:51,847 --> 00:46:56,750 ...to translate nucleic acid information into protein information. 638 00:46:56,952 --> 00:47:01,321 Any tree could read my genetic code. 639 00:47:02,257 --> 00:47:04,782 How did such astonishing similarities come about? 640 00:47:04,993 --> 00:47:09,123 Why are we cousins to the trees? 641 00:47:09,331 --> 00:47:12,459 Would life on some other planet use proteins? 642 00:47:12,668 --> 00:47:17,605 The same proteins? The same nucleic acids? The same genetic code? 643 00:47:18,307 --> 00:47:21,140 The usual explanation is that we are... 644 00:47:21,343 --> 00:47:24,904 ...all of us, trees and people... 645 00:47:25,113 --> 00:47:29,948 ...anglerfish, slime molds, bacteria... 646 00:47:30,152 --> 00:47:33,781 ...all descended from a single and common instance... 647 00:47:33,989 --> 00:47:36,856 ...of the origin of life 4 billion years ago... 648 00:47:37,059 --> 00:47:39,220 ...in the early days of our planet. 649 00:47:40,062 --> 00:47:41,859 Now, how... 650 00:47:42,064 --> 00:47:45,465 ...did the molecules of life arise? 651 00:47:47,035 --> 00:47:48,400 (THUNDER) 652 00:47:51,473 --> 00:47:53,236 (LIGHTNING BUZZES) 653 00:48:03,819 --> 00:48:06,117 In a laboratory at Cornell University... 654 00:48:06,555 --> 00:48:10,184 ...we mix together the gases and waters of the primitive Earth... 655 00:48:10,392 --> 00:48:11,825 ...supply some energy... 656 00:48:12,027 --> 00:48:15,622 ...and see if we can make the stuff of life. 657 00:48:28,243 --> 00:48:32,475 But what was the early atmosphere made of, ordinary air? 658 00:48:32,681 --> 00:48:34,706 If we start with our present atmosphere... 659 00:48:34,916 --> 00:48:37,316 ...the experiment is a dismal failure. 660 00:48:37,519 --> 00:48:40,317 Instead of making proteins and nucleic acids... 661 00:48:40,522 --> 00:48:43,923 ...all we make is smog, a backwards step. 662 00:48:44,126 --> 00:48:46,185 Why doesn't such an experiment work? 663 00:48:46,395 --> 00:48:49,956 Because the air of today contains molecular oxygen. 664 00:48:50,165 --> 00:48:52,463 But oxygen is made by plants. 665 00:48:52,667 --> 00:48:56,296 It's obvious that there were no plants before the origin of life. 666 00:48:56,505 --> 00:48:58,735 We mustn't use oxygen in our experiments... 667 00:48:58,940 --> 00:49:01,807 ...because there wasn't any in the early atmosphere. 668 00:49:07,115 --> 00:49:11,347 This is reasonable because the cosmos is made mostly of hydrogen... 669 00:49:11,553 --> 00:49:13,020 ...which gobbles oxygen up. 670 00:49:13,221 --> 00:49:17,317 The Earth's low gravity has allowed most of our hydrogen gas... 671 00:49:17,526 --> 00:49:21,223 ...to trickle away to space. There's almost none left. 672 00:49:22,197 --> 00:49:23,630 But 4 billion years ago... 673 00:49:23,832 --> 00:49:26,892 ...our atmosphere was full of hydrogen-rich gases: 674 00:49:27,102 --> 00:49:29,593 Methane, ammonia, water vapor. 675 00:49:29,805 --> 00:49:32,273 These are the gases we should use. 676 00:49:37,245 --> 00:49:40,214 Taking great care to ensure the purity of these gases... 677 00:49:40,415 --> 00:49:44,545 ...my colleague, Bishun Khare, pumps them from their holding flasks. 678 00:49:58,433 --> 00:50:01,129 An experiment like this was first performed... 679 00:50:01,336 --> 00:50:05,363 ...by Stanley Miller and Harold Urey in the 1950s. 680 00:50:06,575 --> 00:50:08,668 (GASES FIZZLE) 681 00:50:21,389 --> 00:50:25,985 The starting gases are now introduced into a large reaction vessel. 682 00:50:26,194 --> 00:50:30,392 We could shine ultraviolet light, simulating the early sun. 683 00:50:30,599 --> 00:50:31,964 But in this experiment... 684 00:50:32,200 --> 00:50:34,464 ...the gases will be sparked... 685 00:50:34,669 --> 00:50:38,605 ...as the primitive atmosphere was by early lightning. 686 00:50:52,187 --> 00:50:53,984 (LIGHTNING BUZZES) 687 00:50:59,294 --> 00:51:03,754 After only a few hours, the interior of the reaction vessel... 688 00:51:03,965 --> 00:51:07,093 ...becomes streaked with a strange brown pigment... 689 00:51:07,302 --> 00:51:10,829 ...a rich collection of complex organic molecules... 690 00:51:11,039 --> 00:51:15,601 ...including the building blocks of the proteins and the nucleic acids. 691 00:51:17,812 --> 00:51:21,441 Under the right conditions, these building blocks assemble themselves... 692 00:51:21,650 --> 00:51:26,110 ...into molecules resembling little proteins and little nucleic acids. 693 00:51:26,321 --> 00:51:30,849 These nucleic acids can even make identical copies of themselves. 694 00:51:36,298 --> 00:51:40,701 In this vessel are the notes of the music of life... 695 00:51:40,902 --> 00:51:43,769 ...although not yet the music itself. 696 00:51:46,308 --> 00:51:48,742 Now, no one, so far... 697 00:51:48,944 --> 00:51:53,313 ...has mixed together the gases and waters of the primitive Earth... 698 00:51:53,515 --> 00:51:58,452 ...and at the end of the experiment had something crawl out of the flask. 699 00:51:58,687 --> 00:52:02,316 There's still much to be understood about the origin of life... 700 00:52:02,524 --> 00:52:04,856 ...including the origin of the genetic code. 701 00:52:05,060 --> 00:52:08,518 But we've only been at such experiments for 30 years. 702 00:52:08,730 --> 00:52:12,029 Nature's had a 4-billion-year head start. 703 00:52:12,233 --> 00:52:16,829 Incidentally, there's nothing in such experiments that's unique to the Earth. 704 00:52:17,038 --> 00:52:20,633 The gases we start with, the energy sources we use... 705 00:52:20,842 --> 00:52:23,572 ...are entirely common through the cosmos. 706 00:52:23,778 --> 00:52:28,306 So chemical reactions something like these must be responsible for... 707 00:52:28,516 --> 00:52:30,677 ...the organic matter in interstellar space... 708 00:52:30,885 --> 00:52:33,251 ...and the amino acids in the meteorites. 709 00:52:33,455 --> 00:52:36,288 Similar chemical reactions must have occurred... 710 00:52:36,491 --> 00:52:39,927 ...on a billion other worlds in the Milky Way galaxy. 711 00:52:40,128 --> 00:52:44,087 Look how easy it is to make great globs of this stuff. 712 00:52:44,299 --> 00:52:48,565 The molecules of life fill the cosmos. 713 00:52:49,170 --> 00:52:50,228 Now... 714 00:52:50,772 --> 00:52:53,605 What would life elsewhere look like? 715 00:52:53,808 --> 00:52:57,801 Even if it had an identical molecular chemistry to life on Earth... 716 00:52:58,013 --> 00:52:59,844 ...which I very much doubt... 717 00:53:00,048 --> 00:53:03,609 ...it could not be very similar in form... 718 00:53:03,818 --> 00:53:06,082 ...to familiar organisms on the Earth. 719 00:53:06,287 --> 00:53:09,450 The random character of the evolutionary process... 720 00:53:09,658 --> 00:53:14,493 ...must create elsewhere creatures very different from any that we know. 721 00:53:16,264 --> 00:53:18,892 Think of a world something like Jupiter... 722 00:53:19,100 --> 00:53:23,662 ...with an atmosphere rich in hydrogen, helium, methane, water and ammonia... 723 00:53:23,872 --> 00:53:25,533 ...in which organic molecules might be... 724 00:53:25,740 --> 00:53:28,470 ...falling from the skies like manna from heaven... 725 00:53:28,677 --> 00:53:31,237 ...like the products of the Miller-Urey experiment. 726 00:53:31,446 --> 00:53:33,937 Could there be life on such a world? 727 00:53:34,883 --> 00:53:38,444 There's a special problem. The atmosphere is turbulent... 728 00:53:38,653 --> 00:53:42,282 ...and down deep, before we ever come to a surface, it's very hot. 729 00:53:42,490 --> 00:53:46,153 If you're not careful, you'll be carried down and fried. 730 00:53:46,361 --> 00:53:49,194 If you reproduce before you're fried... 731 00:53:49,397 --> 00:53:54,096 ...turbulence will carry your offspring into the higher and cooler layers. 732 00:53:54,302 --> 00:53:59,001 Such organisms could be very little. We call them sinkers. 733 00:54:00,442 --> 00:54:03,570 The physicist E.E. Salpeter and I at Cornell... 734 00:54:03,778 --> 00:54:06,372 ...have calculated something about the life... 735 00:54:06,581 --> 00:54:09,072 ...that might exist on such a world. 736 00:54:11,119 --> 00:54:14,384 Vast living balloons could stay buoyant... 737 00:54:14,589 --> 00:54:17,752 ...by pumping heavy gases from their interiors... 738 00:54:17,959 --> 00:54:19,950 ...or by keeping their insides warm. 739 00:54:20,161 --> 00:54:22,891 They might eat the organic molecules in the air... 740 00:54:23,098 --> 00:54:24,963 ...or make their own with sunlight. 741 00:54:25,166 --> 00:54:28,499 We call these creatures floaters. 742 00:54:30,171 --> 00:54:32,537 We imagine floaters kilometers across... 743 00:54:32,741 --> 00:54:36,177 ...enormously larger than the greatest whale that ever was... 744 00:54:36,377 --> 00:54:39,778 ...beings the size of cities. 745 00:54:39,981 --> 00:54:43,883 We conceive of them arrayed in great, lazy herds... 746 00:54:44,085 --> 00:54:45,916 ...as far as the eye can see... 747 00:54:46,121 --> 00:54:50,820 ...concentrated in the updrafts in the enormous sea of clouds. 748 00:54:51,025 --> 00:54:55,291 But there can be other creatures in this alien environment: hunters. 749 00:54:57,165 --> 00:54:59,326 Hunters are fast and maneuverable. 750 00:54:59,534 --> 00:55:02,833 They eat the floaters, both for their organic molecules... 751 00:55:03,037 --> 00:55:05,130 ...and for their store of pure hydrogen. 752 00:55:05,340 --> 00:55:07,035 But there can't be many hunters... 753 00:55:07,242 --> 00:55:11,906 ...because if they destroy all the floaters, they themselves will perish. 754 00:55:15,450 --> 00:55:18,578 Physics and chemistry permit such life forms. 755 00:55:18,787 --> 00:55:21,255 Art presents them with a certain reality... 756 00:55:21,456 --> 00:55:25,119 ...but nature is not obliged to follow our speculations. 757 00:55:25,326 --> 00:55:29,922 If there are billions of inhabited worlds in the Milky Way galaxy... 758 00:55:30,365 --> 00:55:34,859 ...then I think it's likely there are a few places which might have... 759 00:55:35,069 --> 00:55:36,468 ...hunters... 760 00:55:36,671 --> 00:55:39,469 ...and floaters and sinkers. 761 00:55:40,708 --> 00:55:44,940 Biology is more like history than it is like physics. 762 00:55:45,146 --> 00:55:48,206 You have to know the past to understand the present. 763 00:55:48,416 --> 00:55:52,580 There is no predictive theory of biology, nor is there for history. 764 00:55:52,787 --> 00:55:54,152 The reason is the same: 765 00:55:54,355 --> 00:55:57,688 Both subjects are still too complicated for us. 766 00:55:57,892 --> 00:56:00,520 But we can understand ourselves much better... 767 00:56:00,728 --> 00:56:02,958 ...by understanding other cases. 768 00:56:04,632 --> 00:56:08,090 The study of a single instance of extraterrestrial life... 769 00:56:08,303 --> 00:56:11,795 No matter how humble, a microbe would be just fine. 770 00:56:12,006 --> 00:56:14,941 ...will de-provincialize biology. 771 00:56:15,143 --> 00:56:18,738 It will show us what else is possible. 772 00:56:20,215 --> 00:56:25,152 We've heard so far the voice of life on only a single world... 773 00:56:25,386 --> 00:56:27,752 ...but for the first time, as we shall see... 774 00:56:27,956 --> 00:56:31,414 ...we've begun a serious scientific search... 775 00:56:31,626 --> 00:56:33,856 ...for the cosmic fugue. 776 00:56:44,072 --> 00:56:47,337 Recently, we've learned more about the origin of life. 777 00:56:47,542 --> 00:56:49,669 Do you remember RNA... 778 00:56:49,878 --> 00:56:53,245 ...that nucleic acid that our cells use as messengers... 779 00:56:53,448 --> 00:56:56,576 ...carrying the genetic information out of the cell nucleus? 780 00:56:57,252 --> 00:57:01,484 Well, it's been found that RNA, like protein... 781 00:57:01,689 --> 00:57:03,953 ...can control chemical reactions... 782 00:57:04,158 --> 00:57:07,821 ...as well as reproduce itself, which proteins can't do. 783 00:57:08,029 --> 00:57:11,260 Many scientists now wonder if the first life on Earth... 784 00:57:11,466 --> 00:57:13,627 ...was an RNA molecule. 785 00:57:13,835 --> 00:57:17,293 It now seems feasible that key molecular building blocks... 786 00:57:17,505 --> 00:57:21,441 ...for the origin of life, fell out of the skies 4 billion years ago. 787 00:57:22,210 --> 00:57:26,613 Comets have been found to have a lot of organic molecules in them... 788 00:57:26,814 --> 00:57:31,148 ...and they fell in huge numbers on the primitive Earth. 789 00:57:32,253 --> 00:57:35,086 We also mention the extinction of the dinosaurs... 790 00:57:35,290 --> 00:57:39,226 ...and most of the other species on Earth about 65 million years ago. 791 00:57:39,427 --> 00:57:43,591 We now know that a large comet hit the Earth at just that time. 792 00:57:43,798 --> 00:57:48,735 The dust pall from that collision must've cooled and darkened the Earth... 793 00:57:49,070 --> 00:57:52,005 ...perhaps killing all the dinosaurs, but sparing... 794 00:57:52,206 --> 00:57:56,666 ...the small, furry mammals who were our ancestors. 795 00:57:56,878 --> 00:58:00,746 Other cometary mass extinctions in other epochs seem likely. 796 00:58:00,949 --> 00:58:05,283 If true, this would mean that comets have been the bringers... 797 00:58:05,486 --> 00:58:07,954 ...both of life and death.