1 00:00:01,068 --> 00:00:03,269 Viewers like you make this program possible. 2 00:00:03,270 --> 00:00:05,339 Support your local PBS station. 3 00:00:16,617 --> 00:00:19,319 {\an8}Every new life on Earth 4 00:00:19,320 --> 00:00:24,125 is here thanks to a fundamental drive to reproduce. 5 00:00:25,860 --> 00:00:28,295 The game of life is all about reproduction. 6 00:00:29,530 --> 00:00:31,731 If you don't reproduce, 7 00:00:31,732 --> 00:00:34,368 your genetic line goes extinct. 8 00:00:35,603 --> 00:00:38,338 But while there are many ways to do it... 9 00:00:38,339 --> 00:00:41,040 Across the tree of life, we see 10 00:00:41,041 --> 00:00:43,243 so many different ways of reproducing, 11 00:00:43,244 --> 00:00:44,412 it's mind-boggling. 12 00:00:45,446 --> 00:00:49,216 ...one way took evolution to the next level. 13 00:00:50,451 --> 00:00:53,987 The evolution of sex was one of the most important steps 14 00:00:53,988 --> 00:00:56,256 in the entire history of life. 15 00:00:58,526 --> 00:01:03,096 Sexual reproduction turbocharged the engine of evolution. 16 00:01:03,097 --> 00:01:05,999 Sex gives us variety, variety, variety. 17 00:01:06,000 --> 00:01:08,334 Every new generation 18 00:01:08,335 --> 00:01:10,371 is a roll of the genetic dice. 19 00:01:11,572 --> 00:01:13,841 Leading to astonishing innovations... 20 00:01:15,009 --> 00:01:18,244 {\an8}BHART-ANJAN BHULLAR: Eggs are protective capsules more complex 21 00:01:18,245 --> 00:01:20,847 {\an8}than anything that could be produced today 22 00:01:20,848 --> 00:01:22,683 {\an8}using machine technology. 23 00:01:23,951 --> 00:01:27,287 {\an8}...and eye-catching transformations. 24 00:01:27,288 --> 00:01:31,591 {\an8}Evolution doesn't just create what needs to survive, 25 00:01:31,592 --> 00:01:32,693 it can create what's beautiful. 26 00:01:33,727 --> 00:01:37,664 {\an8}Sex fueled the color, the beauty, 27 00:01:37,665 --> 00:01:39,699 {\an8}and the downright bizarre stuff 28 00:01:39,700 --> 00:01:40,733 we see in the world today. 29 00:01:40,734 --> 00:01:43,570 Sex made the modern world. 30 00:01:43,571 --> 00:01:45,004 Sex made us. 31 00:01:45,005 --> 00:01:48,708 How did sex create the wondrous world 32 00:01:48,709 --> 00:01:50,977 we see around us? 33 00:01:50,978 --> 00:01:53,313 "Evolution: The Mating Game," 34 00:01:53,314 --> 00:01:56,216 right now, on "NOVA." 35 00:02:25,779 --> 00:02:27,881 Standing nine feet tall 36 00:02:27,882 --> 00:02:30,817 and weighing up to 300 pounds... 37 00:02:33,120 --> 00:02:36,789 ...the ostrich is the largest bird on Earth 38 00:02:36,790 --> 00:02:41,261 and the fastest animal on two legs. 39 00:02:41,262 --> 00:02:42,996 Ostriches are remarkable. 40 00:02:42,997 --> 00:02:46,667 They can reach up to 43 miles an hour. 41 00:02:47,835 --> 00:02:49,536 A single stride 42 00:02:49,537 --> 00:02:50,970 when they're running at full speed 43 00:02:50,971 --> 00:02:52,605 is, like, the length of a car. 44 00:02:56,110 --> 00:02:58,344 {\an8}The legs of an ostrich are so powerful, 45 00:02:58,345 --> 00:03:00,013 {\an8}they can deliver kicks 46 00:03:00,014 --> 00:03:01,848 {\an8}that can kill with a single blow. 47 00:03:05,119 --> 00:03:07,655 {\an8}But the ostrich isn't just a fighter. 48 00:03:09,323 --> 00:03:12,226 {\an8}It's also a lover. 49 00:03:20,568 --> 00:03:22,769 After an elaborate courtship ritual, 50 00:03:22,770 --> 00:03:26,707 females can lay up to 18 eggs. 51 00:03:28,409 --> 00:03:30,143 Ostrich eggs are so cool. 52 00:03:30,144 --> 00:03:33,446 I marvel at the size of it-- it is huge. 53 00:03:33,447 --> 00:03:37,050 This egg right here weighs about as much 54 00:03:37,051 --> 00:03:38,586 as two dozen chicken eggs. 55 00:03:39,620 --> 00:03:41,487 It's the largest egg 56 00:03:41,488 --> 00:03:43,289 laid by any animal alive today. 57 00:03:45,392 --> 00:03:50,029 In about 40 days, a single fertilized egg cell 58 00:03:50,030 --> 00:03:54,567 transforms into a two-pound, ten-inch-tall chick. 59 00:03:59,073 --> 00:04:02,209 Its DNA is a mixture of its parents'. 60 00:04:05,713 --> 00:04:10,149 No chick exactly like it has ever existed before 61 00:04:10,150 --> 00:04:12,052 or will ever exist again. 62 00:04:14,388 --> 00:04:18,391 So, how did the ostrich and its astonishing egg 63 00:04:18,392 --> 00:04:19,793 come to be? 64 00:04:24,465 --> 00:04:25,565 To answer that question, 65 00:04:25,566 --> 00:04:27,434 we have to go back. 66 00:04:29,136 --> 00:04:30,904 Way back. 67 00:04:38,045 --> 00:04:41,881 To meet the ancient ancestors of the ostrich... 68 00:04:46,153 --> 00:04:47,186 {\an8}...who lived 69 00:04:47,187 --> 00:04:48,888 {\an8}and died 70 00:04:48,889 --> 00:04:50,890 {\an8}and reproduced 71 00:04:50,891 --> 00:04:54,028 {\an8}in an unbroken chain of life... 72 00:04:56,830 --> 00:05:00,534 ...stretching back billions of years. 73 00:05:04,138 --> 00:05:08,441 From its powerful legs to its unique feathers 74 00:05:08,442 --> 00:05:10,410 and giant eggs, 75 00:05:10,411 --> 00:05:15,882 the ostrich tells the epic story of how sex unleashed 76 00:05:15,883 --> 00:05:18,852 the full creative power of evolution. 77 00:05:54,388 --> 00:05:58,659 These are some of the world's earliest living things. 78 00:06:00,594 --> 00:06:03,230 Simple single cells. 79 00:06:08,936 --> 00:06:13,773 And there's one thing they are especially good at: 80 00:06:13,774 --> 00:06:15,442 reproducing. 81 00:06:17,244 --> 00:06:19,879 But nearly four billion years ago, 82 00:06:19,880 --> 00:06:25,052 there are no sexes, no mating, no eggs. 83 00:06:26,520 --> 00:06:31,425 These cells reproduce simply by creating a copy of themselves. 84 00:06:34,728 --> 00:06:35,996 A clone. 85 00:06:37,264 --> 00:06:38,831 Cloning is a great way 86 00:06:38,832 --> 00:06:40,967 {\an8}to make a perfect copy, 87 00:06:40,968 --> 00:06:42,870 {\an8}as if you were going to a photocopying machine. 88 00:06:44,805 --> 00:06:47,573 {\an8}All it does is make a copy of its DNA 89 00:06:47,574 --> 00:06:51,144 {\an8}and then it makes a copy of all the other contents of the cell, 90 00:06:51,145 --> 00:06:53,212 {\an8}and then it splits itself in half 91 00:06:53,213 --> 00:06:54,914 {\an8}and creates two cells, 92 00:06:54,915 --> 00:06:56,149 {\an8}and those two cells 93 00:06:56,150 --> 00:06:58,385 {\an8}are almost identical to each other. 94 00:07:00,821 --> 00:07:05,092 Cloning is how bacteria and many other microbes reproduce. 95 00:07:06,260 --> 00:07:08,996 And it still works incredibly well for them. 96 00:07:10,164 --> 00:07:13,432 They have a tremendous diversity of ways of living. 97 00:07:13,433 --> 00:07:16,202 There are microbes that live in hot vents 98 00:07:16,203 --> 00:07:18,271 at the bottom of the ocean. 99 00:07:18,272 --> 00:07:20,907 There are microbes that can eat methane 100 00:07:20,908 --> 00:07:24,111 and microbes that live in sea ice in the Arctic. 101 00:07:25,813 --> 00:07:27,313 Through cloning, 102 00:07:27,314 --> 00:07:29,549 single-celled life forms like this one 103 00:07:29,550 --> 00:07:31,717 prosper and multiply. 104 00:07:31,718 --> 00:07:33,186 And they might well have remained 105 00:07:33,187 --> 00:07:35,689 the only form of life on Earth. 106 00:07:37,991 --> 00:07:41,561 {\an8}But then, around two billion years ago, 107 00:07:41,562 --> 00:07:44,331 a new type of cell emerges. 108 00:07:58,412 --> 00:08:01,614 On the surface, this cell might seem similar 109 00:08:01,615 --> 00:08:03,917 to all the microbes that came before it. 110 00:08:05,285 --> 00:08:07,920 But in fact, it's the beginning 111 00:08:07,921 --> 00:08:12,391 of a new branch on the tree of life, 112 00:08:12,392 --> 00:08:16,395 the common ancestor of every animal, plant, and fungus 113 00:08:16,396 --> 00:08:18,298 alive today. 114 00:08:20,634 --> 00:08:23,269 And it has a superpower. 115 00:08:27,207 --> 00:08:30,276 When it gets close to another of its species... 116 00:08:33,847 --> 00:08:36,483 ...their cell membranes can fuse. 117 00:08:38,252 --> 00:08:41,821 They combine and mix their DNA together 118 00:08:41,822 --> 00:08:45,191 and eventually divide into new offspring, 119 00:08:45,192 --> 00:08:47,394 different from their parents. 120 00:08:49,930 --> 00:08:52,833 This is the earliest form of sex. 121 00:08:54,835 --> 00:08:58,238 And life would never be the same again. 122 00:09:00,040 --> 00:09:02,475 Imagine this is one parent-- it's all red. 123 00:09:02,476 --> 00:09:04,243 And if it cloned, 124 00:09:04,244 --> 00:09:06,045 {\an8}that's the only type of offspring it would have. 125 00:09:06,046 --> 00:09:07,813 {\an8}Maybe, occasionally, mutations. 126 00:09:07,814 --> 00:09:10,750 {\an8}If it were this individual, it'd be all black. 127 00:09:10,751 --> 00:09:12,585 And if it cloned, that's all you'd get. 128 00:09:12,586 --> 00:09:14,120 What sex does is, 129 00:09:14,121 --> 00:09:16,689 it allows those cards to be mixed up, 130 00:09:16,690 --> 00:09:18,958 to shuffle them, to have offspring... 131 00:09:18,959 --> 00:09:21,627 ...that are different from any of the parents. 132 00:09:21,628 --> 00:09:24,297 And that introduces new varieties, 133 00:09:24,298 --> 00:09:27,000 combinations that weren't there in the parents. 134 00:09:28,435 --> 00:09:29,936 Through sex, 135 00:09:29,937 --> 00:09:32,939 there is an almost limitless number of possible combinations 136 00:09:32,940 --> 00:09:34,541 of the parents' genes. 137 00:09:35,976 --> 00:09:38,978 Meaning the offspring of sexual reproduction 138 00:09:38,979 --> 00:09:40,948 are completely unique. 139 00:09:42,115 --> 00:09:44,050 That reshuffling of genes produces 140 00:09:44,051 --> 00:09:47,253 an enormous amount of variation upon which evolution can work. 141 00:09:47,254 --> 00:09:48,821 And so it can accelerate 142 00:09:48,822 --> 00:09:51,357 the entire evolutionary process enormously. 143 00:09:51,358 --> 00:09:53,893 We can never rerun the tape of evolution, 144 00:09:53,894 --> 00:09:58,431 but I do think that if sexual reproduction never evolved, 145 00:09:58,432 --> 00:10:00,933 then complex life would not have evolved. 146 00:10:00,934 --> 00:10:03,602 And by that, I mean the type of life form that we are. 147 00:10:03,603 --> 00:10:04,937 Lots of different body parts, 148 00:10:04,938 --> 00:10:07,373 trillions of cells all working together. 149 00:10:07,374 --> 00:10:10,710 When sexual reproduction came on the scene, 150 00:10:10,711 --> 00:10:12,378 it enabled organisms 151 00:10:12,379 --> 00:10:15,581 to shuffle around their genes to evolve new structures. 152 00:10:15,582 --> 00:10:17,117 That changed everything. 153 00:10:18,652 --> 00:10:21,320 This pioneering new way to reproduce 154 00:10:21,321 --> 00:10:24,256 sends evolution into overdrive. 155 00:10:31,665 --> 00:10:34,067 And as the ages pass... 156 00:10:35,235 --> 00:10:37,437 ...some cells join forces... 157 00:10:38,705 --> 00:10:42,375 ...building bigger, more complex organisms 158 00:10:42,376 --> 00:10:48,248 made up of thousands, millions, and billions of cells. 159 00:10:50,117 --> 00:10:51,550 Until eventually, 160 00:10:51,551 --> 00:10:54,653 after around one-and-a-half billion years 161 00:10:54,654 --> 00:10:56,490 of sexual reproduction... 162 00:11:00,861 --> 00:11:05,297 {\an8}...the Cambrian seas are teeming with complex life. 163 00:11:05,298 --> 00:11:10,269 {\an8}From primitive fish, algae, and sponges 164 00:11:10,270 --> 00:11:13,172 to ancient relatives of insects, 165 00:11:13,173 --> 00:11:16,910 the drive to reproduce is as strong as ever. 166 00:11:24,651 --> 00:11:29,156 But as life got more complicated, so did sex. 167 00:11:32,259 --> 00:11:35,227 South Africa is a fantastic place to be a paleobiologist, 168 00:11:35,228 --> 00:11:39,932 because we have a wide range of different types of fossils. 169 00:11:39,933 --> 00:11:44,136 Enele Twala studies fossils to understand 170 00:11:44,137 --> 00:11:45,739 how ancient creatures lived. 171 00:11:47,274 --> 00:11:50,009 A great example is a trilobite. 172 00:11:50,010 --> 00:11:52,278 {\an8}So, this specific trilobite 173 00:11:52,279 --> 00:11:54,680 {\an8}was found right here in South Africa, 174 00:11:54,681 --> 00:11:57,616 and this is one of 20,000 species of trilobites 175 00:11:57,617 --> 00:11:58,885 that have been found so far. 176 00:12:00,053 --> 00:12:02,655 And we can see that its body is made up 177 00:12:02,656 --> 00:12:05,524 of multiple segments 178 00:12:05,525 --> 00:12:06,492 and, by looking at these features 179 00:12:06,493 --> 00:12:07,693 and how they're structured, 180 00:12:07,694 --> 00:12:09,728 we can infer that trilobites 181 00:12:09,729 --> 00:12:11,931 were most likely walking on the seafloor. 182 00:12:11,932 --> 00:12:15,601 This one here is called an eocrinoid, 183 00:12:15,602 --> 00:12:17,603 and it is 510 million years old. 184 00:12:17,604 --> 00:12:22,041 We can tell a lot about how it lived based on how it looks. 185 00:12:22,042 --> 00:12:24,276 So it was firmly attached to the seafloor, 186 00:12:24,277 --> 00:12:26,579 and it has these finger-like protrusions 187 00:12:26,580 --> 00:12:29,516 that most likely helped it catch its food. 188 00:12:30,684 --> 00:12:34,320 {\an8}But the elaborate bodies of Cambrian creatures 189 00:12:34,321 --> 00:12:36,223 {\an8}had a catch. 190 00:12:39,126 --> 00:12:42,061 {\an8}Some were anchored to the seafloor, 191 00:12:42,062 --> 00:12:43,896 {\an8}away from potential mates, 192 00:12:43,897 --> 00:12:46,733 {\an8}and some had hard shells. 193 00:12:48,702 --> 00:12:51,036 {\an8}With complex forms like these, 194 00:12:51,037 --> 00:12:54,473 {\an8}fusing entire bodies to exchange DNA, 195 00:12:54,474 --> 00:12:56,375 {\an8}like their single-celled ancestors, 196 00:12:56,376 --> 00:12:58,345 {\an8}is simply impossible. 197 00:13:00,313 --> 00:13:02,648 They could only evolve in the first place 198 00:13:02,649 --> 00:13:08,053 thanks to an entirely different way of having sex: 199 00:13:08,054 --> 00:13:12,092 using specialized cells to do the fusing for them. 200 00:13:15,962 --> 00:13:19,498 By the Cambrian, some life had already been doing this 201 00:13:19,499 --> 00:13:22,569 for around 500 million years. 202 00:13:25,105 --> 00:13:27,741 These tiny specks are eggs. 203 00:13:29,576 --> 00:13:33,013 And these even tinier ones are sperm. 204 00:13:35,248 --> 00:13:38,918 Evolution can and often does favor specialization. 205 00:13:38,919 --> 00:13:41,453 Basically a way to divide roles up 206 00:13:41,454 --> 00:13:44,290 and to hyper-focus on doing specific things. 207 00:13:44,291 --> 00:13:46,125 We can think about it at the cellular level. 208 00:13:46,126 --> 00:13:50,563 Egg cells, sperm cells-- those are hyper-specialized cells. 209 00:13:50,564 --> 00:13:53,599 Really, their only job is for reproduction. 210 00:13:55,535 --> 00:13:58,904 As life grew larger and more complex, 211 00:13:58,905 --> 00:14:01,674 this division of labor allowed the parents 212 00:14:01,675 --> 00:14:03,809 to live on separately, 213 00:14:03,810 --> 00:14:06,178 while contributing some of their genetic code 214 00:14:06,179 --> 00:14:09,015 to each sex cell, or gamete. 215 00:14:10,417 --> 00:14:13,018 A gamete is a single cell 216 00:14:13,019 --> 00:14:15,521 that carries half of the information 217 00:14:15,522 --> 00:14:16,755 from your parents 218 00:14:16,756 --> 00:14:19,491 and combines with another gamete through sex, 219 00:14:19,492 --> 00:14:20,926 sexual reproduction, 220 00:14:20,927 --> 00:14:23,162 to make the baby offspring 221 00:14:23,163 --> 00:14:24,798 that then goes on to start a new life. 222 00:14:26,833 --> 00:14:30,502 Just like the first sexually reproducing organisms, 223 00:14:30,503 --> 00:14:32,805 these individual gamete cells 224 00:14:32,806 --> 00:14:36,041 could fuse membranes and combine DNA 225 00:14:36,042 --> 00:14:39,878 to create new, unique offspring. 226 00:14:39,879 --> 00:14:41,080 The earliest gametes 227 00:14:41,081 --> 00:14:43,048 were probably similar in size and shape. 228 00:14:43,049 --> 00:14:44,383 They didn't differ that much. 229 00:14:44,384 --> 00:14:45,651 But then over time, 230 00:14:45,652 --> 00:14:48,454 they evolved to be separate eggs and sperm, 231 00:14:48,455 --> 00:14:49,521 and this allowed them 232 00:14:49,522 --> 00:14:51,091 to specialize for different functions. 233 00:14:52,158 --> 00:14:53,525 And what did they specialize on? 234 00:14:53,526 --> 00:14:56,829 Well, we think that the sperm, the smaller cell, 235 00:14:56,830 --> 00:14:59,331 specialized on moving around 236 00:14:59,332 --> 00:15:02,234 and increasing the chances of encountering an egg, 237 00:15:02,235 --> 00:15:04,036 whereas the egg specialized 238 00:15:04,037 --> 00:15:06,805 on being nutritious and feeding the young. 239 00:15:10,443 --> 00:15:13,345 {\an8}But the evolution of eggs and sperm 240 00:15:13,346 --> 00:15:15,014 {\an8}did not necessarily mean 241 00:15:15,015 --> 00:15:18,018 {\an8}that every creature was either female or male. 242 00:15:20,287 --> 00:15:22,254 This snail is extraordinary, 243 00:15:22,255 --> 00:15:26,292 because any individual can produce both sperm and eggs. 244 00:15:26,293 --> 00:15:29,029 We call these sorts of animals hermaphrodites. 245 00:15:31,164 --> 00:15:32,965 What's interesting about garden snails is, 246 00:15:32,966 --> 00:15:36,502 when they encounter another member of the same species, 247 00:15:36,503 --> 00:15:38,937 they can both produce fertilized eggs. 248 00:15:38,938 --> 00:15:41,006 So this allows them to essentially double 249 00:15:41,007 --> 00:15:43,008 the number of offspring that they could have. 250 00:15:43,009 --> 00:15:45,478 It's a very efficient strategy for reproduction. 251 00:15:48,081 --> 00:15:49,515 Understanding the origin 252 00:15:49,516 --> 00:15:51,583 of what we think of as male and female 253 00:15:51,584 --> 00:15:53,052 is really constantly changing. 254 00:15:53,053 --> 00:15:56,088 And because of its efficiency, we think 255 00:15:56,089 --> 00:15:58,457 that the earliest animals might have combined sexes 256 00:15:58,458 --> 00:16:00,893 in the same body in just the same way. 257 00:16:02,962 --> 00:16:06,699 {\an8}Some scientists believe that for the earliest animals, 258 00:16:06,700 --> 00:16:09,969 {\an8}being a hermaphrodite might have been the norm. 259 00:16:12,539 --> 00:16:15,207 {\an8}They theorize that during the early Cambrian, 260 00:16:15,208 --> 00:16:18,178 {\an8}something happens to upset the status quo. 261 00:16:19,412 --> 00:16:22,514 {\an8}A process that likely happens multiple times 262 00:16:22,515 --> 00:16:24,451 {\an8}in many different species. 263 00:16:26,419 --> 00:16:29,588 {\an8}And it's plausible that one of those species 264 00:16:29,589 --> 00:16:31,523 {\an8}is an animal like this. 265 00:16:36,129 --> 00:16:38,665 This is a vetulicolian. 266 00:16:45,472 --> 00:16:49,208 About the size of a mouse, it doesn't have eyes, 267 00:16:49,209 --> 00:16:52,711 but it does have a primitive spinal cord, 268 00:16:52,712 --> 00:16:56,949 which makes it a distant cousin of vertebrates like the ostrich. 269 00:16:58,651 --> 00:17:00,487 And all of us. 270 00:17:02,922 --> 00:17:07,226 For a hermaphrodite, finding a mate is relatively easy. 271 00:17:07,227 --> 00:17:10,562 Any other member of the species will do. 272 00:17:14,334 --> 00:17:16,970 But for this pair, there's a problem. 273 00:17:21,641 --> 00:17:25,344 One of them has a mutation that switches off its ability 274 00:17:25,345 --> 00:17:27,547 to produce one type of gamete. 275 00:17:28,581 --> 00:17:30,650 In this case, sperm. 276 00:17:35,021 --> 00:17:38,792 The eggs it releases are now its only means of reproduction. 277 00:17:40,493 --> 00:17:43,663 And some of those eggs now carry the same mutation. 278 00:17:47,500 --> 00:17:50,169 Meaning some descendants will also 279 00:17:50,170 --> 00:17:52,571 only be able to produce eggs. 280 00:17:55,909 --> 00:17:58,277 They are no longer hermaphrodites. 281 00:17:58,278 --> 00:18:02,214 Instead, they are something different: 282 00:18:02,215 --> 00:18:03,349 females. 283 00:18:05,385 --> 00:18:07,319 Biological sex is incredibly complex. 284 00:18:07,320 --> 00:18:09,955 We have male, female, and intersex organisms. 285 00:18:09,956 --> 00:18:12,958 But generally, when we talk about organisms that make eggs, 286 00:18:12,959 --> 00:18:14,526 we usually refer to them as female, 287 00:18:14,527 --> 00:18:15,894 and when an organism makes sperm, 288 00:18:15,895 --> 00:18:17,430 we generally refer to it as male. 289 00:18:18,531 --> 00:18:21,533 We know that hermaphrodites occasionally mutate 290 00:18:21,534 --> 00:18:25,271 and produce just one type of sexual function, say females. 291 00:18:27,006 --> 00:18:29,741 The evolution of males and females is still a mystery, 292 00:18:29,742 --> 00:18:31,276 and the science is still evolving, 293 00:18:31,277 --> 00:18:32,277 but there are plausible theories 294 00:18:32,278 --> 00:18:34,646 for how this might have occurred. 295 00:18:34,647 --> 00:18:36,482 It's very energetically expensive 296 00:18:36,483 --> 00:18:38,384 to make both sperm and eggs. 297 00:18:39,853 --> 00:18:42,621 {\an8}So, by specializing on sperm and eggs, 298 00:18:42,622 --> 00:18:45,390 {\an8}oftentimes, those species have come to be better at it, 299 00:18:45,391 --> 00:18:48,160 better at making millions to billions of sperm 300 00:18:48,161 --> 00:18:50,929 that are highly able to swim and move. 301 00:18:50,930 --> 00:18:53,565 By specializing on being an egg producer, 302 00:18:53,566 --> 00:18:56,468 eggs can be huge-- so huge, for example, 303 00:18:56,469 --> 00:18:59,171 that an ostrich egg, when it first forms, 304 00:18:59,172 --> 00:19:00,439 it's a single cell. 305 00:19:00,440 --> 00:19:02,308 That's very specialized. 306 00:19:04,010 --> 00:19:05,244 For modern animals, 307 00:19:05,245 --> 00:19:08,981 the balance lies in favor of specialization, 308 00:19:08,982 --> 00:19:12,818 meaning that around 95% of animal species 309 00:19:12,819 --> 00:19:17,122 have distinct males and females. 310 00:19:17,123 --> 00:19:19,358 But elsewhere on the tree of life, 311 00:19:19,359 --> 00:19:22,261 sexual reproduction can look radically different. 312 00:19:22,262 --> 00:19:24,530 If you look in the plant world, 313 00:19:24,531 --> 00:19:26,031 {\an8}it's the other way around. 314 00:19:26,032 --> 00:19:30,202 {\an8}About 95% of plants have both in the same individual, 315 00:19:30,203 --> 00:19:32,671 {\an8}like this hibiscus flower, where, if you open it up, 316 00:19:32,672 --> 00:19:36,341 {\an8}you can see both the parts that'll produce pollen 317 00:19:36,342 --> 00:19:39,211 and that will receive pollen, 318 00:19:39,212 --> 00:19:41,280 and so they can fertilize themselves 319 00:19:41,281 --> 00:19:43,682 if they have failed to find a mate. 320 00:19:43,683 --> 00:19:44,783 And that's really helpful. 321 00:19:44,784 --> 00:19:46,752 If they can't move around and find a mate, 322 00:19:46,753 --> 00:19:48,654 then it's better to do both. 323 00:19:51,190 --> 00:19:53,058 Even among animals, 324 00:19:53,059 --> 00:19:55,328 nature remains flexible. 325 00:19:57,397 --> 00:20:00,966 Some dads guard the eggs 326 00:20:00,967 --> 00:20:03,036 and raise the young. 327 00:20:05,538 --> 00:20:08,241 {\an8}Some fish can change sex. 328 00:20:10,410 --> 00:20:14,080 And occasionally, males even get pregnant. 329 00:20:15,315 --> 00:20:17,950 Seahorses are a wonderful example 330 00:20:17,951 --> 00:20:21,553 in which the female actually puts the eggs into the male, 331 00:20:21,554 --> 00:20:24,189 who then carries them and has those babies. 332 00:20:24,190 --> 00:20:26,858 So even though we have these sex categories, 333 00:20:26,859 --> 00:20:30,195 what those mean for the animals can be wildly different. 334 00:20:30,196 --> 00:20:31,964 Sexual reproduction, 335 00:20:31,965 --> 00:20:33,265 like the world around us, 336 00:20:33,266 --> 00:20:34,934 is incredibly diverse. 337 00:20:36,569 --> 00:20:38,604 And species accomplish that process 338 00:20:38,605 --> 00:20:40,339 of sexual reproduction 339 00:20:40,340 --> 00:20:43,743 in so many different ways, it's mind-boggling. 340 00:20:44,811 --> 00:20:48,881 Evolution can produce all sorts of quirky outcomes. 341 00:20:49,983 --> 00:20:51,617 But for most animals, 342 00:20:51,618 --> 00:20:55,788 the specialization of the sexes proves a winning formula... 343 00:20:57,557 --> 00:21:01,294 ...setting the egg on its path to greatness. 344 00:21:07,700 --> 00:21:10,169 As the generations pass... 345 00:21:12,605 --> 00:21:16,108 {\an8}...animals continue to evolve, 346 00:21:16,109 --> 00:21:19,845 {\an8}adopting new behaviors, 347 00:21:19,846 --> 00:21:24,116 adapting to new habitats, 348 00:21:24,117 --> 00:21:28,988 spreading out across the oceans. 349 00:21:39,399 --> 00:21:44,369 {\an8}Until, around 150 million years later, 350 00:21:44,370 --> 00:21:48,774 {\an8}some have even ventured into a new frontier: 351 00:21:48,775 --> 00:21:52,378 {\an8}the freshwater streams of the Devonian. 352 00:21:54,313 --> 00:21:57,282 {\an8}If a time machine dropped you into the Devonian period, 353 00:21:57,283 --> 00:21:59,651 {\an8}it would be a very different world. 354 00:21:59,652 --> 00:22:00,919 There'd be no mammals, 355 00:22:00,920 --> 00:22:03,255 no birds, no flowering plants. 356 00:22:05,558 --> 00:22:08,294 But there are plenty of fish. 357 00:22:10,630 --> 00:22:13,031 {\an8}Like this ancestor of the ostrich, 358 00:22:13,032 --> 00:22:15,735 {\an8}searching for a safe place to lay her eggs. 359 00:22:19,372 --> 00:22:23,376 With a streamlined body, she battles against the current. 360 00:22:25,745 --> 00:22:28,214 But her group is being followed. 361 00:22:29,348 --> 00:22:32,050 It was a fish-eat-fish world, 362 00:22:32,051 --> 00:22:33,051 and there were small fish 363 00:22:33,052 --> 00:22:34,087 all the way up to huge fish. 364 00:22:35,822 --> 00:22:38,023 And those big fish at the top of the food chain, 365 00:22:38,024 --> 00:22:42,194 some had what were basically, were scissor blades for jaws. 366 00:22:42,195 --> 00:22:44,930 Going swimming in the Devonian would have been 367 00:22:44,931 --> 00:22:46,933 putting your life at risk immediately. 368 00:22:48,568 --> 00:22:51,103 It would have been a constant battle, 369 00:22:51,104 --> 00:22:53,772 not only for your own survival, but especially for your eggs. 370 00:22:57,143 --> 00:22:59,612 {\an8}Eggs are packed full of nutrients. 371 00:23:03,483 --> 00:23:06,385 {\an8}A tempting prospect for hungry mouths. 372 00:23:10,723 --> 00:23:14,326 So, if the fish don't find a safer place to spawn, 373 00:23:14,327 --> 00:23:16,996 all their eggs could be eaten. 374 00:23:19,966 --> 00:23:22,902 But these fish have something that others lack. 375 00:23:28,307 --> 00:23:32,377 Strong fleshy fins, known as lobed fins, 376 00:23:32,378 --> 00:23:35,481 that might help them move along the riverbed. 377 00:23:38,251 --> 00:23:43,088 And allow some of them to do something other fish can't do. 378 00:23:53,566 --> 00:23:57,770 Leave the river and crawl on land. 379 00:24:01,774 --> 00:24:03,842 If you're a fish 380 00:24:03,843 --> 00:24:05,377 and you've lived your entire evolutionary history 381 00:24:05,378 --> 00:24:06,645 in the water, 382 00:24:06,646 --> 00:24:08,513 why come up on land? 383 00:24:08,514 --> 00:24:10,649 Well, there's a couple different possibilities. 384 00:24:10,650 --> 00:24:14,619 It might have been because they were chasing new prey. 385 00:24:14,620 --> 00:24:17,022 There are these bugs that are crawling around on land 386 00:24:17,023 --> 00:24:18,757 that nothing else is really eating. 387 00:24:18,758 --> 00:24:20,826 But it could have been 388 00:24:20,827 --> 00:24:22,794 because they were looking for new areas 389 00:24:22,795 --> 00:24:24,230 in order to reproduce. 390 00:24:26,299 --> 00:24:28,066 Beyond the main river 391 00:24:28,067 --> 00:24:31,469 lies an isolated network of ponds and streams, 392 00:24:31,470 --> 00:24:33,973 where aquatic predators can't follow. 393 00:24:35,174 --> 00:24:38,343 Perfect for allowing fish to spawn in safety. 394 00:24:38,344 --> 00:24:41,913 If they can go from pond to pond, 395 00:24:41,914 --> 00:24:43,081 they can protect their eggs, 396 00:24:43,082 --> 00:24:45,483 and therefore, the next generation 397 00:24:45,484 --> 00:24:47,419 are more likely to survive, 398 00:24:47,420 --> 00:24:49,120 and very importantly, 399 00:24:49,121 --> 00:24:50,756 more likely to reproduce. 400 00:24:53,259 --> 00:24:56,828 Meaning that the need to find safe places to spawn 401 00:24:56,829 --> 00:24:58,931 has profound consequences. 402 00:25:00,233 --> 00:25:03,001 Because fish with the strongest fins 403 00:25:03,002 --> 00:25:06,239 are more likely to reach the predator-free pools. 404 00:25:08,841 --> 00:25:11,377 {\an8}They have more surviving offspring... 405 00:25:13,045 --> 00:25:15,448 {\an8}...which inherit their strong fins. 406 00:25:19,252 --> 00:25:22,855 {\an8}Over countless generations, bones get longer. 407 00:25:24,457 --> 00:25:26,425 {\an8}Digits and joints emerge. 408 00:25:27,760 --> 00:25:29,695 {\an8}Better for walking on land. 409 00:25:33,466 --> 00:25:37,270 {\an8}Until eventually, after millions of years... 410 00:25:39,805 --> 00:25:43,041 {\an8}...fins have become limbs. 411 00:25:46,279 --> 00:25:48,981 {\an8}And they are fish no more. 412 00:25:52,685 --> 00:25:55,921 {\an8}Paleontology is like a treasure hunt. 413 00:25:55,922 --> 00:25:57,422 {\an8}It's very exciting. 414 00:25:57,423 --> 00:26:00,625 When you find a fossil of this quality, 415 00:26:00,626 --> 00:26:03,028 it's the find of a lifetime. 416 00:26:03,029 --> 00:26:05,096 This is a cast of two individuals 417 00:26:05,097 --> 00:26:06,799 of an animal called Seymouria. 418 00:26:09,068 --> 00:26:10,402 The anatomy of Seymouria 419 00:26:10,403 --> 00:26:13,606 represents the early amphibious stage very well. 420 00:26:14,640 --> 00:26:17,376 And some of its features are remarkably familiar. 421 00:26:18,411 --> 00:26:20,645 {\an8}Here you can see a very well-developed hand. 422 00:26:20,646 --> 00:26:23,248 And you can see, right next to mine, 423 00:26:23,249 --> 00:26:27,118 five well-developed digits, just like you and me. 424 00:26:27,119 --> 00:26:30,322 Our pattern of five digits 425 00:26:30,323 --> 00:26:33,225 goes back over 300 million years. 426 00:26:34,427 --> 00:26:36,728 This is basically a formula that's very widely shared. 427 00:26:36,729 --> 00:26:38,129 And the reason it is is not 428 00:26:38,130 --> 00:26:39,931 because it's evolved over and over again, 429 00:26:39,932 --> 00:26:42,268 but because it goes back to a common ancestor. 430 00:26:43,636 --> 00:26:45,437 From lizards 431 00:26:45,438 --> 00:26:47,405 to elephants 432 00:26:47,406 --> 00:26:50,709 to ostriches to us, 433 00:26:50,710 --> 00:26:53,812 all land vertebrates trace their heritage 434 00:26:53,813 --> 00:26:57,882 back to those first four-legged pioneers. 435 00:26:57,883 --> 00:27:01,619 {\an8}We have all of this shared evolutionary history. 436 00:27:01,620 --> 00:27:03,488 So we look very different on the outside, 437 00:27:03,489 --> 00:27:06,758 but if you figure out on the inside what makes us tick, 438 00:27:06,759 --> 00:27:09,027 we're pretty similar. 439 00:27:09,028 --> 00:27:12,397 And so to me, what's most magical about evolution 440 00:27:12,398 --> 00:27:16,101 is that we can have this incredible difference, 441 00:27:16,102 --> 00:27:18,603 but we are all connected, 442 00:27:18,604 --> 00:27:20,171 and you can't understand evolution 443 00:27:20,172 --> 00:27:24,142 without recognizing those two parts of the story. 444 00:27:29,515 --> 00:27:31,951 For tens of millions of years... 445 00:27:34,120 --> 00:27:38,557 ...amphibians reign as the planet's dominant land animals. 446 00:27:44,196 --> 00:27:46,465 {\an8}But they have a weakness. 447 00:27:53,172 --> 00:27:56,374 This newt-like creature has four strong legs, 448 00:27:56,375 --> 00:27:59,178 inherited from her river-dwelling ancestors. 449 00:28:00,746 --> 00:28:04,016 She can move easily in water and on land. 450 00:28:06,318 --> 00:28:10,388 But like many animals here, she's in trouble. 451 00:28:10,389 --> 00:28:14,159 Drier periods have caused the pool she calls home 452 00:28:14,160 --> 00:28:17,229 to become little more than a muddy puddle. 453 00:28:19,065 --> 00:28:21,666 To understand why this is such a problem, 454 00:28:21,667 --> 00:28:24,569 we only have to look at amphibians today. 455 00:28:27,573 --> 00:28:28,973 What we know of amphibian skin 456 00:28:28,974 --> 00:28:30,875 we know mostly from the amphibians 457 00:28:30,876 --> 00:28:32,210 that are alive today, 458 00:28:32,211 --> 00:28:35,180 and living amphibians like frogs and salamanders 459 00:28:35,181 --> 00:28:38,583 must keep their skin moist to survive. 460 00:28:38,584 --> 00:28:41,720 They can't dry out, otherwise it's deadly for them. 461 00:28:42,822 --> 00:28:44,189 And it's not just their skin. 462 00:28:44,190 --> 00:28:48,126 Their eggs need water, too. 463 00:28:48,127 --> 00:28:50,395 The first vertebrates that moved onto land 464 00:28:50,396 --> 00:28:51,796 were almost certainly still 465 00:28:51,797 --> 00:28:53,298 laying their eggs in the water. 466 00:28:53,299 --> 00:28:55,600 Just like living amphibians, 467 00:28:55,601 --> 00:28:57,402 their eggs were somewhat gelatinous, 468 00:28:57,403 --> 00:28:58,403 and therefore 469 00:28:58,404 --> 00:29:00,505 very prone to drying out 470 00:29:00,506 --> 00:29:03,008 if they were to be laid on dry land. 471 00:29:05,177 --> 00:29:08,347 So, amphibians are still very much tied to the water. 472 00:29:11,650 --> 00:29:14,553 In dry conditions, many amphibians struggle. 473 00:29:15,955 --> 00:29:18,891 But this one has managed to hang on. 474 00:29:22,728 --> 00:29:24,696 The survival of this individual's offspring 475 00:29:24,697 --> 00:29:27,433 doesn't hinge on luck alone. 476 00:29:29,702 --> 00:29:32,804 Laying eggs in the right spot helps, 477 00:29:32,805 --> 00:29:35,975 as does the unique mix of genes she has inherited. 478 00:29:38,677 --> 00:29:40,512 When the environment changes, 479 00:29:40,513 --> 00:29:43,415 that's when the diversity of a population becomes important. 480 00:29:45,384 --> 00:29:48,620 Because sexual reproduction leads to each individual 481 00:29:48,621 --> 00:29:50,622 being genetically unique, 482 00:29:50,623 --> 00:29:53,826 it makes a species as a whole more resilient. 483 00:29:54,994 --> 00:29:57,028 {\an8}Some people use the phrase 484 00:29:57,029 --> 00:29:58,329 {\an8}"survival of the fittest" 485 00:29:58,330 --> 00:30:00,365 {\an8}to describe evolution. 486 00:30:00,366 --> 00:30:02,433 It doesn't necessarily mean the strongest 487 00:30:02,434 --> 00:30:05,137 or the smartest or the most aggressive. 488 00:30:07,606 --> 00:30:10,775 It's simply the ones that are best suited 489 00:30:10,776 --> 00:30:13,144 to the environment in which they lived, 490 00:30:13,145 --> 00:30:16,014 and so they're likely to have more offspring, 491 00:30:16,015 --> 00:30:18,716 and therefore there's more like them 492 00:30:18,717 --> 00:30:20,285 in future generations. 493 00:30:23,489 --> 00:30:26,391 {\an8}This individual's unique mix of genes 494 00:30:26,392 --> 00:30:29,861 {\an8}determines how likely her eggs are to survive 495 00:30:29,862 --> 00:30:31,530 {\an8}in a drying world. 496 00:30:32,898 --> 00:30:36,735 {\an8}And hers are a tiny bit better at retaining moisture. 497 00:30:38,437 --> 00:30:41,674 A small difference that can mean everything. 498 00:30:43,509 --> 00:30:44,742 The slightest variation 499 00:30:44,743 --> 00:30:46,811 in the structure or composition of the egg 500 00:30:46,812 --> 00:30:50,715 is all it takes for her eggs to survive 501 00:30:50,716 --> 00:30:52,584 when others' don't. 502 00:30:56,855 --> 00:30:59,958 {\an8}Over countless generations, 503 00:30:59,959 --> 00:31:02,795 {\an8}the surface of the egg begins to change. 504 00:31:05,264 --> 00:31:08,633 {\an8}It gets tougher, thicker, 505 00:31:08,634 --> 00:31:12,238 {\an8}and special membranes form to prevent moisture loss. 506 00:31:16,008 --> 00:31:19,311 {\an8}The egg gets larger and stronger... 507 00:31:22,214 --> 00:31:26,117 {\an8}...until, after millions of years, 508 00:31:26,118 --> 00:31:29,488 something extraordinary has emerged. 509 00:31:34,994 --> 00:31:37,228 Something that paleobiologists 510 00:31:37,229 --> 00:31:39,931 like Anusuya Chinsamy-Turan 511 00:31:39,932 --> 00:31:44,236 begin to see in the ancient rocks of South Africa. 512 00:31:45,838 --> 00:31:49,107 CHINSAMY-TURAN: In the Karoo basin, we have an almost 513 00:31:49,108 --> 00:31:53,911 128-million-year sequence of rocks. 514 00:31:53,912 --> 00:31:58,750 So we can study how organisms evolved through that time. 515 00:32:01,220 --> 00:32:03,488 Discovered among these rocks 516 00:32:03,489 --> 00:32:06,124 were some of the oldest recorded fossils 517 00:32:06,125 --> 00:32:10,128 of what are known as amniotic eggs, 518 00:32:10,129 --> 00:32:14,799 an evolutionary innovation that persists to this day. 519 00:32:14,800 --> 00:32:19,170 CHINSAMY-TURAN: So this, I'm sure you will recognize, is a chicken egg. 520 00:32:19,171 --> 00:32:21,105 It has this very hard shell. 521 00:32:21,106 --> 00:32:22,974 You can't see it with your naked eye, 522 00:32:22,975 --> 00:32:27,345 but there are almost 20,000 tiny little pores 523 00:32:27,346 --> 00:32:28,746 in this eggshell. 524 00:32:28,747 --> 00:32:32,917 And these allow that little developing embryo inside 525 00:32:32,918 --> 00:32:34,485 to breathe. 526 00:32:34,486 --> 00:32:38,190 But now, if we crack open the same egg... 527 00:32:39,825 --> 00:32:43,829 ...inside, they are just absolutely marvelous structures. 528 00:32:45,230 --> 00:32:48,933 We can see, of course, the yolk, 529 00:32:48,934 --> 00:32:53,037 and this is the most nutritious part of the egg. 530 00:32:53,038 --> 00:32:56,374 It provides food for the developing embryo. 531 00:32:56,375 --> 00:33:00,945 And the white of the egg acts as a shock absorber 532 00:33:00,946 --> 00:33:04,849 and prevents that embryo from getting injured. 533 00:33:04,850 --> 00:33:06,918 If the egg was fertilized, 534 00:33:06,919 --> 00:33:09,587 immediately surrounding the embryo 535 00:33:09,588 --> 00:33:12,056 would be what we call the amniotic fluid, 536 00:33:12,057 --> 00:33:15,026 and this would keep the embryo developing 537 00:33:15,027 --> 00:33:17,462 in its own private little pond. 538 00:33:17,463 --> 00:33:22,133 So, we have a really wonderful innovation. 539 00:33:22,134 --> 00:33:26,237 Kept moist and protected until they hatch, 540 00:33:26,238 --> 00:33:29,408 {\an8}the young emerge as miniature adults. 541 00:33:31,377 --> 00:33:32,744 CHINSAMY-TURAN: This is the egg 542 00:33:32,745 --> 00:33:36,180 that allowed these descendants of amphibians 543 00:33:36,181 --> 00:33:39,283 to venture further and further away from water, 544 00:33:39,284 --> 00:33:40,918 because, unlike amphibians, 545 00:33:40,919 --> 00:33:43,321 they didn't have to return to the water to reproduce. 546 00:33:45,991 --> 00:33:47,392 The ancestors of the ostrich 547 00:33:47,393 --> 00:33:49,361 were amphibians no more. 548 00:33:50,562 --> 00:33:53,931 They'd evolved into something new: 549 00:33:53,932 --> 00:33:56,200 amniotes. 550 00:34:00,105 --> 00:34:03,641 The amniotes are one of the most important groups of animals 551 00:34:03,642 --> 00:34:05,009 that have ever existed, 552 00:34:05,010 --> 00:34:07,779 and they represent one of the great branching points 553 00:34:07,780 --> 00:34:08,814 in the tree of life. 554 00:34:10,549 --> 00:34:13,251 One branch of amniotes gave rise 555 00:34:13,252 --> 00:34:15,888 to reptiles, dinosaurs, and birds. 556 00:34:21,293 --> 00:34:24,695 And the other branch went on to become mammals, 557 00:34:24,696 --> 00:34:27,232 most of whom now incubate their eggs internally. 558 00:34:29,802 --> 00:34:31,335 Even humans, 559 00:34:31,336 --> 00:34:33,337 we have all the trappings of an egg, 560 00:34:33,338 --> 00:34:38,076 but we've just modified that to have this amazing gestation. 561 00:34:40,512 --> 00:34:45,951 Today, there are over 25,000 living species of amniotes. 562 00:34:47,352 --> 00:34:49,887 {\an8}And all owe their existence 563 00:34:49,888 --> 00:34:53,291 {\an8}to the evolution of the amniotic egg. 564 00:34:54,693 --> 00:34:55,893 But for the amniotes 565 00:34:55,894 --> 00:34:58,663 that would evolve into reptiles, 566 00:34:58,664 --> 00:35:01,299 one other anatomical breakthrough 567 00:35:01,300 --> 00:35:03,301 was key to their success. 568 00:35:03,302 --> 00:35:05,269 One of the biggest differences, 569 00:35:05,270 --> 00:35:06,604 if you look at a reptile 570 00:35:06,605 --> 00:35:08,172 versus an amphibian today, 571 00:35:08,173 --> 00:35:09,474 is the skin. 572 00:35:09,475 --> 00:35:12,210 Amphibian skin, it needs water, 573 00:35:12,211 --> 00:35:13,744 it, it feels moist. 574 00:35:13,745 --> 00:35:17,381 But a reptile's skin, it feels dry, it's scally, 575 00:35:17,382 --> 00:35:19,918 it actually helps keep water in. 576 00:35:21,487 --> 00:35:23,287 Armed with scaly skin, 577 00:35:23,288 --> 00:35:27,693 reptiles began exploring a world beyond the water. 578 00:35:29,328 --> 00:35:31,062 But the amniotic egg 579 00:35:31,063 --> 00:35:33,598 that would be pivotal to their success 580 00:35:33,599 --> 00:35:35,867 did pose a problem. 581 00:35:35,868 --> 00:35:38,336 A shelled egg is a great thing to do 582 00:35:38,337 --> 00:35:39,437 to lay on land, 583 00:35:39,438 --> 00:35:40,838 but there's a trick. 584 00:35:40,839 --> 00:35:43,407 A shelled egg can't be fertilized 585 00:35:43,408 --> 00:35:46,277 after it's been laid outside the mother. 586 00:35:46,278 --> 00:35:48,112 So in order to do that, 587 00:35:48,113 --> 00:35:51,082 the first fully terrestrial vertebrates 588 00:35:51,083 --> 00:35:54,385 needed to have sex in a brand-new way. 589 00:35:58,824 --> 00:36:00,858 {\an8}Part of the solution is found 590 00:36:00,859 --> 00:36:04,162 {\an8}on almost all reptile and mammal species today. 591 00:36:07,432 --> 00:36:09,868 It begins to grow early in development. 592 00:36:11,970 --> 00:36:15,874 A tiny outgrowth, like an extra limb. 593 00:36:17,409 --> 00:36:20,678 It's a penis. 594 00:36:20,679 --> 00:36:23,981 The origin of the penis was a major moment in evolution 595 00:36:23,982 --> 00:36:26,118 {\an8}that allowed internal fertilization. 596 00:36:27,653 --> 00:36:29,320 {\an8}The egg and sperm could come together 597 00:36:29,321 --> 00:36:32,356 before the fertilized egg got a shell. 598 00:36:32,357 --> 00:36:34,759 This form of reproduction 599 00:36:34,760 --> 00:36:38,095 that is specialized for terrestrial vertebrates, 600 00:36:38,096 --> 00:36:40,331 that's what we call copulation. 601 00:36:40,332 --> 00:36:42,400 Some of the early evidence 602 00:36:42,401 --> 00:36:43,401 of internal fertilization 603 00:36:43,402 --> 00:36:44,402 that we have 604 00:36:44,403 --> 00:36:45,469 happened amongst armored fish 605 00:36:45,470 --> 00:36:46,804 many millions of years earlier. 606 00:36:46,805 --> 00:36:48,773 But it was really important 607 00:36:48,774 --> 00:36:50,576 for life thriving on land. 608 00:36:57,249 --> 00:37:01,319 Today, penises come in all shapes and sizes 609 00:37:01,320 --> 00:37:02,887 across the animal kingdom. 610 00:37:10,529 --> 00:37:12,897 The blue whale has the largest, 611 00:37:12,898 --> 00:37:15,901 almost ten feet long. 612 00:37:18,303 --> 00:37:20,771 The gorilla has one of the smallest, 613 00:37:20,772 --> 00:37:23,174 compared to his body size. 614 00:37:25,711 --> 00:37:29,047 Duck penises are shaped like corkscrews. 615 00:37:30,616 --> 00:37:33,952 And those of cats are covered in barbs. 616 00:37:35,587 --> 00:37:37,855 But for vertebrates that evolved a penis, 617 00:37:37,856 --> 00:37:39,992 it was only half the story. 618 00:37:41,360 --> 00:37:42,560 A penis by itself 619 00:37:42,561 --> 00:37:44,562 doesn't do anyone any good. 620 00:37:44,563 --> 00:37:46,197 It takes two to tango. 621 00:37:46,198 --> 00:37:49,467 And so the female reproductive anatomy 622 00:37:49,468 --> 00:37:52,903 had to co-evolve with the evolution of the penis. 623 00:37:52,904 --> 00:37:56,140 And that way, the two of them together 624 00:37:56,141 --> 00:37:58,209 could make certain that those eggs 625 00:37:58,210 --> 00:37:59,477 could be fertilized 626 00:37:59,478 --> 00:38:02,279 and it didn't matter if you were near a pond 627 00:38:02,280 --> 00:38:05,049 or up in a tree or out in the desert. 628 00:38:06,618 --> 00:38:08,786 With their sophisticated eggs 629 00:38:08,787 --> 00:38:11,456 and a new way to fertilize them... 630 00:38:12,924 --> 00:38:15,594 ...reptiles spread out across the planet. 631 00:38:18,463 --> 00:38:21,832 {\an8}But the anatomical changes that allowed them to do so 632 00:38:21,833 --> 00:38:23,635 {\an8}had another consequence. 633 00:38:24,770 --> 00:38:26,037 It's obvious 634 00:38:26,038 --> 00:38:27,838 that the evolution of copulation 635 00:38:27,839 --> 00:38:29,473 changed the anatomy of the animals, 636 00:38:29,474 --> 00:38:30,875 the way they looked. 637 00:38:30,876 --> 00:38:32,576 But what's less obvious, 638 00:38:32,577 --> 00:38:34,178 but at least as important, 639 00:38:34,179 --> 00:38:36,615 is, it changed the way that they behaved. 640 00:38:39,851 --> 00:38:42,486 {\an8}And just 100 million years later... 641 00:38:44,022 --> 00:38:46,724 {\an8}...we can see this in action. 642 00:38:51,396 --> 00:38:55,299 This is Coelophysis, an early dinosaur. 643 00:39:03,308 --> 00:39:05,476 Like his amphibious ancestors, 644 00:39:05,477 --> 00:39:09,013 he has strong legs for walking on land, 645 00:39:09,014 --> 00:39:12,683 but he walks on two instead of four. 646 00:39:15,587 --> 00:39:18,489 And it's thought his scaly reptilian skin 647 00:39:18,490 --> 00:39:21,192 might have been covered in a layer of fuzz. 648 00:39:26,531 --> 00:39:28,567 Right now, he's on the hunt. 649 00:39:32,003 --> 00:39:33,338 Not for a meal... 650 00:39:36,742 --> 00:39:38,509 ...but for a mate. 651 00:39:40,812 --> 00:39:44,248 There are plenty of females around, 652 00:39:44,249 --> 00:39:46,650 but the tricky part is getting their attention. 653 00:39:54,092 --> 00:39:56,494 As eggs evolved to be larger, 654 00:39:56,495 --> 00:39:59,163 each one became more costly to produce, 655 00:39:59,164 --> 00:40:01,866 so females laid fewer of them, 656 00:40:01,867 --> 00:40:04,802 making each choice of a mate vital. 657 00:40:09,074 --> 00:40:11,142 So, how does this male make himself 658 00:40:11,143 --> 00:40:13,544 stand out from the crowd? 659 00:40:18,617 --> 00:40:21,886 Scientists suspect that some dinosaurs did 660 00:40:21,887 --> 00:40:25,823 what many animals still do today: 661 00:40:25,824 --> 00:40:27,725 show off. 662 00:40:31,897 --> 00:40:34,466 It might have started with head bobbing. 663 00:40:36,568 --> 00:40:38,470 Followed by twirls. 664 00:40:42,073 --> 00:40:44,141 But to really impress her, 665 00:40:44,142 --> 00:40:46,845 he has one more trick up his sleeve. 666 00:40:52,651 --> 00:40:55,821 Will this be enough to make her choose him? 667 00:40:58,356 --> 00:40:59,757 In the game of life, 668 00:40:59,758 --> 00:41:02,294 survival is just the first test. 669 00:41:04,129 --> 00:41:06,564 If you make it to adulthood, 670 00:41:06,565 --> 00:41:09,000 that's when you can be a contender in the mating game. 671 00:41:10,402 --> 00:41:13,037 If you don't get chosen, your genes represent 672 00:41:13,038 --> 00:41:15,739 an evolutionary dead end-- they go nowhere. 673 00:41:18,677 --> 00:41:21,111 And the need to get chosen 674 00:41:21,112 --> 00:41:23,714 has led to some utterly bizarre features 675 00:41:23,715 --> 00:41:25,150 in modern animals. 676 00:41:26,751 --> 00:41:30,054 From chameleons to crabs, we see exaggerated features 677 00:41:30,055 --> 00:41:32,823 throughout the animal kingdom. 678 00:41:32,824 --> 00:41:35,426 {\an8}Think about those weird noses of the proboscis monkeys, 679 00:41:35,427 --> 00:41:38,028 the horns of rhinos, 680 00:41:38,029 --> 00:41:40,665 the enormous antlers of moose. 681 00:41:41,833 --> 00:41:43,801 The fiddler crab that has a giant claw 682 00:41:43,802 --> 00:41:46,404 that can be half of its body weight. 683 00:41:48,006 --> 00:41:49,541 They're bizarre and beautiful. 684 00:41:50,575 --> 00:41:52,710 You could go on and on about these structures 685 00:41:52,711 --> 00:41:54,979 that just seem so inconceivable, 686 00:41:54,980 --> 00:41:57,515 but they are essential for reproduction. 687 00:42:04,256 --> 00:42:06,423 Fuzzy arm fringes like this 688 00:42:06,424 --> 00:42:09,261 might have been just as essential in the mating game. 689 00:42:10,462 --> 00:42:13,097 If females find them attractive, 690 00:42:13,098 --> 00:42:16,700 this Coelophysis is more likely to be chosen, 691 00:42:16,701 --> 00:42:19,004 meaning his genes get passed down. 692 00:42:20,639 --> 00:42:24,141 And some of his offspring go on to have fuzzy offspring 693 00:42:24,142 --> 00:42:25,443 {\an8}of their own. 694 00:42:28,013 --> 00:42:30,381 {\an8}Over countless generations, 695 00:42:30,382 --> 00:42:34,551 {\an8}those fuzzy strands become exaggerated, 696 00:42:34,552 --> 00:42:37,489 {\an8}getting longer and wider. 697 00:42:39,958 --> 00:42:42,693 {\an8}The strands split into multiple filaments... 698 00:42:47,966 --> 00:42:51,670 {\an8}...making an even bigger surface area for display. 699 00:42:53,872 --> 00:42:58,677 {\an8}New colors and patterns emerge, attracting even more attention. 700 00:43:01,046 --> 00:43:05,316 {\an8}Until eventually, after tens of millions of years, 701 00:43:05,317 --> 00:43:09,787 {\an8}those fuzzy strands are so transformed, 702 00:43:09,788 --> 00:43:11,723 {\an8}they've become something new. 703 00:43:15,894 --> 00:43:19,064 This dinosaur now has more than just fuzz. 704 00:43:20,765 --> 00:43:24,802 He is adorned in elaborate, seductive feathers. 705 00:43:27,372 --> 00:43:31,408 {\an8}And once these new tools for flirting arrived, 706 00:43:31,409 --> 00:43:33,610 {\an8}they were here to stay. 707 00:43:41,019 --> 00:43:42,853 If you think about birds, 708 00:43:42,854 --> 00:43:46,623 the single most distinctive attribute 709 00:43:46,624 --> 00:43:47,758 that sets them apart 710 00:43:47,759 --> 00:43:49,994 from anything else alive today 711 00:43:49,995 --> 00:43:52,263 is the presence of feathers. 712 00:43:55,066 --> 00:43:58,435 {\an8}Home to over 850 bird species, 713 00:43:58,436 --> 00:44:01,372 {\an8}South Africa is a paradise 714 00:44:01,373 --> 00:44:05,476 {\an8}for paleontologist and bird enthusiast Daniel Field. 715 00:44:07,045 --> 00:44:08,579 I love birds. 716 00:44:08,580 --> 00:44:10,447 It's hard to explain why. 717 00:44:10,448 --> 00:44:13,351 I fell in love with them when I was about nine years old. 718 00:44:14,619 --> 00:44:17,421 I've got a feather here, which comes from a species 719 00:44:17,422 --> 00:44:19,591 called the helmeted guinea fowl. 720 00:44:20,859 --> 00:44:23,594 {\an8}Which is a common bird in this sort of open habitat 721 00:44:23,595 --> 00:44:25,262 {\an8}in Southern Africa. 722 00:44:25,263 --> 00:44:26,663 {\an8}And these beautiful feathers 723 00:44:26,664 --> 00:44:29,600 {\an8}serve a really important function 724 00:44:29,601 --> 00:44:31,101 {\an8}in display. 725 00:44:37,008 --> 00:44:40,978 This comes from the tail of a common pheasant. 726 00:44:40,979 --> 00:44:43,313 In the U.K., where I live, 727 00:44:43,314 --> 00:44:46,050 these birds are extremely abundant. 728 00:44:46,051 --> 00:44:48,419 It's greatly elongated 729 00:44:48,420 --> 00:44:52,856 and it's got these beautiful, complex patterns on it, also, 730 00:44:52,857 --> 00:44:56,627 and the primary function for a feather like this is, 731 00:44:56,628 --> 00:44:58,662 well, sexiness. 732 00:45:01,966 --> 00:45:03,901 {\an8}And scientists have strong evidence 733 00:45:03,902 --> 00:45:06,637 {\an8}that birds' use of feathers to attract females 734 00:45:06,638 --> 00:45:08,506 {\an8}has a long history. 735 00:45:10,275 --> 00:45:12,543 {\an8}This is an image of a fossil 736 00:45:12,544 --> 00:45:16,080 {\an8}from about 160 million years ago from China. 737 00:45:16,081 --> 00:45:17,948 {\an8}It's called Epidexipteryx, 738 00:45:17,949 --> 00:45:20,017 {\an8}and it's a fascinating feathered dinosaur. 739 00:45:20,018 --> 00:45:24,455 {\an8}We can see these bizarre, elongate feathers 740 00:45:24,456 --> 00:45:26,657 {\an8}coming off from its tail area. 741 00:45:26,658 --> 00:45:30,761 {\an8}The only conceivable function of these particular feathers 742 00:45:30,762 --> 00:45:32,629 {\an8}is that they would have been useful 743 00:45:32,630 --> 00:45:34,465 in courtship or display. 744 00:45:34,466 --> 00:45:36,233 And that's quite fascinating, 745 00:45:36,234 --> 00:45:40,270 because that is an attribute of living birds 746 00:45:40,271 --> 00:45:43,107 that, thanks to Epidexipteryx, we can say 747 00:45:43,108 --> 00:45:46,844 may extend all the way back 160 million years. 748 00:45:49,914 --> 00:45:52,583 Early barbed feathers had a central shaft... 749 00:45:55,353 --> 00:45:58,056 ...with equal-length barbs on either side. 750 00:45:59,124 --> 00:46:02,626 But around 150 million years ago, 751 00:46:02,627 --> 00:46:05,896 a new type of feather appeared in the fossil record: 752 00:46:05,897 --> 00:46:08,432 one with a subtle change 753 00:46:08,433 --> 00:46:12,136 that would alter the destiny of the dinosaurs. 754 00:46:12,137 --> 00:46:14,771 We see the appearance 755 00:46:14,772 --> 00:46:16,740 of an asymmetrical feather. 756 00:46:16,741 --> 00:46:20,444 So this feather comes from a goose, 757 00:46:20,445 --> 00:46:21,712 and you can clearly see 758 00:46:21,713 --> 00:46:24,281 that one edge of the feather is much narrower 759 00:46:24,282 --> 00:46:26,351 and it's also stiffer. 760 00:46:27,619 --> 00:46:29,153 This is probably one of those features 761 00:46:29,154 --> 00:46:31,622 that just arose by chance in the first place, 762 00:46:31,623 --> 00:46:35,259 but it had really important consequences, 763 00:46:35,260 --> 00:46:37,561 because this sort of asymmetrical shape 764 00:46:37,562 --> 00:46:39,463 shifts the center of pressure 765 00:46:39,464 --> 00:46:41,765 towards the leading edge of the feather, 766 00:46:41,766 --> 00:46:46,603 creating a much more stable, aerodynamic surface for flight. 767 00:46:50,642 --> 00:46:54,846 From fuzz to flirting to flight... 768 00:46:56,314 --> 00:46:58,849 ...over 50 million years of the mating game 769 00:46:58,850 --> 00:47:01,151 resulted in feathered dinosaurs 770 00:47:01,152 --> 00:47:02,987 filling the skies. 771 00:47:04,889 --> 00:47:07,291 For nearly 100 million years, 772 00:47:07,292 --> 00:47:10,394 they thrived alongside their ground-dwelling cousins. 773 00:47:12,697 --> 00:47:14,698 And they were there to witness 774 00:47:14,699 --> 00:47:16,968 one of our planet's darkest days. 775 00:47:27,478 --> 00:47:28,879 {\an8}After the asteroid impact, 776 00:47:28,880 --> 00:47:31,683 the world would have been a terrible place to be. 777 00:47:32,951 --> 00:47:34,552 Whole food chains collapsed. 778 00:47:36,254 --> 00:47:37,588 But seeds can stay in the soil 779 00:47:37,589 --> 00:47:40,290 even if an island is obliterated by a volcano, 780 00:47:40,291 --> 00:47:41,291 and the same would have been true 781 00:47:41,292 --> 00:47:42,292 with the asteroid, 782 00:47:42,293 --> 00:47:43,727 so if you could eat seeds, 783 00:47:43,728 --> 00:47:46,030 that might have been your ticket to surviving. 784 00:47:47,031 --> 00:47:48,966 Within just a few years, 785 00:47:48,967 --> 00:47:52,803 three-quarters of all plant and animal species on Earth 786 00:47:52,804 --> 00:47:54,638 died out. 787 00:47:54,639 --> 00:47:57,007 One of the greatest myths in all of science 788 00:47:57,008 --> 00:48:00,110 is this idea that the dinosaurs went extinct. 789 00:48:00,111 --> 00:48:02,579 But it's not quite accurate, 790 00:48:02,580 --> 00:48:05,616 because one type of dinosaur did make it through. 791 00:48:05,617 --> 00:48:06,750 And these were the dinosaurs 792 00:48:06,751 --> 00:48:09,186 with feathers and wings and beaks-- 793 00:48:09,187 --> 00:48:11,888 the dinosaurs that we call birds. 794 00:48:11,889 --> 00:48:14,491 Today's birds evolved from dinosaurs, 795 00:48:14,492 --> 00:48:16,693 they are part of the dinosaur family tree, 796 00:48:16,694 --> 00:48:19,796 and they were the only dinosaur survivors of that asteroid. 797 00:48:22,033 --> 00:48:23,735 But birds didn't just survive. 798 00:48:25,203 --> 00:48:26,604 They thrived. 799 00:48:28,373 --> 00:48:31,942 Today, there are almost twice as many species of birds 800 00:48:31,943 --> 00:48:34,412 as there are of mammals. 801 00:48:35,647 --> 00:48:37,748 Some are so skilled at flying 802 00:48:37,749 --> 00:48:40,785 that they can stay airborne for months at a time. 803 00:48:44,389 --> 00:48:46,256 While others have shrunk to become 804 00:48:46,257 --> 00:48:49,027 the smallest dinosaurs to ever live. 805 00:48:52,630 --> 00:48:54,965 But in the aftermath of the extinction, 806 00:48:54,966 --> 00:48:58,035 large ground predators were gone. 807 00:48:58,036 --> 00:49:00,971 And with flight being energetically expensive, 808 00:49:00,972 --> 00:49:04,107 some birds gave up on it altogether. 809 00:49:09,881 --> 00:49:12,516 This is Pseudocrypturus. 810 00:49:12,517 --> 00:49:14,385 It's about the size of a pheasant. 811 00:49:16,321 --> 00:49:19,357 Birds like it foraged on the forest floor. 812 00:49:21,025 --> 00:49:23,026 And as the generations passed, 813 00:49:23,027 --> 00:49:25,063 some of their descendants grew bigger. 814 00:49:26,264 --> 00:49:29,701 Their legs got longer and better for walking. 815 00:49:33,504 --> 00:49:36,373 Until, 20 million years ago, 816 00:49:36,374 --> 00:49:38,041 an iconic animal emerged. 817 00:49:42,947 --> 00:49:45,083 The largest bird alive today. 818 00:49:51,689 --> 00:49:53,023 When I see an ostrich, 819 00:49:53,024 --> 00:49:56,059 I see that the story of dinosaur evolution 820 00:49:56,060 --> 00:49:59,262 is not over, but is ongoing. 821 00:50:01,032 --> 00:50:03,300 Ostriches are just a fantastic symbol 822 00:50:03,301 --> 00:50:05,535 of evolutionary change and innovation. 823 00:50:05,536 --> 00:50:06,703 Here you have 824 00:50:06,704 --> 00:50:08,672 a surviving dinosaur covered in feathers 825 00:50:08,673 --> 00:50:11,041 that evolved from flighted ancestors. 826 00:50:12,510 --> 00:50:15,145 {\an8}Their flying days are long gone, 827 00:50:15,146 --> 00:50:16,913 {\an8}but their feathers remain, 828 00:50:16,914 --> 00:50:20,585 {\an8}still playing a starring role in the mating game. 829 00:50:22,420 --> 00:50:24,788 Instead of being used for flying, 830 00:50:24,789 --> 00:50:26,623 the wings are now used in mating displays. 831 00:50:26,624 --> 00:50:27,924 And that's a prime example 832 00:50:27,925 --> 00:50:30,127 of how evolution is always tinkering, 833 00:50:30,128 --> 00:50:32,196 always changing, always adapting. 834 00:50:33,398 --> 00:50:36,233 Their vast, nutrient-rich eggs 835 00:50:36,234 --> 00:50:39,703 are more than just cradles for new life, 836 00:50:39,704 --> 00:50:42,205 {\an8}they are time capsules, 837 00:50:42,206 --> 00:50:46,643 {\an8}holding the story of countless generations. 838 00:50:46,644 --> 00:50:50,814 {\an8}The fact that this is the largest single cell 839 00:50:50,815 --> 00:50:52,182 {\an8}in the animal kingdom 840 00:50:52,183 --> 00:50:54,684 {\an8}is an extraordinary testament 841 00:50:54,685 --> 00:50:59,256 to how far evolution has brought us 842 00:50:59,257 --> 00:51:00,957 from our single-celled ancestors 843 00:51:00,958 --> 00:51:03,760 to the complex organisms we are today. 844 00:51:07,398 --> 00:51:10,400 For billions of years, 845 00:51:10,401 --> 00:51:13,336 sexual reproduction 846 00:51:13,337 --> 00:51:16,007 has shaped life on Earth. 847 00:51:20,978 --> 00:51:24,481 Without sex, we wouldn't have beautiful feathers, 848 00:51:24,482 --> 00:51:26,583 or antlers, or horns, 849 00:51:26,584 --> 00:51:28,953 or courtship displays, or bird calls. 850 00:51:30,254 --> 00:51:33,290 All this diversity evolved 851 00:51:33,291 --> 00:51:35,592 because that sexual creature long ago 852 00:51:35,593 --> 00:51:38,161 evolved to mix their genes 853 00:51:38,162 --> 00:51:41,665 and enable each evolutionary branch of this tree of life 854 00:51:41,666 --> 00:51:43,867 to meander in its own direction. 855 00:51:45,770 --> 00:51:48,138 When I look at life on our planet, 856 00:51:48,139 --> 00:51:49,306 when I look at nature, 857 00:51:49,307 --> 00:51:52,042 what I'm just endlessly in awe of 858 00:51:52,043 --> 00:51:55,111 is how life continues to create itself. 859 00:51:55,112 --> 00:51:57,347 Every moment, every day, 860 00:51:57,348 --> 00:51:59,683 evolution brings us something novel, 861 00:51:59,684 --> 00:52:01,918 and I think that's gorgeous.