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 00:00:14,749 --> 00:00:17,317 {\an8}The world is teeming with life 4 00:00:17,318 --> 00:00:19,886 {\an8}in all shapes and sizes. 5 00:00:19,887 --> 00:00:22,222 We see bodies that can fly, 6 00:00:22,223 --> 00:00:24,457 burrow, swim. 7 00:00:24,458 --> 00:00:25,959 Some have bones, 8 00:00:25,960 --> 00:00:27,494 some don't. 9 00:00:27,495 --> 00:00:29,062 Just about any form you can imagine 10 00:00:29,063 --> 00:00:31,498 you'll find if you look. 11 00:00:33,968 --> 00:00:36,603 And this great variety of body shapes 12 00:00:36,604 --> 00:00:40,273 can be traced back to a single origin. 13 00:00:40,274 --> 00:00:41,508 The earliest forms of life 14 00:00:41,509 --> 00:00:44,377 had no arms, heads, ears, noses, or mouths. 15 00:00:44,378 --> 00:00:45,979 They were single cells. 16 00:00:45,980 --> 00:00:48,715 So how did life get from the microscopic 17 00:00:48,716 --> 00:00:52,318 to a world of giants? 18 00:00:52,319 --> 00:00:55,388 {\an8}When the first cells found a way to stick together, 19 00:00:55,389 --> 00:00:56,456 {\an8}it paved the way 20 00:00:56,457 --> 00:00:57,857 {\an8}for bodies to form. 21 00:00:57,858 --> 00:00:59,626 {\an8}And at that point, 22 00:00:59,627 --> 00:01:02,663 the whole diversity of animals just goes through the roof. 23 00:01:03,798 --> 00:01:06,933 But to get bigger and more complex, 24 00:01:06,934 --> 00:01:09,336 bodies needed to harness more power... 25 00:01:10,304 --> 00:01:12,806 Oxygen was a game changer in terms of body plans, 26 00:01:12,807 --> 00:01:15,308 {\an8}in terms of complexity. 27 00:01:15,309 --> 00:01:18,111 {\an8}...team up with other organisms... 28 00:01:18,112 --> 00:01:19,813 {\an8}My body is actually home 29 00:01:19,814 --> 00:01:22,048 {\an8}to trillions of little microbes. 30 00:01:22,049 --> 00:01:24,751 {\an8}...and evolve new tools. 31 00:01:24,752 --> 00:01:26,319 {\an8}It's when you see something 32 00:01:26,320 --> 00:01:28,154 {\an8}like this on your lap 33 00:01:28,155 --> 00:01:31,157 that you can really appreciate just how enormous they are. 34 00:01:32,827 --> 00:01:35,929 How did life evolve from a single cell 35 00:01:35,930 --> 00:01:39,500 to the spectacular array of shapes we see today? 36 00:01:40,968 --> 00:01:45,038 "Evolution: Body Builders." 37 00:01:45,039 --> 00:01:48,442 Right now, on "NOVA." 38 00:02:20,508 --> 00:02:22,542 The elephant has to be 39 00:02:22,543 --> 00:02:25,679 one of evolution's strangest creatures... 40 00:02:28,682 --> 00:02:31,351 ...with an extraordinary nose 41 00:02:31,352 --> 00:02:34,087 that's as dexterous as a hand. 42 00:02:34,088 --> 00:02:37,223 The trunk is really remarkable. 43 00:02:37,224 --> 00:02:39,325 {\an8}It has the strength to move a tree, 44 00:02:39,326 --> 00:02:42,862 {\an8}but it can also function in such a fine way 45 00:02:42,863 --> 00:02:44,598 {\an8}that it can pick up a single blade of grass. 46 00:02:46,233 --> 00:02:49,669 Enormous tusks that never stop growing. 47 00:02:49,670 --> 00:02:53,006 The tusks can grow up to ten-and-a-half feet long, 48 00:02:53,007 --> 00:02:56,709 and together, they can weigh more than 440 pounds. 49 00:02:59,179 --> 00:03:00,814 And a gigantic appetite. 50 00:03:02,249 --> 00:03:05,351 A huge amount of energy is needed to fuel an elephant. 51 00:03:05,352 --> 00:03:08,354 We're talking feeding for 18 hours a day 52 00:03:08,355 --> 00:03:12,092 {\an8}and eating four percent of their body weight in plants. 53 00:03:13,427 --> 00:03:17,063 But perhaps their most striking trait of all 54 00:03:17,064 --> 00:03:19,232 is their size. 55 00:03:19,233 --> 00:03:20,934 Elephants define 56 00:03:20,935 --> 00:03:24,304 what we understand about being big. 57 00:03:28,108 --> 00:03:30,210 {\an8}Their feet are like tree trunks. 58 00:03:31,345 --> 00:03:33,880 {\an8}Some of them weigh up to maybe 11 tons-- 59 00:03:33,881 --> 00:03:35,149 {\an8}that's as much as a school bus. 60 00:03:37,551 --> 00:03:41,121 So how did this creature's enormous body come to be? 61 00:03:44,091 --> 00:03:46,059 Elephants are the largest living animals 62 00:03:46,060 --> 00:03:48,761 that we have on the land today, 63 00:03:48,762 --> 00:03:53,067 but they are all just built of many, many, many cells. 64 00:03:54,468 --> 00:03:56,235 Cells are ultimately 65 00:03:56,236 --> 00:03:58,137 the fundamental building blocks of bodies. 66 00:03:58,138 --> 00:04:01,207 The elephant is really a marvel of evolution, 67 00:04:01,208 --> 00:04:02,475 because it reflects 68 00:04:02,476 --> 00:04:05,579 a quadrillion cells all working together. 69 00:04:07,414 --> 00:04:10,383 But where did those cells come from 70 00:04:10,384 --> 00:04:13,620 and how did they come together to form bodies? 71 00:04:16,490 --> 00:04:19,325 To answer that, 72 00:04:19,326 --> 00:04:21,494 we have to reach back into the distant past... 73 00:04:25,199 --> 00:04:28,668 {\an8}...to a time long before the first elephants, 74 00:04:28,669 --> 00:04:32,905 {\an8}before the first mammals, 75 00:04:32,906 --> 00:04:35,843 {\an8}before there were any animals at all... 76 00:04:37,945 --> 00:04:40,747 ...four billion years back, 77 00:04:40,748 --> 00:04:43,416 to some of the earliest life on Earth. 78 00:04:44,952 --> 00:04:47,053 This is the extraordinary story 79 00:04:47,054 --> 00:04:52,458 {\an8}of how evolution transformed a single cell 80 00:04:52,459 --> 00:04:56,463 {\an8}into the largest land animal alive today. 81 00:05:22,856 --> 00:05:27,660 Violent and unpredictable, 82 00:05:27,661 --> 00:05:31,765 the young Earth is hostile to life. 83 00:05:33,067 --> 00:05:34,400 But beneath the waves, 84 00:05:34,401 --> 00:05:37,137 simple cells cling on... 85 00:05:40,007 --> 00:05:42,575 ...locked in a relentless struggle for survival. 86 00:05:42,576 --> 00:05:44,911 Most are doomed, 87 00:05:44,912 --> 00:05:47,781 becoming evolutionary dead ends. 88 00:05:49,616 --> 00:05:53,320 But some scientists think one cell is different. 89 00:05:55,556 --> 00:05:57,924 This is LUCA, 90 00:05:57,925 --> 00:06:00,359 the elephant's ancient ancestor 91 00:06:00,360 --> 00:06:05,565 and, in fact, the ancestor of all life on Earth. 92 00:06:05,566 --> 00:06:08,401 The Last Universal Common Ancestor, 93 00:06:08,402 --> 00:06:10,136 or LUCA for short, 94 00:06:10,137 --> 00:06:12,538 {\an8}is the organism that gave rise, 95 00:06:12,539 --> 00:06:15,408 {\an8}quite literally, to all life we see today. 96 00:06:15,409 --> 00:06:18,177 {\an8}It wasn't necessarily the first form of life. 97 00:06:18,178 --> 00:06:19,746 {\an8}It almost certainly wasn't. 98 00:06:19,747 --> 00:06:21,414 {\an8}But it's the one that got by. 99 00:06:21,415 --> 00:06:25,985 {\an8}LUCA, that great single common ancestor of all life today, 100 00:06:25,986 --> 00:06:29,389 {\an8}that was the starting point for bodies. 101 00:06:31,225 --> 00:06:35,662 Scientists don't know for sure how LUCA's lineage survives. 102 00:06:36,830 --> 00:06:39,399 Perhaps it's just lucky. 103 00:06:40,968 --> 00:06:44,738 But what we do know is that LUCA can copy itself. 104 00:06:46,740 --> 00:06:49,208 Each new generation inherits the traits 105 00:06:49,209 --> 00:06:52,311 that helped LUCA persist, 106 00:06:52,312 --> 00:06:56,616 but the process is not perfect. 107 00:06:56,617 --> 00:07:01,854 Small mutations cause some copies to turn out differently. 108 00:07:01,855 --> 00:07:05,024 These tiny glitches in the genetic code 109 00:07:05,025 --> 00:07:06,726 may be mistakes, 110 00:07:06,727 --> 00:07:10,496 but sometimes, mistakes can be useful. 111 00:07:10,497 --> 00:07:12,632 Maybe it's a mutation 112 00:07:12,633 --> 00:07:13,800 that makes an organism 113 00:07:13,801 --> 00:07:15,334 a little bit bigger than it might've been, 114 00:07:15,335 --> 00:07:16,335 or a little bit faster 115 00:07:16,336 --> 00:07:17,336 than it might've been. 116 00:07:17,337 --> 00:07:18,671 A little mutation here, 117 00:07:18,672 --> 00:07:19,772 a little mutation there, 118 00:07:19,773 --> 00:07:23,476 that's how evolution works. 119 00:07:23,477 --> 00:07:24,944 {\an8}Evolution isn't goal-directed. 120 00:07:24,945 --> 00:07:26,279 {\an8}It doesn't have a purpose. 121 00:07:26,280 --> 00:07:28,314 {\an8}It's built entirely on these tiny little mistakes 122 00:07:28,315 --> 00:07:30,717 that allow life to take a new pathway. 123 00:07:32,152 --> 00:07:36,722 The journey to more complex bodies has begun. 124 00:07:36,723 --> 00:07:39,192 Generation after generation, 125 00:07:39,193 --> 00:07:41,727 small mutations accumulate, 126 00:07:41,728 --> 00:07:44,664 becoming big differences. 127 00:07:44,665 --> 00:07:48,634 LUCA is the seed from which the tree of life as we know it 128 00:07:48,635 --> 00:07:50,236 begins to grow. 129 00:07:58,679 --> 00:08:00,948 As time passes... 130 00:08:03,116 --> 00:08:06,386 ...LUCA's descendants flourish and spread. 131 00:08:09,990 --> 00:08:13,059 Some adapt to capture sunlight, 132 00:08:13,060 --> 00:08:14,928 the ancestors of plants. 133 00:08:17,431 --> 00:08:19,799 Others evolve to become fungi. 134 00:08:24,271 --> 00:08:26,639 Many stay small and simple, 135 00:08:26,640 --> 00:08:30,444 giving rise to the hidden microbial world. 136 00:08:31,478 --> 00:08:35,916 But all of them descendants of that sole ancestor. 137 00:08:37,885 --> 00:08:41,153 Although complex bodies still lie far in the future, 138 00:08:41,154 --> 00:08:44,724 life on Earth is already thriving. 139 00:08:48,762 --> 00:08:51,998 Until disaster strikes. 140 00:08:55,469 --> 00:08:57,803 Natural variations in the Earth's climate 141 00:08:57,804 --> 00:09:00,707 plunge the planet into a deep freeze. 142 00:09:06,446 --> 00:09:11,550 {\an8}This period of global cooling is so extreme, 143 00:09:11,551 --> 00:09:14,855 {\an8}it's called Snowball Earth. 144 00:09:19,126 --> 00:09:21,761 {\an8}Snowball Earth was a time when ice sheets 145 00:09:21,762 --> 00:09:24,030 swept all the way to almost the Equator 146 00:09:24,031 --> 00:09:26,565 and our seas were almost entirely frozen over. 147 00:09:26,566 --> 00:09:28,968 It was an extremely cold time 148 00:09:28,969 --> 00:09:31,837 and it was an extremely harsh environment for life to survive. 149 00:09:37,444 --> 00:09:39,078 It's not the cold of Snowball Earth 150 00:09:39,079 --> 00:09:40,713 that was the problem per se, 151 00:09:40,714 --> 00:09:43,316 but it was the fact that the amount of food available 152 00:09:43,317 --> 00:09:44,518 really tanked. 153 00:09:46,086 --> 00:09:49,822 Living during Snowball Earth was the ultimate survival test. 154 00:09:49,823 --> 00:09:51,590 It would've been really hard 155 00:09:51,591 --> 00:09:53,193 to find enough food to make a living. 156 00:09:55,929 --> 00:09:58,831 Floating in these ice-bound oceans, 157 00:09:58,832 --> 00:10:00,566 one of LUCA's descendants 158 00:10:00,567 --> 00:10:02,669 is desperately searching for a meal. 159 00:10:07,107 --> 00:10:10,543 Scientists call this hypothetical tiny organism 160 00:10:10,544 --> 00:10:12,346 Urchoanozoan. 161 00:10:17,851 --> 00:10:19,952 Urchoanozoa can be thought of 162 00:10:19,953 --> 00:10:22,723 as the last single-celled ancestor of animals. 163 00:10:24,458 --> 00:10:26,726 {\an8}This is still a relatively simple organism, 164 00:10:26,727 --> 00:10:28,995 {\an8}but it had a few features that set it apart 165 00:10:28,996 --> 00:10:30,896 {\an8}from other single-celled organisms. 166 00:10:30,897 --> 00:10:32,531 So it would've had a flagellum, 167 00:10:32,532 --> 00:10:36,268 which is like a whip-like tail sticking out of one end, 168 00:10:36,269 --> 00:10:38,338 and this would have powered it through the water. 169 00:10:40,841 --> 00:10:42,775 Over billions of years, 170 00:10:42,776 --> 00:10:45,245 life evolved these new traits. 171 00:10:47,514 --> 00:10:50,783 But one thing hasn't changed. 172 00:10:50,784 --> 00:10:55,755 Urchoanozoan is still tiny, just a single cell, 173 00:10:55,756 --> 00:10:59,025 and there are only so many ways a single cell can find food. 174 00:11:01,795 --> 00:11:03,929 But in its battle for survival, 175 00:11:03,930 --> 00:11:09,502 Urchoanozoan will change the shape of life on Earth forever. 176 00:11:23,850 --> 00:11:26,652 To illustrate the innovation 177 00:11:26,653 --> 00:11:29,655 that transformed life all those millions of years ago, 178 00:11:29,656 --> 00:11:33,025 evolutionary biologist Will Ratcliff 179 00:11:33,026 --> 00:11:35,428 doesn't need a time machine, 180 00:11:35,429 --> 00:11:38,432 just a remarkable creature alive today. 181 00:11:39,800 --> 00:11:41,167 {\an8}Wow. 182 00:11:41,168 --> 00:11:43,270 {\an8}Look at this. 183 00:11:44,938 --> 00:11:46,306 {\an8}There are thousands of ants here. 184 00:11:51,178 --> 00:11:53,179 {\an8}Look at this little ant. 185 00:11:53,180 --> 00:11:56,248 Alone, doesn't stand much of a chance, does it? 186 00:11:56,249 --> 00:11:58,118 It's currently biting into my finger. 187 00:11:59,186 --> 00:12:01,087 I won't lie, it's somewhat painful, 188 00:12:01,088 --> 00:12:03,189 but let's face it, it's not much of a match. 189 00:12:03,190 --> 00:12:04,724 I could squish it right now if I wanted to. 190 00:12:06,560 --> 00:12:08,360 This is a fact of life: 191 00:12:08,361 --> 00:12:11,263 when you're on your own, you have to make it all work. 192 00:12:11,264 --> 00:12:13,099 You have to find food, reproduce, 193 00:12:13,100 --> 00:12:14,701 survive much larger opponents. 194 00:12:15,869 --> 00:12:17,903 The superpower of these ants, however, 195 00:12:17,904 --> 00:12:19,206 is that they're not alone. 196 00:12:20,707 --> 00:12:24,009 Driver ants join forces in colonies 197 00:12:24,010 --> 00:12:26,513 up to 20 million strong. 198 00:12:28,448 --> 00:12:30,349 And as they move through the forest, 199 00:12:30,350 --> 00:12:33,385 even prey that is thousands of times larger than the ants are 200 00:12:33,386 --> 00:12:37,357 is quickly overwhelmed if it cannot run, hop, or fly away. 201 00:12:39,259 --> 00:12:42,228 {\an8}In fact, right here, there seems to be an insect 202 00:12:42,229 --> 00:12:46,166 {\an8}that has been balled over by about 500 ants. 203 00:12:49,769 --> 00:12:52,171 They overcome the limitations of size 204 00:12:52,172 --> 00:12:54,107 through sheer strength in numbers. 205 00:12:55,775 --> 00:12:57,143 And collectively, 206 00:12:57,144 --> 00:12:59,378 they're able to move through this forest 207 00:12:59,379 --> 00:13:00,546 finding prey, 208 00:13:00,547 --> 00:13:03,249 dismembering it joint by joint, 209 00:13:03,250 --> 00:13:05,084 bringing it back to the nest, 210 00:13:05,085 --> 00:13:06,919 feeding it to their offspring. 211 00:13:06,920 --> 00:13:09,054 And together, they are able to accomplish 212 00:13:09,055 --> 00:13:11,624 what any one individual ant could not. 213 00:13:11,625 --> 00:13:12,992 And in fact, 214 00:13:12,993 --> 00:13:16,862 they've hit upon a common motif in evolutionary biology, 215 00:13:16,863 --> 00:13:18,797 which is that with numbers, 216 00:13:18,798 --> 00:13:20,333 organisms gain new abilities. 217 00:13:23,436 --> 00:13:24,837 When you're tiny, 218 00:13:24,838 --> 00:13:28,108 cooperation can bring big rewards. 219 00:13:30,043 --> 00:13:32,745 Over 600 million years ago, 220 00:13:32,746 --> 00:13:35,447 beneath the ice of Snowball Earth, 221 00:13:35,448 --> 00:13:39,352 it's thought Urchoanozoan is different. 222 00:13:41,721 --> 00:13:44,990 The sticky molecules that cover its surface 223 00:13:44,991 --> 00:13:48,995 allow it to do something its ancestors haven't done before. 224 00:13:51,498 --> 00:13:56,268 For a single cell, this trait barely matters. 225 00:13:56,269 --> 00:13:58,704 But when it's time to divide, 226 00:13:58,705 --> 00:14:00,806 instead of drifting apart, 227 00:14:00,807 --> 00:14:04,543 the two cells remain stuck together. 228 00:14:07,614 --> 00:14:09,583 {\an8}And they multiply... 229 00:14:11,751 --> 00:14:16,355 {\an8}...passing this ability on to their descendants 230 00:14:16,356 --> 00:14:19,826 {\an8}until a vast alliance of cells forms. 231 00:14:23,463 --> 00:14:28,100 {\an8}Some scientists call this hypothetical colony 232 00:14:28,101 --> 00:14:29,936 the Death Star. 233 00:14:34,674 --> 00:14:39,178 Much larger than a single cell, 234 00:14:39,179 --> 00:14:42,581 it can do something extraordinary. 235 00:14:47,654 --> 00:14:49,855 By beating their flagella in tandem, 236 00:14:49,856 --> 00:14:53,058 the cells create water currents 237 00:14:53,059 --> 00:14:57,897 that suck in food, like smaller microbes, more efficiently. 238 00:15:03,036 --> 00:15:06,639 A collection of cells all working together, 239 00:15:06,640 --> 00:15:10,042 {\an8}this is the first multicellular organism 240 00:15:10,043 --> 00:15:13,512 {\an8}and a major evolutionary step 241 00:15:13,513 --> 00:15:16,316 {\an8}toward animal bodies as we know them. 242 00:15:19,753 --> 00:15:21,787 Like every other revolution in evolution, 243 00:15:21,788 --> 00:15:23,656 multicellularity was an accident. 244 00:15:23,657 --> 00:15:25,090 It was a mutation that allowed cells 245 00:15:25,091 --> 00:15:26,526 to stick together. 246 00:15:27,994 --> 00:15:30,462 Multicellularity happened many times 247 00:15:30,463 --> 00:15:31,897 across all different 248 00:15:31,898 --> 00:15:33,565 groups of organisms, 249 00:15:33,566 --> 00:15:37,503 {\an8}so multicellularity in general originated 250 00:15:37,504 --> 00:15:39,838 {\an8}probably 25 or more times, 251 00:15:39,839 --> 00:15:41,106 {\an8}but in animals, 252 00:15:41,107 --> 00:15:44,209 {\an8}there was only one moment of multicellularity. 253 00:15:44,210 --> 00:15:45,411 {\an8}For animals, 254 00:15:45,412 --> 00:15:46,979 it turned out to be a winning strategy, 255 00:15:46,980 --> 00:15:48,647 because once they became multicellular, 256 00:15:48,648 --> 00:15:50,116 they never went back. 257 00:15:52,152 --> 00:15:54,386 {\an8}The so-called Death Star 258 00:15:54,387 --> 00:15:55,921 {\an8}can gather food more effectively 259 00:15:55,922 --> 00:15:58,457 {\an8}than its single-celled cousins, 260 00:15:58,458 --> 00:16:01,628 {\an8}thriving in even the harshest conditions. 261 00:16:03,363 --> 00:16:05,497 {\an8}But this is just the start 262 00:16:05,498 --> 00:16:08,301 {\an8}of what a multicellular organism can do. 263 00:16:13,773 --> 00:16:16,275 {\an8}The advantages of working together 264 00:16:16,276 --> 00:16:18,611 {\an8}are evident in nature today. 265 00:16:24,351 --> 00:16:25,651 {\an8}These ants are not 266 00:16:25,652 --> 00:16:27,086 {\an8}a simple collection of cells. 267 00:16:27,087 --> 00:16:28,620 {\an8}Instead, 268 00:16:28,621 --> 00:16:30,756 {\an8}they are an organized, 269 00:16:30,757 --> 00:16:31,957 {\an8}integrated society, 270 00:16:31,958 --> 00:16:35,228 where every ant has its role to play. 271 00:16:36,663 --> 00:16:39,998 Look at the size of these soldiers. 272 00:16:39,999 --> 00:16:42,634 They are at least ten times larger 273 00:16:42,635 --> 00:16:44,002 than their worker sisters. 274 00:16:44,003 --> 00:16:45,137 These soldiers, 275 00:16:45,138 --> 00:16:47,139 {\an8}with their massive mandibles, 276 00:16:47,140 --> 00:16:48,374 {\an8}defend the colony. 277 00:16:48,375 --> 00:16:50,042 The much smaller workers 278 00:16:50,043 --> 00:16:53,379 find food and raise the offspring. 279 00:16:53,380 --> 00:16:56,081 They dedicate their energy and their lives 280 00:16:56,082 --> 00:16:57,617 {\an8}to the colony's success. 281 00:17:01,221 --> 00:17:05,057 Specialization is a crucial evolutionary milestone. 282 00:17:05,058 --> 00:17:06,291 And much like these ants, 283 00:17:06,292 --> 00:17:09,128 in early multicellular organisms, 284 00:17:09,129 --> 00:17:12,464 cells began to take on specialized roles 285 00:17:12,465 --> 00:17:14,566 and get new functions 286 00:17:14,567 --> 00:17:16,870 that their single-celled ancestors couldn't have done. 287 00:17:19,439 --> 00:17:24,576 When groups of cells began taking on different roles, 288 00:17:24,577 --> 00:17:27,313 they unleashed the full power of multicellularity. 289 00:17:28,815 --> 00:17:30,048 If you are a single cell, 290 00:17:30,049 --> 00:17:31,884 you have to do everything all the time. 291 00:17:31,885 --> 00:17:35,120 If you are a collection of multiple different cells, 292 00:17:35,121 --> 00:17:37,356 then you can develop organs and tissues 293 00:17:37,357 --> 00:17:39,825 which can specialize to do different things 294 00:17:39,826 --> 00:17:42,261 and give you an advantage in the world that you live in. 295 00:17:42,262 --> 00:17:43,729 So you can have some cells 296 00:17:43,730 --> 00:17:46,965 that become very specialist at gathering food, 297 00:17:46,966 --> 00:17:49,201 other cells that are involved in motion, 298 00:17:49,202 --> 00:17:51,970 other cells that are involved in reproduction. 299 00:17:51,971 --> 00:17:54,740 And this specialization really leads to 300 00:17:54,741 --> 00:17:56,942 more efficiency and more possibility. 301 00:17:56,943 --> 00:17:59,912 This was a turning point in the evolution of life, 302 00:17:59,913 --> 00:18:01,346 because it allowed for 303 00:18:01,347 --> 00:18:03,015 the explosion of shapes that we see 304 00:18:03,016 --> 00:18:04,784 on Earth today. 305 00:18:09,122 --> 00:18:11,523 Over millions of years, 306 00:18:11,524 --> 00:18:14,359 the descendants of the Death Star's cells 307 00:18:14,360 --> 00:18:16,628 continue to specialize. 308 00:18:20,934 --> 00:18:23,268 Sex cells, nerve cells, 309 00:18:23,269 --> 00:18:26,738 and muscle cells evolve, 310 00:18:26,739 --> 00:18:31,677 until what began as a chance mutation 311 00:18:31,678 --> 00:18:35,482 has become something revolutionary. 312 00:18:38,618 --> 00:18:41,554 {\an8}The age of bodies has arrived. 313 00:18:49,629 --> 00:18:53,031 Just 100 million years after those cells 314 00:18:53,032 --> 00:18:55,201 first joined forces... 315 00:18:58,438 --> 00:18:59,638 {\an8}...the oceans are filled 316 00:18:59,639 --> 00:19:02,841 {\an8}with many different body shapes and forms. 317 00:19:06,813 --> 00:19:09,181 Among them, 318 00:19:09,182 --> 00:19:12,952 an ancient ancestor of the elephant. 319 00:19:15,488 --> 00:19:17,256 No tusks... 320 00:19:19,425 --> 00:19:21,361 no trunk. 321 00:19:25,131 --> 00:19:27,933 Just a tiny worm, 322 00:19:27,934 --> 00:19:29,902 a fraction of an inch long, 323 00:19:29,903 --> 00:19:32,071 wriggling across the sea floor. 324 00:19:33,873 --> 00:19:36,108 Compared to what came before it, 325 00:19:36,109 --> 00:19:38,810 it's incredibly complex, 326 00:19:38,811 --> 00:19:41,314 but it's still tiny. 327 00:19:43,883 --> 00:19:46,885 Bigger, stronger, more powerful bodies 328 00:19:46,886 --> 00:19:51,189 require something that ancient creatures lacked... 329 00:19:51,190 --> 00:19:52,358 oxygen. 330 00:19:54,127 --> 00:19:57,796 Oxygen allows animals to survive. 331 00:19:57,797 --> 00:20:02,167 Animals need oxygen in order to eat, in order to move. 332 00:20:02,168 --> 00:20:05,571 Oxygen is like rocket fuel for living things. 333 00:20:05,572 --> 00:20:07,005 It's highly reactive, 334 00:20:07,006 --> 00:20:08,874 {\an8}it's highly energetic, 335 00:20:08,875 --> 00:20:11,210 {\an8}and it was a game changer for the evolution of bodies. 336 00:20:12,445 --> 00:20:17,015 Animals use oxygen to convert food into energy. 337 00:20:17,016 --> 00:20:21,653 But in these ancient oceans, oxygen is scarce, 338 00:20:21,654 --> 00:20:26,458 and early animals have limited ways to harness it. 339 00:20:26,459 --> 00:20:27,993 Simpler animals 340 00:20:27,994 --> 00:20:29,928 {\an8}would get oxygen primarily 341 00:20:29,929 --> 00:20:31,763 {\an8}by absorbing it through their skin. 342 00:20:31,764 --> 00:20:33,632 In fact, there are lots of animals today 343 00:20:33,633 --> 00:20:36,001 which still breathe through their skin. 344 00:20:36,002 --> 00:20:37,636 So, earthworms breathe 345 00:20:37,637 --> 00:20:40,138 through their skin, but there are limitations. 346 00:20:40,139 --> 00:20:41,807 You need to stay moist, 347 00:20:41,808 --> 00:20:43,075 {\an8}so you have to stay either 348 00:20:43,076 --> 00:20:44,843 {\an8}in the water or near a water environment 349 00:20:44,844 --> 00:20:46,278 {\an8}to stay damp. 350 00:20:46,279 --> 00:20:47,980 {\an8}There's also constraints on how big you can get 351 00:20:47,981 --> 00:20:50,283 {\an8}if you're only absorbing oxygen through your skin. 352 00:20:52,352 --> 00:20:55,087 Unable to absorb much oxygen, 353 00:20:55,088 --> 00:20:58,624 some early animals are small and slow. 354 00:20:58,625 --> 00:21:03,128 But this little worm has something special. 355 00:21:03,129 --> 00:21:06,565 Not all structures, when they appear, are brand new. 356 00:21:06,566 --> 00:21:09,635 Evolution will often act on previous structures, 357 00:21:09,636 --> 00:21:10,970 which might be used for a different purpose. 358 00:21:15,141 --> 00:21:18,577 The worm's sides are lined with tiny pores 359 00:21:18,578 --> 00:21:22,247 that it uses to filter food. 360 00:21:22,248 --> 00:21:25,817 But this worm has a special advantage, 361 00:21:25,818 --> 00:21:29,689 making it stronger and more likely to survive. 362 00:21:32,692 --> 00:21:33,825 Its pores 363 00:21:33,826 --> 00:21:37,162 are slightly wrinkled on the inside, 364 00:21:37,163 --> 00:21:40,432 meaning more surface area to filter food, 365 00:21:40,433 --> 00:21:44,537 and also oxygen from the water. 366 00:21:50,476 --> 00:21:53,746 {\an8}It passes these wrinkles on to its offspring. 367 00:21:55,481 --> 00:21:57,382 {\an8}Over generations, 368 00:21:57,383 --> 00:22:00,218 {\an8}they deepen into folds, 369 00:22:00,219 --> 00:22:03,722 {\an8}greatly increasing their surface area. 370 00:22:03,723 --> 00:22:08,493 {\an8}Until pores that were once used for feeding 371 00:22:08,494 --> 00:22:10,697 {\an8}are repurposed for breathing. 372 00:22:12,665 --> 00:22:16,035 {\an8}It's the beginning of gills. 373 00:22:17,270 --> 00:22:19,037 {\an8}Gills were one of the major 374 00:22:19,038 --> 00:22:20,238 {\an8}evolutionary novelties 375 00:22:20,239 --> 00:22:21,907 {\an8}in the history of 376 00:22:21,908 --> 00:22:23,909 {\an8}animal evolution on Earth, 377 00:22:23,910 --> 00:22:26,578 because it allowed animals to get more oxygen, 378 00:22:26,579 --> 00:22:28,413 and anything that allows 379 00:22:28,414 --> 00:22:31,817 an animal to get more oxygen is a win. 380 00:22:31,818 --> 00:22:34,553 I can illustrate the power of gills 381 00:22:34,554 --> 00:22:37,489 using the art of origami. 382 00:22:37,490 --> 00:22:39,624 So, if you imagine this is a surface that's being used 383 00:22:39,625 --> 00:22:41,793 to take up oxygen from the water, 384 00:22:41,794 --> 00:22:44,029 we have this surface area 385 00:22:44,030 --> 00:22:45,964 that's available for oxygen uptake, 386 00:22:45,965 --> 00:22:47,499 and that's kind of limiting. 387 00:22:47,500 --> 00:22:52,471 But if instead we have a folded shape of the same size, 388 00:22:52,472 --> 00:22:55,207 you can actually increase that surface area 389 00:22:55,208 --> 00:22:57,175 massively in the same space, 390 00:22:57,176 --> 00:22:58,810 and enhance the ability 391 00:22:58,811 --> 00:23:00,613 to take up oxygen from the water. 392 00:23:01,881 --> 00:23:06,718 {\an8}So, simply folding tissue to increase the surface area 393 00:23:06,719 --> 00:23:08,988 is such a beautiful solution to a complex problem. 394 00:23:10,623 --> 00:23:14,259 {\an8}Gills supercharge the uptake of oxygen, 395 00:23:14,260 --> 00:23:19,264 {\an8}and more oxygen means more energy harnessed from food. 396 00:23:19,265 --> 00:23:21,933 {\an8}Gills were an absolute gamechanger. 397 00:23:21,934 --> 00:23:23,602 {\an8}They allowed for much, much 398 00:23:23,603 --> 00:23:25,537 {\an8}more easy energy capture, 399 00:23:25,538 --> 00:23:29,207 which fueled faster, bigger, stronger bodies. 400 00:23:29,208 --> 00:23:31,409 And you can see the success of gills because they're present 401 00:23:31,410 --> 00:23:33,946 in so many different animal groups today. 402 00:23:35,281 --> 00:23:37,149 Gills come in many shapes 403 00:23:37,150 --> 00:23:40,252 and sizes... 404 00:23:40,253 --> 00:23:43,655 {\an8}from the tiny clown fish 405 00:23:43,656 --> 00:23:47,392 to the giant whale shark. 406 00:23:47,393 --> 00:23:51,329 Some have buried them deep inside their bodies. 407 00:23:51,330 --> 00:23:55,634 Others carry them on the outside. 408 00:23:55,635 --> 00:23:58,170 Gills allowed animals to grow bigger, 409 00:23:58,171 --> 00:24:01,506 more complex bodies than ever before. 410 00:24:05,978 --> 00:24:10,782 And in fact every amphibian, reptile, bird, 411 00:24:10,783 --> 00:24:12,751 and mammal alive today, 412 00:24:12,752 --> 00:24:15,887 including the elephant, and us, 413 00:24:15,888 --> 00:24:19,425 descended from ancestors with gills. 414 00:24:22,929 --> 00:24:24,897 So, if gills were such a good idea, 415 00:24:26,032 --> 00:24:29,035 why did so many animals lose them? 416 00:24:33,306 --> 00:24:34,840 In the waterways of Kenya 417 00:24:34,841 --> 00:24:38,877 lies something that may hold the answer. 418 00:24:38,878 --> 00:24:42,113 In Kenya, fish have to be 419 00:24:42,114 --> 00:24:45,685 adapted to flooding and dry seasons. 420 00:24:47,353 --> 00:24:50,189 Some droughts can last months and even years. 421 00:24:52,658 --> 00:24:54,993 Biologist Wanja Dorothy Nyingi 422 00:24:54,994 --> 00:24:59,030 is an expert on Kenya's freshwater fish species. 423 00:24:59,031 --> 00:25:00,799 When the water begins to recede, 424 00:25:00,800 --> 00:25:02,033 then a fish species has a choice. 425 00:25:02,034 --> 00:25:04,736 {\an8}It can either recede with the water 426 00:25:04,737 --> 00:25:06,805 {\an8}to a bigger stream of water 427 00:25:06,806 --> 00:25:08,306 or a lake, 428 00:25:08,307 --> 00:25:11,344 or adapt to that drop of water levels. 429 00:25:16,215 --> 00:25:19,084 This fish has a remarkable adaptation 430 00:25:19,085 --> 00:25:21,954 that allows it to survive in the shallowest of pools. 431 00:25:23,289 --> 00:25:25,490 Meet Protopterus Aethiopicus, 432 00:25:25,491 --> 00:25:28,194 locally known as Kamongo. 433 00:25:30,830 --> 00:25:32,464 Like all fishes, 434 00:25:32,465 --> 00:25:35,133 the Kamongo has gills, 435 00:25:35,134 --> 00:25:38,170 but it also has something else. 436 00:25:39,839 --> 00:25:41,207 Oh, my gosh, look at that. 437 00:25:43,709 --> 00:25:46,311 That's a gulp, that's how it breathes. 438 00:25:46,312 --> 00:25:47,680 That was amazing. 439 00:25:49,248 --> 00:25:52,083 It gulps air through its mouth, 440 00:25:52,084 --> 00:25:54,419 drawing it into a special sack, 441 00:25:54,420 --> 00:25:57,923 deep inside its body. 442 00:25:57,924 --> 00:26:00,025 Normally, fish 443 00:26:00,026 --> 00:26:02,060 would breathe through their gills. 444 00:26:02,061 --> 00:26:03,161 But this fish 445 00:26:03,162 --> 00:26:05,497 is very suited to these extremes 446 00:26:05,498 --> 00:26:07,633 because it has lungs. 447 00:26:09,201 --> 00:26:11,170 It is a lungfish. 448 00:26:13,406 --> 00:26:14,940 The lung breathing takes up 449 00:26:14,941 --> 00:26:17,509 over 90% of its oxygen requirements. 450 00:26:17,510 --> 00:26:19,911 And the gills, though functional, 451 00:26:19,912 --> 00:26:24,250 are mostly for excreting carbon dioxide. 452 00:26:26,485 --> 00:26:28,453 {\an8}This remarkable air-gulping ability 453 00:26:28,454 --> 00:26:32,624 {\an8}first appeared around 400 million years ago, 454 00:26:32,625 --> 00:26:34,926 {\an8}as a backup system, 455 00:26:34,927 --> 00:26:37,562 helping fish to survive in low-oxygen waters 456 00:26:37,563 --> 00:26:39,632 or brief droughts. 457 00:26:40,900 --> 00:26:43,736 But when combined with another key adaptation, 458 00:26:44,770 --> 00:26:48,541 lungs unlocked an entirely new world. 459 00:26:50,443 --> 00:26:52,344 You can see how it uses its limbs. 460 00:26:52,345 --> 00:26:54,412 {\an8}Unlike normal fish fins, 461 00:26:54,413 --> 00:26:57,449 {\an8}they are muscular and very fleshy. 462 00:26:57,450 --> 00:26:59,451 They have the capacity 463 00:26:59,452 --> 00:27:04,389 to crawl onto land. 464 00:27:04,390 --> 00:27:06,257 This fish is very important 465 00:27:06,258 --> 00:27:08,526 when we speak about evolution, 466 00:27:08,527 --> 00:27:10,462 because it is our link 467 00:27:10,463 --> 00:27:12,397 to the transition 468 00:27:12,398 --> 00:27:17,169 that we made from life in water to life on land. 469 00:27:27,580 --> 00:27:31,383 The descendants of that tiny worm 470 00:27:31,384 --> 00:27:33,786 {\an8}gave rise to fishes with gills. 471 00:27:37,356 --> 00:27:39,691 And eventually lungs, 472 00:27:39,692 --> 00:27:41,760 that not only helped them 473 00:27:41,761 --> 00:27:46,498 survive, but allowed them to do something extraordinary-- 474 00:27:46,499 --> 00:27:50,736 leave the water and live on land. 475 00:27:52,438 --> 00:27:54,506 {\an8}So, the first animals that came out onto the land 476 00:27:54,507 --> 00:27:56,007 {\an8}were probably bugs, 477 00:27:56,008 --> 00:27:59,411 {\an8}and then not long after that, vertebrates, backboned animals. 478 00:27:59,412 --> 00:28:01,947 And they looked a lot like fishes with limbs. 479 00:28:05,184 --> 00:28:09,087 I love the vertebrate transition to land because 480 00:28:09,088 --> 00:28:12,090 {\an8}it's really fishes coming onto land, 481 00:28:12,091 --> 00:28:14,325 {\an8}a fish out of water. 482 00:28:14,326 --> 00:28:17,595 And one of my favorite fossils is this Elpistostegalian. 483 00:28:17,596 --> 00:28:18,997 This was a big animal, 484 00:28:18,998 --> 00:28:20,465 something like this, 485 00:28:20,466 --> 00:28:22,600 a head this large, had a five-foot body. 486 00:28:22,601 --> 00:28:26,137 We know that they had these pre-adaptations to live on land. 487 00:28:26,138 --> 00:28:27,772 They have upward-facing eyes. 488 00:28:27,773 --> 00:28:30,675 It breathed through its nostrils into 489 00:28:30,676 --> 00:28:32,343 these big lungs that it had. 490 00:28:32,344 --> 00:28:33,912 And some of them start 491 00:28:33,913 --> 00:28:36,548 having limbs that really start resembling 492 00:28:36,549 --> 00:28:38,884 our hands and our feet. 493 00:28:39,919 --> 00:28:41,953 Scientists don't know exactly why 494 00:28:41,954 --> 00:28:44,123 those first fishes came onto land. 495 00:28:45,291 --> 00:28:49,060 But they do know that they had four fleshy fins. 496 00:28:49,061 --> 00:28:51,830 And these would lay the foundations, 497 00:28:51,831 --> 00:28:53,631 not just for the elephant, 498 00:28:53,632 --> 00:28:56,235 but for every land vertebrate that's ever lived. 499 00:28:58,003 --> 00:29:01,739 This is known as the tetrapod body plan. 500 00:29:01,740 --> 00:29:04,409 Once the tetrapod blueprint was in place, 501 00:29:04,410 --> 00:29:07,879 {\an8}then that blueprint was free to be modified, 502 00:29:07,880 --> 00:29:10,949 to adapt, to change in so many different ways. 503 00:29:10,950 --> 00:29:13,618 And that's produced things as varied as birds, which use 504 00:29:13,619 --> 00:29:15,221 their arms to fly. 505 00:29:16,422 --> 00:29:18,123 It's produced whales, 506 00:29:18,124 --> 00:29:19,958 which have gone back into the water, 507 00:29:19,959 --> 00:29:22,428 that use those limbs to swim. 508 00:29:23,762 --> 00:29:25,730 And even some tetrapods, like snakes, 509 00:29:25,731 --> 00:29:27,766 have lost their limbs entirely. 510 00:29:29,602 --> 00:29:31,436 Every paw, 511 00:29:31,437 --> 00:29:33,404 hoof, 512 00:29:33,405 --> 00:29:34,906 hand 513 00:29:34,907 --> 00:29:36,508 and foot on the planet today, 514 00:29:36,509 --> 00:29:39,978 including the elephant's and ours, 515 00:29:39,979 --> 00:29:43,815 all descended from those pioneering fishes 516 00:29:43,816 --> 00:29:47,285 {\an8}that left the water and walked on land. 517 00:29:52,091 --> 00:29:55,126 In today's world, we see an incredible 518 00:29:55,127 --> 00:29:58,329 diversity of body plans that are 519 00:29:58,330 --> 00:30:01,433 derived from the original tetrapod plan. 520 00:30:02,434 --> 00:30:04,869 Every habitat you go to almost has 521 00:30:04,870 --> 00:30:06,137 a tetrapod living in it 522 00:30:06,138 --> 00:30:07,772 and has a tetrapod that's adapted to live 523 00:30:07,773 --> 00:30:09,708 in these really interesting habitats. 524 00:30:13,812 --> 00:30:17,415 But having four limbs isn't the only successful body plan 525 00:30:17,416 --> 00:30:18,684 out there. 526 00:30:19,818 --> 00:30:20,952 There are hexapods, 527 00:30:20,953 --> 00:30:22,554 like six-limbed insects, 528 00:30:22,555 --> 00:30:24,956 decapods, lobsters 529 00:30:24,957 --> 00:30:27,392 and crabs and shrimp. 530 00:30:27,393 --> 00:30:30,361 There are animals with dozens of limbs on either side, 531 00:30:30,362 --> 00:30:33,097 like centipedes, for example. 532 00:30:33,098 --> 00:30:34,933 From no limbs 533 00:30:34,934 --> 00:30:37,569 to hundreds of limbs, 534 00:30:37,570 --> 00:30:40,605 evolution has shaped animal bodies into a myriad 535 00:30:40,606 --> 00:30:42,473 of different forms. 536 00:30:46,645 --> 00:30:49,848 But tetrapods have one major advantage. 537 00:30:52,551 --> 00:30:56,021 They're really good at getting big. 538 00:30:58,657 --> 00:31:00,491 In the generations that pass 539 00:31:00,492 --> 00:31:04,662 after those first fishes left the water, 540 00:31:04,663 --> 00:31:09,535 {\an8}they evolve stronger skeletons to support their bodies on land. 541 00:31:11,904 --> 00:31:14,373 {\an8}The most efficient lungs on the planet. 542 00:31:15,608 --> 00:31:19,144 {\an8}And robust joints to carry more weight. 543 00:31:20,112 --> 00:31:23,249 {\an8}And these traits allow some of them to evolve... 544 00:31:25,117 --> 00:31:27,486 {\an8}...into giants. 545 00:31:35,694 --> 00:31:39,698 {\an8}Beings so large few predators dare challenge them. 546 00:31:41,467 --> 00:31:44,969 A huge body can store more fat and water. 547 00:31:50,075 --> 00:31:52,043 And tall animals can access food 548 00:31:52,044 --> 00:31:54,513 that smaller animals can't reach. 549 00:31:58,350 --> 00:32:01,653 Thanks in part to their enormous size, 550 00:32:01,654 --> 00:32:04,957 giant dinosaurs dominate the planet. 551 00:32:06,225 --> 00:32:09,861 Dinosaurs persisted for 180 million years, 552 00:32:09,862 --> 00:32:12,030 and they became the largest animals 553 00:32:12,031 --> 00:32:14,065 to have ever lived on the land. 554 00:32:14,066 --> 00:32:16,668 So, in terms of size 555 00:32:16,669 --> 00:32:18,436 and diversification, 556 00:32:18,437 --> 00:32:22,775 dinosaurs may well be the pinnacle of tetrapod evolution. 557 00:32:24,677 --> 00:32:26,879 While the dinosaurs dominate... 558 00:32:30,015 --> 00:32:33,419 ...the ancestors of the elephant are still tiny. 559 00:32:37,456 --> 00:32:40,224 They share the same four-limbed body plan, 560 00:32:40,225 --> 00:32:42,661 inherited from those pioneering fish. 561 00:32:44,963 --> 00:32:48,000 But on a much smaller scale than the dinosaurs. 562 00:32:50,369 --> 00:32:52,670 They're called Afrotherians, 563 00:32:52,671 --> 00:32:54,940 because they evolved in Africa. 564 00:33:01,480 --> 00:33:03,514 In this land of giants, 565 00:33:03,515 --> 00:33:06,819 their small size may seem like a disadvantage. 566 00:33:09,855 --> 00:33:12,491 But that's about to change. 567 00:33:14,293 --> 00:33:15,860 Whether it's volcanism 568 00:33:15,861 --> 00:33:18,029 or an asteroid impact or climate change, 569 00:33:18,030 --> 00:33:21,200 the environment can absolutely be ruthless. 570 00:33:27,873 --> 00:33:29,741 Sudden environmental changes 571 00:33:29,742 --> 00:33:32,577 {\an8}can completely change the challenges 572 00:33:32,578 --> 00:33:34,046 {\an8}that animals have to overcome. 573 00:33:35,914 --> 00:33:38,483 One adaptation might be good 574 00:33:38,484 --> 00:33:40,418 right now, but the environment 575 00:33:40,419 --> 00:33:43,288 might change, and it might change drastically. 576 00:33:44,323 --> 00:33:46,190 What was previously a really useful 577 00:33:46,191 --> 00:33:47,692 {\an8}adaptation, like being really big, 578 00:33:47,693 --> 00:33:49,161 {\an8}can suddenly become a really big hindrance. 579 00:33:50,162 --> 00:33:52,296 So, when it came to making it through mass extinctions, 580 00:33:52,297 --> 00:33:54,099 size really mattered. 581 00:33:55,934 --> 00:33:58,669 An asteroid devastates the planet. 582 00:33:58,670 --> 00:34:00,305 Few survive. 583 00:34:01,874 --> 00:34:04,209 The giant dinosaurs are finished. 584 00:34:05,444 --> 00:34:09,113 In the end, their size is part of their downfall. 585 00:34:13,819 --> 00:34:15,920 In the aftermath of the extinction, 586 00:34:15,921 --> 00:34:18,623 anything that was above the size of, like, 587 00:34:18,624 --> 00:34:20,426 a big raccoon would have gone extinct. 588 00:34:21,727 --> 00:34:23,227 When you're really, really big, 589 00:34:23,228 --> 00:34:24,962 that means you're sort of specialized. 590 00:34:24,963 --> 00:34:27,632 And when the environment changes, specialized animals 591 00:34:27,633 --> 00:34:31,035 {\an8}are often, uh, subjected to a greater 592 00:34:31,036 --> 00:34:33,872 {\an8}deal of pressure than those that are small and generalist. 593 00:34:35,407 --> 00:34:36,707 In the post-meteor world, 594 00:34:36,708 --> 00:34:38,242 vegetation would have disappeared, 595 00:34:38,243 --> 00:34:40,278 entire food sources would have collapsed. 596 00:34:40,279 --> 00:34:42,313 It was much better to be small, to be flexible, 597 00:34:42,314 --> 00:34:44,148 to be agile, being small is a 598 00:34:44,149 --> 00:34:46,317 really successful, uh, survival strategy. 599 00:34:46,318 --> 00:34:49,454 And it's those animals living in the shadows 600 00:34:49,455 --> 00:34:51,923 sometimes that come on to dominate Earth, 601 00:34:51,924 --> 00:34:54,192 and we see that time and time again 602 00:34:54,193 --> 00:34:57,028 when we look at the history of life on planet Earth. 603 00:34:57,029 --> 00:35:01,467 The survivors sometimes come from very unexpected places. 604 00:35:09,842 --> 00:35:11,442 In this new age, 605 00:35:11,443 --> 00:35:14,980 the small furry mammals find a world of opportunity. 606 00:35:18,484 --> 00:35:19,750 And in Africa, 607 00:35:19,751 --> 00:35:22,987 the elephant's ancestors are among those 608 00:35:22,988 --> 00:35:24,723 that make it through. 609 00:35:28,627 --> 00:35:30,795 Agile and adaptable, 610 00:35:30,796 --> 00:35:33,698 their small body size and sheer number 611 00:35:33,699 --> 00:35:35,234 are key to their success. 612 00:35:36,368 --> 00:35:39,404 And so is their flexible diet. 613 00:35:41,907 --> 00:35:43,207 These Afrotherians will eat 614 00:35:43,208 --> 00:35:44,710 almost anything. 615 00:35:46,445 --> 00:35:47,913 Including plants. 616 00:35:49,515 --> 00:35:52,183 Plants seem like easy prey. 617 00:35:52,184 --> 00:35:54,920 They don't run, they don't hide. 618 00:35:56,054 --> 00:35:58,389 But that doesn't mean they're defenseless. 619 00:35:58,390 --> 00:35:59,957 Plants have evolved 620 00:35:59,958 --> 00:36:01,793 {\an8}all kinds of ways to protect themselves. 621 00:36:03,629 --> 00:36:04,996 Thorns, bark, 622 00:36:04,997 --> 00:36:06,631 even chemicals. 623 00:36:06,632 --> 00:36:08,566 All of this is warfare. 624 00:36:08,567 --> 00:36:09,935 It's to keep from being eaten. 625 00:36:12,104 --> 00:36:14,138 But even if you can get past their defenses, 626 00:36:14,139 --> 00:36:17,276 there's another problem with eating plants. 627 00:36:18,110 --> 00:36:21,313 They contain a lot of cellulose. 628 00:36:23,315 --> 00:36:26,183 Cellulose is this fantastic building block 629 00:36:26,184 --> 00:36:29,053 {\an8}of a molecule that's made by a bunch of plants. 630 00:36:29,054 --> 00:36:32,857 {\an8}Think of it as like the structural part of a plant, 631 00:36:32,858 --> 00:36:35,226 the scaffolding-- it helps 632 00:36:35,227 --> 00:36:36,727 give plants their strength. 633 00:36:36,728 --> 00:36:38,763 It helps them stay upright, 634 00:36:38,764 --> 00:36:41,600 and it's really, really, really hard to break down. 635 00:36:43,302 --> 00:36:46,471 {\an8}Even today's plant eaters can't fully digest it. 636 00:36:47,606 --> 00:36:49,674 {\an8}After a full trip through the gut, 637 00:36:49,675 --> 00:36:52,277 {\an8}nearly half the meal emerges intact. 638 00:36:56,615 --> 00:36:58,816 What we have here is a bit of horse dung. 639 00:36:58,817 --> 00:37:01,586 If we take a closer look, 640 00:37:01,587 --> 00:37:03,754 you can clearly see the problem; there's actually 641 00:37:03,755 --> 00:37:06,657 {\an8}a lot of fibrous material still in here. 642 00:37:06,658 --> 00:37:09,393 {\an8}There are little strands of grass here and there. 643 00:37:09,394 --> 00:37:11,996 {\an8}The grass has been chewed, it's been digested 644 00:37:11,997 --> 00:37:14,966 and yet it's still incredibly intact. 645 00:37:14,967 --> 00:37:16,834 So, just goes to show 646 00:37:16,835 --> 00:37:18,636 it's tough being a plant eater. 647 00:37:21,773 --> 00:37:24,710 So how did early Afrotherians manage it? 648 00:37:26,745 --> 00:37:28,746 Throughout evolutionary history, 649 00:37:28,747 --> 00:37:31,849 plenty of animals have fed on plants, 650 00:37:31,850 --> 00:37:34,886 but few have been able to break them down unaided. 651 00:37:36,488 --> 00:37:37,755 They have relied on 652 00:37:37,756 --> 00:37:40,124 an unseen partnership with something 653 00:37:40,125 --> 00:37:43,562 that they swallow along with their food. 654 00:37:47,199 --> 00:37:48,633 Most animals lack 655 00:37:48,634 --> 00:37:49,967 the enzyme cellulase, 656 00:37:49,968 --> 00:37:52,704 which allows them to break down cellulose. 657 00:37:53,705 --> 00:37:56,107 But they have a secret weapon inside their guts. 658 00:37:56,108 --> 00:37:58,042 Microbes. 659 00:37:58,043 --> 00:37:59,810 And some are actually specialists 660 00:37:59,811 --> 00:38:01,780 in breaking down cellulose. 661 00:38:03,281 --> 00:38:05,616 Today, almost every plant-eating animal 662 00:38:05,617 --> 00:38:10,321 {\an8}has outsourced digestion of plant matter to microbes. 663 00:38:10,322 --> 00:38:13,792 {\an8}It's what's known as a "symbiotic relationship." 664 00:38:16,028 --> 00:38:18,329 These relationships are so important that, 665 00:38:18,330 --> 00:38:21,666 over time, evolution drives both species 666 00:38:21,667 --> 00:38:24,101 to become more and more interlocked. 667 00:38:24,102 --> 00:38:26,505 They effectively can't live without each other. 668 00:38:27,773 --> 00:38:29,206 Without the communities 669 00:38:29,207 --> 00:38:31,709 of microbes living in their guts, 670 00:38:31,710 --> 00:38:34,646 most animals wouldn't be able to survive. 671 00:38:35,647 --> 00:38:37,149 Including us. 672 00:38:38,684 --> 00:38:39,984 My body is actually 673 00:38:39,985 --> 00:38:42,621 home to trillions of little microbes. 674 00:38:44,056 --> 00:38:46,190 And that's the microbiome. 675 00:38:46,191 --> 00:38:47,658 In fact there are 676 00:38:47,659 --> 00:38:50,061 trillions of these cells in my gut alone, 677 00:38:50,062 --> 00:38:51,996 four-and-a-half pounds in fact. 678 00:38:51,997 --> 00:38:55,332 Microbes love to live in the guts of organisms 679 00:38:55,333 --> 00:38:59,336 because it's kind of like an all-you-can-eat buffet. 680 00:38:59,337 --> 00:39:01,739 {\an8}The microbes don't have to move in any way shape or form. 681 00:39:01,740 --> 00:39:03,974 {\an8}The animals do the job of eating, 682 00:39:03,975 --> 00:39:06,178 the microbes do the job of breaking it down. 683 00:39:09,081 --> 00:39:11,148 Thanks to their gut microbes, 684 00:39:11,149 --> 00:39:13,785 the early Afrotherians thrive. 685 00:39:16,988 --> 00:39:21,459 Spreading out across Africa, moving into many of the habitats 686 00:39:21,460 --> 00:39:24,096 left open by the giant dinosaurs. 687 00:39:25,931 --> 00:39:28,232 When a lot of animals disappear, 688 00:39:28,233 --> 00:39:30,568 {\an8}when there is a mass extinction, let's say, 689 00:39:30,569 --> 00:39:33,939 {\an8}that means a lot of niches, a lot of jobs open up. 690 00:39:34,973 --> 00:39:37,775 An animal's niche is what it eats, it's where it lives, 691 00:39:37,776 --> 00:39:39,744 it's what it does, it's how it behaves. 692 00:39:39,745 --> 00:39:40,878 And animals can be 693 00:39:40,879 --> 00:39:42,580 really specialized for particular niches, 694 00:39:42,581 --> 00:39:45,216 to eat a particular kind of leaf. 695 00:39:45,217 --> 00:39:47,151 {\an8}Or hummingbirds can be specialized 696 00:39:47,152 --> 00:39:49,820 to have long beaks that allow them to extract nectar 697 00:39:49,821 --> 00:39:51,756 in a way that no other animal does. 698 00:39:51,757 --> 00:39:53,758 So, after a large mass extinction event, 699 00:39:53,759 --> 00:39:56,594 the surviving animals will then evolve often times 700 00:39:56,595 --> 00:39:58,295 to fill these jobs and fill these roles, 701 00:39:58,296 --> 00:39:59,997 um, and that can result in 702 00:39:59,998 --> 00:40:03,602 some really impressive adaptations and innovations. 703 00:40:08,006 --> 00:40:11,175 Some of the descendants of those Afrotherian survivors 704 00:40:11,176 --> 00:40:13,644 still live in Africa today, 705 00:40:13,645 --> 00:40:17,249 their bodies sculpted by the niches they inhabit. 706 00:40:19,985 --> 00:40:21,185 Growing up in Kenya, 707 00:40:21,186 --> 00:40:23,522 there was wildlife all around us. 708 00:40:24,823 --> 00:40:26,157 I got so curious 709 00:40:26,158 --> 00:40:29,059 {\an8}about how they live, how they survive, 710 00:40:29,060 --> 00:40:33,965 {\an8}and that sparked my interest in becoming a biologist. 711 00:40:36,535 --> 00:40:38,903 Biologist Winnie Kiiru 712 00:40:38,904 --> 00:40:42,339 has dedicated her life to understanding and protecting 713 00:40:42,340 --> 00:40:44,942 some of the Afrotherians that survive 714 00:40:44,943 --> 00:40:46,443 to this day. 715 00:40:46,444 --> 00:40:49,781 The Afrotherians are very unique and interesting. 716 00:40:53,285 --> 00:40:55,887 Look at this animal, this is the hyrax. 717 00:40:57,322 --> 00:40:59,658 Looks very much like a rodent, so furry. 718 00:41:02,394 --> 00:41:04,628 A small, nimble plant eater, 719 00:41:04,629 --> 00:41:07,097 the hyrax has retained many 720 00:41:07,098 --> 00:41:08,732 of the characteristics 721 00:41:08,733 --> 00:41:11,002 that helped its ancestors survive. 722 00:41:14,172 --> 00:41:16,040 They climb very well. 723 00:41:16,041 --> 00:41:19,944 They have these amazing soft pads on the bottom of their feet 724 00:41:19,945 --> 00:41:21,278 that are kept 725 00:41:21,279 --> 00:41:22,980 lubricated by their sweat glands, 726 00:41:22,981 --> 00:41:27,151 that allow them to move very well in the trees, 727 00:41:27,152 --> 00:41:29,520 and they can reach the top canopy, 728 00:41:29,521 --> 00:41:32,489 where they're able to find all these different leaves. 729 00:41:32,490 --> 00:41:34,725 And because of being high up, 730 00:41:34,726 --> 00:41:36,961 they can also look out for predators. 731 00:41:36,962 --> 00:41:40,866 The tree hyrax body is shaped for life in the canopies. 732 00:41:42,167 --> 00:41:44,001 They are very well adapted to the niche 733 00:41:44,002 --> 00:41:46,770 that they have found. 734 00:41:46,771 --> 00:41:49,073 And their success is obvious, 735 00:41:49,074 --> 00:41:51,775 because they are very prolific around Nairobi. 736 00:41:51,776 --> 00:41:54,012 You find a lot of them in the gardens here. 737 00:41:56,314 --> 00:41:59,683 While these hyraxes adapted to live in the trees, 738 00:41:59,684 --> 00:42:03,588 other Afrotherians moved into entirely different habitats. 739 00:42:04,756 --> 00:42:07,658 Even if they look like rodents, some of their 740 00:42:07,659 --> 00:42:10,461 closest relatives are actually elephants. 741 00:42:10,462 --> 00:42:12,597 Very unlikely relatives. 742 00:42:14,032 --> 00:42:15,866 Although closely related, 743 00:42:15,867 --> 00:42:18,235 hyraxes and elephants seem to have 744 00:42:18,236 --> 00:42:20,537 completely different bodies, 745 00:42:20,538 --> 00:42:23,174 because they evolved to live in different niches. 746 00:42:25,143 --> 00:42:28,412 The Afrotherians had an excellent advantage 747 00:42:28,413 --> 00:42:32,716 because they were isolated on the African continent 748 00:42:32,717 --> 00:42:36,887 and had very little competition, and so they could actually 749 00:42:36,888 --> 00:42:39,291 populate a whole lot of different niches. 750 00:42:40,525 --> 00:42:43,394 Over countless generations, 751 00:42:43,395 --> 00:42:46,163 the Afrotherians adapted to a wide variety of different 752 00:42:46,164 --> 00:42:48,999 environments and lifestyles, making them 753 00:42:49,000 --> 00:42:53,371 one of Earth's most eclectic groups of animals. 754 00:42:55,173 --> 00:42:58,208 The manatees went into the water. 755 00:42:58,209 --> 00:43:01,111 The elephants went off into the savannahs. 756 00:43:01,112 --> 00:43:05,549 The hyraxes eat all these leaves, high up in the trees. 757 00:43:05,550 --> 00:43:08,319 And then you have the elephant shrews 758 00:43:08,320 --> 00:43:11,188 that went out and ate insects. 759 00:43:11,189 --> 00:43:13,857 And the aardvarks with their long snouts that are able to go 760 00:43:13,858 --> 00:43:16,260 and find food in the termite mounds. 761 00:43:16,261 --> 00:43:18,329 So, with all these 762 00:43:18,330 --> 00:43:19,730 different niches, 763 00:43:19,731 --> 00:43:21,967 it meant they were not competing against each other. 764 00:43:26,037 --> 00:43:29,707 Over time, the early Afrotherians 765 00:43:29,708 --> 00:43:31,942 spread across Africa, 766 00:43:31,943 --> 00:43:34,178 adapting to different niches 767 00:43:34,179 --> 00:43:37,014 that shape their bodies in different ways. 768 00:43:40,385 --> 00:43:44,389 {\an8}Some evolve bigger guts to process more plant material. 769 00:43:45,824 --> 00:43:47,359 {\an8}And a bigger gut... 770 00:43:48,660 --> 00:43:50,929 {\an8}...means a bigger body size. 771 00:44:02,007 --> 00:44:06,143 {\an8}This stocky creature is around three feet tall, 772 00:44:06,144 --> 00:44:09,747 {\an8}much larger than its furry ancestors. 773 00:44:09,748 --> 00:44:11,715 {\an8}It spends most of its time 774 00:44:11,716 --> 00:44:13,418 {\an8}browsing on plants. 775 00:44:14,686 --> 00:44:18,455 {\an8}It's an early Proboscidean, a relative 776 00:44:18,456 --> 00:44:19,924 {\an8}of the elephant. 777 00:44:21,159 --> 00:44:23,193 {\an8}It's evolved elongated incisors 778 00:44:23,194 --> 00:44:25,730 {\an8}for tearing through dense vegetation. 779 00:44:27,732 --> 00:44:31,269 {\an8}But big teeth can have their drawbacks. 780 00:44:33,972 --> 00:44:36,073 Okay, right, so I'll put that on there. 781 00:44:36,074 --> 00:44:37,908 Okay, I got it-- watch your fingers. Like that? 782 00:44:37,909 --> 00:44:38,976 You got it? Okay? Yeah, I got it. 783 00:44:38,977 --> 00:44:40,744 There we go. 784 00:44:40,745 --> 00:44:42,880 Evolution doesn't have a goal. 785 00:44:42,881 --> 00:44:46,383 {\an8}It's not working in a straight line towards some end. 786 00:44:46,384 --> 00:44:49,620 {\an8}All right, yep-- wait, oh! 787 00:44:49,621 --> 00:44:51,756 Evolution finds solutions to problems. 788 00:44:53,258 --> 00:44:55,260 And I got it, yes, comfortable. 789 00:44:56,895 --> 00:45:00,297 {\an8}This is the skull of a walrus. 790 00:45:00,298 --> 00:45:02,466 These tusks can be really long. 791 00:45:02,467 --> 00:45:05,869 They can be up to three feet from the very top 792 00:45:05,870 --> 00:45:08,238 all the way to the bottom. 793 00:45:08,239 --> 00:45:11,175 These are used for mating and for courtship. 794 00:45:11,176 --> 00:45:13,977 It's hard for these to eat with these big tusks in the way. 795 00:45:13,978 --> 00:45:16,613 {\an8}So, what they have to do is they stick their snouts 796 00:45:16,614 --> 00:45:18,682 {\an8}into the mud and then find the little crabs 797 00:45:18,683 --> 00:45:22,853 {\an8}and clams and mollusks, tucking their tusks behind them. 798 00:45:22,854 --> 00:45:24,988 {\an8}Sometimes with evolution you end up 799 00:45:24,989 --> 00:45:28,292 with really strange body plans that you couldn't predict, 800 00:45:28,293 --> 00:45:30,160 that don't seem the obvious 801 00:45:30,161 --> 00:45:31,728 body plan to evolve on Earth. 802 00:45:31,729 --> 00:45:34,264 So, evolution is always a compromise. 803 00:45:34,265 --> 00:45:36,433 Sometimes you solve one problem, 804 00:45:36,434 --> 00:45:37,635 you create another. 805 00:45:39,404 --> 00:45:42,606 Like the walrus, these Proboscideans have 806 00:45:42,607 --> 00:45:45,709 muscular noses that they use to push food towards 807 00:45:45,710 --> 00:45:47,278 their mouths. 808 00:45:49,981 --> 00:45:52,749 But this individual was born with a slightly 809 00:45:52,750 --> 00:45:55,787 longer one, giving it a tiny advantage. 810 00:45:56,955 --> 00:45:59,923 It may not look like much, but it's 811 00:45:59,924 --> 00:46:03,027 the start of something entirely new. 812 00:46:06,664 --> 00:46:10,434 {\an8}Over countless generations, the muscular nose 813 00:46:10,435 --> 00:46:12,804 {\an8}gives better access to food... 814 00:46:14,706 --> 00:46:17,141 {\an8}...fueling an even bigger body, 815 00:46:17,142 --> 00:46:20,744 {\an8}which in turn, drives a longer nose, 816 00:46:20,745 --> 00:46:25,082 {\an8}capable of reaching all the way to the ground. 817 00:46:25,083 --> 00:46:28,452 {\an8}Until, after millions of years, one of the most 818 00:46:28,453 --> 00:46:32,757 {\an8}unusual bodies in the animal kingdom arrives. 819 00:46:37,629 --> 00:46:40,465 This is Palaeomastodon. 820 00:46:41,399 --> 00:46:44,202 It roamed the Earth around 30 million years ago. 821 00:46:45,703 --> 00:46:48,272 It has the trunk and the tusks. 822 00:46:48,273 --> 00:46:50,774 Big ears are still a work in progress, 823 00:46:50,775 --> 00:46:54,612 but it's a body shape that looks a lot like an elephant's. 824 00:46:57,282 --> 00:47:00,851 Palaeomastodon is one of the many extinct ancient relatives 825 00:47:00,852 --> 00:47:02,453 of the elephant. 826 00:47:03,655 --> 00:47:05,389 Originating in Africa, 827 00:47:05,390 --> 00:47:08,358 these relatives soon spread across the globe, 828 00:47:08,359 --> 00:47:10,527 adapting to new niches, 829 00:47:10,528 --> 00:47:12,931 which continue to shape their bodies. 830 00:47:14,766 --> 00:47:18,468 {\an8}Some have bizarre jaw extensions. 831 00:47:18,469 --> 00:47:21,940 Others evolve downward-curving tusks. 832 00:47:25,109 --> 00:47:27,711 And of course, the mammoths, 833 00:47:27,712 --> 00:47:31,048 those hairy icons of the ice age. 834 00:47:31,049 --> 00:47:34,818 With huge guts for digesting masses of plants, 835 00:47:34,819 --> 00:47:38,422 the Proboscideans become some of the largest mammals 836 00:47:38,423 --> 00:47:41,159 to have ever walked the Earth. 837 00:47:45,663 --> 00:47:48,332 Um... 838 00:47:48,333 --> 00:47:50,067 Does that work? Like that? 839 00:47:50,068 --> 00:47:52,036 That's amazing. 840 00:47:53,271 --> 00:47:55,172 This is a replica mammoth tusk. 841 00:47:55,173 --> 00:47:57,074 Uh, it's made of polystyrene, 842 00:47:57,075 --> 00:48:01,245 but even made of polystyrene, it is pretty heavy. 843 00:48:01,246 --> 00:48:02,746 A tusk is really 844 00:48:02,747 --> 00:48:04,982 just an overgrown incisor. 845 00:48:04,983 --> 00:48:06,717 So, that's your front teeth here. 846 00:48:06,718 --> 00:48:08,719 Tusks come in all shapes and sizes, 847 00:48:08,720 --> 00:48:10,287 but it's when you see something 848 00:48:10,288 --> 00:48:12,289 like this on your lap 849 00:48:12,290 --> 00:48:15,760 that you can really appreciate just how enormous they are. 850 00:48:20,098 --> 00:48:22,733 But of this once-diverse group, 851 00:48:22,734 --> 00:48:26,436 some 180 species strong, 852 00:48:26,437 --> 00:48:28,406 only three remain. 853 00:48:30,541 --> 00:48:32,377 The Asian elephant. 854 00:48:33,578 --> 00:48:36,481 The African forest elephant. 855 00:48:39,217 --> 00:48:43,054 And the largest land animal on the planet today. 856 00:48:45,723 --> 00:48:48,626 The African savannah elephant. 857 00:48:55,266 --> 00:48:57,668 If you look at an elephant, I think you will see 858 00:48:57,669 --> 00:49:00,437 three things immediately that stand out. 859 00:49:00,438 --> 00:49:03,507 Their enormous size, their incredible tusks, 860 00:49:03,508 --> 00:49:08,179 and their amazing, unusual, and unique trunk. 861 00:49:10,214 --> 00:49:13,884 It is the ultimate evolutionary multi-tool-- so, 862 00:49:13,885 --> 00:49:15,652 {\an8}you're using your trunk to breathe, 863 00:49:15,653 --> 00:49:17,654 {\an8}you're using it to pick up things from the ground 864 00:49:17,655 --> 00:49:19,890 and bring it to your mouth-- it's amazing. 865 00:49:19,891 --> 00:49:23,193 And it is this extraordinary example 866 00:49:23,194 --> 00:49:25,029 of how evolution works. 867 00:49:27,665 --> 00:49:30,967 With a body that's made of a million billion cells, 868 00:49:30,968 --> 00:49:34,304 and powered by some of the largest lungs 869 00:49:34,305 --> 00:49:35,640 of any land animal... 870 00:49:37,108 --> 00:49:40,410 ...a gut up to 120 feet long, 871 00:49:40,411 --> 00:49:43,047 packed full of microbes for digesting plants... 872 00:49:44,515 --> 00:49:46,784 {\an8}...tusks that never stop growing... 873 00:49:48,519 --> 00:49:52,156 {\an8}...and a nose like nothing else on the planet... 874 00:49:53,858 --> 00:49:56,226 ...the elephant is one of evolution's 875 00:49:56,227 --> 00:49:59,663 most extraordinary creations. 876 00:49:59,664 --> 00:50:02,699 There's something about elephants that speaks to humans. 877 00:50:02,700 --> 00:50:04,134 Maybe it's because they 878 00:50:04,135 --> 00:50:05,702 have been our constant companions 879 00:50:05,703 --> 00:50:07,838 on our own evolutionary journey. 880 00:50:07,839 --> 00:50:11,108 They share a close relationship with us 881 00:50:11,109 --> 00:50:12,609 and an environment with us. 882 00:50:12,610 --> 00:50:15,512 They've been worshipped, and that's clear 883 00:50:15,513 --> 00:50:17,247 from a long history 884 00:50:17,248 --> 00:50:20,618 of, of human interactions with these beautiful creatures. 885 00:50:21,986 --> 00:50:24,287 Four billion years of evolution 886 00:50:24,288 --> 00:50:26,691 shaped the elephant we see today. 887 00:50:29,961 --> 00:50:32,663 But its story is far from over. 888 00:50:33,865 --> 00:50:37,034 Today, humans have become a driving force. 889 00:50:38,603 --> 00:50:42,773 By poaching elephants in huge numbers for their ivory tusks, 890 00:50:42,774 --> 00:50:45,443 we are changing the course of their evolution. 891 00:50:46,878 --> 00:50:48,745 We're starting to see that 892 00:50:48,746 --> 00:50:51,515 {\an8}humans poaching elephants for their tusks 893 00:50:51,516 --> 00:50:55,085 {\an8}is favoring those few elephants 894 00:50:55,086 --> 00:50:57,989 that are naturally tuskless. 895 00:50:59,223 --> 00:51:01,558 There is such strong selection pressure 896 00:51:01,559 --> 00:51:04,327 on some populations that it looks like 897 00:51:04,328 --> 00:51:05,929 there is now an advantage 898 00:51:05,930 --> 00:51:09,666 to those elephants to not have the tusks 899 00:51:09,667 --> 00:51:11,635 that have been part of their evolutionary history 900 00:51:11,636 --> 00:51:14,337 for millions and millions of years, 901 00:51:14,338 --> 00:51:19,444 and the best explanation for that is us and poaching. 902 00:51:22,280 --> 00:51:25,082 But evolution never stands still. 903 00:51:26,217 --> 00:51:28,318 If we get to a place where we're not 904 00:51:28,319 --> 00:51:31,254 hunting tusked elephants, 905 00:51:31,255 --> 00:51:33,924 and having tusks is advantageous, 906 00:51:33,925 --> 00:51:36,693 they'll come back-- that's how evolution works. 907 00:51:40,031 --> 00:51:42,999 {\an8}The elephant is not the end of the journey, 908 00:51:43,000 --> 00:51:47,270 {\an8}just the latest chapter in a four-billion-year story 909 00:51:47,271 --> 00:51:50,341 that began with a single cell. 910 00:51:52,076 --> 00:51:53,910 For me, it's so humbling to know 911 00:51:53,911 --> 00:51:57,080 that we're just part of this puzzle of evolution, 912 00:51:57,081 --> 00:51:58,381 this tree of life. 913 00:51:58,382 --> 00:52:00,417 All life on Earth is related, 914 00:52:00,418 --> 00:52:02,552 and evolution helps us not just 915 00:52:02,553 --> 00:52:04,921 to understand who's related to whom, 916 00:52:04,922 --> 00:52:08,658 but how we got to where we are, and why we are the way we are.