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}NARRATOR: 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 ANDREW GILLIS: 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 00:00:31,499 --> 00:00:33,967 ♪ ♪ 12 00:00:33,968 --> 00:00:36,603 NARRATOR: And this great variety of body shapes 13 00:00:36,604 --> 00:00:40,273 can be traced back to a single origin. 14 00:00:40,274 --> 00:00:41,508 JEFFREY MARLOW: The earliest forms of life 15 00:00:41,509 --> 00:00:44,377 had no arms, heads, ears, noses, or mouths. 16 00:00:44,378 --> 00:00:45,979 They were single cells. 17 00:00:45,980 --> 00:00:48,715 NARRATOR: So how did life get from the microscopic 18 00:00:48,716 --> 00:00:52,318 to a world of giants? 19 00:00:52,319 --> 00:00:55,388 {\an8}RIKA ANDERSON: When the first cells found a way to stick together, 20 00:00:55,389 --> 00:00:56,456 {\an8}it paved the way 21 00:00:56,457 --> 00:00:57,857 {\an8}for bodies to form. 22 00:00:57,858 --> 00:00:59,626 {\an8}PETER GIRGUIS: And at that point, 23 00:00:59,627 --> 00:01:02,663 the whole diversity of animals just goes through the roof. 24 00:01:03,798 --> 00:01:06,933 NARRATOR: But to get bigger and more complex, 25 00:01:06,934 --> 00:01:09,336 bodies needed to harness more power... 26 00:01:10,304 --> 00:01:12,806 MARY DROSER: Oxygen was a game changer in terms of body plans, 27 00:01:12,807 --> 00:01:15,308 {\an8}in terms of complexity. 28 00:01:15,309 --> 00:01:18,111 {\an8}NARRATOR: ...team up with other organisms... 29 00:01:18,112 --> 00:01:19,813 {\an8}My body is actually home 30 00:01:19,814 --> 00:01:22,048 {\an8}to trillions of little microbes. 31 00:01:22,049 --> 00:01:24,751 {\an8}NARRATOR: ...and evolve new tools. 32 00:01:24,752 --> 00:01:26,319 {\an8}VICTORIA HERRIDGE: It's when you see something 33 00:01:26,320 --> 00:01:28,154 {\an8}like this on your lap 34 00:01:28,155 --> 00:01:31,157 that you can really appreciate just how enormous they are. 35 00:01:31,158 --> 00:01:32,826 ♪ ♪ 36 00:01:32,827 --> 00:01:35,929 NARRATOR: How did life evolve from a single cell 37 00:01:35,930 --> 00:01:39,500 to the spectacular array of shapes we see today? 38 00:01:40,968 --> 00:01:45,038 "Evolution: Body Builders." 39 00:01:45,039 --> 00:01:48,442 Right now, on "NOVA." 40 00:01:51,912 --> 00:01:56,951 ♪ ♪ 41 00:02:07,828 --> 00:02:09,663 (elephant trumpets) 42 00:02:12,466 --> 00:02:16,070 ♪ ♪ 43 00:02:18,472 --> 00:02:20,507 (rumbling) 44 00:02:20,508 --> 00:02:22,542 NARRATOR: The elephant has to be 45 00:02:22,543 --> 00:02:25,679 one of evolution's strangest creatures... 46 00:02:28,682 --> 00:02:31,351 ...with an extraordinary nose 47 00:02:31,352 --> 00:02:34,087 that's as dexterous as a hand. 48 00:02:34,088 --> 00:02:37,223 GILLIS: The trunk is really remarkable. 49 00:02:37,224 --> 00:02:39,325 {\an8}It has the strength to move a tree, 50 00:02:39,326 --> 00:02:42,862 {\an8}but it can also function in such a fine way 51 00:02:42,863 --> 00:02:44,598 {\an8}that it can pick up a single blade of grass. 52 00:02:46,233 --> 00:02:49,669 NARRATOR: Enormous tusks that never stop growing. 53 00:02:49,670 --> 00:02:53,006 STEVE BRUSATTE: The tusks can grow up to ten-and-a-half feet long, 54 00:02:53,007 --> 00:02:56,709 and together, they can weigh more than 440 pounds. 55 00:02:56,710 --> 00:02:59,178 ♪ ♪ 56 00:02:59,179 --> 00:03:00,814 NARRATOR: And a gigantic appetite. 57 00:03:02,249 --> 00:03:05,351 SURESH SINGH: A huge amount of energy is needed to fuel an elephant. 58 00:03:05,352 --> 00:03:08,354 We're talking feeding for 18 hours a day 59 00:03:08,355 --> 00:03:12,092 {\an8}and eating four percent of their body weight in plants. 60 00:03:13,427 --> 00:03:17,063 NARRATOR: But perhaps their most striking trait of all 61 00:03:17,064 --> 00:03:19,232 is their size. 62 00:03:19,233 --> 00:03:20,934 HERRIDGE: Elephants define 63 00:03:20,935 --> 00:03:24,304 what we understand about being big. 64 00:03:24,305 --> 00:03:28,107 {\an8}♪ ♪ 65 00:03:28,108 --> 00:03:30,210 {\an8}Their feet are like tree trunks. 66 00:03:31,345 --> 00:03:33,880 {\an8}HERRIDGE: Some of them weigh up to maybe 11 tons-- 67 00:03:33,881 --> 00:03:35,149 {\an8}that's as much as a school bus. 68 00:03:37,551 --> 00:03:41,121 NARRATOR: So how did this creature's enormous body come to be? 69 00:03:42,423 --> 00:03:44,090 (rumbles) 70 00:03:44,091 --> 00:03:46,059 Elephants are the largest living animals 71 00:03:46,060 --> 00:03:48,761 that we have on the land today, 72 00:03:48,762 --> 00:03:53,067 but they are all just built of many, many, many cells. 73 00:03:54,468 --> 00:03:56,235 BRUSATTE: Cells are ultimately 74 00:03:56,236 --> 00:03:58,137 the fundamental building blocks of bodies. 75 00:03:58,138 --> 00:04:01,207 GILLIS: The elephant is really a marvel of evolution, 76 00:04:01,208 --> 00:04:02,475 because it reflects 77 00:04:02,476 --> 00:04:05,579 a quadrillion cells all working together. 78 00:04:07,414 --> 00:04:10,383 NARRATOR: But where did those cells come from 79 00:04:10,384 --> 00:04:13,620 and how did they come together to form bodies? 80 00:04:13,621 --> 00:04:16,489 ♪ ♪ 81 00:04:16,490 --> 00:04:19,325 To answer that, 82 00:04:19,326 --> 00:04:21,494 we have to reach back into the distant past... 83 00:04:21,495 --> 00:04:25,198 ♪ ♪ 84 00:04:25,199 --> 00:04:28,668 {\an8}...to a time long before the first elephants, 85 00:04:28,669 --> 00:04:32,905 {\an8}before the first mammals, 86 00:04:32,906 --> 00:04:35,843 {\an8}before there were any animals at all... 87 00:04:37,945 --> 00:04:40,747 ...four billion years back, 88 00:04:40,748 --> 00:04:43,416 to some of the earliest life on Earth. 89 00:04:43,417 --> 00:04:44,951 ♪ ♪ 90 00:04:44,952 --> 00:04:47,053 This is the extraordinary story 91 00:04:47,054 --> 00:04:52,458 {\an8}of how evolution transformed a single cell 92 00:04:52,459 --> 00:04:56,463 {\an8}into the largest land animal alive today. 93 00:05:00,167 --> 00:05:06,206 ♪ ♪ 94 00:05:14,148 --> 00:05:18,185 {\an8}♪ ♪ 95 00:05:22,856 --> 00:05:27,660 Violent and unpredictable, 96 00:05:27,661 --> 00:05:31,765 the young Earth is hostile to life. 97 00:05:33,067 --> 00:05:34,400 But beneath the waves, 98 00:05:34,401 --> 00:05:37,137 simple cells cling on... 99 00:05:40,007 --> 00:05:42,575 ...locked in a relentless struggle for survival. 100 00:05:42,576 --> 00:05:44,911 Most are doomed, 101 00:05:44,912 --> 00:05:47,781 becoming evolutionary dead ends. 102 00:05:49,616 --> 00:05:53,320 But some scientists think one cell is different. 103 00:05:55,556 --> 00:05:57,924 This is LUCA, 104 00:05:57,925 --> 00:06:00,359 the elephant's ancient ancestor 105 00:06:00,360 --> 00:06:05,565 and, in fact, the ancestor of all life on Earth. 106 00:06:05,566 --> 00:06:08,401 GIRGUIS: The Last Universal Common Ancestor, 107 00:06:08,402 --> 00:06:10,136 or LUCA for short, 108 00:06:10,137 --> 00:06:12,538 {\an8}is the organism that gave rise, 109 00:06:12,539 --> 00:06:15,408 {\an8}quite literally, to all life we see today. 110 00:06:15,409 --> 00:06:18,177 {\an8}MARLOW: It wasn't necessarily the first form of life. 111 00:06:18,178 --> 00:06:19,746 {\an8}It almost certainly wasn't. 112 00:06:19,747 --> 00:06:21,414 {\an8}But it's the one that got by. 113 00:06:21,415 --> 00:06:25,985 {\an8}LUCA, that great single common ancestor of all life today, 114 00:06:25,986 --> 00:06:29,389 {\an8}that was the starting point for bodies. 115 00:06:31,225 --> 00:06:35,662 NARRATOR: Scientists don't know for sure how LUCA's lineage survives. 116 00:06:36,830 --> 00:06:39,399 Perhaps it's just lucky. 117 00:06:40,968 --> 00:06:44,738 But what we do know is that LUCA can copy itself. 118 00:06:46,740 --> 00:06:49,208 Each new generation inherits the traits 119 00:06:49,209 --> 00:06:52,311 that helped LUCA persist, 120 00:06:52,312 --> 00:06:56,616 but the process is not perfect. 121 00:06:56,617 --> 00:07:01,854 Small mutations cause some copies to turn out differently. 122 00:07:01,855 --> 00:07:05,024 These tiny glitches in the genetic code 123 00:07:05,025 --> 00:07:06,726 may be mistakes, 124 00:07:06,727 --> 00:07:10,496 but sometimes, mistakes can be useful. 125 00:07:10,497 --> 00:07:12,632 DROSER: Maybe it's a mutation 126 00:07:12,633 --> 00:07:13,800 that makes an organism 127 00:07:13,801 --> 00:07:15,334 a little bit bigger than it might've been, 128 00:07:15,335 --> 00:07:16,335 or a little bit faster 129 00:07:16,336 --> 00:07:17,336 than it might've been. 130 00:07:17,337 --> 00:07:18,671 A little mutation here, 131 00:07:18,672 --> 00:07:19,772 a little mutation there, 132 00:07:19,773 --> 00:07:23,476 that's how evolution works. 133 00:07:23,477 --> 00:07:24,944 {\an8}Evolution isn't goal-directed. 134 00:07:24,945 --> 00:07:26,279 {\an8}It doesn't have a purpose. 135 00:07:26,280 --> 00:07:28,314 {\an8}It's built entirely on these tiny little mistakes 136 00:07:28,315 --> 00:07:30,717 that allow life to take a new pathway. 137 00:07:32,152 --> 00:07:36,722 NARRATOR: The journey to more complex bodies has begun. 138 00:07:36,723 --> 00:07:39,192 Generation after generation, 139 00:07:39,193 --> 00:07:41,727 small mutations accumulate, 140 00:07:41,728 --> 00:07:44,664 becoming big differences. 141 00:07:44,665 --> 00:07:48,634 LUCA is the seed from which the tree of life as we know it 142 00:07:48,635 --> 00:07:50,236 begins to grow. 143 00:07:50,237 --> 00:07:54,808 ♪ ♪ 144 00:07:58,679 --> 00:08:00,948 As time passes... 145 00:08:03,116 --> 00:08:06,386 ...LUCA's descendants flourish and spread. 146 00:08:09,990 --> 00:08:13,059 Some adapt to capture sunlight, 147 00:08:13,060 --> 00:08:14,928 the ancestors of plants. 148 00:08:17,431 --> 00:08:19,799 Others evolve to become fungi. 149 00:08:19,800 --> 00:08:24,270 ♪ ♪ 150 00:08:24,271 --> 00:08:26,639 Many stay small and simple, 151 00:08:26,640 --> 00:08:30,444 giving rise to the hidden microbial world. 152 00:08:31,478 --> 00:08:35,916 But all of them descendants of that sole ancestor. 153 00:08:37,885 --> 00:08:41,153 Although complex bodies still lie far in the future, 154 00:08:41,154 --> 00:08:44,724 life on Earth is already thriving. 155 00:08:44,725 --> 00:08:48,761 ♪ ♪ 156 00:08:48,762 --> 00:08:51,998 Until disaster strikes. 157 00:08:51,999 --> 00:08:55,468 ♪ ♪ 158 00:08:55,469 --> 00:08:57,803 Natural variations in the Earth's climate 159 00:08:57,804 --> 00:09:00,707 plunge the planet into a deep freeze. 160 00:09:02,109 --> 00:09:06,445 {\an8}♪ ♪ 161 00:09:06,446 --> 00:09:11,550 {\an8}This period of global cooling is so extreme, 162 00:09:11,551 --> 00:09:14,855 {\an8}it's called Snowball Earth. 163 00:09:19,126 --> 00:09:21,761 {\an8}ANDERSON: Snowball Earth was a time when ice sheets 164 00:09:21,762 --> 00:09:24,030 swept all the way to almost the Equator 165 00:09:24,031 --> 00:09:26,565 and our seas were almost entirely frozen over. 166 00:09:26,566 --> 00:09:28,968 It was an extremely cold time 167 00:09:28,969 --> 00:09:31,837 and it was an extremely harsh environment for life to survive. 168 00:09:31,838 --> 00:09:37,443 ♪ ♪ 169 00:09:37,444 --> 00:09:39,078 GIRGUIS: It's not the cold of Snowball Earth 170 00:09:39,079 --> 00:09:40,713 that was the problem per se, 171 00:09:40,714 --> 00:09:43,316 but it was the fact that the amount of food available 172 00:09:43,317 --> 00:09:44,518 really tanked. 173 00:09:46,086 --> 00:09:49,822 FRANKIE DUNN: Living during Snowball Earth was the ultimate survival test. 174 00:09:49,823 --> 00:09:51,590 It would've been really hard 175 00:09:51,591 --> 00:09:53,193 to find enough food to make a living. 176 00:09:55,929 --> 00:09:58,831 NARRATOR: Floating in these ice-bound oceans, 177 00:09:58,832 --> 00:10:00,566 one of LUCA's descendants 178 00:10:00,567 --> 00:10:02,669 is desperately searching for a meal. 179 00:10:07,107 --> 00:10:10,543 Scientists call this hypothetical tiny organism 180 00:10:10,544 --> 00:10:12,346 Urchoanozoan. 181 00:10:17,851 --> 00:10:19,952 GILLIS: Urchoanozoa can be thought of 182 00:10:19,953 --> 00:10:22,723 as the last single-celled ancestor of animals. 183 00:10:24,458 --> 00:10:26,726 {\an8}This is still a relatively simple organism, 184 00:10:26,727 --> 00:10:28,995 {\an8}but it had a few features that set it apart 185 00:10:28,996 --> 00:10:30,896 {\an8}from other single-celled organisms. 186 00:10:30,897 --> 00:10:32,531 DUNN: So it would've had a flagellum, 187 00:10:32,532 --> 00:10:36,268 which is like a whip-like tail sticking out of one end, 188 00:10:36,269 --> 00:10:38,338 and this would have powered it through the water. 189 00:10:40,841 --> 00:10:42,775 NARRATOR: Over billions of years, 190 00:10:42,776 --> 00:10:45,245 life evolved these new traits. 191 00:10:47,514 --> 00:10:50,783 But one thing hasn't changed. 192 00:10:50,784 --> 00:10:55,755 Urchoanozoan is still tiny, just a single cell, 193 00:10:55,756 --> 00:10:59,025 and there are only so many ways a single cell can find food. 194 00:10:59,026 --> 00:11:01,794 ♪ ♪ 195 00:11:01,795 --> 00:11:03,929 But in its battle for survival, 196 00:11:03,930 --> 00:11:09,502 Urchoanozoan will change the shape of life on Earth forever. 197 00:11:09,503 --> 00:11:12,272 ♪ ♪ 198 00:11:15,342 --> 00:11:18,445 (birds chirping) 199 00:11:23,850 --> 00:11:26,652 To illustrate the innovation 200 00:11:26,653 --> 00:11:29,655 that transformed life all those millions of years ago, 201 00:11:29,656 --> 00:11:33,025 evolutionary biologist Will Ratcliff 202 00:11:33,026 --> 00:11:35,428 doesn't need a time machine, 203 00:11:35,429 --> 00:11:38,432 just a remarkable creature alive today. 204 00:11:39,800 --> 00:11:41,167 {\an8}(laughs): Wow. 205 00:11:41,168 --> 00:11:43,270 {\an8}Look at this. 206 00:11:44,938 --> 00:11:46,306 {\an8}There are thousands of ants here. 207 00:11:51,178 --> 00:11:53,179 {\an8}Look at this little ant. 208 00:11:53,180 --> 00:11:56,248 Alone, doesn't stand much of a chance, does it? 209 00:11:56,249 --> 00:11:58,118 It's currently biting into my finger. 210 00:11:59,186 --> 00:12:01,087 I won't lie, it's somewhat painful, 211 00:12:01,088 --> 00:12:03,189 but let's face it, it's not much of a match. 212 00:12:03,190 --> 00:12:04,724 I could squish it right now if I wanted to. 213 00:12:06,560 --> 00:12:08,360 This is a fact of life: 214 00:12:08,361 --> 00:12:11,263 when you're on your own, you have to make it all work. 215 00:12:11,264 --> 00:12:13,099 You have to find food, reproduce, 216 00:12:13,100 --> 00:12:14,701 survive much larger opponents. 217 00:12:15,869 --> 00:12:17,903 The superpower of these ants, however, 218 00:12:17,904 --> 00:12:19,206 is that they're not alone. 219 00:12:20,707 --> 00:12:24,009 NARRATOR: Driver ants join forces in colonies 220 00:12:24,010 --> 00:12:26,513 up to 20 million strong. 221 00:12:28,448 --> 00:12:30,349 RATCLIFF: And as they move through the forest, 222 00:12:30,350 --> 00:12:33,385 even prey that is thousands of times larger than the ants are 223 00:12:33,386 --> 00:12:37,357 is quickly overwhelmed if it cannot run, hop, or fly away. 224 00:12:39,259 --> 00:12:42,228 {\an8}In fact, right here, there seems to be an insect 225 00:12:42,229 --> 00:12:46,166 {\an8}that has been balled over by about 500 ants. 226 00:12:49,769 --> 00:12:52,171 They overcome the limitations of size 227 00:12:52,172 --> 00:12:54,107 through sheer strength in numbers. 228 00:12:55,775 --> 00:12:57,143 And collectively, 229 00:12:57,144 --> 00:12:59,378 they're able to move through this forest 230 00:12:59,379 --> 00:13:00,546 finding prey, 231 00:13:00,547 --> 00:13:03,249 dismembering it joint by joint, 232 00:13:03,250 --> 00:13:05,084 bringing it back to the nest, 233 00:13:05,085 --> 00:13:06,919 feeding it to their offspring. 234 00:13:06,920 --> 00:13:09,054 And together, they are able to accomplish 235 00:13:09,055 --> 00:13:11,624 what any one individual ant could not. 236 00:13:11,625 --> 00:13:12,992 And in fact, 237 00:13:12,993 --> 00:13:16,862 they've hit upon a common motif in evolutionary biology, 238 00:13:16,863 --> 00:13:18,797 which is that with numbers, 239 00:13:18,798 --> 00:13:20,333 organisms gain new abilities. 240 00:13:23,436 --> 00:13:24,837 NARRATOR: When you're tiny, 241 00:13:24,838 --> 00:13:28,108 cooperation can bring big rewards. 242 00:13:30,043 --> 00:13:32,745 Over 600 million years ago, 243 00:13:32,746 --> 00:13:35,447 beneath the ice of Snowball Earth, 244 00:13:35,448 --> 00:13:39,352 it's thought Urchoanozoan is different. 245 00:13:41,721 --> 00:13:44,990 The sticky molecules that cover its surface 246 00:13:44,991 --> 00:13:48,995 allow it to do something its ancestors haven't done before. 247 00:13:51,498 --> 00:13:56,268 For a single cell, this trait barely matters. 248 00:13:56,269 --> 00:13:58,704 But when it's time to divide, 249 00:13:58,705 --> 00:14:00,806 instead of drifting apart, 250 00:14:00,807 --> 00:14:04,543 the two cells remain stuck together. 251 00:14:04,544 --> 00:14:07,613 {\an8}♪ ♪ 252 00:14:07,614 --> 00:14:09,583 {\an8}And they multiply... 253 00:14:11,751 --> 00:14:16,355 {\an8}...passing this ability on to their descendants 254 00:14:16,356 --> 00:14:19,826 {\an8}until a vast alliance of cells forms. 255 00:14:20,994 --> 00:14:23,462 {\an8}♪ ♪ 256 00:14:23,463 --> 00:14:28,100 {\an8}Some scientists call this hypothetical colony 257 00:14:28,101 --> 00:14:29,936 the Death Star. 258 00:14:34,674 --> 00:14:39,178 Much larger than a single cell, 259 00:14:39,179 --> 00:14:42,581 it can do something extraordinary. 260 00:14:42,582 --> 00:14:47,653 ♪ ♪ 261 00:14:47,654 --> 00:14:49,855 By beating their flagella in tandem, 262 00:14:49,856 --> 00:14:53,058 the cells create water currents 263 00:14:53,059 --> 00:14:57,897 that suck in food, like smaller microbes, more efficiently. 264 00:15:03,036 --> 00:15:06,639 A collection of cells all working together, 265 00:15:06,640 --> 00:15:10,042 {\an8}this is the first multicellular organism 266 00:15:10,043 --> 00:15:13,512 {\an8}and a major evolutionary step 267 00:15:13,513 --> 00:15:16,316 {\an8}toward animal bodies as we know them. 268 00:15:19,753 --> 00:15:21,787 Like every other revolution in evolution, 269 00:15:21,788 --> 00:15:23,656 multicellularity was an accident. 270 00:15:23,657 --> 00:15:25,090 It was a mutation that allowed cells 271 00:15:25,091 --> 00:15:26,526 to stick together. 272 00:15:27,994 --> 00:15:30,462 DROSER: Multicellularity happened many times 273 00:15:30,463 --> 00:15:31,897 across all different 274 00:15:31,898 --> 00:15:33,565 groups of organisms, 275 00:15:33,566 --> 00:15:37,503 {\an8}so multicellularity in general originated 276 00:15:37,504 --> 00:15:39,838 {\an8}probably 25 or more times, 277 00:15:39,839 --> 00:15:41,106 {\an8}but in animals, 278 00:15:41,107 --> 00:15:44,209 {\an8}there was only one moment of multicellularity. 279 00:15:44,210 --> 00:15:45,411 {\an8}ANDERSON: For animals, 280 00:15:45,412 --> 00:15:46,979 it turned out to be a winning strategy, 281 00:15:46,980 --> 00:15:48,647 because once they became multicellular, 282 00:15:48,648 --> 00:15:50,116 they never went back. 283 00:15:52,152 --> 00:15:54,386 {\an8}NARRATOR: The so-called Death Star 284 00:15:54,387 --> 00:15:55,921 {\an8}can gather food more effectively 285 00:15:55,922 --> 00:15:58,457 {\an8}than its single-celled cousins, 286 00:15:58,458 --> 00:16:01,628 {\an8}thriving in even the harshest conditions. 287 00:16:03,363 --> 00:16:05,497 {\an8}But this is just the start 288 00:16:05,498 --> 00:16:08,301 {\an8}of what a multicellular organism can do. 289 00:16:10,770 --> 00:16:13,772 {\an8}(birds twittering) 290 00:16:13,773 --> 00:16:16,275 {\an8}The advantages of working together 291 00:16:16,276 --> 00:16:18,611 {\an8}are evident in nature today. 292 00:16:19,913 --> 00:16:24,350 {\an8}(rustling) 293 00:16:24,351 --> 00:16:25,651 {\an8}These ants are not 294 00:16:25,652 --> 00:16:27,086 {\an8}a simple collection of cells. 295 00:16:27,087 --> 00:16:28,620 {\an8}Instead, 296 00:16:28,621 --> 00:16:30,756 {\an8}they are an organized, 297 00:16:30,757 --> 00:16:31,957 {\an8}integrated society, 298 00:16:31,958 --> 00:16:35,228 where every ant has its role to play. 299 00:16:36,663 --> 00:16:39,998 Look at the size of these soldiers. 300 00:16:39,999 --> 00:16:42,634 They are at least ten times larger 301 00:16:42,635 --> 00:16:44,002 than their worker sisters. 302 00:16:44,003 --> 00:16:45,137 These soldiers, 303 00:16:45,138 --> 00:16:47,139 {\an8}with their massive mandibles, 304 00:16:47,140 --> 00:16:48,374 {\an8}defend the colony. 305 00:16:48,375 --> 00:16:50,042 The much smaller workers 306 00:16:50,043 --> 00:16:53,379 find food and raise the offspring. 307 00:16:53,380 --> 00:16:56,081 They dedicate their energy and their lives 308 00:16:56,082 --> 00:16:57,617 {\an8}to the colony's success. 309 00:17:01,221 --> 00:17:05,057 Specialization is a crucial evolutionary milestone. 310 00:17:05,058 --> 00:17:06,291 And much like these ants, 311 00:17:06,292 --> 00:17:09,128 in early multicellular organisms, 312 00:17:09,129 --> 00:17:12,464 cells began to take on specialized roles 313 00:17:12,465 --> 00:17:14,566 and get new functions 314 00:17:14,567 --> 00:17:16,870 that their single-celled ancestors couldn't have done. 315 00:17:19,439 --> 00:17:24,576 NARRATOR: When groups of cells began taking on different roles, 316 00:17:24,577 --> 00:17:27,313 they unleashed the full power of multicellularity. 317 00:17:28,815 --> 00:17:30,048 If you are a single cell, 318 00:17:30,049 --> 00:17:31,884 you have to do everything all the time. 319 00:17:31,885 --> 00:17:35,120 If you are a collection of multiple different cells, 320 00:17:35,121 --> 00:17:37,356 then you can develop organs and tissues 321 00:17:37,357 --> 00:17:39,825 which can specialize to do different things 322 00:17:39,826 --> 00:17:42,261 and give you an advantage in the world that you live in. 323 00:17:42,262 --> 00:17:43,729 GILLIS: So you can have some cells 324 00:17:43,730 --> 00:17:46,965 that become very specialist at gathering food, 325 00:17:46,966 --> 00:17:49,201 other cells that are involved in motion, 326 00:17:49,202 --> 00:17:51,970 other cells that are involved in reproduction. 327 00:17:51,971 --> 00:17:54,740 And this specialization really leads to 328 00:17:54,741 --> 00:17:56,942 more efficiency and more possibility. 329 00:17:56,943 --> 00:17:59,912 This was a turning point in the evolution of life, 330 00:17:59,913 --> 00:18:01,346 because it allowed for 331 00:18:01,347 --> 00:18:03,015 the explosion of shapes that we see 332 00:18:03,016 --> 00:18:04,784 on Earth today. 333 00:18:06,753 --> 00:18:09,121 ♪ ♪ 334 00:18:09,122 --> 00:18:11,523 NARRATOR: Over millions of years, 335 00:18:11,524 --> 00:18:14,359 the descendants of the Death Star's cells 336 00:18:14,360 --> 00:18:16,628 continue to specialize. 337 00:18:16,629 --> 00:18:20,933 ♪ ♪ 338 00:18:20,934 --> 00:18:23,268 Sex cells, nerve cells, 339 00:18:23,269 --> 00:18:26,738 and muscle cells evolve, 340 00:18:26,739 --> 00:18:31,677 until what began as a chance mutation 341 00:18:31,678 --> 00:18:35,482 has become something revolutionary. 342 00:18:38,618 --> 00:18:41,554 {\an8}The age of bodies has arrived. 343 00:18:49,629 --> 00:18:53,031 Just 100 million years after those cells 344 00:18:53,032 --> 00:18:55,201 first joined forces... 345 00:18:58,438 --> 00:18:59,638 {\an8}...the oceans are filled 346 00:18:59,639 --> 00:19:02,841 {\an8}with many different body shapes and forms. 347 00:19:02,842 --> 00:19:06,812 {\an8}♪ ♪ 348 00:19:06,813 --> 00:19:09,181 Among them, 349 00:19:09,182 --> 00:19:12,952 an ancient ancestor of the elephant. 350 00:19:15,488 --> 00:19:17,256 No tusks... 351 00:19:17,257 --> 00:19:19,424 (booming echo) 352 00:19:19,425 --> 00:19:21,361 no trunk. 353 00:19:25,131 --> 00:19:27,933 Just a tiny worm, 354 00:19:27,934 --> 00:19:29,902 a fraction of an inch long, 355 00:19:29,903 --> 00:19:32,071 wriggling across the sea floor. 356 00:19:33,873 --> 00:19:36,108 Compared to what came before it, 357 00:19:36,109 --> 00:19:38,810 it's incredibly complex, 358 00:19:38,811 --> 00:19:41,314 but it's still tiny. 359 00:19:43,883 --> 00:19:46,885 Bigger, stronger, more powerful bodies 360 00:19:46,886 --> 00:19:51,189 require something that ancient creatures lacked... 361 00:19:51,190 --> 00:19:52,358 oxygen. 362 00:19:54,127 --> 00:19:57,796 DROSER: Oxygen allows animals to survive. 363 00:19:57,797 --> 00:20:02,167 Animals need oxygen in order to eat, in order to move. 364 00:20:02,168 --> 00:20:05,571 BRUSATTE: Oxygen is like rocket fuel for living things. 365 00:20:05,572 --> 00:20:07,005 It's highly reactive, 366 00:20:07,006 --> 00:20:08,874 {\an8}it's highly energetic, 367 00:20:08,875 --> 00:20:11,210 {\an8}and it was a game changer for the evolution of bodies. 368 00:20:12,445 --> 00:20:17,015 NARRATOR: Animals use oxygen to convert food into energy. 369 00:20:17,016 --> 00:20:21,653 But in these ancient oceans, oxygen is scarce, 370 00:20:21,654 --> 00:20:26,458 and early animals have limited ways to harness it. 371 00:20:26,459 --> 00:20:27,993 GILLIS: Simpler animals 372 00:20:27,994 --> 00:20:29,928 {\an8}would get oxygen primarily 373 00:20:29,929 --> 00:20:31,763 {\an8}by absorbing it through their skin. 374 00:20:31,764 --> 00:20:33,632 DUNN: In fact, there are lots of animals today 375 00:20:33,633 --> 00:20:36,001 which still breathe through their skin. 376 00:20:36,002 --> 00:20:37,636 So, earthworms breathe 377 00:20:37,637 --> 00:20:40,138 through their skin, but there are limitations. 378 00:20:40,139 --> 00:20:41,807 SAM GILES: You need to stay moist, 379 00:20:41,808 --> 00:20:43,075 {\an8}so you have to stay either 380 00:20:43,076 --> 00:20:44,843 {\an8}in the water or near a water environment 381 00:20:44,844 --> 00:20:46,278 {\an8}to stay damp. 382 00:20:46,279 --> 00:20:47,980 {\an8}There's also constraints on how big you can get 383 00:20:47,981 --> 00:20:50,283 {\an8}if you're only absorbing oxygen through your skin. 384 00:20:52,352 --> 00:20:55,087 NARRATOR: Unable to absorb much oxygen, 385 00:20:55,088 --> 00:20:58,624 some early animals are small and slow. 386 00:20:58,625 --> 00:21:03,128 But this little worm has something special. 387 00:21:03,129 --> 00:21:06,565 Not all structures, when they appear, are brand new. 388 00:21:06,566 --> 00:21:09,635 Evolution will often act on previous structures, 389 00:21:09,636 --> 00:21:10,970 which might be used for a different purpose. 390 00:21:15,141 --> 00:21:18,577 NARRATOR: The worm's sides are lined with tiny pores 391 00:21:18,578 --> 00:21:22,247 that it uses to filter food. 392 00:21:22,248 --> 00:21:25,817 But this worm has a special advantage, 393 00:21:25,818 --> 00:21:29,689 making it stronger and more likely to survive. 394 00:21:32,692 --> 00:21:33,825 Its pores 395 00:21:33,826 --> 00:21:37,162 are slightly wrinkled on the inside, 396 00:21:37,163 --> 00:21:40,432 meaning more surface area to filter food, 397 00:21:40,433 --> 00:21:44,537 and also oxygen from the water. 398 00:21:47,306 --> 00:21:50,475 {\an8}♪ ♪ 399 00:21:50,476 --> 00:21:53,746 {\an8}It passes these wrinkles on to its offspring. 400 00:21:55,481 --> 00:21:57,382 {\an8}Over generations, 401 00:21:57,383 --> 00:22:00,218 {\an8}they deepen into folds, 402 00:22:00,219 --> 00:22:03,722 {\an8}greatly increasing their surface area. 403 00:22:03,723 --> 00:22:08,493 {\an8}Until pores that were once used for feeding 404 00:22:08,494 --> 00:22:10,697 {\an8}are repurposed for breathing. 405 00:22:12,665 --> 00:22:16,035 {\an8}It's the beginning of gills. 406 00:22:17,270 --> 00:22:19,037 {\an8}Gills were one of the major 407 00:22:19,038 --> 00:22:20,238 {\an8}evolutionary novelties 408 00:22:20,239 --> 00:22:21,907 {\an8}in the history of 409 00:22:21,908 --> 00:22:23,909 {\an8}animal evolution on Earth, 410 00:22:23,910 --> 00:22:26,578 because it allowed animals to get more oxygen, 411 00:22:26,579 --> 00:22:28,413 and anything that allows 412 00:22:28,414 --> 00:22:31,817 an animal to get more oxygen is a win. 413 00:22:31,818 --> 00:22:34,553 GILLIS: I can illustrate the power of gills 414 00:22:34,554 --> 00:22:37,489 using the art of origami. 415 00:22:37,490 --> 00:22:39,624 So, if you imagine this is a surface that's being used 416 00:22:39,625 --> 00:22:41,793 to take up oxygen from the water, 417 00:22:41,794 --> 00:22:44,029 we have this surface area 418 00:22:44,030 --> 00:22:45,964 that's available for oxygen uptake, 419 00:22:45,965 --> 00:22:47,499 and that's kind of limiting. 420 00:22:47,500 --> 00:22:52,471 But if instead we have a folded shape of the same size, 421 00:22:52,472 --> 00:22:55,207 you can actually increase that surface area 422 00:22:55,208 --> 00:22:57,175 massively in the same space, 423 00:22:57,176 --> 00:22:58,810 and enhance the ability 424 00:22:58,811 --> 00:23:00,613 to take up oxygen from the water. 425 00:23:01,881 --> 00:23:06,718 {\an8}So, simply folding tissue to increase the surface area 426 00:23:06,719 --> 00:23:08,988 is such a beautiful solution to a complex problem. 427 00:23:10,623 --> 00:23:14,259 {\an8}NARRATOR: Gills supercharge the uptake of oxygen, 428 00:23:14,260 --> 00:23:19,264 {\an8}and more oxygen means more energy harnessed from food. 429 00:23:19,265 --> 00:23:21,933 {\an8}DUNN: Gills were an absolute gamechanger. 430 00:23:21,934 --> 00:23:23,602 {\an8}They allowed for much, much 431 00:23:23,603 --> 00:23:25,537 {\an8}more easy energy capture, 432 00:23:25,538 --> 00:23:29,207 which fueled faster, bigger, stronger bodies. 433 00:23:29,208 --> 00:23:31,409 And you can see the success of gills because they're present 434 00:23:31,410 --> 00:23:33,946 in so many different animal groups today. 435 00:23:35,281 --> 00:23:37,149 NARRATOR: Gills come in many shapes 436 00:23:37,150 --> 00:23:40,252 and sizes... 437 00:23:40,253 --> 00:23:43,655 {\an8}from the tiny clown fish 438 00:23:43,656 --> 00:23:47,392 to the giant whale shark. 439 00:23:47,393 --> 00:23:51,329 Some have buried them deep inside their bodies. 440 00:23:51,330 --> 00:23:55,634 Others carry them on the outside. 441 00:23:55,635 --> 00:23:58,170 Gills allowed animals to grow bigger, 442 00:23:58,171 --> 00:24:01,506 more complex bodies than ever before. 443 00:24:01,507 --> 00:24:05,977 ♪ ♪ 444 00:24:05,978 --> 00:24:10,782 And in fact every amphibian, reptile, bird, 445 00:24:10,783 --> 00:24:12,751 and mammal alive today, 446 00:24:12,752 --> 00:24:15,887 including the elephant, and us, 447 00:24:15,888 --> 00:24:19,425 descended from ancestors with gills. 448 00:24:22,929 --> 00:24:24,897 So, if gills were such a good idea, 449 00:24:26,032 --> 00:24:29,035 why did so many animals lose them? 450 00:24:33,306 --> 00:24:34,840 In the waterways of Kenya 451 00:24:34,841 --> 00:24:38,877 lies something that may hold the answer. 452 00:24:38,878 --> 00:24:42,113 WANJA DOROTHY NYINGI: In Kenya, fish have to be 453 00:24:42,114 --> 00:24:45,685 adapted to flooding and dry seasons. 454 00:24:47,353 --> 00:24:50,189 Some droughts can last months and even years. 455 00:24:52,658 --> 00:24:54,993 NARRATOR: Biologist Wanja Dorothy Nyingi 456 00:24:54,994 --> 00:24:59,030 is an expert on Kenya's freshwater fish species. 457 00:24:59,031 --> 00:25:00,799 NYINGI: When the water begins to recede, 458 00:25:00,800 --> 00:25:02,033 then a fish species has a choice. 459 00:25:02,034 --> 00:25:04,736 {\an8}It can either recede with the water 460 00:25:04,737 --> 00:25:06,805 {\an8}to a bigger stream of water 461 00:25:06,806 --> 00:25:08,306 or a lake, 462 00:25:08,307 --> 00:25:11,344 or adapt to that drop of water levels. 463 00:25:16,215 --> 00:25:19,084 NARRATOR: This fish has a remarkable adaptation 464 00:25:19,085 --> 00:25:21,954 that allows it to survive in the shallowest of pools. 465 00:25:23,289 --> 00:25:25,490 Meet Protopterus Aethiopicus, 466 00:25:25,491 --> 00:25:28,194 locally known as Kamongo. 467 00:25:30,830 --> 00:25:32,464 NARRATOR: Like all fishes, 468 00:25:32,465 --> 00:25:35,133 the Kamongo has gills, 469 00:25:35,134 --> 00:25:38,170 but it also has something else. 470 00:25:39,839 --> 00:25:41,207 NYINGI: Oh, my gosh, look at that. 471 00:25:43,709 --> 00:25:46,311 That's a gulp, that's how it breathes. 472 00:25:46,312 --> 00:25:47,680 That was amazing. 473 00:25:49,248 --> 00:25:52,083 NARRATOR: It gulps air through its mouth, 474 00:25:52,084 --> 00:25:54,419 drawing it into a special sack, 475 00:25:54,420 --> 00:25:57,923 deep inside its body. 476 00:25:57,924 --> 00:26:00,025 NYINGI: Normally, fish 477 00:26:00,026 --> 00:26:02,060 would breathe through their gills. 478 00:26:02,061 --> 00:26:03,161 But this fish 479 00:26:03,162 --> 00:26:05,497 is very suited to these extremes 480 00:26:05,498 --> 00:26:07,633 because it has lungs. 481 00:26:09,201 --> 00:26:11,170 It is a lungfish. 482 00:26:13,406 --> 00:26:14,940 The lung breathing takes up 483 00:26:14,941 --> 00:26:17,509 over 90% of its oxygen requirements. 484 00:26:17,510 --> 00:26:19,911 And the gills, though functional, 485 00:26:19,912 --> 00:26:24,250 are mostly for excreting carbon dioxide. 486 00:26:26,485 --> 00:26:28,453 {\an8}NARRATOR: This remarkable air-gulping ability 487 00:26:28,454 --> 00:26:32,624 {\an8}first appeared around 400 million years ago, 488 00:26:32,625 --> 00:26:34,926 {\an8}as a backup system, 489 00:26:34,927 --> 00:26:37,562 helping fish to survive in low-oxygen waters 490 00:26:37,563 --> 00:26:39,632 or brief droughts. 491 00:26:40,900 --> 00:26:43,736 But when combined with another key adaptation, 492 00:26:44,770 --> 00:26:48,541 lungs unlocked an entirely new world. 493 00:26:50,443 --> 00:26:52,344 You can see how it uses its limbs. 494 00:26:52,345 --> 00:26:54,412 {\an8}Unlike normal fish fins, 495 00:26:54,413 --> 00:26:57,449 {\an8}they are muscular and very fleshy. 496 00:26:57,450 --> 00:26:59,451 They have the capacity 497 00:26:59,452 --> 00:27:04,389 to crawl onto land. 498 00:27:04,390 --> 00:27:06,257 This fish is very important 499 00:27:06,258 --> 00:27:08,526 when we speak about evolution, 500 00:27:08,527 --> 00:27:10,462 because it is our link 501 00:27:10,463 --> 00:27:12,397 to the transition 502 00:27:12,398 --> 00:27:17,169 that we made from life in water to life on land. 503 00:27:23,909 --> 00:27:27,579 ♪ ♪ 504 00:27:27,580 --> 00:27:31,383 NARRATOR: The descendants of that tiny worm 505 00:27:31,384 --> 00:27:33,786 {\an8}gave rise to fishes with gills. 506 00:27:37,356 --> 00:27:39,691 And eventually lungs, 507 00:27:39,692 --> 00:27:41,760 that not only helped them 508 00:27:41,761 --> 00:27:46,498 survive, but allowed them to do something extraordinary-- 509 00:27:46,499 --> 00:27:50,736 leave the water and live on land. 510 00:27:52,438 --> 00:27:54,506 {\an8}So, the first animals that came out onto the land 511 00:27:54,507 --> 00:27:56,007 {\an8}were probably bugs, 512 00:27:56,008 --> 00:27:59,411 {\an8}and then not long after that, vertebrates, backboned animals. 513 00:27:59,412 --> 00:28:01,947 And they looked a lot like fishes with limbs. 514 00:28:05,184 --> 00:28:09,087 PROSANTA CHAKRABARTY: I love the vertebrate transition to land because 515 00:28:09,088 --> 00:28:12,090 {\an8}it's really fishes coming onto land, 516 00:28:12,091 --> 00:28:14,325 {\an8}a fish out of water. 517 00:28:14,326 --> 00:28:17,595 And one of my favorite fossils is this Elpistostegalian. 518 00:28:17,596 --> 00:28:18,997 This was a big animal, 519 00:28:18,998 --> 00:28:20,465 something like this, 520 00:28:20,466 --> 00:28:22,600 a head this large, had a five-foot body. 521 00:28:22,601 --> 00:28:26,137 We know that they had these pre-adaptations to live on land. 522 00:28:26,138 --> 00:28:27,772 They have upward-facing eyes. 523 00:28:27,773 --> 00:28:30,675 It breathed through its nostrils into 524 00:28:30,676 --> 00:28:32,343 these big lungs that it had. 525 00:28:32,344 --> 00:28:33,912 And some of them start 526 00:28:33,913 --> 00:28:36,548 having limbs that really start resembling 527 00:28:36,549 --> 00:28:38,884 our hands and our feet. 528 00:28:39,919 --> 00:28:41,953 NARRATOR: Scientists don't know exactly why 529 00:28:41,954 --> 00:28:44,123 those first fishes came onto land. 530 00:28:45,291 --> 00:28:49,060 But they do know that they had four fleshy fins. 531 00:28:49,061 --> 00:28:51,830 And these would lay the foundations, 532 00:28:51,831 --> 00:28:53,631 not just for the elephant, 533 00:28:53,632 --> 00:28:56,235 but for every land vertebrate that's ever lived. 534 00:28:58,003 --> 00:29:01,739 This is known as the tetrapod body plan. 535 00:29:01,740 --> 00:29:04,409 BRUSATTE: Once the tetrapod blueprint was in place, 536 00:29:04,410 --> 00:29:07,879 {\an8}then that blueprint was free to be modified, 537 00:29:07,880 --> 00:29:10,949 to adapt, to change in so many different ways. 538 00:29:10,950 --> 00:29:13,618 And that's produced things as varied as birds, which use 539 00:29:13,619 --> 00:29:15,221 their arms to fly. 540 00:29:16,422 --> 00:29:18,123 It's produced whales, 541 00:29:18,124 --> 00:29:19,958 which have gone back into the water, 542 00:29:19,959 --> 00:29:22,428 that use those limbs to swim. 543 00:29:23,762 --> 00:29:25,730 And even some tetrapods, like snakes, 544 00:29:25,731 --> 00:29:27,766 have lost their limbs entirely. 545 00:29:29,602 --> 00:29:31,436 NARRATOR: Every paw, 546 00:29:31,437 --> 00:29:33,404 hoof, 547 00:29:33,405 --> 00:29:34,906 hand 548 00:29:34,907 --> 00:29:36,508 and foot on the planet today, 549 00:29:36,509 --> 00:29:39,978 including the elephant's and ours, 550 00:29:39,979 --> 00:29:43,815 all descended from those pioneering fishes 551 00:29:43,816 --> 00:29:47,285 {\an8}that left the water and walked on land. 552 00:29:47,286 --> 00:29:52,090 {\an8}♪ ♪ 553 00:29:52,091 --> 00:29:55,126 SUMIDA: In today's world, we see an incredible 554 00:29:55,127 --> 00:29:58,329 diversity of body plans that are 555 00:29:58,330 --> 00:30:01,433 derived from the original tetrapod plan. 556 00:30:02,434 --> 00:30:04,869 KORY EVANS: Every habitat you go to almost has 557 00:30:04,870 --> 00:30:06,137 a tetrapod living in it 558 00:30:06,138 --> 00:30:07,772 and has a tetrapod that's adapted to live 559 00:30:07,773 --> 00:30:09,708 in these really interesting habitats. 560 00:30:13,812 --> 00:30:17,415 NARRATOR: But having four limbs isn't the only successful body plan 561 00:30:17,416 --> 00:30:18,684 out there. 562 00:30:19,818 --> 00:30:20,952 SUMIDA: There are hexapods, 563 00:30:20,953 --> 00:30:22,554 like six-limbed insects, 564 00:30:22,555 --> 00:30:24,956 decapods, lobsters 565 00:30:24,957 --> 00:30:27,392 and crabs and shrimp. 566 00:30:27,393 --> 00:30:30,361 There are animals with dozens of limbs on either side, 567 00:30:30,362 --> 00:30:33,097 like centipedes, for example. 568 00:30:33,098 --> 00:30:34,933 NARRATOR: From no limbs 569 00:30:34,934 --> 00:30:37,569 to hundreds of limbs, 570 00:30:37,570 --> 00:30:40,605 evolution has shaped animal bodies into a myriad 571 00:30:40,606 --> 00:30:42,473 of different forms. 572 00:30:42,474 --> 00:30:46,644 ♪ ♪ 573 00:30:46,645 --> 00:30:49,848 But tetrapods have one major advantage. 574 00:30:52,551 --> 00:30:56,021 They're really good at getting big. 575 00:30:57,056 --> 00:30:58,656 ♪ ♪ 576 00:30:58,657 --> 00:31:00,491 In the generations that pass 577 00:31:00,492 --> 00:31:04,662 after those first fishes left the water, 578 00:31:04,663 --> 00:31:09,535 {\an8}they evolve stronger skeletons to support their bodies on land. 579 00:31:11,904 --> 00:31:14,373 {\an8}The most efficient lungs on the planet. 580 00:31:15,608 --> 00:31:19,144 {\an8}And robust joints to carry more weight. 581 00:31:20,112 --> 00:31:23,249 {\an8}And these traits allow some of them to evolve... 582 00:31:25,117 --> 00:31:27,486 {\an8}...into giants. 583 00:31:30,322 --> 00:31:35,693 {\an8}(wind whistling) 584 00:31:35,694 --> 00:31:39,698 {\an8}Beings so large few predators dare challenge them. 585 00:31:41,467 --> 00:31:44,969 A huge body can store more fat and water. 586 00:31:44,970 --> 00:31:50,074 ♪ ♪ 587 00:31:50,075 --> 00:31:52,043 And tall animals can access food 588 00:31:52,044 --> 00:31:54,513 that smaller animals can't reach. 589 00:31:58,350 --> 00:32:01,653 Thanks in part to their enormous size, 590 00:32:01,654 --> 00:32:04,957 giant dinosaurs dominate the planet. 591 00:32:06,225 --> 00:32:09,861 SUMIDA: Dinosaurs persisted for 180 million years, 592 00:32:09,862 --> 00:32:12,030 and they became the largest animals 593 00:32:12,031 --> 00:32:14,065 to have ever lived on the land. 594 00:32:14,066 --> 00:32:16,668 So, in terms of size 595 00:32:16,669 --> 00:32:18,436 and diversification, 596 00:32:18,437 --> 00:32:22,775 dinosaurs may well be the pinnacle of tetrapod evolution. 597 00:32:24,677 --> 00:32:26,879 NARRATOR: While the dinosaurs dominate... 598 00:32:30,015 --> 00:32:33,419 ...the ancestors of the elephant are still tiny. 599 00:32:37,456 --> 00:32:40,224 They share the same four-limbed body plan, 600 00:32:40,225 --> 00:32:42,661 inherited from those pioneering fish. 601 00:32:44,963 --> 00:32:48,000 But on a much smaller scale than the dinosaurs. 602 00:32:50,369 --> 00:32:52,670 They're called Afrotherians, 603 00:32:52,671 --> 00:32:54,940 because they evolved in Africa. 604 00:32:56,442 --> 00:33:00,179 ♪ ♪ 605 00:33:01,480 --> 00:33:03,514 In this land of giants, 606 00:33:03,515 --> 00:33:06,819 their small size may seem like a disadvantage. 607 00:33:09,855 --> 00:33:12,491 But that's about to change. 608 00:33:14,293 --> 00:33:15,860 DROSER: Whether it's volcanism 609 00:33:15,861 --> 00:33:18,029 or an asteroid impact or climate change, 610 00:33:18,030 --> 00:33:21,200 the environment can absolutely be ruthless. 611 00:33:27,873 --> 00:33:29,741 SINGH: Sudden environmental changes 612 00:33:29,742 --> 00:33:32,577 {\an8}can completely change the challenges 613 00:33:32,578 --> 00:33:34,046 {\an8}that animals have to overcome. 614 00:33:35,914 --> 00:33:38,483 CHAKRABARTY: One adaptation might be good 615 00:33:38,484 --> 00:33:40,418 right now, but the environment 616 00:33:40,419 --> 00:33:43,288 might change, and it might change drastically. 617 00:33:44,323 --> 00:33:46,190 GILES: What was previously a really useful 618 00:33:46,191 --> 00:33:47,692 {\an8}adaptation, like being really big, 619 00:33:47,693 --> 00:33:49,161 {\an8}can suddenly become a really big hindrance. 620 00:33:50,162 --> 00:33:52,296 So, when it came to making it through mass extinctions, 621 00:33:52,297 --> 00:33:54,099 size really mattered. 622 00:33:55,934 --> 00:33:58,669 NARRATOR: An asteroid devastates the planet. 623 00:33:58,670 --> 00:34:00,305 Few survive. 624 00:34:01,874 --> 00:34:04,209 The giant dinosaurs are finished. 625 00:34:05,444 --> 00:34:09,113 In the end, their size is part of their downfall. 626 00:34:09,114 --> 00:34:13,818 ♪ ♪ 627 00:34:13,819 --> 00:34:15,920 GILES: In the aftermath of the extinction, 628 00:34:15,921 --> 00:34:18,623 anything that was above the size of, like, 629 00:34:18,624 --> 00:34:20,426 a big raccoon would have gone extinct. 630 00:34:21,727 --> 00:34:23,227 SUMIDA: When you're really, really big, 631 00:34:23,228 --> 00:34:24,962 that means you're sort of specialized. 632 00:34:24,963 --> 00:34:27,632 And when the environment changes, specialized animals 633 00:34:27,633 --> 00:34:31,035 {\an8}are often, uh, subjected to a greater 634 00:34:31,036 --> 00:34:33,872 {\an8}deal of pressure than those that are small and generalist. 635 00:34:35,407 --> 00:34:36,707 GILES: In the post-meteor world, 636 00:34:36,708 --> 00:34:38,242 vegetation would have disappeared, 637 00:34:38,243 --> 00:34:40,278 entire food sources would have collapsed. 638 00:34:40,279 --> 00:34:42,313 It was much better to be small, to be flexible, 639 00:34:42,314 --> 00:34:44,148 to be agile, being small is a 640 00:34:44,149 --> 00:34:46,317 really successful, uh, survival strategy. 641 00:34:46,318 --> 00:34:49,454 DROSER: And it's those animals living in the shadows 642 00:34:49,455 --> 00:34:51,923 sometimes that come on to dominate Earth, 643 00:34:51,924 --> 00:34:54,192 and we see that time and time again 644 00:34:54,193 --> 00:34:57,028 when we look at the history of life on planet Earth. 645 00:34:57,029 --> 00:35:01,467 The survivors sometimes come from very unexpected places. 646 00:35:06,238 --> 00:35:09,841 ♪ ♪ 647 00:35:09,842 --> 00:35:11,442 NARRATOR: In this new age, 648 00:35:11,443 --> 00:35:14,980 the small furry mammals find a world of opportunity. 649 00:35:18,484 --> 00:35:19,750 And in Africa, 650 00:35:19,751 --> 00:35:22,987 the elephant's ancestors are among those 651 00:35:22,988 --> 00:35:24,723 that make it through. 652 00:35:28,627 --> 00:35:30,795 Agile and adaptable, 653 00:35:30,796 --> 00:35:33,698 their small body size and sheer number 654 00:35:33,699 --> 00:35:35,234 are key to their success. 655 00:35:36,368 --> 00:35:39,404 And so is their flexible diet. 656 00:35:41,907 --> 00:35:43,207 These Afrotherians will eat 657 00:35:43,208 --> 00:35:44,710 almost anything. 658 00:35:46,445 --> 00:35:47,913 Including plants. 659 00:35:49,515 --> 00:35:52,183 Plants seem like easy prey. 660 00:35:52,184 --> 00:35:54,920 They don't run, they don't hide. 661 00:35:56,054 --> 00:35:58,389 But that doesn't mean they're defenseless. 662 00:35:58,390 --> 00:35:59,957 MARLOW: Plants have evolved 663 00:35:59,958 --> 00:36:01,793 {\an8}all kinds of ways to protect themselves. 664 00:36:03,629 --> 00:36:04,996 GIRGUIS: Thorns, bark, 665 00:36:04,997 --> 00:36:06,631 even chemicals. 666 00:36:06,632 --> 00:36:08,566 All of this is warfare. 667 00:36:08,567 --> 00:36:09,935 It's to keep from being eaten. 668 00:36:12,104 --> 00:36:14,138 NARRATOR: But even if you can get past their defenses, 669 00:36:14,139 --> 00:36:17,276 there's another problem with eating plants. 670 00:36:18,110 --> 00:36:21,313 They contain a lot of cellulose. 671 00:36:23,315 --> 00:36:26,183 GIRGUIS: Cellulose is this fantastic building block 672 00:36:26,184 --> 00:36:29,053 {\an8}of a molecule that's made by a bunch of plants. 673 00:36:29,054 --> 00:36:32,857 {\an8}Think of it as like the structural part of a plant, 674 00:36:32,858 --> 00:36:35,226 the scaffolding-- it helps 675 00:36:35,227 --> 00:36:36,727 give plants their strength. 676 00:36:36,728 --> 00:36:38,763 It helps them stay upright, 677 00:36:38,764 --> 00:36:41,600 and it's really, really, really hard to break down. 678 00:36:43,302 --> 00:36:46,471 {\an8}NARRATOR: Even today's plant eaters can't fully digest it. 679 00:36:47,606 --> 00:36:49,674 {\an8}After a full trip through the gut, 680 00:36:49,675 --> 00:36:52,277 {\an8}nearly half the meal emerges intact. 681 00:36:56,615 --> 00:36:58,816 SINGH: What we have here is a bit of horse dung. 682 00:36:58,817 --> 00:37:01,586 If we take a closer look, 683 00:37:01,587 --> 00:37:03,754 you can clearly see the problem; there's actually 684 00:37:03,755 --> 00:37:06,657 {\an8}a lot of fibrous material still in here. 685 00:37:06,658 --> 00:37:09,393 {\an8}There are little strands of grass here and there. 686 00:37:09,394 --> 00:37:11,996 {\an8}The grass has been chewed, it's been digested 687 00:37:11,997 --> 00:37:14,966 and yet it's still incredibly intact. 688 00:37:14,967 --> 00:37:16,834 So, just goes to show 689 00:37:16,835 --> 00:37:18,636 it's tough being a plant eater. 690 00:37:18,637 --> 00:37:21,772 ♪ ♪ 691 00:37:21,773 --> 00:37:24,710 NARRATOR: So how did early Afrotherians manage it? 692 00:37:26,745 --> 00:37:28,746 Throughout evolutionary history, 693 00:37:28,747 --> 00:37:31,849 plenty of animals have fed on plants, 694 00:37:31,850 --> 00:37:34,886 but few have been able to break them down unaided. 695 00:37:36,488 --> 00:37:37,755 They have relied on 696 00:37:37,756 --> 00:37:40,124 an unseen partnership with something 697 00:37:40,125 --> 00:37:43,562 that they swallow along with their food. 698 00:37:47,199 --> 00:37:48,633 SINGH: Most animals lack 699 00:37:48,634 --> 00:37:49,967 the enzyme cellulase, 700 00:37:49,968 --> 00:37:52,704 which allows them to break down cellulose. 701 00:37:53,705 --> 00:37:56,107 But they have a secret weapon inside their guts. 702 00:37:56,108 --> 00:37:58,042 Microbes. 703 00:37:58,043 --> 00:37:59,810 And some are actually specialists 704 00:37:59,811 --> 00:38:01,780 in breaking down cellulose. 705 00:38:03,281 --> 00:38:05,616 NARRATOR: Today, almost every plant-eating animal 706 00:38:05,617 --> 00:38:10,321 {\an8}has outsourced digestion of plant matter to microbes. 707 00:38:10,322 --> 00:38:13,792 {\an8}It's what's known as a "symbiotic relationship." 708 00:38:16,028 --> 00:38:18,329 These relationships are so important that, 709 00:38:18,330 --> 00:38:21,666 over time, evolution drives both species 710 00:38:21,667 --> 00:38:24,101 to become more and more interlocked. 711 00:38:24,102 --> 00:38:26,505 They effectively can't live without each other. 712 00:38:27,773 --> 00:38:29,206 NARRATOR: Without the communities 713 00:38:29,207 --> 00:38:31,709 of microbes living in their guts, 714 00:38:31,710 --> 00:38:34,646 most animals wouldn't be able to survive. 715 00:38:35,647 --> 00:38:37,149 Including us. 716 00:38:38,684 --> 00:38:39,984 SINGH: My body is actually 717 00:38:39,985 --> 00:38:42,621 home to trillions of little microbes. 718 00:38:44,056 --> 00:38:46,190 And that's the microbiome. 719 00:38:46,191 --> 00:38:47,658 In fact there are 720 00:38:47,659 --> 00:38:50,061 trillions of these cells in my gut alone, 721 00:38:50,062 --> 00:38:51,996 four-and-a-half pounds in fact. 722 00:38:51,997 --> 00:38:55,332 Microbes love to live in the guts of organisms 723 00:38:55,333 --> 00:38:59,336 because it's kind of like an all-you-can-eat buffet. 724 00:38:59,337 --> 00:39:01,739 {\an8}The microbes don't have to move in any way shape or form. 725 00:39:01,740 --> 00:39:03,974 {\an8}The animals do the job of eating, 726 00:39:03,975 --> 00:39:06,178 the microbes do the job of breaking it down. 727 00:39:09,081 --> 00:39:11,148 NARRATOR: Thanks to their gut microbes, 728 00:39:11,149 --> 00:39:13,785 the early Afrotherians thrive. 729 00:39:16,988 --> 00:39:21,459 Spreading out across Africa, moving into many of the habitats 730 00:39:21,460 --> 00:39:24,096 left open by the giant dinosaurs. 731 00:39:25,931 --> 00:39:28,232 BRUSATTE: When a lot of animals disappear, 732 00:39:28,233 --> 00:39:30,568 {\an8}when there is a mass extinction, let's say, 733 00:39:30,569 --> 00:39:33,939 {\an8}that means a lot of niches, a lot of jobs open up. 734 00:39:34,973 --> 00:39:37,775 GILES: An animal's niche is what it eats, it's where it lives, 735 00:39:37,776 --> 00:39:39,744 it's what it does, it's how it behaves. 736 00:39:39,745 --> 00:39:40,878 And animals can be 737 00:39:40,879 --> 00:39:42,580 really specialized for particular niches, 738 00:39:42,581 --> 00:39:45,216 to eat a particular kind of leaf. 739 00:39:45,217 --> 00:39:47,151 {\an8}Or hummingbirds can be specialized 740 00:39:47,152 --> 00:39:49,820 to have long beaks that allow them to extract nectar 741 00:39:49,821 --> 00:39:51,756 in a way that no other animal does. 742 00:39:51,757 --> 00:39:53,758 EVANS: So, after a large mass extinction event, 743 00:39:53,759 --> 00:39:56,594 the surviving animals will then evolve often times 744 00:39:56,595 --> 00:39:58,295 to fill these jobs and fill these roles, 745 00:39:58,296 --> 00:39:59,997 um, and that can result in 746 00:39:59,998 --> 00:40:03,602 some really impressive adaptations and innovations. 747 00:40:08,006 --> 00:40:11,175 NARRATOR: Some of the descendants of those Afrotherian survivors 748 00:40:11,176 --> 00:40:13,644 still live in Africa today, 749 00:40:13,645 --> 00:40:17,249 their bodies sculpted by the niches they inhabit. 750 00:40:19,985 --> 00:40:21,185 WINNIE KIIRU: Growing up in Kenya, 751 00:40:21,186 --> 00:40:23,522 there was wildlife all around us. 752 00:40:24,823 --> 00:40:26,157 I got so curious 753 00:40:26,158 --> 00:40:29,059 {\an8}about how they live, how they survive, 754 00:40:29,060 --> 00:40:33,965 {\an8}and that sparked my interest in becoming a biologist. 755 00:40:36,535 --> 00:40:38,903 NARRATOR: Biologist Winnie Kiiru 756 00:40:38,904 --> 00:40:42,339 has dedicated her life to understanding and protecting 757 00:40:42,340 --> 00:40:44,942 some of the Afrotherians that survive 758 00:40:44,943 --> 00:40:46,443 to this day. 759 00:40:46,444 --> 00:40:49,781 KIIRU: The Afrotherians are very unique and interesting. 760 00:40:53,285 --> 00:40:55,887 Look at this animal, this is the hyrax. 761 00:40:57,322 --> 00:40:59,658 Looks very much like a rodent, so furry. 762 00:41:02,394 --> 00:41:04,628 NARRATOR: A small, nimble plant eater, 763 00:41:04,629 --> 00:41:07,097 the hyrax has retained many 764 00:41:07,098 --> 00:41:08,732 of the characteristics 765 00:41:08,733 --> 00:41:11,002 that helped its ancestors survive. 766 00:41:14,172 --> 00:41:16,040 KIIRU: They climb very well. 767 00:41:16,041 --> 00:41:19,944 They have these amazing soft pads on the bottom of their feet 768 00:41:19,945 --> 00:41:21,278 that are kept 769 00:41:21,279 --> 00:41:22,980 lubricated by their sweat glands, 770 00:41:22,981 --> 00:41:27,151 that allow them to move very well in the trees, 771 00:41:27,152 --> 00:41:29,520 and they can reach the top canopy, 772 00:41:29,521 --> 00:41:32,489 where they're able to find all these different leaves. 773 00:41:32,490 --> 00:41:34,725 And because of being high up, 774 00:41:34,726 --> 00:41:36,961 they can also look out for predators. 775 00:41:36,962 --> 00:41:40,866 NARRATOR: The tree hyrax body is shaped for life in the canopies. 776 00:41:42,167 --> 00:41:44,001 KIIRU: They are very well adapted to the niche 777 00:41:44,002 --> 00:41:46,770 that they have found. 778 00:41:46,771 --> 00:41:49,073 And their success is obvious, 779 00:41:49,074 --> 00:41:51,775 because they are very prolific around Nairobi. 780 00:41:51,776 --> 00:41:54,012 You find a lot of them in the gardens here. 781 00:41:56,314 --> 00:41:59,683 NARRATOR: While these hyraxes adapted to live in the trees, 782 00:41:59,684 --> 00:42:03,588 other Afrotherians moved into entirely different habitats. 783 00:42:04,756 --> 00:42:07,658 KIIRU: Even if they look like rodents, some of their 784 00:42:07,659 --> 00:42:10,461 closest relatives are actually elephants. 785 00:42:10,462 --> 00:42:12,597 Very unlikely relatives. 786 00:42:14,032 --> 00:42:15,866 NARRATOR: Although closely related, 787 00:42:15,867 --> 00:42:18,235 hyraxes and elephants seem to have 788 00:42:18,236 --> 00:42:20,537 completely different bodies, 789 00:42:20,538 --> 00:42:23,174 because they evolved to live in different niches. 790 00:42:25,143 --> 00:42:28,412 The Afrotherians had an excellent advantage 791 00:42:28,413 --> 00:42:32,716 because they were isolated on the African continent 792 00:42:32,717 --> 00:42:36,887 and had very little competition, and so they could actually 793 00:42:36,888 --> 00:42:39,291 populate a whole lot of different niches. 794 00:42:40,525 --> 00:42:43,394 NARRATOR: Over countless generations, 795 00:42:43,395 --> 00:42:46,163 the Afrotherians adapted to a wide variety of different 796 00:42:46,164 --> 00:42:48,999 environments and lifestyles, making them 797 00:42:49,000 --> 00:42:53,371 one of Earth's most eclectic groups of animals. 798 00:42:55,173 --> 00:42:58,208 KIIRU: The manatees went into the water. 799 00:42:58,209 --> 00:43:01,111 The elephants went off into the savannahs. 800 00:43:01,112 --> 00:43:05,549 The hyraxes eat all these leaves, high up in the trees. 801 00:43:05,550 --> 00:43:08,319 And then you have the elephant shrews 802 00:43:08,320 --> 00:43:11,188 that went out and ate insects. 803 00:43:11,189 --> 00:43:13,857 And the aardvarks with their long snouts that are able to go 804 00:43:13,858 --> 00:43:16,260 and find food in the termite mounds. 805 00:43:16,261 --> 00:43:18,329 So, with all these 806 00:43:18,330 --> 00:43:19,730 different niches, 807 00:43:19,731 --> 00:43:21,967 it meant they were not competing against each other. 808 00:43:26,037 --> 00:43:29,707 NARRATOR: Over time, the early Afrotherians 809 00:43:29,708 --> 00:43:31,942 spread across Africa, 810 00:43:31,943 --> 00:43:34,178 adapting to different niches 811 00:43:34,179 --> 00:43:37,014 that shape their bodies in different ways. 812 00:43:37,015 --> 00:43:38,048 ♪ ♪ 813 00:43:38,049 --> 00:43:39,417 (thunder rumbling) 814 00:43:40,385 --> 00:43:44,389 {\an8}Some evolve bigger guts to process more plant material. 815 00:43:45,824 --> 00:43:47,359 {\an8}And a bigger gut... 816 00:43:48,660 --> 00:43:50,929 {\an8}...means a bigger body size. 817 00:44:02,007 --> 00:44:06,143 {\an8}This stocky creature is around three feet tall, 818 00:44:06,144 --> 00:44:09,747 {\an8}much larger than its furry ancestors. 819 00:44:09,748 --> 00:44:11,715 {\an8}It spends most of its time 820 00:44:11,716 --> 00:44:13,418 {\an8}browsing on plants. 821 00:44:14,686 --> 00:44:18,455 {\an8}It's an early Proboscidean, a relative 822 00:44:18,456 --> 00:44:19,924 {\an8}of the elephant. 823 00:44:21,159 --> 00:44:23,193 {\an8}It's evolved elongated incisors 824 00:44:23,194 --> 00:44:25,730 {\an8}for tearing through dense vegetation. 825 00:44:27,732 --> 00:44:31,269 {\an8}But big teeth can have their drawbacks. 826 00:44:33,972 --> 00:44:36,073 Okay, right, so I'll put that on there. 827 00:44:36,074 --> 00:44:37,908 Okay, I got it-- watch your fingers. Like that? 828 00:44:37,909 --> 00:44:38,976 You got it? Okay? Yeah, I got it. 829 00:44:38,977 --> 00:44:40,744 There we go. 830 00:44:40,745 --> 00:44:42,880 CHAKRABARTY: Evolution doesn't have a goal. 831 00:44:42,881 --> 00:44:46,383 {\an8}It's not working in a straight line towards some end. 832 00:44:46,384 --> 00:44:49,620 {\an8}All right, yep-- wait, oh! 833 00:44:49,621 --> 00:44:51,756 Evolution finds solutions to problems. 834 00:44:53,258 --> 00:44:55,260 And I got it, yes, comfortable. 835 00:44:56,895 --> 00:45:00,297 {\an8}This is the skull of a walrus. 836 00:45:00,298 --> 00:45:02,466 These tusks can be really long. 837 00:45:02,467 --> 00:45:05,869 They can be up to three feet from the very top 838 00:45:05,870 --> 00:45:08,238 all the way to the bottom. 839 00:45:08,239 --> 00:45:11,175 These are used for mating and for courtship. 840 00:45:11,176 --> 00:45:13,977 It's hard for these to eat with these big tusks in the way. 841 00:45:13,978 --> 00:45:16,613 {\an8}So, what they have to do is they stick their snouts 842 00:45:16,614 --> 00:45:18,682 {\an8}into the mud and then find the little crabs 843 00:45:18,683 --> 00:45:22,853 {\an8}and clams and mollusks, tucking their tusks behind them. 844 00:45:22,854 --> 00:45:24,988 {\an8}DROSER: Sometimes with evolution you end up 845 00:45:24,989 --> 00:45:28,292 with really strange body plans that you couldn't predict, 846 00:45:28,293 --> 00:45:30,160 that don't seem the obvious 847 00:45:30,161 --> 00:45:31,728 body plan to evolve on Earth. 848 00:45:31,729 --> 00:45:34,264 GIRGUIS: So, evolution is always a compromise. 849 00:45:34,265 --> 00:45:36,433 Sometimes you solve one problem, 850 00:45:36,434 --> 00:45:37,635 you create another. 851 00:45:39,404 --> 00:45:42,606 NARRATOR: Like the walrus, these Proboscideans have 852 00:45:42,607 --> 00:45:45,709 muscular noses that they use to push food towards 853 00:45:45,710 --> 00:45:47,278 their mouths. 854 00:45:49,981 --> 00:45:52,749 But this individual was born with a slightly 855 00:45:52,750 --> 00:45:55,787 longer one, giving it a tiny advantage. 856 00:45:56,955 --> 00:45:59,923 It may not look like much, but it's 857 00:45:59,924 --> 00:46:03,027 the start of something entirely new. 858 00:46:06,664 --> 00:46:10,434 {\an8}Over countless generations, the muscular nose 859 00:46:10,435 --> 00:46:12,804 {\an8}gives better access to food... 860 00:46:14,706 --> 00:46:17,141 {\an8}...fueling an even bigger body, 861 00:46:17,142 --> 00:46:20,744 {\an8}which in turn, drives a longer nose, 862 00:46:20,745 --> 00:46:25,082 {\an8}capable of reaching all the way to the ground. 863 00:46:25,083 --> 00:46:28,452 {\an8}Until, after millions of years, one of the most 864 00:46:28,453 --> 00:46:32,757 {\an8}unusual bodies in the animal kingdom arrives. 865 00:46:37,629 --> 00:46:40,465 This is Palaeomastodon. 866 00:46:41,399 --> 00:46:44,202 It roamed the Earth around 30 million years ago. 867 00:46:45,703 --> 00:46:48,272 It has the trunk and the tusks. 868 00:46:48,273 --> 00:46:50,774 Big ears are still a work in progress, 869 00:46:50,775 --> 00:46:54,612 but it's a body shape that looks a lot like an elephant's. 870 00:46:57,282 --> 00:47:00,851 Palaeomastodon is one of the many extinct ancient relatives 871 00:47:00,852 --> 00:47:02,453 of the elephant. 872 00:47:03,655 --> 00:47:05,389 Originating in Africa, 873 00:47:05,390 --> 00:47:08,358 these relatives soon spread across the globe, 874 00:47:08,359 --> 00:47:10,527 adapting to new niches, 875 00:47:10,528 --> 00:47:12,931 which continue to shape their bodies. 876 00:47:14,766 --> 00:47:18,468 {\an8}Some have bizarre jaw extensions. 877 00:47:18,469 --> 00:47:21,940 Others evolve downward-curving tusks. 878 00:47:25,109 --> 00:47:27,711 And of course, the mammoths, 879 00:47:27,712 --> 00:47:31,048 those hairy icons of the ice age. 880 00:47:31,049 --> 00:47:34,818 With huge guts for digesting masses of plants, 881 00:47:34,819 --> 00:47:38,422 the Proboscideans become some of the largest mammals 882 00:47:38,423 --> 00:47:41,159 to have ever walked the Earth. 883 00:47:45,663 --> 00:47:48,332 Um... (chuckles) 884 00:47:48,333 --> 00:47:50,067 Does that work? Like that? 885 00:47:50,068 --> 00:47:52,036 WOMAN: That's amazing. (chuckles) 886 00:47:53,271 --> 00:47:55,172 HERRIDGE: This is a replica mammoth tusk. 887 00:47:55,173 --> 00:47:57,074 Uh, it's made of polystyrene, 888 00:47:57,075 --> 00:48:01,245 but even made of polystyrene, it is pretty heavy. 889 00:48:01,246 --> 00:48:02,746 A tusk is really 890 00:48:02,747 --> 00:48:04,982 just an overgrown incisor. 891 00:48:04,983 --> 00:48:06,717 So, that's your front teeth here. 892 00:48:06,718 --> 00:48:08,719 Tusks come in all shapes and sizes, 893 00:48:08,720 --> 00:48:10,287 but it's when you see something 894 00:48:10,288 --> 00:48:12,289 like this on your lap 895 00:48:12,290 --> 00:48:15,760 that you can really appreciate just how enormous they are. 896 00:48:16,828 --> 00:48:20,097 ♪ ♪ 897 00:48:20,098 --> 00:48:22,733 NARRATOR: But of this once-diverse group, 898 00:48:22,734 --> 00:48:26,436 some 180 species strong, 899 00:48:26,437 --> 00:48:28,406 only three remain. 900 00:48:30,541 --> 00:48:32,377 The Asian elephant. 901 00:48:33,578 --> 00:48:36,481 The African forest elephant. 902 00:48:39,217 --> 00:48:43,054 And the largest land animal on the planet today. 903 00:48:45,723 --> 00:48:48,626 The African savannah elephant. 904 00:48:55,266 --> 00:48:57,668 HERRIDGE: If you look at an elephant, I think you will see 905 00:48:57,669 --> 00:49:00,437 three things immediately that stand out. 906 00:49:00,438 --> 00:49:03,507 Their enormous size, their incredible tusks, 907 00:49:03,508 --> 00:49:08,179 and their amazing, unusual, and unique trunk. 908 00:49:10,214 --> 00:49:13,884 DROSER: It is the ultimate evolutionary multi-tool-- so, 909 00:49:13,885 --> 00:49:15,652 {\an8}you're using your trunk to breathe, 910 00:49:15,653 --> 00:49:17,654 {\an8}you're using it to pick up things from the ground 911 00:49:17,655 --> 00:49:19,890 and bring it to your mouth-- it's amazing. 912 00:49:19,891 --> 00:49:23,193 And it is this extraordinary example 913 00:49:23,194 --> 00:49:25,029 of how evolution works. 914 00:49:27,665 --> 00:49:30,967 NARRATOR: With a body that's made of a million billion cells, 915 00:49:30,968 --> 00:49:34,304 and powered by some of the largest lungs 916 00:49:34,305 --> 00:49:35,640 of any land animal... 917 00:49:37,108 --> 00:49:40,410 ...a gut up to 120 feet long, 918 00:49:40,411 --> 00:49:43,047 packed full of microbes for digesting plants... 919 00:49:44,515 --> 00:49:46,784 {\an8}...tusks that never stop growing... 920 00:49:48,519 --> 00:49:52,156 {\an8}...and a nose like nothing else on the planet... 921 00:49:53,858 --> 00:49:56,226 ...the elephant is one of evolution's 922 00:49:56,227 --> 00:49:59,663 most extraordinary creations. 923 00:49:59,664 --> 00:50:02,699 There's something about elephants that speaks to humans. 924 00:50:02,700 --> 00:50:04,134 Maybe it's because they 925 00:50:04,135 --> 00:50:05,702 have been our constant companions 926 00:50:05,703 --> 00:50:07,838 on our own evolutionary journey. 927 00:50:07,839 --> 00:50:11,108 CHAKRABARTY: They share a close relationship with us 928 00:50:11,109 --> 00:50:12,609 and an environment with us. 929 00:50:12,610 --> 00:50:15,512 They've been worshipped, and that's clear 930 00:50:15,513 --> 00:50:17,247 from a long history 931 00:50:17,248 --> 00:50:20,618 of, of human interactions with these beautiful creatures. 932 00:50:21,986 --> 00:50:24,287 NARRATOR: Four billion years of evolution 933 00:50:24,288 --> 00:50:26,691 shaped the elephant we see today. 934 00:50:29,961 --> 00:50:32,663 But its story is far from over. 935 00:50:33,865 --> 00:50:37,034 Today, humans have become a driving force. 936 00:50:38,603 --> 00:50:42,773 By poaching elephants in huge numbers for their ivory tusks, 937 00:50:42,774 --> 00:50:45,443 we are changing the course of their evolution. 938 00:50:46,878 --> 00:50:48,745 GIRGUIS: We're starting to see that 939 00:50:48,746 --> 00:50:51,515 {\an8}humans poaching elephants for their tusks 940 00:50:51,516 --> 00:50:55,085 {\an8}is favoring those few elephants 941 00:50:55,086 --> 00:50:57,989 that are naturally tuskless. 942 00:50:59,223 --> 00:51:01,558 HERRIDGE: There is such strong selection pressure 943 00:51:01,559 --> 00:51:04,327 on some populations that it looks like 944 00:51:04,328 --> 00:51:05,929 there is now an advantage 945 00:51:05,930 --> 00:51:09,666 to those elephants to not have the tusks 946 00:51:09,667 --> 00:51:11,635 that have been part of their evolutionary history 947 00:51:11,636 --> 00:51:14,337 for millions and millions of years, 948 00:51:14,338 --> 00:51:19,444 and the best explanation for that is us and poaching. 949 00:51:22,280 --> 00:51:25,082 NARRATOR: But evolution never stands still. 950 00:51:26,217 --> 00:51:28,318 GIRGUIS: If we get to a place where we're not 951 00:51:28,319 --> 00:51:31,254 hunting tusked elephants, 952 00:51:31,255 --> 00:51:33,924 and having tusks is advantageous, 953 00:51:33,925 --> 00:51:36,693 they'll come back-- that's how evolution works. 954 00:51:36,694 --> 00:51:40,030 ♪ ♪ 955 00:51:40,031 --> 00:51:42,999 {\an8}NARRATOR: The elephant is not the end of the journey, 956 00:51:43,000 --> 00:51:47,270 {\an8}just the latest chapter in a four-billion-year story 957 00:51:47,271 --> 00:51:50,341 that began with a single cell. 958 00:51:52,076 --> 00:51:53,910 CHAKRABARTY: For me, it's so humbling to know 959 00:51:53,911 --> 00:51:57,080 that we're just part of this puzzle of evolution, 960 00:51:57,081 --> 00:51:58,381 this tree of life. 961 00:51:58,382 --> 00:52:00,417 All life on Earth is related, 962 00:52:00,418 --> 00:52:02,552 and evolution helps us not just 963 00:52:02,553 --> 00:52:04,921 to understand who's related to whom, 964 00:52:04,922 --> 00:52:08,658 but how we got to where we are, and why we are the way we are. 965 00:52:08,659 --> 00:52:11,229 ♪ ♪ 966 00:52:33,317 --> 00:52:38,322 ♪ ♪ 967 00:52:54,272 --> 00:52:59,310 ♪ ♪