[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:07.09,0:00:10.67,Default,,0000,0000,0000,,You know that little pink thing nestled \Nin the corner of your eye?
Dialogue: 0,0:00:10.67,0:00:13.100,Default,,0000,0000,0000,,It’s actually the remnant \Nof a third eyelid.
Dialogue: 0,0:00:13.100,0:00:16.15,Default,,0000,0000,0000,,Known as the “plica semilunaris,”
Dialogue: 0,0:00:16.15,0:00:19.37,Default,,0000,0000,0000,,it’s much more prominent in birds \Nand a few mammals,
Dialogue: 0,0:00:19.37,0:00:24.46,Default,,0000,0000,0000,,and functions like a windshield wiper \Nto keep dust and debris out of their eyes.
Dialogue: 0,0:00:24.46,0:00:26.80,Default,,0000,0000,0000,,But in humans, it doesn’t work.
Dialogue: 0,0:00:26.80,0:00:31.48,Default,,0000,0000,0000,,It’s vestigial, meaning it no longer \Nserves its original purpose.
Dialogue: 0,0:00:31.48,0:00:35.68,Default,,0000,0000,0000,,There are several other vestigial \Nstructures like the plica semilunaris
Dialogue: 0,0:00:35.68,0:00:37.01,Default,,0000,0000,0000,,in the human body.
Dialogue: 0,0:00:37.01,0:00:41.45,Default,,0000,0000,0000,,Most of these became vestigial long \Nbefore homo sapiens existed,
Dialogue: 0,0:00:41.45,0:00:45.88,Default,,0000,0000,0000,,quietly riding along from one of \Nour ancestor species to the next.
Dialogue: 0,0:00:45.88,0:00:49.14,Default,,0000,0000,0000,,But why have they stuck \Naround for so long?
Dialogue: 0,0:00:49.14,0:00:53.08,Default,,0000,0000,0000,,To answer this question, it helps to \Nunderstand natural selection.
Dialogue: 0,0:00:53.08,0:00:55.50,Default,,0000,0000,0000,,Natural selection simply means that traits
Dialogue: 0,0:00:55.50,0:00:59.76,Default,,0000,0000,0000,,which help an organism survive and \Nreproduce in a given environment
Dialogue: 0,0:00:59.76,0:01:02.92,Default,,0000,0000,0000,,are more likely to make it to the \Nnext generation.
Dialogue: 0,0:01:02.92,0:01:07.88,Default,,0000,0000,0000,,As the environment changes, traits \Nthat were once useful can become harmful.
Dialogue: 0,0:01:07.88,0:01:10.65,Default,,0000,0000,0000,,Those traits are often selected against,
Dialogue: 0,0:01:10.65,0:01:14.11,Default,,0000,0000,0000,,meaning they gradually disappear \Nfrom the population.
Dialogue: 0,0:01:14.11,0:01:19.46,Default,,0000,0000,0000,,But if a trait isn’t actively harmful, \Nit might not get selected against,
Dialogue: 0,0:01:19.46,0:01:23.06,Default,,0000,0000,0000,,and stick around even \Nthough it isn’t useful.
Dialogue: 0,0:01:23.06,0:01:24.56,Default,,0000,0000,0000,,Take the tailbone.
Dialogue: 0,0:01:24.56,0:01:28.45,Default,,0000,0000,0000,,Evolutionary biologists think that as the \Nclimate got drier
Dialogue: 0,0:01:28.45,0:01:30.27,Default,,0000,0000,0000,,and grasslands popped up,
Dialogue: 0,0:01:30.27,0:01:35.37,Default,,0000,0000,0000,,our tail-bearing ancestors left the trees \Nand started walking on land.
Dialogue: 0,0:01:35.37,0:01:37.43,Default,,0000,0000,0000,,The tails that had helped them \Nin the trees
Dialogue: 0,0:01:37.43,0:01:40.46,Default,,0000,0000,0000,,began to disrupt their ability \Nto walk on land.
Dialogue: 0,0:01:40.46,0:01:44.72,Default,,0000,0000,0000,,So individuals with mutations that reduced\Nthe length of their tails
Dialogue: 0,0:01:44.72,0:01:47.50,Default,,0000,0000,0000,,became more successful at life on land,
Dialogue: 0,0:01:47.50,0:01:51.98,Default,,0000,0000,0000,,surviving long enough to pass their short \Ntails on to the next generation.
Dialogue: 0,0:01:51.98,0:01:55.89,Default,,0000,0000,0000,,The change was likely gradual over \Nmillions of years until,
Dialogue: 0,0:01:55.89,0:01:57.84,Default,,0000,0000,0000,,about 20 million years ago,
Dialogue: 0,0:01:57.84,0:02:02.24,Default,,0000,0000,0000,,our ancestors’ external tails disappeared \Naltogether.
Dialogue: 0,0:02:02.24,0:02:07.36,Default,,0000,0000,0000,,Today, we know human embryos have tails\Nthat dissolve as the embryo develops.
Dialogue: 0,0:02:07.36,0:02:09.94,Default,,0000,0000,0000,,But the stubby tailbone sticks around,
Dialogue: 0,0:02:09.94,0:02:12.30,Default,,0000,0000,0000,,probably because it doesn’t \Ncause any harm—
Dialogue: 0,0:02:12.30,0:02:14.87,Default,,0000,0000,0000,,in fact, it serves a more minor function
Dialogue: 0,0:02:14.87,0:02:18.08,Default,,0000,0000,0000,,as the anchor point for certain \Nother muscles.
Dialogue: 0,0:02:18.08,0:02:23.39,Default,,0000,0000,0000,,Up to 85% of people have a vestigial \Nmuscle called the “palmaris longus.”
Dialogue: 0,0:02:23.39,0:02:24.58,Default,,0000,0000,0000,,To see if you do,
Dialogue: 0,0:02:24.58,0:02:29.01,Default,,0000,0000,0000,,put your hand down on a flat surface \Nand touch your pinkie to your thumb.
Dialogue: 0,0:02:29.01,0:02:32.82,Default,,0000,0000,0000,,If you see a little band pop up in the \Nmiddle of your wrist,
Dialogue: 0,0:02:32.82,0:02:36.98,Default,,0000,0000,0000,,that’s the tendon that attaches to this \Nnow-defunct muscle.
Dialogue: 0,0:02:36.98,0:02:42.24,Default,,0000,0000,0000,,In this case, the fact that not everyone \Nhas it has helped us trace its function.
Dialogue: 0,0:02:42.24,0:02:46.40,Default,,0000,0000,0000,,Vestigial traits can persist when there’s \Nno incentive to lose them—
Dialogue: 0,0:02:46.40,0:02:49.25,Default,,0000,0000,0000,,but since there’s also no incentive \Nto keep them,
Dialogue: 0,0:02:49.25,0:02:52.63,Default,,0000,0000,0000,,random mutations will sometimes still\Neliminate them
Dialogue: 0,0:02:52.63,0:02:54.54,Default,,0000,0000,0000,,from part of the population.
Dialogue: 0,0:02:54.54,0:02:56.34,Default,,0000,0000,0000,,Looking at our primate relatives,
Dialogue: 0,0:02:56.34,0:02:59.92,Default,,0000,0000,0000,,we can see that the palmaris longus \Nis sometimes absent
Dialogue: 0,0:02:59.92,0:03:02.83,Default,,0000,0000,0000,,in those that spend more time on the land,
Dialogue: 0,0:03:02.83,0:03:07.09,Default,,0000,0000,0000,,but always present in those that spend \Nmore time in trees.
Dialogue: 0,0:03:07.09,0:03:10.82,Default,,0000,0000,0000,,So we think it used to help us swing \Nfrom branch to branch,
Dialogue: 0,0:03:10.82,0:03:14.46,Default,,0000,0000,0000,,and became unnecessary when \Nwe moved down to land.
Dialogue: 0,0:03:14.46,0:03:19.20,Default,,0000,0000,0000,,The appendix, meanwhile, may once have \Nbeen part of the intestinal system
Dialogue: 0,0:03:19.20,0:03:22.96,Default,,0000,0000,0000,,our ancestors used for digesting \Nplant materials.
Dialogue: 0,0:03:22.96,0:03:27.70,Default,,0000,0000,0000,,As their diets changed, those parts of the\Nintestinal system began to shrink.
Dialogue: 0,0:03:27.70,0:03:32.77,Default,,0000,0000,0000,,Unlike other vestigial structures, though,\Nthe appendix isn’t always harmless—
Dialogue: 0,0:03:32.77,0:03:35.60,Default,,0000,0000,0000,,it can become dangerously inflamed.
Dialogue: 0,0:03:35.60,0:03:39.71,Default,,0000,0000,0000,,For most of human history, a burst \Nappendix could be a death sentence.
Dialogue: 0,0:03:39.71,0:03:41.73,Default,,0000,0000,0000,,So why did it stick around?
Dialogue: 0,0:03:41.73,0:03:45.38,Default,,0000,0000,0000,,It’s possible that it was very slowly on \Nits way out,
Dialogue: 0,0:03:45.38,0:03:49.20,Default,,0000,0000,0000,,or that mutations simply hadn’t arisen \Nto make it smaller.
Dialogue: 0,0:03:49.20,0:03:51.85,Default,,0000,0000,0000,,Or maybe it has other benefits—
Dialogue: 0,0:03:51.85,0:03:57.93,Default,,0000,0000,0000,,for example, it might still be a reservoir\Nof bacteria that helps us break down food.
Dialogue: 0,0:03:57.93,0:04:02.37,Default,,0000,0000,0000,,But the fact is, we’re not really sure why\Nthe appendix persists.
Dialogue: 0,0:04:02.37,0:04:04.95,Default,,0000,0000,0000,,Evolution is an imperfect process.
Dialogue: 0,0:04:04.95,0:04:10.86,Default,,0000,0000,0000,,Human beings are the result of millions of\Nyears of trial, error, and random chance—
Dialogue: 0,0:04:10.86,0:04:15.17,Default,,0000,0000,0000,,and we’re full of evolutionary relics \Nto remind us of that.