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  • You know that little pink thing nestled in the corner of your eye?

  • It's actually the remnant of a third eyelid.

  • Known as theplica semilunaris,”

  • it's much more prominent in birds and a few mammals,

  • and functions like a windshield wiper to keep dust and debris out of their eyes.

  • But in humans, it doesn't work.

  • It's vestigial, meaning it no longer serves its original purpose.

  • There are several other vestigial structures like the plica semilunaris

  • in the human body.

  • Most of these became vestigial long before homo sapiens existed,

  • quietly riding along from one of our ancestor species to the next.

  • But why have they stuck around for so long?

  • To answer this question, it helps to understand natural selection.

  • Natural selection simply means that traits

  • which help an organism survive and reproduce in a given environment

  • are more likely to make it to the next generation.

  • As the environment changes, traits that were once useful can become harmful.

  • Those traits are often selected against,

  • meaning they gradually disappear from the population.

  • But if a trait isn't actively harmful, it might not get selected against,

  • and stick around even though it isn't useful.

  • Take the tailbone.

  • Evolutionary biologists think that as the climate got drier

  • and grasslands popped up,

  • our tail-bearing ancestors left the trees and started walking on land.

  • The tails that had helped them in the trees

  • began to disrupt their ability to walk on land.

  • So individuals with mutations that reduced the length of their tails

  • became more successful at life on land,

  • surviving long enough to pass their short tails on to the next generation.

  • The change was likely gradual over millions of years until,

  • about 20 million years ago,

  • our ancestors' external tails disappeared altogether.

  • Today, we know human embryos have tails that dissolve as the embryo develops.

  • But the stubby tailbone sticks around,

  • probably because it doesn't cause any harm

  • in fact, it serves a more minor function

  • as the anchor point for certain other muscles.

  • Up to 85% of people have a vestigial muscle called thepalmaris longus.”

  • To see if you do,

  • put your hand down on a flat surface and touch your pinkie to your thumb.

  • If you see a little band pop up in the middle of your wrist,

  • that's the tendon that attaches to this now-defunct muscle.

  • In this case, the fact that not everyone has it has helped us trace its function.

  • Vestigial traits can persist when there's no incentive to lose them

  • but since there's also no incentive to keep them,

  • random mutations will sometimes still eliminate them

  • from part of the population.

  • Looking at our primate relatives,

  • we can see that the palmaris longus is sometimes absent

  • in those that spend more time on the land,

  • but always present in those that spend more time in trees.

  • So we think it used to help us swing from branch to branch,

  • and became unnecessary when we moved down to land.

  • The appendix, meanwhile, may once have been part of the intestinal system

  • our ancestors used for digesting plant materials.

  • As their diets changed, those parts of the intestinal system began to shrink.

  • Unlike other vestigial structures, though, the appendix isn't always harmless

  • it can become dangerously inflamed.

  • For most of human history, a burst appendix could be a death sentence.

  • So why did it stick around?

  • It's possible that it was very slowly on its way out,

  • or that mutations simply hadn't arisen to make it smaller.

  • Or maybe it has other benefits

  • for example, it might still be a reservoir of bacteria that helps us break down food.

  • But the fact is, we're not really sure why the appendix persists.

  • Evolution is an imperfect process.

  • Human beings are the result of millions of years of trial, error, and random chance

  • and we're full of evolutionary relics to remind us of that.

You know that little pink thing nestled in the corner of your eye?

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B2 中高級 美國腔

人類為什麼會有第三眼皮? (Why do humans have a third eyelid? - Dorsa Amir)

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    Judy chan 發佈於 2021 年 01 月 14 日
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