Why do humans have a third eyelid? - Dorsa Amir

譯者: Lilian Chiu 審譯者: Harper Chang

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

你可知道在你的眼睛角落 有塊粉紅色的小東西？

It’s actually the remnant of a third eyelid.

它其實是第三個眼瞼 殘留下來的部分。

Known as the “plica semilunaris,”

就是一般所知的「半月皺襞」，

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

在鳥類和少數幾種哺乳類 動物的眼睛上更明顯，

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

它的功能類似雨刷， 把眼睛上的塵土和殘骸清除。

But in humans, it doesn’t work.

但它在人類的眼睛上則沒有用途。

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

它已經退化了，

意即它不再具有原本的功能。

There are several other vestigial structures like the plica semilunaris

在人類的身體上，還有好幾個

類似半月皺襞的退化結構。

in the human body.

Most of these became vestigial long before homo sapiens existed,

它們大部分都在現代人 出現之前就已經退化了，

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

默默地隨著我們某一個 古老物種進到下一個物種。

But why have they stuck around for so long?

但，它們為什麼能留這麼久？

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

要回答這個問題， 需要先了解自然天擇。

Natural selection simply means that traits

自然天擇的意思就是， 在既定的環境中，

which help an organism survive and reproduce in a given environment

對有機體生存及繁衍有利的特徵

are more likely to make it to the next generation.

更有可能會繼續傳到下一代。

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

隨著環境改變，曾經有用的特徵 也可能會變成有害的特質。

Those traits are often selected against,

那些特徵通常會被篩選掉，

meaning they gradually disappear from the population.

即它們會從物種中漸漸消失。

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

但，不會主動造成傷害的特徵，

就可能不會被篩掉，

and stick around even though it isn’t useful.

即使沒有用也會繼續留下來。

Take the tailbone.

以尾椎骨為例。

Evolutionary biologists think that as the climate got drier

演化生物學家認為， 隨著氣候變得越來越乾燥，

and grasslands popped up,

草地也漸漸冒出來，

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

我們有尾巴的祖先離開了樹木， 開始在地面上行走。

The tails that had helped them in the trees

他們的尾巴有助於在樹上活動，

began to disrupt their ability to walk on land.

卻阻擾了他們在地面上的行走。

So individuals with mutations that reduced the length of their tails

所以，尾巴因突變而縮短的個體

became more successful at life on land,

更能成功地在地面上過生活，

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

存活得夠久，足以把 他們的短尾巴傳給下一代。

The change was likely gradual over millions of years until,

這種改變可能是花了 數百萬年逐漸形成的，

about 20 million years ago,

一直到大約兩千萬年前，

our ancestors’ external tails disappeared altogether.

我們祖先的外部尾巴全都不見了。

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

現今，我們知道人類 在胚胎時具有尾巴，

隨著胚胎發展尾巴會漸消失。

But the stubby tailbone sticks around,

但粗短的硬尾椎骨會留下來，

probably because it doesn’t cause any harm—

可能是因為它不會 造成任何傷害——

in fact, it serves a more minor function

事實上，它還有一個小功能，

as the anchor point for certain other muscles.

是某些其他肌肉的定錨點。

Up to 85% of people have a vestigial muscle called the “palmaris longus.”

高達 85% 的人 有一條退化的肌肉，

叫做「掌長肌」。

To see if you do,

若想知道你有沒有， 把你的手放到平面上，

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

把小指和大拇指碰在一起。

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

如果你發現手腕中間 有一條肌腱跑出來，

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

那條肌腱連結的 正是已經沒功能的掌長肌。

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

並非人人都有這條肌肉，

這個事實能幫助我們 追溯它的功能。

Vestigial traits can persist when there’s no incentive to lose them—

如果丟棄退化的特徵沒有任何好處， 它可能會被留下來——

but since there’s also no incentive to keep them,

不過，因為留下它也沒有好處，

random mutations will sometimes still eliminate them

隨機突變有時仍然會

把它從某部分人身上除去。

from part of the population.

Looking at our primate relatives,

研究我們的靈長類親戚就可以發現，

we can see that the palmaris longus is sometimes absent

沒有掌長肌的個體 通常花在地面上的時間較多，

in those that spend more time on the land,

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

較長待在樹木上的個體 則總是會具有掌長肌。

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

所以，我們認為這條肌肉 以前能協助我們在樹枝間飛盪，

and became unnecessary when we moved down to land.

當我們搬到地面上之後就沒必要了。

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

至於闌尾，它過去可能 是腸道系統的一部分，

our ancestors used for digesting plant materials.

我們的祖先用它來消化植物食材。

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

隨著他們的飲食改變， 腸道系統的那些部分便開始萎縮。

Unlike other vestigial structures, though, the appendix isn’t always harmless—

不過，和其他退化的結構不同， 闌尾不見得總是無害的——

it can become dangerously inflamed.

它有可能發炎造成危險。

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

在人類的大半歷史上， 闌尾破掉就等同於宣判死刑了。

So why did it stick around?

所以為什麼它還會留下來？

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

有可能它正非常緩慢地被排除，

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

或者就是還沒有發生突變讓它變小。

Or maybe it has other benefits—

或者它有其他益處——

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

比如，能協助我們分解食物的細菌 就可能住在那裡。

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

但事實是，我們尚不清楚 闌尾為什麼仍然存在。

Evolution is an imperfect process.

演化是個不完美的過程。

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

人類是數百萬年

試驗、錯誤、隨機可能性的結果，

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

我們身上演化的遺跡 永遠提醒著我們這一點。