Why are human bodies asymmetrical? - Leo Q. Wan

譯者: Ann Chen 審譯者: Max Chern

Symmetry is everywhere in nature,

對稱在自然界裡無所不在

and we usually associate it with beauty:

我們通常把它和美聯想在一起：

a perfectly shaped leaf,

一個完美對稱的葉片

or a butterfly with intricate patterns mirrored on each wing.

或在雙翅上有精緻對稱花紋的蝴蝶

But it turns out that asymmetry is pretty important, too,

但事實上，不對稱性也很重要

and more common than you might think,

而且可能比你想像的更常見

from crabs with one giant pincer claw

從只有一支巨螯的螃蟹

to snail species whose shells' always coil in the same direction.

到殼總是繞同一方向的蝸牛類

Some species of beans only climb up their trellises clockwise,

有些豆類只依順時針方向盤上棚架

others, only counterclockwise,

其它則依逆時針方向

and even though the human body looks pretty symmetrical on the outside,

雖然人體從外觀來看是對稱的

it's a different story on the inside.

但在體內卻是另一回事了

Most of your vital organs are arranged asymmetrically.

大多數重要器官是不對稱排列

The heart, stomach, spleen, and pancreas lie towards the left.

心臟、胃、脾臟和胰臟位置偏左

The gallbladder and most of your liver are on the right.

膽囊和大部分的肝臟則在右邊

Even your lungs are different.

連肺臟也是不一樣

The left one has two lobes, and the right one has three.

左邊的有兩葉，但右邊的有三葉

The two sides of your brain look similar, but function differently.

腦的兩半球看起來相似， 但功能卻不同

Making sure this asymmetry is distributed the right way is critical.

確保這不對稱性正確地分佈 是很重要的

If all your internal organs are flipped, a condition called situs inversus,

如果你的內臟器官左右翻轉， 這現象稱為「內臟逆轉」

it's often harmless.

通常是無害的

But incomplete reversals can be fatal,

但不完全的翻轉可能會致命

especially if the heart is involved.

特別是牽涉到心臟

But where does this asymmetry come from,

但這不對稱性是如何產生的？──

since a brand-new embryo looks identical on the right and left.

既然全新的胚胎 看起來左右相同的話

One theory focuses on a small pit on the embryo

有一理論著重於胚胎上的一個小凹槽

called a node.

稱為「原節」

The node is lined with tiny hairs called cilia,

原節的內膜上佈滿細小毛髮， 稱為「纖毛」

while tilt away from the head and whirl around rapidly,

這些纖毛傾離頭部，並快速旋轉

all in the same direction.

且全部朝同一方向

This synchronized rotation pushes fluid from the right side of the embryo

這同步的旋轉 推動液體從胚胎右邊

to the left.

流到左邊

On the node's left-hand rim,

在原節的左邊緣

other cilia sense this fluid flow

其他的纖毛感受到這液體流動

and activate specific genes on the embryo's left side.

並激活胚胎左半邊的特定基因

These genes direct the cells to make certain proteins,

這些基因指示細胞產生特定的蛋白質

and in just a few hours,

在數小時之內

the right and left sides of the embryo are chemically different.

胚胎左右兩邊的化學成分已經不同

Even though they still look the same,

即使它們看起來仍然一樣

these chemical differences are eventually translated into asymmetric organs.

這些化學成分差異 最後會轉變成不對稱器官

Asymmetry shows up in the heart first.

不對稱性是先在心臟顯現

It begins as a straight tube along the center of the embryo,

它在胚胎中央一開始是一個直管

but when the embryo is around three weeks old,

當胚胎約三週大時

the tube starts to bend into a c-shape

管子開始凹成 C 形

and rotate towards the right side of the body.

並向身體右側旋轉

It grows different structures on each side,

它左右兩邊長成不同的結構

eventually turning into the familiar asymmetric heart.

且最終變成 我們所熟知的不對稱心臟

Meanwhile, the other major organs emerge from a central tube

同時，其他主要器官從中央管形成

and grow towards their ultimate positions.

並朝最後的所在位置成長

But some organisms, like pigs, don't have those embryonic cilia

但有些生物 ─像豬─ 沒有胚胎纖維

and still have asymmetric internal organs.

卻還是有不對稱的內臟

Could all cells be intrinsically asymmetric?

是不是細胞本質上就不對稱呢？

Probably.

有可能

Bacterial colonies grow lacy branches that all curl in the same direction,

細菌菌落長成帶狀分枝， 它們都朝同一方向捲曲

and human cells cultured inside a ring-shaped boundary

人類細胞在環狀中空容器裡培養

tend to line up like the ridges on a cruller.

往往排成像油煎餅的斜紋隆起

If we zoom in even more,

如果放更大一些來看

we see that many of cells' basic building blocks,

可見許多細胞的基本組成部份

like nucleic acids, proteins, and sugars, are inherently asymmetric.

像核酸、蛋白質和醣類 都是天生不對稱

Proteins have complex asymmetric shapes,

蛋白質有複雜不對稱的形狀

and those proteins control which way cells migrate

這些蛋白質控制細胞轉移的方向

and which way embryonic cilia twirl.

以及胚胎纖毛的轉向

These biomolecules have a property called chirality,

這些生物分子有個特性， 稱為「手性」

which means that a molecule and its mirror image aren't identical.

意即：一個分子與其鏡像不相同

Like your right and left hands, they look the same,

像你的左右手看起來是一樣

but trying to put your right in your left glove proves they're not.

但試著把你的右手放入左手套裡， 証實它們是不一樣的

This asymmetry at the molecular level is reflected in asymmetric cells,

這分子層面上的不對稱 反映在不對稱的細胞、

asymmetric embryos,

不對稱的胚胎

and finally asymmetric organisms.

及最後不對稱的生物

So while symmetry may be beautiful,

所以雖然對稱是美的

asymmetry holds an allure of its own,

不對稱也有自己的吸引力

found in its graceful whirls,

顯現在其優雅的旋轉、

its organized complexity,

排列有序的複雜結構

and its striking imperfections.

及精彩動人的不完美

翻譯：Sophie Wu、Ann Chen