Symmetry is everywhere in nature, and we usually associate it with beauty: a perfectly shaped leaf, or a butterfly with intricate patterns mirrored on each wing. But it turns out that asymmetry is pretty important, too, and more common than you might think, from crabs with one giant pincer claw to snail species whose shells' always coil in the same direction. Some species of beans only climb up their trellises clockwise, others, only counterclockwise, and even though the human body looks pretty symmetrical on the outside, it's a different story on the inside. Most of your vital organs are arranged asymmetrically. The heart, stomach, spleen, and pancreas lie towards the left. The gallbladder and most of your liver are on the right. Even your lungs are different. The left one has two lobes, and the right one has three. The two sides of your brain look similar, but function differently. Making sure this asymmetry is distributed the right way is critical. If all your internal organs are flipped, a condition called situs inversus, it's often harmless. But incomplete reversals can be fatal, especially if the heart is involved. But where does this asymmetry come from, since a brand-new embryo looks identical on the right and left. One theory focuses on a small pit on the embryo called a node. The node is lined with tiny hairs called cilia, while tilt away from the head and whirl around rapidly, all in the same direction. This synchronized rotation pushes fluid from the right side of the embryo to the left. On the node's left-hand rim, other cilia sense this fluid flow and activate specific genes on the embryo's left side. These genes direct the cells to make certain proteins, and in just a few hours, the right and left sides of the embryo are chemically different. Even though they still look the same, these chemical differences are eventually translated into asymmetric organs. Asymmetry shows up in the heart first. It begins as a straight tube along the center of the embryo, but when the embryo is around three weeks old, the tube starts to bend into a c-shape and rotate towards the right side of the body. It grows different structures on each side, eventually turning into the familiar asymmetric heart. Meanwhile, the other major organs emerge from a central tube and grow towards their ultimate positions. But some organisms, like pigs, don't have those embryonic cilia and still have asymmetric internal organs. Could all cells be intrinsically asymmetric? Probably. Bacterial colonies grow lacy branches that all curl in the same direction, and human cells cultured inside a ring-shaped boundary tend to line up like the ridges on a cruller. If we zoom in even more, we see that many of cells' basic building blocks, like nucleic acids, proteins, and sugars, are inherently asymmetric. Proteins have complex asymmetric shapes, and those proteins control which way cells migrate and which way embryonic cilia twirl. These biomolecules have a property called chirality, which means that a molecule and its mirror image aren't identical. Like your right and left hands, they look the same, but trying to put your right in your left glove proves they're not. This asymmetry at the molecular level is reflected in asymmetric cells, asymmetric embryos, and finally asymmetric organisms. So while symmetry may be beautiful, asymmetry holds an allure of its own, found in its graceful whirls, its organized complexity, and its striking imperfections.
對稱在自然界裡無所不在 我們通常把它和美聯想在一起: 一個完美對稱的葉片 或在雙翅上有精緻對稱花紋的蝴蝶 但事實上,不對稱性也很重要 而且可能比你想像的更常見 從只有一支巨螯的螃蟹 到殼總是繞同一方向的蝸牛類 有些豆類只依順時針方向盤上棚架 其它則依逆時針方向 雖然人體從外觀來看是對稱的 但在體內卻是另一回事了 大多數重要器官是不對稱排列 心臟、胃、脾臟和胰臟位置偏左 膽囊和大部分的肝臟則在右邊 連肺臟也是不一樣 左邊的有兩葉,但右邊的有三葉 腦的兩半球看起來相似, 但功能卻不同 確保這不對稱性正確地分佈 是很重要的 如果你的內臟器官左右翻轉, 這現象稱為「內臟逆轉」 通常是無害的 但不完全的翻轉可能會致命 特別是牽涉到心臟 但這不對稱性是如何產生的?── 既然全新的胚胎 看起來左右相同的話 有一理論著重於胚胎上的一個小凹槽 稱為「原節」 原節的內膜上佈滿細小毛髮, 稱為「纖毛」 這些纖毛傾離頭部,並快速旋轉 且全部朝同一方向 這同步的旋轉 推動液體從胚胎右邊 流到左邊 在原節的左邊緣 其他的纖毛感受到這液體流動 並激活胚胎左半邊的特定基因 這些基因指示細胞產生特定的蛋白質 在數小時之內 胚胎左右兩邊的化學成分已經不同 即使它們看起來仍然一樣 這些化學成分差異 最後會轉變成不對稱器官 不對稱性是先在心臟顯現 它在胚胎中央一開始是一個直管 當胚胎約三週大時 管子開始凹成 C 形 並向身體右側旋轉 它左右兩邊長成不同的結構 且最終變成 我們所熟知的不對稱心臟 同時,其他主要器官從中央管形成 並朝最後的所在位置成長 但有些生物 ─像豬─ 沒有胚胎纖維 卻還是有不對稱的內臟 是不是細胞本質上就不對稱呢? 有可能 細菌菌落長成帶狀分枝, 它們都朝同一方向捲曲 人類細胞在環狀中空容器裡培養 往往排成像油煎餅的斜紋隆起 如果放更大一些來看 可見許多細胞的基本組成部份 像核酸、蛋白質和醣類 都是天生不對稱 蛋白質有複雜不對稱的形狀 這些蛋白質控制細胞轉移的方向 以及胚胎纖毛的轉向 這些生物分子有個特性, 稱為「手性」 意即:一個分子與其鏡像不相同 像你的左右手看起來是一樣 但試著把你的右手放入左手套裡, 証實它們是不一樣的 這分子層面上的不對稱 反映在不對稱的細胞、 不對稱的胚胎 及最後不對稱的生物 所以雖然對稱是美的 不對稱也有自己的吸引力 顯現在其優雅的旋轉、 排列有序的複雜結構 及精彩動人的不完美