How did teeth evolve? - Peter S. Ungar

譯者: Helen Chang 審譯者: S Sung

You may take them for granted, but your teeth are a marvel.

或許你把牙齒視為理所當然 但它們是個奇蹟

They break up all your food over the course of your life,

在你一生中，它們弄碎你所有的食物

while being strong enough to withstand breakage themselves.

而且自身夠強大，能承受破損

And they’re formed using only the raw materials

而且形成它們的原料

只來自它們先前所研磨的食物

from the food they grind down in the first place.

What’s behind their impressive strength?

它們令人印象深刻的力量背後是什麼？

Teeth rely on an ingenious structure that makes them both hard and tough.

牙齒靠著巧妙的結構 使得它們既硬且韌

Hardness can be thought of as the ability to resist a crack from starting,

硬度被視為​防止開始產生裂縫的能力

while toughness is what stops the crack from spreading

而韌性則阻止裂縫蔓延

Very few materials have both properties.

少有材料兼具這兩種屬性

For instance, glass is hard but not tough,

例如玻璃硬但不韌

while leather is tough but not hard.

而皮革則是韌而不硬

Teeth manage both by having two layers:

牙齒兩者兼具因為有兩層

a hard external cap of enamel, made up almost entirely of a calcium phosphate,

幾乎全由磷酸鈣組成的堅硬琺瑯外層

and beneath it, a tougher layer of dentin,

裡面有更堅硬的一層牙質

partly formed from organic fibers that make it flexible.

部分的組成是有機纖維使它有彈性

This amazing structure is created by two types of cells:

這了不起的結構由兩類細胞造成

ameloblasts that secrete enamel

分泌釉質的「造釉母細胞」

and odontoblasts that secrete dentin.

和分泌牙本質的「造牙本質細胞」

As they form teeth, odontoblasts move inward,

在形成牙齒時 造牙本質細胞向內移動

while ameloblasts move out and slough off when they hit the surface.

而造釉母細胞則向外移 一旦到了表面就會脫落

For enamel, this process produces long, thin strands,

這過程會產生長又薄的琺瑯質串

each about 60 nanometers in diameter.

每個直徑約 60 奈米

That’s one one-thousandth the width of a human hair.

是人髮千分之一的寬度

Those are bundled into rods, packed together,

緊緊綁成一捆捆

tens of thousands per square millimeter,

每平方毫米裡頭有數萬捆

to form the shield-like enamel layer.

形成盾狀的琺瑯層

Once this process is finished, your enamel can’t repair itself again

一旦完成這個過程 你的琺瑯不能再自我修復

because all the cells that make it are lost,

因為所有的生成細胞全消失了

so we’re lucky that enamel can’t be easily destroyed.

所以我們很幸運 因為琺瑯質不易被破壞

Odontoblasts use a more complex process, but unlike ameloblasts, they stick around,

形成造牙本質細胞的過程更複雜

但與造釉母細胞不同

它們還留著 在你一生中持續分泌牙本質

continuing to secrete dentin throughout your life.

Despite the differences in teeth across the mammalian order,

儘管哺乳類動物牙齒的差異

the underlying process of tooth growth is the same whether it’s for lions,

牙齒生長的基本過程相同

不論是獅子、袋鼠、大象

kangaroos,

elephants,

or us.

或者我們

What changes is how nature sculpts the shape of the tooth,

變化的是自然給牙齒形狀的造型

altering the folding and growth patterns

改變折疊和增長的模式

to suit the distinct diets of different species.

以適應不同物種的飲食模式

Cows have flat molar teeth with parallel ridges for grinding tough grasses.

牛有平行的平坦臼齒用於磨硬草

Cats have sharp crested molars, like blades, for shearing meat and sinew.

貓有像刀片一樣鋒利的尖頭臼齒 用來剪切肉和筋

Pigs have blunt, thick ones, useful for crushing hard roots and seeds.

豬有鈍、厚的臼齒 用於粉碎堅硬的根和種子

The myriad molars of modern mammals

現代哺乳動物的無數臼齒

can be traced back to a common form called “tribosphenic,"

可以追溯到常見的「三尖齒」這形式

which first appeared during the dinosaur age.

最先出現在恐龍時代

In the 19th Century, paleontologist Edward Drinker Cope

十九世紀古生物學家 愛德華 · 德林克 · 科普

developed the basic model for how this form evolved.

開發了這演變形式的基本模型

He hypothesized that it started with a cone-like tooth,

他假設始於一個錐形的牙齒

as we see in many fishes, amphibians, and reptiles.

正如我們在許多魚類 兩棲動物和爬行動物中看到的那樣

Small cusps were then added, so the tooth had three in a row,

然後加入小尖頭 使牙齒有連續三個尖頭

aligned front to back, and connected by crests.

前後對齊，由齒冠連接著

Over time, the cusps were pushed out of line to make triangular crowns.

隨著時間推移，牙尖被推開 不再呈一直線，而是形成三角形的冠

Adjacent teeth formed a continuous zigzag of crests for slicing and dicing.

相鄰的牙齒形成連續鋸齒形用於切割

A low shelf then formed at the back of each set of teeth,

然後每組牙齒的後面形成低架子

which became a platform for crushing.

成了壓碎的平台

As Cope realized, the tribosphenic molar served as the jumping-off point

正如科普所意識到的，三尖臼齒成了

for the radiation of specialized forms to follow,

隨後各專門牙齒的起點

each shaped by evolutionary needs.

每個都隨著進化需求而塑造

Straighten the crests and remove the shelf,

弄平波峰並去掉低架

and you’ve got the conveniently bladed teeth of cats and dogs.

就有了貓和狗好用的牙齒

Remove the front cusp, raise the shelf, and you’ve got our human molars.

去掉前尖，抬高低架 就成了人類的臼齒

A few additional tweaks get you a horse or cow tooth.

一些額外的調整成了馬或牛的牙齒

Some details in Cope’s intuitive hypothesis proved wrong.

某些科普直觀假設的細節是錯的

But in the fossil record,

但在化石記錄中

there are examples of teeth that look just as he predicted

有些牙齒就像他所預測的樣子

and we can trace the molars of all living mammals back to that primitive form.

所有現存哺乳類動物的臼齒 都可被追溯到他的原始模型

Today, the ability to consume diverse forms of food

今天能夠食用不同形式食物的能力

enables mammals to survive in habitats

使哺乳動物能夠在各種棲息地中生存

ranging from mountain peaks and ocean depths

從山峰、海洋深處

to rainforests and deserts.

到熱帶雨林和沙漠

So the success of our biological class is due in no small measure

所以我們生物綱的成功 有相當的程度是在於

to the remarkable strength and adaptability

哺乳類動物卑微臼齒的 卓越實力和適應性

of the humble mammalian molar.