They break up all your food over the course of your life, while being strong enough to withstand breakage themselves.

牙齒嚼碎你生活中所有的食物，而且它非常堅固，可免於毀損。

And they're formed using only the raw materials from the food they grind down in the first place.

而牙齒的組成源自被它們磨碎的食物的原物料。

What's behind their impressive strength?

它們擁有如此驚人力量的原因為何？

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

牙齒擁有巧妙的結構，使得它們又硬又強韌。

Hardness can be thought of as the ability to resist a crack from starting, while toughness is what stops the crack from spreading.

硬度可用來預防裂縫產生，而韌性則可防止裂縫擴散。

Very few materials have both properties.

很有東西兼具這兩種屬性。

For instance, glass is hard but not tough, while leather is tough but not hard.

例如，玻璃堅硬卻沒有韌性，而皮革則是有韌性卻不硬。

Teeth manage both by having two layers: a hard external cap of enamel, made up almost entirely of a calcium phosphate, and beneath it, a tougher layer of dentin, partly formed from organic fibers that make it flexible.

牙齒因為有兩層而兩者兼具：幾乎全由磷酸鈣組成的堅硬琺瑯質外層，裡面是更堅硬的牙質，部分組成為使其擁有彈性的有機纖維。

This amazing structure is created by two types of cells: ameloblasts that secrete enamel and odontoblasts that secrete dentin.

這種驚人的結構由兩種細胞生成：分泌釉質的「造釉母細胞」和分泌牙本質的「造牙本質母細胞」。

As they form teeth, odontoblasts move inward, while ameloblasts move out and slough off when they hit the surface.

牙齒形成時，造牙本質母細胞向內移動，造釉母細胞則向外移，一旦到達表面就會脫落。

For enamel, this process produces long, thin strands, each about 60 nanometers in diameter.

這過程會產生長又薄的琺瑯質串，每個直徑約 60 奈米。

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

此寬度相當於人髮的千分之一。

Those are bundled into rods, packed together, tens of thousands per square millimeter, to form the shield-like enamel layer.

緊緊綁成一捆捆，每平方毫米裡頭有數萬捆，形成盾狀的琺瑯層。

Once this process is finished, your enamel can't repair itself again because all the cells that make it are lost, so we're lucky that enamel can't be easily destroyed.

一旦完成這個過程，琺瑯質就不能再自我修復，因為其生成細胞已經消失了，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, continuing to secrete dentin throughout your life.

造牙本質母細胞的生成過程更複雜，但與造釉母細胞不同，它們在你一生中持續分泌牙本質。

Despite the differences in teeth across the mammalian order, the underlying process of tooth growth is the same whether it's for lions, kangaroos, elephants or us.

儘管哺乳類動物的牙齒各不相同，牙齒生長的過程卻相同，不論是獅子、袋鼠、大象或人類。

What changes is how nature sculpts the shape of the tooth, altering the folding and growth patterns to suit the distinct diets of different species.

產生變化的是打自然賦予牙齒的形狀，改變牙齒的弧度和生長模式，以適應不同物種的飲食習慣。

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

牛具有平坦的臼齒和平行的齒脊以磨碎韌草。

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

貓具有刀片般鋒利的尖頭臼齒，用來切斷肉和筋。

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

豬具有又鈍又厚的臼齒，用來粉碎堅硬的根和種子。

The myriad molars of modern mammals can be traced back to a common form called "tribosphenic", which first appeared during the dinosaur age.

各種現代哺乳類動物的臼齒可追溯到一種常見形狀，名為「三尖齒」，最早出現在恐龍時期。

In the 19th century, paleontologist Edward Drinker Cope developed the basic model for how this form evolved.

十九世紀，古生物學家 Edward Drinker Cope 提出其進化的基本模型。

He hypothesized that it started with a cone-like tooth, as we see in many fishes, amphibians, and reptiles.

他假設牙齒的初始形狀為錐形，就像許多魚類、兩棲動物和爬行動物的牙齒。

Small cusps were then added, so the tooth had three in a row, aligned front to back, and connected by crests.

接下來出現小牙尖，一排有三個，前後對齊，由齒冠連接。

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

隨著時間推移，牙尖受到推擠而不再是一直線，形成三角形的齒冠。

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

相鄰的牙齒形成連續的鋸齒狀齒冠，用來進行各種切割。

A low shelf then formed at the back of each set of teeth, which became a platform for crushing.

然後每組牙齒的最後面變成較低的頰棚，專門用來壓碎食物。

As Cope realized, the tribosphenic molar served as the jumping-off point for the radiation of specialized forms to follow, each shaped by evolutionary needs.

正如Cope的發現，三尖齒為各種牙齒的起點，隨著進化需求演變成不同齒型。

Straighten the crests and remove the shelf, and you've got the conveniently bladed teeth of cats and dogs.

去掉牙脊和頰棚，就成了貓和狗鋒利的牙齒。

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

去掉牙尖，抬高頰棚，就成了人類的臼齒。

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

調整一下就會形成馬或牛的牙齒。

Some details in Cope's intuitive hypothesis proved wrong.

Cope提出的直觀假設有些細節是錯的。

But in the fossil record, there are examples of teeth that look just as he predicted and we can trace the molars of all living mammals back to that primitive form.

但在化石記錄中，有些牙齒就如同他的預測，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 enables mammals to survive in habitats ranging from mountain peaks and ocean depths to rainforests and deserts.

食用各種食物的能力使哺乳類動物能在各種棲息地中生存，包括山間、海洋深處、熱帶雨林和沙漠。

So, the success of our biological class is due in no small measure to the remarkable strength and adaptability of the humble mammalian molar.

所以哺乳類動物的生命得以延續，有相當程度要歸功於小小的臼齒具備優勢和適應性。