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You may take them for granted, but your teeth are a marvel.
你也许不太重视它们，但是你的牙齿是非凡之物。
They break up all your food over the course of your life, while being strong enough to withstand breakage themselves.
它们把你一生所吃的食物咬碎，与此同时却足够坚固使自己不会损坏。
And they're formed using only the raw materials from the food they grind down in the first place.
组成它们的原材料全部来自于起初磨碎的食物。
What's behind their impressive strength?
是什么使牙齿拥有异常的强度？
Teeth rely on an ingenious structure that makes them both hard and tough.
牙齿依靠其巧妙的构造来使它们坚硬和强韧。
Hardness can be thought of as the ability to resist a crack from starting,
坚硬可看作是抵御出现破裂的能力，
while toughness is what stops the crack from spreading.
而强韧则表示能够阻止破裂进一步扩散。
Very few materials have both properties.
世上极少有材料兼备这两种特性。
For instance, glass is hard but not tough, while leather is tough but not hard.
举个例子，玻璃坚固却不强韧，皮革则强韧却不坚固。
Teeth manage both by having two layers: a hard external cap of enamel, made up almost entirely of a calcium phosphate,
牙齿兼备两种特性因为它由两层组成：外层由坚硬的牙釉质包围，几乎完全由磷酸钙组成，
and beneath it, a tougher layer of dentin, partly formed from organic fibers that make it flexible.
牙釉质之下，有一层更加强韧的牙本质，部分由有机纤维组成，它们能够使牙齿强韧。
This amazing structure is created by two types of cells:
这奇妙的构造是由两种细胞生成的：
ameloblasts that secrete enamel and odontoblasts that secrete dentin.
分泌牙釉的成釉细胞以及分泌牙本质的成牙本质细胞。
As they form teeth, odontoblasts move inward, while ameloblasts move out and slough off when they hit the surface.
在形成牙齿时，成牙本质细胞向牙内部扩散，成釉细胞则向牙的外部生长，并在抵达牙齿表面时脱落。
For enamel, this process produces long, thin strands, each about 60 nanometers in diameter.
在生成牙釉质的过程中会产生细长的纤维状物，每条纤维的直径宽约60纳米。
That's one one-thousandth the width of a human hair.
相当于人类发丝直径的千分之一。
Those are bundled into rods, packed together, tens of thousands per square millimeter, to form the shield-like enamel layer.
这些物质集合成捆，紧密地排列起来，每平方毫米可达到上万个，组成如盾牌般坚硬的牙釉质层。
Once this process is finished, your enamel can't repair itself again
一旦这个过程完成，你的牙釉不会再自我修复，
because all the cells that make it are lost, so we're lucky that enamel can't be easily destroyed.
因为所有制造牙釉质的细胞都已经消失，所以我们很幸运牙釉质并不容易遭到破坏。
Odontoblasts use a more complex process,
成牙本质细胞则需要更复杂的过程，
but unlike ameloblasts, they stick around, continuing to secrete dentin throughout your life.
但与成釉细胞不同，它们不会消失，会在我们有生之年继续分泌牙本质。
Despite the differences in teeth across the mammalian order,
虽然在哺乳目中各类生物的牙齿不同，
the underlying process of tooth growth is the same whether it's for lions, kangaroos, elephants, or us.
但是牙齿的基本生长过程则一样，不论是狮子、袋鼠、大象，还是人类自己。
What changes is how nature sculpts the shape of the tooth,
不同之处在于大自然如何塑造牙齿的形状，
altering the folding and growth patterns to suit the distinct diets of different species.
改变牙齿的折层和生长模式，来契合各个物种不同的饮食习惯。
Cows have flat molar teeth with parallel ridges for grinding tough grasses.
牛的臼齿扁平，有平行的牙脊来咬碎坚韧的草。
Cats have sharp crested molars, like blades, for shearing meat and sinew.
猫的臼齿呈波形，如锋刃般尖锐，用来撕碎肌肉和韧带。
Pigs have blunt, thick ones, useful for crushing hard roots and seeds.
猪有又钝又厚的牙齿，有利于压碎坚硬的根和种子。
The myriad molars of modern mammals can be traced back to a common form called “tribosphenic,"
现代哺乳类的臼齿千变万化，可以追溯到一个同源形式，叫做“磨楔式齿”，
which first appeared during the dinosaur age.
这种牙齿最早在恐龙时代出现。
In the 19th Century, paleontologist Edward Drinker Cope developed the basic model for how this form evolved.
19世纪，古生物学家爱德华·德林克·科普发展出了一个基本模型，用以表示该形式的演化过程。
He hypothesized that it started with a cone-like tooth, as we see in many fishes, amphibians, and reptiles.
他推测牙齿起初都呈尖状，正如我们在鱼类、两栖类与爬行类动物中所见到的那样。
Small cusps were then added, so the tooth had three in a row, aligned front to back, and connected by crests.
后来，更小的尖端出现，所以一颗牙齿有成排的三个尖头，从前往后排列，并由凹槽相连接。
Over time, the cusps were pushed out of line to make triangular crowns.
随着时间推移，这些尖端逐渐移位，形成了三角形的牙冠。
Adjacent teeth formed a continuous zigzag of crests for slicing and dicing.
相邻的牙齿则形成连贯性的之字形的凹槽，用来切碎和咬碎食物。
A low shelf then formed at the back of each set of teeth, which became a platform for crushing.
之后，每颗牙齿的后部都形成了一个低牙架，这为压碎食物提供了一个平台。
As Cope realized, the tribosphenic molar served as the jumping-off point
科普意识到，以磨楔式臼齿这一模式作为跳板，
for the radiation of specialized forms to follow, each shaped by evolutionary needs.
大量不同的牙齿形式在此基础上衍生，每种牙齿的塑造都由进化的需求完成。
Straighten the crests and remove the shelf, and you've got the conveniently bladed teeth of cats and dogs.
把凹槽拉直，移除牙架，你就得到猫狗的尖齿。
Remove the front cusp, raise the shelf, and you've got our human molars.
移除前尖端，提升牙架，你就得到我们人类的臼齿。
A few additional tweaks get you a horse or cow tooth.
轻微扭转调整，你就可以得到马或牛的牙齿。
Some details in Cope's intuitive hypothesis proved wrong.
在科普凭直觉提出的推论中，有些细节是错误的。
But in the fossil record, there are examples of teeth that look just as he predicted
但是化石记录显示，有些牙齿样本与他推测的一样，
and we can trace the molars of all living mammals back to that primitive form.
并且所有哺乳生物的臼齿也可以追溯到那个原始形式。
Today, the ability to consume diverse forms of food enables mammals to survive in habitats
如今，哺乳类动物可以摄入多元化食物的这种能力，使它们可以在多种栖息地生存，
ranging from mountain peaks and ocean depths to rainforests and deserts.
从高山顶峰到深海底层，从热带雨林到炙热沙漠。
So the success of our biological class is due in
所以，我们所属的哺乳纲动物之所以成功，
no small measure to the remarkable strength and adaptability of the humble mammalian molar.
大部分要归功于不起眼的哺乳类臼齿所具有的惊人的强度和适应性。
