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牙齿的进化

How did teeth evolve? - Peter S. Ungar

“牙齿由坚硬的物质构成 它们禁得住火的灼烧 水的浸泡 甚至能在坟墓里留存几个世纪”-玛丽·奥托 《牙齿》
“The teeth are made from stern stuff. They can withstand floods, fires, even centuries in the grave.”–Mary Otto, Teeth
你可能认为他们是理所当然的
You may take them for granted,
但你的牙齿是个奇迹
but your teeth are a marvel.
它们在你的生命过程中把你所有的食物都分解了
They break up all your food over the course of your life,
同时又能强壮到足以承受自身破损
while being strong enough to withstand breakage themselves.
并且他们仅仅由最初磨碎的
And they’re formed using only the raw materials
食物里的原料构成
from the food they grind down in the first place.
是什么让它们有了令人钦佩的力量呢?
What’s behind their impressive strength?
牙齿依靠其巧妙的结构使得它们坚硬且坚韧
Teeth rely on an ingenious structure that makes them both hard and tough.
硬度可以被认为是一开始承受破裂的能力
Hardness can be thought of as the ability to resist a crack from starting,
而韧度就是阻止裂缝扩张的能力
while toughness is what stops the crack from spreading.
几乎没有材料能同时拥有这两种性能
Very few materials have both properties.
例如 玻璃硬但不韧
For instance, glass is hard but not tough,
而皮革则韧但不硬
while leather is tough but not hard.
牙齿都由两部分组成:
Teeth manage both by having two layers:
一部分是几乎全部由
a hard external cap of enamel
磷酸钙构成的坚硬瓷釉外帽
made up almost entirely of a calcium phosphate,
在它下面 是一层更坚硬的牙质
and beneath it, a tougher layer of dentin,
部分由使其变得柔韧的有机纤维构成
partly formed from organic fibers that make it flexible.
这种令人惊讶的结构是由两种细胞创造的:
This amazing structure is created by two types of cells:
造釉细胞分泌瓷釉
ameloblasts that secrete enamel
成牙质细胞分泌牙质
and odontoblasts that secrete dentin.
在它们塑造牙齿的时候 成牙质细胞向内移动
As they form teeth, odontoblasts move inward,
而造釉细胞向外移动 它们碰到表面时就会脱落
while ameloblasts move out and slough off when they hit the surface.
对于瓷釉来说 这个过程产生了许多细长的线
For enamel, this process produces long thin strands,
每根直径大约为60毫微米
each about 60 nanometers in diameter.
相当于人类头发宽度的千分之一
That’s one one-thousandth the width of a human hair.
它们被捆绑成条 然后打包在一起
Those are bundled into rods, packed together,
每平方毫米就有成千上万条线
tens of thousands per square millimeter
构成了像盾一样的瓷釉层
to form the shield-like enamel layer.
一旦这个过程结束
Once this process is finished,
你的瓷釉层就无法再次自行修复
your enamel can’t repair itself again,
因为所有制造它的细胞都失活了
because all the cells that make it are lost,
所以我们该庆幸瓷釉层不能被轻易破坏
so we’re lucky that enamel can’t be easily destroyed.
成牙质细胞使用更复杂的程序
Odontoblasts use a more complex process,
但和瓷釉细胞不同 它们黏附在周围
but unlike ameloblasts, they stick around,
继续在你的生命里分泌牙质
continuing to secrete dentin throughout your life.
尽管哺乳动物的牙齿各有不同
Despite the differences in teeth across the mammalian order,
但牙齿生长的内在过程都是一样的
the underlying process of tooth growth is the same,
无论是狮子 袋鼠 大象还是我们
whether it’s for lions, kangaroos, elephants or us.
不同的是大自然如何雕刻牙齿的形状
What changes is how nature sculpts the shape of the tooth,
改变折叠和生长模式
altering the folding and growth patterns
以适应不同物种的饮食差异
to suit the distinct diets of different species.
牛拥有扁平的臼齿 上面有平行的脊以便磨硬草
Cows have flat molar teeth with parallel ridges for grinding tough grasses.
猫拥有尖峰般锐利的臼齿 就像刀片
Cats have sharp crested molars, like blades,
可以切开筋肉
for shearing meat and sinew.
猪拥有粗大的钝齿
Pigs have blunt, thick ones,
有利于粉碎硬根和种子
useful for crushing hard roots and seeds.
当代哺乳动物多种多样的臼齿
The myriad molars of modern mammals
都可以追溯到一种共同的形态叫“磨蝶臼齿型”
can be traced back to a common formcalled “tribosphenic”,
这最早出现在恐龙时代
which first appeared during the dinosaur age.
在19世纪 古生物学者爱德华·德林克科佩
In the 19th Century, paleontologist Edward Drinker Cope
开发出了关于该形态如何演变的基础模型
developed the basic model for how this form evolved.
他假设牙齿最开始的形态是圆锥状
He hypothesized that it started with a cone-like tooth,
正如我们看到的很多鱼类 两栖动物和爬行动物的牙齿
as we see in many fishes, amphibians, and reptiles.
然后增加了小的尖角
Small cusps were then added,
因此一排有三颗牙齿
so the tooth had three in a row,
前后成一直线
aligned front to back,
与波峰相连
and connected by crests.
随着时间的流逝 小尖角被挤出了线
Over time, the cusps were pushed out of line
形成了三角冠
to make triangular crowns.
相邻的牙齿形成了连续的Z字形波峰
Adjacent teeth formed a continuous zigzag of crests
用于咬断和咬碎食物
for slicing and dicing.
然后每颗牙齿背部形成了低架子
A low shelf then formed at the back of each set of teeth,
成为了压碎食物的平台
which became a platform for crushing.
正如科佩所言 磨蝶臼齿型
As Cope realized, the tribosphenic molar
相当于之后的各种特殊形态
served as the jumping-off point
发展中的一个分叉点
for the radiation of specialized forms to follow,
每一种都是为进化需求发展而成
each shaped by evolutionary needs.
把波峰拉直再把架子移走
Straighten the crests and remove the shelf,
你就得到了猫和狗的方便的刃齿
and you’ve got the conveniently bladed teeth of cats and dogs.
把前牙尖移走 再拉高架子
Remove the front cusp, raise the shelf,
你就得到了人类的臼齿
and you’ve got our human molars.
稍微扭转一下就得到了马或牛的牙齿
A few additional tweaks get you a horse or cow tooth.
科佩凭直觉作出的假设中有一些细节被证明是错误的
Some details in Cope’s intuitive hypothesis proved wrong.
但在化石记录中
But in the fossil record,
从牙齿的化石例子看来又和他预言的一样
there are examples of teeth that look just as he predicted.
我们能从所有现存哺乳动物的臼齿
And we can trace the molars of all living mammals
追溯到原始臼齿形态
back to that primitive form.
今天 拥有消灭各种各样食物的能力
Today, the ability to consume diverse forms of food
使得哺乳动物得以存活在从
enables mammals to survive in habitats
高山和深海到
ranging from mountain peaks and ocean depths
热带雨林和沙漠的栖息地中
to rainforests and deserts.
因此生物研究的成功
So the success of our biological class
很大程度上要归结于
is due in no small measure to
低级哺乳动物臼齿的非凡力量和适应力
the remarkable strength and adaptability of the humble mammalian molar.
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视频概述

牙齿是如何形成又是如何进化成当前形态的呢?

听录译者

jm

翻译译者

kayra

审核员

审核员JC

视频来源

https://www.youtube.com/watch?v=wrPEjEqURJg

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