In 2017 alone,

單單 2017 年，

Americans spent $129 billion on dental visits,

美國人便花費 1 千 2 百 90 億美元看牙醫，

and an estimated 90% of adults in the US have fillings.

而且約有 90% 美國成年人有補牙。

But many animals don't have this problem

然而，許多動物卻沒有這樣的問題，

because they can regrow their teeth,

因為牠們的牙齒可以重新生長，

replacing old damaged ones with brand new pearly whites.

以新生的珍白牙齒取代受損的老舊牙齒。

Wouldn't it be great if you could do that?

如果人類的牙齒也有這功能，那不是很棒嗎？

Well, there's actually an important trade-off

這樣的麻煩其實隱含著

for this nuisance.

重要的取捨。

Only a handful of mammals can regrow teeth multiple times

相較於 5 萬種爬蟲類和魚類，

compared to the 50,000 species of reptiles and fish.

僅有少數哺乳類動物可以多次重新生牙。

Take geckos for example.

好比說壁虎，

They replace all 100 or so teeth every three to four months,

牠們每 3 至 4 個月便會換掉所有的牙齒，大約 100 顆左右，

and since geckos can live for six to 10 years,

由於壁虎可以活到 6 至 10 年，

they'll grow anywhere from 1,800

牠們一生可以長 1 千 8 百顆至

to 4,000 teeth in a lifetime.

4 千顆牙齒。

That's all thanks to a special type of cell in their gums

這都要感謝牠們的牙齦裡一種特殊的細胞，

called stem cells.

稱為幹細胞。

Stem cells are handy because they can morph into different

幹細胞非常有用，因為它們可以合成不同細胞，

cells when needed,

尤其在有需要時，

like tooth stem cells to build new teeth.

譬如牙齒幹細胞可以生成新的牙齒。

Humans have these stem cells when we're younger,

人類年輕的時候擁有這些幹細胞，

but after our adult teeth grow in,

但當我們的成年牙齒生長後，

the stem cells die and disappear.

這些幹細胞就會死去消逝。

To understand why, let's take a journey back in time

想要了解原因，讓我們回到過去，

to about 320 million years ago when mammals

大約 3 億 2 千萬年前，當時哺乳類

and reptiles split off.

和爬蟲類分開。

In addition to many obvious differences,

除了許多明顯外貌上的差異，

another change that eventually emerged was tooth shape.

另一個永久差異是牙齒結構。

Reptiles, for example, are what's called generalists,

以爬蟲類來說，又稱廣適者，

meaning they eat the animals they can get their teeth on.

意味牠們可以把吃下肚的動物的牙齒歸於己用，

And for that, they needed teeth with the same size and shape

為此，牠們需要同樣尺寸結構的牙齒，

to keep prey from escaping their mouths.

以防止獵物從牠們的嘴裡逃出。

Mammals, on the other hand, developed more specific diets,

另一方面，哺乳類發展固定的獵食習慣，

like grazers who only eat grass and hunters who rip flesh

譬如食草動物只吃草，獵人只取用獵物

from their kills.

身上的肉。

As a result, mammals evolved different-shaped teeth

因此，哺乳類演化成不同結構的牙齒

for different purposes.

有其不同的目的。

And it's this difference that could also explain

這樣的差異便可以解釋

why mammals can't regrow more teeth.

為何哺乳類不能重新生長更多的牙齒。

Now, let's say you could regrow your molars multiple times, for example.

現在，假設說你的臼齒可以重新生長好幾次。

It's important that the top and bottom sets match up.

上排和下排的牙齒能夠咬合是非常重要的。

Otherwise, they can't grind food as efficiently.

如果不這麼做，就不能正確有效率地咀嚼。

That sounds good in principle, but with each new set,

聽起來好像理論上可行，但是每長一顆新牙，

there's a risk that the regrown teeth won't line up.

這些新牙會有無法排列整齊的風險。

So the leading theory is that adult humans can't regrow

所以研究指出成人無法生長新牙，

our teeth because it was better for survival

是因為如要生存，

to only grow one well-aligned adult set.

只要生長一組排列良好的成人牙齒即可。

However, if you still wish you could regrow a tooth,

然而，如果你還是希望能重新長牙，

there may be a way in the future.

或許未來可行。

Using lasers and drugs, scientists have helped rats and mice

科學家已經透過雷射以及藥物，幫助老鼠

regrow damaged tissue in cavity-ridden teeth

在蛀牙的牙洞中重新修復受損的組織。

with the idea that if you could regrow tooth tissue,

以這樣的做法來說，如果你的牙齒組織可以重新生長，

you could eventually regrow entire teeth,

最終必定也可以長出新的一副牙齒，

though no human testing has been done on that yet.

不過目前這個實驗還沒有做過人類測試就是了。

So keep flossing and keep seeing your dentist.

所以還是繼續用牙線清潔牙齒以及定期看牙醫吧。

At least for now.

至少目前來說是必要的。