Basically, everyone has them: fish, birds,

基本上，大家都有尾巴：魚類，鳥類

most reptiles, and even some of our closest relatives.

大多數爬蟲類，甚至是我們的近親。

So why are we missing out?

那麼，為什麼我們的尾巴不見了？

Since tails first evolved at least 500 million years ago,

自從 5 億年前有尾巴以來

they've taken on every role imaginable.

它們就承擔了所有可以想像的角色。

Geckos use them to store fat.

壁虎用它來儲存脂肪。

Birds use them to steer through the air,

鳥類用它在飛行時掌控方向

and rattlesnakes use them to scare off predators.

響尾蛇用它來嚇跑掠食者。

But for most mammals, they serve one major purpose:

但是對於大多數哺乳動物來說，它有一個主要的功能：

balance.

平衡。

Yet, as you get closer to humans on the evolutionary tree,

然而，在進化成人類的過程中

tails disappear. Gorillas don't have them,

尾巴消失了。大猩猩沒有尾巴

and neither do chimps or any other apes,

黑猩猩或任何其他的猿類也沒有

including us, of course.

當然，包括我們在內。

To understand why, take a look at how we walk.

要了解原因，請看看我們如何走路。

Some of us primates crouch with our chest held diagonally

有些靈長類動物是蹲著

to the ground. Others like gibbons and humans

胸部傾斜地貼近面行走。 其他像是長臂猿和人類

can walk completely upright.

可以完全直立行走。

Now, walking like this gives us a huge advantage

這樣的走路姿勢，給我們帶來了莫大的好處

because unlike four-legged animals,

因為不同於四條腿動物

which have to pour energy into every step they take,

它們每走一步都必須耗費精力

two legs take advantage of gravity,

兩條腿走路則可以利用地心引力

which does some of the work for us.

地心引力減輕了一些我們走路的負擔。

You see, each time we take a step,

你知道，每當我們邁出一步

gravity pulls us forward.

地心引力就會拉動我們前進。

The end result is that when we walk,

最終的結果是，當我們走路時

we use around 25% less energy than walking on all fours.

我們使用的能量比用四肢走路要少 25％。

And in the wild, every ounce of energy you save

在野外，你儲存的每一分能量

can mean the difference between survival and starvation.

都可能是影響生存或飢餓的關鍵。

But this way of getting around also totally

但這種走路方式

eliminates the need for a tail

也完全除去了對尾巴的需求

because even though a human head

因為即使一個人的頭部

weighs a hefty 5 kilograms,

重達 5 公斤

it sits on top of the body when you walk,

當你走路時，它是位在身體的最上方

not in front, so you don't need a tail as a counterbalance.

而不是前方，所以你不需要用到尾巴來調節平衡。

Pretty disappointing, huh?

很令人失望，是吧？

That being said, you can still see a reminder

儘管如此，你仍然可以看到證據顯示

of a time when our ancient primate ancestors had one.

我們古代的靈長類祖先，曾經有過這麼一段時間是有尾巴的。

Just look at a human spine.

只要看看人類的脊柱就知道了。

You can see how the last few bones

你可以看到脊柱最尾端

are partially fused together.

非完整組合的幾塊骨頭。

That's your tailbone.

這就是你的尾骨。

It's all that's left of our tail,

我們的尾巴只剩下這些了

and, yes, it's sad and pathetic, and you can't wag it.

沒錯，很可悲，而且你還不能搖它。

Now, in rare cases, babies are born

在很罕見的情況下，嬰兒出生時

with what looks like a tail,

長著像尾巴的東西

but that's not what's really going on.

但那並不是真正的尾巴。

Most often, these tails are actually tumors,

通常，這些尾巴實際上是腫瘤

cysts, or even a parasitic twin.

囊腫，甚至可能是連體嬰。

Even more occasionally, they're a true

更少見的是，它們是真正

outgrowth of the spine but are completely boneless,

從脊柱長出來的，只是沒有骨頭

a soft tube made entirely of fat and tissue.

是一種完全由脂肪和組織構成的管狀物。

These types of tails usually form as a birth defect,

這些類型的尾巴通常是一種天生的缺陷

a deformity of the spine called spina bifida.

是一種發生在脊柱的病變稱為脊柱裂。

And in these situations, doctors will surgically

在這種情況下，醫生會在不傷害嬰兒的情形下

remove the tails with no harm to the baby.

用手術將尾巴切除。

But as cool as it might sound to have an extra limb

聽起來似乎很酷，我們多出了一肢，可以利用它

to swing through the trees or keep mosquitoes away,

在樹上盪來盪去，或是驅趕蚊子

we are who we are today because, well, we don't.

我們之所以成為現在的我們，因為我們沒有尾巴。