Why is it that we study other animals

為何是我們研究其它動物的腦

instead of them studying us?

而不是我們的被研究呢？

What does a human brain have or do

人腦擁有什麼

that no other brain does?

其它動物的腦沒有的呢？

When I became interested in these questions about 10 years ago,

十幾年前我開始 對這類的問題感興趣

scientists thought they knew what different brains were made of.

那時的科學家自認已知差異所在

Though it was based on very little evidence,

即便可依據的證據非常少

many scientists thought that all mammalian brains,

許多科學家認為所有哺乳類的腦

including the human brain,

包括人類的頭腦

were made in the same way,

都有著相同的構造

with a number of neurons that was always

有著一定數量的神經元

proportional to the size of the brain.

而數量與腦的大小成正比例

This means that two brains of the same size,

這表示同樣大小的兩個腦

like these two, with a respectable 400 grams,

例如這兩約400公克的

should have similar numbers of neurons.

應有差不多數量的神經元

Now, if neurons are the functional

如果神經元是腦裡面

information processing units of the brain,

訊息處理的最小單位

then the owners of these two brains

那這兩腦的主人

should have similar cognitive abilities.

應該有著差不多一樣的認知能力

And yet, one is a chimp,

然而一個是黑猩猩的

and the other is a cow.

另一個是乳牛的

Now maybe cows have a really rich

或許乳牛有著

internal mental life and are so smart

心靈極豐富的內在美

that they choose not to let us realize it,

聰明到不讓我們發現這些

but we eat them.

但我們吃牠們

I think most people will agree

我想大部分的人會同意

that chimps are capable of much more complex,

黑猩猩比乳牛更有能力處理

elaborate and flexible behaviors than cows are.

複雜、精細且有彈性的行為

So this is a first indication that the

所以這意味著

"all brains are made the same way" scenario

「所有腦都有相同的構造」的想法

is not quite right.

不完全正確

But let's play along.

但先讓我們繼續下去

If all brains were made the same way

如果所有腦的構造都是一樣的

and you were to compare animals with brains of different sizes,

你比較動物之間不同大小的腦

larger brains should always have more neurons

那較大的腦就該有較多的神經元

than smaller brains, and the larger the brain,

腦愈大的話

the more cognitively able its owner should be.

其主人也該有較強的認知能力

So the largest brain around should also be

所以世上最大的腦

the most cognitively able.

都該有最強大的認知能力

And here comes the bad news:

但壞消息來了

Our brain, not the largest one around.

我們人類的腦不是最大的

It seems quite vexing.

這聽起來有點傷腦筋

Our brain weighs between 1.2 and 1.5 kilos,

我們人類的腦大約是 1.2~1.5 公斤

but elephant brains weigh between four and five kilos,

但大象的腦是 4~5 公斤

and whale brains can weigh up to nine kilos,

而鯨的腦重達 9 公斤

which is why scientists used to resort to saying

這就是為何科學家總是說

that our brain must be special

我們的腦一定是特別

to explain our cognitive abilities.

這有便於解釋我們的認知能力

It must be really extraordinary,

我們的腦一定是很非比尋常

an exception to the rule.

排除在一般規則外

Theirs may be bigger, but ours is better,

牠們的腦或許更大但我們的更好

and it could be better, for example,

舉例來說

in that it seems larger than it should be,

我們的腦比它應該有的樣子更大

with a much larger cerebral cortex than we should have

而且依我們身軀的大小來看

for the size of our bodies.

它有著超過應有的大腦皮質數量

So that would give us extra cortex

這些「過多」的大腦皮質讓我們

to do more interesting things than just operating the body.

做出比操控身軀更多有趣的事情

That's because the size of the brain

因為通常腦的大小

usually follows the size of the body.

跟身材的大小有關係

So the main reason for saying that

主要會說「我們的腦

our brain is larger than it should be

比它應有的樣子更大」

actually comes from comparing ourselves

是來自於

to great apes.

我們跟大猩猩作的比較

Gorillas can be two to three times larger than we are,

大猩猩的身材比我們大上二到三倍

so their brains should also be larger than ours,

所以牠們的腦也該比我們的大才對

but instead it's the other way around.

事實上卻相反

Our brain is three times larger than a gorilla brain.

我們的腦比大猩猩的大上三倍

The human brain also seems special

從能量消耗的角度來看

in the amount of energy that it uses.

人類的腦也是很特別的

Although it weighs only two percent of the body,

雖然腦只佔身體重量的 2%

it alone uses 25 percent of all the energy

光是它就消耗了身體

that your body requires to run per day.

每天需要25% 的能量

That's 500 calories out of a total of 2,000 calories,

也就是一天 2,000 卡路里中的 500 卡

just to keep your brain working.

都是花在腦的運作上

So the human brain is larger than it should be,

人腦不僅比它應有的大小還大

it uses much more energy than it should,

且還消耗掉更多能量

so it's special.

所以說它是特別的

And this is where the story started to bother me.

這也是一開始困擾我的地方

In biology, we look for rules

在生物學上我們尋找規則

that apply to all animals and to life in general,

可以大體上適用於所有的動物和生命

so why should the rules of evolution

但為什麼演化的規則

apply to everybody else but not to us?

適用於所有生物，但不包括我們呢?

Maybe the problem was with the basic assumption

或許問題就在於

that all brains are made in the same way.

「所有腦的構造都是一樣的」這個假設上

Maybe two brains of a similar size

或許兩個大小相近的腦

can actually be made of very different numbers of neurons.

其實可以有著非常不同數量的神經元

Maybe a very large brain

或許一個非常大的腦

does not necessarily have more neurons

並不需要比一般大小的腦

than a more modest-sized brain.

需要更多的神經元

Maybe the human brain actually has the most neurons

或許不管人腦的大小

of any brain, regardless of its size,

都比其他生物擁有最多的神經元

especially in the cerebral cortex.

尤其是在大腦皮質這部分

So this to me became

這些讓我想到一個

the important question to answer:

待解答的重要問題：

how many neurons does the human brain have,

人類的腦有多少神經元？

and how does that compare to other animals?

答案可如何與其它動物做比較呢？

Now, you may have heard or read somewhere

你可能聽過或在哪讀到

that we have 100 billion neurons,

我們有一千億個神經元

so 10 years ago, I asked my colleagues

十前年我問我的同事

if they knew where this number came from.

知不知這數字哪來的

But nobody did.

結果沒人知道

I've been digging through the literature

我翻閱所有的文獻

for the original reference for that number,

想找出這數字最原始的出處

and I could never find it.

但我也沒辦法找到

It seems that nobody had actually ever counted

看來似乎沒有人真正算出過

the number of neurons in the human brain,

人類的腦究竟有多少個神經元

or in any other brain for that matter.

其它的腦也一樣沒數出來過

So I came up with my own way to count cells in the brain,

所以我用自己的方法來數

and it essentially consists of

這方法基本上是

dissolving that brain into soup.

把大腦溶解在一碗湯裡

It works like this:

就像這樣

You take a brain, or parts of that brain,

你拿出一個或部分的腦

and you dissolve it in detergent,

用某種溶劑溶解它

which destroys the cell membranes

破壞掉細胞壁

but keeps the cell nuclei intact,

但保留細胞核完整

so you end up with a suspension of free nuclei

這樣你就有自由懸浮的細胞核

that looks like this,

看起來就會像這樣

like a clear soup.

一碗清湯

This soup contains all the nuclei

這碗湯內有一隻鼠腦

that once were a mouse brain.

所有的細胞核

Now, the beauty of a soup is that because it is soup,

這碗湯的好處就在於因為它是一碗湯

you can agitate it and make those nuclei

你可攪動它

be distributed homogeneously in the liquid,

讓細胞核均勻散布在液體中

so that now by looking under the microscope

然後只需要在顯微鏡之下

at just four or five samples of this homogeneous solution,

觀察這碗湯中四到五個樣本

you can count nuclei, and therefore tell

數算細胞核的數量

how many cells that brain had.

就可得知這個腦有多少個細胞了

It's simple, it's straightforward,

這既簡單又直接

and it's really fast.

而且迅速

So we've used that method to count neurons

所以我們利用這方法

in dozens of different species so far,

計數十種不同物種的神經元

and it turns out that all brains

得到的結果是

are not made the same way.

所有的腦都不具有相同的構造

Take rodents and primates, for instance:

以嚙齒動物和靈長類為例

In larger rodent brains, the average size

較大隻的嚙齒動物腦中

of the neuron increases,

其神經元的平均大小變大

so the brain inflates very rapidly

腦就可迅速變大

and gains size much faster than it gains neurons.

腦變大的速度比增加神經元快

But primate brains gain neurons

但靈長類的腦

without the average neuron becoming any larger,

可在神經元沒有變大的情況下增加數量

which is a very economical way

這使得增加腦的神經元數量上

to add neurons to your brain.

變得非常有效率

The result is that a primate brain

結果顯示同樣大小的腦

will always have more neurons than a rodent brain of the same size,

靈長類的神經元數量都會超過嚙齒類的

and the larger the brain,

而且腦愈大

the larger this difference will be.

這差異就顯得愈大

Well, what about our brain then?

那我們的腦呢？

We found that we have, on average,

平均來講

86 billion neurons,

我們有 860億個神經元

16 billion of which are in the cerebral cortex,

其中的 160億都在大腦皮質裡

and if you consider that the cerebral cortex

如果你認為大腦皮質

is the seat of functions like

的功能中心是像

awareness and logical and abstract reasoning,

認知 、邏輯 、抽象理解力

and that 16 billion is the most neurons

那腦皮層中的160億就比其他

that any cortex has,

任何大腦皮質擁有最多的數量了

I think this is the simplest explanation

我認為這是我們人類卓越的認知能力

for our remarkable cognitive abilities.

最簡單的解釋了

But just as important is what the 86 billion neurons mean.

同樣重要的是這860億個神經元意味著什?

Because we found that the relationship

因為我們發現到

between the size of the brain and its number of neurons

腦的大小和神經元數量的關係

could be described mathematically,

可以用數學的方式描述

we could calculate what a human brain

我們可計算出

would look like if it was made like a rodent brain.

如果人腦的構造像嚙齒動物的會怎麼樣呢?

So, a rodent brain with 86 billion neurons

若嚙齒動物的腦有 860億的神經元

would weigh 36 kilos.

體重就會重達 36公斤

That's not possible.

這是不可能的事情

A brain that huge would be crushed

這麼大的一個腦

by its own weight,

會被自己的重量壓垮

and this impossible brain would go

要這麼重大的腦

in the body of 89 tons.

搭配一個 89公噸的身體

I don't think it looks like us.

是件讓我難以想像的東西

So this brings us to a very important conclusion already,

這為我們今天下一個很重要的結論

which is that we are not rodents.

我們不是嚙齒動物

The human brain is not a large rat brain.

人腦沒有嚙齒動物搬的大腦

Compared to a rat, we might seem special, yes,

比起老鼠我們的看起來特別多了

but that's not a fair comparison to make,

但這樣比較並不公平

given that we know that we are not rodents.

因為我們知道我們不是嚙齒動物

We are primates,

我們是靈長類

so the correct comparison is to other primates.

所以跟其它的靈長類動物作比較才正確

And there, if you do the math,

所以計算一下的話

you find that a generic primate

你會發現一般靈長類動物

with 86 billion neurons

擁有860億的神經元

would have a brain of about 1.2 kilos,

腦就會重約 1.2公斤

which seems just right,

搭配重達 66公斤的身軀

in a body of some 66 kilos,

看起來似乎合理

which in my case is exactly right,

這剛好是我自身的例子

which brings us to a very unsurprising

這結果雖然一點也不意外

but still incredibly important conclusion:

但還是下了一個很重要的結論：

I am a primate.

我是靈長類動物

And all of you are primates.

你們全部也是靈長類動物

And so was Darwin.

達爾文也是

I love to think that Darwin would have really appreciated this.

我猜想達爾文也會很喜歡這項結論

His brain, like ours,

他的腦跟我們一樣

was made in the image of other primate brains.

構造都是與其它靈長類相同的

So the human brain may be remarkable, yes,

沒錯，人腦或許較卓越

but it is not special in its number of neurons.

但在神經元的數量方面就沒什特別的

It is just a large primate brain.

它不過是大的靈長類的頭腦

I think that's a very humbling and sobering thought

我想這是一個非常謙虛和中肯的想法

that should remind us of our place in nature.

提醒我們在大自然中扮演的角色

Why does it cost so much energy, then?

為何它會消耗如此多的能量？

Well, other people have figured out

有人已經估算出

how much energy the human brain

人類和其它動物的腦

and that of other species costs,

所消耗的能量

and now that we knew how many neurons

現在了解每種腦有多少神經元

each brain was made of, we could do the math.

我們就可做些計算

And it turns out that both human

結果發現人類和其它動物的腦

and other brains cost about the same,

消耗方式差不多一樣

an average of six calories per billion neurons per day.

每天平均每 10億個神經元消耗 6卡

So the total energetic cost of a brain

一個腦所消耗的能量

is a simple, linear function

和它的神經元數量

of its number of neurons,

成了一個簡單的線性關係

and it turns out that the human brain

所以人腦消耗的能量

costs just as much energy as you would expect.

就跟你估算的一樣多

So the reason why the human brain

所以人腦為什麼

costs so much energy is simply because

會消耗許多能量

it has a huge number of neurons,

單純是因為它有較多神經元的關係

and because we are primates

作為靈長類

with many more neurons for a given body size

比起身體大小類似的動物

than any other animal,

我們擁有更多的神經元

the relative cost of our brain is large,

腦也就消耗較多能量

but just because we're primates, not because we're special.

這單純是因為我們是靈長類動物的關係 不是因為我們特別

Last question, then:

最後一個問題：

how did we come by this remarkable number of neurons,

我們是如何有這麼多的神經元？

and in particular, if great apes

明確地說

are larger than we are,

如果大猩猩身體比我們大

why don't they have a larger brain than we do, with more neurons?

為何沒有較大的腦和更多的神經元呢？

When we realized how much expensive it is

當理解到擁有許多神經元的同時

to have a lot of neurons in the brain, I figured,

也就需要燃燒更多的能量時

maybe there's a simple reason.

我推測或許這就是原因－

They just can't afford the energy

大猩猩無法同時供給能量

for both a large body and a large number of neurons.

應付龐大的身軀和大量的神經元

So we did the math.

我們計算過

We calculated on the one hand

一方面計算出

how much energy a primate gets per day

大猩猩一天從食物中

from eating raw foods,

獲得多少能量

and on the other hand, how much energy

另一方面計算

a body of a certain size costs

特定大小的身軀要消耗多少能量

and how much energy a brain of a certain number of neurons costs,

還有一定數量的神經元所消耗的能量

and we looked for the combinations

然後我們一併查看

of body size and number of brain neurons

如果牠一天進食固定的時數

that a primate could afford

牠可以提供多少能量

if it ate a certain number of hours per day.

給多大的身軀和多少的神經元數量

And what we found is that

我們發現到

because neurons are so expensive,

因為神經元消耗太多

there is a tradeoff between body size and number of neurons.

這是身軀大小和神經元數量之間的妥協

So a primate that eats eight hours per day