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What is so special about the human brain?
Why is it that we study other animals
instead of them studying us?
What does a human brain have or do
that no other brain does?
When I became interested in these questions about 10 years ago,
scientists thought they knew what different brains were made of.
Though it was based on very little evidence,
many scientists thought that all mammalian brains,
including the human brain,
were made in the same way,
with a number of neurons that was always
proportional to the size of the brain.
This means that two brains of the same size,
like these two, with a respectable 400 grams,
should have similar numbers of neurons.
Now, if neurons are the functional
information processing units of the brain,
then the owners of these two brains
should have similar cognitive abilities.
And yet, one is a chimp,
and the other is a cow.
Now maybe cows have a really rich
internal mental life and are so smart
that they choose not to let us realize it,
but we eat them.
I think most people will agree
that chimps are capable of much more complex,
elaborate and flexible behaviors than cows are.
So this is a first indication that the
"all brains are made the same way" scenario
is not quite right.
But let's play along.
If all brains were made the same way
and you were to compare animals with brains of different sizes,
larger brains should always have more neurons
than smaller brains, and the larger the brain,
the more cognitively able its owner should be.
So the largest brain around should also be
the most cognitively able.
And here comes the bad news:
Our brain, not the largest one around.
It seems quite vexing.
Our brain weighs between 1.2 and 1.5 kilos,
but elephant brains weigh between four and five kilos,
and whale brains can weigh up to nine kilos,
which is why scientists used to resort to saying
that our brain must be special
to explain our cognitive abilities.
It must be really extraordinary,
an exception to the rule.
Theirs may be bigger, but ours is better,
and it could be better, for example,
in that it seems larger than it should be,
with a much larger cerebral cortex than we should have
for the size of our bodies.
So that would give us extra cortex
to do more interesting things than just operating the body.
That's because the size of the brain
usually follows the size of the body.
So the main reason for saying that
our brain is larger than it should be
actually comes from comparing ourselves
to great apes.
Gorillas can be two to three times larger than we are,
so their brains should also be larger than ours,
but instead it's the other way around.
Our brain is three times larger than a gorilla brain.
The human brain also seems special
in the amount of energy that it uses.
Although it weighs only two percent of the body,
it alone uses 25 percent of all the energy
that your body requires to run per day.
That's 500 calories out of a total of 2,000 calories,
just to keep your brain working.
So the human brain is larger than it should be,
it uses much more energy than it should,
so it's special.
And this is where the story started to bother me.
In biology, we look for rules
that apply to all animals and to life in general,
so why should the rules of evolution
apply to everybody else but not to us?
Maybe the problem was with the basic assumption
that all brains are made in the same way.
Maybe two brains of a similar size
can actually be made of very different numbers of neurons.
Maybe a very large brain
does not necessarily have more neurons
than a more modest-sized brain.
Maybe the human brain actually has the most neurons
of any brain, regardless of its size,
especially in the cerebral cortex.
So this to me became
the important question to answer:
how many neurons does the human brain have,
and how does that compare to other animals?
Now, you may have heard or read somewhere
that we have 100 billion neurons,
so 10 years ago, I asked my colleagues
if they knew where this number came from.
But nobody did.
I've been digging through the literature
for the original reference for that number,
and I could never find it.
It seems that nobody had actually ever counted
the number of neurons in the human brain,
or in any other brain for that matter.
So I came up with my own way to count cells in the brain,
and it essentially consists of
dissolving that brain into soup.
It works like this:
You take a brain, or parts of that brain,
and you dissolve it in detergent,
which destroys the cell membranes
but keeps the cell nuclei intact,
so you end up with a suspension of free nuclei
that looks like this,
like a clear soup.
This soup contains all the nuclei
that once were a mouse brain.
Now, the beauty of a soup is that because it is soup,
you can agitate it and make those nuclei
be distributed homogeneously in the liquid,
so that now by looking under the microscope
at just four or five samples of this homogeneous solution,
you can count nuclei, and therefore tell
how many cells that brain had.
It's simple, it's straightforward,
and it's really fast.
So we've used that method to count neurons
in dozens of different species so far,
and it turns out that all brains
are not made the same way.
Take rodents and primates, for instance:
In larger rodent brains, the average size
of the neuron increases,
so the brain inflates very rapidly
and gains size much faster than it gains neurons.
But primate brains gain neurons
without the average neuron becoming any larger,
which is a very economical way
to add neurons to your brain.
The result is that a primate brain
will always have more neurons than a rodent brain of the same size,
and the larger the brain,
the larger this difference will be.
Well, what about our brain then?
We found that we have, on average,
86 billion neurons,
16 billion of which are in the cerebral cortex,
and if you consider that the cerebral cortex
is the seat of functions like
awareness and logical and abstract reasoning,
and that 16 billion is the most neurons
that any cortex has,
I think this is the simplest explanation
for our remarkable cognitive abilities.
But just as important is what the 86 billion neurons mean.
Because we found that the relationship
between the size of the brain and its number of neurons
could be described mathematically,
we could calculate what a human brain
would look like if it was made like a rodent brain.
So, a rodent brain with 86 billion neurons
would weigh 36 kilos.
That's not possible.
A brain that huge would be crushed
by its own weight,
and this impossible brain would go
in the body of 89 tons.
I don't think it looks like us.
So this brings us to a very important conclusion already,
which is that we are not rodents.
The human brain is not a large rat brain.
Compared to a rat, we might seem special, yes,
but that's not a fair comparison to make,
given that we know that we are not rodents.
We are primates,
so the correct comparison is to other primates.
And there, if you do the math,
you find that a generic primate
with 86 billion neurons
would have a brain of about 1.2 kilos,
which seems just right,
in a body of some 66 kilos,
which in my case is exactly right,
which brings us to a very unsurprising
but still incredibly important conclusion:
I am a primate.
And all of you are primates.
And so was Darwin.
I love to think that Darwin would have really appreciated this.
His brain, like ours,
was made in the image of other primate brains.
So the human brain may be remarkable, yes,
but it is not special in its number of neurons.
It is just a large primate brain.
I think that's a very humbling and sobering thought
that should remind us of our place in nature.
Why does it cost so much energy, then?
Well, other people have figured out
how much energy the human brain
and that of other species costs,
and now that we knew how many neurons
each brain was made of, we could do the math.
And it turns out that both human
and other brains cost about the same,
an average of six calories per billion neurons per day.
So the total energetic cost of a brain
is a simple, linear function
of its number of neurons,
and it turns out that the human brain
costs just as much energy as you would expect.
So the reason why the human brain
costs so much energy is simply because
it has a huge number of neurons,
and because we are primates
with many more neurons for a given body size
than any other animal,
the relative cost of our brain is large,
but just because we're primates, not because we're special.
Last question, then:
how did we come by this remarkable number of neurons,
and in particular, if great apes
are larger than we are,
why don't they have a larger brain than we do, with more neurons?
When we realized how much expensive it is
to have a lot of neurons in the brain, I figured,
maybe there's a simple reason.
They just can't afford the energy
for both a large body and a large number of neurons.
So we did the math.
We calculated on the one hand
how much energy a primate gets per day
from eating raw foods,
and on the other hand, how much energy
a body of a certain size costs
and how much energy a brain of a certain number of neurons costs,
and we looked for the combinations
of body size and number of brain neurons
that a primate could afford
if it ate a certain number of hours per day.
And what we found is that
because neurons are so expensive,
there is a tradeoff between body size and number of neurons.
So a primate that eats eight hours per day
can afford at most 53 billion neurons,
but then its body cannot be any bigger
than 25 kilos.
To weigh any more than that,
it has to give up neurons.
So it's either a large body
or a large number of neurons.
When you eat like a primate,
you can't afford both.
One way out of this metabolic limitation
would be to spend even more hours per day eating,
but that gets dangerous,
and past a certain point, it's just not possible.
Gorillas and orangutans, for instance,
afford about 30 billion neurons
by spending eight and a half hours per day eating,
and that seems to be about as much as they can do.
Nine hours of feeding per day
seems to be the practical limit for a primate.
What about us?
With our 86 billion neurons
and 60 to 70 kilos of body mass,
we should have to spend over nine hours
per day every single day feeding,
which is just not feasible.
If we ate like a primate,
we should not be here.
How did we get here, then?
Well, if our brain costs just as much energy
as it should, and if we can't spend
every waking hour of the day feeding,
then the only alternative, really,
is to somehow get more energy
out of the same foods.
And remarkably, that matches exactly
what our ancestors are believed to have invented
one and a half million years ago,
when they invented cooking.
To cook is to use fire
to pre-digest foods outside of your body.
Cooked foods are softer, so they're easier to chew
and to turn completely into mush in your mouth,
so that allows them to be completely digested
and absorbed in your gut,
which makes them yield much more energy in much less time.
So cooking frees time for us to do
much more interesting things with our day
and with our neurons
than just thinking about food,
looking for food, and gobbling down food
all day long.
So because of cooking, what once was
a major liability, this large,
dangerously expensive brain with a lot of neurons,
could now become a major asset,
now that we could both afford the energy for a lot of neurons
and the time to do interesting things with them.
So I think this explains why the human brain
grew to become so large so fast in evolution,
all of the while remaining just a primate brain.
With this large brain now affordable by cooking,
we went rapidly from raw foods to culture,
agriculture, civilization, grocery stores,
electricity, refrigerators,
all of those things that nowadays
allow us to get all the energy we need
for the whole day in a single sitting
at your favorite fast food joint.
So what once was a solution
now became the problem,
and ironically, we look for the solution in raw food.
So what is the human advantage?
What is it that we have
that no other animal has?
My answer is that we have the largest number
of neurons in the cerebral cortex,
and I think that's the simplest explanation
for our remarkable cognitive abilities.
And what is it that we do that no other animal does,
and which I believe was fundamental
to allow us to reach that large,
largest number of neurons in the cortex?
In two words, we cook.
No other animal cooks its food. Only humans do.
And I think that's how we got to become human.
Studying the human brain changed the way I think about food.
I now look at my kitchen,
and I bow to it,
and I thank my ancestors for coming up
with the invention that probably made us humans.
Thank you very much.
(Applause)
コツ:単語をクリックしてすぐ意味を調べられます!

読み込み中…

【TED】スザーナ・エルクラーノ=アウゼル: 人の脳は、何がそんなに特別なのでしょうか? (What is so special about the human brain? | Suzana Herculano-Houzel)

6179 タグ追加 保存
Max Lin 2015 年 10 月 26 日 に公開
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