What is so special about the human brain?

The human brain is puzzling -- it is curiously large given the size of our bodies, uses a tremendous amount of energy for its weight and has a bizarrely dense cerebral cortex. But: why? Neuroscientist Suzana Herculano-Houzel puts on her detective's cap and leads us through this mystery. By making "brain soup," she arrives at a startling conclusion.

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)