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TEDGlobal 2013

Suzana Herculano-Houzel: What is so special about the human brain?

June 12, 2013

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.

Suzana Herculano-Houzel - Neuroscientist
Suzana Herculano-Houzel shrunk the human brain by 14 billion neurons -- by developing a new way to count them. Full bio

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Double-click the English subtitles below to play the video.
What is so special about the human brain?
00:12
Why is it that we study other animals
00:15
instead of them studying us?
00:17
What does a human brain have or do
00:19
that no other brain does?
00:21
When I became interested
in these questions about 10 years ago,
00:23
scientists thought they knew
what different brains were made of.
00:25
Though it was based on very little evidence,
00:28
many scientists thought that all mammalian brains,
00:30
including the human brain,
00:32
were made in the same way,
00:34
with a number of neurons that was always
00:35
proportional to the size of the brain.
00:37
This means that two brains of the same size,
00:39
like these two, with a respectable 400 grams,
00:41
should have similar numbers of neurons.
00:45
Now, if neurons are the functional
00:47
information processing units of the brain,
00:49
then the owners of these two brains
00:52
should have similar cognitive abilities.
00:53
And yet, one is a chimp,
00:56
and the other is a cow.
00:59
Now maybe cows have a really rich
01:01
internal mental life and are so smart
01:04
that they choose not to let us realize it,
01:06
but we eat them.
01:10
I think most people will agree
01:11
that chimps are capable of much more complex,
01:13
elaborate and flexible behaviors than cows are.
01:15
So this is a first indication that the
01:18
"all brains are made the same way" scenario
01:20
is not quite right.
01:22
But let's play along.
01:23
If all brains were made the same way
01:25
and you were to compare animals
with brains of different sizes,
01:27
larger brains should always have more neurons
01:29
than smaller brains,
and the larger the brain,
01:32
the more cognitively able its owner should be.
01:34
So the largest brain around should also be
01:37
the most cognitively able.
01:39
And here comes the bad news:
01:41
Our brain, not the largest one around.
01:43
It seems quite vexing.
01:46
Our brain weighs between 1.2 and 1.5 kilos,
01:47
but elephant brains weigh between four and five kilos,
01:50
and whale brains can weigh up to nine kilos,
01:53
which is why scientists used to resort to saying
01:56
that our brain must be special
02:01
to explain our cognitive abilities.
02:03
It must be really extraordinary,
02:06
an exception to the rule.
02:09
Theirs may be bigger, but ours is better,
02:11
and it could be better, for example,
02:14
in that it seems larger than it should be,
02:16
with a much larger cerebral cortex
than we should have
02:18
for the size of our bodies.
02:21
So that would give us extra cortex
02:22
to do more interesting things
than just operating the body.
02:24
That's because the size of the brain
02:27
usually follows the size of the body.
02:28
So the main reason for saying that
02:31
our brain is larger than it should be
02:33
actually comes from comparing ourselves
02:35
to great apes.
02:37
Gorillas can be two to three times larger than we are,
02:39
so their brains should also be larger than ours,
02:41
but instead it's the other way around.
02:44
Our brain is three times larger than a gorilla brain.
02:46
The human brain also seems special
02:48
in the amount of energy that it uses.
02:51
Although it weighs only two percent of the body,
02:53
it alone uses 25 percent of all the energy
02:56
that your body requires to run per day.
02:59
That's 500 calories out of a total of 2,000 calories,
03:01
just to keep your brain working.
03:05
So the human brain is larger than it should be,
03:07
it uses much more energy than it should,
03:10
so it's special.
03:12
And this is where the story started to bother me.
03:14
In biology, we look for rules
03:16
that apply to all animals and to life in general,
03:18
so why should the rules of evolution
03:21
apply to everybody else but not to us?
03:23
Maybe the problem was with the basic assumption
03:26
that all brains are made in the same way.
03:28
Maybe two brains of a similar size
03:30
can actually be made of
very different numbers of neurons.
03:32
Maybe a very large brain
03:34
does not necessarily have more neurons
03:36
than a more modest-sized brain.
03:38
Maybe the human brain
actually has the most neurons
03:40
of any brain, regardless of its size,
03:43
especially in the cerebral cortex.
03:46
So this to me became
03:48
the important question to answer:
03:49
how many neurons does the human brain have,
03:51
and how does that compare to other animals?
03:54
Now, you may have heard or read somewhere
03:56
that we have 100 billion neurons,
03:58
so 10 years ago, I asked my colleagues
04:01
if they knew where this number came from.
04:03
But nobody did.
04:04
I've been digging through the literature
04:06
for the original reference for that number,
04:07
and I could never find it.
04:09
It seems that nobody had actually ever counted
04:11
the number of neurons in the human brain,
04:14
or in any other brain for that matter.
04:16
So I came up with my own way
to count cells in the brain,
04:18
and it essentially consists of
04:22
dissolving that brain into soup.
04:24
It works like this:
04:27
You take a brain, or parts of that brain,
04:29
and you dissolve it in detergent,
04:32
which destroys the cell membranes
04:34
but keeps the cell nuclei intact,
04:35
so you end up with a suspension of free nuclei
04:38
that looks like this,
04:41
like a clear soup.
04:43
This soup contains all the nuclei
04:44
that once were a mouse brain.
04:46
Now, the beauty of a soup is that because it is soup,
04:48
you can agitate it and make those nuclei
04:51
be distributed homogeneously in the liquid,
04:54
so that now by looking under the microscope
04:56
at just four or five samples
of this homogeneous solution,
04:58
you can count nuclei, and therefore tell
05:02
how many cells that brain had.
05:04
It's simple, it's straightforward,
05:06
and it's really fast.
05:08
So we've used that method to count neurons
05:09
in dozens of different species so far,
05:11
and it turns out that all brains
05:13
are not made the same way.
05:15
Take rodents and primates, for instance:
05:18
In larger rodent brains, the average size
05:20
of the neuron increases,
05:22
so the brain inflates very rapidly
05:24
and gains size much faster than it gains neurons.
05:26
But primate brains gain neurons
05:30
without the average neuron becoming any larger,
05:31
which is a very economical way
05:34
to add neurons to your brain.
05:35
The result is that a primate brain
05:37
will always have more neurons than
a rodent brain of the same size,
05:39
and the larger the brain,
05:42
the larger this difference will be.
05:43
Well, what about our brain then?
05:46
We found that we have, on average,
05:48
86 billion neurons,
05:49
16 billion of which are in the cerebral cortex,
05:51
and if you consider that the cerebral cortex
05:54
is the seat of functions like
05:56
awareness and logical and abstract reasoning,
05:59
and that 16 billion is the most neurons
06:02
that any cortex has,
06:05
I think this is the simplest explanation
06:08
for our remarkable cognitive abilities.
06:10
But just as important is what
the 86 billion neurons mean.
06:13
Because we found that the relationship
06:16
between the size of the brain
and its number of neurons
06:18
could be described mathematically,
06:20
we could calculate what a human brain
06:22
would look like if it was made like a rodent brain.
06:24
So, a rodent brain with 86 billion neurons
06:27
would weigh 36 kilos.
06:30
That's not possible.
06:33
A brain that huge would be crushed
06:35
by its own weight,
06:37
and this impossible brain would go
06:38
in the body of 89 tons.
06:40
I don't think it looks like us.
06:43
So this brings us to a very important
conclusion already,
06:45
which is that we are not rodents.
06:48
The human brain is not a large rat brain.
06:51
Compared to a rat, we might seem special, yes,
06:54
but that's not a fair comparison to make,
06:57
given that we know that we are not rodents.
06:59
We are primates,
07:01
so the correct comparison is to other primates.
07:02
And there, if you do the math,
07:05
you find that a generic primate
07:06
with 86 billion neurons
07:09
would have a brain of about 1.2 kilos,
07:11
which seems just right,
07:14
in a body of some 66 kilos,
07:16
which in my case is exactly right,
07:18
which brings us to a very unsurprising
07:20
but still incredibly important conclusion:
07:23
I am a primate.
07:26
And all of you are primates.
07:27
And so was Darwin.
07:30
I love to think that Darwin
would have really appreciated this.
07:32
His brain, like ours,
07:35
was made in the image of other primate brains.
07:37
So the human brain may be remarkable, yes,
07:40
but it is not special in its number of neurons.
07:43
It is just a large primate brain.
07:46
I think that's a very humbling and sobering thought
07:47
that should remind us of our place in nature.
07:50
Why does it cost so much energy, then?
07:53
Well, other people have figured out
07:56
how much energy the human brain
07:58
and that of other species costs,
07:59
and now that we knew how many neurons
08:00
each brain was made of, we could do the math.
08:02
And it turns out that both human
08:04
and other brains cost about the same,
08:06
an average of six calories
per billion neurons per day.
08:09
So the total energetic cost of a brain
08:13
is a simple, linear function
08:15
of its number of neurons,
08:17
and it turns out that the human brain
08:18
costs just as much energy as you would expect.
08:21
So the reason why the human brain
08:25
costs so much energy is simply because
08:27
it has a huge number of neurons,
08:28
and because we are primates
08:30
with many more neurons for a given body size
08:32
than any other animal,
08:34
the relative cost of our brain is large,
08:36
but just because we're primates,
not because we're special.
08:39
Last question, then:
08:42
how did we come by this
remarkable number of neurons,
08:43
and in particular, if great apes
08:47
are larger than we are,
08:48
why don't they have a larger brain
than we do, with more neurons?
08:50
When we realized how much expensive it is
08:54
to have a lot of neurons in the brain, I figured,
08:56
maybe there's a simple reason.
08:58
They just can't afford the energy
09:00
for both a large body
and a large number of neurons.
09:02
So we did the math.
09:05
We calculated on the one hand
09:06
how much energy a primate gets per day
09:08
from eating raw foods,
09:10
and on the other hand, how much energy
09:11
a body of a certain size costs
09:13
and how much energy a brain of a
certain number of neurons costs,
09:15
and we looked for the combinations
09:18
of body size and number of brain neurons
09:20
that a primate could afford
09:22
if it ate a certain number of hours per day.
09:24
And what we found is that
09:26
because neurons are so expensive,
09:28
there is a tradeoff between
body size and number of neurons.
09:30
So a primate that eats eight hours per day
09:33
can afford at most 53 billion neurons,
09:36
but then its body cannot be any bigger
09:39
than 25 kilos.
09:41
To weigh any more than that,
09:43
it has to give up neurons.
09:44
So it's either a large body
09:46
or a large number of neurons.
09:49
When you eat like a primate,
09:50
you can't afford both.
09:52
One way out of this metabolic limitation
09:54
would be to spend even more hours per day eating,
09:56
but that gets dangerous,
09:59
and past a certain point, it's just not possible.
10:01
Gorillas and orangutans, for instance,
10:03
afford about 30 billion neurons
10:05
by spending eight and a half hours per day eating,
10:07
and that seems to be about as much as they can do.
10:10
Nine hours of feeding per day
10:13
seems to be the practical limit for a primate.
10:15
What about us?
10:18
With our 86 billion neurons
10:20
and 60 to 70 kilos of body mass,
10:21
we should have to spend over nine hours
10:24
per day every single day feeding,
10:28
which is just not feasible.
10:31
If we ate like a primate,
10:34
we should not be here.
10:35
How did we get here, then?
10:38
Well, if our brain costs just as much energy
10:40
as it should, and if we can't spend
10:42
every waking hour of the day feeding,
10:44
then the only alternative, really,
10:48
is to somehow get more energy
10:50
out of the same foods.
10:52
And remarkably, that matches exactly
10:54
what our ancestors are believed to have invented
10:57
one and a half million years ago,
11:01
when they invented cooking.
11:02
To cook is to use fire
11:05
to pre-digest foods outside of your body.
11:07
Cooked foods are softer, so they're easier to chew
11:11
and to turn completely into mush in your mouth,
11:14
so that allows them to be completely digested
11:16
and absorbed in your gut,
11:18
which makes them yield much more
energy in much less time.
11:20
So cooking frees time for us to do
11:23
much more interesting things with our day
11:26
and with our neurons
11:28
than just thinking about food,
11:29
looking for food, and gobbling down food
11:31
all day long.
11:33
So because of cooking, what once was
11:34
a major liability, this large,
11:37
dangerously expensive brain with a lot of neurons,
11:39
could now become a major asset,
11:42
now that we could both afford
the energy for a lot of neurons
11:44
and the time to do interesting things with them.
11:47
So I think this explains why the human brain
11:50
grew to become so large so fast in evolution,
11:52
all of the while remaining just a primate brain.
11:55
With this large brain now affordable by cooking,
11:59
we went rapidly from raw foods to culture,
12:02
agriculture, civilization, grocery stores,
12:05
electricity, refrigerators,
12:07
all of those things that nowadays
12:09
allow us to get all the energy we need
12:11
for the whole day in a single sitting
12:13
at your favorite fast food joint.
12:15
So what once was a solution
12:18
now became the problem,
12:21
and ironically, we look for the solution in raw food.
12:22
So what is the human advantage?
12:28
What is it that we have
12:31
that no other animal has?
12:32
My answer is that we have the largest number
12:35
of neurons in the cerebral cortex,
12:37
and I think that's the simplest explanation
12:38
for our remarkable cognitive abilities.
12:40
And what is it that we do that no other animal does,
12:42
and which I believe was fundamental
12:45
to allow us to reach that large,
12:47
largest number of neurons in the cortex?
12:50
In two words, we cook.
12:53
No other animal cooks its food. Only humans do.
12:55
And I think that's how we got to become human.
12:58
Studying the human brain changed
the way I think about food.
13:01
I now look at my kitchen,
13:04
and I bow to it,
13:05
and I thank my ancestors for coming up
13:07
with the invention that probably made us humans.
13:09
Thank you very much.
13:11
(Applause)
13:13

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Suzana Herculano-Houzel - Neuroscientist
Suzana Herculano-Houzel shrunk the human brain by 14 billion neurons -- by developing a new way to count them.

Why you should listen

How many neurons make a human brain? For years, the answer has been (give or take) 100 billion. But neuroscientist Suzana Herculano-Houzel decided to count them herself. Her research approach involved dissolving four human brains (donated to science) into a homogeneous mixture -- in her lab at the Institute of Biomedical Sciences in Rio de Janeiro, they call it "brain soup." She then took a sample of the mix, counted the number of cell nuclei belonging to neurons, and scaled that up. Result: the human brain has about 86 billion neurons, 14 billion fewer than assumed -- but intriguingly, far more than other animals, relative to brain size.

She suggests that it was the invention of cooking by our ancestors -- which makes food yield much more metabolic energy -- that allowed humans to develop the largest primate brain. She's now working on elephant and whale brains to test her hypothesis.

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