0:00:01.024,0:00:03.540
What is so special about the human brain?

0:00:03.540,0:00:05.660
Why is it that we study other animals

0:00:05.660,0:00:07.660
instead of them studying us?

0:00:07.660,0:00:09.580
What does a human brain have or do

0:00:09.580,0:00:11.420
that no other brain does?

0:00:11.420,0:00:14.220
When I became interested[br]in these questions about 10 years ago,

0:00:14.220,0:00:15.700
scientists thought they knew

0:00:15.700,0:00:17.380
what different brains were made of.

0:00:17.380,0:00:19.140
Though it was based on very little evidence,

0:00:19.140,0:00:21.020
many scientists thought that all mammalian brains,

0:00:21.020,0:00:22.820
including the human brain,

0:00:22.820,0:00:24.260
were made in the same way,

0:00:24.260,0:00:25.700
with a number of neurons that was always

0:00:25.700,0:00:27.700
proportional to the size of the brain.

0:00:27.700,0:00:29.700
This means that two brains of the same size,

0:00:29.700,0:00:33.140
like these two, with a respectable 400 grams,

0:00:33.140,0:00:35.860
should have similar numbers of neurons.

0:00:35.860,0:00:37.780
Now, if neurons are the functional

0:00:37.780,0:00:40.620
information processing units of the brain,

0:00:40.620,0:00:42.260
then the owners of these two brains

0:00:42.260,0:00:44.820
should have similar cognitive abilities.

0:00:44.820,0:00:47.260
And yet, one is a chimp,

0:00:47.260,0:00:50.020
and the other is a cow.

0:00:50.020,0:00:52.260
Now maybe cows have a really rich

0:00:52.260,0:00:54.620
internal mental life and are so smart

0:00:54.620,0:00:58.380
that they choose not to let us realize it,

0:00:58.380,0:01:00.300
but we eat them.

0:01:00.300,0:01:01.660
I think most people will agree

0:01:01.660,0:01:03.660
that chimps are capable of much more complex,

0:01:03.660,0:01:06.500
elaborate, and flexible behaviors than cows are.

0:01:06.500,0:01:08.540
So this is a first indication that the

0:01:08.540,0:01:10.420
"all brains are made the same way" scenario

0:01:10.420,0:01:11.860
is not quite right.

0:01:11.860,0:01:13.700
But let's play along.

0:01:13.700,0:01:15.460
If all brains were made the same way

0:01:15.460,0:01:17.820
and you were to compare animals[br]with brains of different sizes,

0:01:17.820,0:01:20.340
larger brains should always have more neurons

0:01:20.340,0:01:21.940
than smaller brains,

0:01:21.940,0:01:23.340
and the larger the brain,

0:01:23.340,0:01:25.500
the more cognitively able its owner should be.

0:01:25.500,0:01:27.620
So the largest brain around should also be

0:01:27.620,0:01:30.140
the most cognitively able.

0:01:30.140,0:01:31.860
And here comes the bad news:

0:01:31.860,0:01:34.340
our brain, not the largest one around.

0:01:34.340,0:01:36.140
It seems quite vexing.

0:01:36.140,0:01:38.860
Our brain weighs between 1.2 and 1.5 kilos,

0:01:38.860,0:01:42.100
but elephant brains weigh between four and five kilos,

0:01:42.100,0:01:44.500
and whale brains can weigh up to nine kilos,

0:01:44.500,0:01:49.180
which is why scientists used to resort to saying

0:01:49.180,0:01:51.380
that our brain must be special

0:01:51.380,0:01:54.380
to explain our cognitive abilities.

0:01:54.380,0:01:57.220
It must be really extraordinary,

0:01:57.220,0:01:59.380
an exception to the rule.

0:01:59.380,0:02:03.100
Theirs may be bigger, but ours is better,

0:02:03.100,0:02:04.900
and it could be better for example

0:02:04.900,0:02:06.700
in that it seems larger than it should be,

0:02:06.700,0:02:09.260
with a much larger cerebral cortex[br]than it should have,

0:02:09.260,0:02:10.860
for the size of our bodies.

0:02:10.860,0:02:12.620
So that would give us extra cortex

0:02:12.620,0:02:15.580
to do more interesting things[br]than just operating the body.

0:02:15.580,0:02:17.260
That's because the size of the brain

0:02:17.260,0:02:19.300
usually follows the size of the body.

0:02:19.300,0:02:21.540
So the main reason for saying that

0:02:21.540,0:02:23.780
our brain is larger than it should be

0:02:23.780,0:02:25.660
actually comes from comparing ourselves

0:02:25.660,0:02:27.300
to great apes.

0:02:27.300,0:02:29.940
Gorillas can be two to three times larger than we are,

0:02:29.940,0:02:32.300
so their brains should also be larger than ours,

0:02:32.300,0:02:34.260
but instead it's the other way around.

0:02:34.260,0:02:37.980
Our brain is three times larger than a gorilla brain.

0:02:37.980,0:02:39.700
The human brain also seems special

0:02:39.700,0:02:41.500
in the amount of energy that it uses.

0:02:41.500,0:02:44.220
Although it weighs only two percent of the body,

0:02:44.220,0:02:46.940
it alone uses 25 percent of all the energy

0:02:46.940,0:02:49.940
that your body requires to run per day.

0:02:49.940,0:02:52.940
That's 500 calories out of a total of 2,000 calories,

0:02:52.940,0:02:55.900
just to keep your brain working.

0:02:55.900,0:02:58.780
So the human brain is larger than it should be,

0:02:58.780,0:03:00.900
it uses much more energy than it should,

0:03:00.900,0:03:02.820
so it's special.

0:03:02.820,0:03:05.020
And this is where the story started to bother me.

0:03:05.020,0:03:06.660
In biology, we look for rules

0:03:06.660,0:03:09.460
that apply to all animals and to life in general,

0:03:09.460,0:03:11.340
so why should the rules of evolution

0:03:11.340,0:03:14.860
apply to everybody else but not to us?

0:03:14.860,0:03:16.980
Maybe the problem was with the basic assumption

0:03:16.980,0:03:19.220
that all brains are made in the same way.

0:03:19.220,0:03:21.020
Maybe two brains of a similar size

0:03:21.020,0:03:23.180
can actually be made of[br]very different numbers of neurons.

0:03:23.180,0:03:24.940
Maybe a very large brain

0:03:24.940,0:03:26.780
does not necessarily have more neurons

0:03:26.780,0:03:29.140
than a more modest-sized brain.

0:03:29.140,0:03:31.780
Maybe the human brain actually has the most neurons

0:03:31.780,0:03:34.260
of any brain, regardless of its size,

0:03:34.260,0:03:36.740
especially in the cerebral cortex.

0:03:36.740,0:03:38.860
So this to me became the important question

0:03:38.860,0:03:40.420
to answer:

0:03:40.420,0:03:42.220
how many neurons does the human brain have,

0:03:42.220,0:03:44.620
and how does that compare to other animals?

0:03:44.620,0:03:46.980
Now, you may have heard or read somewhere

0:03:46.980,0:03:49.060
than we have a hundred billion neurons,

0:03:49.060,0:03:51.300
so 10 years ago, I asked my colleagues

0:03:51.300,0:03:53.540
if they knew where this number came from.

0:03:53.540,0:03:55.060
But nobody did.

0:03:55.060,0:03:56.420
I've been digging through the literature

0:03:56.420,0:03:58.140
for the original reference for that number,

0:03:58.140,0:04:00.060
and I could never find it.

0:04:00.060,0:04:02.660
It seems that nobody had actually ever counted

0:04:02.660,0:04:04.380
the number of neurons in the human brain,

0:04:04.380,0:04:06.820
or in any other brain for that matter.

0:04:06.820,0:04:10.260
So I came up with my own way[br]to count cells in the brain,

0:04:10.260,0:04:12.260
and it essentially consists of

0:04:12.260,0:04:14.660
dissolving that brain into soup.

0:04:14.660,0:04:17.465
It works like this:

0:04:17.465,0:04:20.500
you take a brain, or parts of that brain,

0:04:20.500,0:04:22.460
and you dissolve it in detergent,

0:04:22.460,0:04:24.260
which destroys the cell membranes

0:04:24.260,0:04:26.312
but keeps the cell nuclei intact,

0:04:26.312,0:04:29.100
so you end up with a suspension of free nuclei

0:04:29.100,0:04:31.340
that looks like this,

0:04:31.340,0:04:33.060
like a clear soup.

0:04:33.060,0:04:34.860
This soup contains all the nuclei

0:04:34.860,0:04:36.620
that once were a mouse brain.

0:04:36.620,0:04:39.420
Now, the beauty of a soup is that because it is soup,

0:04:39.420,0:04:42.580
you can agitate it and make those nuclei

0:04:42.580,0:04:44.580
be distributed homogeneously in the liquid,

0:04:44.580,0:04:46.500
so that now by looking under the microscope

0:04:46.500,0:04:50.260
at just four or five samples[br]of this homogeneous solution,

0:04:50.260,0:04:53.180
you can count nuclei, and therefore tell

0:04:53.180,0:04:55.060
how many cells that brain had.

0:04:55.060,0:04:56.700
It's simple, it's straightforward,

0:04:56.700,0:04:58.180
and it's really fast.

0:04:58.180,0:05:00.100
So we've used that method to count neurons

0:05:00.100,0:05:02.180
in dozens of different species so far,

0:05:02.180,0:05:03.900
and it turns out that all brains

0:05:03.900,0:05:06.220
are not made the same way.

0:05:06.220,0:05:08.500
Take rodents and primates, for instance:

0:05:08.500,0:05:10.860
in larger rodent brains, the average size

0:05:10.860,0:05:12.700
of the neuron increases,

0:05:12.700,0:05:14.900
so the brain inflates very rapidly

0:05:14.900,0:05:18.220
and gains size much faster than it gains neurons.

0:05:18.220,0:05:20.260
But primate brains gain neurons

0:05:20.260,0:05:22.580
without the average neuron becoming any larger,

0:05:22.580,0:05:24.460
which is a very economical way

0:05:24.460,0:05:26.220
to add neurons to your brain.

0:05:26.220,0:05:27.940
The result is that a primate brain

0:05:27.940,0:05:30.860
will always have more neurons than[br]a rodent brain of the same size,

0:05:30.860,0:05:32.420
and the larger the brain,

0:05:32.420,0:05:34.540
the larger this difference will be.

0:05:34.540,0:05:36.460
Well, what about our brain then?

0:05:36.460,0:05:38.220
We found that we have, on average,

0:05:38.220,0:05:40.020
86 billion neurons,

0:05:40.020,0:05:42.620
16 billion of which are in the cerebral cortex,

0:05:42.620,0:05:44.700
and if you consider that the cerebral cortex

0:05:44.700,0:05:47.380
is the seat of functions like

0:05:47.380,0:05:51.100
awareness and logical and abstract reasoning,

0:05:51.100,0:05:53.900
and that 16 billion is the most neurons

0:05:53.900,0:05:56.660
that any cortex has,

0:05:56.660,0:05:58.500
I think this is the simplest explanation

0:05:58.500,0:06:01.660
for our remarkable cognitive abilities.

0:06:01.660,0:06:04.940
But just as important is what[br]the 86 billion neurons mean.

0:06:04.940,0:06:06.500
Because we found that the relationship

0:06:06.500,0:06:08.607
between the size of the brain[br]and its number of neurons

0:06:08.607,0:06:10.740
could be described mathematically,

0:06:10.740,0:06:12.820
we could calculate what a human brain

0:06:12.820,0:06:15.340
would look like if it was made like a rodent brain.

0:06:15.340,0:06:18.500
So, a rodent brain with 86 billion neurons

0:06:18.500,0:06:23.020
would weigh 36 kilos.

0:06:23.020,0:06:24.300
That's not possible.

0:06:24.300,0:06:25.820
A brain that huge would be crushed

0:06:25.820,0:06:27.020
by its own weight,

0:06:27.020,0:06:28.500
and this impossible brain would go

0:06:28.500,0:06:31.180
in the body of 89 tons.

0:06:31.180,0:06:34.220
I don't think it looks like us.

0:06:34.220,0:06:36.620
So this brings us to a very important[br]conclusion already,

0:06:36.620,0:06:39.420
which is that we are not rodents.

0:06:39.420,0:06:42.580
The human brain is not a large rat brain.

0:06:42.580,0:06:45.300
Compared to a rat, we might seem special, yes,

0:06:45.300,0:06:47.260
but that's not a fair comparison to make,

0:06:47.260,0:06:49.780
given that we know that we are not rodents.

0:06:49.780,0:06:51.540
We are primates,

0:06:51.540,0:06:54.020
so the correct comparison is to other primates.

0:06:54.020,0:06:55.540
And there, if you do the math,

0:06:55.540,0:06:57.540
you find that a generic primate

0:06:57.540,0:06:59.500
with 86 billion neurons

0:06:59.500,0:07:02.580
would have a brain of about 1.2 kilos,

0:07:02.580,0:07:04.500
which seems just right,

0:07:04.500,0:07:06.580
in a body of some 66 kilos,

0:07:06.580,0:07:08.980
which in my case is exactly right,

0:07:08.980,0:07:11.660
which brings us to a very unsurprising

0:07:11.660,0:07:14.860
but still incredibly important conclusion:

0:07:14.860,0:07:16.540
I am a primate.

0:07:16.540,0:07:19.100
And all of you are primates.

0:07:19.100,0:07:21.140
And so was Darwin.

0:07:21.140,0:07:23.140
I love to think that Darwin[br]would have really appreciated this.

0:07:23.140,0:07:25.660
His brain, like ours,

0:07:25.660,0:07:29.140
was made in the image of other primate brains.

0:07:29.140,0:07:31.180
So the human brain may be remarkable, yes,

0:07:31.180,0:07:34.254
but it is not special in its number of neurons.

0:07:34.254,0:07:36.140
It is just a large primate brain.

0:07:36.140,0:07:37.940
I think that's a very humbling

0:07:37.940,0:07:39.580
and sobering thought

0:07:39.580,0:07:42.340
that should remind us of our place in nature.

0:07:42.340,0:07:44.460
Why does it cost so much energy, then?

0:07:44.460,0:07:46.260
Well, other people have figured out

0:07:46.260,0:07:47.980
how much energy the human brain

0:07:47.980,0:07:49.540
and that of other species cost,

0:07:49.540,0:07:50.900
and now that we knew how many neurons

0:07:50.900,0:07:53.180
each brain was made of, we could do the math.

0:07:53.180,0:07:54.940
And it turns out that both human

0:07:54.940,0:07:57.900
and other brains cost about the same,

0:07:57.900,0:08:01.260
an average of six calories per billion neurons per day.

0:08:01.260,0:08:03.460
So the total energetic cost of a brain

0:08:03.460,0:08:05.540
is a simple, linear function

0:08:05.540,0:08:07.249
of its number of neurons,

0:08:07.249,0:08:09.140
and it turns out that the human brain

0:08:09.140,0:08:13.180
costs just as much energy as you would expect.

0:08:13.180,0:08:15.180
So the reason why the human brain

0:08:15.180,0:08:17.020
costs so much energy is simply because

0:08:17.020,0:08:19.020
it has a huge number of neurons,

0:08:19.020,0:08:20.860
and because we are primates

0:08:20.860,0:08:22.820
with many more neurons for a given body size

0:08:22.820,0:08:24.540
than any other animal,

0:08:24.540,0:08:28.020
the relative cost of our brain is large,

0:08:28.020,0:08:31.020
but just because we're primates,[br]not because we're special.

0:08:31.020,0:08:32.580
Last question, then:

0:08:32.580,0:08:35.540
how did we come by this[br]remarkable number of neurons,

0:08:35.540,0:08:37.580
and in particular, if great apes

0:08:37.580,0:08:39.060
are larger than we are,

0:08:39.060,0:08:41.379
why don't they have a larger brain than we do

0:08:41.379,0:08:42.900
with more neurons?

0:08:42.900,0:08:44.620
When we realized how much expensive it is

0:08:44.620,0:08:46.460
to have a lot of neurons in the brain, I figured,

0:08:46.460,0:08:48.940
maybe there's a simple reason.

0:08:48.940,0:08:50.700
They just can't afford the energy

0:08:50.700,0:08:53.620
for both a larger body and a large number of neurons.

0:08:53.620,0:08:55.140
So we did the math.

0:08:55.140,0:08:56.540
We calculated on the one hand

0:08:56.540,0:08:58.620
how much energy a primate gets per day

0:08:58.620,0:09:00.340
from eating raw foods,

0:09:00.340,0:09:01.900
and on the other hand, how much energy

0:09:01.900,0:09:03.940
a body of a certain size costs

0:09:03.940,0:09:06.900
and how much energy a brain of a[br]certain number of neurons costs,

0:09:06.900,0:09:08.540
and we looked for the combinations

0:09:08.540,0:09:10.460
of body size and number of brain neurons

0:09:10.460,0:09:12.020
that a primate could afford

0:09:12.020,0:09:15.060
if it ate a certain number of hours per day.

0:09:15.060,0:09:17.140
And what we found is that

0:09:17.140,0:09:18.620
because neurons are so expensive,

0:09:18.620,0:09:21.820
there is a tradeoff between[br]body size and number of neurons.

0:09:21.820,0:09:24.340
So a primate that eats eight hours per day

0:09:24.340,0:09:27.580
can afford at most 53 billion neurons,

0:09:27.580,0:09:29.460
but then its body cannot be any bigger

0:09:29.460,0:09:31.340
than 25 kilos.

0:09:31.340,0:09:33.100
To weigh any more than that,

0:09:33.100,0:09:34.900
it has to give up neurons.

0:09:34.900,0:09:37.460
So it's either a large body

0:09:37.460,0:09:39.140
or a large number of neurons.

0:09:39.140,0:09:40.660
When you eat like a primate,

0:09:40.660,0:09:42.620
you can't afford both.

0:09:42.620,0:09:44.660
One way out of this metabolic limitation

0:09:44.660,0:09:47.340
would be to spend even more hours per day eating,

0:09:47.340,0:09:49.300
but that gets dangerous,

0:09:49.300,0:09:51.220
and past a certain point, it's just not possible.

0:09:51.220,0:09:53.540
Gorillas and orangutans, for instance,

0:09:53.540,0:09:55.540
afford about 30 billion neurons

0:09:55.540,0:09:58.460
by spending eight and a half hours a day eating,

0:09:58.460,0:10:01.500
and that seems to be about as much as they can do.

0:10:01.500,0:10:03.260
Nine hours of feeding per day

0:10:03.260,0:10:07.100
seems to be the practical limit for a primate.

0:10:07.100,0:10:08.460
What about us?

0:10:08.460,0:10:10.460
With our 86 billion neurons

0:10:10.460,0:10:12.420
and 60 to 70 kilos of body mass,

0:10:12.420,0:10:16.380
we should have to spend over nine hours

0:10:16.380,0:10:20.140
per day every single day feeding,

0:10:20.140,0:10:21.980
which is just not feasible.

0:10:21.980,0:10:23.860
If we ate like a primate,

0:10:23.860,0:10:26.820
we should not be here.

0:10:26.820,0:10:28.500
How did we get here, then?

0:10:28.500,0:10:31.020
Well, if our brain costs just as much energy

0:10:31.020,0:10:32.820
as it should, and if we can't spend

0:10:32.820,0:10:36.540
every waking hour of the day feeding,

0:10:36.540,0:10:38.420
then the only alternative, really,

0:10:38.420,0:10:40.300
is to somehow get more energy

0:10:40.300,0:10:42.100
out of the same foods.

0:10:42.100,0:10:45.900
And remarkably, that matches exactly

0:10:45.900,0:10:49.060
what our ancestors are believed to have invented

0:10:49.060,0:10:51.100
one and a half million years ago,

0:10:51.100,0:10:53.620
when they invented cooking.

0:10:53.620,0:10:55.820
To cook is to use fire

0:10:55.820,0:10:59.020
to re-digest foods outside of your body.

0:10:59.020,0:11:01.780
Cooked foods are softer, so they're easier to chew

0:11:01.780,0:11:04.820
and to turn completely into mush in your mouth,

0:11:04.820,0:11:06.820
so that allows them to be completely digested

0:11:06.820,0:11:08.500
and absorbed in your gut,

0:11:08.500,0:11:12.140
which makes them yield much more[br]energy in much less time.

0:11:12.140,0:11:14.460
So cooking frees time for us to do

0:11:14.460,0:11:16.660
much more interesting things with our day

0:11:16.660,0:11:18.340
and with our neurons

0:11:18.340,0:11:20.060
than just thinking about food,

0:11:20.060,0:11:21.780
looking for food, and gobbling down food

0:11:21.780,0:11:23.220
all day long.

0:11:23.220,0:11:25.380
So because of cooking, what once was

0:11:25.380,0:11:27.380
a major liability, this large,

0:11:27.380,0:11:30.260
dangerously expensive brain with a lot of neurons,

0:11:30.260,0:11:32.620
could now become a major asset,

0:11:32.620,0:11:34.900
now that we could both afford the energy

0:11:34.900,0:11:36.380
for a lot of neurons

0:11:36.380,0:11:38.621
and the time to do interesting things with them.

0:11:38.621,0:11:40.580
So I think this explains why the human brain

0:11:40.580,0:11:43.620
grew to become so large so fast in evolution,

0:11:43.620,0:11:47.180
all of the while remaining just a primate brain.

0:11:47.180,0:11:50.060
With this large brain now affordable by cooking,

0:11:50.060,0:11:53.220
we went rapidly from raw foods to culture,

0:11:53.220,0:11:55.580
agriculture, civilization, grocery stores,

0:11:55.580,0:11:57.620
electricity, refrigerators,

0:11:57.620,0:11:59.380
all of those things that nowadays

0:11:59.380,0:12:01.300
allow us to get all the energy we need

0:12:01.300,0:12:04.020
for the whole day in a single sitting

0:12:04.020,0:12:07.020
at your favorite fast food joint.

0:12:07.020,0:12:09.180
So what once was a solution

0:12:09.180,0:12:11.140
now became the problem,

0:12:11.140,0:12:16.460
and ironically, we look for the solution in raw food.

0:12:16.460,0:12:19.860
So what is the human advantage?

0:12:19.860,0:12:21.180
What is it that we have

0:12:21.180,0:12:23.220
that no other animal has?

0:12:23.220,0:12:25.540
My answer is that we have the largest number

0:12:25.540,0:12:27.180
of neurons in the cerebral cortex,

0:12:27.180,0:12:28.900
and I think that's the simplest explanation

0:12:28.900,0:12:31.098
for our remarkable cognitive abilities.

0:12:31.098,0:12:33.940
And what is it that we do that no other animal does,

0:12:33.940,0:12:36.140
and which I believe was fundamental

0:12:36.140,0:12:38.900
to allow us to reach that large,

0:12:38.900,0:12:41.460
largest number of neurons in the cortex?

0:12:41.460,0:12:43.460
In two words, we cook.

0:12:43.460,0:12:47.180
No other animal cooks its food. Only humans do.

0:12:47.180,0:12:50.020
And I think that's how we got to become human.

0:12:50.020,0:12:51.787
Studying the human brain changed the way

0:12:51.787,0:12:53.100
I think about food.

0:12:53.100,0:12:54.540
I now look in my kitchen,

0:12:54.540,0:12:56.180
and I bow to it,

0:12:56.180,0:12:57.700
and I thank my ancestors for coming up

0:12:57.700,0:12:59.460
with the invention that probably made us humans.

0:12:59.460,0:13:01.500
Thank you very much.

0:13:01.500,0:13:07.714
(Applause)