0:00:00.946,0:00:03.340
What is so special about the human brain?

0:00:03.340,0:00:05.552
Why is it that we study other animals

0:00:05.552,0:00:07.673
instead of them studying us?

0:00:07.673,0:00:09.471
What does a human brain have or do

0:00:09.471,0:00:11.204
that no other brain does?

0:00:11.204,0:00:14.052
When I became interested[br]in these questions about 10 years ago,

0:00:14.052,0:00:16.994
scientists thought they knew[br]what different brains were made of.

0:00:16.994,0:00:18.723
Though it was based on very little evidence,

0:00:18.723,0:00:20.910
many scientists thought that all mammalian brains,

0:00:20.910,0:00:22.635
including the human brain,

0:00:22.635,0:00:23.935
were made in the same way,

0:00:23.935,0:00:25.437
with a number of neurons that was always

0:00:25.437,0:00:27.668
proportional to the size of the brain.

0:00:27.668,0:00:29.744
This means that two brains of the same size,

0:00:29.744,0:00:33.292
like these two, with a respectable 400 grams,

0:00:33.292,0:00:35.844
should have similar numbers of neurons.

0:00:35.844,0:00:37.640
Now, if neurons are the functional

0:00:37.640,0:00:40.543
information processing units of the brain,

0:00:40.543,0:00:42.135
then the owners of these two brains

0:00:42.135,0:00:44.864
should have similar cognitive abilities.

0:00:44.864,0:00:47.213
And yet, one is a chimp,

0:00:47.213,0:00:49.850
and the other is a cow.

0:00:49.850,0:00:52.212
Now maybe cows have a really rich

0:00:52.212,0:00:54.465
internal mental life and are so smart

0:00:54.465,0:00:58.268
that they choose not to let us realize it,

0:00:58.268,0:00:59.713
but we eat them.

0:00:59.713,0:01:01.412
I think most people will agree

0:01:01.412,0:01:03.413
that chimps are capable of much more complex,

0:01:03.413,0:01:06.469
elaborate and flexible behaviors than cows are.

0:01:06.469,0:01:08.401
So this is a first indication that the

0:01:08.401,0:01:10.434
"all brains are made the same way" scenario

0:01:10.434,0:01:12.105
is not quite right.

0:01:12.105,0:01:13.451
But let's play along.

0:01:13.451,0:01:15.229
If all brains were made the same way

0:01:15.229,0:01:18.141
and you were to compare animals[br]with brains of different sizes,

0:01:18.141,0:01:20.323
larger brains should always have more neurons

0:01:20.323,0:01:22.646
than smaller brains,[br]and the larger the brain,

0:01:22.646,0:01:25.668
the more cognitively able its owner should be.

0:01:25.668,0:01:27.804
So the largest brain around should also be

0:01:27.804,0:01:30.093
the most cognitively able.

0:01:30.093,0:01:31.674
And here comes the bad news:

0:01:31.674,0:01:34.431
Our brain, not the largest one around.

0:01:34.431,0:01:35.986
It seems quite vexing.

0:01:35.986,0:01:38.721
Our brain weighs between 1.2 and 1.5 kilos,

0:01:38.721,0:01:42.038
but elephant brains weigh between four and five kilos,

0:01:42.038,0:01:44.623
and whale brains can weigh up to nine kilos,

0:01:44.623,0:01:49.361
which is why scientists used to resort to saying

0:01:49.361,0:01:51.518
that our brain must be special

0:01:51.518,0:01:54.380
to explain our cognitive abilities.

0:01:54.380,0:01:57.326
It must be really extraordinary,

0:01:57.326,0:01:59.303
an exception to the rule.

0:01:59.303,0:02:02.529
Theirs may be bigger, but ours is better,

0:02:02.529,0:02:04.404
and it could be better, for example,

0:02:04.404,0:02:06.515
in that it seems larger than it should be,

0:02:06.515,0:02:09.213
with a much larger cerebral cortex[br]than we should have

0:02:09.213,0:02:10.752
for the size of our bodies.

0:02:10.752,0:02:12.404
So that would give us extra cortex

0:02:12.404,0:02:15.364
to do more interesting things[br]than just operating the body.

0:02:15.364,0:02:17.043
That's because the size of the brain

0:02:17.043,0:02:19.376
usually follows the size of the body.

0:02:19.376,0:02:21.693
So the main reason for saying that

0:02:21.693,0:02:23.780
our brain is larger than it should be

0:02:23.780,0:02:25.552
actually comes from comparing ourselves

0:02:25.552,0:02:27.238
to great apes.

0:02:27.238,0:02:29.832
Gorillas can be two to three times larger than we are,

0:02:29.832,0:02:32.253
so their brains should also be larger than ours,

0:02:32.253,0:02:34.198
but instead it's the other way around.

0:02:34.198,0:02:37.133
Our brain is three times larger than a gorilla brain.

0:02:37.133,0:02:39.253
The human brain also seems special

0:02:39.253,0:02:41.500
in the amount of energy that it uses.

0:02:41.500,0:02:44.249
Although it weighs only two percent of the body,

0:02:44.249,0:02:47.692
it alone uses 25 percent of all the energy

0:02:47.692,0:02:50.155
that your body requires to run per day.

0:02:50.155,0:02:53.508
That's 500 calories out of a total of 2,000 calories,

0:02:53.508,0:02:55.913
just to keep your brain working.

0:02:55.913,0:02:58.841
So the human brain is larger than it should be,

0:02:58.841,0:03:00.867
it uses much more energy than it should,

0:03:00.867,0:03:02.325
so it's special.

0:03:02.325,0:03:04.896
And this is where the story started to bother me.

0:03:04.896,0:03:06.567
In biology, we look for rules

0:03:06.567,0:03:09.413
that apply to all animals and to life in general,

0:03:09.413,0:03:11.263
so why should the rules of evolution

0:03:11.263,0:03:14.905
apply to everybody else but not to us?

0:03:14.905,0:03:17.102
Maybe the problem was with the basic assumption

0:03:17.102,0:03:18.973
that all brains are made in the same way.

0:03:18.973,0:03:20.572
Maybe two brains of a similar size

0:03:20.572,0:03:23.163
can actually be made of[br]very different numbers of neurons.

0:03:23.163,0:03:24.770
Maybe a very large brain

0:03:24.770,0:03:26.610
does not necessarily have more neurons

0:03:26.610,0:03:28.831
than a more modest-sized brain.

0:03:28.831,0:03:31.840
Maybe the human brain [br]actually has the most neurons

0:03:31.840,0:03:34.413
of any brain, regardless of its size,

0:03:34.413,0:03:36.507
especially in the cerebral cortex.

0:03:36.507,0:03:38.059
So this to me became

0:03:38.059,0:03:39.788
the important question to answer:

0:03:39.788,0:03:42.188
how many neurons does the human brain have,

0:03:42.188,0:03:44.710
and how does that compare to other animals?

0:03:44.710,0:03:47.133
Now, you may have heard or read somewhere

0:03:47.133,0:03:49.244
that we have 100 billion neurons,

0:03:49.244,0:03:51.330
so 10 years ago, I asked my colleagues

0:03:51.330,0:03:53.169
if they knew where this number came from.

0:03:53.169,0:03:54.627
But nobody did.

0:03:54.627,0:03:56.080
I've been digging through the literature

0:03:56.080,0:03:58.138
for the original reference for that number,

0:03:58.138,0:03:59.921
and I could never find it.

0:03:59.921,0:04:02.660
It seems that nobody had actually ever counted

0:04:02.660,0:04:04.194
the number of neurons in the human brain,

0:04:04.194,0:04:06.693
or in any other brain for that matter.

0:04:06.693,0:04:10.228
So I came up with my own way[br]to count cells in the brain,

0:04:10.228,0:04:12.336
and it essentially consists of

0:04:12.336,0:04:15.581
dissolving that brain into soup.

0:04:15.581,0:04:17.618
It works like this:

0:04:17.618,0:04:20.745
You take a brain, or parts of that brain,

0:04:20.745,0:04:22.489
and you dissolve it in detergent,

0:04:22.489,0:04:23.998
which destroys the cell membranes

0:04:23.998,0:04:26.434
but keeps the cell nuclei intact,

0:04:26.434,0:04:29.574
so you end up with a suspension of free nuclei

0:04:29.574,0:04:31.355
that looks like this,

0:04:31.355,0:04:32.628
like a clear soup.

0:04:32.628,0:04:34.519
This soup contains all the nuclei

0:04:34.519,0:04:36.573
that once were a mouse brain.

0:04:36.573,0:04:39.603
Now, the beauty of a soup is that because it is soup,

0:04:39.603,0:04:42.501
you can agitate it and make those nuclei

0:04:42.501,0:04:44.472
be distributed homogeneously in the liquid,

0:04:44.472,0:04:46.453
so that now by looking under the microscope

0:04:46.453,0:04:50.536
at just four or five samples[br]of this homogeneous solution,

0:04:50.536,0:04:53.072
you can count nuclei, and therefore tell

0:04:53.072,0:04:54.751
how many cells that brain had.

0:04:54.751,0:04:56.376
It's simple, it's straightforward,

0:04:56.376,0:04:57.810
and it's really fast.

0:04:57.810,0:04:59.807
So we've used that method to count neurons

0:04:59.807,0:05:02.118
in dozens of different species so far,

0:05:02.118,0:05:03.807
and it turns out that all brains

0:05:03.807,0:05:06.389
are not made the same way.

0:05:06.389,0:05:08.576
Take rodents and primates, for instance:

0:05:08.576,0:05:10.860
In larger rodent brains, the average size

0:05:10.860,0:05:12.546
of the neuron increases,

0:05:12.546,0:05:15.145
so the brain inflates very rapidly

0:05:15.145,0:05:18.310
and gains size much faster than it gains neurons.

0:05:18.310,0:05:20.013
But primate brains gain neurons

0:05:20.013,0:05:22.472
without the average neuron becoming any larger,

0:05:22.472,0:05:23.997
which is a very economical way

0:05:23.997,0:05:25.650
to add neurons to your brain.

0:05:25.650,0:05:27.414
The result is that a primate brain

0:05:27.414,0:05:30.581
will always have more neurons than[br]a rodent brain of the same size,

0:05:30.581,0:05:32.064
and the larger the brain,

0:05:32.064,0:05:34.278
the larger this difference will be.

0:05:34.278,0:05:36.381
Well, what about our brain then?

0:05:36.381,0:05:38.065
We found that we have, on average,

0:05:38.065,0:05:39.850
86 billion neurons,

0:05:39.850,0:05:42.849
16 billion of which are in the cerebral cortex,

0:05:42.849,0:05:44.930
and if you consider that the cerebral cortex

0:05:44.930,0:05:48.024
is the seat of functions like

0:05:48.024,0:05:51.161
awareness and logical and abstract reasoning,

0:05:51.161,0:05:54.145
and that 16 billion is the most neurons

0:05:54.145,0:05:56.766
that any cortex has,

0:05:56.766,0:05:58.484
I think this is the simplest explanation

0:05:58.484,0:06:01.626
for our remarkable cognitive abilities.

0:06:01.626,0:06:04.847
But just as important is what[br]the 86 billion neurons mean.

0:06:04.847,0:06:06.376
Because we found that the relationship

0:06:06.376,0:06:08.728
between the size of the brain[br]and its number of neurons

0:06:08.728,0:06:10.355
could be described mathematically,

0:06:10.355,0:06:12.573
we could calculate what a human brain

0:06:12.573,0:06:15.247
would look like if it was made like a rodent brain.

0:06:15.247,0:06:18.821
So, a rodent brain with 86 billion neurons

0:06:18.821,0:06:21.942
would weigh 36 kilos.

0:06:21.942,0:06:23.575
That's not possible.

0:06:23.575,0:06:25.481
A brain that huge would be crushed

0:06:25.481,0:06:26.665
by its own weight,

0:06:26.665,0:06:28.252
and this impossible brain would go

0:06:28.252,0:06:32.023
in the body of 89 tons.

0:06:32.023,0:06:34.157
I don't think it looks like us.

0:06:34.157,0:06:36.710
So this brings us to a very important[br]conclusion already,

0:06:36.710,0:06:39.357
which is that we are not rodents.

0:06:39.357,0:06:42.625
The human brain is not a large rat brain.

0:06:42.625,0:06:45.253
Compared to a rat, we might seem special, yes,

0:06:45.253,0:06:47.473
but that's not a fair comparison to make,

0:06:47.473,0:06:49.562
given that we know that we are not rodents.

0:06:49.562,0:06:50.952
We are primates,

0:06:50.952,0:06:53.726
so the correct comparison is to other primates.

0:06:53.726,0:06:54.953
And there, if you do the math,

0:06:54.953,0:06:57.739
you find that a generic primate

0:06:57.739,0:06:59.669
with 86 billion neurons

0:06:59.669,0:07:02.672
would have a brain of about 1.2 kilos,

0:07:02.672,0:07:04.561
which seems just right,

0:07:04.561,0:07:06.533
in a body of some 66 kilos,

0:07:06.533,0:07:09.178
which in my case is exactly right,

0:07:09.178,0:07:11.798
which brings us to a very unsurprising

0:07:11.798,0:07:14.767
but still incredibly important conclusion:

0:07:14.767,0:07:16.108
I am a primate.

0:07:16.108,0:07:18.806
And all of you are primates.

0:07:18.806,0:07:20.706
And so was Darwin.

0:07:20.706,0:07:23.646
I love to think that Darwin[br]would have really appreciated this.

0:07:23.646,0:07:25.613
His brain, like ours,

0:07:25.613,0:07:29.124
was made in the image of other primate brains.

0:07:29.124,0:07:31.426
So the human brain may be remarkable, yes,

0:07:31.426,0:07:34.207
but it is not special in its number of neurons.

0:07:34.207,0:07:36.062
It is just a large primate brain.

0:07:36.062,0:07:39.106
I think that's a very humbling and sobering thought

0:07:39.106,0:07:42.169
that should remind us of our place in nature.

0:07:42.169,0:07:44.813
Why does it cost so much energy, then?

0:07:44.813,0:07:46.260
Well, other people have figured out

0:07:46.260,0:07:47.763
how much energy the human brain

0:07:47.763,0:07:49.170
and that of other species costs,

0:07:49.170,0:07:50.822
and now that we knew how many neurons

0:07:50.822,0:07:53.164
each brain was made of, we could do the math.

0:07:53.164,0:07:55.030
And it turns out that both human

0:07:55.030,0:07:57.853
and other brains cost about the same,

0:07:57.853,0:08:01.274
an average of six calories [br]per billion neurons per day.

0:08:01.274,0:08:03.413
So the total energetic cost of a brain

0:08:03.413,0:08:05.447
is a simple, linear function

0:08:05.447,0:08:07.156
of its number of neurons,

0:08:07.156,0:08:09.339
and it turns out that the human brain

0:08:09.339,0:08:13.180
costs just as much energy as you would expect.

0:08:13.180,0:08:15.271
So the reason why the human brain

0:08:15.271,0:08:16.943
costs so much energy is simply because

0:08:16.943,0:08:18.926
it has a huge number of neurons,

0:08:18.926,0:08:20.411
and because we are primates

0:08:20.411,0:08:22.910
with many more neurons for a given body size

0:08:22.910,0:08:24.432
than any other animal,

0:08:24.432,0:08:27.972
the relative cost of our brain is large,

0:08:27.972,0:08:30.973
but just because we're primates,[br]not because we're special.

0:08:30.973,0:08:32.163
Last question, then:

0:08:32.163,0:08:35.278
how did we come by this[br]remarkable number of neurons,

0:08:35.278,0:08:37.178
and in particular, if great apes

0:08:37.178,0:08:38.733
are larger than we are,

0:08:38.733,0:08:42.498
why don't they have a larger brain [br]than we do, with more neurons?

0:08:42.498,0:08:44.588
When we realized how much expensive it is

0:08:44.588,0:08:46.921
to have a lot of neurons in the brain, I figured,

0:08:46.921,0:08:48.924
maybe there's a simple reason.

0:08:48.924,0:08:50.607
They just can't afford the energy

0:08:50.607,0:08:53.557
for both a large body [br]and a large number of neurons.

0:08:53.557,0:08:54.986
So we did the math.

0:08:54.986,0:08:56.586
We calculated on the one hand

0:08:56.586,0:08:58.527
how much energy a primate gets per day

0:08:58.527,0:08:59.877
from eating raw foods,

0:08:59.877,0:09:01.914
and on the other hand, how much energy

0:09:01.914,0:09:03.678
a body of a certain size costs

0:09:03.678,0:09:07.065
and how much energy a brain of a[br]certain number of neurons costs,

0:09:07.065,0:09:08.554
and we looked for the combinations

0:09:08.554,0:09:10.965
of body size and number of brain neurons

0:09:10.965,0:09:12.235
that a primate could afford

0:09:12.235,0:09:14.783
if it ate a certain number of hours per day.

0:09:14.783,0:09:16.599
And what we found is that

0:09:16.599,0:09:18.311
because neurons are so expensive,

0:09:18.311,0:09:21.681
there is a tradeoff between[br]body size and number of neurons.

0:09:21.681,0:09:24.632
So a primate that eats eight hours per day

0:09:24.632,0:09:27.656
can afford at most 53 billion neurons,

0:09:27.656,0:09:29.383
but then its body cannot be any bigger

0:09:29.383,0:09:31.337
than 25 kilos.

0:09:31.337,0:09:33.038
To weigh any more than that,

0:09:33.038,0:09:34.807
it has to give up neurons.

0:09:34.807,0:09:37.460
So it's either a large body

0:09:37.460,0:09:38.955
or a large number of neurons.

0:09:38.955,0:09:40.320
When you eat like a primate,

0:09:40.320,0:09:42.556
you can't afford both.

0:09:42.556,0:09:44.520
One way out of this metabolic limitation

0:09:44.520,0:09:47.921
would be to spend even more hours per day eating,

0:09:47.921,0:09:49.284
but that gets dangerous,

0:09:49.284,0:09:52.003
and past a certain point, it's just not possible.

0:09:52.003,0:09:53.540
Gorillas and orangutans, for instance,

0:09:53.540,0:09:55.463
afford about 30 billion neurons

0:09:55.463,0:09:58.429
by spending eight and a half hours per day eating,

0:09:58.429,0:10:01.545
and that seems to be about as much as they can do.

0:10:01.545,0:10:03.336
Nine hours of feeding per day

0:10:03.336,0:10:06.607
seems to be the practical limit for a primate.

0:10:06.607,0:10:08.398
What about us?

0:10:08.398,0:10:09.998
With our 86 billion neurons

0:10:09.998,0:10:13.033
and 60 to 70 kilos of body mass,

0:10:13.033,0:10:16.594
we should have to spend over nine hours

0:10:16.594,0:10:20.169
per day every single day feeding,

0:10:20.169,0:10:22.208
which is just not feasible.

0:10:22.208,0:10:24.042
If we ate like a primate,

0:10:24.042,0:10:26.295
we should not be here.

0:10:26.295,0:10:28.422
How did we get here, then?

0:10:28.422,0:10:31.157
Well, if our brain costs just as much energy

0:10:31.157,0:10:32.880
as it should, and if we can't spend

0:10:32.880,0:10:36.554
every waking hour of the day feeding,

0:10:36.554,0:10:38.465
then the only alternative, really,

0:10:38.465,0:10:40.423
is to somehow get more energy

0:10:40.423,0:10:42.376
out of the same foods.

0:10:42.376,0:10:46.176
And remarkably, that matches exactly

0:10:46.176,0:10:49.213
what our ancestors are believed to have invented

0:10:49.213,0:10:51.052
one and a half million years ago,

0:10:51.052,0:10:53.834
when they invented cooking.

0:10:53.834,0:10:55.804
To cook is to use fire

0:10:55.804,0:10:59.604
to pre-digest foods outside of your body.

0:10:59.604,0:11:02.210
Cooked foods are softer, so they're easier to chew

0:11:02.210,0:11:04.773
and to turn completely into mush in your mouth,

0:11:04.773,0:11:06.925
so that allows them to be completely digested

0:11:06.925,0:11:08.361
and absorbed in your gut,

0:11:08.361,0:11:12.016
which makes them yield much more[br]energy in much less time.

0:11:12.016,0:11:14.505
So cooking frees time for us to do

0:11:14.505,0:11:16.567
much more interesting things with our day

0:11:16.567,0:11:18.047
and with our neurons

0:11:18.047,0:11:19.952
than just thinking about food,

0:11:19.952,0:11:21.656
looking for food, and gobbling down food

0:11:21.656,0:11:22.881
all day long.

0:11:22.881,0:11:25.380
So because of cooking, what once was

0:11:25.380,0:11:27.733
a major liability, this large,

0:11:27.733,0:11:30.766
dangerously expensive brain with a lot of neurons,

0:11:30.766,0:11:32.802
could now become a major asset,

0:11:32.802,0:11:36.053
now that we could both afford [br]the energy for a lot of neurons

0:11:36.056,0:11:38.559
and the time to do interesting things with them.

0:11:38.559,0:11:40.533
So I think this explains why the human brain

0:11:40.533,0:11:43.773
grew to become so large so fast in evolution,

0:11:43.773,0:11:47.670
all of the while remaining just a primate brain.

0:11:47.670,0:11:50.444
With this large brain now affordable by cooking,

0:11:50.444,0:11:53.357
we went rapidly from raw foods to culture,

0:11:53.357,0:11:55.963
agriculture, civilization, grocery stores,

0:11:55.963,0:11:57.588
electricity, refrigerators,

0:11:57.588,0:11:59.195
all of those things that nowadays

0:11:59.195,0:12:01.237
allow us to get all the energy we need

0:12:01.237,0:12:04.019
for the whole day in a single sitting

0:12:04.019,0:12:06.973
at your favorite fast food joint.

0:12:06.973,0:12:09.410
So what once was a solution

0:12:09.410,0:12:11.109
now became the problem,

0:12:11.109,0:12:16.626
and ironically, we look for the solution in raw food.

0:12:16.626,0:12:19.182
So what is the human advantage?

0:12:19.182,0:12:20.685
What is it that we have

0:12:20.685,0:12:23.220
that no other animal has?

0:12:23.220,0:12:25.568
My answer is that we have the largest number

0:12:25.568,0:12:27.040
of neurons in the cerebral cortex,

0:12:27.040,0:12:28.884
and I think that's the simplest explanation

0:12:28.884,0:12:30.897
for our remarkable cognitive abilities.

0:12:30.897,0:12:34.124
And what is it that we do that no other animal does,

0:12:34.124,0:12:36.093
and which I believe was fundamental

0:12:36.093,0:12:39.176
to allow us to reach that large,

0:12:39.176,0:12:41.398
largest number of neurons in the cortex?

0:12:41.398,0:12:43.613
In two words, we cook.

0:12:43.613,0:12:47.179
No other animal cooks its food. Only humans do.

0:12:47.179,0:12:50.049
And I think that's how we got to become human.

0:12:50.049,0:12:52.509
Studying the human brain changed [br]the way I think about food.

0:12:52.509,0:12:54.154
I now look at my kitchen,

0:12:54.154,0:12:55.624
and I bow to it,

0:12:55.624,0:12:57.329
and I thank my ancestors for coming up

0:12:57.329,0:12:59.229
with the invention that probably made us humans.

0:12:59.229,0:13:01.361
Thank you very much.

0:13:01.361,0:13:07.714
(Applause)