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What is so special about the human brain?

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Why is it that we study other animals

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instead of them studying us?

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What does a human brain have or do

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that no other brain does?

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When I became interested
in these questions about 10 years ago,

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scientists thought they knew
what different brains were made of.

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Though it was based on very little evidence,

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many scientists thought that all mammalian brains,

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including the human brain,

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were made in the same way,

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with a number of neurons that was always

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proportional to the size of the brain.

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This means that two brains of the same size,

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like these two, with a respectable 400 grams,

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should have similar numbers of neurons.

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Now, if neurons are the functional

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information processing units of the brain,

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then the owners of these two brains

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should have similar cognitive abilities.

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And yet, one is a chimp,

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and the other is a cow.

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Now maybe cows have a really rich

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internal mental life and are so smart

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that they choose not to let us realize it,

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but we eat them.

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I think most people will agree

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that chimps are capable of much more complex,

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elaborate and flexible behaviors than cows are.

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So this is a first indication that the

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"all brains are made the same way" scenario

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is not quite right.

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But let's play along.

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If all brains were made the same way

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and you were to compare animals
with brains of different sizes,

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larger brains should always have more neurons

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than smaller brains,
and the larger the brain,

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the more cognitively able its owner should be.

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So the largest brain around should also be

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the most cognitively able.

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And here comes the bad news:

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Our brain, not the largest one around.

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It seems quite vexing.

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Our brain weighs between 1.2 and 1.5 kilos,

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but elephant brains weigh between four and five kilos,

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and whale brains can weigh up to nine kilos,

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which is why scientists used to resort to saying

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that our brain must be special

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to explain our cognitive abilities.

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It must be really extraordinary,

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an exception to the rule.

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Theirs may be bigger, but ours is better,

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and it could be better, for example,

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in that it seems larger than it should be,

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with a much larger cerebral cortex
than we should have

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for the size of our bodies.

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So that would give us extra cortex

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to do more interesting things
than just operating the body.

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That's because the size of the brain

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usually follows the size of the body.

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So the main reason for saying that

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our brain is larger than it should be

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actually comes from comparing ourselves

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to great apes.

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Gorillas can be two to three times larger than we are,

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so their brains should also be larger than ours,

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but instead it's the other way around.

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Our brain is three times larger than a gorilla brain.

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The human brain also seems special

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in the amount of energy that it uses.

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Although it weighs only two percent of the body,

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it alone uses 25 percent of all the energy

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that your body requires to run per day.

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That's 500 calories out of a total of 2,000 calories,

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just to keep your brain working.

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So the human brain is larger than it should be,

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it uses much more energy than it should,

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so it's special.

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And this is where the story started to bother me.

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In biology, we look for rules

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that apply to all animals and to life in general,

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so why should the rules of evolution

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apply to everybody else but not to us?

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Maybe the problem was with the basic assumption

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that all brains are made in the same way.

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Maybe two brains of a similar size

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can actually be made of
very different numbers of neurons.

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Maybe a very large brain

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does not necessarily have more neurons

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than a more modest-sized brain.

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Maybe the human brain 
actually has the most neurons

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of any brain, regardless of its size,

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especially in the cerebral cortex.

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So this to me became

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the important question to answer:

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how many neurons does the human brain have,

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and how does that compare to other animals?

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Now, you may have heard or read somewhere

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that we have 100 billion neurons,

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so 10 years ago, I asked my colleagues

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if they knew where this number came from.

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But nobody did.

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I've been digging through the literature

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for the original reference for that number,

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and I could never find it.

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It seems that nobody had actually ever counted

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the number of neurons in the human brain,

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or in any other brain for that matter.

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So I came up with my own way
to count cells in the brain,

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and it essentially consists of

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dissolving that brain into soup.

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It works like this:

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You take a brain, or parts of that brain,

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and you dissolve it in detergent,

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which destroys the cell membranes

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but keeps the cell nuclei intact,

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so you end up with a suspension of free nuclei

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that looks like this,

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like a clear soup.

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This soup contains all the nuclei

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that once were a mouse brain.

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Now, the beauty of a soup is that because it is soup,

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you can agitate it and make those nuclei

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be distributed homogeneously in the liquid,

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so that now by looking under the microscope

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at just four or five samples
of this homogeneous solution,

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you can count nuclei, and therefore tell

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how many cells that brain had.

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It's simple, it's straightforward,

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and it's really fast.

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So we've used that method to count neurons

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in dozens of different species so far,

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and it turns out that all brains

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are not made the same way.

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Take rodents and primates, for instance:

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In larger rodent brains, the average size

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of the neuron increases,

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so the brain inflates very rapidly

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and gains size much faster than it gains neurons.

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But primate brains gain neurons

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without the average neuron becoming any larger,

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which is a very economical way

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to add neurons to your brain.

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The result is that a primate brain

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will always have more neurons than
a rodent brain of the same size,

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and the larger the brain,

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the larger this difference will be.

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Well, what about our brain then?

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We found that we have, on average,

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86 billion neurons,

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16 billion of which are in the cerebral cortex,

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and if you consider that the cerebral cortex

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is the seat of functions like

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awareness and logical and abstract reasoning,

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and that 16 billion is the most neurons

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that any cortex has,

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I think this is the simplest explanation

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for our remarkable cognitive abilities.

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But just as important is what
the 86 billion neurons mean.

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Because we found that the relationship

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between the size of the brain
and its number of neurons

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could be described mathematically,

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we could calculate what a human brain

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would look like if it was made like a rodent brain.

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So, a rodent brain with 86 billion neurons

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would weigh 36 kilos.

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That's not possible.

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A brain that huge would be crushed

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by its own weight,

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and this impossible brain would go

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in the body of 89 tons.

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I don't think it looks like us.

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So this brings us to a very important
conclusion already,

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which is that we are not rodents.

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The human brain is not a large rat brain.

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Compared to a rat, we might seem special, yes,

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but that's not a fair comparison to make,

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given that we know that we are not rodents.

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We are primates,

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so the correct comparison is to other primates.

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And there, if you do the math,

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you find that a generic primate

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with 86 billion neurons

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would have a brain of about 1.2 kilos,

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which seems just right,

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in a body of some 66 kilos,

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which in my case is exactly right,

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which brings us to a very unsurprising

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but still incredibly important conclusion:

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I am a primate.

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And all of you are primates.

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And so was Darwin.

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I love to think that Darwin
would have really appreciated this.

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His brain, like ours,

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was made in the image of other primate brains.

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So the human brain may be remarkable, yes,

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but it is not special in its number of neurons.

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It is just a large primate brain.

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I think that's a very humbling and sobering thought

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that should remind us of our place in nature.

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Why does it cost so much energy, then?

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Well, other people have figured out

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how much energy the human brain

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and that of other species costs,

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and now that we knew how many neurons

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each brain was made of, we could do the math.

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And it turns out that both human

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and other brains cost about the same,

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an average of six calories 
per billion neurons per day.

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So the total energetic cost of a brain

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is a simple, linear function

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of its number of neurons,

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and it turns out that the human brain

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costs just as much energy as you would expect.

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So the reason why the human brain

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costs so much energy is simply because

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it has a huge number of neurons,

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and because we are primates

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with many more neurons for a given body size

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than any other animal,

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the relative cost of our brain is large,

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but just because we're primates,
not because we're special.

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Last question, then:

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how did we come by this
remarkable number of neurons,

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and in particular, if great apes

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are larger than we are,

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why don't they have a larger brain 
than we do, with more neurons?

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When we realized how much expensive it is

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to have a lot of neurons in the brain, I figured,

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maybe there's a simple reason.

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They just can't afford the energy

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for both a large body 
and a large number of neurons.

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So we did the math.

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We calculated on the one hand

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how much energy a primate gets per day

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from eating raw foods,

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and on the other hand, how much energy

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a body of a certain size costs

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and how much energy a brain of a
certain number of neurons costs,

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and we looked for the combinations

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of body size and number of brain neurons

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that a primate could afford

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if it ate a certain number of hours per day.

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And what we found is that

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because neurons are so expensive,

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there is a tradeoff between
body size and number of neurons.

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So a primate that eats eight hours per day

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can afford at most 53 billion neurons,

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but then its body cannot be any bigger

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than 25 kilos.

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To weigh any more than that,

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it has to give up neurons.

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So it's either a large body

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or a large number of neurons.

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When you eat like a primate,

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you can't afford both.

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One way out of this metabolic limitation

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would be to spend even more hours per day eating,

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but that gets dangerous,

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and past a certain point, it's just not possible.

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Gorillas and orangutans, for instance,

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afford about 30 billion neurons

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by spending eight and a half hours per day eating,

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and that seems to be about as much as they can do.

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Nine hours of feeding per day

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seems to be the practical limit for a primate.

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What about us?

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With our 86 billion neurons

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and 60 to 70 kilos of body mass,

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we should have to spend over nine hours

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per day every single day feeding,

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which is just not feasible.

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If we ate like a primate,

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we should not be here.

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How did we get here, then?

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Well, if our brain costs just as much energy

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as it should, and if we can't spend

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every waking hour of the day feeding,

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then the only alternative, really,

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is to somehow get more energy

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out of the same foods.

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And remarkably, that matches exactly

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what our ancestors are believed to have invented

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one and a half million years ago,

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when they invented cooking.

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To cook is to use fire

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to pre-digest foods outside of your body.

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Cooked foods are softer, so they're easier to chew

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and to turn completely into mush in your mouth,

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so that allows them to be completely digested

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and absorbed in your gut,

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which makes them yield much more
energy in much less time.

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So cooking frees time for us to do

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much more interesting things with our day

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and with our neurons

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than just thinking about food,

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looking for food, and gobbling down food

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all day long.

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So because of cooking, what once was

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a major liability, this large,

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dangerously expensive brain with a lot of neurons,

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could now become a major asset,

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now that we could both afford 
the energy for a lot of neurons

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and the time to do interesting things with them.

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So I think this explains why the human brain

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grew to become so large so fast in evolution,

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all of the while remaining just a primate brain.

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With this large brain now affordable by cooking,

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we went rapidly from raw foods to culture,

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agriculture, civilization, grocery stores,

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electricity, refrigerators,

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all of those things that nowadays

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allow us to get all the energy we need

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for the whole day in a single sitting

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at your favorite fast food joint.

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So what once was a solution

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now became the problem,

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and ironically, we look for the solution in raw food.

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So what is the human advantage?

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What is it that we have

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that no other animal has?

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My answer is that we have the largest number

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of neurons in the cerebral cortex,

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and I think that's the simplest explanation

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for our remarkable cognitive abilities.

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And what is it that we do that no other animal does,

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and which I believe was fundamental

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to allow us to reach that large,

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largest number of neurons in the cortex?

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In two words, we cook.

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No other animal cooks its food. Only humans do.

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And I think that's how we got to become human.

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Studying the human brain changed 
the way I think about food.

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I now look at my kitchen,

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and I bow to it,

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and I thank my ancestors for coming up

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with the invention that probably made us humans.

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Thank you very much.

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(Applause)