TEDGlobal 2013
Suzana Herculano-Houzel: What is so special about the human brain?
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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
Suzana Herculano-Houzel shrunk the human brain by 14 billion neurons -- by developing a new way to count them. Full bio
Double-click the English transcript below to play the video.
00:12
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,
in these questions about 10 years ago,
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scientists thought they knew
what different brains were made of.
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,
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,
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
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.
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.
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
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,
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,
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,
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.
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
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,
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.
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.
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.
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,
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?
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.
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,
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.
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.
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
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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12:23
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.
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)
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ABOUT THE SPEAKER
Suzana Herculano-Houzel - NeuroscientistSuzana 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.
Suzana Herculano-Houzel | Speaker | TED.com