Ted Talks - Suzana Herculano Houzel - O que há de tão especial sobre o cérebro humano?

what is so special about the human brain why is it that we study other animals instead of them studying us what does a human brain have or do that no other brain does when I became interested in these questions about 10 years ago scientists thought they knew what different brains were made of though it was based on very little evidence many scientists thought that all Mamon brains including the human brain were made in the same way with a number of neurons that was always proportional to the size of the brain this means that two brains

of the same size like these two of a respectable 400 G should have similar numbers of neurons now if neurons are the functional information processing units of the brain then the owners of these two brains should have similar cognitive abilities and yet one is a chimp and the other is a cow now maybe cows have a really rich internal mental life and are so smart that they choose not to let us realize it but we eat them I think most people will agree that chimps are capable of much more complex elaborate and flexible behaviors than

cows are so this is the first indication that the all brains were made the same way scenario is not quite right but let's play along if all brains were made the same way and you were to compare animals with brains of different sizes larger brains should always have more neurons than smaller brains and the larger the brain the more cognitively able its owner should be so the largest brain around should also be the most cognitively able and here comes the bad news our brain not the largest one around it seems quite vexing our brain weighs

between 1.2 and 1.5 kilos but elephant brains weigh between four and 5 kilos and whale brains can weigh up to 9 kilos which is why scientists used to sort to saying that our brain must be special to explain our cognitive abilities it must be really extraordinary an exception to the rule theirs may be bigger but ours is better and it could be better for example and that it seems larger than it should be with a much larger cerebral cortex than we should have for the size of our body so that would give us extra cortex

to do more interesting things than just operating the body that's because the size of the brain usually fall follows the size of the body so the main reason for saying that our brain is larger than it should be actually comes from comparing ourselves to great apes gorillas can be two to three times larger than we are so their brain should also be larger than ours but instead it's the other way around our brain is three times larger than a gorilla brain the human brain also seems special in the amount of energy that it uses although

it weighs only 2% of the body it alone uses 25% of all the energy that your body requires to run per day that's 500 calories out of a total of 2,000 calories just to keep your brain working so the human brain is larger than it should be it uses much more energy than it should so it's special and this is where the story started to bother me in biology we look for rules that apply to all animals and to life in general so why should the rules of evolution apply to everybody else but not to

us maybe the problem was with the basic assumption that all brains are are made in the same way maybe two brains of a similar size can actually be made of very different numbers of neurons maybe a very large Brain does not necessarily have more neurons than a more modes sized brain maybe the human brain actually has the most neurons of any brain regardless of its size especially in the cerebral cortex so this to me became the important question to answer how many neurons does the human brain have and how does that compare to other animals

now you may have heard or read somewhere that we have 100 billion neurons so 10 years ago I asked my colleagues if they knew where this number came from but nobody did I went digging through the literature for the original reference for that number and I could never find it it seems that nobody had actually ever counted the number of neurons in the human brain or in any other brain for that matter so I came up with my own way to count cells in the brain and it essentially consists of dissolving that brain into soup

it works like this you take a brain or parts of that brain and you dissolve it in detergent which destroys the cell membranes but keeps the cell nuclei intact so you end up with a suspension of free nuclei that looks like this like a clear soup this soup contains all the nuclei that once were a mouse brain now the beauty of a soup is that because it is soup you can agitate it and make those nuclei be distributed homogeneously in the liquid so that now by looking under the microscope at just four or five samples

of this homogeneous solution You Can Count nuclei and therefore tell how many cells that brain had it's simple it's straightforward and it's really fast so we've used that method to count neurons in dozens of different species so far and it turns out that all brains are not made the same way take rodents and primates for instance in larger rodent brains the average size of the neuron increases so the brain inflates very rapidly and gains size much faster than it gains noral neurons but primate brains gain neurons without the average neuron becoming any larger which is

a very economical way to add neurons to your brain the result is that a primate brain will always have more neurons than a rodent brain of the same size and the larger the brain the larger this difference will be well what about our brain then we found that we have on average 86 billion neurons 16 billion of which are in the cerebral cortex and if you consider that the cerebral cortex is the seat of functions like um awareness and logical and abstract reasoning in that 16 billion is the most neurons that any cortex has I

think this is a simplest explanation for our remarkable cognitive abilities but just as important is what the 86 billion neurons mean because we found that the relationship between the size of the brain and its number of neurons could be described mathematically we could calculate what a human brain would look like if it was made like a rodent brain so a rodent brain with 80 6 billion neurons would weigh 36 kilos that's not possible a brain that huge would be crushed by its own weight and this impossible brain would go in the body of 89 tons

I don't think it looks like us so this brings us to a very important conclusion already which is that we are not rodents the human brain is not a large rat brain compared to a rat we might seem special yes but that's not a fair comparison to make given that we know that we are not rodents we are primates so the correct comparison is to other primates and there if you do the math you find that a generic primate with 86 billion neurons would have a brain of about 1.2 kilos which seems just right in

a body of some 66 kilos which in my case is exactly right which brings us to a very unsurprising but still incredibly important conclusion I am a primate and all of you are primates and so was Darwin I love to think that Darwin would have really appreciated this his brain like ours was made in the image of other primate brains so the human brain may be remarkable yes but it is not special in its number of neurons it is just a large primate brain I think that's a very humbling and sobering thought that should remind

us of our place in nature why does it cost so much energy then well other people have figured out how much energy do human brain and that of other species costs and now that we knew how many neurons each brain was made of we could do the math and it turns out that both human and other uh brains cost about the same an average of six calories per billion neurons per day so the total energetic cost of a brain is a simple linear function of its number of neurons and it turns out that the human

brain costs just as much energy as you would expect so the reason why the human brain costs so much energy is simply because it has a huge number of neurons and because we are primates with many more neurons for a given body size than any other animal the relative cost of our brain is large but just because we're primates not because we're special let's question then how did we come by this remarkable number of neurons and in particular if great apes are larger than we are why don't they have a larger brain than we do

with more neurons when we realized how expensive it is to have a lot of neurons in the brain I figured maybe there's a simple reason they just can't afford the energy for both a large body and a large number of neurons so we did the math we calculated on the one hand how much energy a primate gets per day from eating raw foods and on the other hand how much energy a body of a certain size costs and how much energy a brain of a certain number of neurons costs and we looked for the combinations

of body size and number of brain neurons that a primate could afford if it ate a certain number of hours per day and what we found is that because neurons are so expensive there is a tradeoff between body size and number of neurons so a primate that eats 8 hours per day can afford at most 53 billion neurons but then its body cannot be any bigger than 25 kilos to weigh any more than that it has to give up neurons so it's either a large body or a large number of neurons when you eat like

a primate you can't afford both one way out of this metabolic limitation would be to spend even more hours per day eating but that gets dangerous and past a certain point is just not possible gorillas and oranga tanks for instance afford about 30 billion neurons by spending 8 and 1 half hours per day eating and that seems to be about as much as they can't do nine hours of feeding per day seems to be the Practical limit for a primate what about us with our 86 billion neurons and 60 to 70 kilos of body mass

we should have to spend over 9 hours per day every single day feeding which is just not feasible if we ate like a primate we should not be here how did we get here then well if our brain costs just as much energy as it should and if we can't spend every every waking hour of the day um feeding then the only alternative really is to somehow get more energy out of the same foods and um remarkably that matches exactly what our ancestors are believed to have invented one and a half million years ago when

they invented cooking to cook is to use fire to predigest Foods outside of your body cooked foods are softer so they're easier to chew and to turn completely into mush in your mouth so that allows them to be completely digested and absorbed in your gut which makes them yield much more energy and much less time so cooking free time for us to do much more interesting things with our day and with our neurons than just thinking about food looking for food and gobbling down food all day long so because of cooking what once was a

major liability this large dangerously expensive brain with a lot of neurons could now become a major asset now that we could both afford the energy for a lot of neurons and the time to do interesting things with them so I think this explains why the human brain grew to become so large so fast in evolution all the while remaining just a primate brain with this large Brain now affordable by cooking we went rapidly from raw foods to culture agriculture civilization grocery stores electricity refrigerators all those things that nowadays allow us to get all the energy

we need for the whole day in a single sitting at your favorite fast food joint so what once was a solution now became the problem and ironically we look for the solution in raw food so what is the human Advantage what is it that we have that no other animal has my answer is we have the largest number of neurons in the cerebral cortex and I think that's the simplest explanation for our remarkable cognitive abilities and what is it that we do that no other animal does and which I believe was fundamental to allow us

to reach that large largest number of neurons in the cortex in two words we cook no other animal Cooks its food only humans do and I think that's how we got to become human studying the human brain changed the way I think about food and I'll look at my kitchen and I bow to it and I thank my ancestors for coming up with the invention that probably made us humans thank you very much [Music] [Applause] [Music]