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 mammalian 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, with a respectable 400 grams, 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 a first indication that the "all brains are made the same way" scenario is not quite right.
Que ten de especial o cerebro humano? Por que estudamos os outros animais mais eles non nos estudan a nós? Que ten ou fai o cerebro humano que non fai nin ten ningún outro? Cando comecei a interesarme nestas cuestións hai uns 10 anos, os científicos crían saber de que estaban feitos os cerebros. Malia basearse en pequenas evidencias, moitos científicos crían que todos os cerebros de mamíferos, incluíndo o cerebro humano, estaban feitos igual, cun número de neuronas que era sempre proporcional ao tamaño do cerebro. Isto significa que dous cerebros do mesmo tamaño, como estes dous, cos seus 400 gramos, deberían ter un número semellante de neuronas. Pero se as neuronas son as unidades funcionais de procesamento de información do cerebro, os donos deses dous cerebros deberían ter habilidades cognitivas semellantes. E porén, un é un chimpancé, e o outro é unha vaca. Ben, é posible que as vacas teñan unha vizosa vida interior e sexan tan intelixentes que prefiren que non nos deamos de conta, pero comémolas. Penso que a maioría estará de acordo en que os chimpancés poden ter un comportamento moito máis complexo, elaborado e flexible ca unha vaca. Este é un primeiro indicio de que "todos os cerebros están feitos da mesma maneira" non é exactamente así.
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 five kilos, and whale brains can weigh up to nine kilos, which is why scientists used to resort 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, in that it seems larger than it should be, with a much larger cerebral cortex than we should have for the size of our bodies. 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 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 brains should also be larger than ours, but instead it's the other way around. Our brain is three times larger than a gorilla brain.
Pero sigamos. Se todos os cerebros estivesen feitos da mesma maneira e comparásemos animais con cerebros de tamaños diferentes, os máis grandes deberían ter máis neuronas cós máis pequenos, e canto maior for o cerebro, máis habilidades cognitivas debería ter o dono. Así que o cerebro máis grande debería ser tamén o de maior capacidade cognitiva. E aquí veñen as malas noticias: O noso cerebro non é o máis grande. Resulta un pouco desacougante. O noso cerebro pesa entre 1,2 e 1,5 kg, pero o cerebro dun elefante pesa entre 4 e 5 kg e o das baleas pode pesar ata 9 kg, é por isto que os científicos, para explicar as nosas habilidades cognitivas, acababan por dicir que o noso cerebro tiña que ser especial. Tiña que ser extraordinario de verdade, unha excepción á regra. O deles pode ser maior pero o noso é mellor, e podería ser mellor, por exemplo, porque parece maior do que debería ser, cun córtex cerebral moito maior do que deberiamos ter para o tamaño dos nosos corpos. Iso daríanos córtex extra para facer cousas máis interesantes amais de manexar o corpo. Iso é porque o tamaño do cerebro normalmente está relacionado co tamaño do corpo. Así que a principal razón para dicir que o noso cerebro é maior do que debería ser vén en realidade de comparármonos cos grandes simios. Os gorilas poden ser de dúas a tres veces meirandes ca nós, así que os seus cerebros deberían ser tamén meirandes cós nosos, pero é ao revés. O noso cerebro é tres veces máis grande có cerebro dun gorila.
The human brain also seems special in the amount of energy that it uses. Although it weighs only two percent of the body, it alone uses 25 percent 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.
O cerebro humano tamén semella especial na cantidade de enerxía que usa. Aínda que só pesa o 2 % do corpo, usa o 25 % de toda a enerxía que o corpo require para funcionar cada día. Iso son 500 calorías dun total de 2000 calorías, só para manter o cerebro funcionando.
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 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 modest-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?
Entón, o cerebro humano é máis grande do que debería, usa moita máis enerxía do que debería e, polo tanto, é especial. E aquí é onde a historia empezou a intrigarme. En bioloxía buscamos regras que se apliquen a todos os animais e á vida en xeral. Por que as regras da evolución se ían aplicar a todo o mundo menos a nós? Quizais o problema era a asunción básica de todos os cerebros estaren feitos igual. Quizais dous cerebros de tamaño similar poden ter un número moi diferente de neuronas. Quizais un cerebro moi grande non ten por que ter máis neuronas ca un cerebro de tamaño máis modesto. Quizais o cerebro humano ten máis neuronas ca calquera outro cerebro a pesar do seu tamaño, especialmente no córtex cerebral. Así que para min, isto converteuse na pregunta que había que resolver: Cantas neuronas ten o cerebro humano, e como se compara co doutros animais?
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've been 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.
Ben, seguramente escoitastes ou lestes que temos 100 mil millóns de neuronas, así que hai 10 anos, pregunteilles aos meus colegas se sabían de onde saíra este número. Pero ninguén o sabía. Estiven investigando na literatura a referencia orixinal deste número, e nunca cheguei a atopala. Semella que ninguén chegou a contar o número de neuronas do cerebro humano ou de ningún outro, para o caso.
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 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 awareness and logical and abstract reasoning, and that 16 billion is the most neurons that any cortex has, I think this is the 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 86 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.
Así que inventei o meu propio xeito de contar células no cerebro e esencialmente consiste en disolver ese cerebro en sopa. Funciona así: Cóllese un cerebro, ou partes del, e disólvese en deterxente, que destrúe as membranas das células pero mantén o núcleo celular intacto, e o resultado é unha suspensión de núcleos soltos que se parece a isto, unha sopa clara. Esta sopa contén todos os núcleos que un día compuxeron o cerebro dun rato. Agora ben, a beleza dunha sopa é que ao ser sopa pódese abanear e facer que eses núcleos se distribúan no líquido homoxeneamente, así que agora ao mirar debaixo do microscopio catro ou cinco mostras desta solución homoxénea pódense contar núcleos e dese xeito dicir cantas células tiña ese cerebro. É simple, é sinxelo, e é moi rápido. Así que usamos ese método para contar neuronas en ducias de especies diferentes e resulta que os cerebros non están todos feitos do mesmo xeito. Collamos os roedores e os primates, por exemplo. Nos cerebros de roedores grandes, o tamaño medio das neuronas increméntase, así que o cerebro medra rapidamente e gaña tamaño moito máis rápido do que gaña neuronas. Pero os cerebros dos primates gañan neuronas sen que o tamaño medio destas aumente, o que é un xeito moi económico de engadir neuronas ao cerebro. O resultado é que o cerebro dun primate sempre terá máis neuronas có cerebro dun roedor do mesmo tamaño, e canto maior for o cerebro, maior será esta diferenza. Ben, entón que pasa co noso cerebro? Resulta que temos, de media, 86 mil millóns de neuronas, e destas, 16 mil millóns están no córtex cerebral. Se se considera que o córtex cerebral é a sede de funcións como a consciencia e o razoamento lóxico e abstracto, e que 16 mil millóns é o máximo de neuronas que ten un córtex, penso que esta é a explicación máis simple para as nosas extraordinarias habilidades cognitivas. Pero tan importante é o que significan os 86 mil millóns de neuronas. Porque descubrimos que a relación entre o tamaño do cerebro e o número de neuronas poderíase describir matematicamente, poderiamos calcular como sería un cerebro humano se fose como o cerebro dun roedor. Así que o cerebro dun roedor con 86 mil millóns de neuronas pesaría 36 kg. Isto non é posible. Un cerebro tan enorme acabaría esmagado polo seu propio peso. E a este cerebro imposible corresponderíalle un corpo de 89 toneladas. Non creo que se pareza a nós.
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.
Así que isto xa nos leva a unha conclusión moi importante: nós non somos roedores. O cerebro humano non é o cerebro dunha rata grande. Comparados cunha rata, poderiamos parecer especiais, si, pero non é unha comparación moi xusta, dado que sabemos que non somos roedores. Somos primates, así que a comparación correcta é con outros primates. E aí, se se fan a contas, resulta que un primate normal con 86 mil millóns de neuronas tería un cerebro de case 1,2 kg que encaixa perfectamente nun corpo duns 66 quilos, que no meu caso é exactamente ese. Isto lévanos a unha conclusión que non sorprende moito pero que é moi importante: Son un primate. E todos vós sodes 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.
E Darwin tamén o era. Encántame pensar que Darwin apreciaría moito isto. O seu cerebro, coma o noso, estaba feito á imaxe do cerebro doutros primates.
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.
Así que o cerebro humano pode ser extraordinario, si, pero non é especial no número de neuronas que ten. É só un cerebro de primate grande. Penso que ese é un pensamento moi humilde e realista que nos debería lembrar o noso lugar na natureza.
Why does it cost so much energy, then? Well, other people have figured out how much energy the 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 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.
Por que precisa tanta enerxía, entón? Ben, outros descubriron canta enerxía gasta o cerebro humano e o doutras especies, e agora que sabemos cantas neuronas ten cada cerebro, podemos sacar conclusións. E resulta que tanto o cerebro humano coma outros cerebros gastan o mesmo, unha media de seis calorías por cada mil millóns de neuronas por día. Así que o custo total de enerxía dun cerebro é unha función simple, lineal, do seu número de neuronas, e resulta que o cerebro humano gasta tanta enerxía como poderiamos agardar. Así que a razón pola que o cerebro humano gasta tanta enerxía é simplemente porque ten un número enorme de neuronas, e porque somos primates con moitas máis neuronas para unha masa corporal determinada ca calquera outro animal. O custo relativo do noso cerebro é grande pero só porque somos primates, non por sermos especiais.
Last 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 much 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.
Unha última cuestión, entón: como chegamos a este extraordinario número de neuronas? E, en particular, se os grandes primates son máis grandes ca nós, por que non teñen un cerebro máis grande có noso con máis neuronas? Cando nos demos de conta do que custa ter tantas neuronas no cerebro, pensei: talvez hai unha razón simple. Eles non poden permitirse a enerxía para un corpo grande e un gran número de neuronas. Así que fixemos contas. Por unha parte, calculamos canta enerxía consegue ao día un primate comendo alimentos crus e, por outra parte, canta enerxía precisa un corpo dun determinado tamaño e canta enerxía precisa un cerebro dun determinado número de neuronas, e buscamos as combinacións de tamaño de corpo e número de neuronas que un primate podería permitirse se comese durante certo número de horas por día.
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 eight 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.
E descubrimos que, como as neuronas precisan tanta enerxía, hai unha compensación entre o tamaño do corpo e o número de neuronas. Así que un primate que come oito horas cada día pode ter como moito 53 mil millóns de neuronas pero entón o seu corpo non pode superar os 25 kg. Para pesar algo máis, ten que renunciar a neuronas. Así que ou se ten un corpo grande ou un número alto de neuronas. Se comes coma un primate non podes ter as dúas cousas.
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, it's just not possible. Gorillas and orangutans, for instance, afford about 30 billion neurons by spending eight and a half hours per day eating, and that seems to be about as much as they can do. Nine hours of feeding per day seems to be the practical limit for a primate.
Unha forma de superar esta limitación metabólica sería pasar aínda máis horas comendo cada día. pero iso é perigoso e, pasado certo punto, non é posible. Os gorilas e os orangutáns, por exemplo, poden permitirse uns 30 mil millóns de neuronas botando oito horas e media comendo cada día, e isto é o máximo ao que poden chegar. Nove horas comendo ao día parece ser o límite práctico para un primate.
What about us? With our 86 billion neurons and 60 to 70 kilos of body mass, we should have to spend over nine hours per day every single day feeding, which is just not feasible. If we ate like a primate, we should not be here.
Que hai de nós? Cos nosos 86 mil millóns de neuronas e 60 ou 70 kg de masa corporal, deberiamos ter que pasar unhas nove horas comendo cada día, o que non é factible. Se comésemos coma un primate, non estariamos aquí.
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 waking hour of the day feeding, then the only alternative, really, is to somehow get more energy out of the same foods. And 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 pre-digest 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 in much less time. So cooking frees 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.
Daquela, como chegamos aquí? Ben, se o noso cerebro gasta toda a enerxía que se supón, e se non podemos pasar todo o día comendo, entón a única alternativa, e conseguir, dalgún xeito, máis enerxía da mesma comida. E, sorprendentemente, iso encaixa co que se supón que inventaron os nosos devanceiros hai un millón e medio de anos, ao inventaren a cociña. Cociñar é usar o lume para predixerir alimentos fóra do noso corpo. Os alimentos cociñados son máis brandos e, por tanto, máis fáciles de mastigar e de converter en papa na boca, isto permítenos dixerilos completamente e absorbelos no intestino, o que fai que desprendan moita máis enerxía en moito menos tempo. Así que cociñar dános tempo libre para facermos cousas máis interesantes co noso día e coas nosas neuronas que pensar só en comer, buscando comida e engulindo todo o día.
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 of 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 of 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.
Grazas á cociña, o que unha vez foi unha carga pesada, este cerebro enorme e perigosamente custoso cun montón de neuronas, puido converterse nun gran recurso, ao poder permitírmonos a enerxía para moitas neuronas e o tempo para facer cousas interesantes con elas. Así que penso que isto explica por que o cerebro humano evolucionou tanto e tan rápido, mentres seguía sendo só un cerebro de primate. Con este cerebro enorme que nos podiamos permitir grazas á cociña, pasamos rapidamente da comida crúa á cultura, agricultura, civilización, tendas, electricidade, refrixeradores, todas esas cousas que agora nos permiten conseguir a enerxía precisa para o día enteiro mentres sentamos no noso bar preferido de comida rápida. (Risos) Así que o que unha vez foi a solución agora convértese no problema, e, ironicamente, buscamos a solución na comida crúa.
So what is the human advantage? What is it that we have that no other animal has? My answer is that 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.
Así que cal é a vantaxe humana? Que é iso que temos que ningún outro animal ten? A miña resposta é que temos o maior número de neuronas no córtex cerebral e creo que é a explicación máis sinxela para as nosas extraordinarias habilidades cognitivas. E que facemos nós que ningún outro animal fai e que penso que é fundamental para permitirnos acadar ese número tan grande de neuronas no córtex? En dúas palabras, nós cociñamos. Ningún outro animal cociña. Só o fan os humanos. E penso que é así como chegamos a ser humanos.
Studying the human brain changed the way I think about food. I now 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. (Applause)
Estudando o cerebro humano cambiou a miña forma de ver a comida. Eu agora miro a miña cociña e inclínome perante ela e doulles grazas aos meus devanceiros por dar co invento que probablemente nos fixo humanos. Moitas grazas. (Aplausos )