What is so special about the human brain? | Suzana Herculano-Houzel

Translator: Carme Paz Reviewer: Serv. de Norm. Lingüística U. de Santiago de Compostela

What is so special about the human brain?

Que ten de especial o cerebro humano?

Why is it that we study other animals

Por que estudamos os outros animais

instead of them studying us?

mais eles non nos estudan a nós?

What does a human brain have or do

Que ten ou fai o cerebro humano

that no other brain does?

que non fai nin ten ningún outro?

Cando comecei a interesarme nestas cuestións hai uns 10 anos,

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

os científicos crían saber de que estaban feitos os cerebros.

scientists thought they knew what different brains were made of.

Though it was based on very little evidence,

Malia basearse en pequenas evidencias,

many scientists thought that all mammalian brains,

moitos científicos crían que todos os cerebros de mamíferos,

including the human brain,

incluíndo o cerebro humano,

were made in the same way,

with a number of neurons that was always

estaban feitos igual,

cun número de neuronas que era

proportional to the size of the brain.

sempre proporcional ao tamaño do cerebro.

This means that two brains of the same size,

Isto significa que dous cerebros do mesmo tamaño,

like these two, with a respectable 400 grams,

como estes dous, cos seus 400 gramos,

should have similar numbers of neurons.

deberían ter un número semellante de neuronas.

Now, if neurons are the functional

Pero se as neuronas son as unidades funcionais

information processing units of the brain,

de procesamento de información do cerebro,

then the owners of these two brains

os donos deses dous cerebros

should have similar cognitive abilities.

deberían ter habilidades cognitivas semellantes.

And yet, one is a chimp,

E porén, un é un chimpancé,

and the other is a cow.

e o outro é unha vaca.

Now maybe cows have a really rich

Ben, é posible que as vacas teñan unha vizosa vida interior

internal mental life and are so smart

e sexan tan intelixentes

that they choose not to let us realize it,

que prefiren que non nos deamos de conta,

but we eat them.

pero comémolas.

I think most people will agree

Penso que a maioría estará de acordo

that chimps are capable of much more complex,

en que os chimpancés poden ter

un comportamento moito máis complexo, elaborado e flexible ca unha vaca.

elaborate and flexible behaviors than cows are.

So this is a first indication that the

Este é un primeiro indicio de que

"all brains are made the same way" scenario

"todos os cerebros están feitos da mesma maneira"

is not quite right.

non é exactamente así.

Pero sigamos.

But let's play along.

If all brains were made the same way

Se todos os cerebros estivesen feitos da mesma maneira

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

e comparásemos animais con cerebros de tamaños diferentes,

larger brains should always have more neurons

os máis grandes deberían ter máis neuronas

than smaller brains, and the larger the brain,

cós máis pequenos, e canto maior for o cerebro,

the more cognitively able its owner should be.

máis habilidades cognitivas debería ter o dono.

So the largest brain around should also be

Así que o cerebro máis grande debería ser tamén

the most cognitively able.

o de maior capacidade cognitiva.

E aquí veñen as malas noticias:

And here comes the bad news:

Our brain, not the largest one around.

O noso cerebro non é o máis grande.

It seems quite vexing.

Resulta un pouco desacougante.

Our brain weighs between 1.2 and 1.5 kilos,

O noso cerebro pesa entre 1,2 e 1,5 kg,

but elephant brains weigh between four and five kilos,

pero o cerebro dun elefante pesa entre 4 e 5 kg

and whale brains can weigh up to nine kilos,

e o das baleas pode pesar ata 9 kg,

which is why scientists used to resort to saying

é por isto que os científicos, para explicar as nosas habilidades cognitivas,

that our brain must be special

acababan por dicir

to explain our cognitive abilities.

que o noso cerebro tiña que ser especial.

It must be really extraordinary,

Tiña que ser extraordinario de verdade,

an exception to the rule.

unha excepción á regra.

Theirs may be bigger, but ours is better,

O deles pode ser maior pero o noso é mellor,

and it could be better, for example,

e podería ser mellor, por exemplo,

in that it seems larger than it should be,

porque parece maior do que debería ser,

with a much larger cerebral cortex than we should have

cun córtex cerebral moito maior do que deberiamos ter

for the size of our bodies.

para o tamaño dos nosos corpos.

So that would give us extra cortex

Iso daríanos córtex extra

to do more interesting things than just operating the body.

para facer cousas máis interesantes amais de manexar o corpo.

That's because the size of the brain

Iso é porque o tamaño do cerebro

normalmente está relacionado co tamaño do corpo.

usually follows the size of the body.

So the main reason for saying that

Así que a principal razón para dicir que

our brain is larger than it should be

o noso cerebro é maior do que debería ser

actually comes from comparing ourselves

vén en realidade de comparármonos

to great apes.

cos grandes simios.

Os gorilas poden ser de dúas a tres veces meirandes ca nós,

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

so their brains should also be larger than ours,

así que os seus cerebros deberían ser tamén meirandes cós nosos,

but instead it's the other way around.

pero é ao revés.

O noso cerebro é tres veces máis grande có cerebro dun gorila.

Our brain is three times larger than a gorilla brain.

The human brain also seems special

O cerebro humano tamén semella especial

in the amount of energy that it uses.

na cantidade de enerxía que usa.

Although it weighs only two percent of the body,

Aínda que só pesa o 2 % do corpo,

it alone uses 25 percent of all the energy

usa o 25 % de toda a enerxía

that your body requires to run per day.

que o corpo require para funcionar cada día.

Iso son 500 calorías dun total de 2000 calorías,

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

just to keep your brain working.

só para manter o cerebro funcionando.

So the human brain is larger than it should be,

Entón, o cerebro humano é máis grande do que debería,

it uses much more energy than it should,

usa moita máis enerxía do que debería

so it's special.

e, polo tanto, é especial.

And this is where the story started to bother me.

E aquí é onde a historia empezou a intrigarme.

In biology, we look for rules

En bioloxía buscamos regras

that apply to all animals and to life in general,

que se apliquen a todos os animais e á vida en xeral.

so why should the rules of evolution

Por que as regras da evolución

apply to everybody else but not to us?

se ían aplicar a todo o mundo menos a nós?

Maybe the problem was with the basic assumption

Quizais o problema era a asunción básica

de todos os cerebros estaren feitos igual.

that all brains are made in the same way.

Maybe two brains of a similar size

Quizais dous cerebros de tamaño similar

can actually be made of very different numbers of neurons.

poden ter un número moi diferente de neuronas.

Maybe a very large brain

Quizais un cerebro moi grande

does not necessarily have more neurons

non ten por que ter máis neuronas

than a more modest-sized brain.

ca un cerebro de tamaño máis modesto.

Maybe the human brain actually has the most neurons

Quizais o cerebro humano ten máis neuronas ca calquera outro cerebro

of any brain, regardless of its size,

a pesar do seu tamaño,

especially in the cerebral cortex.

especialmente no córtex cerebral.

So this to me became

Así que para min, isto converteuse

the important question to answer:

na pregunta que había que resolver:

how many neurons does the human brain have,

Cantas neuronas ten o cerebro humano,

and how does that compare to other animals?

e como se compara co doutros animais?

Now, you may have heard or read somewhere

Ben, seguramente escoitastes ou lestes

que temos 100 mil millóns de neuronas,

that we have 100 billion neurons,

así que hai 10 anos, pregunteilles aos meus colegas

so 10 years ago, I asked my colleagues

if they knew where this number came from.

se sabían de onde saíra este número.

But nobody did.

Pero ninguén o sabía.

I've been digging through the literature

Estiven investigando na literatura

for the original reference for that number,

a referencia orixinal deste número,

e nunca cheguei a atopala.

and I could never find it.

It seems that nobody had actually ever counted

Semella que ninguén chegou a contar

the number of neurons in the human brain,

o número de neuronas do cerebro humano

or in any other brain for that matter.

ou de ningún outro, para o caso.

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

Así que inventei o meu propio xeito de contar células no cerebro

and it essentially consists of

e esencialmente consiste en

dissolving that brain into soup.

disolver ese cerebro en sopa.

It works like this:

Funciona así:

You take a brain, or parts of that brain,

Cóllese un cerebro, ou partes del,

and you dissolve it in detergent,

e disólvese en deterxente,

which destroys the cell membranes

que destrúe as membranas das células

but keeps the cell nuclei intact,

pero mantén o núcleo celular intacto,

so you end up with a suspension of free nuclei

e o resultado é unha suspensión de núcleos soltos

that looks like this,

que se parece a isto,

like a clear soup.

unha sopa clara.

This soup contains all the nuclei

Esta sopa contén todos os núcleos

that once were a mouse brain.

que un día compuxeron o cerebro dun rato.

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

Agora ben, a beleza dunha sopa é que ao ser sopa

you can agitate it and make those nuclei

pódese abanear e facer que eses núcleos

be distributed homogeneously in the liquid,

se distribúan no líquido homoxeneamente,

so that now by looking under the microscope

así que agora ao mirar debaixo do microscopio

at just four or five samples of this homogeneous solution,

catro ou cinco mostras desta solución homoxénea

you can count nuclei, and therefore tell

pódense contar núcleos e dese xeito dicir

cantas células tiña ese cerebro.

how many cells that brain had.

It's simple, it's straightforward,

É simple, é sinxelo,

and it's really fast.

e é moi rápido.

So we've used that method to count neurons

Así que usamos ese método para contar neuronas

in dozens of different species so far,

en ducias de especies diferentes

e resulta que os cerebros

and it turns out that all brains

are not made the same way.

non están todos feitos do mesmo xeito.

Take rodents and primates, for instance:

Collamos os roedores e os primates, por exemplo.

In larger rodent brains, the average size

Nos cerebros de roedores grandes, o tamaño medio

of the neuron increases,

das neuronas increméntase,

so the brain inflates very rapidly

así que o cerebro medra rapidamente

e gaña tamaño moito máis rápido do que gaña neuronas.

and gains size much faster than it gains neurons.

But primate brains gain neurons

Pero os cerebros dos primates gañan neuronas

without the average neuron becoming any larger,

sen que o tamaño medio destas aumente,

which is a very economical way

o que é un xeito moi económico

to add neurons to your brain.

de engadir neuronas ao cerebro.

The result is that a primate brain

O resultado é que o cerebro dun primate

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

sempre terá máis neuronas có cerebro dun roedor do mesmo tamaño,

and the larger the brain,

e canto maior for o cerebro,

the larger this difference will be.

maior será esta diferenza.

Well, what about our brain then?

Ben, entón que pasa co noso cerebro?

We found that we have, on average,

Resulta que temos, de media,

86 mil millóns de neuronas,

86 billion neurons,

16 billion of which are in the cerebral cortex,

e destas, 16 mil millóns están no córtex cerebral.

and if you consider that the cerebral cortex

Se se considera que o córtex cerebral

is the seat of functions like

é a sede de funcións como a consciencia

awareness and logical and abstract reasoning,

e o razoamento lóxico e abstracto,

e que 16 mil millóns é o máximo de neuronas

and that 16 billion is the most neurons

that any cortex has,

que ten un córtex,

I think this is the simplest explanation

penso que esta é a explicación máis simple

for our remarkable cognitive abilities.

para as nosas extraordinarias habilidades cognitivas.

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

Pero tan importante é o que significan os 86 mil millóns de neuronas.

Because we found that the relationship

Porque descubrimos que a relación

between the size of the brain and its number of neurons

entre o tamaño do cerebro e o número de neuronas

could be described mathematically,

poderíase describir matematicamente,

we could calculate what a human brain

poderiamos calcular como sería un cerebro humano

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

se fose como o cerebro dun roedor.

So, a rodent brain with 86 billion neurons

Así que o cerebro dun roedor con 86 mil millóns de neuronas

would weigh 36 kilos.

pesaría 36 kg.

That's not possible.

Isto non é posible.

A brain that huge would be crushed

Un cerebro tan enorme acabaría esmagado

by its own weight,

polo seu propio peso.

and this impossible brain would go

E a este cerebro imposible corresponderíalle

in the body of 89 tons.

un corpo de 89 toneladas.

Non creo que se pareza a nós.

I don't think it looks like us.

Así que isto xa nos leva a unha conclusión moi importante:

So this brings us to a very important conclusion already,

which is that we are not rodents.

nós non somos roedores.

The human brain is not a large rat brain.

O cerebro humano non é o cerebro dunha rata grande.

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

Comparados cunha rata, poderiamos parecer especiais, si,

but that's not a fair comparison to make,

pero non é unha comparación moi xusta,

given that we know that we are not rodents.

dado que sabemos que non somos roedores.

We are primates,

Somos primates,

so the correct comparison is to other primates.

así que a comparación correcta é con outros primates.

And there, if you do the math,

E aí, se se fan a contas,

you find that a generic primate

resulta que un primate normal

with 86 billion neurons

con 86 mil millóns de neuronas

would have a brain of about 1.2 kilos,

tería un cerebro de case 1,2 kg

which seems just right,

que encaixa perfectamente

in a body of some 66 kilos,

nun corpo duns 66 quilos,

which in my case is exactly right,

que no meu caso é exactamente ese.

Isto lévanos a unha conclusión que non sorprende moito

which brings us to a very unsurprising

but still incredibly important conclusion:

pero que é moi importante:

I am a primate.

Son un primate.

And all of you are primates.

E todos vós sodes primates.

And so was Darwin.

E Darwin tamén o era.

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

Encántame pensar que Darwin apreciaría moito isto.

His brain, like ours,

O seu cerebro, coma o noso,

was made in the image of other primate brains.

estaba feito á imaxe do cerebro doutros primates.

Así que o cerebro humano pode ser extraordinario, si,

So the human brain may be remarkable, yes,

but it is not special in its number of neurons.

pero non é especial no número de neuronas que ten.

It is just a large primate brain.

É só un cerebro de primate grande.

I think that's a very humbling and sobering thought

Penso que ese é un pensamento moi humilde e realista

que nos debería lembrar o noso lugar na natureza.

that should remind us of our place in nature.

Why does it cost so much energy, then?

Por que precisa tanta enerxía, entón?

Well, other people have figured out

Ben, outros descubriron

how much energy the human brain

canta enerxía gasta

and that of other species costs,

o cerebro humano e o doutras especies,

and now that we knew how many neurons

e agora que sabemos cantas neuronas

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

ten cada cerebro, podemos sacar conclusións.

And it turns out that both human

E resulta que tanto o cerebro humano

and other brains cost about the same,

coma outros cerebros gastan o mesmo,

an average of six calories per billion neurons per day.

unha media de seis calorías por cada mil millóns de neuronas por día.

So the total energetic cost of a brain

Así que o custo total de enerxía dun cerebro

is a simple, linear function

é unha función simple, lineal,

of its number of neurons,

do seu número de neuronas,

e resulta que o cerebro humano

and it turns out that the human brain

costs just as much energy as you would expect.

gasta tanta enerxía como poderiamos agardar.

So the reason why the human brain

Así que a razón pola que o cerebro humano

costs so much energy is simply because

gasta tanta enerxía é simplemente porque

it has a huge number of neurons,

ten un número enorme de neuronas,

and because we are primates

e porque somos primates

with many more neurons for a given body size

con moitas máis neuronas para unha masa corporal determinada

than any other animal,

ca calquera outro animal.

the relative cost of our brain is large,

O custo relativo do noso cerebro é grande

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

pero só porque somos primates, non por sermos especiais.

Last question, then:

Unha última cuestión, entón:

how did we come by this remarkable number of neurons,

como chegamos a este extraordinario número de neuronas?

and in particular, if great apes

E, en particular, se os grandes primates

are larger than we are,

son máis grandes ca nós,

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

por que non teñen un cerebro máis grande có noso con máis neuronas?

When we realized how much expensive it is

Cando nos demos de conta do que custa

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

ter tantas neuronas no cerebro,

maybe there's a simple reason.

pensei: talvez hai unha razón simple.

They just can't afford the energy

Eles non poden permitirse a enerxía

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

para un corpo grande e un gran número de neuronas.

So we did the math.

Así que fixemos contas.

We calculated on the one hand

Por unha parte, calculamos

how much energy a primate gets per day

canta enerxía consegue ao día un primate

from eating raw foods,

comendo alimentos crus

and on the other hand, how much energy

e, por outra parte, canta enerxía

a body of a certain size costs

precisa un corpo dun determinado tamaño

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

e canta enerxía precisa un cerebro dun determinado número de neuronas,

and we looked for the combinations

e buscamos as combinacións

of body size and number of brain neurons

de tamaño de corpo e número de neuronas

that a primate could afford

que un primate podería permitirse

if it ate a certain number of hours per day.

se comese durante certo número de horas por día.

And what we found is that

E descubrimos que,

because neurons are so expensive,

como as neuronas precisan tanta enerxía,

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

hai unha compensación entre o tamaño do corpo e o número de neuronas.

So a primate that eats eight hours per day

Así que un primate que come oito horas cada día

can afford at most 53 billion neurons,

pode ter como moito 53 mil millóns de neuronas

but then its body cannot be any bigger

pero entón o seu corpo non pode

than 25 kilos.

superar os 25 kg.

To weigh any more than that,

Para pesar algo máis,

ten que renunciar a neuronas.

it has to give up neurons.

So it's either a large body

Así que ou se ten un corpo grande

or a large number of neurons.

ou un número alto de neuronas.

When you eat like a primate,

Se comes coma un primate

you can't afford both.

non podes ter as dúas cousas.

One way out of this metabolic limitation

Unha forma de superar esta limitación metabólica

would be to spend even more hours per day eating,

sería pasar aínda máis horas comendo cada día.

but that gets dangerous,

pero iso é perigoso

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

e, pasado certo punto, non é posible.

Os gorilas e os orangutáns, por exemplo,

Gorillas and orangutans, for instance,

afford about 30 billion neurons

poden permitirse uns 30 mil millóns de neuronas

by spending eight and a half hours per day eating,

botando oito horas e media comendo cada día,

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

e isto é o máximo ao que poden chegar.

Nine hours of feeding per day

Nove horas comendo ao día

seems to be the practical limit for a primate.

parece ser o límite práctico para un primate.

What about us?

Que hai de nós?

With our 86 billion neurons

Cos nosos 86 mil millóns de neuronas

and 60 to 70 kilos of body mass,

e 60 ou 70 kg de masa corporal,

deberiamos ter que pasar unhas nove horas

we should have to spend over nine hours

per day every single day feeding,

comendo cada día,

o que non é factible.

which is just not feasible.

If we ate like a primate,

Se comésemos coma un primate,

non estariamos aquí.

we should not be here.

How did we get here, then?

Daquela, como chegamos aquí?

Well, if our brain costs just as much energy

Ben, se o noso cerebro gasta toda a enerxía

que se supón, e se non podemos pasar

as it should, and if we can't spend

every waking hour of the day feeding,

todo o día comendo,

then the only alternative, really,

entón a única alternativa,

is to somehow get more energy

e conseguir, dalgún xeito, máis enerxía

out of the same foods.

da mesma comida.

And remarkably, that matches exactly

E, sorprendentemente, iso encaixa

co que se supón que inventaron os nosos devanceiros

what our ancestors are believed to have invented

one and a half million years ago,

hai un millón e medio de anos,

ao inventaren a cociña.

when they invented cooking.

To cook is to use fire

Cociñar é usar o lume

to pre-digest foods outside of your body.

para predixerir alimentos fóra do noso corpo.

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

Os alimentos cociñados son máis brandos e, por tanto, máis fáciles de mastigar

and to turn completely into mush in your mouth,

e de converter en papa na boca,

so that allows them to be completely digested

isto permítenos dixerilos completamente

and absorbed in your gut,

e absorbelos no intestino,

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

o que fai que desprendan moita máis enerxía en moito menos tempo.

Así que cociñar dános tempo libre para facermos

So cooking frees time for us to do

much more interesting things with our day

cousas máis interesantes co noso día

and with our neurons

e coas nosas neuronas

que pensar só en comer,

than just thinking about food,

looking for food, and gobbling down food

buscando comida e engulindo

all day long.

todo o día.

So because of cooking, what once was

Grazas á cociña, o que unha vez foi

a major liability, this large,

unha carga pesada, este cerebro enorme

dangerously expensive brain with a lot of neurons,

e perigosamente custoso cun montón de neuronas,

could now become a major asset,

puido converterse nun gran recurso,

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

ao poder permitírmonos a enerxía para moitas neuronas

e o tempo para facer cousas interesantes con elas.

and the time to do interesting things with them.

So I think this explains why the human brain

Así que penso que isto explica por que o cerebro humano

grew to become so large so fast in evolution,

evolucionou tanto e tan rápido,

all of the while remaining just a primate brain.

mentres seguía sendo só un cerebro de primate.

With this large brain now affordable by cooking,

Con este cerebro enorme que nos podiamos permitir grazas á cociña,

we went rapidly from raw foods to culture,

pasamos rapidamente da comida crúa á cultura,

agricultura, civilización, tendas,

agriculture, civilization, grocery stores,

electricity, refrigerators,

electricidade, refrixeradores,

all of those things that nowadays

todas esas cousas que agora

allow us to get all the energy we need

nos permiten conseguir a enerxía precisa

for the whole day in a single sitting

para o día enteiro mentres sentamos

no noso bar preferido de comida rápida.

at your favorite fast food joint.

(Risos)

So what once was a solution

Así que o que unha vez foi a solución

now became the problem,

agora convértese no problema,

e, ironicamente, buscamos a solución na comida crúa.

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

So what is the human advantage?

Así que cal é a vantaxe humana?

What is it that we have

Que é iso que temos

that no other animal has?

que ningún outro animal ten?

My answer is that we have the largest number

A miña resposta é que temos o maior número

of neurons in the cerebral cortex,

de neuronas no córtex cerebral

e creo que é a explicación máis sinxela

and I think that's the simplest explanation

for our remarkable cognitive abilities.

para as nosas extraordinarias habilidades cognitivas.

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

E que facemos nós que ningún outro animal fai

e que penso que é fundamental

and which I believe was fundamental

para permitirnos acadar ese número

to allow us to reach that large,

tan grande de neuronas no córtex?

largest number of neurons in the cortex?

In two words, we cook.

En dúas palabras, nós cociñamos.

No other animal cooks its food. Only humans do.

Ningún outro animal cociña. Só o fan os humanos.

E penso que é así como chegamos a ser humanos.

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

Estudando o cerebro humano cambiou a miña forma de ver a comida.

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

I now look at my kitchen,

Eu agora miro a miña cociña

and I bow to it,

e inclínome perante ela

and I thank my ancestors for coming up

e doulles grazas aos meus devanceiros por dar

with the invention that probably made us humans.

co invento que probablemente nos fixo humanos.

Thank you very much.

Moitas grazas.

(Applause)

(Aplausos )