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.
Zer da giza burmuina hain berezia egiten duena? Zergatik ikertzen ditugu guk beste animaliak eta haiek ez gaituzte gu ikertzen? Zer du giza burmuinak edo zer egiten du besteenak egiten ez duenik? Gai hauetaz interesatzen hasi nintzenean, duela 10 urte, zientifikoek uste zuten bazekitela garunak nola eginda zeuden. Ebidentzia gutxi izan arren, zientifiko askok zioten ugaztunen garunak, baita gizakiena ere, berdin osatuak zeudela eta garunaren tamainarekiko proportzionala zen neurona kopurua zutela. Hau da, tamaina bereko bi garunek, 400 gramoko bi garun hauek esaterako, neurona kopuru bertsua izan beharko luketela. Baina neuronak garuneko informazioaren prozesamenduaren unitate funtzionalak badira, garun bi hauen jabeek gaitasun kognitibo antzekoak izan beharko lituzkete. Baina bata txinpantzea da, eta bestea behi bat. Posible da behiek barne bizitza sakon bat izatea, eta hain azkarrak izanik guri hortaz ohartzen ez uztea erabaki izana, baina guk eurak jaten ditugu. Nirekin ados egongo zarete txinpantzeek behiek baino jokabide konplexu, elaboratu eta malguagoak burutzeko gai direla. "Garun guztiak modu berean osatuak daude" esaldia oso zuzena ez dela erakusten du lehen puntu honek.
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.
Baina jarrai dezagun. Garun denak berdin osatuak baleude, tamaina desberdineko garundun animaliak konparatuz gero, garun handiek txikiek baino neurona gehiago izan beharko lituzkete. Gero eta handiagoa, orduan eta gaitasun kognitibo handiagoa izango luke garunaren jabeak. Hori horrela balitz, garun handienak gaitasun kognitibo handiena izango luke. Eta, hemen datoz berri txarrak: Gure garuna ez da guztietan handiena. Honek nahasgarria dirudi. Gure garunak 1, 2 eta 1,5 kg artean pisatzen du, elefanteen garunak 4 eta 5 kg artean pisatzen du, eta baleen garunak 9 kg arteko pisua har dezake. Guzti honengatik, zientifikoek gure garuna berezia zela esaten zuten gure gaitasun kognitiboak azaltzean. Benetan ezohikoa izan behar du bai, arauaren salbuespena. Beraiena handiagoa izango da bai, baina gurea hobea da, eta hobea izan zitekeen, hau da, izan behar lukeena baino handiagoa omen delako, gure gorputzen tamainagatik izan beharko genukeena baino kortex askoz handiagoa baitugu. Garun kortex estra omen dugu, gorputza kudeatzeko baino gauza interesgarriagoak egiteko. Normalean, garunaren tamaina gorputzaren tamainaren araberakoa da. Beraz, gure garuna beharko lukeena baino handiagoa dela esateko arrazoi nagusia, beste hominidoekin egindako konparaketetatik dator. Gorilak gu baino 2 edo 3 aldiz handiagoak izan daitezke. Bere garunak gureak baino handiagoak izan beharko lukete baina aurkakoa gertatzen da. Gure garuna gorilena baino hiru aldiz handiagoa da.
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.
Giza garuna berezia dirudi ere erabiltzen duen energia kantitateagatik. Gorputzaren pisuaren %2a soilik izan arren, berak bakarrik gorputzak eguneroko funtzioetarako darabilen energia guztiaren %25 behar du. Hori 2000 kalorietatik 500 litzateke, garuna funtzionamenduan izateko bakarrik.
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?
Beraz, giza garuna beharko lukeena baino handiagoa da, behar lukeena baino energia gehiago erabiltzen du, eta berezia da. Orain istorio hau gogaikarri bihurtzen hasi zen. Biologian arauak bilatzen ditugu, animalia guztiei eta bizitzari oro har aplikatzekoak. Zergatik aplikatu beharko zaizkie eboluzioaren arauak espezie guztiei guri izan ezik? Agian arazoa oinarrizko ustean zegoen, agian garun denak ez daude berdin osatuak. Agian tamaina bereko bi garun neurona kantitate ezberdinez osatuta egon daitezke. Oso handia den garun batek agian ez du txikiago batek baino neurona gehiago izan behar. Agian gizakion garunak neurona kantitate handiena du bere tamainak axolarik izan gabe, batez ere kortexean. Ondorioz, guzti hau erantzun beharreko kontu garrantzitsu bihurtu zen. Zenbat neurona ditu giza garunak, eta nola konpara dezakegu beste animaliekin? Posible da zuetako batzuk nonbait irakurri edo entzun izana
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.
100 mila milioi neurona ditugula. Duela 10 urte, lankideei galdetu nien ea bazekiten zifra hori nondik zetorren. Inork ez zekien. Zifra horren jatorriaren bila jardun nintzen erreferentzia-bibliografian, baina sekula ez nuen aurkitu. Badirudi inork ez zuela benetan giza garunaren neurona kopurua zenbatu, ez eta beste edozein garunetako neurona kopurua ere.
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.
Garuneko zelulak kontatu ahal izateko formula bat sortu nuen. Oinarrian zera egin behar da: garuna disolbatu, zopa bihurtu arte. Horrela funtzionatzen du: garun bat edo garun zati batzuk hartu eta garbigarritan disolbatzen da. Honek ehunak suntsitzen ditu baina zelularen nukleoa osorik mantentzen du. Horrela, nukleoarekin soilik geratzen zara. Nukleoak itxura hau du, zopa baten antzekoa da. Zopa honek, arratoi baten garuna zenak, nukleo guztiak ditu. Zoparen edertasuna zera da, zopa denez, berau nahastu dezakezula, eta horrela nukleo guzti horiek likidotan homogeneoki banatu. Soluzio homogeneo honen lau edo bost lagin mikroskopiotik behatzean, nukleoak kontatu ahal dira, eta garun horrek zenbat zelula zituen jakin. Sinplea da, erraza, eta benetan azkarra. Metodo honekin dozenaka espezieren neuronak kontatu ditugu, eta zera ikusi dugu, garunak ez daudela modu berean osatuta. Azter ditzagun karraskariak eta primateak. Karraskarien garun handienetan, handitu egiten da neuronen batezbesteko tamaina. Beraz, garuna azkar puzten da eta tamainaz handitzen da, neuronak gehitu baino. Primateen garunak, berriz, neuronak gehitzen ditu neuronen tamaina handitu barik. Garunari neuronak gehitzeko modu oso ekonomikoa da. Beraz, primate baten garunak neurona gehiago izango du tamaina bereko karraskari batenak baino. Garuna gero eta handiagoa izan, gero eta handiagoa izango da aldea. Zer gertatzen da gure garunean? Zera aurkitu dugu: batez beste 86 mila milioi neurona ditugu. Hauetako 16 mila milioi garun kortexean aurkitzen dira, Kontuan hartuz kortexean daudela kontzientzia, arrazonamendu logikoa, abstraktua eta antzeko funtzioak, eta ez dagoela 16 mila milioi neurona baino gehiago duen kortexik, uste dut hau dela gure aparteko gaitasun kognitiboen azalpen sinpleena. 86 mila milioi neurona horien esanahia ere garrantzitsua da. Aurkitu genuen garunaren tamainaren eta bere neurona kopuruaren arteko erlazioa matematikoki deskribatu daitekeela. Giza garun bat karraskari batena balitz nolakoa izango zen kalkulatu ahal izan genuen. Karraskari baten garunak 86 mila milioi neurona izango balitu, 36 kg pisatuko lituzke. Hori ezta posible. Hain garun handia bere pisuak zanpatuko luke, eta garun hau 89 tonatako gorputz batekin etorriko litzake. Horrek ez luke gure antzik izango. Guzti honek ondorio nagusi batera garamatza:
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.
ez garela karraskariak. Giza garuna ez da arratoi garun erraldoi bat. Arratoi batekin alderatuta bereziak izan gaitezke, baina ez da alderaketa justua, baitakigu ez garela karraskariak. Primateak gara, beraz alderaketa justua beste primateekin litzateke. Kalkuluak hau ematen du: 86 mila milioi neurona lituzkeen primate batek gutxi gora behera 1,2 kg-ko garuna edukiko luke onargarria dena, eta 66 kg-ko gorputza. Horixe da nire kasua hain zuzen ere. Guzti honek harrigarria ez, baina garrantzitsua den beste ondorio batera garamatza: primate bat naiz. Eta zuek guztiak primateak zarete.
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.
Darwin ere primatea zen. Darwinek hau benetan gustukoa izango zuen. Bere garuna, gurea bezala, beste primateen garunaren itxurara egina zegoen.
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.
Beraz, giza garuna apartekoa da baina ez da berezia neurona kopuruaren aldetik. Primate handi baten garuna baino ez da. Pentsamendu apal eta erakusgarria dela uste dut, naturan dugun lekua gogoratzen digun pentsamendua.
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.
Zergatik behar du hainbeste energia? Beste batzuk ikertu dute gizakiaren eta beste espezieren garunek behar duten energia. Badakigunez garunen neurona kopurua, kalkulua egin dezakegu. Aurkitu da giza garunak eta beste garunek kantitate bera erabiltzen dutela: batezbeste 6 kaloria eguneko mila milioi neuronako. Beraz, garun baten gastu energetikoa neurona kantitatearen funtzio lineal sinple bat da, eta giza garunak espero zitekeen beste energia erabiltzen du. Beraz, giza garunak hainbeste energia erabiltzearen arrazoia sinplea da, neurona kantitate ikaragarria dugula. Primateak garelako, neurona asko ditugu gure tamainarako beste animaliekin alderatuta. Gure garunaren gastu erlatiboa izugarria da baina hori primateak garelako da, ez bereziak garelako.
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.
Beraz, azken galdera: Nola heldu gara neurona kopuru izugarri horretara? Eta, are gehiago, primateak gu baino handiagoak izanik, zergatik ez dute gurea baino handiagoa eta neurona gehiago dituen garuna? Garunean hainbeste neurona izateak ze gastu zekarren ikusita, arrazoi sinple bat zegoela bururatu zitzaidan. Beraiek ez dute nahikoa energia gorputz handia eta neurona kopuru handia mantentzeko. Kalkulua egin genuen. Primate batek janari gordina janez eguneko zenbat energia lortzen duen kalkulatu genuen. Gero, zenbat energia beharko lukeen tamaina zehatz bateko gorputz batek eta neurona kopuru konkretu batek kalkulatu genuen. Konbinaketak behatu genituen, primate baten gorputz tamaina eta garun-neurona kopuruaren arteko konbinaketak egunean ordu zehatz batzuk jaten emanez gero.
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.
Zera aurkitu genuen: neuronak hain garestiak direnez, gorputzaren tamaina eta neurona kantitatea konpentsatzen dira. Egunean 8 ordu jaten pasatzen dituen primate batek gehienez ere 53 mila milioi neurona izan ditzake, baina bere gorputza ezin da 25 kg baino handiagoa izan. Hori baino astunagoa izateko, neurona gutxiago izan behar ditu. Beraz, edo gorputz handi bat daukagu, edo neurona kopuru handi bat. Primate batek bezala jaten baduzu, ezin dituzu bi gauzak izan.
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.
Muga metaboliko honetatik askatzeko modu bat eguneko ordu gehiago jaten pasatzea litzateke. Baina hori arriskutsua da, eta puntu batetik aurrera ezinezkoa. Gorilek eta orangutanek esaterako, 30 mila milioi neurona dituzte, eta egunean 8 ordu eta erdi jaten pasatzen dituzte. Hori da antza lor dezaketen gehiena. Badirudi egunean 9 ordu jaten pasatzea dela primate baten muga praktikoa.
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.
Eta gurea? Gure 86 mila milioi neuronekin eta gure 60-70 kg-ko gorputzekin, egunean 9 ordu baino gehiago pasa beharko genituzke egunero gure elikadura beharrak asetzen, eta hau ezinezkoa da. Primateek bezala jango bagenu, ez ginateke hemen egongo.
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.
Orduan, nola heldu ginen hona? Gure garunak hainbeste energia erabiltzen badu, eta ezin badugu jaten eman esna gauden ordu bakoitza, dugun aukera bakarra alimentu berdinetatik energia gehiago lortzea da. Harrigarria badirudi ere, hori bat dator duela milioi eta erdi urte gure arbasoek asmatu zutela uste denarekin, janaria prestatzearekin alegia. Janaria prestatzea sua erabiliz elikagaiak gorputzetik kanpo aurrez liseritzea da. Elikagai egosiak leunagoak dira, murtxikatzeko errazagoak eta errazago bihurtzen dira papila ahoan. Horrela estomagoan erabat liseritu eta xurgatzen dira. Honela, denbora gutxiagoan energia gehiago sor dezakegu. Beraz, janaria prestatzeak neuronekin gauza interesgarriagoak egiteko denbora ematen digu. Ez dugu etengabe janarian, janaria biltzean eta jatean pentsatu behar.
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.
Beraz, janaria prestatzen dugulako, behin eragozpen handi bat izan zen garun handi hau neuronez beteta eta energia gastu handia duen garun hau, oso baliagarri bihurtu da, neurona piloarentzako energia lortu dezakegulako eta eurekin gauza interesgarriak egiteko astia izan. Honek azaltzen du eboluzioan zehar zergatik hain azkar hazi zen giza garuna, handia egin arte, primate garuna izaten jarraitzen zuen bitartean. Janaria prestatzeak garun handi hau eskuragarri egin eta elikagai gordinetatik kulturara, nekazaritzara, zibilizaziora, dendetara, elektrizitatera, hozkailuetara, eta egun ditugun gauzetara heldu gara. Honela gauza gara egun guztia iraun ahal izateko energia janari azkarreko gustuko lekuetan kolpetik lortzeko. Behin konponbide bat izan zena orain arazo bihurtu da. Ironikoki, orain konponbidea elikagai gordinetan bilatzen dugu.
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.
Zein da gizakiaren abantaila? Zer daukagu beste animaliek ez daukatena? Nire erantzuna hau da: garun kortexean neurona kopuru handiena dugu eta uste dut hori dela gure gaitasun kognitibo ezohikoen azalpen sinpleena. Zer egiten dugu beste animaliek egiten ez dutena, nire ustez oinarrizkoa dena, garun kortexean ditugun neurona kantitate izugarri horrek posible egiten duena? Bi hitzetan: janaria prestatu. Beste animaliarik ez dago janaria prestatzen duenik. Gizakiek bakarrik egiten dute. Horrela heldu ginen gizaki izatera.
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)
Giza garuna ikertzeak elikagaiez nuen pentsaera aldatu dit. Orain sukaldea begiratu eta gurtu egiten dut. Arbasoei eskertzen diet ziurrenik gizaki egin gaituen asmakizuna sortzeagatik. Mila esker. (Txaloak)