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← What is so special about the human brain?

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Showing Revision 5 created 11/27/2013 by Morton Bast.

  1. What is so special about the human brain?
  2. Why is it that we study other animals
  3. instead of them studying us?
  4. What does a human brain have or do
  5. that no other brain does?
  6. When I became interested
    in these questions about 10 years ago,
  7. scientists thought they knew
    what different brains were made of.
  8. Though it was based on very little evidence,
  9. many scientists thought that all mammalian brains,
  10. including the human brain,
  11. were made in the same way,
  12. with a number of neurons that was always
  13. proportional to the size of the brain.
  14. This means that two brains of the same size,
  15. like these two, with a respectable 400 grams,
  16. should have similar numbers of neurons.
  17. Now, if neurons are the functional
  18. information processing units of the brain,
  19. then the owners of these two brains
  20. should have similar cognitive abilities.
  21. And yet, one is a chimp,
  22. and the other is a cow.
  23. Now maybe cows have a really rich
  24. internal mental life and are so smart
  25. that they choose not to let us realize it,
  26. but we eat them.
  27. I think most people will agree
  28. that chimps are capable of much more complex,
  29. elaborate and flexible behaviors than cows are.
  30. So this is a first indication that the
  31. "all brains are made the same way" scenario
  32. is not quite right.
  33. But let's play along.

  34. If all brains were made the same way
  35. and you were to compare animals
    with brains of different sizes,
  36. larger brains should always have more neurons
  37. than smaller brains,
    and the larger the brain,
  38. the more cognitively able its owner should be.
  39. So the largest brain around should also be
  40. the most cognitively able.
  41. And here comes the bad news:
  42. Our brain, not the largest one around.
  43. It seems quite vexing.
  44. Our brain weighs between 1.2 and 1.5 kilos,
  45. but elephant brains weigh between four and five kilos,
  46. and whale brains can weigh up to nine kilos,
  47. which is why scientists used to resort to saying
  48. that our brain must be special
  49. to explain our cognitive abilities.
  50. It must be really extraordinary,
  51. an exception to the rule.
  52. Theirs may be bigger, but ours is better,
  53. and it could be better, for example,
  54. in that it seems larger than it should be,
  55. with a much larger cerebral cortex
    than we should have
  56. for the size of our bodies.
  57. So that would give us extra cortex
  58. to do more interesting things
    than just operating the body.
  59. That's because the size of the brain
  60. usually follows the size of the body.
  61. So the main reason for saying that
  62. our brain is larger than it should be
  63. actually comes from comparing ourselves
  64. to great apes.
  65. Gorillas can be two to three times larger than we are,
  66. so their brains should also be larger than ours,
  67. but instead it's the other way around.
  68. Our brain is three times larger than a gorilla brain.
  69. The human brain also seems special

  70. in the amount of energy that it uses.
  71. Although it weighs only two percent of the body,
  72. it alone uses 25 percent of all the energy
  73. that your body requires to run per day.
  74. That's 500 calories out of a total of 2,000 calories,
  75. just to keep your brain working.
  76. So the human brain is larger than it should be,

  77. it uses much more energy than it should,
  78. so it's special.
  79. And this is where the story started to bother me.
  80. In biology, we look for rules
  81. that apply to all animals and to life in general,
  82. so why should the rules of evolution
  83. apply to everybody else but not to us?
  84. Maybe the problem was with the basic assumption
  85. that all brains are made in the same way.
  86. Maybe two brains of a similar size
  87. can actually be made of
    very different numbers of neurons.
  88. Maybe a very large brain
  89. does not necessarily have more neurons
  90. than a more modest-sized brain.
  91. Maybe the human brain
    actually has the most neurons
  92. of any brain, regardless of its size,
  93. especially in the cerebral cortex.
  94. So this to me became
  95. the important question to answer:
  96. how many neurons does the human brain have,
  97. and how does that compare to other animals?
  98. Now, you may have heard or read somewhere

  99. that we have 100 billion neurons,
  100. so 10 years ago, I asked my colleagues
  101. if they knew where this number came from.
  102. But nobody did.
  103. I've been digging through the literature
  104. for the original reference for that number,
  105. and I could never find it.
  106. It seems that nobody had actually ever counted
  107. the number of neurons in the human brain,
  108. or in any other brain for that matter.
  109. So I came up with my own way
    to count cells in the brain,

  110. and it essentially consists of
  111. dissolving that brain into soup.
  112. It works like this:
  113. You take a brain, or parts of that brain,
  114. and you dissolve it in detergent,
  115. which destroys the cell membranes
  116. but keeps the cell nuclei intact,
  117. so you end up with a suspension of free nuclei
  118. that looks like this,
  119. like a clear soup.
  120. This soup contains all the nuclei
  121. that once were a mouse brain.
  122. Now, the beauty of a soup is that because it is soup,
  123. you can agitate it and make those nuclei
  124. be distributed homogeneously in the liquid,
  125. so that now by looking under the microscope
  126. at just four or five samples
    of this homogeneous solution,
  127. you can count nuclei, and therefore tell
  128. how many cells that brain had.
  129. It's simple, it's straightforward,
  130. and it's really fast.
  131. So we've used that method to count neurons
  132. in dozens of different species so far,
  133. and it turns out that all brains
  134. are not made the same way.
  135. Take rodents and primates, for instance:
  136. In larger rodent brains, the average size
  137. of the neuron increases,
  138. so the brain inflates very rapidly
  139. and gains size much faster than it gains neurons.
  140. But primate brains gain neurons
  141. without the average neuron becoming any larger,
  142. which is a very economical way
  143. to add neurons to your brain.
  144. The result is that a primate brain
  145. will always have more neurons than
    a rodent brain of the same size,
  146. and the larger the brain,
  147. the larger this difference will be.
  148. Well, what about our brain then?
  149. We found that we have, on average,
  150. 86 billion neurons,
  151. 16 billion of which are in the cerebral cortex,
  152. and if you consider that the cerebral cortex
  153. is the seat of functions like
  154. awareness and logical and abstract reasoning,
  155. and that 16 billion is the most neurons
  156. that any cortex has,
  157. I think this is the simplest explanation
  158. for our remarkable cognitive abilities.
  159. But just as important is what
    the 86 billion neurons mean.
  160. Because we found that the relationship
  161. between the size of the brain
    and its number of neurons
  162. could be described mathematically,
  163. we could calculate what a human brain
  164. would look like if it was made like a rodent brain.
  165. So, a rodent brain with 86 billion neurons
  166. would weigh 36 kilos.
  167. That's not possible.
  168. A brain that huge would be crushed
  169. by its own weight,
  170. and this impossible brain would go
  171. in the body of 89 tons.
  172. I don't think it looks like us.
  173. So this brings us to a very important
    conclusion already,

  174. which is that we are not rodents.
  175. The human brain is not a large rat brain.
  176. Compared to a rat, we might seem special, yes,
  177. but that's not a fair comparison to make,
  178. given that we know that we are not rodents.
  179. We are primates,
  180. so the correct comparison is to other primates.
  181. And there, if you do the math,
  182. you find that a generic primate
  183. with 86 billion neurons
  184. would have a brain of about 1.2 kilos,
  185. which seems just right,
  186. in a body of some 66 kilos,
  187. which in my case is exactly right,
  188. which brings us to a very unsurprising
  189. but still incredibly important conclusion:
  190. I am a primate.
  191. And all of you are primates.
  192. And so was Darwin.

  193. I love to think that Darwin
    would have really appreciated this.
  194. His brain, like ours,
  195. was made in the image of other primate brains.
  196. So the human brain may be remarkable, yes,

  197. but it is not special in its number of neurons.
  198. It is just a large primate brain.
  199. I think that's a very humbling and sobering thought
  200. that should remind us of our place in nature.
  201. Why does it cost so much energy, then?

  202. Well, other people have figured out
  203. how much energy the human brain
  204. and that of other species costs,
  205. and now that we knew how many neurons
  206. each brain was made of, we could do the math.
  207. And it turns out that both human
  208. and other brains cost about the same,
  209. an average of six calories
    per billion neurons per day.
  210. So the total energetic cost of a brain
  211. is a simple, linear function
  212. of its number of neurons,
  213. and it turns out that the human brain
  214. costs just as much energy as you would expect.
  215. So the reason why the human brain
  216. costs so much energy is simply because
  217. it has a huge number of neurons,
  218. and because we are primates
  219. with many more neurons for a given body size
  220. than any other animal,
  221. the relative cost of our brain is large,
  222. but just because we're primates,
    not because we're special.
  223. Last question, then:

  224. how did we come by this
    remarkable number of neurons,
  225. and in particular, if great apes
  226. are larger than we are,
  227. why don't they have a larger brain
    than we do, with more neurons?
  228. When we realized how much expensive it is
  229. to have a lot of neurons in the brain, I figured,
  230. maybe there's a simple reason.
  231. They just can't afford the energy
  232. for both a large body
    and a large number of neurons.
  233. So we did the math.
  234. We calculated on the one hand
  235. how much energy a primate gets per day
  236. from eating raw foods,
  237. and on the other hand, how much energy
  238. a body of a certain size costs
  239. and how much energy a brain of a
    certain number of neurons costs,
  240. and we looked for the combinations
  241. of body size and number of brain neurons
  242. that a primate could afford
  243. if it ate a certain number of hours per day.
  244. And what we found is that

  245. because neurons are so expensive,
  246. there is a tradeoff between
    body size and number of neurons.
  247. So a primate that eats eight hours per day
  248. can afford at most 53 billion neurons,
  249. but then its body cannot be any bigger
  250. than 25 kilos.
  251. To weigh any more than that,
  252. it has to give up neurons.
  253. So it's either a large body
  254. or a large number of neurons.
  255. When you eat like a primate,
  256. you can't afford both.
  257. One way out of this metabolic limitation

  258. would be to spend even more hours per day eating,
  259. but that gets dangerous,
  260. and past a certain point, it's just not possible.
  261. Gorillas and orangutans, for instance,
  262. afford about 30 billion neurons
  263. by spending eight and a half hours per day eating,
  264. and that seems to be about as much as they can do.
  265. Nine hours of feeding per day
  266. seems to be the practical limit for a primate.
  267. What about us?

  268. With our 86 billion neurons
  269. and 60 to 70 kilos of body mass,
  270. we should have to spend over nine hours
  271. per day every single day feeding,
  272. which is just not feasible.
  273. If we ate like a primate,
  274. we should not be here.
  275. How did we get here, then?

  276. Well, if our brain costs just as much energy
  277. as it should, and if we can't spend
  278. every waking hour of the day feeding,
  279. then the only alternative, really,
  280. is to somehow get more energy
  281. out of the same foods.
  282. And remarkably, that matches exactly
  283. what our ancestors are believed to have invented
  284. one and a half million years ago,
  285. when they invented cooking.
  286. To cook is to use fire
  287. to pre-digest foods outside of your body.
  288. Cooked foods are softer, so they're easier to chew
  289. and to turn completely into mush in your mouth,
  290. so that allows them to be completely digested
  291. and absorbed in your gut,
  292. which makes them yield much more
    energy in much less time.
  293. So cooking frees time for us to do
  294. much more interesting things with our day
  295. and with our neurons
  296. than just thinking about food,
  297. looking for food, and gobbling down food
  298. all day long.
  299. So because of cooking, what once was

  300. a major liability, this large,
  301. dangerously expensive brain with a lot of neurons,
  302. could now become a major asset,
  303. now that we could both afford
    the energy for a lot of neurons
  304. and the time to do interesting things with them.
  305. So I think this explains why the human brain
  306. grew to become so large so fast in evolution,
  307. all of the while remaining just a primate brain.
  308. With this large brain now affordable by cooking,
  309. we went rapidly from raw foods to culture,
  310. agriculture, civilization, grocery stores,
  311. electricity, refrigerators,
  312. all of those things that nowadays
  313. allow us to get all the energy we need
  314. for the whole day in a single sitting
  315. at your favorite fast food joint.
  316. So what once was a solution
  317. now became the problem,
  318. and ironically, we look for the solution in raw food.
  319. So what is the human advantage?

  320. What is it that we have
  321. that no other animal has?
  322. My answer is that we have the largest number
  323. of neurons in the cerebral cortex,
  324. and I think that's the simplest explanation
  325. for our remarkable cognitive abilities.
  326. And what is it that we do that no other animal does,
  327. and which I believe was fundamental
  328. to allow us to reach that large,
  329. largest number of neurons in the cortex?
  330. In two words, we cook.
  331. No other animal cooks its food. Only humans do.
  332. And I think that's how we got to become human.
  333. Studying the human brain changed
    the way I think about food.

  334. I now look at my kitchen,
  335. and I bow to it,
  336. and I thank my ancestors for coming up
  337. with the invention that probably made us humans.
  338. Thank you very much.
  339. (Applause)