[Script Info]
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[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.95,0:00:03.34,Default,,0000,0000,0000,,What is so special about the human brain?
Dialogue: 0,0:00:03.34,0:00:05.55,Default,,0000,0000,0000,,Why is it that we study other animals
Dialogue: 0,0:00:05.55,0:00:07.67,Default,,0000,0000,0000,,instead of them studying us?
Dialogue: 0,0:00:07.67,0:00:09.47,Default,,0000,0000,0000,,What does a human brain have or do
Dialogue: 0,0:00:09.47,0:00:11.20,Default,,0000,0000,0000,,that no other brain does?
Dialogue: 0,0:00:11.20,0:00:14.05,Default,,0000,0000,0000,,When I became interested\Nin these questions about 10 years ago,
Dialogue: 0,0:00:14.05,0:00:16.99,Default,,0000,0000,0000,,scientists thought they knew\Nwhat different brains were made of.
Dialogue: 0,0:00:16.99,0:00:18.72,Default,,0000,0000,0000,,Though it was based on very little evidence,
Dialogue: 0,0:00:18.72,0:00:20.91,Default,,0000,0000,0000,,many scientists thought that all mammalian brains,
Dialogue: 0,0:00:20.91,0:00:22.64,Default,,0000,0000,0000,,including the human brain,
Dialogue: 0,0:00:22.64,0:00:23.94,Default,,0000,0000,0000,,were made in the same way,
Dialogue: 0,0:00:23.94,0:00:25.44,Default,,0000,0000,0000,,with a number of neurons that was always
Dialogue: 0,0:00:25.44,0:00:27.67,Default,,0000,0000,0000,,proportional to the size of the brain.
Dialogue: 0,0:00:27.67,0:00:29.74,Default,,0000,0000,0000,,This means that two brains of the same size,
Dialogue: 0,0:00:29.74,0:00:33.29,Default,,0000,0000,0000,,like these two, with a respectable 400 grams,
Dialogue: 0,0:00:33.29,0:00:35.84,Default,,0000,0000,0000,,should have similar numbers of neurons.
Dialogue: 0,0:00:35.84,0:00:37.64,Default,,0000,0000,0000,,Now, if neurons are the functional
Dialogue: 0,0:00:37.64,0:00:40.54,Default,,0000,0000,0000,,information processing units of the brain,
Dialogue: 0,0:00:40.54,0:00:42.14,Default,,0000,0000,0000,,then the owners of these two brains
Dialogue: 0,0:00:42.14,0:00:44.86,Default,,0000,0000,0000,,should have similar cognitive abilities.
Dialogue: 0,0:00:44.86,0:00:47.21,Default,,0000,0000,0000,,And yet, one is a chimp,
Dialogue: 0,0:00:47.21,0:00:49.85,Default,,0000,0000,0000,,and the other is a cow.
Dialogue: 0,0:00:49.85,0:00:52.21,Default,,0000,0000,0000,,Now maybe cows have a really rich
Dialogue: 0,0:00:52.21,0:00:54.46,Default,,0000,0000,0000,,internal mental life and are so smart
Dialogue: 0,0:00:54.46,0:00:58.27,Default,,0000,0000,0000,,that they choose not to let us realize it,
Dialogue: 0,0:00:58.27,0:00:59.71,Default,,0000,0000,0000,,but we eat them.
Dialogue: 0,0:00:59.71,0:01:01.41,Default,,0000,0000,0000,,I think most people will agree
Dialogue: 0,0:01:01.41,0:01:03.41,Default,,0000,0000,0000,,that chimps are capable of much more complex,
Dialogue: 0,0:01:03.41,0:01:06.47,Default,,0000,0000,0000,,elaborate and flexible behaviors than cows are.
Dialogue: 0,0:01:06.47,0:01:08.40,Default,,0000,0000,0000,,So this is a first indication that the
Dialogue: 0,0:01:08.40,0:01:10.43,Default,,0000,0000,0000,,"all brains are made the same way" scenario
Dialogue: 0,0:01:10.43,0:01:12.10,Default,,0000,0000,0000,,is not quite right.
Dialogue: 0,0:01:12.10,0:01:13.45,Default,,0000,0000,0000,,But let's play along.
Dialogue: 0,0:01:13.45,0:01:15.23,Default,,0000,0000,0000,,If all brains were made the same way
Dialogue: 0,0:01:15.23,0:01:18.14,Default,,0000,0000,0000,,and you were to compare animals\Nwith brains of different sizes,
Dialogue: 0,0:01:18.14,0:01:20.32,Default,,0000,0000,0000,,larger brains should always have more neurons
Dialogue: 0,0:01:20.32,0:01:22.65,Default,,0000,0000,0000,,than smaller brains,\Nand the larger the brain,
Dialogue: 0,0:01:22.65,0:01:25.67,Default,,0000,0000,0000,,the more cognitively able its owner should be.
Dialogue: 0,0:01:25.67,0:01:27.80,Default,,0000,0000,0000,,So the largest brain around should also be
Dialogue: 0,0:01:27.80,0:01:30.09,Default,,0000,0000,0000,,the most cognitively able.
Dialogue: 0,0:01:30.09,0:01:31.67,Default,,0000,0000,0000,,And here comes the bad news:
Dialogue: 0,0:01:31.67,0:01:34.43,Default,,0000,0000,0000,,Our brain, not the largest one around.
Dialogue: 0,0:01:34.43,0:01:35.99,Default,,0000,0000,0000,,It seems quite vexing.
Dialogue: 0,0:01:35.99,0:01:38.72,Default,,0000,0000,0000,,Our brain weighs between 1.2 and 1.5 kilos,
Dialogue: 0,0:01:38.72,0:01:42.04,Default,,0000,0000,0000,,but elephant brains weigh between four and five kilos,
Dialogue: 0,0:01:42.04,0:01:44.62,Default,,0000,0000,0000,,and whale brains can weigh up to nine kilos,
Dialogue: 0,0:01:44.62,0:01:49.36,Default,,0000,0000,0000,,which is why scientists used to resort to saying
Dialogue: 0,0:01:49.36,0:01:51.52,Default,,0000,0000,0000,,that our brain must be special
Dialogue: 0,0:01:51.52,0:01:54.38,Default,,0000,0000,0000,,to explain our cognitive abilities.
Dialogue: 0,0:01:54.38,0:01:57.33,Default,,0000,0000,0000,,It must be really extraordinary,
Dialogue: 0,0:01:57.33,0:01:59.30,Default,,0000,0000,0000,,an exception to the rule.
Dialogue: 0,0:01:59.30,0:02:02.53,Default,,0000,0000,0000,,Theirs may be bigger, but ours is better,
Dialogue: 0,0:02:02.53,0:02:04.40,Default,,0000,0000,0000,,and it could be better, for example,
Dialogue: 0,0:02:04.40,0:02:06.52,Default,,0000,0000,0000,,in that it seems larger than it should be,
Dialogue: 0,0:02:06.52,0:02:09.21,Default,,0000,0000,0000,,with a much larger cerebral cortex\Nthan we should have
Dialogue: 0,0:02:09.21,0:02:10.75,Default,,0000,0000,0000,,for the size of our bodies.
Dialogue: 0,0:02:10.75,0:02:12.40,Default,,0000,0000,0000,,So that would give us extra cortex
Dialogue: 0,0:02:12.40,0:02:15.36,Default,,0000,0000,0000,,to do more interesting things\Nthan just operating the body.
Dialogue: 0,0:02:15.36,0:02:17.04,Default,,0000,0000,0000,,That's because the size of the brain
Dialogue: 0,0:02:17.04,0:02:19.38,Default,,0000,0000,0000,,usually follows the size of the body.
Dialogue: 0,0:02:19.38,0:02:21.69,Default,,0000,0000,0000,,So the main reason for saying that
Dialogue: 0,0:02:21.69,0:02:23.78,Default,,0000,0000,0000,,our brain is larger than it should be
Dialogue: 0,0:02:23.78,0:02:25.55,Default,,0000,0000,0000,,actually comes from comparing ourselves
Dialogue: 0,0:02:25.55,0:02:27.24,Default,,0000,0000,0000,,to great apes.
Dialogue: 0,0:02:27.24,0:02:29.83,Default,,0000,0000,0000,,Gorillas can be two to three times larger than we are,
Dialogue: 0,0:02:29.83,0:02:32.25,Default,,0000,0000,0000,,so their brains should also be larger than ours,
Dialogue: 0,0:02:32.25,0:02:34.20,Default,,0000,0000,0000,,but instead it's the other way around.
Dialogue: 0,0:02:34.20,0:02:37.13,Default,,0000,0000,0000,,Our brain is three times larger than a gorilla brain.
Dialogue: 0,0:02:37.13,0:02:39.25,Default,,0000,0000,0000,,The human brain also seems special
Dialogue: 0,0:02:39.25,0:02:41.50,Default,,0000,0000,0000,,in the amount of energy that it uses.
Dialogue: 0,0:02:41.50,0:02:44.25,Default,,0000,0000,0000,,Although it weighs only two percent of the body,
Dialogue: 0,0:02:44.25,0:02:47.69,Default,,0000,0000,0000,,it alone uses 25 percent of all the energy
Dialogue: 0,0:02:47.69,0:02:50.16,Default,,0000,0000,0000,,that your body requires to run per day.
Dialogue: 0,0:02:50.16,0:02:53.51,Default,,0000,0000,0000,,That's 500 calories out of a total of 2,000 calories,
Dialogue: 0,0:02:53.51,0:02:55.91,Default,,0000,0000,0000,,just to keep your brain working.
Dialogue: 0,0:02:55.91,0:02:58.84,Default,,0000,0000,0000,,So the human brain is larger than it should be,
Dialogue: 0,0:02:58.84,0:03:00.87,Default,,0000,0000,0000,,it uses much more energy than it should,
Dialogue: 0,0:03:00.87,0:03:02.32,Default,,0000,0000,0000,,so it's special.
Dialogue: 0,0:03:02.32,0:03:04.90,Default,,0000,0000,0000,,And this is where the story started to bother me.
Dialogue: 0,0:03:04.90,0:03:06.57,Default,,0000,0000,0000,,In biology, we look for rules
Dialogue: 0,0:03:06.57,0:03:09.41,Default,,0000,0000,0000,,that apply to all animals and to life in general,
Dialogue: 0,0:03:09.41,0:03:11.26,Default,,0000,0000,0000,,so why should the rules of evolution
Dialogue: 0,0:03:11.26,0:03:14.90,Default,,0000,0000,0000,,apply to everybody else but not to us?
Dialogue: 0,0:03:14.90,0:03:17.10,Default,,0000,0000,0000,,Maybe the problem was with the basic assumption
Dialogue: 0,0:03:17.10,0:03:18.97,Default,,0000,0000,0000,,that all brains are made in the same way.
Dialogue: 0,0:03:18.97,0:03:20.57,Default,,0000,0000,0000,,Maybe two brains of a similar size
Dialogue: 0,0:03:20.57,0:03:23.16,Default,,0000,0000,0000,,can actually be made of\Nvery different numbers of neurons.
Dialogue: 0,0:03:23.16,0:03:24.77,Default,,0000,0000,0000,,Maybe a very large brain
Dialogue: 0,0:03:24.77,0:03:26.61,Default,,0000,0000,0000,,does not necessarily have more neurons
Dialogue: 0,0:03:26.61,0:03:28.83,Default,,0000,0000,0000,,than a more modest-sized brain.
Dialogue: 0,0:03:28.83,0:03:31.84,Default,,0000,0000,0000,,Maybe the human brain \Nactually has the most neurons
Dialogue: 0,0:03:31.84,0:03:34.41,Default,,0000,0000,0000,,of any brain, regardless of its size,
Dialogue: 0,0:03:34.41,0:03:36.51,Default,,0000,0000,0000,,especially in the cerebral cortex.
Dialogue: 0,0:03:36.51,0:03:38.06,Default,,0000,0000,0000,,So this to me became
Dialogue: 0,0:03:38.06,0:03:39.79,Default,,0000,0000,0000,,the important question to answer:
Dialogue: 0,0:03:39.79,0:03:42.19,Default,,0000,0000,0000,,how many neurons does the human brain have,
Dialogue: 0,0:03:42.19,0:03:44.71,Default,,0000,0000,0000,,and how does that compare to other animals?
Dialogue: 0,0:03:44.71,0:03:47.13,Default,,0000,0000,0000,,Now, you may have heard or read somewhere
Dialogue: 0,0:03:47.13,0:03:49.24,Default,,0000,0000,0000,,that we have 100 billion neurons,
Dialogue: 0,0:03:49.24,0:03:51.33,Default,,0000,0000,0000,,so 10 years ago, I asked my colleagues
Dialogue: 0,0:03:51.33,0:03:53.17,Default,,0000,0000,0000,,if they knew where this number came from.
Dialogue: 0,0:03:53.17,0:03:54.63,Default,,0000,0000,0000,,But nobody did.
Dialogue: 0,0:03:54.63,0:03:56.08,Default,,0000,0000,0000,,I've been digging through the literature
Dialogue: 0,0:03:56.08,0:03:58.14,Default,,0000,0000,0000,,for the original reference for that number,
Dialogue: 0,0:03:58.14,0:03:59.92,Default,,0000,0000,0000,,and I could never find it.
Dialogue: 0,0:03:59.92,0:04:02.66,Default,,0000,0000,0000,,It seems that nobody had actually ever counted
Dialogue: 0,0:04:02.66,0:04:04.19,Default,,0000,0000,0000,,the number of neurons in the human brain,
Dialogue: 0,0:04:04.19,0:04:06.69,Default,,0000,0000,0000,,or in any other brain for that matter.
Dialogue: 0,0:04:06.69,0:04:10.23,Default,,0000,0000,0000,,So I came up with my own way\Nto count cells in the brain,
Dialogue: 0,0:04:10.23,0:04:12.34,Default,,0000,0000,0000,,and it essentially consists of
Dialogue: 0,0:04:12.34,0:04:15.58,Default,,0000,0000,0000,,dissolving that brain into soup.
Dialogue: 0,0:04:15.58,0:04:17.62,Default,,0000,0000,0000,,It works like this:
Dialogue: 0,0:04:17.62,0:04:20.74,Default,,0000,0000,0000,,You take a brain, or parts of that brain,
Dialogue: 0,0:04:20.74,0:04:22.49,Default,,0000,0000,0000,,and you dissolve it in detergent,
Dialogue: 0,0:04:22.49,0:04:23.100,Default,,0000,0000,0000,,which destroys the cell membranes
Dialogue: 0,0:04:23.100,0:04:26.43,Default,,0000,0000,0000,,but keeps the cell nuclei intact,
Dialogue: 0,0:04:26.43,0:04:29.57,Default,,0000,0000,0000,,so you end up with a suspension of free nuclei
Dialogue: 0,0:04:29.57,0:04:31.36,Default,,0000,0000,0000,,that looks like this,
Dialogue: 0,0:04:31.36,0:04:32.63,Default,,0000,0000,0000,,like a clear soup.
Dialogue: 0,0:04:32.63,0:04:34.52,Default,,0000,0000,0000,,This soup contains all the nuclei
Dialogue: 0,0:04:34.52,0:04:36.57,Default,,0000,0000,0000,,that once were a mouse brain.
Dialogue: 0,0:04:36.57,0:04:39.60,Default,,0000,0000,0000,,Now, the beauty of a soup is that because it is soup,
Dialogue: 0,0:04:39.60,0:04:42.50,Default,,0000,0000,0000,,you can agitate it and make those nuclei
Dialogue: 0,0:04:42.50,0:04:44.47,Default,,0000,0000,0000,,be distributed homogeneously in the liquid,
Dialogue: 0,0:04:44.47,0:04:46.45,Default,,0000,0000,0000,,so that now by looking under the microscope
Dialogue: 0,0:04:46.45,0:04:50.54,Default,,0000,0000,0000,,at just four or five samples\Nof this homogeneous solution,
Dialogue: 0,0:04:50.54,0:04:53.07,Default,,0000,0000,0000,,you can count nuclei, and therefore tell
Dialogue: 0,0:04:53.07,0:04:54.75,Default,,0000,0000,0000,,how many cells that brain had.
Dialogue: 0,0:04:54.75,0:04:56.38,Default,,0000,0000,0000,,It's simple, it's straightforward,
Dialogue: 0,0:04:56.38,0:04:57.81,Default,,0000,0000,0000,,and it's really fast.
Dialogue: 0,0:04:57.81,0:04:59.81,Default,,0000,0000,0000,,So we've used that method to count neurons
Dialogue: 0,0:04:59.81,0:05:02.12,Default,,0000,0000,0000,,in dozens of different species so far,
Dialogue: 0,0:05:02.12,0:05:03.81,Default,,0000,0000,0000,,and it turns out that all brains
Dialogue: 0,0:05:03.81,0:05:06.39,Default,,0000,0000,0000,,are not made the same way.
Dialogue: 0,0:05:06.39,0:05:08.58,Default,,0000,0000,0000,,Take rodents and primates, for instance:
Dialogue: 0,0:05:08.58,0:05:10.86,Default,,0000,0000,0000,,In larger rodent brains, the average size
Dialogue: 0,0:05:10.86,0:05:12.55,Default,,0000,0000,0000,,of the neuron increases,
Dialogue: 0,0:05:12.55,0:05:15.14,Default,,0000,0000,0000,,so the brain inflates very rapidly
Dialogue: 0,0:05:15.14,0:05:18.31,Default,,0000,0000,0000,,and gains size much faster than it gains neurons.
Dialogue: 0,0:05:18.31,0:05:20.01,Default,,0000,0000,0000,,But primate brains gain neurons
Dialogue: 0,0:05:20.01,0:05:22.47,Default,,0000,0000,0000,,without the average neuron becoming any larger,
Dialogue: 0,0:05:22.47,0:05:23.100,Default,,0000,0000,0000,,which is a very economical way
Dialogue: 0,0:05:23.100,0:05:25.65,Default,,0000,0000,0000,,to add neurons to your brain.
Dialogue: 0,0:05:25.65,0:05:27.41,Default,,0000,0000,0000,,The result is that a primate brain
Dialogue: 0,0:05:27.41,0:05:30.58,Default,,0000,0000,0000,,will always have more neurons than\Na rodent brain of the same size,
Dialogue: 0,0:05:30.58,0:05:32.06,Default,,0000,0000,0000,,and the larger the brain,
Dialogue: 0,0:05:32.06,0:05:34.28,Default,,0000,0000,0000,,the larger this difference will be.
Dialogue: 0,0:05:34.28,0:05:36.38,Default,,0000,0000,0000,,Well, what about our brain then?
Dialogue: 0,0:05:36.38,0:05:38.06,Default,,0000,0000,0000,,We found that we have, on average,
Dialogue: 0,0:05:38.06,0:05:39.85,Default,,0000,0000,0000,,86 billion neurons,
Dialogue: 0,0:05:39.85,0:05:42.85,Default,,0000,0000,0000,,16 billion of which are in the cerebral cortex,
Dialogue: 0,0:05:42.85,0:05:44.93,Default,,0000,0000,0000,,and if you consider that the cerebral cortex
Dialogue: 0,0:05:44.93,0:05:48.02,Default,,0000,0000,0000,,is the seat of functions like
Dialogue: 0,0:05:48.02,0:05:51.16,Default,,0000,0000,0000,,awareness and logical and abstract reasoning,
Dialogue: 0,0:05:51.16,0:05:54.14,Default,,0000,0000,0000,,and that 16 billion is the most neurons
Dialogue: 0,0:05:54.14,0:05:56.77,Default,,0000,0000,0000,,that any cortex has,
Dialogue: 0,0:05:56.77,0:05:58.48,Default,,0000,0000,0000,,I think this is the simplest explanation
Dialogue: 0,0:05:58.48,0:06:01.63,Default,,0000,0000,0000,,for our remarkable cognitive abilities.
Dialogue: 0,0:06:01.63,0:06:04.85,Default,,0000,0000,0000,,But just as important is what\Nthe 86 billion neurons mean.
Dialogue: 0,0:06:04.85,0:06:06.38,Default,,0000,0000,0000,,Because we found that the relationship
Dialogue: 0,0:06:06.38,0:06:08.73,Default,,0000,0000,0000,,between the size of the brain\Nand its number of neurons
Dialogue: 0,0:06:08.73,0:06:10.36,Default,,0000,0000,0000,,could be described mathematically,
Dialogue: 0,0:06:10.36,0:06:12.57,Default,,0000,0000,0000,,we could calculate what a human brain
Dialogue: 0,0:06:12.57,0:06:15.25,Default,,0000,0000,0000,,would look like if it was made like a rodent brain.
Dialogue: 0,0:06:15.25,0:06:18.82,Default,,0000,0000,0000,,So, a rodent brain with 86 billion neurons
Dialogue: 0,0:06:18.82,0:06:21.94,Default,,0000,0000,0000,,would weigh 36 kilos.
Dialogue: 0,0:06:21.94,0:06:23.58,Default,,0000,0000,0000,,That's not possible.
Dialogue: 0,0:06:23.58,0:06:25.48,Default,,0000,0000,0000,,A brain that huge would be crushed
Dialogue: 0,0:06:25.48,0:06:26.66,Default,,0000,0000,0000,,by its own weight,
Dialogue: 0,0:06:26.66,0:06:28.25,Default,,0000,0000,0000,,and this impossible brain would go
Dialogue: 0,0:06:28.25,0:06:32.02,Default,,0000,0000,0000,,in the body of 89 tons.
Dialogue: 0,0:06:32.02,0:06:34.16,Default,,0000,0000,0000,,I don't think it looks like us.
Dialogue: 0,0:06:34.16,0:06:36.71,Default,,0000,0000,0000,,So this brings us to a very important\Nconclusion already,
Dialogue: 0,0:06:36.71,0:06:39.36,Default,,0000,0000,0000,,which is that we are not rodents.
Dialogue: 0,0:06:39.36,0:06:42.62,Default,,0000,0000,0000,,The human brain is not a large rat brain.
Dialogue: 0,0:06:42.62,0:06:45.25,Default,,0000,0000,0000,,Compared to a rat, we might seem special, yes,
Dialogue: 0,0:06:45.25,0:06:47.47,Default,,0000,0000,0000,,but that's not a fair comparison to make,
Dialogue: 0,0:06:47.47,0:06:49.56,Default,,0000,0000,0000,,given that we know that we are not rodents.
Dialogue: 0,0:06:49.56,0:06:50.95,Default,,0000,0000,0000,,We are primates,
Dialogue: 0,0:06:50.95,0:06:53.73,Default,,0000,0000,0000,,so the correct comparison is to other primates.
Dialogue: 0,0:06:53.73,0:06:54.95,Default,,0000,0000,0000,,And there, if you do the math,
Dialogue: 0,0:06:54.95,0:06:57.74,Default,,0000,0000,0000,,you find that a generic primate
Dialogue: 0,0:06:57.74,0:06:59.67,Default,,0000,0000,0000,,with 86 billion neurons
Dialogue: 0,0:06:59.67,0:07:02.67,Default,,0000,0000,0000,,would have a brain of about 1.2 kilos,
Dialogue: 0,0:07:02.67,0:07:04.56,Default,,0000,0000,0000,,which seems just right,
Dialogue: 0,0:07:04.56,0:07:06.53,Default,,0000,0000,0000,,in a body of some 66 kilos,
Dialogue: 0,0:07:06.53,0:07:09.18,Default,,0000,0000,0000,,which in my case is exactly right,
Dialogue: 0,0:07:09.18,0:07:11.80,Default,,0000,0000,0000,,which brings us to a very unsurprising
Dialogue: 0,0:07:11.80,0:07:14.77,Default,,0000,0000,0000,,but still incredibly important conclusion:
Dialogue: 0,0:07:14.77,0:07:16.11,Default,,0000,0000,0000,,I am a primate.
Dialogue: 0,0:07:16.11,0:07:18.81,Default,,0000,0000,0000,,And all of you are primates.
Dialogue: 0,0:07:18.81,0:07:20.71,Default,,0000,0000,0000,,And so was Darwin.
Dialogue: 0,0:07:20.71,0:07:23.65,Default,,0000,0000,0000,,I love to think that Darwin\Nwould have really appreciated this.
Dialogue: 0,0:07:23.65,0:07:25.61,Default,,0000,0000,0000,,His brain, like ours,
Dialogue: 0,0:07:25.61,0:07:29.12,Default,,0000,0000,0000,,was made in the image of other primate brains.
Dialogue: 0,0:07:29.12,0:07:31.43,Default,,0000,0000,0000,,So the human brain may be remarkable, yes,
Dialogue: 0,0:07:31.43,0:07:34.21,Default,,0000,0000,0000,,but it is not special in its number of neurons.
Dialogue: 0,0:07:34.21,0:07:36.06,Default,,0000,0000,0000,,It is just a large primate brain.
Dialogue: 0,0:07:36.06,0:07:39.11,Default,,0000,0000,0000,,I think that's a very humbling and sobering thought
Dialogue: 0,0:07:39.11,0:07:42.17,Default,,0000,0000,0000,,that should remind us of our place in nature.
Dialogue: 0,0:07:42.17,0:07:44.81,Default,,0000,0000,0000,,Why does it cost so much energy, then?
Dialogue: 0,0:07:44.81,0:07:46.26,Default,,0000,0000,0000,,Well, other people have figured out
Dialogue: 0,0:07:46.26,0:07:47.76,Default,,0000,0000,0000,,how much energy the human brain
Dialogue: 0,0:07:47.76,0:07:49.17,Default,,0000,0000,0000,,and that of other species costs,
Dialogue: 0,0:07:49.17,0:07:50.82,Default,,0000,0000,0000,,and now that we knew how many neurons
Dialogue: 0,0:07:50.82,0:07:53.16,Default,,0000,0000,0000,,each brain was made of, we could do the math.
Dialogue: 0,0:07:53.16,0:07:55.03,Default,,0000,0000,0000,,And it turns out that both human
Dialogue: 0,0:07:55.03,0:07:57.85,Default,,0000,0000,0000,,and other brains cost about the same,
Dialogue: 0,0:07:57.85,0:08:01.27,Default,,0000,0000,0000,,an average of six calories \Nper billion neurons per day.
Dialogue: 0,0:08:01.27,0:08:03.41,Default,,0000,0000,0000,,So the total energetic cost of a brain
Dialogue: 0,0:08:03.41,0:08:05.45,Default,,0000,0000,0000,,is a simple, linear function
Dialogue: 0,0:08:05.45,0:08:07.16,Default,,0000,0000,0000,,of its number of neurons,
Dialogue: 0,0:08:07.16,0:08:09.34,Default,,0000,0000,0000,,and it turns out that the human brain
Dialogue: 0,0:08:09.34,0:08:13.18,Default,,0000,0000,0000,,costs just as much energy as you would expect.
Dialogue: 0,0:08:13.18,0:08:15.27,Default,,0000,0000,0000,,So the reason why the human brain
Dialogue: 0,0:08:15.27,0:08:16.94,Default,,0000,0000,0000,,costs so much energy is simply because
Dialogue: 0,0:08:16.94,0:08:18.93,Default,,0000,0000,0000,,it has a huge number of neurons,
Dialogue: 0,0:08:18.93,0:08:20.41,Default,,0000,0000,0000,,and because we are primates
Dialogue: 0,0:08:20.41,0:08:22.91,Default,,0000,0000,0000,,with many more neurons for a given body size
Dialogue: 0,0:08:22.91,0:08:24.43,Default,,0000,0000,0000,,than any other animal,
Dialogue: 0,0:08:24.43,0:08:27.97,Default,,0000,0000,0000,,the relative cost of our brain is large,
Dialogue: 0,0:08:27.97,0:08:30.97,Default,,0000,0000,0000,,but just because we're primates,\Nnot because we're special.
Dialogue: 0,0:08:30.97,0:08:32.16,Default,,0000,0000,0000,,Last question, then:
Dialogue: 0,0:08:32.16,0:08:35.28,Default,,0000,0000,0000,,how did we come by this\Nremarkable number of neurons,
Dialogue: 0,0:08:35.28,0:08:37.18,Default,,0000,0000,0000,,and in particular, if great apes
Dialogue: 0,0:08:37.18,0:08:38.73,Default,,0000,0000,0000,,are larger than we are,
Dialogue: 0,0:08:38.73,0:08:42.50,Default,,0000,0000,0000,,why don't they have a larger brain \Nthan we do, with more neurons?
Dialogue: 0,0:08:42.50,0:08:44.59,Default,,0000,0000,0000,,When we realized how much expensive it is
Dialogue: 0,0:08:44.59,0:08:46.92,Default,,0000,0000,0000,,to have a lot of neurons in the brain, I figured,
Dialogue: 0,0:08:46.92,0:08:48.92,Default,,0000,0000,0000,,maybe there's a simple reason.
Dialogue: 0,0:08:48.92,0:08:50.61,Default,,0000,0000,0000,,They just can't afford the energy
Dialogue: 0,0:08:50.61,0:08:53.56,Default,,0000,0000,0000,,for both a large body \Nand a large number of neurons.
Dialogue: 0,0:08:53.56,0:08:54.99,Default,,0000,0000,0000,,So we did the math.
Dialogue: 0,0:08:54.99,0:08:56.59,Default,,0000,0000,0000,,We calculated on the one hand
Dialogue: 0,0:08:56.59,0:08:58.53,Default,,0000,0000,0000,,how much energy a primate gets per day
Dialogue: 0,0:08:58.53,0:08:59.88,Default,,0000,0000,0000,,from eating raw foods,
Dialogue: 0,0:08:59.88,0:09:01.91,Default,,0000,0000,0000,,and on the other hand, how much energy
Dialogue: 0,0:09:01.91,0:09:03.68,Default,,0000,0000,0000,,a body of a certain size costs
Dialogue: 0,0:09:03.68,0:09:07.06,Default,,0000,0000,0000,,and how much energy a brain of a\Ncertain number of neurons costs,
Dialogue: 0,0:09:07.06,0:09:08.55,Default,,0000,0000,0000,,and we looked for the combinations
Dialogue: 0,0:09:08.55,0:09:10.96,Default,,0000,0000,0000,,of body size and number of brain neurons
Dialogue: 0,0:09:10.96,0:09:12.24,Default,,0000,0000,0000,,that a primate could afford
Dialogue: 0,0:09:12.24,0:09:14.78,Default,,0000,0000,0000,,if it ate a certain number of hours per day.
Dialogue: 0,0:09:14.78,0:09:16.60,Default,,0000,0000,0000,,And what we found is that
Dialogue: 0,0:09:16.60,0:09:18.31,Default,,0000,0000,0000,,because neurons are so expensive,
Dialogue: 0,0:09:18.31,0:09:21.68,Default,,0000,0000,0000,,there is a tradeoff between\Nbody size and number of neurons.
Dialogue: 0,0:09:21.68,0:09:24.63,Default,,0000,0000,0000,,So a primate that eats eight hours per day
Dialogue: 0,0:09:24.63,0:09:27.66,Default,,0000,0000,0000,,can afford at most 53 billion neurons,
Dialogue: 0,0:09:27.66,0:09:29.38,Default,,0000,0000,0000,,but then its body cannot be any bigger
Dialogue: 0,0:09:29.38,0:09:31.34,Default,,0000,0000,0000,,than 25 kilos.
Dialogue: 0,0:09:31.34,0:09:33.04,Default,,0000,0000,0000,,To weigh any more than that,
Dialogue: 0,0:09:33.04,0:09:34.81,Default,,0000,0000,0000,,it has to give up neurons.
Dialogue: 0,0:09:34.81,0:09:37.46,Default,,0000,0000,0000,,So it's either a large body
Dialogue: 0,0:09:37.46,0:09:38.96,Default,,0000,0000,0000,,or a large number of neurons.
Dialogue: 0,0:09:38.96,0:09:40.32,Default,,0000,0000,0000,,When you eat like a primate,
Dialogue: 0,0:09:40.32,0:09:42.56,Default,,0000,0000,0000,,you can't afford both.
Dialogue: 0,0:09:42.56,0:09:44.52,Default,,0000,0000,0000,,One way out of this metabolic limitation
Dialogue: 0,0:09:44.52,0:09:47.92,Default,,0000,0000,0000,,would be to spend even more hours per day eating,
Dialogue: 0,0:09:47.92,0:09:49.28,Default,,0000,0000,0000,,but that gets dangerous,
Dialogue: 0,0:09:49.28,0:09:52.00,Default,,0000,0000,0000,,and past a certain point, it's just not possible.
Dialogue: 0,0:09:52.00,0:09:53.54,Default,,0000,0000,0000,,Gorillas and orangutans, for instance,
Dialogue: 0,0:09:53.54,0:09:55.46,Default,,0000,0000,0000,,afford about 30 billion neurons
Dialogue: 0,0:09:55.46,0:09:58.43,Default,,0000,0000,0000,,by spending eight and a half hours per day eating,
Dialogue: 0,0:09:58.43,0:10:01.54,Default,,0000,0000,0000,,and that seems to be about as much as they can do.
Dialogue: 0,0:10:01.54,0:10:03.34,Default,,0000,0000,0000,,Nine hours of feeding per day
Dialogue: 0,0:10:03.34,0:10:06.61,Default,,0000,0000,0000,,seems to be the practical limit for a primate.
Dialogue: 0,0:10:06.61,0:10:08.40,Default,,0000,0000,0000,,What about us?
Dialogue: 0,0:10:08.40,0:10:09.100,Default,,0000,0000,0000,,With our 86 billion neurons
Dialogue: 0,0:10:09.100,0:10:13.03,Default,,0000,0000,0000,,and 60 to 70 kilos of body mass,
Dialogue: 0,0:10:13.03,0:10:16.59,Default,,0000,0000,0000,,we should have to spend over nine hours
Dialogue: 0,0:10:16.59,0:10:20.17,Default,,0000,0000,0000,,per day every single day feeding,
Dialogue: 0,0:10:20.17,0:10:22.21,Default,,0000,0000,0000,,which is just not feasible.
Dialogue: 0,0:10:22.21,0:10:24.04,Default,,0000,0000,0000,,If we ate like a primate,
Dialogue: 0,0:10:24.04,0:10:26.30,Default,,0000,0000,0000,,we should not be here.
Dialogue: 0,0:10:26.30,0:10:28.42,Default,,0000,0000,0000,,How did we get here, then?
Dialogue: 0,0:10:28.42,0:10:31.16,Default,,0000,0000,0000,,Well, if our brain costs just as much energy
Dialogue: 0,0:10:31.16,0:10:32.88,Default,,0000,0000,0000,,as it should, and if we can't spend
Dialogue: 0,0:10:32.88,0:10:36.55,Default,,0000,0000,0000,,every waking hour of the day feeding,
Dialogue: 0,0:10:36.55,0:10:38.46,Default,,0000,0000,0000,,then the only alternative, really,
Dialogue: 0,0:10:38.46,0:10:40.42,Default,,0000,0000,0000,,is to somehow get more energy
Dialogue: 0,0:10:40.42,0:10:42.38,Default,,0000,0000,0000,,out of the same foods.
Dialogue: 0,0:10:42.38,0:10:46.18,Default,,0000,0000,0000,,And remarkably, that matches exactly
Dialogue: 0,0:10:46.18,0:10:49.21,Default,,0000,0000,0000,,what our ancestors are believed to have invented
Dialogue: 0,0:10:49.21,0:10:51.05,Default,,0000,0000,0000,,one and a half million years ago,
Dialogue: 0,0:10:51.05,0:10:53.83,Default,,0000,0000,0000,,when they invented cooking.
Dialogue: 0,0:10:53.83,0:10:55.80,Default,,0000,0000,0000,,To cook is to use fire
Dialogue: 0,0:10:55.80,0:10:59.60,Default,,0000,0000,0000,,to pre-digest foods outside of your body.
Dialogue: 0,0:10:59.60,0:11:02.21,Default,,0000,0000,0000,,Cooked foods are softer, so they're easier to chew
Dialogue: 0,0:11:02.21,0:11:04.77,Default,,0000,0000,0000,,and to turn completely into mush in your mouth,
Dialogue: 0,0:11:04.77,0:11:06.92,Default,,0000,0000,0000,,so that allows them to be completely digested
Dialogue: 0,0:11:06.92,0:11:08.36,Default,,0000,0000,0000,,and absorbed in your gut,
Dialogue: 0,0:11:08.36,0:11:12.02,Default,,0000,0000,0000,,which makes them yield much more\Nenergy in much less time.
Dialogue: 0,0:11:12.02,0:11:14.50,Default,,0000,0000,0000,,So cooking frees time for us to do
Dialogue: 0,0:11:14.50,0:11:16.57,Default,,0000,0000,0000,,much more interesting things with our day
Dialogue: 0,0:11:16.57,0:11:18.05,Default,,0000,0000,0000,,and with our neurons
Dialogue: 0,0:11:18.05,0:11:19.95,Default,,0000,0000,0000,,than just thinking about food,
Dialogue: 0,0:11:19.95,0:11:21.66,Default,,0000,0000,0000,,looking for food, and gobbling down food
Dialogue: 0,0:11:21.66,0:11:22.88,Default,,0000,0000,0000,,all day long.
Dialogue: 0,0:11:22.88,0:11:25.38,Default,,0000,0000,0000,,So because of cooking, what once was
Dialogue: 0,0:11:25.38,0:11:27.73,Default,,0000,0000,0000,,a major liability, this large,
Dialogue: 0,0:11:27.73,0:11:30.77,Default,,0000,0000,0000,,dangerously expensive brain with a lot of neurons,
Dialogue: 0,0:11:30.77,0:11:32.80,Default,,0000,0000,0000,,could now become a major asset,
Dialogue: 0,0:11:32.80,0:11:36.05,Default,,0000,0000,0000,,now that we could both afford \Nthe energy for a lot of neurons
Dialogue: 0,0:11:36.06,0:11:38.56,Default,,0000,0000,0000,,and the time to do interesting things with them.
Dialogue: 0,0:11:38.56,0:11:40.53,Default,,0000,0000,0000,,So I think this explains why the human brain
Dialogue: 0,0:11:40.53,0:11:43.77,Default,,0000,0000,0000,,grew to become so large so fast in evolution,
Dialogue: 0,0:11:43.77,0:11:47.67,Default,,0000,0000,0000,,all of the while remaining just a primate brain.
Dialogue: 0,0:11:47.67,0:11:50.44,Default,,0000,0000,0000,,With this large brain now affordable by cooking,
Dialogue: 0,0:11:50.44,0:11:53.36,Default,,0000,0000,0000,,we went rapidly from raw foods to culture,
Dialogue: 0,0:11:53.36,0:11:55.96,Default,,0000,0000,0000,,agriculture, civilization, grocery stores,
Dialogue: 0,0:11:55.96,0:11:57.59,Default,,0000,0000,0000,,electricity, refrigerators,
Dialogue: 0,0:11:57.59,0:11:59.20,Default,,0000,0000,0000,,all of those things that nowadays
Dialogue: 0,0:11:59.20,0:12:01.24,Default,,0000,0000,0000,,allow us to get all the energy we need
Dialogue: 0,0:12:01.24,0:12:04.02,Default,,0000,0000,0000,,for the whole day in a single sitting
Dialogue: 0,0:12:04.02,0:12:06.97,Default,,0000,0000,0000,,at your favorite fast food joint.
Dialogue: 0,0:12:06.97,0:12:09.41,Default,,0000,0000,0000,,So what once was a solution
Dialogue: 0,0:12:09.41,0:12:11.11,Default,,0000,0000,0000,,now became the problem,
Dialogue: 0,0:12:11.11,0:12:16.63,Default,,0000,0000,0000,,and ironically, we look for the solution in raw food.
Dialogue: 0,0:12:16.63,0:12:19.18,Default,,0000,0000,0000,,So what is the human advantage?
Dialogue: 0,0:12:19.18,0:12:20.68,Default,,0000,0000,0000,,What is it that we have
Dialogue: 0,0:12:20.68,0:12:23.22,Default,,0000,0000,0000,,that no other animal has?
Dialogue: 0,0:12:23.22,0:12:25.57,Default,,0000,0000,0000,,My answer is that we have the largest number
Dialogue: 0,0:12:25.57,0:12:27.04,Default,,0000,0000,0000,,of neurons in the cerebral cortex,
Dialogue: 0,0:12:27.04,0:12:28.88,Default,,0000,0000,0000,,and I think that's the simplest explanation
Dialogue: 0,0:12:28.88,0:12:30.90,Default,,0000,0000,0000,,for our remarkable cognitive abilities.
Dialogue: 0,0:12:30.90,0:12:34.12,Default,,0000,0000,0000,,And what is it that we do that no other animal does,
Dialogue: 0,0:12:34.12,0:12:36.09,Default,,0000,0000,0000,,and which I believe was fundamental
Dialogue: 0,0:12:36.09,0:12:39.18,Default,,0000,0000,0000,,to allow us to reach that large,
Dialogue: 0,0:12:39.18,0:12:41.40,Default,,0000,0000,0000,,largest number of neurons in the cortex?
Dialogue: 0,0:12:41.40,0:12:43.61,Default,,0000,0000,0000,,In two words, we cook.
Dialogue: 0,0:12:43.61,0:12:47.18,Default,,0000,0000,0000,,No other animal cooks its food. Only humans do.
Dialogue: 0,0:12:47.18,0:12:50.05,Default,,0000,0000,0000,,And I think that's how we got to become human.
Dialogue: 0,0:12:50.05,0:12:52.51,Default,,0000,0000,0000,,Studying the human brain changed \Nthe way I think about food.
Dialogue: 0,0:12:52.51,0:12:54.15,Default,,0000,0000,0000,,I now look at my kitchen,
Dialogue: 0,0:12:54.15,0:12:55.62,Default,,0000,0000,0000,,and I bow to it,
Dialogue: 0,0:12:55.62,0:12:57.33,Default,,0000,0000,0000,,and I thank my ancestors for coming up
Dialogue: 0,0:12:57.33,0:12:59.23,Default,,0000,0000,0000,,with the invention that probably made us humans.
Dialogue: 0,0:12:59.23,0:13:01.36,Default,,0000,0000,0000,,Thank you very much.
Dialogue: 0,0:13:01.36,0:13:07.71,Default,,0000,0000,0000,,(Applause)