What is so special about the human brain?
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0:01 - 0:03What is so special about the human brain?
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0:03 - 0:06Why is it that we study other animals
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0:06 - 0:08instead of them studying us?
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0:08 - 0:09What does a human brain have or do
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0:09 - 0:11that no other brain does?
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0:11 - 0:14When I became interested
in these questions about 10 years ago, -
0:14 - 0:17scientists thought they knew
what different brains were made of. -
0:17 - 0:19Though it was based on very little evidence,
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0:19 - 0:21many scientists thought that all mammalian brains,
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0:21 - 0:23including the human brain,
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0:23 - 0:24were made in the same way,
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0:24 - 0:25with a number of neurons that was always
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0:25 - 0:28proportional to the size of the brain.
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0:28 - 0:30This means that two brains of the same size,
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0:30 - 0:33like these two, with a respectable 400 grams,
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0:33 - 0:36should have similar numbers of neurons.
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0:36 - 0:38Now, if neurons are the functional
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0:38 - 0:41information processing units of the brain,
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0:41 - 0:42then the owners of these two brains
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0:42 - 0:45should have similar cognitive abilities.
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0:45 - 0:47And yet, one is a chimp,
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0:47 - 0:50and the other is a cow.
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0:50 - 0:52Now maybe cows have a really rich
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0:52 - 0:54internal mental life and are so smart
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0:54 - 0:58that they choose not to let us realize it,
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0:58 - 1:00but we eat them.
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1:00 - 1:01I think most people will agree
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1:01 - 1:03that chimps are capable of much more complex,
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1:03 - 1:06elaborate and flexible behaviors than cows are.
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1:06 - 1:08So this is a first indication that the
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1:08 - 1:10"all brains are made the same way" scenario
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1:10 - 1:12is not quite right.
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1:12 - 1:13But let's play along.
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1:13 - 1:15If all brains were made the same way
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1:15 - 1:18and you were to compare animals
with brains of different sizes, -
1:18 - 1:20larger brains should always have more neurons
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1:20 - 1:23than smaller brains,
and the larger the brain, -
1:23 - 1:26the more cognitively able its owner should be.
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1:26 - 1:28So the largest brain around should also be
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1:28 - 1:30the most cognitively able.
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1:30 - 1:32And here comes the bad news:
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1:32 - 1:34Our brain, not the largest one around.
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1:34 - 1:36It seems quite vexing.
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1:36 - 1:39Our brain weighs between 1.2 and 1.5 kilos,
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1:39 - 1:42but elephant brains weigh between four and five kilos,
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1:42 - 1:45and whale brains can weigh up to nine kilos,
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1:45 - 1:49which is why scientists used to resort to saying
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1:49 - 1:52that our brain must be special
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1:52 - 1:54to explain our cognitive abilities.
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1:54 - 1:57It must be really extraordinary,
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1:57 - 1:59an exception to the rule.
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1:59 - 2:03Theirs may be bigger, but ours is better,
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2:03 - 2:04and it could be better, for example,
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2:04 - 2:07in that it seems larger than it should be,
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2:07 - 2:09with a much larger cerebral cortex
than we should have -
2:09 - 2:11for the size of our bodies.
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2:11 - 2:12So that would give us extra cortex
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2:12 - 2:15to do more interesting things
than just operating the body. -
2:15 - 2:17That's because the size of the brain
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2:17 - 2:19usually follows the size of the body.
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2:19 - 2:22So the main reason for saying that
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2:22 - 2:24our brain is larger than it should be
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2:24 - 2:26actually comes from comparing ourselves
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2:26 - 2:27to great apes.
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2:27 - 2:30Gorillas can be two to three times larger than we are,
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2:30 - 2:32so their brains should also be larger than ours,
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2:32 - 2:34but instead it's the other way around.
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2:34 - 2:37Our brain is three times larger than a gorilla brain.
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2:37 - 2:39The human brain also seems special
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2:39 - 2:42in the amount of energy that it uses.
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2:42 - 2:44Although it weighs only two percent of the body,
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2:44 - 2:48it alone uses 25 percent of all the energy
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2:48 - 2:50that your body requires to run per day.
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2:50 - 2:54That's 500 calories out of a total of 2,000 calories,
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2:54 - 2:56just to keep your brain working.
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2:56 - 2:59So the human brain is larger than it should be,
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2:59 - 3:01it uses much more energy than it should,
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3:01 - 3:02so it's special.
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3:02 - 3:05And this is where the story started to bother me.
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3:05 - 3:07In biology, we look for rules
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3:07 - 3:09that apply to all animals and to life in general,
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3:09 - 3:11so why should the rules of evolution
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3:11 - 3:15apply to everybody else but not to us?
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3:15 - 3:17Maybe the problem was with the basic assumption
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3:17 - 3:19that all brains are made in the same way.
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3:19 - 3:21Maybe two brains of a similar size
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3:21 - 3:23can actually be made of
very different numbers of neurons. -
3:23 - 3:25Maybe a very large brain
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3:25 - 3:27does not necessarily have more neurons
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3:27 - 3:29than a more modest-sized brain.
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3:29 - 3:32Maybe the human brain
actually has the most neurons -
3:32 - 3:34of any brain, regardless of its size,
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3:34 - 3:37especially in the cerebral cortex.
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3:37 - 3:38So this to me became
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3:38 - 3:40the important question to answer:
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3:40 - 3:42how many neurons does the human brain have,
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3:42 - 3:45and how does that compare to other animals?
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3:45 - 3:47Now, you may have heard or read somewhere
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3:47 - 3:49that we have 100 billion neurons,
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3:49 - 3:51so 10 years ago, I asked my colleagues
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3:51 - 3:53if they knew where this number came from.
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3:53 - 3:55But nobody did.
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3:55 - 3:56I've been digging through the literature
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3:56 - 3:58for the original reference for that number,
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3:58 - 4:00and I could never find it.
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4:00 - 4:03It seems that nobody had actually ever counted
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4:03 - 4:04the number of neurons in the human brain,
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4:04 - 4:07or in any other brain for that matter.
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4:07 - 4:10So I came up with my own way
to count cells in the brain, -
4:10 - 4:12and it essentially consists of
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4:12 - 4:16dissolving that brain into soup.
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4:16 - 4:18It works like this:
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4:18 - 4:21You take a brain, or parts of that brain,
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4:21 - 4:22and you dissolve it in detergent,
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4:22 - 4:24which destroys the cell membranes
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4:24 - 4:26but keeps the cell nuclei intact,
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4:26 - 4:30so you end up with a suspension of free nuclei
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4:30 - 4:31that looks like this,
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4:31 - 4:33like a clear soup.
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4:33 - 4:35This soup contains all the nuclei
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4:35 - 4:37that once were a mouse brain.
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4:37 - 4:40Now, the beauty of a soup is that because it is soup,
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4:40 - 4:43you can agitate it and make those nuclei
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4:43 - 4:44be distributed homogeneously in the liquid,
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4:44 - 4:46so that now by looking under the microscope
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4:46 - 4:51at just four or five samples
of this homogeneous solution, -
4:51 - 4:53you can count nuclei, and therefore tell
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4:53 - 4:55how many cells that brain had.
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4:55 - 4:56It's simple, it's straightforward,
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4:56 - 4:58and it's really fast.
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4:58 - 5:00So we've used that method to count neurons
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5:00 - 5:02in dozens of different species so far,
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5:02 - 5:04and it turns out that all brains
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5:04 - 5:06are not made the same way.
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5:06 - 5:09Take rodents and primates, for instance:
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5:09 - 5:11In larger rodent brains, the average size
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5:11 - 5:13of the neuron increases,
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5:13 - 5:15so the brain inflates very rapidly
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5:15 - 5:18and gains size much faster than it gains neurons.
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5:18 - 5:20But primate brains gain neurons
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5:20 - 5:22without the average neuron becoming any larger,
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5:22 - 5:24which is a very economical way
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5:24 - 5:26to add neurons to your brain.
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5:26 - 5:27The result is that a primate brain
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5:27 - 5:31will always have more neurons than
a rodent brain of the same size, -
5:31 - 5:32and the larger the brain,
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5:32 - 5:34the larger this difference will be.
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5:34 - 5:36Well, what about our brain then?
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5:36 - 5:38We found that we have, on average,
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5:38 - 5:4086 billion neurons,
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5:40 - 5:4316 billion of which are in the cerebral cortex,
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5:43 - 5:45and if you consider that the cerebral cortex
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5:45 - 5:48is the seat of functions like
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5:48 - 5:51awareness and logical and abstract reasoning,
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5:51 - 5:54and that 16 billion is the most neurons
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5:54 - 5:57that any cortex has,
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5:57 - 5:58I think this is the simplest explanation
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5:58 - 6:02for our remarkable cognitive abilities.
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6:02 - 6:05But just as important is what
the 86 billion neurons mean. -
6:05 - 6:06Because we found that the relationship
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6:06 - 6:09between the size of the brain
and its number of neurons -
6:09 - 6:10could be described mathematically,
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6:10 - 6:13we could calculate what a human brain
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6:13 - 6:15would look like if it was made like a rodent brain.
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6:15 - 6:19So, a rodent brain with 86 billion neurons
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6:19 - 6:22would weigh 36 kilos.
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6:22 - 6:24That's not possible.
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6:24 - 6:25A brain that huge would be crushed
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6:25 - 6:27by its own weight,
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6:27 - 6:28and this impossible brain would go
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6:28 - 6:32in the body of 89 tons.
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6:32 - 6:34I don't think it looks like us.
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6:34 - 6:37So this brings us to a very important
conclusion already, -
6:37 - 6:39which is that we are not rodents.
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6:39 - 6:43The human brain is not a large rat brain.
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6:43 - 6:45Compared to a rat, we might seem special, yes,
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6:45 - 6:47but that's not a fair comparison to make,
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6:47 - 6:50given that we know that we are not rodents.
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6:50 - 6:51We are primates,
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6:51 - 6:54so the correct comparison is to other primates.
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6:54 - 6:55And there, if you do the math,
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6:55 - 6:58you find that a generic primate
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6:58 - 7:00with 86 billion neurons
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7:00 - 7:03would have a brain of about 1.2 kilos,
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7:03 - 7:05which seems just right,
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7:05 - 7:07in a body of some 66 kilos,
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7:07 - 7:09which in my case is exactly right,
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7:09 - 7:12which brings us to a very unsurprising
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7:12 - 7:15but still incredibly important conclusion:
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7:15 - 7:16I am a primate.
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7:16 - 7:19And all of you are primates.
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7:19 - 7:21And so was Darwin.
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7:21 - 7:24I love to think that Darwin
would have really appreciated this. -
7:24 - 7:26His brain, like ours,
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7:26 - 7:29was made in the image of other primate brains.
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7:29 - 7:31So the human brain may be remarkable, yes,
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7:31 - 7:34but it is not special in its number of neurons.
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7:34 - 7:36It is just a large primate brain.
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7:36 - 7:39I think that's a very humbling and sobering thought
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7:39 - 7:42that should remind us of our place in nature.
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7:42 - 7:45Why does it cost so much energy, then?
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7:45 - 7:46Well, other people have figured out
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7:46 - 7:48how much energy the human brain
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7:48 - 7:49and that of other species costs,
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7:49 - 7:51and now that we knew how many neurons
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7:51 - 7:53each brain was made of, we could do the math.
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7:53 - 7:55And it turns out that both human
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7:55 - 7:58and other brains cost about the same,
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7:58 - 8:01an average of six calories
per billion neurons per day. -
8:01 - 8:03So the total energetic cost of a brain
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8:03 - 8:05is a simple, linear function
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8:05 - 8:07of its number of neurons,
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8:07 - 8:09and it turns out that the human brain
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8:09 - 8:13costs just as much energy as you would expect.
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8:13 - 8:15So the reason why the human brain
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8:15 - 8:17costs so much energy is simply because
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8:17 - 8:19it has a huge number of neurons,
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8:19 - 8:20and because we are primates
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8:20 - 8:23with many more neurons for a given body size
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8:23 - 8:24than any other animal,
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8:24 - 8:28the relative cost of our brain is large,
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8:28 - 8:31but just because we're primates,
not because we're special. -
8:31 - 8:32Last question, then:
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8:32 - 8:35how did we come by this
remarkable number of neurons, -
8:35 - 8:37and in particular, if great apes
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8:37 - 8:39are larger than we are,
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8:39 - 8:42why don't they have a larger brain
than we do, with more neurons? -
8:42 - 8:45When we realized how much expensive it is
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8:45 - 8:47to have a lot of neurons in the brain, I figured,
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8:47 - 8:49maybe there's a simple reason.
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8:49 - 8:51They just can't afford the energy
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8:51 - 8:54for both a large body
and a large number of neurons. -
8:54 - 8:55So we did the math.
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8:55 - 8:57We calculated on the one hand
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8:57 - 8:59how much energy a primate gets per day
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8:59 - 9:00from eating raw foods,
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9:00 - 9:02and on the other hand, how much energy
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9:02 - 9:04a body of a certain size costs
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9:04 - 9:07and how much energy a brain of a
certain number of neurons costs, -
9:07 - 9:09and we looked for the combinations
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9:09 - 9:11of body size and number of brain neurons
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9:11 - 9:12that a primate could afford
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9:12 - 9:15if it ate a certain number of hours per day.
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9:15 - 9:17And what we found is that
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9:17 - 9:18because neurons are so expensive,
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9:18 - 9:22there is a tradeoff between
body size and number of neurons. -
9:22 - 9:25So a primate that eats eight hours per day
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9:25 - 9:28can afford at most 53 billion neurons,
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9:28 - 9:29but then its body cannot be any bigger
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9:29 - 9:31than 25 kilos.
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9:31 - 9:33To weigh any more than that,
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9:33 - 9:35it has to give up neurons.
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9:35 - 9:37So it's either a large body
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9:37 - 9:39or a large number of neurons.
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9:39 - 9:40When you eat like a primate,
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9:40 - 9:43you can't afford both.
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9:43 - 9:45One way out of this metabolic limitation
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9:45 - 9:48would be to spend even more hours per day eating,
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9:48 - 9:49but that gets dangerous,
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9:49 - 9:52and past a certain point, it's just not possible.
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9:52 - 9:54Gorillas and orangutans, for instance,
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9:54 - 9:55afford about 30 billion neurons
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9:55 - 9:58by spending eight and a half hours per day eating,
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9:58 - 10:02and that seems to be about as much as they can do.
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10:02 - 10:03Nine hours of feeding per day
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10:03 - 10:07seems to be the practical limit for a primate.
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10:07 - 10:08What about us?
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10:08 - 10:10With our 86 billion neurons
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10:10 - 10:13and 60 to 70 kilos of body mass,
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10:13 - 10:17we should have to spend over nine hours
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10:17 - 10:20per day every single day feeding,
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10:20 - 10:22which is just not feasible.
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10:22 - 10:24If we ate like a primate,
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10:24 - 10:26we should not be here.
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10:26 - 10:28How did we get here, then?
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10:28 - 10:31Well, if our brain costs just as much energy
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10:31 - 10:33as it should, and if we can't spend
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10:33 - 10:37every waking hour of the day feeding,
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10:37 - 10:38then the only alternative, really,
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10:38 - 10:40is to somehow get more energy
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10:40 - 10:42out of the same foods.
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10:42 - 10:46And remarkably, that matches exactly
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10:46 - 10:49what our ancestors are believed to have invented
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10:49 - 10:51one and a half million years ago,
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10:51 - 10:54when they invented cooking.
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10:54 - 10:56To cook is to use fire
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10:56 - 11:00to pre-digest foods outside of your body.
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11:00 - 11:02Cooked foods are softer, so they're easier to chew
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11:02 - 11:05and to turn completely into mush in your mouth,
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11:05 - 11:07so that allows them to be completely digested
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11:07 - 11:08and absorbed in your gut,
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11:08 - 11:12which makes them yield much more
energy in much less time. -
11:12 - 11:15So cooking frees time for us to do
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11:15 - 11:17much more interesting things with our day
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11:17 - 11:18and with our neurons
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11:18 - 11:20than just thinking about food,
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11:20 - 11:22looking for food, and gobbling down food
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11:22 - 11:23all day long.
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11:23 - 11:25So because of cooking, what once was
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11:25 - 11:28a major liability, this large,
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11:28 - 11:31dangerously expensive brain with a lot of neurons,
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11:31 - 11:33could now become a major asset,
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11:33 - 11:36now that we could both afford
the energy for a lot of neurons -
11:36 - 11:39and the time to do interesting things with them.
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11:39 - 11:41So I think this explains why the human brain
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11:41 - 11:44grew to become so large so fast in evolution,
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11:44 - 11:48all of the while remaining just a primate brain.
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11:48 - 11:50With this large brain now affordable by cooking,
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11:50 - 11:53we went rapidly from raw foods to culture,
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11:53 - 11:56agriculture, civilization, grocery stores,
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11:56 - 11:58electricity, refrigerators,
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11:58 - 11:59all of those things that nowadays
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11:59 - 12:01allow us to get all the energy we need
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12:01 - 12:04for the whole day in a single sitting
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12:04 - 12:07at your favorite fast food joint.
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12:07 - 12:09So what once was a solution
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12:09 - 12:11now became the problem,
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12:11 - 12:17and ironically, we look for the solution in raw food.
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12:17 - 12:19So what is the human advantage?
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12:19 - 12:21What is it that we have
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12:21 - 12:23that no other animal has?
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12:23 - 12:26My answer is that we have the largest number
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12:26 - 12:27of neurons in the cerebral cortex,
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12:27 - 12:29and I think that's the simplest explanation
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12:29 - 12:31for our remarkable cognitive abilities.
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12:31 - 12:34And what is it that we do that no other animal does,
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12:34 - 12:36and which I believe was fundamental
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12:36 - 12:39to allow us to reach that large,
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12:39 - 12:41largest number of neurons in the cortex?
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12:41 - 12:44In two words, we cook.
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12:44 - 12:47No other animal cooks its food. Only humans do.
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12:47 - 12:50And I think that's how we got to become human.
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12:50 - 12:53Studying the human brain changed
the way I think about food. -
12:53 - 12:54I now look at my kitchen,
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12:54 - 12:56and I bow to it,
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12:56 - 12:57and I thank my ancestors for coming up
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12:57 - 12:59with the invention that probably made us humans.
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12:59 - 13:01Thank you very much.
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13:01 - 13:08(Applause)
- Title:
- What is so special about the human brain?
- Speaker:
- Suzana Herculano-Houzel
- Description:
-
The human brain is puzzling -- it is curiously large given the size of our bodies, uses a tremendous amount of energy for its weight and has a bizarrely dense cerebral cortex. But: why? Neuroscientist Suzana Herculano-Houzel puts on her detective's cap and leads us through this mystery. By making "brain soup," she arrives at a startling conclusion.
- Video Language:
- English
- Team:
- closed TED
- Project:
- TEDTalks
- Duration:
- 13:31
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