What Makes The Human Brain Unique?

Transcript

GUY RAZ, HOST:

It's the TED Radio Hour from NPR. I'm Guy Raz. And on the show today, The Unknown Brain - the mystery of how billions of neurons make us who we are.

SUZANA HERCULANO-HOUZEL: You know, when I was starting this whole thing and just creating the method, the lab I worked at had a blender - a kitchen blender just sitting there on top of a shelf. And I remember walking in and looking at that and thinking, am I really going to throw a human brain into a blender?

RAZ: This is Suzana.

HERCULANO-HOUZEL: Suzana Herculano-Houzel.

RAZ: She's a neuroscientist.

HERCULANO-HOUZEL: And a professor at the Federal University of Rio de Janeiro in Brazil.

RAZ: And that blender?

HERCULANO-HOUZEL: Just sitting there on a top of a shelf.

RAZ: What led her to the blender was a question that had never really been answered - why us? What does a human brain have that no other brain does? Why did we become the dominant species on Earth?

HERCULANO-HOUZEL: The human brain must be special in some way compared to every single other brain on earth.

RAZ: But how? Well, one popular idea was a number.

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UNIDENTIFIED MAN #1: There are 100 billion neurons in the adult human brain.

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UNIDENTIFIED MAN #2: One hundred billion neurons...

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UNIDENTIFIED WOMAN #1: A hundred....

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UNIDENTIFIED MAN #3: One hundred...

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UNIDENTIFIED MAN #4: One hundred billion...

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UNIDENTIFIED WOMAN #2: Billion...

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UNIDENTIFIED MAN #3: Billion neurons...

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UNIDENTIFIED MAN #5: One hundred billion neurons.

RAZ: One hundred billion neurons. And we'd always thought that was more than any other brain on the planet. But believe it or not, no one had ever actually counted them before until Suzana did.

So how'd you do it?

HERCULANO-HOUZEL: Not in the blender. (Laughter). What you use is...

RAZ: OK, so a blender would be a little crude. Instead, she used a special detergent to dissolve a brain...

HERCULANO-HOUZEL: Turn it into soup.

RAZ: But leave the remnants of cells behind to count, not one by one of course, but using an equation. And...

HERCULANO-HOUZEL: How many nuclei you find in the given volume.

RAZ: OK, so what's the answer? How many?

HERCULANO-HOUZEL: We found an average of 86 billion neurons.

RAZ: Eighty-six billion neurons in the human brain, which is pretty close to that 100 billion estimate.

HERCULANO-HOUZEL: But I like to point out the 14 billion that are missing are an entire baboon brain.

RAZ: Oh, wow.

HERCULANO-HOUZEL: That's a lot of neurons. If we had the mythical 100 billion neurons, we would be leaning towards the extraordinary really. But, like I said, that's an entire baboon brain away.

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RAZ: Eighty-six billion neurons didn't explain what makes us special. In fact, it seemed to suggest, compared to our primate cousins, we're pretty ordinary. Here's Suzana's TED Talk.

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HERCULANO-HOUZEL: 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. 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 4 and 5 kilos, and whale brains can weigh up to 9 kilos. 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 brain should also be larger than ours. But instead it's the other way around. Our brain is three times larger than a gorilla brain. The human brain also seems special in the amount of energy that it uses. Although it weighs only 2 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. 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.

RAZ: So Suzana asked another question. Why does our brain burn so much energy? And what she found is that it's not about how many neurons we have, but where those neurons are located. Sixteen of our 86 billion neurons are clustered in a part of the brain known as the cerebral cortex.

HERCULANO-HOUZEL: So the cerebral cortex is really responsible for all those things that we like to think of as superior cognitive abilities - the ability to plan ahead, to look back, to learn from your mistakes. So what is so remarkable about the human brain is that we manage to have a number of neurons in the cerebral cortex that is many times larger than any other animal has.

RAZ: Wow, so 16 billion neurons in our cerebral cortex. How many - I don't know. Like, how many does a mouse have?

HERCULANO-HOUZEL: A mouse has about 30 million.

RAZ: And an elephant?

HERCULANO-HOUZEL: An elephant has 5.6 billion neurons.

RAZ: Wow. What about, like, an ape?

HERCULANO-HOUZEL: Nine billion neurons in the cerebral cortex.

RAZ: So how did we get, like, almost twice as many as an ape?

HERCULANO-HOUZEL: Well, the answer is that to add more neurons to your brain, you need more energy. A brain with more neurons costs more energy. And gorillas and orangutans eat raw foods - leaves, fruit, bark. They spent around eight and a half hours per day every single day collecting food, eating food and looking for food. And with the amount of energy that they get from that food, they can just sustain the large bodies that they have and the number of neurons that they have.

Now, what about humans? With the brain that we have today, we would have to spend more than nine and a half hours per day looking for food and eating food. So somewhere back in our history - in our evolutionary history, our ancestors must have found a way to modify what we eat in a way that gives you many more calories than just raw food.

RAZ: Yeah.

HERCULANO-HOUZEL: And what we know that does that beautifully is cooking.

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RAZ: Cooking.

HERCULANO-HOUZEL: Cooking.

RAZ: We are who we are because of cooking.

HERCULANO-HOUZEL: We learned to modify the food that we eat, and that allowed us to just cram more neurons inside the brain.

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HERCULANO-HOUZEL: To cook is to use fire to predigest 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. 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 the while remaining just a primate brain.

RAZ: This just, like, completely changes the way I'm going to see "Top Chef" from now on.

HERCULANO-HOUZEL: I know. It completely changed the way I look at my kitchen.

RAZ: Yeah.

HERCULANO-HOUZEL: It's - the act of cooking has become something of not a miracle, but something to be revered.

RAZ: Yeah, yeah. So how does this, like - how does this change the way you think about what it means to be human?

HERCULANO-HOUZEL: I think it's very humbling. We're animals. We're just another species. It so happens that we were able to pack so much processing power in our cerebral cortex that we're even at that point now of looking at ourselves and gaining insight on how we work.

RAZ: That's amazing that we are studying essentially this thing that runs us.

HERCULANO-HOUZEL: Right, which I think is a lot of fun that some philosophers used to say that, you know, it's impossible. How can you ever use your very brain to understand how your brain works? And, well, neuroscience has proven them wrong.

RAZ: That's neuroscientist Suzana Herculano-Houzel. Check out her full talk at ted.npr.org. Transcript provided by NPR, Copyright NPR.