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From one perspective, the human brain is a masterpiece; and from another perspective, it's three pounds of jelly. Now, it's the subject of our series The Human Edge, which explores evolutionary changes that have helped people to dominate the planet.

Today, NPR's Jon Hamilton reports on how evolution assembled a remarkable brain from some very primitive parts.

JON HAMILTON: Lots of people use superlatives to describe the human brain. Then there are scientists like David Linden at Johns Hopkins University in Baltimore.

Dr. DAVID LINDEN (Neuroscience, Johns Hopkins University): Although the things it can do are very wonderful and impressive, its design is very poor engineering in many respects.

HAMILTON: Linden says there's a simple explanation: evolution.

Mr. LINDEN: In evolution, you never build something new if you can adapt something you've already got. It's the ultimate tinkerer and the ultimate cheapskate.

HAMILTON: So, our brain has been put together with parts from jellyfish and lizards and mice. Linden says these parts may have been Okay for their original owners, but they aren't so great for us. Take brain cells.

Mr. LINDEN: They are slow. They are inefficient. They leak signals to their neighbors. And consequently, if you want to build clever human us, with these very suboptimal parts, the only way to do it is to build a brain that is simply enormous and massively interconnected.

HAMILTON: And it's still slow. Linden says getting a simple message from our feet to our brain can take a long time.

Mr. LINDEN: For example, if there were a giant with her head in Baltimore and her toe off the coast of South Africa, and she was bit by a great white shark on the toe on Monday, she wouldn't feel it until Wednesday, and she wouldn't jerk her toe until Saturday.

HAMILTON: Why the lag? Linden says it's because we're still using a communication system developed 600 million years ago - by jellyfish.

They were the first animal to have any sort of nervous system.

Unidentified child: There's a jellyfish.

Mr. CHET SHERWOOD (George Washington University): Jellyfish.

HAMILTON: I'm at the National Zoo with Chet Sherwood, who studies brain evolution at The George Washington University. Sherwood says jellyfish don't have a brain.

Mr. SHERWOOD: Jellyfish have a nervous system, but it would be best described as a nerve net.

HAMILTON: A loose network of nerves that carry simple messages like, start eating, when something gets caught in the jelly's tentacles.

Jellyfish don't exactly live life in the fast lane, so the nerve messages can travel slowly; the ones on a telephone wire move a million times faster. But Sherwood says once evolution had come up with a messaging system, it kept using it.

Mr. SHERWOOD: The nerves, and the manner in which signals are sent, is similar to what we have ourselves.

David Linden says our brains are also limited by design features we share with lizards. Evolution's tinkering gave them the brain they needed to hunt and survive in a tough world. And, Linden says, our brains still have that ancient wiring.

Mr. LINDEN: If I threw a baseball at you, you're going to reflexively duck your head, and there's nothing you can do to override that with your conscious mind. It is hard-wired and you're going to do it. And your ancient, subconscious, lizard-like visual system is doing that task.

HAMILTON: Unfortunately, that sort of fight or flight response isn't great for most social interactions.

(Soundbite of kids talking)

HAMILTON: At the zoo, Chet Sherwood and I see a young primate in a T-shirt talking to an iguana on the other side of the glass. The iguana does not look happy.

Mr. SHERWOOD: He's doing a head gesture. Looks like he's trying to communicate something to one of these zoo guests.

HAMILTON: It looks a little aggressive.

Mr. SHERWOOD: It does.

HAMILTON: A lizard brain is about survival - it controls heart rate and breathing, it processes information from the eyes and ears and mouth. When mammals like mice came along, the lizard brain didn't go away. Linden says it became the brain stem, which is perched on top of the spine. Then evolution slapped more brain on top of that.

Mr. LINDEN: It's like adding scoops to an ice cream cone. So, if you imagine the lizard brain as a single-scoop ice cream cone, the way you make a mouse brain out of a lizard brain isn't to throw the cone and the first scoop away and start over and make a banana split; rather, it's to put a second scoop on top of the first scoop.

HAMILTON: That second scoop helped give mammals more memory and a wider range of emotions. You can see that at the zoo in the naked mole rats. Chet Sherwood and I watched them scamper through a labyrinth of plastic tubes.

Mr. SHERWOOD: It's got a brain that has a neocortex, which is the most expanded part of the forebrain, which is shared by all mammals.

HAMILTON: The neocortex allows naked mole rats to do things a lizard can't, like using experiences to anticipate danger instead of just responding to it.

At a zoo, you can cover millions of years of evolution in a few steps. That's what Chet Sherwood and I do when we walk from the mole rats to the gorillas, where a mother is watching over her baby.

Mr. SHERWOOD: They are sitting closely together. The mother's looking out at the crowd.

HAMILTON: Sherwood says if a brain is like an ice cream cone, apes got the third scoop. It's the part that allows these gorillas to reason and live much more complicated lives.

Mr. SHERWOOD: In these brains, you can find all of the very same parts that you would see in a human brain.

HAMILTON: But there's a difference - the brain of an adult human is about three times the size of a gorilla brain.

Much of the size difference appears after birth. The human brain continues to grow rapidly for the first five years. David Linden says it takes 20 years before all the circuits are laid out and connected up.

Mr. LINDEN: A miracle happens. You have enough neurons in this cortical circuit, massively interconnected, and somehow, what emerges from that are these amazing human traits - the ability for me to know what you are thinking based on social cues that you give me, other forms of observational learning and high-level cognition.

HAMILTON: In one sense, we've had to pay a heavy cost for our big, inefficient brains: childbirth is difficult, childhood is long, our brains consume 20 percent of the calories we eat. But Linden says these adaptations turn out to have some surprising payoffs, like romantic love.

Mr. LINDEN: If our neurons weren't such lousy processors and we didn't need 100 billion of them massively interconnected in order to make a clever brain out of such lousy parts, then we wouldn't have such a long childhood while that brain grew up and matured after birth, and it wouldn't drive the dominant human mating system - we wouldn't have love.

HAMILTON: An integral part of our three pounds of inefficient jelly.

Jon Hamilton, NPR News.

(Soundbite of music)

INSKEEP: If you want to find out more about our Human Edge series, send your brain over to our website. Let it discover what has made us the most versatile and powerful species on Earth - for better or worse. That's at NPR.org/Science. And you can tune in this afternoon as we explore another aspect of our minds -the ability of humans to think with symbols.

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INSKEEP: This is NPR News.

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