From Primitive Parts, A Highly Evolved Human Brain

Enlarge this image toggle caption Courtesy of the Allen Institute for Brain Science Courtesy of the Allen Institute for Brain Science

From one perspective, the human brain is a masterpiece. From another, it's 3 pounds of inefficient jelly. Both views are accurate, and that's because our remarkable brain has been assembled from some very primitive parts.

"Although the things it can do are very wonderful and impressive, its design is very poor engineering in many respects," says David Linden, a professor of neuroscience at Johns Hopkins University in Baltimore, the author of The Accidental Mind.

Linden says there's a simple explanation: evolution.

Enlarge this image toggle caption Courtesy of the Allen Institute for Brain Science Courtesy of the Allen Institute for Brain Science

"In evolution, you never build something new if you can adapt something you've already got," he says. "It's the ultimate tinkerer and the ultimate cheapskate."

Our brain has been put together with parts from jellyfish and lizards and mice, Linden says. These parts may have been OK for their original owners, he says, but they aren't ideal for us.

Take brain cells, for example.

"They are slow. They are inefficient. They leak signals to their neighbors," Linden says. "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."

And that means it's very slow. Linden says getting a simple message from our feet to our brain can take a remarkably long time. To get a sense of just how long, he says, imagine a giant with her head in Baltimore and her toe off the coast of South Africa. If a shark bit that toe on Monday, Linden says, "she wouldn't feel it until Wednesday, and she wouldn't jerk her toe until Saturday."

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

In evolution, you never build something new if you can adapt something you've already got.

Deep Down, We're Lizards

Jellyfish don't have a brain, but they were the first animal to have any sort of nervous system. It's a loose network of nerves called a "nerve net," says Chet Sherwood, who studies brain evolution at The George Washington University in Washington, D.C.

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

"The nerves and the manner in which signals are sent is similar to what we have ourselves," he says.

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

"If I throw 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," Linden says. "Your ancient, subconscious, lizard-like visual system is doing that task."

Our Layered Brain The human brain relies heavily on structures found in lower animals. These functions play key roles in our everyday life. Stephanie d'Otreppe, Jon Hamilton/NPR

Three Scoops Of Ice Cream

A lizard brain is about survival -- it controls heart rate and breathing, and processes information from the eyes and ears and mouth.

When mammals like mice came along, the lizard brain didn't go away. It simply became the brain stem, which is perched on top of the spine, Linden says.

Then evolution slapped more brain on top of the brain stem.

"It's like adding scoops to an ice cream cone," Linden says. "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."

That second scoop gave mammals more memory and a wider range of emotions. It also allows a mouse to do things a lizard can't, like using experiences to anticipate danger instead of just responding to it.

To create the brain found in apes, Sherwood says, evolution added a third scoop. It allows apes to reason and live much more complicated lives than mice.

"In these brains, you can find all of the very same parts that you would see in a human brain," Sherwood says. But there's a difference -- the brain of an adult human is about three times the size of a gorilla brain.

The Cost Of A Big Brain

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

"A miracle happens," he says. "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."

In one sense, we've had to pay a heavy cost for our big, inefficient brains: Childbirth is difficult, childhood is long, and 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.

"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," Linden says.

And without that long childhood, he says, evolution wouldn't have equipped us with the force that bonds parents together to protect their children.

"We wouldn't have love," Linden says.