Among mammals, the brain keeps its three major components, but with two new structures. The neocerebellum ("new cerebellum") is added to the cerebellum, looking much like a fungal growth at the base of the brain, and the neocortex ("new cortex") grows out of the front of the forebrain. In most mammals, these new additions are not particularly large relative to the brain stem. In primates they are much larger, and in the human they are so large that the original brain stem is almost completely hidden by this large convoluted mass of grey neural matter.
American evolutionary paleontologist Stephen Jay Gould provided a term for this phenomenon--exaptation. He made a major contribution to our understanding of evolution by insisting that we distinguish adaptation, the evolutionary process through which adaptedly complex structures and behaviors are progressively fine-tuned by natural selection with no marked change in the structure's or behavior's function, from exaptation, through which structures and behaviors originally selected for one function become involved in another, possibly quite unrelated, function. Exaptation makes it difficult if not impossible to understand why our brain evolved as it did. Although the brain allows us to speak, sing, dance, laugh, design computers, and solve differential equations, these and other abilities may well be accidental side effects of its evolution."
Professor Gary Cziko
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.
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.
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.
(Source - NPR)