a new class of creatures would start to fill the vacuum created as more and more of their roommates died out.
The change was enough to shake us out of our comfortable trees, but it wasn’t violent enough to kill us when we landed. Landing was only the beginning of the hard work, however. Faced with grasslands rather than trees, we were rudely introduced to the idea of “flat.” We quickly discovered that our new digs were already occupied. The locals had co-opted the food sources, and most of them were stronger and faster than we were. It is disconcerting to think that we started our evolutionary journey on an unfamiliar horizontal plane with the words “Eat me, I’m prey” taped to our evolutionary butts.
You might suspect that the odds against our survival were great. You would be right. The founding population of our direct ancestors is not thought to have been much larger than 2,000 individuals; some think the group was as small as a few hundred. How, then, did we go from such a wobbly, fragile minority population to a staggering tide of humanity seven billion strong and growing?
There is only one way, according to Richard Potts, director of the Human Origins Program at the Smithsonian’s National Museum of Natural History. We gave up on stability. We began not to care about consistency within a given habitat, because consistency wasn’t an option. We adapted to variation itself. Those unable to rapidly solve new problems or learn from mistakes didn’t survive long enough to pass on their genes. The net effect of this evolution was that rather than becoming stronger, we became smarter. It was a brilliant strategy. We went on to conquer other ecological niches in Africa. Then we took over the world.
Potts’s theory predicts some fairly simple things about human learning. It predicts interactions between two powerful features of the brain: a database in which to store a fund of knowledge, and the ability to improvise off that database. One allows us to know when we’ve made mistakes. The other allows us to learn from them. Both give us the ability to add new information under rapidly changing conditions. And both are relevant to the way we design classrooms and cubicles. We’ll uncover more about this database in the Memory chapter.
Bigger and bigger brains
Adapting to variation provides a context for symbolic reasoning, but it hardly explains our unique ability to invent calculus and write romance novels. After all, many animals create a database of knowledge, and many of them make tools, which they use creatively. Still, it is not as if chimpanzees write symphonies badly and we write them well. Chimps can’t write them at all, and we can write ones that make people spend their life savings on subscriptions to the New York Philharmonic. There must have been something else in our evolutionary history that gave rise to unique human thinking.
One of the random genetic mutations that gave us an adaptive advantage involved walking upright on two legs. Because the trees were gone or going, we needed to travel increasingly long distances between food sources. Walking on two legs instead of four both freed up our hands and used fewer calories. It was energy-efficient. Our ancestral bodies used the energy surplus not to pump up our muscles but to pump up our minds.
This led to the masterpiece of evolution, the region that distinguishes humans from all other creatures. It is a specialized area of the frontal lobe, just behind the forehead, called the prefrontal cortex. What does the prefrontal cortex do? We got our first hints from a man named Phineas Gage, who suffered the most famous occupational injury in the history of brain science.
Gage was a popular foreman of a railroad construction crew. He was funny, clever, hardworking, and responsible, the kind of guy any father would be proud to call “son-in-law.” On September 13, 1848, he set an explosives charge in the hole of a rock using a tamping iron, a three-foot rod about an inch in diameter. The charge blew the rod into Gage’s head. It entered just under the eye and destroyed most of his prefrontal cortex. Miraculously, Gage survived. But he became tactless, impulsive, and profane. He left his family and wandered aimlessly from job to job. His friends said he was no longer Gage.
When damage occurs to a specific brain region, we know that any observed behavioral abnormality must in some way be linked to that region’s function. I describe several such cases throughout the book for this reason. Gage’s case was the first real evidence that the prefrontal cortex governs several uniquely human cognitive talents, called “executive functions”: solving problems, maintaining attention, and inhibiting emotional impulses. In short, this region controls many of the behaviors that separate us from other animals (and from teenagers).
Three brains in one
The prefrontal cortex, however, is only the newest addition to the brain. Three brains are tucked inside your head, and parts of their structure took millions of years to design. Your most ancient neural structure is the brain stem, or “lizard brain.” This rather insulting label reflects the fact that the brain stem functions the same way in you as in a Gila monster. The brain stem controls most of your body’s housekeeping chores: breathing, heart rate, sleeping, waking. Lively as Las Vegas, these neurons are always active, keeping your brain buzzing along whether you’re napping or wide awake.
Sitting atop your brain stem is your “mammalian brain.” It appears in you the same way it does in many mammals, such as house cats, which is how it got its name. It has more to do with your animal survival than with your human potential. Most of its functions involve what some researchers call the “four Fs”: fighting, feeding, fleeing, and … reproductive behavior. Several parts of the mammalian brain play a large role in the Brain Rules.
The amygdala allows you to feel rage. Or fear. Or pleasure. Or memories of past experiences of rage, fear, or pleasure. The amygdala is responsible for both the creation of emotions and the memories they generate. We’ll explore the powerful effects of emotions, and how to harness them, in the Attention chapter.
The hippocampus converts your short-term memories into longer-term forms. The Memory chapter covers the surprising way that happens, and the key to remembering.
The thalamus is one of the most active, well-connected parts of the brain—a control tower for the senses. Sitting squarely in the center of your brain, it processes and routes signals sent from nearly every corner of your sensory universe. We’ll return to this bizarre, complex process in the Sensory Integration chapter.
Folded atop all of this is your “human brain,” a layer called the cortex. Unfolded, this layer would be about the size of a baby blanket, with a thickness ranging from that of blotting paper to that of heavy-duty cardboard. It is in deep electrical communication with the interior. Neurons spark to life, then suddenly blink off, then fire again. Complex circuits of electrical information crackle in coordinated, repeated patterns, racing to communicate their information along large neural highways that branch suddenly into thousands of exits. As we’ll see in the Wiring chapter, these branches are different in every single one of us. Each region of the cortex is highly specialized, with sections for speech, for vision, for memory.
You wouldn’t know all this just by looking at the brain. The cortex looks homogenous, somewhat like the shell of a walnut, which fooled anatomists for hundreds of years. Then World War I happened. It was the first major conflict where medical advances allowed large numbers of combatants to survive shrapnel injuries. Some of these injuries penetrated only to the periphery of the brain, destroying tiny regions of the cortex while leaving everything else intact. Enough soldiers were hurt that scientists could study in detail the injuries and the truly strange behaviors that resulted. Eventually, scientists were able to make a complete structure–function map of the brain. They were able to see that the brain had, over eons, become three.
Scientists found that as our brains evolved, our heads did, too: They were getting bigger all the time. But the pelvis—and birth canal—can be only so wide, which is bonkers if you are giving birth to children with larger and larger heads. A lot of mothers and babies died
