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DeLoache and 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 even 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 made human thinking unique.
    One of the random genetic mutations that gave us an adaptive advantage involved learning to walk upright. The trees were gone or going, so we had to deal with something new in our experience: walking increasingly long distances between food sources. That eventually involved the specialized use of our two legs. Bipedalism was an excellent solution to a vanishing rainforest. But it was also a major change. At the very least, it meant refashioning the pelvis so that it no longer propelled the back legs forward (which is what it does for great apes). Instead, the pelvis had to be re-imagined as a load-bearing device capable of keeping the head above the grass (which is what it does for you). Walking on two legs had several consequences. For one thing, it freed up our hands. For another, it was energy-efficient. It used fewer calories than walking on four legs. Our ancestral bodies used the energy surplus not to pump up our muscles but to pump up our minds— to the point that our modern-day brain, 2 percent of our body weight, sucks up 20 percent of the energy we consume.
    These changes in the structure of the brain 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.
    We got our first hints about its function from a man named Phineas Gage, who suffered the most famous occupational injury in the history of brain science. The injury didn’t kill him, but his family probably wished it had. Gage was a popular foreman of a railroad construction crew. He was funny, clever, hardworking, and responsible, the kind of man any dad 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 3-foot rod about an inch in diameter. The charge blew the rod into Gage’s head, entering just under the eye and destroying 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.
    This was the first real evidence that the prefrontal cortex governs several uniquely human cognitive talents, called “executive functions”: maintaining attention, solving problems, and inhibiting emotional impulses. In short, this region controls many of the behaviors that separate us from other animals. And from teenagers.
    meet your brain
    The prefrontal cortex 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. (This “triune theory of the brain” is one of several models scientists use to describe the brain’s overarching structural organization.) 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 in you as in a gila monster. The brain stem controls most of your body’s housekeeping chores. Its neurons regulate breathing, heart rate, sleeping, and waking. Lively as Las Vegas, they are always active, keeping your brain purring along whether you’re napping or wide awake.
    Sitting atop your brain stem is what looks like a sculpture of a scorpion carrying a slightly puckered egg on its back. The Paleomammalian brain appears in you the same way it does in many mammals, such as house