To better grasp the potent hold of the emotions on the thinking mind—and why feeling and reason are so readily at war—consider how the brain evolved. Human brains, with their three pounds or so of cells and neural juices, are about triple the size of those in our nearest cousins in evolution, the nonhuman primates. Over millions of years of evolution, the brain has grown from the bottom up, with its higher centers developing as elaborations of lower, more ancient parts. (The growth of the brain in the human embryo roughly retraces this evolutionary course.)

The most primitive part of the brain, shared with all species that have more than a minimal nervous system, is the brainstem surrounding the top of the spinal cord. This root brain regulates basic life functions like breathing and the metabolism of the body's other organs, as well as controlling stereotyped reactions and movements. This primitive brain cannot be said to think or learn; rather it is a set of preprogrammed regulators that keep the body running as it should and reacting in a way that ensures survival. This brain reigned supreme in the Age of the Reptiles: Picture a snake hissing to signal the threat of an attack.

From the most primitive root, the brainstem, emerged the emotional centers. Millions of years later in evolution, from these emotional areas evolved the thinking brain or "neocortex," the great bulb of convoluted tissues that make up the top layers. The fact that the thinking brain grew from the emotional reveals much about the relationship of thought to feeling; there was an emotional brain long before there was a rational one.

The most ancient root of our emotional life is in the sense of smell, or, more precisely, in the olfactory lobe, the cells that take in and analyze smell. Every living entity, be it nutritious, poisonous, sexual partner, predator or prey, has a distinctive molecular signature that can be carried in the wind. In those primitive times smell commended itself as a paramount sense for survival.

From the olfactory lobe the ancient centers for emotion began to evolve, eventually growing large enough to encircle the top of the brainstem. In its rudimentary stages, the olfactory center was composed of little more than thin layers of neurons gathered to analyze smell. One layer of cells took in what was smelled and sorted it out into the relevant categories: edible or toxic, sexually available, enemy or meal. A second layer of cells sent reflexive messages throughout the nervous system telling the body what to do: bite, spit, approach, flee, chase.10

With the arrival of the first mammals came new, key layers of the emotional brain. These, surrounding the brainstem, look roughly like a bagel with a bite taken out at the bottom where the brainstem nestles into them. Because this part of the brain rings and borders the brainstem, it was called the "limbic" system, from "limbus," the Latin word for "ring." This new neural territory added emotions proper to the brain's repertoire.11 When we are in the grip of craving or fury, head-over- heels in love or recoiling in dread, it is the limbic system that has us in its grip.

As it evolved, the limbic system refined two powerful tools: learning and memory. These revolutionary advances allowed an animal to be much smarter in its choices for survival, and to fine- tune its responses to adapt to changing demands rather than having invariable and automatic reactions. If a food led to sickness, it could be avoided next time. Decisions like knowing what to eat and what to spurn were still determined largely through smell; the connections between the olfactory bulb and the limbic system now took on the tasks of making distinctions among smells and recognizing them, comparing a present smell with past ones, and so discriminating good from bad. This was done by the "rhinencephalon," literally, the "nose brain," a part of the limbic wiring, and the rudimentary basis of the neocortex, the thinking brain.

About 100 million years ago the brain in mammals took a great growth spurt. Piled on top of the thin two-layered cortex—the regions that plan, comprehend what is sensed, coordinate movement— several new layers of brain cells were added to form the neocortex. In contrast to the ancient brain's two-layered cortex, the neocortex offered an extraordinary intellectual edge.

The Homo sapiens neocortex, so much larger than in any other species, has added all that is distinctly human. The neocortex is the seat of thought; it contains the centers that put together and comprehend what the senses perceive. It adds to a feeling what we think about it—and allows us to have feelings about ideas, art, symbols, imaginings.

In evolution the neocortex allowed a judicious fine-tuning that no doubt has made enormous advantages in an organism's ability to survive adversity, making it more likely that its progeny would in turn pass on the genes that contain that same neural circuitry. The survival edge is due to the neocortex's talent for strategizing, long-term planning, and other mental wiles. Beyond that, the triumphs of art, of civilization and culture, are all fruits of the neocortex.

This new addition to the brain allowed the addition of nuance to emotional life. Take love. Limbic structures generate feelings of pleasure and sexual desire—the emotions that feed sexual passion. But the addition of the neocortex and its connections to the limbic system allowed for the mother-child bond that is the basis of the family unit and the long-term commitment to childrearing that makes human development possible. (Species that have no neocortex, such as reptiles, lack maternal affection; when their young hatch, the newborns must hide to avoid being cannibalized.) In humans the protective bond between parent and child allows much of maturation to go on over the course of a long childhood—during which the brain continues to develop.

As we proceed up the phylogenetic scale from reptile to rhesus to human, the sheer mass of the neocortex increases; with that increase comes a geometric rise in the interconnections in brain circuitry. The larger the number of such connections, the greater the range of possible responses. The neocortex allows for the subtlety and complexity of emotional life, such as the ability to have feelings about our feelings. There is more neocortex-to-limbic system in primates than in other species—and vastly more in humans—suggesting why we are able to display a far greater range of reactions to our emotions, and more nuance. While a rabbit or rhesus has a restricted set of typical responses to fear, the larger human neocortex allows a far more nimble repertoire—including calling 999. The more complex the social system, the more essential is such flexibility—and there is no more complex social world than our own.12

But these higher centers do not govern all of emotional life; in crucial matters of the heart—and most especially in emotional emergencies—they can be said to defer to the limbic system. Because so many of the brain's higher centers sprouted from or extended the scope of the limbic area, the emotional brain plays a crucial role in neural architecture. As the root from which the newer brain grew, the emotional areas are intertwined via myriad connecting circuits to all parts of the neocortex. This gives the emotional centers immense power to influence the functioning of the rest of the brain— including its centers for thought.