One early spring day I was driving along a highway over a mountain pass in Colorado, when a snow flurry suddenly blotted out the car a few lengths ahead of me. As I peered ahead I couldn't make out anything; the swirling snow was now a blinding whiteness. Pressing my foot on the brake, I could feel anxiety flood my body and hear the thumping of my heart.

The anxiety built to full fear: I pulled over to the side of the road, waiting for the flurry to pass. A half hour later the snow stopped, visibility returned, and I continued on my way—only to be stopped a few hundred yards down the road, where an ambulance crew was helping a passenger in a car that had rear-ended a slower car in front; the collision blocked the highway. If I had continued driving in the blinding snow, I probably would have hit them.

The caution fear forced on me that day may have saved my life. Like a rabbit frozen in terror at the hint of a passing fox—or a protomammal hiding from a marauding dinosaur—I was overtaken by an internal state that compelled me to stop, pay attention, and take heed of a coming clanger.

All emotions are, in essence, impulses to act, the instant plans for handling life that evolution has instilled in us. The very root of the word emotion is motere, the Latin verb "to move," plus the prefix "e-" to connote "move away," suggesting that a tendency to act is implicit in every emotion. That emotions lead to actions is most obvious in watching animals or children; it is only in "civilized" adults we so often find the great anomaly in the animal kingdom, emotions—root impulses to act— divorced from obvious reaction.

In our emotional repertoire each emotion plays a unique role, as revealed by their distinctive biological signatures (see Appendix A for details on "basic" emotions). With new methods to peer into the body and brain, researchers are discovering more physiological details of how each emotion prepares the body for a very different kind of response:

• With anger blood flows to the hands, making it easier to grasp a weapon or strike at a foe; heart rate increases, and a rush of hormones such as adrenaline generates a pulse of energy strong enough for vigorous action.

• With fear blood goes to the large skeletal muscles, such as in the legs, making it easier to flee— and making the face blanch as blood is shunted away from it (creating the feeling that the blood "runs cold"). At the same time, the body freezes, if only for a moment, perhaps allowing time to gauge whether hiding might be a better reaction. Circuits in the brain's emotional centers trigger a flood of hormones that put the body on general alert, making it edgy and ready for action, and attention fixates on the threat at hand, the better to evaluate what response to make.

• Among the main biological changes in happiness is an increased activity in a brain center that inhibits negative feelings and fosters an increase in available energy, and a quieting of those that generate worrisome thought. But there is no particular shift in physiology save a quiescence, which makes the body recover more quickly from the biological arousal of upsetting emotions. This configuration offers the body a general rest, as well as readiness and enthusiasm for whatever task is at hand and for striving toward a great variety of goals.

• Love, tender feelings, and sexual satisfaction entail parasympathetic arousal—the physiological opposite of the "fight-or-flight" mobilization shared by fear and anger. The parasympathetic pattern, dubbed the "relaxation response," is a body wide set of reactions that generates a general state of calm and contentment, facilitating cooperation.

• The lifting of the eyebrows in surprise allows the taking in of a larger visual sweep and also

permits more light to strike the retina. This offers more information about the unexpected event, making it easier to figure out exactly what is going on and concoct the best plan for action.

• Around the world an expression of disgust looks the same, and sends the identical message: something is offensive in taste or smell, or metaphorically so. The facial expression of disgust—the upper lip curled to the side as the nose wrinkles slightly—suggests a primordial attempt, as Darwin observed, to close the nostrils against a noxious odor or to spit out a poisonous food.

• A main function for sadness is to help adjust to a significant loss, such as the death of someone close or a major disappointment. Sadness brings a drop in energy and enthusiasm for life's activities, particularly diversions and pleasures, and, as it deepens and approaches depression, slows the body's metabolism. This introspective withdrawal creates the opportunity to mourn a loss or frustrated hope, grasp its consequences for one's life, and, as energy returns, plan new beginnings. This loss of energy may well have kept saddened—and vulnerable—early humans close to home, where they were safer.

These biological propensities to act are shaped further by our life experience and our culture. For instance, universally the loss of a loved one elicits sadness and grief. But how we show our grieving —how emotions are displayed or held back for private moments—is molded by culture, as are which particular people in our lives fall into the category of "loved ones" to be mourned.

The protracted period of evolution when these emotional responses were hammered into shape was certainly a harsher reality than most humans endured as a species after the dawn of recorded history. It was a time when few infants survived to childhood and few adults to thirty years, when predators could strike at any moment, when the vagaries of droughts and floods meant the difference between starvation and survival. But with the coming of agriculture and even the most rudimentary human societies, the odds for survival began to change dramatically. In the last ten thousand years, when these advances took hold throughout the world, the ferocious pressures that had held the human population in check eased steadily.

Those same pressures had made our emotional responses so valuable for survival; as they waned, so did the goodness of fit of parts of our emotional repertoire. While in the ancient past a hair-trigger anger may have offered a crucial edge for survival, the availability of automatic weaponry to thirteen- year-olds has made it too often a disastrous reaction.