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Imagine you are driving on a mountain road. You enjoy the greenery and the great view opening from the mountain, but the heavy dark clouds are gathering above you anticipating the tropical storm. You get a little anxious but decide to continue driving up your path. Then, all of a sudden, the bright lightning blinds you, and, seconds later, you hear loud thunder somewhere very close. The strong wind blows and throws your car off course. You grab the steering wheel tightly as you become more anxious.

You slow down but continue driving, feeling your heart beating fast. The tropical rain starts pouring really hard. You can’t see the road as you press your foot on the brake. Trying to control the overwhelming feeling and repeating to yourself that it’s just the rain, you feel fear flood your entire body. Unable to control it, you pull over to the side of the road, waiting for the storm to pass.

A half an hour later as the rain stops and visibility returns, you continue on your way—only to be stopped a few hundred yards down the road, where an ambulance crew is helping a passenger in a car that had rear-ended a slower car in front; the collision blocked the highway. If you had continued driving in the blinding storm, you probably would have hit them.

The fear that you experienced saved your life. Like a rabbit frozen in fear of the fox—or a first human hiding from a dinosaur—you were overtaken by an internal state that made you stop, pay attention, and hide from the coming danger. All emotions are, in essence, impulses to act that evolution has set up in us. Each emotion plays a unique role. With new methods to look into the body and brain, researchers are discovering more 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 attack the enemy. When we experience anger, our heart rate increases and a rush of hormones such as adrenaline generates a pulse of energy strong enough for fast action.

With fear blood goes to the large muscles, such as in the legs, making it easier to run away. At the same time, the body freezes, if only for a moment, perhaps allowing time to think whether hiding might be a better reaction. The brain’s emotional center triggers hormones that put the body on general alert, making it edgy and ready for action, and attention sharpens.

Among the main biological changes in happiness is an increased activity in the brain center that stops negative feelings and increases available energy, and a quieting of those that generate worrisome thoughts. It 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 and friendship are the opposite of the “fight-or-flight” response produced by fear and anger. It is called a “relaxation response,” when a body-wide set of reactions generates a general state of calm and contentment, preparing us for cooperation.

The lifting of the eyebrows in surprise allows eyesight to cover a larger area and also permits more light. This offers more information about the unexpected event, making it easier to figure out exactly what is going on and generate the best action plan.

Around the world, an expression of disgust looks the same and sends the identical message: something is offensive in taste or smell. The facial expression of disgust—the upper lip curled to the side as the nose wrinkles slightly—suggests an attempt to

The main function of 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 enthusiasm for life’s activities and slows the body’s metabolism. This loss of energy kept early humans close to home, where they were safer.

In the past, the dangers of the world made emotional responses valuable for survival. They kept our ancestors close to home in moments of danger and produced energy to fight or escape enemies. Emotions also gave early humans opportunities to recover and restore their life power. In modern times, humans continue expressing emotions exactly in the same ways as thousands of years ago, even though they play less role in our survival these days.