What are some of these real or perceived stressors with which we continually do battle? Coping with rush-hour traffic, job and financial difficulties, troubled relationships, and family problems are just a few of hundreds of stressful stimuli that can be identified.

Anxiety over personal problems (will I be able to pay the rent this month?), or more global concerns (will there be another war?) is another type of stress that we all encounter much too often.

Nonetheless, anxiety and these other stressors are not immediate threats to survival, even if they do raise our blood pressure a bit now and then. Of greater concern is that the internal defense mechanisms of the body respond to these types of psychological stimuli in the same way as they would respond to life-threatening ones.

Why is this unfortunate? Because over the long haul, excess release of potent stress-fighting factors like the adrenal-gland hormones cortisol and epinephrine (also known as adrenaline) can suppress the immune system, cause ulcers, produce muscle atrophy, elevate blood sugar, place excessive demands on the heart, and eventually lead to the death of certain brain cells.

A person in the midst of a divorce does not require the hormonal, neuronal, and metabolic responses of someone who falls through thin ice on a wintry pond--yet in both cases the same internal changes are occurring.

Why do emotionally stressful events elicit the same chemical changes in our bodies as do events that are actual threats to survival? The answer may lie in a comparison of stress as we know it today and stress as it must have been when vertebrate animals were first evolving.

Are we really any more "stressed out" than our prehistoric ancestors? Presumably not, since the defense mechanisms that developed in mammals like ourselves did so very early in the evolution of life. We even see similar biological responses to stress in non-mammalian vertebrates like birds and reptiles.

These defenses consist of hormonal and neuronal signals that increase breathing, accelerate heart rate, increase blood pressure, increase the liver's ability to pump sugar into the bloodstream, and open up blood vessels in the large muscles to maximize the delivery of nutrients and oxygen.

The net effect is an animal that has lots of fuel in its blood, a more forceful heart to pump the blood around, plenty of oxygen, and efficient muscles. For an antelope in the wild that has spotted a nearby lion, these changes are exactly what the antelope needs to avoid becoming a meal.

Not surprisingly, then, animals evolved internal mechanisms to combat the stresses of infection, starvation, dehydration and pain, to name a few. Cortisol breaks down bone, muscle, fat, and other body tissues to provide material for the liver to convert into sugar. This sugar, essentially formed by the body's own self-digestion, can supply the needs of the heart and brain during a crisis. The natural pain-killer endorphin developed to combat severe pain.

Picture the antelope being attacked by the lion, but escaping to live another day. Its endorphin would allow the animal to cope with the pain of its wound, if only temporarily, and continue with the herd. Other hormones enable the kidney to retain more water than normal during periods of drought and dehydration.

All of these varied measures are short-term responses to very different types of stress, but they act in a concerted way to give an organism a fighting chance to get back on its feet.

Imagining the types of stress our paleolithic forebears must have encountered makes our daily aggravations seem much less overwhelming. Prior to the advent of agriculture, the typical cave-dweller would rarely have had the luxury of a steady and nutritious diet. On the contrary, malnutrition, vitamin and mineral deficiencies, even starvation would have been extremely common in the winter months, and sporadic dehydration from lack of clean or available water may have been common in the summer.

Hypothermia was a constant threat in the winter, especially in northern climes during the many ice ages. Injuries and infections that resulted from untreated minor wounds or parasite invasion would not only have been physiologically stressful but often lethal. Anthropological data suggest that our ancestors suffered many of the same maladies that continue to plague us today (arthritis, back problems, tooth decay, osteoporosis, to name a few).

However, as stressful as those conditions are for modern man, they would have been far more stressful at a time when no medical treatment of any kind was available.

What about the other type of stress that is not life-threatening, but is perceived to be of potential danger? When the antelope spotted the lion, there was not yet physical damage to the antelope's body. Nonetheless, the hormonal systems responded as if the damage was already done, in anticipation of impending doom. If the crisis were luckily averted, a complex system of hormonal feedback loops would apply a brake on the stress response to prevent unabated secretion of cortisol and other stress hormones.