In an interview, Dr. Gut, who lives in Sweden, said: ''I think that depression is a normal mechanism. It's an attempt toward adaptation to a problem.''

Dr. Nesse and Dr. Gut are not the first to ponder what evolutionary function depression -- and its close relative, sadness or low mood -- might serve. Thinkers from Schopenhauer to Freud have offered their views. Over the years, scientists have speculated variously that depression represents a plea for help, a strategy for manipulating others into providing resources, a signal of submission or yielding in conflict, or a way to conserve an organism's energy and resources in hard times. Other investigators see no point in viewing depression as anything other than a malfunction of brain chemicals.

Dr. Nesse, however, is one of a growing number of scientists who over the past decade have systematically tried to bring a Darwinian perspective to medicine, hoping to learn more about how evolution has shaped humans' vulnerability to disease.

The approach is yielding a richer understanding of illness, and in particular, is helping scientists distinguish between diseases and medical conditions that developed as defenses against other more serious threats to survival.

Coughing, for example, is not a disease in itself but is the body's attempt to rid itself of bacteria in the lungs. Diarrhea and vomiting, though unpleasant, also serve as defenses, evolving as ways to evade danger, and thus to preserve fitness. And the ability to feel pain is essential for an organism's survival. ''People who are born without any capacity to feel pain are usually dead by early adulthood,'' Dr. Nesse said.

Similarly, emotions like anxiety, fear and depression can be viewed as defensive strategies. Such defenses make sense, Dr. Nesse noted, because they have a low cost, especially when compared with their potential for protection. Vomiting, for instance, involves only the loss of few hundred calories, but might save an animal's life if the substance ingested is poisonous. In the same way, being frightened and anxious when no threat exists is a small price to pay for the readiness to fight or flee when true danger looms. Yet these tactics also sometimes go awry, and illness results: chronic pain syndromes or panic attacks, for example.

Dr. Nesse believes the need to gain a deeper understanding of what purpose depression might serve is especially pressing at a time when the illness is so prevalent. Surveys indicate that 10 percent of Americans suffer from clinical depression; millions more endure a darkness that is not severe enough to earn a diagnosis but still interferes with their lives.