Depression is a predictable response to certain types of life occurrences, such as loss of status, divorce, or death of a child or spouse. These are events that signal a loss of reproductive ability or potential, or that did so in human′s ancestral environment. Depression can be seen as an adaptive response, in the sense that it causes an individual to turn away from the earlier (and reproductively unsuccessful) modes of behavior.

A depressed mood is common during illnesses, such as influenza. It has been argued that this is an evolved mechanism that assists the individual in recovering by limiting his/her physical activity.The occurrence of low-level depression during the winter months, or seasonal affective disorder, may have been adaptive in the past, by limiting physical activity at times when food was scarce.It is argued that humans have retained the instinct to experience low mood during the winter months, even if the availability of food is no longer determined by the weather.

http://en.wikipedia.org/wiki/Depression_(mood)

Dr. Henry Emmons, M.D:

The first thing, and if I could tell people to do just one thing for depression and for anxiety it would be this - exercise.We all know it. We all know it′s good for us and it truly is. It is so good for the brain as well as the rest of the body, and it is as helpful if not more helpful than medication for most kinds of depression, for all but the most serious kinds of depression.So people who have, for example, who have a genetic predisposition to depression or anxiety, if they go for a period of time of sleeping poorly or not sleeping enough, it′s the most common immediate trigger for their illness.

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