As usual, orginal text in black and comments in blue.

Paul Keedwell on the good that depression can bring | Society | The Guardian

Here is some of the text:

To explain why depression has not been "bred out" through Darwinian natural selection, theories have suggested that rather than being a defect, depression could be a defence against the chronic stress that misguided people can put themselves under. It is possible that depression defends us against the tendency to deny our true needs by chasing unobtainable goals and helps to bring these needs into sharper focus. More specifically, the proposed benefits are as follows: removal from a stressful situation, introspection, problem solving, the development of a new perspective, and reintegrating this with the community upon recovery.
This sequence is repeated in the legends, myths, tribal beliefs, and religions of the world that tell the story of solitary exploration in adversity leading to personal growth.
Depression may bring about a "rebirth" because it removes self-delusion. There is some evidence from scientific studies to show that depressed people are rather more realistic in their thinking than "healthy" individuals - the phenomenon of "depressive realism". It prompted the scientific journalist Kyla Dunn to write: "One cognitive symptom of depression might be the loss of optimistic, self-enhancing biases that normally protect healthy people against assaults to their self-esteem. In many instances, depressives may simply be judging themselves and the world much more accurately than non-depressed people, and finding it not a pretty place."
With recovery, and with the lifting of mood, a new kind of truth could emerge that lies somewhere between the overly optimistic and the overly negative. The new truth would be devoid of blind optimism: a more humble assessment of the depressed person's own capability, containing a more balanced picture of his or her perceived strengths and limitations.
Other functions have been proposed by evolutionary psychologists. Ed Hagan has suggested that getting depressed is like going on strike for better pay and conditions - it is a way of making the wider social network more aware of your needs. Paul Watson has suggested that the sobering quality of depression makes us more aware of any changes that might need to occur in that social network.
Coping strategies
Depression may have forced our ancestors to look again at their strengths and their limitations, their coping strategies, their direction, their priorities, their supports. Regardless of the reason for falling into depression, the journey has the potential to make us better equipped, in a general sense, for life. If we are too busy to think and feel, to be mindful, depression might represent the first opportunity to take an honest inventory of ourselves. If the modern world prevents us from learning from depression, perhaps it is the fault of the modern world and not this ubiquitous human condition.

Don't really agree with all this as it doesn't seem to have a fundamental basis which "depression" leverages off.

Depression is likely to be more of a survival mechanism. That’s why people suffer from SAD (Seasonal affective disorder). When the night closes in and winter arrives depression creates the need to stay close to the group. When its sunlight and summer the elation allows the ability to travel further from the clan. Women suffer more from depression as it’s a submissive mechanism which affords protection to the expression of vulnerability as well as the obvious staying close to the clan.

Closely bound clans very likely had far better survival rates. The death of clan members creates depression bringing the clan closer and more likely to work together = survival.

Depression as an analogy is like vertigo... if you're not scared of heights you’re more likely to die... same to with venturing too far from the clan.

I think too much psychology focuses on the near term of human development, we've been around for near 1 million years or so... we are far more animal past than modern man.