Could depression be a form of adaptation?

The most recent theory on depression is that it could be some type of adaptation. I have written regarding this idea before in a post titled Evolutionary origins of the depression epidemic. The author of the book in that previous post was making a similar argument. He believed that depression was not a pathological condition but a way of adapting to stressful conditions. For example if a person lost a job and became depressed the depression could perhaps be a way of conserving energy or the depression might be suggesting that a person should change their goals.

In that previous post I wrote that depression in its milder forms could conceivably be seen as an adaptation however in its more severe forms not very likely. After all severely depressed people have problems functioning on a very basic level and aren't analyzing problems in the most objective or productive manner. Often depression seems to reduce flexibility and brainstorming. The only solution for many severely depressed people is suicide. It is more accurate to say that majorly depressed people obsess more than they analyze.

I, for example, have experienced chronic depression for most of my life and while being rather obsessive and analytical still have not gained  much insight into why I have felt depressed for most of my life. I have experienced many periods where I felt normal and quite productive but it wasn't necessarily due to some kind of insight. Those normal periods just happened and sometimes they occurred in a diurnal pattern or in a menstrual pattern which makes no sense in regard to the theory. Why would one person feel severely depressed in the morning and normal at night for a number of days? Why would I often feel normal to euphoric when I had my period? Various levels in hormones and their effect on neurotransmitters seem more likely.

This most recent research is suggesting the adaptation theory once again only instead of conserving energy depression focuses our energy obsessively on one thing and supposedly solves it.

The research suggests that depression may be a natural condition in which the mind concentrates involuntarily on a complex issue to the point where it allocates resources to analyzing the problem at hand, diminishing concentration on other aspects of living, perhaps giving rise to disrupted eating, sleeping and social interaction that are associated with depression.

This theory seems to assume the idea that depression is due to excessive stress which is debatable. One can feel more stressed pursuing a goal and yet have a reduced level of depression.  For example, I have given up on a number of goals which I found stressful and have not noticed any positive change in my level of depression. In fact, I felt worse for giving up on my goal. A proponent of this theory might also argue that I haven't made the right changes in my life. How does one falsify this theory if this is their answer. Additionally, the proponents of this theory keep pointing out that there is no conclusive evidence of pathology in major depression. True but it doesn't necessarily follow that pathology won't be discovered in the future. The fact that depression increases the likelihood of dementia is sufficient reason to keep looking for pathology in regards to depression.