Having learned about depression briefly during health and psychology classes, I was always taught that it was a result of multiple factors including brain chemistry imbalance, social factors, and mental factors. I was never taught about, nor did I ever consider, that depression could be thought of as a sort of defense mechanism from an evolutionary standpoint.
Of course, any illness is usually a result of multiple factors rather than just one. Stress-related illnesses, which can be due to eating habits, sleep, peace of mind, and exercise, are a perfect example of this. I've always been taught that depression is not ever the result of one factor. The social, mental, and biological factors make perfect sense to me, but when I first began reading that depression could be due to evolution, I must admit I was skeptical. How could people possibly have ever benefited in any way from the effects of depression? I suppose it makes sense though. The reading discusses the possibility that in situations where "winning" was not an option, it would be futile, and possibly dangerous, for humans to even attempt a task. This might lead to the feeling of hopelessness and unwillingness to put forth effort that is associated with depression. Perhaps it was a defense mechanism, a way of exchanging one very unpleasant outcome for a slightly less unpleasant one.
Why, then, is depression so prevalent and detrimental to us now? Like the fight-or-flight response, it could have served a very useful purpose in history, protecting us from negative situations. In our modern-day world, we do experience many more stressors though. Instead of helping us to "yield in a hierarchy conflict," these feelings of pessimism could arise simply from a situation in which we feel bullied or are in some way made to feel inadequate. I imagine these types of situations arise more often for humans now than they did back then, so this response of a low mood and possibly depression is perhaps overused. The best thing we can do right now is to do our best to handle the depression-causing factors that are within our control. In this way, we can exercise more control over other factors and be rewarded with better, happier moods. This is exactly the same way we discussed dealing with stress, and I suggest it because the evolution of stress and depression seem extremely similar to me.