"Depression's Evolutionary Roots"
My line of thought is this: Depression is an extreme example, a pathological inability to escape from what is a perfectly legitimate state of mind when temporary.
We have a brain that is sort of balanced between different impulses, different emotions, essentially different gears that it can shift into to deal with different problems from different perspectives. Temporary depression is a natural response to failures and tragedies. Like they say, it forces us into a more contemplative state of mind, where we examine things more closely. It's when this response is THE constant state of mind, the default operating condition of thought that we have a problem, for other states of mind have their value, too.
