This article comes at the question of "Why Depression?" from a slightly different angle than did Dr. Steve Ilardi. The premise here is that depression must create an evolutionary advantage, or else depressive tendencies would have been bred out.
The working theory is that a depressive episode allows for rumination, or obsessing about a particular complex problem, which then allows the depressive to break that complex problem into its component parts. These parts would then be potentially solvable.
Maybe shamans were depressives who used mind-altering substances to enhance their monomaniacal ruminations and solve the grateful clan's problems.
Or maybe the theory is flawed; maybe depression is like Huntingdon's Disease and doesn't fully manifest its evolutionary non-viability until after individual fecundity ends.