The mystery of depression is not that it exists — the mind, like the flesh, is prone to malfunction. Instead, the paradox of depression has long been its prevalence. While most mental illnesses are extremely rare — schizophrenia, for example, is seen in less than 1 percent of the population — depression is everywhere, as inescapable as the common cold… The persistence of this affliction — and the fact that it seemed to be heritable — posed a serious challenge to Darwin’s…evolutionary theory. If depression was a disorder, then evolution had made a tragic mistake, allowing an illness that impedes reproduction — it leads people to stop having sex and consider suicide — to spread throughout the population…
The alternative, of course, is that depression has a secret purpose and our medical interventions are making a bad situation even worse. Like a fever that helps the immune system fight off infection — increased body temperature sends white blood cells into overdrive — depression might be an unpleasant yet adaptive response to affliction.
Full article on the upside of depression.