Depression is a common and debilitating mental disorder. Estimates suggest that depression affects 5% of the global population, or 350 million people, at any given time and that it is the second leading cause of disability. Furthermore depression is a condition that exists in all age groups, although more prevalent in the aged.
Some evolutionary theorists have suggested mechanisms by which biological malfunctions leading to depression today. For example, it is noted that modern humans exist in physical environments that are radically different from those in which we evolved, and that our new environments interact with our ancient genomes in maladaptive ways.
Evolutionary adaptive explanations of depression share the idea that symptoms occur in response to an environmental stressor. Most hypotheses include a concept of energy limitation, and an attempt to overcome a failure, danger, or shortage of resources. Several hypotheses suggest that depression may be a means to conserve energy during an adverse situation.
Others have suggested that depression may function to help an individual accept their defeat, and signal their submission in order to avoid further enforcement from more dominant individuals. Or that depression may have evolved as a means for one to solicit additional resources from one‘s social network as a cry for help,
I think it is much simpler than that. Depression as we know it was a much less likely state of being in our distant ancestors - otherwise it would have been deselected from our genome. Hominids mission, like all creatures, was to procreate and their main business was eating - not for pleasure but for survival.
This is the key to the emergence of abnormal mood states in civilization today: we eat for pleasure and subsequently the content of that radically different diet causes disease states, including depression.