As far as adaptations go, the most sensible model of depression I've ever run across is the one proposed by Howard Bloom. First, it's imperative to acknowledge

As far as adaptations go, the most sensible model of depression I've ever run across is the one proposed by Howard Bloom. First, it's imperative to acknowledge humans as primarily social creatures, always living in relation to a social context, not self-sustaining individuals. Some of our adaptations affect our functioning as members of a social body, rather than our health as individuals. Succeeding, finding a place, securing resources by trading, creating a context for ourselves within a family and/or community - all these things trigger reward mechanisms in the brain. Doing well? Biologically you'll be stimulated to do even more in a positive feedback loop. Not doing well? Failing to find your place? Then you'll be anti-stimulated towards, essentially, death. you weren't working out, no need to prolong the dysfunction. Of course, this 'adaptation' causes tremendous prolonged suffering as it plays out, in practice.