Depression may have evolved as an adaptive response to signals requiring increased inflammation.

“So stress becomes reliably associated with inflammation. Inflammation induces sickness. But sickness and depression share a lot in common, so over time, what happens is that anything that signals a need for increased inflammation activates a suite of behaviors that in humans over time, also sort of evolves into depression. So it’s an even deeper way of saying that in fact, the link between inflammation and depression may be deeper across evolutionary time and for adaptive purposes, than it actually is in terms of mechanism.”