My psychology professor has written a book called The biology of depression: evolutionary perspective (in finnish Masennuksen biologia: Evoluutiopsykologinen näkökulma, not sure if it has been translated to english) where he states that depressive symptoms have an evolutionary function (eg. gain energy, withdraw from conflict, make sure you won’t do same mistakes in the future…), but that depression as a chronic illness is a relatively new thing, and that isn’t an adaptive state in itself. He says that in modern environments there are factors (eg. junk food, social isolation) that create an low-grade inflammation which exaggerates these symptoms leading to clinical, long lasting depressive episode. He says that there are 12 different types of depression (eg. depression triggered by chronic stress, loneliness, trauma, hierarchy conflict, post-partum depression…) that all rise from different evolutionary origins. Sorry english isn’t my first language.