Depression:

via Evolution and Psychiatry:

"Social competition and rank theories (Price et al, 1994) propose that depression is an adaptive strategy for coping with a decline in social standing or rank. The depressed state is thus conceptualized as a subordinate state that signals both submission to more powerful adversaries and may help in eliciting help from others."

"The analytical rumination hypothesis (Andrews & Anderson Thomson, Jr, 2010) proposes that the depressed state may facilitate solving social dilemmas and decision making."

"Nesse (2019) has proposed that depression facilitates withdrawal and disengagement from unattainable goals with clinical depression arising when the goals are too important to be abandoned."

"Rantala et al (2018) have addressed the clinical heterogeneity within depression and suggested that, based on an evolutionary framework, major depressive disorder consists of twelve different conditions with proximate and ultimate causes, including infection, long-term stress, hierarchy conflict, grief, loneliness, traumatic experiences, post-partum events, romantic rejection, seasonal factors, chemicals, somatic disease and starvation."