Ludwig, A M (1983) The psychobiological functions of dissociation, *American J Clin Hypnosis, 26, 93 - 99* writes of sham death reflex (in animals with slow locomotor abilities) protecting against faster predators ... He compares this to dissociation, which like most mental capacities may be adaptive or maladaptive in nature ...
Now, a number of writers on depression have put forward views relating it to, or likening it to, a dissociative trance-like state. See Alladin A & Heap M (1991), Hypnosis and Depression, in M Heap & W Dryden (eds) Hypnosis: A handbook, publ Milton Keynes, Open Univ Press. Also Yapko MD (1992) Hypnosis and the Treatment of Depressions, Brunner/Mazel, NY....
E.g. - Depression as an inwardly-directed, symptomatic trance, with its self-limiting, self-rejecting, rigid misperceptions of reality, context-inappropriate behavior and poor rapport (Yapko who cites --) Araoz, speaking of *Negative Self-Hypnosis* - see Araoz D L (1981) Negative Self-Hypnosis, in J of Contemp Psychotherapy, 12, 1, Spring/Summer, '81, 45 - 52. See also Alladin & Heap (above) - Alladin holds that when people become very involved in negative self-suggestions & negative imagery, this process can strengthen what Hilgard has termed Subordinate Cognitive Control Structures (what Cognitive Therapy - Beck - calls Schemas); these in turn can develop varying degrees of autonomy, leading to the activity for which they are responsible passing out of voluntary control, partially or completely. It is this loss of control over emotional images which in this model is conceptualised as a dissociative phenomenon.
It may be that what is adaptive about depression is its dissociative aspects, if this model proves to be correct in at least some aspects.