Depression.

[devonin]

Originally Posted by All_That_Chaz

What possible purpose could the ability to become depressed serve, in any time?

Did you read what I said?

"Natural selection doesn't only select positive traits"

Evolution isn't a magic bullet, it doesn't have a 100% success rate in only selecting positive traits and discarding all negative traits. If the abilty of brain chemistry to react in a way that could cause depression didn't severely impact the ability of depressed things to breed, there's no reason to assume that such a particular of ability of brain chemistry to react in that way would magically evolve out.

Further, if the ability to be depressed is a necessary consequence of say...sentience, then it is -absolutely- worth taking depression in exchange. Every trait doesn't exist in a sole vacuum, to be singly taken or not. Many things come together in a suite of things that evolved together, and were accepted together as being of overall benefit to survival.