Speaking as a Canadian, I believe it was from CBC Radio that I learned of the theory that we humans evolved the ability to get depressed because back in the good old cave-dwelling days, losing your appetite, hunkering down, and sleeping nearly all day long for months on end was an effective way of getting through the winter. Russians, Scandinavians, Scots, and yes, even we Canadians, are all reputed dour and gloomy in our own characteristic national ways, whereas tropical peoples are stereotypically cheerful. An interesting counter-example is the case of the Inuit, the most polar yet most cheerful of peoples. No sense in them hunkering down and conserving food stored from the harvest--they have to hunt seals when they can, no matter how cold and dark it gets.