Why Mental Depression Evolved in Humans

Humans can get depressed. So can mammals, and that's about it as far as I know, of the animals that can get depressed. There may be more. So why do we get depressed, why did we evolve this negative mental condition. Well here's my theory. We live in a negative ion world. We "feel more energised" in a negative ion environment than in a positive ion environment. In fact, negative ion generators are available that purport to elevate your mood and your activity. So why does a positive ion environment make us lethargic. Here's my theory. When the weather turns cloudy and rainy, the ion environment turns from negative to positive, making us lethargic. So why is this beneficial to humans. Well let's say that the weather seems a bit cloudy and our ancestor "Claude" doesn't feel like leaving the cave to hunt today. The positive ions made him lethargic. His buddies, don't seem at all bothered by the change in the weather, and in fact a eager for the hunt. So Claude stays in the cave, and his buddies go out for the hunt. Claude's buddies get caught in a storm, catch cold, then pneumonia and die. Claude's "positive ion genes" survive (as do all the other "Claudes" in other caves,) and are passed into the gene pool as a predisposition to lethargy when the ion environment turns positive.