When you look at the life span of the human species, we're still pretty young.
It wasn't so long ago that our ancestors faced threats from disease and predators.
They evolved over time to be best-prepared for threats, and emotions are part of the mix.
But the set of emotions for surviving a hostile world may not be the best emotions for today's world. Psychologist Jonathan Rottenberg explores the ways in which our evolution work against us, in his book The Depths: The Evolutionary Origins of the Depression Epidemic.