http://therumpus.net/2016/01/the-limbic-system-roundup/

"There’s another theory that the fear is simply handed down, like DNA, an archaic notion of defending turf, an accumulation of generations. It’s something our ancestors had control over on the chaotic plains, where wind, drought, fires, tornadoes, hail, and surprise visits from frost had little to halt them. A struggling human is often bent upon the little scratch of power he or she has."