From “Mass Animal Deaths: An Environmental Whodunit,” by James Gorman, New York Times, Jan. 9, 2011:
Michael Shermer, the founding publisher of Skeptic magazine and a Scientific American columnist …uses a common scenario to explain why we believe in things that may not be there — hominids on the savannah hearing a rustling in the tall grass. The one who thinks, “It’s a lion!” and escapes quickly survives to propagate her genes, thus fostering a kind of protective alarmism in her descendants. Another might think, “There’s always some kind of rustling in the tall grass, it’s probably the wind,” and keep on grooming. If he guesses wrong, the downside is being eaten by the lion. Thus, no offspring and no propagation of the “don’t worry, be happy” genes.
Of course, people have both modes of thought, perhaps because rustling is usually caused by the wind, and the hominid who is too alarmist is always running away from nothing and probably too exhausted and too anxiety-ridden to mate. So there’s room for both the wind and the lion in human minds.