Part of the reason that children are afraid of the dark may be that, in our entire evolutionary history up until just a moment ago, they never slept alone. Instead, they nestled safely, protected by an adult - usually Mom. In the enlightened West we stick them alone in a dark room, say goodnight, and have difficulty understanding why they’re sometimes upset. It makes good evolutionary sense for children to have fantasies of scary monsters. In a world stalked by lions and hyenas, such fantasies help prevent defenseless toddlers from wandering too far from their guardians. How can this safety machinery be effective for a vigorous, curious young animal unless it delivers industrial-strength terror? Those who are not afraid of monsters tend not to leave descendants.
- Carl Sagan, The Demon-Haunted World (1997)