I'm pretty far out of my depth here, so I could be off-base. I think it's an evolutionary adaptation. Since the fear response is more or less initiated by the amygdala - a relatively less-derived brain structure - and only afterwards moderated by the cortex, it would seem to make sense that the reaction is a really really old one. As I understand it, perceptual cues are derived from auditory and visual input, then sent to both the cortex and amygdala. The amygdala gets it faster, however. Back when we were a small tasty insectivore the size of my hand, being able to react immediately to potential threats would be a survival benefit. As Joseph LeDoux put it,
quote:
Failing to respond to danger is more costly than responding inappropriately to a benign stimulus. (Ledoux 2002, Emotion, Memory and the Brain, SciAm Feb 2002)
IMO, the fear reaction to spiders, snakes and other things in humans is a hold-over from the time when these organisms WERE significant threats. The fact that many people can overcome them indicates that lack of familiarity - i.e., the identification of these organisms as NOT posing a threat - is why people still fear them.
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