A question is not a claim. The question does make the assumption that there is an evolutionary explanation but does not specify the nature of that explanation. There is in fact ample evidence of innate constraints on fear learning which pre-dispose primates, including humans, to acquire some types of fears more readily than others. This research has been applied to phobias specifically, and some of it has been applied specifically to common phobias for animals such as snakes.
That's interesting. My understanding is there is a huge gap between observing a behavior in humans and saying there is an underlying evolutionary mechanism for promoting that behavior.
I would be willing to take a look at the fossil evidence though.