In college I remember coming across something about this in one of my lab courses. There was an experiment (I'm trying to find it and coming up empty), done with both children and adults, where the subjects would be rapidly shown dozens of card with random images on them. All the images had to do with nature and animal life. Their brains were monitored for certain fear signals, and without fail in both groups, their brains spiked whenever an image of a spider, snake, or a similarly shaped object appeared.
One could argue that an adult may have conditioned themselves to be afraid of snakes and the like, but the fact that children under the age of 5 were showing identical brain activity would suggest and an instinctual underlying rather than a learned behavior. That's not to say every person will fear the animals the same or can't overcome their fear of them. People in India overcome thing by conditioning their children early on to play with and not fear snakes. They sew the mouth of something like a cobra shut and then throw it in with the kid.
Edit: Just opened the link Elite posted, and I'm pretty sure I'm talking about the same thing.