Because our ancestors were far more exposed to danger than we are, humans have acquired genetically influenced phobias. The five most common phobias are the fears of spiders, snakes, heights, germs and having a panic attack in public. The latter makes sense once we take into account how much humans relied on their social standing and reputation amongst their tribe. Those who were afraid of these stimuli would be more likely to avoid them effectively, and thus survive, passing on those phobia genes. Take note that these ancient fears are so ingrained into humans that having phobias of modern dangerous inventions such as guns, motorcycles or electricity are far less common.