The preparedness hypothesis is the belief that humans have a tendency, based on natural selection, to fear things that were a source of danger to our ancestors. This tendency also causes us to react before giving very much thought of it.
For instance, if cleaning a shed or working in the yard you encounter a spider or a snake the first reaction is back off. Likewise, if you are out hiking and spot a bear, lion, or wolf the first reaction is to get away quickly. These automatic reactions are believed to have developed over centuries or millennia as a means of ensuring safety.