Before all else, there was terror. The terror of what goes bump in the night, the terror of that screeching sound outside the entrance of the cave, the terror of the fire from the sky, overwhelming terror. Terror is a truly primal emotion. For as much as an emotion could ‘start’, it probably started as a survival instinct innate in all animals once they were born, but has evolved along with life its self.
To clarify one point: I see an intrinsic and etymological difference between terror and horror. When I use the word horror, I mean the kind of blood and gore one might read in the literary equivalent of a slasher movie; horror doesn’t need a motive in the same way terror does, because horror can be an external thing. Horror is connected with a certain sense of disgust; a connotation terror doesn’t have in the same way.
When I think of terror, I think of something much more personal, more internal. Terror is finding out all your thoughts have been broadcast for the world to see; terror is a loss of control. What terrifies any one person can vary, but there are definite themes I’ve seen. Personally, loss of control is a big one, but there’s also terror revolving around family relationships, terror in the ambiguous, and, so often, terror in the unknown.