It's being adapted into a sci-fi movie. For those who don't want to watch it....basically, vampires evolved alongside humans. And like apex predators (Lions) there aren't a lot of them in relation to their prey (gazelles). They were superior in all aspects to humans, especially intelligence, and hunted humans. As humans evolved and became civilized, they realized certain shapes (right angles) would trigger vampires giving them seizures (thus holding up a cross to a vampire). Eventually the vampires died out and were replaced by humans as the top species on earth.