Over time these folk tales have been subtly changed and have evolved just like an biological organism. Because many of them were not written down until much later, they have been misremembered or reinvented through hundreds of generations.
By looking at how these folk tales have spread and changed it tells us something about human psychology and what sort of things we find memorable.
Jamie Tehrani quoted by Richard Gray at The Telegraph. Fairy tales have ancient origin=
Popular fairy tales and folk stories are more ancient than was previously thought, according research by biologists.
(via protoslacker)