Euhemerus, the 4th Century BCE Greek author, espoused the idea known as Euhemerism that gods and other figures in mythology were originally people who were magnified by the retelling of their stories until they assumed supernatural proportions. This is not a viable approach among scholars today.

One need not have a real seed to yield stories pertaining to the supernatural. People interpret unusual phenomena in their surroundings by looking for extraordinary explanations, and this often causes people to recall supernatural entities in their culture's pantheon. This act of recalling, combined with the retelling of the stories, are sufficient to account for the stories that people tell, internationally, to explain what the supernatural is all about and how it acts - and interacts with people. It's a surprising process on one level, I suppose, until we remember that we are dealing with people. And people are capable of just about anything.

At the moment I am working with Elisabeth Hartmann’s 1936 Die Trollvorstellungen in den Sagen und Märchen der Skandinavischen Völker – The Troll Beliefs in the Legends and Folktales of the Scandinavian Folk, which deals with some of these questions. I also recommend Alan Dundes, The Study of Folklore. Both these sources are a bit old, but while the question you ask is a reasonable one, it has also been around the block for a while (for over two thousand years if we include Euhemerus). In the case of finding an answer to your question, some of the classic mid-twentieth-century publications are best.