How much time do you have? Your question is perilously close to meaningless, for lacking 1) a time machine. 2) adequate historical records. 3) Phsychological reports. 4) Physical evidence,
almost everything we say about the origin and original nature of these myths is conjectural.
Hastein is about right, take a walk in the woods. Its amazing how your imagination can run away with you. If you then, for example, combine that with evidence based ideas, such as the skeletons mentioned by great Bluehead, thats enough to cover some begginings. Another would be the origins of giants. It seems taht there are some exceptionally large old time corpses around, ie well over 6 ft. they dug up quite a few in east central asia a few years ago, and I believe there is evidence that there were some in Judah bothering DAvid 3,000 years ago. So, all it takes is 20 generations of transmission over 2,000 miles, a migrations, an invasion, a round of plague, 3 famines and you have something like what we have today.
Another example would be creation myths. I cant be bothered to go hunting them up just now, but a lot of them are based in specific localities, and rely upon local vagaries of climate, flora and fauna. Imagine that transmitted along trade routes. Anotehr category of story is the one whereby a place name is know, and it means something, but a story is related about why it is called that. I re-read Culhwch and Olwen, a story from the MAbinogion yesterday, and it includes an onomatopeic place name, Mesur-y-Peir, which means in its parts" measure of the cauldron" as being a palce where King Arthur came ashore after invading Ireland and taking the magical Cauldron of the Irish King. Now, the place may have been named that because of some other reason, and along came a bard and thought hed weave it into a grander tale to make the place seem more important. Or maybe some chieftain did come ashore there with some booty from Ireland. Or something. But we cannot really tell.