Theory on origin of mythical peoples

When my mom was a little girl living up in Marin County, she used to hear stories of this "wild man" that lived up in the mountains. Years later she came across a book called "Ishi, the last Yahi". Ishi actually died many years before my mom was born, but like the internet "urban legends", the stories of him were transmitted from child to child long after Ishi died.

The Irish have legends about a people called the "Tuatha de Danann", or "People of the Godess Danu". (I'm pulling this off the web, I'm not Irish myself). From what I've been able to gather, it seems like the Tuatha de Danann were considered a kind of fairy folk, but the stories about them sound more like a people migrating to Ireland from somewhere else and then encountering the people who already live there.

This started me thinking. There were a lot of migrations of different peoples throughout pre-history, as shown in the archeological record. (The only one I specifically remember reading about is the "Beaker People" moving into Europe, but I'm sure there were others).

When people move from one area to another, they're going to have interactions with the people who lived there before them. As time goes on, and the original people are either absorbed or move on, these interactions will be less and less, but the stories of these interactions will live on in the oral tradition of the new people.

If the new people were much more powerful or aggressive than the original people in the area, then the original people will probably hide from the new people, and the stories will reflect this. Mythical peoples like Fairies and Leprechauns are known to be secretive and hard to catch.

On the other hand, if the two peoples are more evenly matched, there will be more violent encounters, and the stories will reflect that. I think stories of giants may be an example of this. In Roman/Greek mythology, the gods vs. the titans may be another example of this, where the gods were the heroes of the new people, and the titans those of the original people.

In either case, encounters with the original people will become fewer and and farther between as time goes on. Eventually the original people will be gone, but the new people won't really know when that happens. To them, it will just have been a long time since an encounter happened. As with Ishi, the stories will live on, and occasionally, someone will report a new encounter, either imagined or made up.

Of course, if the original people are more powerful than the new people, then the new people won't be able to replace the originals. You may still have stories, but they'll be of a different nature.

It would be interesting to look at the history of the various mythical peoples, and compare it to the archeological record of peoples migrations to see if there is any correlation. Even the names by which we refer to these mythical peoples might reflect the names by which these ancient peoples were know.