The question then becomes one of cultural stability over that length of time. I personally can't tell you anything about the cultural and technological stability of of the aboriginal cultures, all I can assume is they were far more stable than europe, north africa, and the middle east. These later groups I listed had an absolutely massive amount of cultural exchange, and other things like mass genocide over the centuries. The exchange of new concepts and ideas is a powerful force in shaping the stories we tell. When two groups of people exist together over longer periods of time with conflicting sets of stories one of the stories tends to fade and die, and most likely a mixed story is created (Christianity ate a lot of Pagan mythos this way).