Earlier quoted context omitted.
> In fact I've heard archaeologists in the past specifically say that no oral tradition can survive intact more than 100 years. Usually this statement is in reference to certain creation myths being relatively modern inventions. Maybe you meant 1000 years? If you really meant 100 years, there's an obvious counterexample: The "Happy Birthday Song", published in 1893. Arguably no one learns it by reading the words and…
You are aware that the Happy Birthday Song has dozens upon dozens of variations (and not just the parody ones), right? The Happy Birthday Song isn't even the Happy Birthday Song! It's Good Morning To All! Happy Birthday lyrics didn't even make it into print until 1912 and that's not even the version that we use today. That version was first published in 1924 by Robert Coleman. The copyrighted version that even credit…
Of course I don't think oral traditions go unchanged for 12,000 years. Some very old oral traditions, like the ones from ancient India, stayed relatively unchanged only because their emphasis shifted suddenly and dramatically from active composition to ritual performance, to the extent that most people had no idea what the actual language used meant. Their opacity became a feature, not a bug.
However, despite the details changing, consistent storylines and story fragments exist across many ancient cultures, sometimes because they are derived from the same narrative source, and other times because of shared experiences across divergent cultures.