Back in February I explained why the ultimate origin of a traditional story isn’t an especially interesting or important question. However, I don’t want anybody to misunderstand what I was saying. I’m not at all claiming that a traditional story can’t be passed down essentially unchanged over long periods of time; I’m saying that it won’t be passed down unchanged, or at all for that matter, without there being a reason.
In societies without writing, like Mesopotamia at the time the Gulf Oasis would have last been above water, stories can’t sit unread in an archive waiting for some scholar to rediscover them at a later time. There’s no possibility of a story surviving unless it’s told. And people don’t just randomly talk into the air; storytellers have a purpose (or purposes) in mind. So a story that nobody has a reason to tell will disappear.
The willingness of the teller to make changes in a story depends critically on the purpose for which it is told. If a story is told purely for entertainment it doesn’t matter how it changes in the telling, as long as the audience finds the result entertaining (compare the fairy tales of Charles Perrault with their Disney descendants to see a great example of this). If the story is part of a magical ritual, or an invocation to a deity who might be offended if everything isn’t done perfectly, there is a very strong incentive to tell the story exactly the same way each time. It’s not enough just to say the story is sacred, however; it’s the purpose that matters. A sacred story used to instruct children in the proper way to behave might be capable of absorbing some changes, even while other parts of the story remain constant. The only way an oral story remains constant over time is if it has to in order to fulfill the purpose for which it is told.
(And, of course, all of the above only applies if we’re considering things from a human perspective. If an omnipotent God wants a story to be passed down unchanged, he is quite capable of causing that to happen.)