We know a great deal about the earliest known (Sumerian) civilization partly because they were able to both read and write and left surviving written (pictographic) records of their culture in stone or clay.

The rest of the world around them did not know how to read or write, or if they did, they left no surviving artifacts or records of those abilities.

So when the Sumerians finally got around to writing down a creation story, who is to say they were the originators of that story? Just because they were one of the earliest civilizations and were able to read and write?

Whose to say the Sumerians didn't in fact receive the creation story second hand or at several removes through oral transmission of accounts given by peoples who couldn't read or write, complete with distortions and exaggerations?