The earliest known written Sumerian accounts of gods or myths date to about 1600 BCE, more than a thousand years after the first known Sumerian writing. It was probably written by a specially educated class of priests, after spoken Sumerian had become extinct in general society, as Latin was used by priests in the Christian religion long after the Roman Empire was dust.
The earliest known Sumerian writing - by a thousand years - is stuff like this:
"
Enlil, king of all the lands, father of all the gods, by his firm command, fixed the border between
Ningirsu and
Šara."
"
Mesilim, king of
Kiš, at the command of
Ištaran, measured the field and set up a stele there."
"Ush, ruler of
Umma, acted unspeakably."
"He ripped out that stele and marched toward the plain of
Lagaš."
"Ningirsu, warrior of Enlil, at his just command, made war with Umma."
"At Enlil's command, he threw his great battle net over it and heaped up burial mounds for it on the plain."
"
Eannatum, ruler of Lagash, uncle of
Entemena, ruler of Lagaš"
"fixed the border with Enakale, ruler of Umma"