[adastra22]

We can follow the development of the Sumerian writing system, so we know it didn't derive from an older source.

While nobody can prove there wasn't some civilization that had writing prior to Sumer (can't prove a negative), it stretches the imagination to think that a civilization ONLY ever wrote on perishable media and not on, say, pottery or building walls. Then died out without transmitting their invention to successor civilizations, all of which we can account for their development of writing.

Also, what perishable media? The invention of papyrus was contemporary with Sumer's clay tablets.