There are a lot of surprising materials that mankind could use and have used. Here are just some: Clay tablets, wax tablets, stone, animal skin parchment, bone, silk, palm leaves, bark, papyrus, bamboo, paper, etc. If they are stored in a dry environment, palm leaves can survive for 400 to 500 years. Papyrus has been found nearly 4000 years old. There must be thousands of 'documents' that have been lost due to decay, so the 'evidence' for early writing is going to be hard to find.