The Sumerian language is one of the earliest known written languages. The “proto-literate” period of Sumerian writing spans c. 3300 to 3000 BC. In this period, records are purely logographic, with phonological content. The oldest document of the proto-literate period is the Kish tablet.
Records with unambiguously linguistic content, identifiably Sumerian, are those found at Jemdet Nasr, dating to the 31st or 30th century BC. From about 2600 BC, the logographic symbols were generalised using a wedge-shaped stylus to impress the shapes into wet clay. This cuneiform (“wedge-shaped”) mode of writing co-existed with the pre-cuneiform archaic mode. Deimel (1922) lists 870 signs used in the Early Dynastic IIIa period (26th century). In the same period the large set of logographic signs had been simplified into a logosyllabic script comprising several hundred signs. The pre-Sargonian period of the 26th to 24th centuries BC is the “Classical Sumerian” stage of the language.
The cuneiform script was adapted to Akkadian writing beginning in the mid third millennium. Our knowledge of Sumerian is based on Akkadian glossaries. During the Sumerian Renaissance (Ur III) of the 21st century BC, Sumerian was written in already highly abstract cuneiform glyphs directly succeeded by Old Assyrian cuneiform.
https://en.wikipedia.org/wiki/Sumerian_language#Development
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