What is the most ancient written language?

Sumerian was spoken in southern Mesopotamia from at least the 4th millennium BC, and was gradually replaced by Akkadian as a spoken language around 2000 BC, though it continued to be used as a ceremonial, literary and scientific language until the 1st century AD. Even though it was later forgotten, today it is well understood thanks to the large amount of written sources left. Scientists know its grammar, vocabulary, and even its dialects and pronunciation.