Tiamat was originally a Sumerian prime mother goddess, representing the salty ocean that existed at the beginning of time. The waters of Tiamat mingled with the waters of Apsu, the fresh water primordial ocean. This resulted in the creation of the first gods.
By the time that the Akkadian language had taken over from the Sumerian language as the main spoken language in Sumer, Tiamat had been transformed from a devoted mother to an evil breeder of dragons.
One story has it that their children fought for supremacy and Apsu decided to kill them. However, Apsu was killed by Ea, "earthly wisdom", who learned of his plan. An avenging Tiamat then produced a new brood of snarling dragons to fight Ea. Ea's son Marduk shot her in the stomach and split her womb to remake the world that she had created.