Mesopotamia, located between the Tigris and Euphrates Rivers, was the site of early agricultural developments and the world's first cities. Key Mesopotamian civilizations included Sumer, Akkad, Assyria, and Babylon. Writing developed from tokens used for accounting to the cuneiform script on clay tablets. Major cities such as Uruk, Ur, and Babylon featured ziggurats, temples, and palaces at their cores and traded extensively by river and overland routes. Recurring themes in Mesopotamian art, literature, and religion included war, powerful rulers, and gods and goddesses like Ishtar.