The Sumerians established one of the earliest known civilizations in Mesopotamia between the Tigris and Euphrates Rivers in the 5th millennium BCE. They developed cuneiform, one of the world's first systems of writing, leaving behind a written record of their achievements. The Sumerians lived in city-states like Ur and Uruk, centered around ziggurats and engaged in irrigation-based agriculture along the rivers. They established complex religious and legal systems and some of the first schools, leaving a lasting cultural legacy despite the original origins and language of the Sumerian people remaining unknown.