Region of southern Mesopotamia and site of the earliest known civilization. It was first settled c. 4500–4000 BC by a non-Semitic people called the Ubaidians, who drained the marshes for agriculture and developed trade. The Sumerians, who spoke a Semitic language that came to dominate the region, arrived c. 3300 BC and established the world's first known cities. These polities evolved into city-states, which eventually developed monarchical systems that later came to be loosely united under a single city, beginning with Kishc. 2800 BC. Thereafter, Kish, Erech, Ur, and Lagash vied for ascendancy for centuries; Nippur emerged as a religous centre. The area came under the control of dynasties from outside the region, beginning with Elam (c. 2530–2450 BC) and later Akkad, led by the Akkadian king Sargon (r. 2334–2279 BC). After the Akkadian dynasty collapsed, the city-states were largely independent until they were reunified under the 3rd dynasty of Ur (22nd–21st century BC). That final Sumerian dynasty declined after being weakened by foreign invasions, and the Sumerians as a distinct political entity disappeared, becoming part of Babylonia in the 18th century BC. The Sumerian legacy includes a number of technological and cultural innovations, including the first known wheeled vehicles, the potter's wheel, a system of writing (cuneiform), and written codes of law.