Which civilization is known as the earliest one in the historical region of southern Mesopotamia?

Sumer is the earliest known civilization in the historical region of southern Mesopotamia, modern-day southern Iraq, during the Chalcolithic and Early Bronze ages, and one of the first civilizations in the world along with Ancient Egypt and the Indus Valley. Living along the valleys of the Tigris and Euphrates, Sumerian farmers were able to grow an abundance of grain and other crops, the surplus of which enabled them to settle in one place. Prehistoric proto-writing dates back before 3000 BC. The earliest texts come from the cities of Uruk and Jemdet Nasr and date to between roughly c. 3500 and c. 3000 BC.

The Sumerian city of Eridu, on the coast of the Persian Gulf, is considered to have been one of the oldest cities, where three separate cultures may have fused: that of peasant Ubaidian farmers, living in mud-brick huts and practicing irrigation; that of mobile nomadic Semitic pastoralists living in black tents and following herds of sheep and goats; and that of fisher folk, living in reed huts in the marshlands, who may have been the ancestors of the Sumerians.