Final answer:
During the early third millennium B.C., the inhabitants of Mesopotamia were a mix of different groups including the Amorites, Elamites, and other foreign invaders. The Sumerians were among the dominant ethnic group in southern Mesopotamia.
Step-by-step explanation:
The inhabitants of Mesopotamia during the early third millennium B.C. were a mix of different groups, including the Amorites, Elamites, and other foreign invaders from the north, east, and west. These groups put pressure on the empire of Mesopotamia, leading to the fall of cities like Sumer and Ur. The Sumerians, who were an inventive people known for their sophisticated writing system, were among the dominant ethnic and linguistic groups in southern Mesopotamia during this time.