Answer: Sumerians
The Sumerians were the first Mesopotamian civilization considered the first and oldest civilization in the world.
The Sumerians settled in the vicinity of the Tigris and Euphrates rivers, near the mouth of the Persian Gulf. They spoke their own language and had their own religion, which was central to their social ordering.
In addition, the Sumerians left the legacy of cuneiform writing, it was considered the first record of the writing of humankind.