A Mysterious Group of People came to settle in southern Mesopotamia, sometime around 5400BC. What is now the modern state of Iraq, the first city of Mesopotamia was founded named Eridu. Although historians have generally regarded this as the world's first city, we have seen this challenged on numerous occasions by recent discoveries too numerous to mention here. Eridu had all the things we ordinarily associate with an ancient city: temples, administrative buildings, housing, agriculture, markets, art, and, of course, walls to keep out unsavoury characters.
The elusive aspect is we have absolutely no idea where they acquired their language, and bizarre language it is, we have no idea what they originally looked like. Their language, which we call Sumerian, and the subsequent Akkadian derivative were linguistic isolates. Sumerian is the oldest known written language on Earth, and any languages it might have derived from or developed alongside have been lost to time.
Figuring out what their baffling ethnic identity based on their art is a doomed effort, because their art was so stylized that a good case could be made that it portrays people of any ethnicity, or the people they encountered. The Sumerian language was not Semitic, and the Akkadian conquests of 2334 BCE disrupted the ethnic and cultural isolation of the Sumerian people. By about 2000 BCE, the Sumerians were speaking Akkadian and the Sumerian and Akkadian civilizations were regarded as a single enterprise.