Around 5400 BCE, settlers in southern Mesopotamia—in what would now be called southeastern Iraq—founded Eridu, which historians generally regard as the world’s first city. It 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 wild animals and bandits.
The mystery of the Sumerians is that we have absolutely no idea where they acquired their language, or what they might have looked like. Their language, which we call Sumerian, was a linguistic isolate—it’s the oldest known written language on Earth, and any languages it might have derived from or developed alongside have been lost to time. The Sumerian people were also, it can be reasoned, ethnically isolated; referring to themselves as the sag gigga (“black-headed people”), they appear to have had no concept of race. And figuring out what their ethnic identity might have been 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.
Culturally, they’re often linked to the Ma’dan (Marsh Arabs) who still live in southern Iraq. But the idea that the Ma’dan are ethnically Sumerian seems a bit unlikely, as 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 people; there is no evidence in any existing texts that they were discouraged from intermarrying, so we can reasonably assume that it was normal to do so. Given that fact, and the 4,000 years of history between then and now, it seems unlikely that anyone living today has more than a tiny amount of Sumerian ancestry.
This doesn’t necessarily mean that we’ll never know how the Sumerian language developed, or where the Sumerians originally came from. There are some ways we might still find things out. It’s possible that older extant texts from the region, written in a proto-Sumerian language, might connect Sumerian with languages that currently seem unrelated. And if any reasonably well-preserved Sumerian bones can be found, which isn’t completely implausible, DNA testing could tell us their ethnic origin. Then again, it’s possible, and, given how little we know about the ancient world, perhaps even probable, that these discoveries will only deepen the mystery.