Human DNA today is the same as 50,000 BCE. There is no doubt there were many dozens and perhaps thousands of civilizations prior to the Sumer civilization, but Sumer is the earliest known, or at least the earliest well known. The Sumer civilization first established between 6500 and 4100 BCE. We know quite a bit about the Sumerians because they immortalized their writing in clay tablets which will be around long after all the paper books on Earth right now have deteriorated. Sadly, we know almost nothing about prior civilizations because very little evidence survived the test of time. The Sumerians spoke and wrote Sumer and starting several millennia into their civilization they started immortalizing their culture on clay. They had an advanced democracy with elected officials, religion, art, wheel, math, philosophy, and language. The Cuneiform script was in use until 100 CE.