Who were the Sumerians?

Sumerians have intrigued archeologists and lay people ever since the discovery of the first ruins in the 19th century. Due to their apparent sudden and profound influence on western culture, wild theories abound about their extra-terrestrial influence if not outright propagation (example: https://discover.hubpages.com/religion-philosophy/annunaki). The simple fact is, archeologists do not know where the Sumerians originated from. Their language is called an isolate, meaning there are no other known related languages, past or present.