I want to know the answer to the following question:
Why were the lands in the region the Sumerian people settled fertile?
All the background I'm provided with is that "The Sumerians were a non-Semitic caucasoid people, and spoke a language isolate; a number of linguists believed they could detect a substrate language beneath Sumerian, because names of some of Sumer's major cities are not Sumerian, revealing influences of earlier inhabitants.". Is that enough to answer the question?
Answer: no