Sumer:

However, some scholars contest the idea of a Proto-Euphratean language or one substrate language. It has been suggested by them and others, that the Sumerian language was originally that of the hunter and fisher peoples, who lived in the marshland and the Eastern Arabia littoral region, and were part of the Arabian bifacial culture. Reliable historical records begin much later; there are none in Sumer of any kind that have been dated before Enmebaragesi (c. 26th century BC). Professor Juris Zarins believes the Sumerians were settled along the coast of Eastern Arabia, today's Persian Gulf region, before it flooded at the end of the Ice Age.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What people lived in river region of Arabia?
unanswerable