The Ancient Sumerians were the most advanced civilization of all time according to most iterations out there. They originally arrived in Mesopotamia by rivers and instantly knew to target Tigris and Euphrates, as they were the best locations to settle in. They were the first civilization known to man to use the bike, build an irrigation system, and even create their own written language in the Middle East.
Nobody knows where they came from per se, but a lot of people believe that they originated from South India.
Since Tigris and Euphrates were a part of the Armenian mountains they were the most bountiful lands that there were in the area which is why they chose these two to begin with.
According to their lore books, all of the ancient Gods attacked humanity because they believed it to be too impure. They caused storms and other cataclysmic events on humanity. Enki, their only supporter, helped humanity by telling Ziusudra to build an enormous ship and get all of the Sumerians on it.
The floods were so bad that even the Gods were afraid of stepping foot on the lands so when the Sumerians proved themselves and Ziusudra personally made an offering to the Gods they accepted it and made him and his wife immortal.
This is where the original story of Noah’s ark came from according to most experts. Each one of their cities had its own deity. You might know of them form their most popular ruler, king Gilgamesh. He is often portrayed next to his lion pet.