Question: Which civilizations developed in the fertile valley between the tigris and euphrates rivers?
Disambiguated questions and answers: question: What type of civilization developed in the fertile valley between the tigris and Euphrates rivers?
Which civilizations formed in the fertile valley between the Tigris and Euphrates rivers?
answer: River Valley Civilization
Mesopotamia and Sumerian
Natural Questions answer: Mesopotamia (from the Wikipedia article "River valley civilization")