The oldest known civilization was believed to have formed in  Mesopotamia near the Tigris and Euphrates rivers. This was believed  to have happened around 4000 B.C. and that … is a fact! Answered. In Geology.he oldest known civilization was believed to have formed in  Mesopotamia near the Tigris and Euphrates rivers. This was believed  to have happened around 4000 B.C. and that … is a fact! Answered. In Geology.