Answer:
As the soil is very fertile, world's earliest civilization grew up in Mesopotamia.
Step-by-step explanation:
Mesopotamia was the earliest river valley civilization which started around 3500 BC. Snow melting in the mountains resulted in floods across the area. These floods were yearly which deposited silt in the soil. This made the land very fertile. This enabled people to come and settle in Mesopotamia.
People gathered here with hunting and slowly settled here with domesticating animals, and later giving attention to agriculture. They joined the two rivers Tigris and Euphrates for irrigation purpose using the fertile soil for fertilization. 'Ur' was the first city in this region to have first literacy.