Mesopotamia and Egypt were early river valley civilizations that relied on irrigation from the Tigris/Euphrates rivers and Nile River respectively. Writing first developed in Mesopotamia in 3500 BC to record taxes, trade, and populations. Mesopotamia was located between the Tigris and Euphrates Rivers and was ruled by various empires over time, developing agriculture, trade, and urban societies. Egypt similarly relied on annual Nile River floods for irrigation and was unified by King Menes in 3100 BC, establishing Egypt's long-lasting civilization.