The first civilizations were located in river valleys, mainly in the Middle East and parts of Asia. The earliest known ancient civilizations emerged in Mesopotamia (between the Tigris and Euphrates rivers, in present-day Iraq), Egypt (along the Nile River), the Indus Valley (in present-day Pakistan and northwest India), and China (along the Yellow River). These civilizations arose around the 4th millennium BCE and are known for developing agriculture, writing systems, and complex urban centers. Mesopotamia, often called the "cradle of civilization," had early city-states such as Sumer, with notable cities like Ur and Eridu. The Indus Valley civilization had major cities like Harappa and Mohenjo-daro. Egypt's civilization developed around the Nile River, building famous structures like the pyramids.