Archaeologists study ancient human settlements and artifacts to learn about past civilizations. River valleys were sites of early permanent settlements during the Neolithic period due to their fertile soil, water sources, and natural protections. Key early civilizations included the Mesopotamians along the Tigris and Euphrates Rivers, the Egyptians in the Nile River valley, the Indus civilization in the Indus River valley, and the Chinese in the Huang He valley.