The first civilizations appeared in major river valleys, where floodplains contained rich soil and therefore the rivers provided irrigation for crops and a way of transportation. Foundational civilizations developed urbanization and complexity without outside influence and without building on a pre-existing civilization, though they didn't all develop simultaneously. Many later civilizations either borrowed elements of, built on, or incorporated—through conquest—other civilizations. Because foundational civilizations arose independently, they're particularly useful to historians and archaeologists who want to know how civilization first developed.
