Ancient civilizations arose in Asia along major river valleys such as the Indus River Valley in South Asia and the Huang He Valley in East Asia. These river valleys provided rich soil for agriculture and natural protections from invasion. Some of the earliest civilizations included the Indus Valley Civilization with major cities of Harappa and Mohenjo-Daro, as well as early Chinese civilizations. In India, Aryan migrations introduced new social hierarchies like the caste system. Major empires later rose like the Mauryan Empire under Asoka, which spread Buddhism throughout Asia. Trade networks also developed linking Asia, like the Silk Road under the Kushan Kingdom.