Ancient India developed along the Indus River where the Harappan civilization established large cities like Mohenjo-Daro and Harappa. Around 1500 BCE, Indo-Aryan tribes invaded from Central Asia, bringing their language and religion. The two cultures merged, establishing Hinduism and the caste system. Buddhism later emerged as a reaction to Hinduism under Siddhartha Gautama in the 6th century BCE. Powerful empires like the Mauryas and Guptas unified large parts of India and promoted Hinduism.