The Indus Valley civilization began around 7000 BCE and reached its peak between 2500-1500 BCE. Two major cities, Harappa and Mohenjo-Daro, had populations of around 40,000 people and were carefully planned with grid-like streets and drainage systems. Around 1500 BCE, Aryan groups began migrating into India, influencing the culture and religion. They developed the caste system and Vedic scriptures like the Upanishads, which influenced Hinduism.