The Indus Valley Civilization began around 7000 BCE and reached its peak around 2500-1500 BCE, centered around the large cities of Harappa and Mohenjo-Daro. These cities had populations of around 40,000 people and were carefully planned with grid-like streets and drainage systems. Around 1500 BCE, Aryan groups migrated into the region, influencing the culture and religion. They established Vedic religion, with its hymns collected in the Rig Veda, and introduced the caste system. The Upanishads later expanded Vedic philosophy, introducing concepts like samsara, karma, and moksha.