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 Harappan culture and language. This led to the Vedic civilization, which established the foundations of Hinduism through sacred texts like the Vedas and Upanishads. The Aryans also developed the caste system and a patriarchal social structure.