The Indus Valley Civilization began around 2500 BCE and was one of the earliest civilizations, discovered in 1856. The cities of 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 migrated into the region, bringing their Vedic religion and social structures like the caste system. This helped contribute to the decline of the Indus civilization, though their cultures blended together to influence early Indian society.