In early second millennium BCE persistent drought caused the population of the Indus Valley to scatter from large urban centres to villages. Around the same time, Indo-Aryan tribes moved into the Punjab from Central Asia in several waves of migration. Their Vedic period (1500-500 BCE) was marked by the composition of the Vedas, large collections of hymns of these tribes. Their varna system, which evovled into the caste system, consisted of a hierarchy of priests, warriors, and free peasants, excluded indigenous peoples by labeling their occupations impure. The pastoral and nomadic Indo-Aryans spread from the Punjab into the Gangetic plain, large swaths of which they deforested for agriculture usage. The composition of Vedic texts ended around 600 BCE, when a new, interregional culture arose. Small chieftaincies, or janapadas, were consolidated into larger states, or mahajanapadas, and a second urbanisation took place. This urbanisation was accompanied by the rise of new ascetic movements in Greater Magadha, including Jainism and Buddhism, which opposed the growing influence of Brahmanism and the primacy of rituals, presided by Brahmin priests, that had come to be associated with Vedic religion,[4] and gave rise to new religious concepts.[5] In response to the succes of these movements, Vedic Brahmanism was synthesised with the preexisting religious cultures of the subcontinent, giving rise to Hinduism.