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During the period 2000–500 BCE, in terms of culture, many regions of the subcontinent transitioned from the Chalcolithic to the Iron Age. The Vedas, the oldest scriptures of Hinduism, were composed during this period, and historians have analysed these to posit a Vedic culture in the Punjab region and the upper Gangetic Plain. Most historians also consider this period to have encompassed several waves of Indo-Aryan migration into the subcontinent from the north-west. The caste system, which created a hierarchy of priests, warriors, and free peasants, but which excluded indigenous peoples by labelling their occupations impure, arose during this period. On the Deccan Plateau, archaeological evidence from this period suggests the existence of a chiefdom stage of political organisation. In southern India, a progression to sedentary life is indicated by the large number of megalithic monuments dating from this period, as well as by nearby traces of agriculture, irrigation tanks, and craft traditions.