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 regions further northwest in several waves of migration. The resulting Vedic period was marked by the composition of the Vedas, large collections of hymns of these tribes whose postulated religious culture, through synthesis with the preexisting religious cultures of the subcontinent, gave rise to Hinduism. The caste system, which created a hierarchy of priests, warriors, and free peasants arose later during this period. Towards the end of the period, around 600 BCE, after the pastoral and nomadic Indo-Aryans spread from the Punjab into the Gangetic plain, large swaths of which they deforested to pave way for agriculture, a second urbanisation took place. The numerous Indo-Aryan realms, or janapadas, were consolidated into larger states, or mahajanapadas. This urbanisation was accompanied by the rise of new ascetic movements, including Jainism and Buddhism, which challenged the primacy of rituals, presided by Brahmin priests, that had come to be associated with Vedic religion,and gave rise to new religious concepts. Most of the Indian subcontinent was conquered by the Maurya Empire during the 4th and 3rd centuries BCE. From the 3rd century BCE onwards Prakrit and Pali literature in the north and the Tamil Sangam literature in southern India started to flourish. Wootzsteel originated in south India in the 3rd century BCE and was exported to foreign countries. During the Classical period, various parts of India were ruled by numerous dynasties for the next 1,500 years, among which the Gupta Empire stands out. This period, witnessing a Hindu religious and intellectual resurgence, is known as the classical or "Golden Age of India". During this period, aspects of Indian civilisation, administration, culture, and religion spread to much of Asia, while kingdoms in southern India had maritime business links with the Middle East and the Mediterranean.