(c.1200–600 BCE), which did not expand east of the Ganga-Yamuya Doab. It differed from the related, yet markedly different, culture of the Central Ganges region, which was associated with the Northern Black Polished Ware and the Mahajanapadas of Kosala and Magadha. In this period the varna system emerged, state Kulke and Rothermund, which in this stage of Indian history were a "hierarchical order of estates which reflected a division of labor among various social classes". The Vedic period estates were four: Brahmin priests and warrior nobility stood on top, free peasants and traders were the third, and slaves, labourers and artisans, many belonging to the indigenous people, were the fourth. This was a period where agriculture, metal, and commodity production, as well as trade, greatly expanded, and the Vedic era texts including the early Upanishads and many Sutras important to later Hindu culture were completed. The Kuru Kingdom, the earliest Vedic "state", was formed by a "super-tribe" which joined several tribes in a new unit. To govern this state, Vedic hymns were collected and transcribed, and new rituals were developed, which formed the now orthodox Śrauta rituals. Two key figures in this process of the development of the Kuru state were the king Parikshit and his successor Janamejaya, transforming this realm into the dominant political and cultural power of northern Iron Age India. The most well-known of the new religious sacrifices that arose in this period were the Ashvamedha (horse sacrifice). This sacrifice involved setting a consecrated horse free to roam the kingdoms for a year. The horse was followed by a chosen band of warriors. The kingdoms and chiefdoms in which the horse wandered had to pay homage or prepare to battle the king to whom the horse belonged. This sacrifice put considerable pressure on inter-state relations in this era. This period saw also the beginning of the social stratification by the use of varna, the division of Vedic society in Brahmins, Kshatriya, Vaishya and Shudra. The Kuru kingdom declined after its defeat by the non-Vedic Salva tribe, and the political center of Vedic culture shifted east, into the Panchala kingdom on the Ganges, under King Keśin Dālbhya (approximately between 900 and 750 BCE). Later, in the 8th or 7th century BCE, the kingdom of Videha emerged as a political center farther to the East, in what is today northern Bihar of India and southeastern Nepal, reaching its prominence