The Vedic period or Vedic age (c. 1500 - c. 600 BCE) is the period in the history of the Indian subcontinent intervening between the end of the urban Indus Valley Civilization, and a second urbanisation which began in c. 600 BCE. It gets its name from the Vedas, which are liturgical texts containing details of life during this period that have been interpreted to be historical and constitute the primary sources for understanding the period. The Vedas were composed and orally transmitted by speakers of an Old Indo-Aryan language who had migrated into the northwestern regions of the Indian subcontinent early in this period. The associated Vedic culture was tribal and pastoral until c. 1200 or 1100 BCE, and centred in the Punjab.

After the 12th century BCE, as the Rigveda had taken its final form, the Vedic society transitioned from semi-nomadic life to settled agriculture. Vedic culture extended into the western Ganges Plain. Possession of horses remained an important priority of Vedic leaders and a remnant of the nomadic lifestyle, resulting in trade routes beyond the Hindu Kush to maintain this supply as horses needed for cavalry and sacrifice could not be bred in India. The Gangetic plains had remained out of bounds to the Vedic tribes because of thick forest cover. After 1000 BCE, the use of iron axes and ploughs became widespread and the jungles could be cleared with ease. This enabled the Vedic Aryans to settle at the western Gangetic plains. Many of the old tribes coalesced to form larger political units.  The Vedic religion was further developed when the Indo-Aryans migrated into the Ganges Plain after c. 1100 BCE and became settled farmers, further syncretising with the native cultures of northern India. 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 Shrauta 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 Kshatriya, Brahmins, Vaishya and Shudra.  The Kuru kingdom declined after its defeat by the non-Vedic Salva tribe, and the political centre of Vedic culture shifted east, into the Panchala kingdom on the Ganges. Later, the kingdom of Videha emerged as a political centre farther to the East, in what is today northern Bihar of India and south eastern Nepal, reaching its prominence under the king Janaka, whose court provided patronage for Brahmin sages and philosophers such as Yajnavalkya, Uddalaka Aruni, and Gargi Vachaknavi.

Using a quote from the above article, answer the following question: how did the society change
the old tribes coalesced to form larger political units.