Explanation:

Towards the end of the Rig Vedic period some transformation took place in the Rig Vedic society. The Aryans gradually migrated from Punjab to the Ganga-Jamuna Valley of Eastern India. In this process of migration the Aryans were in a state of ceaseless warfare with Non-Aryan aborigines. Under the impact of constant war the traditional political and social organization of the Aryans gave way. They were replaced by some new system. In the process of this change the petty tribal kingdoms of the Aryans were merged into the big kingdoms that grew in this period. The petty rulers of these tribal kingdoms sank to the class of warriors and nobility. The Kshatriyas grew from this warrior class. They were required to fight as soldiers of the king of these big states. With the development of civilization the mode of worship and religious sacrifice of the Aryans became elaborate and complicated. Formerly the householders personally performed religious sacrifices. But they now found it difficult due to its complicated process. A class of people called priests who were proficient in scripture and rituals performed the religious sacrifices for the householders. These priests came to be known as Brahamanas. The remaining bulk of the Aryans received protection from the warrior class called the Kshatriyas. As the Kshatriyas were responsible for defence, maintenance of law and order, the rest of the Aryans abandoned the use of arms. They also gave up the practice of performing sacrifices which were done for them by the priests. These general Aryans took agriculture, industry and trade as their occupations. They formed a vis’ or the Vaishya class. The aborigines who were outside the Aryan race were considered as the Sudras. Thus the four classes developed in the Late Rig Vedic Age.
In the Later Vedic period caste system was in a state of semi rigidity. Change of caste became difficult though not absolutely impossible. Hereditary occupations were converted into hereditary castes. There was a tendency to discourage change of occupation. Great importance was attached to purity of descent. In the Later Vedic Age the status of the Vaishya class seems to have deteriorated. The Brahmanical literature denied many rights to Vaishya class. When the Vaishayas were in such a declining state, one can imagine the condition of the Sudras. A Sudra could be expelled at will, to be slain at will and he was a servant of another. The Sudra was not the twice born or ‘Dvija’. A Brahmin could kill a Sudra with impunity. Sudra was considered as a thing, a chattel. He was required to serve the three higher classes.