Caste system in Rig Veda forms the main distinguishing feature of the Hindu society. The Aryans of Central Asia and the Indo-Aryans also in the first two centuries did not follow any caste. It is only during the Rig-Vedic age that the four castes are stated. However the rigidity in the caste system as prevalent in our society or as it appears from the Code of Manu did not exist in the Vedic age.
In Central Asia the term `Brahmana` earlier meant `one expert in the religious rites`. It is assumed that sage Narayana discovered the Brahman (the Supreme Being) all the rishis and devas converted into `Brahmanas.` It is noted that from the counsels of Narada to Mandhata in the Mahabharata all were Brahmanas. The earliest distinction made by the Indo-Aryans was between themselves and the natives and it was on the basis of varna i.e. colour. The Indo-Aryans identified themselves as Svitnya meaning white and the natives Krishna-twach meaning dark.
It is believed that all traditions of the caste system originated from Prithudaka, capital of Prithu, on the sacred River Saraswati. Brahmavarta, presently Bithoor, was earlier the home of the most eminent sages. Their institutions, morality, chaste mannerisms, wisdom and perceptions were so admirable that even the `devas` of the north would descend to learn. Even Manu in his Code (twenty-eighth century BC) had recommended to imitate their morals and ethics. Thus it is significant that the Brahmans, especially those of Bithoor were considered with an eye of supremacy and veneration. Gradually the Kshatriya and Vaishyas also grew out of the Brahmanas. Kshatra or Kshatriya initially meant a saviour, a rescuer from the wrongs. The early Aryan settlers in India were often exploited by the non-Aryan dasyus or dasaa hence the Aryans would come approach to save them and thereafter punish the offenders. The class `Vaisya` originated from settlers and their main profession was trading, cattle-rearing, cultivation, and money-lending.
During 2800 BC Lord Indra, Lord Vishnu and other new Aryan leaders had left Aryavarta and established new states. During this time Society has taken a new shape. The common man no more required to follow diverse professions instead they adopted permanent professions according to their taste and ability. Hence those who followed religion and studied exclusively were termed as Brahmans. Others who were engaged in warfare became Kshatriyas. Some got involved in trade and became Vaishyas. The children of one and the same family were left free to adopt any professions according to their desire and ability. There were thus distinguished on lines of professions and not castes as yet. Inter-marriages and eating together were prevalent among them.
The Brahmanas and the Kshatriyas were fewer in numbers; the Vaishyas formed the major crowd of the Aryan community. Many freedom-loving Dravidians went eastward to Bengal and from there to Kalinga. There they established powerful kingdoms. Few of them were included among the Aryans as rishis and warriors. Many became Vaishyas and the rest were given the status of Sudras.
The first three orders were known as dwija or `twice-born`, whereas the Sudras were considered as `once-born`. The first three orders were almost equally privileged while the Sudras were barred from studying the Vedas or perform other religious sacrifices. They however had the freedom to learn by listening to discourses, or make money through service, trading or other industrial works. They were well honoured and even the elderly men were consulted.
With the passage of time these orders based on professions got framed into castes. Besides these four castes, there was a huge group termed as the Nishadas (hunters). Gradually the caste system became rigid and was asserted to be hereditary. Slowly it became established in the Indo-Gangetic Valley, among the early Aryan tribes, outside India among the Parsees and the Egyptians. Even Alexander and his generals established the caste system in Egypt (330 BC). Today there is existence of numerous castes and sub-castes that have emerged out due to intermarriages, professions and migrations.