What was the first society/civilisation which started caste system in India and when did they start, and why was it even introduced into the society? What was that benefit or advantage of this system that it spread to the whole sub-continent?
Also, it is said that in initial times caste system wasn't very rigid, and it was possible for a person to change his/her caste then when did it become so rigid that changing of caste became almost impossible?
From author's comments:
The Wikipedia article 'Caste system in India' can be a good start but does not help in knowing anything about the time when it originated and society which came up first with this system. There is a huge possibility that it must have originated in different parts of the sub-continent or even continent in different forms and then eventually everything came together but the Wikipedia link gives very little idea of anything. The only thing substantial which can be extracted from it is that there was a huge role of manusmriti book in perfecting it the way we know it today