I find out that may be buddha contradicted when he talked about caste when buddha talked about previous buddha 3 of khattiya and 3 of bamano and in various sutta buddha equalls all caste like in Aggana sutta and in one of sutta buddha says " na jacca hoti chandalo na jacca hoti bamano , kamano chandalo hoti kamano hoti bamano
Please help me 
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I am not sure, I understand the nature of your question, but castes are (or were) real social things, we may approve their existence, or seriously dislike it, but this is as things are.
But Buddha’s Teaching emphasis high moral standards, and in social group as Sangha there is no such thing as castes. But individual member of Sangha, previously were members of certain social groups, including Buddhas.
It is arrogance from my side, to explain why Buddhas usually are born in high castes, but I think it is reasonable. High castes enjoy more authority in society, and people much more easily accept such difficult massage as Dhamma from one who renounces his high social status.
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Yes, that’s an interesting point. In some suttas, it seems like they assume the caste system is always around, whereas in other places it is seen to evolve.
Historically, the four vannas seem to have originated around the end of the Vedic period, perhaps several centuries before the Buddha. But it almost certainly was not around for thousands of years. As for previous world systems and the like, well, all we have is what the suttas say.
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