Thanks AJAY, yes, the theory of ethnicity would scratch it but Darwin's origin of species and any other on caste, racism (in measured terms) and social stratification would do and there are so many. I am doing something quite close, the references are still massive but i may return to your question later with a list if the information is desired.

My own guess would be that caste systems are rooted in language, the idea that there are unchanging essences and that the nouns we employ to talk about differences refer to real categories. But that premise would probably fail in explaining the caste system in Japan, which is apparently only a few hundred years old. The feature of caste that seems universal is that it is based on principles of purity and pollution. The work of Mary Douglas on that topic is helpful. But purity-pollution distinctions exist in many cultures that do not seem to have developed caste systems. So, if there is a truly universal explanation of caste, I hope you will share the name of the theorist who has developed it.