There are many theories as to the origin of caste system in India. Many of these theories take into account skin colour or dominance of Indo-Europeans. I would base my theory on the following assumptions.
1. The initial Indian society was matriarchal.
2. The fundamental of caste system is purity and pollution distinction.
3. There was indeed an Indo-Aryan migration.
4. These Indo-Aryans had a structured and well defined religion in their Indo-European language.
I have already discussed about three most influential migrations to India.
1. Africa->Middle-East->India (Tribals)
2.Middle-East->North-West Indian subcontinent and Southern Central Asia-> Whole India (Dravidians)
3. North-West Indian subcontinent and Southern Central Asia-> South-East Europe(Kurgan)->North-West India-> Whole India(Indo-Aryans or R1A Dravidians)
Later I have discussed about rise of patriarchical society in Kurgan lands.
Masculization of feminine concepts:
Previously, I discussed about dominance of Dravidian religious symbols in Hinduism and marginalization of original Indo-Aryan gods. My argument was it was brought about by dominant Dravidian priestly class among Brahmins.
If one observe other Indo-European societies, there were class systems but nothing akin to caste system based on purity and pollution. The division was based on labour but the requirement was extent of cleanliness in every aspect of life from the higher castes. It's a classic case of female domain of cleanliness getting perverted in the feminized men.
As we know Indo-Aryans were patriarchical. Also, I would expect that there was a gradual migration to India and not invasion. If it's invasion I find it difficult to see how their religion got marginalized. On the other hand, migration gives a proper picture. The patriarchical Indo-Aryans gave rise to a male Dravidian priestly class(or it must be in existence worshipping mostly goddesses), however, the society was matriarchal therefore even the male abstraction of the world was feminine. The resulting hierarchical system when Indo-Aryans and Dravidians merged was conceptually one sided. Dravidian. However, male appropriation of female psyche resulted in the perversion of the whole system.
Indeed, if you observe Brahmin society since the historical times it was mostly passive. It goes against the general expectation from haplogroup R1A people(aggressive and attacking like Pathans atleast after their liberation from Dravidian Hinduism and Buddhism). You can explain this character only from Dravidian angle.
To sum it up all, India's dominant culture, character and people has remained Dravidian throughout the history. Basically, all the religions of India, Hinduism, Buddhism and Jainism, are the abstraction of feminine character of Dravidian India.