Indo-Aryan migration
Palanichamy et al. (2015), elaborating on Kivisild et al. (1999) conclude that "A large proportion of the west Eurasian mtDNA haplogroups observed among the higher-ranked caste groups, their phylogenetic affinity and age estimate indicate recent Indo-Aryan migration to India from west Asia. According to Palanichamy et al. (2015), "the west Eurasian admixture was restricted to caste rank. It is likely that Indo-Aryan migration has influenced the social stratification in the pre-existing populations and helped in building the Hindu caste system, but it should not be inferred that the contemporary Indian caste groups have directly descended from Indo-Aryan immigrants.