Board Thread:Naruto Discussions/@comment-25498180-20141004140328/@comment-4271790-20141004194628

@Redcrusader, it was stated in the manga by Orochimaru and Kabuto's hypothesis that the Rinnegan is a natural evolution of the Sharingan. However, that theory isn't entirely true, since one, we haven't seen Hagoromo with a Sharingan, and two, Kaguya had the Rinnegan (a different one) from the get-go, that was Sharingan-powered. Then it was stated that Senju and Uchiha DNA bring forth the Rinnegan, which was later specified to be Asura and Indra's chakras to bring forth Hagoromo's chakra and the Rinnegan.