Board Thread:Naruto Discussions/@comment-36221354-20180717114536/@comment-36351381-20180802001405

"Do you need the Sharingan to activate the Rinnegan?"

Yes and no.

No - Kaguya ate the chakra fruit. Momoshiki absorbed Kinshiki. The Rinnegan originally, is a dormant Otsutsuki power. Or just a blessing of Nature (The Rinne-sharingan of both Kaguya and Madara are clear examples). Probably anyone who eats the chakra fruit would gain magnificent power and that dojutsu. (Absorbing the god tree is another option to).

Yes - Yes in the cases of Hagoromo and his sons and their reincarnates. Either one of them have to possess, or come into possession of the MS though. These are the instances. Indra + Asura= Hagoromo. If an Indra reincarnate had to take a piece of an Asura reincarnate, while in possession of a pair of MS, the end game is Rinnegan. Vice versa, if an Asura reincarnate had to take a peice of an Indra reincarnate, I don't think the end result would be Rinnegan, because even after Hagoromo learnt sage power from Mount Myoboku, his base 3 tomoe Sharingan eyes never manifested the Rinnegan. The Rinnegan only appeared in his eyes after he awakened the MS, but continually. Indra's spirit would surely manifest the Sharingan in the eyes of that Asura reincarnate. If they evolve that manifested Sharingan to the Mangekyou. Then is when the end game would be Rinnegan. The Asura reincarnate could also just implant the MS eyes of the Indra reincarnate and then boom, Rinnegan.

All of these instances are only half truths though, because what if Hagoromo ate the chakra fruit? I'm pretty sure instead of a mark, he would've manifested the RS. Same thing goes for Indra and his reincarnates. And also for Asura and his reincarnates.