Board Thread:Versus Debates/@comment-33229263-20171019104931/@comment-1512913-20171021114217

You are confused. The people who got the Rinnegan didn't get it because they had the Sharingan, rather because they were Indra incarnates. The hypothesis was only semi-correct, because it operated under the wrong premise, so as said, sample bias. It only SEEMED that Madara's Sharingan transformed into Rinnegan by taking Hashirama's power, hence the whole Uchiha + Senju = Rinnegan hypothesis, which is not correct, because only chakras of Asura and Indra incarnates merged together will do, something Orochimaru and Kabuto had no way knowing, so they worked with limited knowledge.

Second, in no way, is the Sharingan a devolution from Rinnegan or Rinnegan evolution of Sharingan. Yes, it's stated as such on this very wiki, but because of bias.

The Sharingan and Rinnegan are sibling doujutsu to each other, not parent-offspring relationship.

Both are halves of Rinne Sharingan, it just gets messy, because the Rinnegan got split between Asura and Indra, while the Sharingan stayed intact and got passed on to next generations. So when an Indra incarnate, most likely an Uchiha (since Indra isn't confirmed to have fathered any other clans, nor are any 'distantly related to Uchiha' clans known) awakens the Rinnegan by getting his hands on chakra/flesh of Asura incarnate, his Sharingan doesn't become the Rinnegan, rather the Sharingan stays intact and the Rinnegan awakens as a separate doujutsu.

Furthermore, it was strongly implied if not blatantly stated that Black Zetsu attempted to have the Asura incarnates awaken the Rinnegan as well. You might think that this would require an Asura incarnate steal and transplant Sharingan from Indra incarnate to get the Rinnegan, but that's not true. Even buttocks tissue of an Indra incarnate would do just fine for the Asura incarnate to get the Rinnegan, the Sharingan in itself is in no shape a pre-requirement to get the Rinnegan, nor is the Rinnegan and upgrade to the Sharingan.