Board Thread:Theories and Speculation/@comment-30355627-20161218164138/@comment-1512913-20161225085358

Yes, there is, but the tablet isn't worth anything, it was rewritten by Black Zetsu to brainwash Uchiha Clan, so anything written on it shouldn't be taken at face value.

There's literally only 3 reasons why people falsely believe that Rinnegan is final stage of Sharingan/Sharingan is what changes into the Rinnegan or whatever:
 * Wrong Orochimaru and Kabuto's hypothesis taken at face value
 * In the canon, Hagoromo was shown only with Rinnegan and his son Indra with Sharingan, so people assume that the Rinnegan evolved/devolved into Sharingan, rather than that Hagoromo must also have had Sharingan as separate from Rinnegan, which Indra inherited
 * Every post-Hagoromo person who awakened the Rinnegan (excluding Momoshiki) is also known to have had the Sharingan, both cases Indra incarnates Madara and Sasuke, so to people it for some reason seems like the Sharingan is a requirement for the Rinnegan/lesser stage of it and that Madara and Sasuke's Sharingan changed into Rinnegan.

This is not true in either's case. Rinnegan Madara still could access Sharingan, meaning the latter didn't transform into former, but only one doujutsu is usable at a time. And in Sasuke's case, in one eye, power of Sharingan and Rinnegan got merged and are usable at the same time.

This clearly shows that the Sharingan has nothing to do with the Rinnegan and isn't an evolution or devolution or stage of it, they are both separate doujutsu.