Board Thread:Theories and Speculation/@comment-34661853-20181111160651/@comment-1512913-20181115115703

People make a much bigger mess out of it than it really is. The Sharingan and Rinnegan being evolutions/devolutions whatever of each other was nothing but a hypothesis by Kabuto and Orochimaru, which has since been debunked on several accounts.

The Sharingan and Rinnegan are sibling doujutsu to each other, with Rinne Sharingan being the parent. So it doesn't really go Rinne Sharingan > Rinnegan > Sharingan, but Rinne Sharingan > Rinnegan & Sharingan. The Sharingan part got passed down Indra's line, while the Rinnegan genes/chakra got split between Indra and Asura and further got diluted in their descendants, hence why a random Senju's chakra + random Uchiha's chakra doesn't produce the Rinnegan.

Hell, not even the Sharingan is 100% present among the Uchiha, it was stated only a few within the clan ever awakened it, meaning the majority of the Uchiha in history must have been Sharingan-less.