Board Thread:Naruto Discussions/@comment-3232093-20140617193601/@comment-1512913-20140617204739

Not a theory, but hypothesis, which is nothing but an educated guess at best. And the only thing he confirmed was that merging Uchiha and Senju powers makes the Rinnegan, but you ignore that they were originally one in Hagoromo, therefore Sharingan never did evolve into Rinnegan. Hagoromo's Rinnegan got split into Indra's Sharingan and Ashura's power.