Board Thread:Theories and Speculation/@comment-1512913-20161125200227/@comment-1512913-20161127165524

So I suppose you agree with me that Rinnegan isn't part of the Sharingan line, but rather a sibling doujutsu, they are related, but contrary to the common belief, Sharingan doesn't lead to the Rinnegan, not directly in the least. It just so happens that Hagoromo had both the Sharingan and Rinnegan, so getting the other half of his chakra (Asura's) in addition to Indra's chakra grants access to it, but it doesn't evolve the Sharingan into it. As Madara demonstrated, Sharingan was still accessible after him having awakened the Rinnegan, thus no evolution took place, he just awakened a second doujutsu.

About just anyone having Hagoromo's chakra not being enough, I agree, that seems to be the case. Only Asura or Indra incarnate can get Hagoromo's Rinnegan. Obito had Hagoromo's chakra in form of one of Madara's Rinnegan and was also the Ten-Tails jinchuuriki on top of that, yet he didn't naturally awaken a second Rinnegan eye or the Rinne Sharingan, thus access to Hagoromo's full power truly is restricted to incarnates of his sons.

Yet another reason why Orochimaru and Kabuto's hypothesis was incorrect. It's not Uchiha power (Sharingan) + Senju power = Rinnegan nor Sharingan > Rinnegan. It's Indra's chakra (Sharingan and fragment of the Rinnegan power) + Asura's chakra (body power and fragment of the Rinnegan power) = Hagoromo's restored chakra thus the pieces of power merge and the access to the Rinnegan besides the Sharingan is unlocked.

For Momoshiki's Rinnegan, I have zero idea why it's red and on his palms, but I believe Momoshiki was purposely made by Kishimoto to explain this thing to the fanbase, thus I'm basically correct. Another individual with the Rinnegan, but someone who doesn't have Hagoromo's chakra, which says several things:
 * Momoshiki has the Rinnegan by other means than Hagoromo's chakra, so Hagoromo most likely wasn't the first person with Rinnegan, there might have been others besides him on other planets
 * Momoshiki's chakra/power despite having the Rinnegan, more than certainly doesn't also include the Sharingan, unlike Hagoromo's, yet proving that one can have the Rinnegan without having Sharingan, therefore the former isn't progression of the latter.