Board Thread:Versus Debates/@comment-33229263-20171019104931/@comment-9782468-20171021064752

Or, it's in levels.

The Rinnegan is a level before Rinne Sharingan and the Sharingan is a level before that.

So Hagoromo, under his own growth, could manifest up to the Rinnegan (without the Juubi or anything). And it was shown that he evolved it from the Sharingan in the anime.

His kid, Indra, had the eye power and manifested the Sharingan. The extra power required to push it to the Rinnegan lied with his brother, Asura.

That's what makes the most sense to me.