Board Thread:Theories and Speculation/@comment-33485854-20180625233306/@comment-1512913-20180628113518

You don't need a Sharingan to get Rinnegan. Just because Indra's + Asura's chakras = Hagoromo's chakra = Rinnegan, doesn't mean a Sharingan is required. Sharingan just happens to be part of Indra's chakra. It's just the way the power split.

Rinne Sharingan split into Sharingan and Rinnegan in Hagoromo, Indra inherited Sharingan, while the Rinnegan split between both Indra and Asura. It could have split another way too, Rinnegan with no Sharingan etc.