I really don't understand why others insist that families/clans existed only after Rikūdo's foundation of the Shinobi World, logic and common sense entails that these Shinobi Clans were descended from the Pre-Ninjutsu families/clans that preceded them, look at Japanese history, the Iga and Kõga Clans were a clan of hired mercenaries and sellswords before they became the formally organized Shinobi Clans they were known for since.
The Rinnegan that is able to bring forth all Ninjutsu logically comes from the Jūbi which is the source of all Chakra that fuels ninjutsu, having similitude in divine qualities must logically come from the same source: Jūbi
That was after Rikūdo became the Jūbi's Jinchūriki, The Uchiha and Senju being the Yin and the Yang of the Jūbi, prior to that he was an Uzumaki (or possibly a Hyūga hybrid)
Sir Derp, want to support you on something. The first bolded part. It's LOGICAL that there were clans before Rikudou. Rikudou himself was human, not a god, as you have stated, so he was logically BORN. He didn't fall from the sky and then -boom- other humans appeared...
Someone said something above the Byakugan secrets. Isn't it obvious that kishi focused on sharingan, uchiha and senju and left behind the hyuuga and the byakugan, to foreshadow the "secrets"? He left the hyuuga in the back, because they are the ultimate "surprise", to call it like that. Why did the Cloud want to kidnap Hinata, the MAIN branch Hyuuga? It's obvious that hyuuga have deep secrets, which lie in the existence of byakugan. From this point, we link it with rikudou...and then, we have the complete theory.
People should start considering this theories...:vincent: That's all =D Thanks for reading (if someone had the interest) a wall of text =D