Its obvious that the Rinnegan is a purified from of the Juubi's power. It will only appear once the juubi's power is purified and is controlled by the host not by the prisoner. Not to mention that it is very likely that the juubi must be in its final form for it to truly manifest.
Why everyone keeps going on about it being impossible for the Uzumaki to be around at the time of the sage is beyond me. Its likely all the big clans have been around for a great deal of time, certain families around the world can trace their lineage back thousands of years. Royal families or nobles can do this quite easily.
Chakra existed back then before the sage and was likely utilized by practitioners, most likely a part of clans which horded what little knowledge they could glean on the subject of chakra. Chakra wielding Samurai also likely existed, but as can be seen even in Naruto's era they use it in a primitive manner using it in a very limited way.
The Uzumaki likely already had the basics of Fuinjutsu understood or invented in that era. Once the sage was born he pulled all this info together to figure out what the heck was going on with this chakra business. It is impossible for one man no matter how intelligent to single-handedly found an entire school of thought from scratch within his own lifetime. THIS HAS NEVER HAPPENED IN ALL OF HUMAN HISTORY. Every scientist or thinker has always built on what has come before, there has never been a great enough genius to be able to manage such a super human feat. Its like saying the very first scientist discovered physics in its totality and then gave birth to advanced technology ALL WITHIN HIS OWN LIFETIME.
For the sage to come up with the seal he used on the Juubi he needed for there to be a basic school of thought or method ALREADY in place, if he started from scratch its very unlikely that he would have managed to create such an absurdly advanced jutsu without knowing how to make a god damn sealing scroll LOL.
Ergo while the Sage was brilliant he drew upon the resources available to him IE his clan had the knowledge he needed to do what he did within his life time. If he had started from nothing he would have ended with close to nothing, and he would never have become the sage or the bearer of the Rinnegan. Everyone seems to assume that in that era of strife everyone was a dumb ass lol.
From what I can find out about the Sage he had time in his life, ALOT OF TIME, to go around preaching his truth of chakra, IE he didn't stick around most of his life thinking or working in a lab trying to find out what it was. IE he rapidly figured out what it was within his first 20 years of life more or less, sealed the Juubi during or just after that time, and then went off on his world tour lol. Then Split the juubi before his death and created the moon....That's quite the resume, but to make it remotely possible there MUST be a foundation of information/understanding of chakra to work with.
