I was thinking of that, and i wonder if the Uzumaki clan line goes as far as the Sage of the Six Paths. The Seal the Sage used to seal the Juubi is the New Seal Naruto has right now, it is clear that the Sage did not gained the rinnegan after becoming the 10 tailed host, also it is very clear that sealing was done way before jutsus came about. Which brings me to my theories.
Theory 1:
Sealing was done way before ninjutsu came about, but this does not explain as to why it had to be Rikudo who was the only one who sealed the Juubi. It could have been anyone else, since it was clear that the 10 tailed beast was wrecking havoc upon the world for a long amount of time.
Theory 2: Rikudo story is similar to Minato's in the sense that they could have been married to someone who taught them how to use sealing. Maybe he had 3 kids instead of 2, but the third might have been too young, or a female. [considering were talking very feudal here] thus not suitable to have been chosen. [Let's just say its female] Then she came to form the late Uzumaki clan, who perished under war.
I like the last because it might explain 2 things, one is Rikudo suddenly decided to reincarnate into Nagato, in the hopes to meet its real heir Naruto that has now been tasked with his dream of bringing peace to the world, since both his 2 sons were disappointing in their hate and sibling rivalry and 2, if that is Rikudo's seal then the swirl in his stomach was clearly with Rikudo when he sealed the 10 tails...Of course, it overturns Madara so called destiny talk about Naruto destined fight with Sasuke, but maybe it doesn't have to be that way.
And last but not least
Theory 3: The Sage was eating broccoli, Juubi attacks the Sage, the Sage fights back with broccoli, leaving Juubi...a vegetable.