HAMM... im confused,needing help! :/

HI everyone. I have been with a doubt about the history of the rinnegan. supposedly the clan, Uchiha and Usumaki were created by two brothers who were born with a certain power that came from the sage of six paths right? thus we can conclude (I think even the anime concludes) that the junction of the powers would creat the rinnegan. So how can (is in the Manga),the rinnegan is the evolution of the sharingan? And there are rumors of the possibility of Sasuke get this power. Wtf? Please I want your opinions. Im very confused.