I just realised, that can't be possible considering the Fourth Hokage was the person who created the technique which was obviously after the era of the First Hokage... Though that being said, there could always be some sort of technique that allows a person to split another's soul or body into two, which would basically split apart one's own power into halves. With Madara trying to 'fake' his own death by allowing the First to kill him, when his other half planning to resurrect himself at a later date. So basically, Tobi is a shell so to speak and will try to reunite himself with his entire body and gain back all of his abilities he once possessed. It's a stretch, but it's a possible theory.

Enma wrote:

I'm still thinking and using my old theory that Tobi is Obito, who was possessed by Madara's spirit when he was dying. After all Obito was just about showing potential - who is to say, that he wasn't in fact a prodigy.

Now this I've always found hard to believe. If we think back to the point where Obito was close to death, half of his body was a pancake thanks to the boulder that was crushing him. Then after the rocks came tumbling down, he was either a rolled up tube of toothpaste or a full pancake. No way would anyone be able to survive such a cave in like that. Tobi has also been seen fighting with the Fourth Hokage and at the time Kakashi was still in his teen years which would mean that Obito would also be in his teen years, so why was Tobi a full grown man? Right now, I'm finding it hard to believe that it's actually Obito.