Yoda's origins, or his history outside that shown in the films, has never been described.
Along with the true story of how Chewbacca and Han Solo came about, it is one of the themes that has been decreed as 'forbidden' by Lucasfilm. Hence why anything that is considered canon by Lucasfilm has never detailed this.
Will it come to light in Ep 3? I have no idea, but no one has been allowed to do anything thus far.
- S
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