User blog comment:NickFusi0n/A Theory on The Coordinate and The Wall Titans/@comment-121.54.54.248-20140608174101/@comment-24930713-20140608214443

I feel that's highly unlikely. The beast titan didn't know what 3DMG was. So it can't be anyone within the walls. Plus the gear that Captain Ackerman and his crew use is an advanced and better version of the 3DMG that the other military branches use. Not to mention if he was the beast titan, why not just turn all of humanity into titans while they slept. Or destroy them, since he already lives within the walls.

I personally think more of humanity has survived the titans initial attack.(before the story takes place) And is currently living in various areas outside the walls. The "Ymir's people" part is what makes me think it's possible.