User blog comment:MeDeoPal/On the Origin of Titans and Shifters/@comment-155.158.60.209-20131118075732

According to the newest manga the titans are a failed weapon project. At the begining, human civilazation was more advanced than it is the the latter storyline.

Back to the weapon a.k.a. "The Human Extermination Project". The problem was that some humans would lose themselves to the titan body and run amoc, killing everything. Others however would take control of the body and be able to use it. Eren as of now has the same power the eldest titan wielders have. Namely, controlling the titans at will.

As of the ape titan, I believe that it's one of the oldest ones, hence the appearance.

Also, the first titans were able to infect and shape-shift, turning others into titans, which would explain a whole village turning into titans(a theory mentioned in the manga chapter 51).

I dunno if this is correct cannon, but those are my speculations considering the information given.