User blog comment:G4bbster/snk discussion // spoilers/@comment-10733658-20130801082238/@comment-5047068-20130801182110

Having been a fan of the Resident Evil series for a long while, I think I can bring up my reeking suspicion that Grisha Yaeger's syringe (seen in Eren's dream sequence) contains some sort of substance or biotic that can transform humans into titans, or into titan shifters. Given that titans have little or no realistic biology even in the world of Shingeki, and by all scientific accounts, are freaks of nature, my guess is that titans are a radical mutation from a humanoid level.

This is both what I use to explain the familiar titans in Conny's home village, and to explain the seemingly unlimited number of titans outside the walls and now inside Wall Maria. Think: if there were a natural source of whatever substance is capable of causing a human transformation into a titan outside the walls, then any human outside Maria, assuming there are some, would be turned into a titan. Again if there are a large number of humans holed up outside the walls, it's also possible that someone is forcing people to become titans against their will, the way Grisha turned Eren into a titan shifter. If we take the mutagen theory as true (which you don't have to, as it's just my speculation), then the titan shifters (Eren, Reiner, Bertholdt, Annie, Ymir) may have been subjected to a more refined strain of the mutagen. If that were so, you could also take Bertholdt's incredibly large titan form for a mutation. With the mutagen theory, it's possible that the original purpose was to provide humans with longer lifespans and healthier metabolisms, or even special attributes to defy death, given that titans as a result:

-can regenerate any part of their bodies up to an including their entire heads and brains, save for one select point (the napes of their necks).

-literally do not need to eat or drink in order to survive, and consume humans to kill, not to eat.

-can survive complete dismemberment and be unhindered, although pain sensory is an individual factor.

-decompose directly after death, eliminating the need for burial or to dispose of corpses.

As for the Ape Titan, I think that came from outside the walls, possibly to investigate the recent happenings around mankind's fortress, as he knew of the 3DMG, but not of its complete functions, and was also able to speak a human language, further attributing to the idea of transfigurative mutagens and refined strains. It seems to be actively and overtly hostile to humans, but patient.