Attack on Titan

It's recently come to my attention that there isn't a thread for this amazing anime.

So... What are your thoughts on the origin of Titans?

Granted I'm only about halfway through the first season, but I feel like they're 'organic' somehow. They feed off of sunlight, can regenerate and reproduce by eating humans (turning them into other Titans somehow).

I'm also like 99% sure that the Colossal and Armored Titans are also like Eren and are probably main characters in the show right now. Just trying to figure out the best candidates... :3