User blog comment:MeDeoPal/On the Origin of Titans and Shifters/@comment-99.3.104.175-20140705015106

erens father knows how this all happed maybe erens older family that lived 100 years ago made the titans and didnt tell anyone so they kept it a secret. and thats how eren became a titan too in episode 3 when they reach wall rose he was thinking back on what happed to him...when his father gave him the shot to become a titan shifter. his dad even said it was the titan power if u read then subtitles. because it didnt show any scene of his dad and eren in the giant trees and he suddenly got the key and mikasa(thebadass) didnt realize.{like wtf u dont see a key on his neck -_-} and how the others got titan shifter, we dont know yet maybe the shots got passed to certain people who just hated the world like annie and  her father.we dont know about reiner and bertholdt yet. going back to when erens dad gave him the shot... he said iit was for his good and to use it willingly so thats all i got to say. <3 hope this might help you all.