User blog comment:MeDeoPal/On the Origin of Titans and Shifters/@comment-202.160.18.118-20130815161048/@comment-108.236.225.129-20140731034031

Eating berik is what gave yamir the ability the sahpeshift, later on in the manga yamir says that before that happened it was like a nightmare because yamir said she or he was just a titan roaming the walls for roughly 60  years and it was a blur.