From reading various stories mentioned in the mishnah, it seems that students studied directly from their rav. I assume that they reviewed what they studied, and, perhaps, discussed ideas with each other.
But, there doesn't appear to be a mention to chevruta style learning which is common in yeshivot and Batei Midrash, today.
Who thought of this idea and why? What was the reasoning or motive? When did this idea begin?