A lot of people correlate it with humans moving from a vegetarian diet to a omnivorous diet. 1. Higher nutrition levels allowed the brain to grow. 2. Hunting

A lot of people correlate it with humans moving from a vegetarian diet to a omnivorous diet. 1. Higher nutrition levels allowed the brain to grow. 2. Hunting required higher levels of strategy and tactics than picking fruit off trees. 3. Not needing to eat continuously (as we did on vegetation) to get what we needed allowed us time to put our efforts into other things. Now did the diet cause the change, or the change necessitate the change in diet... I don't think we know.