Why bigger brains?
Theories about the origin of the large human brain have focused on many aspects of behavior that were supposed to have driven this change...
Scholars have tended to assume that the reason that brains did not get larger earlier is that they were not needed. A new theory, the "expensive tissue hypothesis" has argued instead that brains could not become larger earlier, because they used up too much of the body's energy -- ounce for ounce, the mammalian brain uses nine times as much energy as the rest of the body, on average. Leslie Aiello and Phillip Wheeler point out that five major organs or organ systems use up 60-70% of the body's energy at rest, although they account for only 7% of the body's total mass. These "expensive" organs are the gut, the heart, the liver, the kidney and the brain. (Lungs are also quite "expensive.") Unless the animal eats a lot more high calorie foods (very unlikely in the case of humans, to judge from the teeth) or one of these organs gets smaller, there is no energy budget left to feed a larger brain.
What got smaller around 2 mya that allowed the brain size to finally increase? The heart, liver and kidney are scaled to body size (mass); they cannot get smaller unless you do. The only remaining possibility is the gut, which could become smaller if foods were either higher quality or partially "digested" outside the body by tools.
Babies, Brains and Bone Marrow
In many ways, humans have guts like other apes. Food travels slowly through the intestines to allow time to absorb nutrients from fibrous plant foods. The human gut differs from those of other apes in one important way - it is far shorter. Mammals with short guts tend to be carnivores, because animal foods are easily digested. With meat, the nutrition comes in smaller, low fibre packages, which means less processing time is needed in the gut. Eating meat may also help to reduce the effects of undigestible or poisonous substances in the plant foods that are eaten.
Modern humans have smaller guts than would be expected if we were ordinary apes, which supports the archaeological evidence that our ancestors embarked on a meat-rich diet. There are even more important repercussions connected with this fundamental change in dietary strategy. The evolution of a smaller gut freed up energy that could be used elsewhere. The most obvious place to benefit from an increased share of the body's energy budget is the brain. The fossil record shows that Homo erectus brains were about twice as big as those of the australopithecines.
So, meat in the diet also allowed the evolution of a larger brain.