It has always been my understanding also that we owe our large brains to the
consumption of animal fats, especially omega 3 rich fats such as that from
animal brains.

It's interesting also that brain contains many other important nutrients. I
once calculated that if a typical animal brain weighs 3 lbs then it takes
only about one brain per day to meet the recommended daily allowance of
vitamin C. While it is unlikely that early humans actually ate a brain per
day, the regular consumption of animal brains would have gone a long way
toward meeting their needs for vitamin C after they left the vitamin C rich
jungles.

-gts