Some scientists claim that our brains grew because of our consumption of meat.
That your typical vegan diet couldn't support the huge amount of energy required by the human brain.
We are smart because we consumed animal flesh.
In the animal kingdom, grass-eating mammals have smaller brains in proportion to their bodies. Manatees are a shining example. They have huge, thick skulls, but the acutal braincase is about the size of a small orange. No wonder they aren't smart enough to learn to avoid the propellers of noisy boats. Their brains are much smaller than a comparably sized dolphin, another aquatic mammal, who eats ONLY meat. The only exception I can think of in the world of herbivorous mammals is the elephant, which is pretty intelligent, but then again, the elephant is VERY large, hence has a fairly large brain.
But the proportion of brain size with regard to body size in mammals (we're not talking birds or reptiles here) is MUCH larger in carnivores. A horse is smart enough to learn stuff, but look at how BIG he is. A large dog probably has a bigger brain than a horse ten times his size. Cats have especially big brains with regard to their body sizes -- interestingly, although the sabertooth tiger was a LOT bigger in body than a modern cougar, and although he had a big skull, the acutal brain case in that skull allowed for a brain perhaps smaller than that of a modern cougar. In other words, the sabertooths weren't all that bright. Maybe not smart enough to stay away from the tar pits!
I think they might be on to something here: The vegans I have known have could not hold a thought for more than a second or two. They were airheads.
I know that if a breastfeeding mother follows a vegan diet, it is recommended that she takes fish oil, in order that the baby's brain not be malformed.
Which may explain why vegans are often mentally *off*! But no, really, with regard to how our brains "grew" to their present size, it is known that any mammal fetus requires protein for the development of the brain cells, and ONLY protein. The higher the protein content in a pregnant mammal's diet, the larger the brain of its offspring. Theoretically, when our ancestors "split" from chimps or apes and our diet changed to one of high protein (as in, meat!), pregnant females started "building" fetuses with larger brains.
Today, when you come across the very STUPID vegan woman who insists on keeping to a vegan diet while pregnant, what her body will do is to rob from her nutrients to "build" the baby. Hence pregnant vegan women are debilitated and weak during their pregnancies, and their babies are often underweight.
Another reason WEAPONS made the man, not the other way around.