I've always thought the metabolic needs must have been just as driving (specifically, being carnivores). Note: not a dig at vegetarians, we're talking about long before the proliferation of tofu and quinoa.

"A gram of brain tissue takes 20 times more energy to grow and maintain than a gram of tissue from the kidney, heart, or liver"

http://news.harvard.edu/gazette/story/2008/04/eating-meat-le...