Want a bigger brain? Then carb load

At least that's how our ancestors did it according to this science article. To paraphrase a credit card commercial: What's in your diet?

"The brains of our ancestors doubled in size between 2 million and 700,000 years ago. Researchers have long credited better stone tools and cooperative hunting: As early humans got better at killing animals and processing meat, they ate a higher quality diet, which gave them more energy more rapidly to fuel the growth of their hungrier brains. 

"Still, researchers have puzzled over how meat did the job. 'For human ancestors to efficiently grow a bigger brain, they needed energy dense foods containing glucose'—a type of sugar—says molecular archaeologist Christina Warinner of Harvard and the Max Planck Institute for the Science of Human History. 'Meat is not a good source of glucose.' The starchy plants gathered by many living hunter-gatherers are an excellent source of glucose, however."