Where did humans get the extra energy to support their large brains?

Actually, I've been wondering some busy afternoons where I'm going to find the energy to hold my head up much less support the brain within. Harvard University primatologist, Richard Wrangham has been wrangling, however, with this long-standing riddle in human evolution, namely what allowed Big Head Todd and friends to rise from the lower primates in order to paint in caves, play rock music, and blog on the Internet.

One word, per Dr. Wrangham: Cooking. Big old brains require big time energy; a resting adult's noggin uses 25% of his/her total energy output. If you have to spend the day picking nits out of each other's fur for supplemental protein, you hardly have time to work with your opposable thumb or to invent Folsom points.

On the other hand, if you cook the nits you pick, you in effect predigest your food, so less energy is spent on digesting and more sugar is available to the brain. Per Wrangham, cooking paved the way for the dramatic expansion of the human brain. He notes:

No human foragers have been recorded as living without cooking, and people who choose a 'raw-foodist' life-style experience low energy and impaired reproductive function. The possibility that cooking is obligatory is supported by calculations suggesting that a diet of raw food could not supply sufficient calories for a normal hunter-gatherer lifestyle.
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Wrangham R etal. Comp Biochem Physiol A Mol Integr Physiol. 2003 Sep;136(1):35-46.