Fermentation

Interesting hypothesis, but it seems very unlikely. Safely fermenting any calorie-dense plant matter in the tropics without airtight containers or other equipment would be more a matter of luck than anything. It would not be a reliable means of increasing caloric intake for an entire population. A much simpler explanation for human brain expansion? Sweet fruit. No tools, no fire, no difficult digestion. Neuroscientist Tony Wright has researched this possibility extensively.