I have had a passing interest in human evolution ever since I became human (don't ask). Well, it seems as though the link between cooking and brain size has become less speculative. Not that it was that bad anyhow, it's simply a bit more obvious.
There seems to be a rather massive shift in human evolution about 1.8 million years. Certainly adaptations for running, taller, less muscle bound, secondary adaptations. Probably all kicked off from fire.
Fire allows cooking. Cooking gives more calories from typically inedible food. Less muscles are needed. Running with improved calories and bipedalism becomes improved strategy for scavenging and for running down prey (you could chase an animal until it can't run marathon style). Hairlessness evolves as to improve sweating ability and regulate heat better for running and existence in the savanna, downside of freezing at night countered by shelter and fire. Humans then are quite efficient. Able to hunt successfully and have a much more broad range of possible foods and do not require large digestive tracts or half a face of muscles to chew (don't let a chimp bite your finger). This extra time, allows humans to sit around the camp fire solving sodukos and jumbles, until their brains got big.
Okay, needs a bit better picture for that last step. Having the resources to do well in many climates why an absolutely massive brain? No other improvement rendered any greater survival rates? Socialization significantly advantageous? Prediction of resources? Necessity to replicate the acts of making tools and fire? Need for complex language to help describe the processes required? Our brains do largely differ in our mirror neuron systems so we are more able to replicate and learn by seeing the acts of others. Needed success in a new arms race between groups, required intelligence to accelerate the evolution of tools and strategies?
Or, if one is to believe the Aquatic Ape Hypothesis, all this research is just poppycock and the truth has something vague to do with water.