You are mistaken if you think that hunting was the primary source of food in times before agriculture. For many tribes it was (and still is, in some parts of the world) a secondary "luxury" which augmented the gathered food. As you say, hunting can easily fail, or game can become rare because of environmental hiccups. This happens more often than you should think - there are times when deer can't be found for weeks. Blood sugar was mainly kept up with berries, roots, and fruit - nibbling all day long. Fat stores are not so easily accessed - if they were, then all-week-fasting would be the way to go for fat-loss (which mostly leads to muscle wasting and short-term water loss, and a mighty yo-yo effect). Yes, one can use intermittent fasting to lose weight, but weight-loss usually only kicks in after 2-3 weeks.

I agree that Cheerios are a bad idea generally, but I'm not pro fasting or skipping meals for weight loss. Fat is stored as an emergency reaction to high blood sugar levels, and you get exactly that when eating more between the fasts (you have to). You constantly expose your brain to too much and too low sugar levels, resulting in highs and lows in your ability to focus (and also your mood).