Off and on over the years (and now almost exclusively) I have subscribed to some form of the paleolithic diet, most notably articulated by Loren Cordain. The basic idea is that human beings have evolved and that, in particular, we have evolved over millions of years to eat a diet that is rich in lean meat and seafood, fruits and vegetables and nuts. We are not well-adapted for a diet high in carbohydrate rich foods like starchy vegetables, legumes, grains or dairy. Even worse are the unnaturally high levels of concentrated sugars and salt.
These basic guidelines give the content of the original human diet. But there is a further question concerning the pattern of eating in hunter gatherer society. Cordain and his colleagues are starting to address this issue (which can be found in his latest newsletter). And it appears that hunter-gatherer dietary patterns are at odds with conventional (though unsubstantiated) conventional wisdom in the health food community. Hunter-gatherers do not "graze" on small meals throughout the day. The typical pattern appears to be one of intermittent fasting: one large meal in the evening (sometimes with a small meal consumed in the morning).
It makes sense to me, that is exactly how I do things when I am out hunting all day. It is only when I sit here behind the computer that I get tempted to hit the fridge for a mid-day meal.