There are big reasons to be skeptical about the claims of Ornish, McDougall, et al., especially given the evidence that low carbohydrate diets have in improving major heart disease and diabetes risk factors.

I don't agree with the OP necessarily that agriculture led to the above diseases (agriculture is a very broad term), but it is true that these are "diseases of civilization" - such disease have conspicuously shown up under Western dietary influence. Cue Gary Taubes and his book, Good Calories / Bad Calories.