Steve: I think the genesis of the paleo diet has to do with the bad things that happened to humans when we went from a hunter-gatherer diet to a grain-based diet with the advent of agriculture. Lower life expectancy, slighter stature, and smaller muscles are documented by anthropologists. Also, there were no overweight cave men, but lots of overweight grain-eaters.
Fast forward to today and with the prevalence of refined sugars and starch in processed food and drinks obesity is epidemic.
Slightly off-topic, but if you frequent Starbucks, next time you're there waiting in line, listen to what people order and compare it to their physique. You won't hear many (if any) skinny people ordering any of the crap with tons of sugar, like a "double mocha frappacino."
