Re: Vegetarians of note
Offhand I would say the same way that they know what dinosaurs ate millions of years ago: first by knowing from the fossil record what plants and animals were available, next from examining cuproliths (fossilized poop, from examining the contents of fossilized intestines, and from examining the bones and fossilized foodstuffs in encampments and around campfires. Well, actually very few dinosaurs had campfires.
: )
From the Dekker encyclopedia:
"Anatomically modern humansm (Homo sapiens_ began to colonize the continents as foraging hunter-fisher-gatherers some 100,000 years ago, but it was not until about 12,000 years ago that farming began to replace foraging as the main mode of human subsistence. It did so first, and very gradually, in the so-called Fertile Crescent of Southwest Asia. In other regions of the world, such as central China, northern tropical Africa, and Mesoamerica, primary (independent) transitions from foraging to farming also occurred, even later than in Southwest Asia, but by 1500 a.d., when Europeans were beginning to expand overseas, most of the worldaposs population (estimated at 350 million) had become dependent on agriculture."
Genetically, man has changed very, very little from the time when man had eaten vegetables, fruits, nuts, roots, tubors, insects, fish and wild game. Grains had not been domesticated and cultivated (much less genetically modified) and made up very little of the diet. Same thing for Cheerios and Happy Meals.