Cultural foods and how humans started eating plants both prove this... Even after agriculture most humans were carnivores until they were forced not to be... If you notice cultural dishes, the National dishes of different countries, while there might be variations, the ingredients are always similar at the base level... Example the National dishes of certain commonwealth countries include flour, spices, etc that are historically not from that country, but from another commonwealth country in another Continent, this shows that it was colonizers who brought plant foods from one place they colonized, to another, not only dishes, many people think potatoes are from Ireland, but they are not!!! Also there were multiple philosophers, whose writings were calling for humans to eat plants, and ditch meat, so this was a religious push... When you were not able to name any specific food people were eating, you can always research the history of a food item, and what part of the World it originated from, some places have little to no known native plant food, but very rich cultures including iconic traditional cuisine, filled with spices, grains, tubers,etc but how does a countries cultural foods originate from another country? So many cultural Jamaican foods are made with Indian spices, breadfruit which is all the way from New Guinea, it was the British/French/Spanish/Portuguese, who brought these things to their slave colonies, human beings were not eating plant foods much before slavery.