Humans should eat lots of veggies, meat when they can get it, and fruit in season. But grains and legumes were/are mostly poisonous and indigestible without (relatively speaking) modern processing and some people’s metabolism cannot run on them well. Some get quite sick. Pockets of genetic groups (asians/middle east) may do better with grains but they are killing the rest.
We seem to be able to get meat pretty regularly these days.