People can live without eating animals (at least without seeking them out; getting small animals in your food is practically inevitable). However you’re right,

People can live without eating animals (at least without seeking them out; getting small animals in your food is practically inevitable). However you’re right, it’s very difficult and impractical without modern agriculture. Apart from a good understanding of macros and vitamin B12, there’s not much else a vegan needs from modernity. B12 should be present in all foods too, but in modernity it’s simply too clean to get enough without supplementing. Even livestock are supplemented with it in feed lots where they aren’t exposed to much outdoor microbe-laden plant matter and water. I’m not sure how well a vegan would do if they just ate plants from a relatively “clean” field without washing the food too much; maybe they’d actually be just fine. Bizarrely enough, even meat eaters can be somewhat low in B12 in the USA. It’s a vitamin that’s generally not in excess in a lot of diets, omnivore or otherwise.