Your body doesn't care where protein comes from, as long as you're getting all the essential amino acids.  Protein is broken down to amino acids, and at the molecular level, it's absolutely identical no matter the source. The amino acid, leucine, from pond scum is exactly the same as leucine from a nice sirloin, is exactly the  same as synthetic leucine. 

The hunter theory is based in circumstances that no longer apply. It's very hard to forage a balanced plant-based protein mix. Protein rich plants are hard to find and you need a variety to get all your nutrients, whereas meat provides a very convenient and accessible package that's available year round in regions with distinct growing seasons. That's really all it is. 

Thanks to that newfangled agriculture, and nutritional awareness, it is perfectly feasible to get what you need without meat. We are no longer hunter-gatherers. Eat meat if you want, but there's no "natural reason" you must.