B12 come from soil bacteria. Most paleo humans got it from eating unsanitized foods including plant foods. The b12 in animal products has low bioavailability as it is bound to protein molecules. In fact most people get their b12 not from animal foods but from fortified foods like cereals because it's not bound to protein thus easier to absord. So no you don't need to eat animal foods just for b12. Theres no nutrients available in animals that is not available in plants which is because herbivores get all their nutrition from plants. In fact it's the opposite there are plenty of nutrients that plant foods have that is not present in animal foods like antioxidants and phytonutrients.