Back to the original question of the thread, my firm belief based on everything I’ve researched about diet and nutrition is that we evolved to need animal foods in our diet, period. This goes way beyond B12 - the majority of the vitamins, minerals, amino acids, essential fatty acids, etc our bodies need come from animal foods.
Before you string me up or post endless links of vegan propaganda pieces (I was going to say articles, but the vast majority of the ones I’ve read skew science to serve an agenda, so I’m sticking with propaganda), hear me out. Yes, plant foods are rich in vitamins & minerals, etc. But by and large they do not exist in the FORM our bodies can utilize. Yes, our bodies can convert many/most/all (not sure?) into forms that we CAN use, but we just don’t do this efficiently compared to true herbivores.
Take omega-3’s, for example. Plant oils are rich in omega-6’s, naturally. The problem with just eating omega-6’s is that we evolved to have a certain balance of omega-3’s to omega-6’s, because the former are anti-inflammatory to the body while the latter are inflammatory. Too many omega-6’s, and you have chronic inflammation problems (like the majority of people in modern societies).
Yes, there are some plant sources of omega-3’s, and all plants have some amount of omega-3’s. But the plant form of omega-3’s is ALA, which is not the form our bodies can use. We need EFA’s or DHA’s, and while our bodies can convert ALA into EFA, it only does so at an efficiency rate of 8%. Cows and other grass-eaters, on the other hand, convert ALA at a rate of 100% efficiency. They were, quiet simply, designed for it.
Not only were we not designed for it, but we were designed to be dependent on herbivores to be our intermediary, converting nutrients from the plant world into the forms that we need to survive. If we had evolved to eat plants exclusively, we would convert the nutrients ourselves just fine - but we don’t.