Comment by the_gastropod

Where do those animals get their B12 from, do you think? (Hint: they don't synthesize it either, they get it from their diet)

Today, because our food / water are so sterilized (good thing!), we don't typically get much bacterial B12 from unclean root vegetables, drinking from streams, etc. So yes, we need to supplement today. Because of advances in cleanliness.