You ultimately get B12 from bacteria. They even culture something called "nutritional yeast" which is relatively high in B12. From what I have heard, they let the bacteria feast on molasses, and the dried resulting flakes are what are known as "nutritional yeast". No actual, higher-order "animals" involved.
It is also possible to get B12 from contaminated (i.e. crapped on) vegetation. Basically, the bacteria in the gut of many animals produce quite a bit of B12 (rats even have a second digestive tract that passes waste through a second time, after the bacteria has a chance to manufacture B12), and so it is present in fecal matter.
There is a good amount of evidence to suggest that large quantities of meat were never expected to be a part of our diets. Some, perhaps. But most people eat far more (nearly ever meal for a lot of people) than we're designed for. For example, full-blown carnivores have a very short digestive tract, which means that rotting meat (and all the bacteria that comes with it) doesn't stay in their systems for very long at all. Our digestive tract, on the other hand, is enormously long. This means that any meat we do eat is bound for a pretty long journey through our system, and this isn't an advantageous thing, hence the reason long digestive tracts were bred out of, or avoided by developing carnivores. Our human propensity and ability to "cook" meat saves us here quite a bit, as we have killed most of the bad bacteria by the time it hits the relatively "long haul" processes of our digestive system. So, if we were eating raw meat (the way all other animals do), then a lot of people continuing with their current levels of meat consumption, and given their long digestive tracts, would likely run into complications.
So yes, we're omnivores. A simple examination of our teeth confirms this. But by no means does this mean that we need meat at every meal, or even every day, or even at all!
As an aside - as far as dairy goes - in what way would it even make sense for us to develop requiring the gestationally produced milk products of an entirely different species? In short... we don't require it. At all. It is concentrated nutrient, sure. But if you listened to (for example) the US FDA at times, you'd walk away thinking we humans need dairy. This simply cannot be possible. Need calcium in some form? Sure. But need "dairy"? No.