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B12 is produced by bacteria that live in the soil and in the intestines of animals, however, in humans it is manufactured too far down the intestinal tract, in the colon, to be absorbed, and is instead excreted in feces. In pre-history we would have got some B12 from pond water, but mostly scavenging/hunting animals. I just think most people existed in a state of nutrient defiency throughout all of history and it's only recently with greenhouse growing and international trade that we can plan to eat a healthy varied diet all year round.

Here's a useful video going over the quanitity of B12 in various edible/drinkable items and their distribution around the globe: https://youtu.be/gkQA1MMk_cI