…it appears that non-pathogenic soil microbes, human small intestinal bacteria, lactobacilli from fermented foods, some sea algae, common mushrooms, and plants grown on soil fertilized with animal manure can all can provide biologically active B12. Any of these could have served as ongoing sources of B12 for prehistoric human ancestors, but modern circumstances may make these non-animal sources of B12 unreliable for modern humans.
I think it safe to assume that our prehistoric ancestors had more contact with soil than we do, sitting on it, sleeping on it, digging in it, and drawing water from sources in contact with the soil. Humans like other primates are apt to touch their own lips from time to time, providing a vector by which soil microbes could enter the human gut.
Humans living in modern industrialized nations typically ingest multiple courses of oral antibiotics over a lifetime, reducing or eliminating the population of B12-producing bacteria residing in the small intestine. All of our prehistoric ancestors would have been breast fed and probably kissed often, which transmits flora from one generation to another, and this transmission would not have been interrupted by antibiotic treatments.
Fermentation of plant foods, particularly fruits, occurs spontaneously in nature, providing another route by which our ancestors may have ingested B12-producing lactobacilli. Our ancestors almost certainly consumed any edible wild mushrooms and all of the plants they ate grew in soils teaming with bacteria and fertilized by fermented organic wastes, providing another B12 source.
All of this information suggests that modern hygiene, indoor lifestyles, antibiotics, and use of chemical rather than biomass fertilizers in farming have reduced the amount of B12 available to humans in modern urban environments from non-animal sources.
Thus, the low availability of B12 from non-animal sources in modern urban environments is an artifact not reflective of preindustrial environments, and it appears probable that our prehistoric ancestors had more non-animal sources present in their environment, like the southern Indians studied by Albert et al. …