Ruminants (cattle, sheep, and goats) get B12 from bacteria which are the primary digesting organisms in multistage stomachs, and which are absorbed in the upper (small) intestine, given feedstock with sufficient cobalt nutrient from the soil in which it was grown. Any B12 produced in human digestive tract occurs in the lower (large) intestine which is too late for absorption into the circulatory system of vitamins and nutrients. This means that what trace production may be produced in human digestion, whether it is increased or reduced by natural exposure to trace cobalt in the soil or water we consume has no effect on metabolic B12. Natural soil and water do not contain sufficient B12 producing organisms to result in this being a primary nutrient source, even in a theoretical 'stone age' level of uncleanliness.

Livestock which are fed grain and leaf forage grown from soils which are deficient in cobalt may not produce sufficient B12 for their own metabolic health and thus most commercially reared livestock will have trace cobalt compounds in the salt lick or mineral amended grain supplements.

These generally do not actually contain B12, as these compounds do not directly pass through the complex digestion of ruminants and would not actually serve the muscle growth targets of the herd, whereas trace cobalt will get those results reliably.

In turn those animals are the nutritional source of B12 for humans, as the two primary bioactive forms, methylcobalamin from cellular cytosol (the internal gel in cells) and adenosylcobalamin from mitochondria.

Vegans can reliably get non-animal sources of B12 from nutritional yeast or from manufactured sources which make the two bioactive forms through bacterial fermentation (typically Pseudomonas denitrificans and Propionibacterium freudenreichii <~ which is the Swiss cheese bubble-causing species) and then stabilized as cyanocobalamin through the addition of potassium cyanide in the presence of nitric acid and heat, which converts back into those two bioactive forms through human digestion.

If we eat meat that was raised to be healthy, then we don't need supplements to get enough B12 to be healthy. If we choose not to eat meat, then sure, supplemental sources will be important to meet metabolic needs. But, dirty roots and cloudy water are not the natural 'wild' source of B12 for humans in any reasonable imagined version of the stone age.