Humans require vitamin B-12, and only animal products reliably provide natural bioactive vitamin B-12, therefore we must be adapted to and dependent upon meat-eating.
Humans have enterohepatic circulation of vitamin B-12.[1 ] As noted by Herbert [2 ], this can allow an initially B-12 replete adult go 20-30 years without vitamin B-12 intake:
“The enterohepatic circulation of vitamin B-12 is very important in vitamin B-12 economy and homeostasis (27). Nonvegetarians normally eat 2-6 mcg of vitamin B-12/d and excrete from their liver into the intestine via their bile 5-10 mcg of vitamin B-12/d. If they have no gastric, pancreatic, or small bowel dysfunction interfering with reabsorption, their bodies reabsorb ~3-5 mcg of bile vitamin B-12/d. Because of this, an efficient enterohepatic circulation keeps the adult vegan, who eats very little vitamin B-12, from developing vitamin B-12 deficiency disease for 20-30 y (27) because even as body stores fall and daily bile vitamin B-12 output falls with body stores to as low as 1 mcg, the percentage of bile vitamin B-12 reabsorbed rises to close to 100%, so that the whole microgram is reabsorbed.”
Currently the National Academy of Sciences recommends that adults consume 2.4 mcg of B12 daily. They calculated that this covers the needs of 98 percent of individuals, but most of us require less than this. The following table shows the B12 contents of commonly consumed animal products:
As I said above, I previously accepted that only animal products reliably provide natural vitamin B-12. Although this is a common belief, and probably a good general rule in modern industrialized nations, I think we have significant evidence that pre-industrial humans had other significant sources of vitamin B-12.
Bacillus megaterium is a common soil bacteria, not pathogenic to humans, and a producer of vitamin B-12. According to Patricia Vany of the Department of Biological Sciences at NIU, B. megaterium occurs in human breast milk.[15, third slide]
Lactobacillus reuturi, a member of the gastrointestinal ecosystems of humans, poultry, swine, and other animals, and present in sourdough culture, produces vitamin B-12. 
[In 2005 Croft et al reported that algae acquire vitamin B12 through a symbiotic relationship with bacteria. (14) ]“A nutritional analysis was conducted on the dietary intake of a group of 6 vegan children aged 7 to 14 who had been living on a vegan diet including brown rice for from 4 to 10
years, and on that of an age-matched control group. In addition, their serum vitamin B12 levels and other data (red blood cell count, hematocrit, hemoglobin, etc.) were determined in the laboratory. In vegans' diets, 2-4 g of nori (dried laver), which contained B12, were consumed daily. Not a single case of symptoms due to B12 deficiency was found. There were no statistically significant differences between the two groups with respect to any of the examination data, including B12 levels (p < 0.05). Therefore, consumption of nori may keep vegans from suffering B12 deficiency.” 
“High concentrations of vitamin B12 were also detected in the flush mushrooms including cups and flats. HPLC and mass spectrometry showed vitamin B12 retention time and mass spectra identical to those of the standard vitamin B12 and those of food products
including beef, beef liver, salmon, egg, and milk but not of the pseudovitamin B12, an inactive corrinoid in humans. The results suggest that the consumer may benefit from the consumption of mushroom to increase intake of this vitamin in the diet.” 
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.
My Fallacious Appeal to 'Nature'
When I previously argued that meat-eating is the 'natural' way to get B12, I committed the fallacy of appeal to nature.
The problem here lies in these underlying assumptions: 1) all 'natural' behaviors are 'health-promoting' behaviors for modern urban humans, and 2) all 'unnatural' behaviors are unhealthful.
Consider these questions:
But is the 'natural' choice of our ancestors the best possible choice for modern humans of the present day?
Simply put, the fact that our ancestors did something then does not tell us that it is the best thing for us to do now.
The fact that our ancestors obtained B12 by a 'natural' route (eating meat) does not tell me that this is the optimal way for me to get B12 in our modern circumstances.
Artificial synthesis of B12 requires about 70 synthesis steps, making it impractical as a method for commercial production of B12. “Therefore, today vitamin B12 is exclusively produced by biosynthetic fermentation processes, using selected and genetically optimized micro-organisms.” 
In short, B12 supplements are the most reliable source of natural B12 in the modern environment.
Microbial B12 Supplements Recommended To People Past 50 Years Of Age
The Linus Pauling Institute at Oregon State University recommends that all people (including omnivores) over the age of 50 take a B12 supplement:
“Also, individuals over the age of 50 should obtain their vitamin B12 in supplements or fortified foods like fortified cereal because of the increased likelihood of food-bound vitamin B12 malabsorption.”
Do we reject this advice because it is not 'natural' to take supplements?
Modern humans typically use their minds to identify their requirements for health and comfort, then develop and use appropriate technology to provide those requirements in the most efficient, safest possible way.
Take your pick, or perhaps you will discover another way.
Thanks to the author/producer of the Primitive Nutrition video series for alerting me to the article on the B12 content mushrooms used in this post.