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All herbivores produce vitamin B12 de novo. Gorillas, for example, are "vegans" so I suppose some human ancestor was also herbivore.

Have we ever been B12 self-producers? If so, why have we lost that ability and do we have to obtain B12 from our diet? Have we just gotten "used to" being omnivores (i.e., we don't need self-production anymore)?

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    $\begingroup$ Can you provide a source for your claim that "all herbivores produce vitamin B12 by themselves"? If that was true, humans would be able to as well, since we consume plenty of vegetables, and some of us are on the vegetarian-vegan spectrum. $\endgroup$ – MattDMo May 16 '15 at 18:58
  • $\begingroup$ en.wikipedia.org/wiki/Vitamin_B12#Foods If cow eats a fly, the species doesn't get carnivore, doesn't? $\endgroup$ – Probably May 16 '15 at 19:47
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    $\begingroup$ from the very first sentence of the section you link to: "Ultimately, animals must obtain vitamin B12 directly or indirectly from bacteria...". Therefore, herbivores don't produce B12 themselves - it comes from bacteria in their gut. I don't understand what your statement about cows and flies means... $\endgroup$ – MattDMo May 16 '15 at 20:07
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    $\begingroup$ Bacteria (and archaea) produce B12, not mammals. Full stop. It is absorbed into the host's body, and passed along when those hosts are consumed by other animals, including humans. The difference between self-production and symbiosis is the point. You claim that herbivores produce B12 by themselves, and that is simply not true, any more than saying humans produce iron because it is found in our hemoglobin is true. Animals have never been B12 producers, because we lack the necessary biochemical pathways. $\endgroup$ – MattDMo May 16 '15 at 22:02
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    $\begingroup$ Do you mean Vitamin C instead? The statement is true for Vitamin C. $\endgroup$ – March Ho May 16 '15 at 22:36

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