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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)?

AliceD
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Probably
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    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. – MattDMo May 16 '15 at 18:58
  • http://en.wikipedia.org/wiki/Vitamin_B12#Foods If cow eats a fly, the species doesn't get carnivore, doesn't? – Probably May 16 '15 at 19:47
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    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... – MattDMo May 16 '15 at 20:07
  • It means that "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" doesn't make any sence. Regarding the difference between self-producing and permanent symbiosis with bacteria in the digestive system you're definitely right, but I don't think it's so significant difference (and if so it has to have some evolutionary explanation)- there is just some mechanism producing B12. – Probably May 16 '15 at 21:22
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    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. – MattDMo May 16 '15 at 22:02
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    Do you mean Vitamin C instead? The statement is true for Vitamin C. – March Ho May 16 '15 at 22:36
  • @MattDMo Ok, but anyway bacteria in animals produce B12, but in humans not. There is no evolutionary process behind it? – Probably May 17 '15 at 21:35
  • Because it would be quite funny if we developed to get B12 only from diary sources and also were able to eat only cooked meat. (you know we would be stuck somewhere between carnivores and herbivores). So no, I didn't mean vitamin C. – Probably May 17 '15 at 21:39
  • There are bacteria in the human gut who produce cobalamins, including vitamin B12. http://www.ncbi.nlm.nih.gov/pubmed/7354869 – skymningen May 18 '15 at 07:34
  • @skymninge Wow, thank you. I was really confused of this from many sources: https://www.google.cz/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=vitamin%20b12%20only%20in%20animal%20products – Probably May 19 '15 at 09:41
  • @skymninge So why humans don't create it enough that vegetarians have to take supplements? – Probably May 19 '15 at 09:42
  • It is still the bacteria creating it. I am not an expert in cobalamin synthesis pathways of bacteria, but I guess they need something which is more prevalent in non-vegetarian food to produce it. After all, the bacteria in you gut mainly eat what you eat. – skymningen May 19 '15 at 09:46
  • @skymninge Wait... so where does the herbivores get it from? Or have they got just more bacteria-producers? – Probably May 19 '15 at 10:55
  • Remember that a herbivore still eats a very different diet compared to a vegetarian. (Raw, high in fiber,...) And they do also have different microbiota, yes. But again, as I am not an expert in the cobalamin synthesis, I cannot explain you the mechanism and thus also not decide how it would be different in a herbivore. – skymningen May 19 '15 at 16:27

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