Is veganism a natural human diet?

In short, no.

When you cannot get all your basic nutritional requirements without a supplement manufactured in a lab, then your diet is not nutritionally complete. The only natural source of Vitamin B12 is meat, dairy, and eggs (unless your vegetables are contaminated with human feces). B12 deficiency causes tiredness, a decreased mental work capacity, weakened concentration and memory, and irritability and depression. If allowed to progress, it causes anemia and nervous system degeneration, resulting in numbness, tingling, impaired sense of smell, loss of appetite, disturbed coordination, and eventually ataxia, or a general loss of muscle coordination and balance. All of this is irreversible.

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Lest you accuse me of bias, all of these sources are from vegan/veggie website, with the exception of the Wiki article on the detailed effects of B12 deficiency.

Also, almost all B12 in supplements is derived from genetically modified bacteria, for those of you who care about GMO's.

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Of course, you can still make the choice to be vegan if you are okay with eating processed foods with a GMO supplement. I don't doubt that many people think that not "exploiting" animals is worth this small price. It is an ethical choice that we are able to make. But suggesting that humans can do without animal any animal products in the diet or that we are naturally vegan is a blatant falsehood. Naturally-speaking, we are without a doubt omnivores.