Short answer it is needed for a few things in all animals, so a source is needed. Most animals can synthesize it. In evolution, sometimes a mutation will take

Short answer it is needed for a few things in all animals, so a source is needed. Most animals can synthesize it. In evolution, sometimes a mutation will take away the ability to synthesize a vitamin. If the organism gets enough of it from diet, this is not disadvantageous and won't be selected out. Vitamin D deficiency is present in all primates. So that give you some notion of how long ago the synthesis was lost. Vitamin C is similar since it is readily available in fresh meat and many plants. It is absent in Haplorhini (the dry-nosed primates).