system
Given the following post, respond as if you're commenting on Reddit:
human
Why did primates evolve to not internally manufacture an essential vitamin, vitamin C, whereas other herbivorous, carnivorous and omnivorous species did not lose this trait?

I understand that we (~~homo~~ homo sapiens sapiens, and other primates) rely on consuming animals and plants that create this vitamin themselves, however, why has natural selection singled us out and (for all intents and purposes) caused us to not produce it ourselves? Why haven't herbivores, which eat flora that is vitamin C rich lose this trait? Same thing goes for carnivores, which eat other fauna (which contain vitamin C themselves from either flora or other fauna)?

Since it is an essential vitamin, and can cause a myriad of problems if there aren't sufficient quantities of it in our body, why have we evolved to not produce it when other animals haven't?

NOTE: I am presuming that the ability to produce this vitamin was present earlier on in our evolutionary timeline (hence why the vast majority of living things have the ability). If this is incorrect (ignore, otherwise), then a similar question: Why are we one of the very few species to not develop this trait, yet?

Thanks guys! :-)
gpt
The ancestors of old world primates lived in an environment that was abundantly rich in vitamin C in the form of citrus fruits. In this environment, being able to produce vitamin C didn't matter and genetic drift lead to it being removed from our genome. It only causes problem for human because the have spread out of this environment.

Source: The Selfish Gene by Richard Dawkins.