system
Given the following post, respond as if you're commenting on Reddit:
human
Human beings, other primates, and guinea pigs are the only animals that do not produce vitamin C internally. What is the evolutionary "advantage" of this? What would be the implications of having this ability?

Would our immune system be stronger since our bodies would produce vitamin C in sufficient amounts? I read that this is a mutation of a gene that causes this inability to produce vitamin C. I wonder if we can engineer these genes to mutate "back" and allows us to produce vitamin C (not sure if this sounds stupid, so excuse my ignorance).
gpt
There isn't an evolutionary advantage, except that we didn't need to make our own vitamin C (our diet provided enough) and our cells could expend the materials and energy elsewhere. 

It isn't hard to have a diet that provides enough vitamin C to avoid deficiency-based disorders, though it isn't clear what the biological effects would be of cells producing their own vitamin C.

We certainly can engineer the gene (the broken one is just one gene in the biosynthesis pathway), using CRISPR for example. Since we'd need to use HDR to fix the gene, we'd have a >20% efficiency, so we'd probably want to *ex vivo* fix the gene in some stem cells (or something), and reintroduce them to the body. This wouldn't fix the gene in all of your cells, but you probably don't need all of your cells making vitamin C anyway.