Vitamin Synthesis

Humans are rare in their inability to synthesize vitamin C. Almost every mammal can do it, with few exceptions like primates and guinea pigs. In fact, humans actually have the genetic coding to make vitamin C, but it’s turned off. A lot of species typically evolve the ability to make vitamins that they can't get easily in their diet. The human body can synthesize its own vitamin D because there aren't many food sources for it, but we can’t make vitamin C because we can find it in a lot of the food we eat. There are lots of other species that make their own vitamin C. It's a tradeoff between needing to find a variety of food and not needing the cellular machines to make more stuff.