Almost every mammal manufactures its own Vitamin C. Humans do not. Between roughly 40 and 25 million years ago, a mutation disabled the GULO gene in one of our primate ancestors, eliminating the final step needed to produce Vitamin C. That defect was passed down to humans and other anthropoid primates and remains with us today.

We survive entirely on whatever Vitamin C we eat. Go long enough without it and the result is scurvy, the disease that devastated sailors on long voyages for centuries before anyone understood why citrus fruit cured it. They were treating a manufacturing defect that had been quietly running in human DNA for millions of years before ships existed.

The gene responsible is still there. It is just broken.