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Humans are one of the few members of the animal kingdom unable to synthesize vitamin C.
In humans, the cause of the loss of ability manufacture vitamin C is that the last enzyme in the synthesis process, L-gulonolactone oxidase, cannot be made because the gene for this enzyme, Pseudogene ΨGULO, is defective. The mutation has not been lethal because vitamin C is abundant in their food sources. It has been found that species with this mutation (including humans) have adapted a vitamin C recycling mechanism to compensate.
It has been noted that the loss of the ability to synthesize ascorbate strikingly parallels the evolutionary loss of the ability to break down uric acid (an organic compound of carbon, nitrogen, oxygen and hydrogen with the formula C5H4N4O3). Uric acid and ascorbate are both strong reducing agents. This has led to the suggestion that in higher primates, uric acid has taken over some of the functions of ascorbate. Ascorbic acid can be oxidised (broken down) in the human body by the enzyme ascorbic acid oxidase.