Why can't humans produce their own vitamin c while animals can?what is in them that we don't have?

Question

Why can't humans produce their own vitamin c while animals can?what is in them that we don't have?

Answer 1

Among the animals that have lost the ability to synthesize vitamin C are simians (specifically the suborder haplorrhini), guinea pigs, a number of species of passerine birds (but not all of them), and in apparently many major families of bats and perhaps all of them. Humans have no enzymatic capability to manufacture 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.

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Answer 2

Just to add my 2 cents' worth. Humans evolved from an arboreal fruit-eating primate. If we stopped eating fruits, we could have become extinct long time ago.