Unneeded genes eventually get bumped off by mutations.

Why can't human apes (and their ape and monkey cousins) manufacture their own vitamin C like some other species can?

Why do our mouse cousins have a much better sense of smell than human apes?

As usual evolution by natural selection explains everything.

Apes (including people) and monkeys don't manufacture their own vitamin C because they don't have to. They have lived for millions of years in an environment where there has been plenty of fruit to eat. When mutations made those genes inactive, the creatures continued to survive and reproduce. Those same creatures would have died if not for the abundance of fruit, but instead they lived and so those inactive genes were passed on to the next generation.

We can't smell like the mice can because our ancient ape ancestors had color vision and so they didn't need a strong sense of smell. Those genes became inactive and were inherited by future generations including our species.

For a much better and much more thorough explanation see the section "Dead Genes" on pages 66 thru 73 of Jerry Coyne's Why Evolution is True.

Christian tards, don't bother with it. You're too bloody stupid to understand anything.

In this blog there are 86 posts about the evidence for evolution.