Christians (or proponents of intelligent design) can you answer this (see details)?

We humans have what is called a GLO gene. This gene produces vitamin C in other species, yet ours does not work. If there were an intelligent designer, why would he (or she . . . or it) design us with a faulty one? The only answer is that it was functional previously, and then became dysfunctional at some stage, after which it was passed on to all people. This is actually true, but it just so happens that our closest primate relatives—orangutans, gorillas, and chimpanzees—have the same faulty GLO gene, unlike other, distant primates (which have functional GLO genes). Kenneth R. Miller, in his book 'Only a Theory: Evolution and the Battle for America’s Soul', writes, "That means that the common ancestor in which the capacity to make vitamin C was originally lost wasn't a human, but a primate—an ancestor that, according to the advocates of intelligent design, we're not supposed to have. And there's the problem. (2008:85-87)"