Well said. I would also point out that, IIRC, L-GLO is merely the last of a series of necessary steps to synthesizing ascorbic acid - in humans and other primates, all the other steps and ingredients necessary are still present, despite the fact that the process is broken due to the missing ingredient. So it's not as though we only have this one broken gene, held in isolation, which may or may not be related to functional genes in other organisms - in addition to the broken homolog in humans, we also have a set of other genes that are homologous to ascorbic acid-producing genes in other organisms.
It's not as though Alice simply has a bag of rotten flour, and we don't know how to begin interpreting that one odd ingredient. Alice also has eggs and butter and sugar and chocolate and shortening, and everything you need for the recipe. Except the flour, of course. Bob and Carol have all those things, plus functional flour, and they're happily baking away - why should we not infer that Alice was once capable of baking a cake too?
If the process is broken, and useless, why are these genes still around, in Darwinian terms?
why should we not infer that Alice was once capable of baking a cake too?
Whether Alice was once capable of baking a cake or not says nothing about whether she is related to Bob and Carol.
Cordially,