That wasn't your original claim. Your original claim was "The conclusion of common descent is built into the bare assumption that the lack of the L-gulano-g-lactone oxidase gene is a "defect", or "nonfunctional" version of a gene that was purportedly functional at some point in human history".
The original function of L-GLO isn't purported, and it isn't an assumption. Try to muster just a smidgen of intellectual honesty.
How do you know this? Were you there at the time? It is an assumption based on the observation of DNA sequences that resemble genes but appear (at present) to be nonfunctional, and which are assumed to be the byproduct of random and useless but not fatal mutations, and which is built entirely on the bare assumption of complete knowledge of the organism and its history, something we do not have because we were not there and could not observe it.
Cordially,