[Thatcherite]

Or, how do I know that these defective genes are not remmants of some earlier optimal Design that has been damaged?

What do you make of the fact that the spread of the damage in this gene and others matches the expectations of common descent? This earlier optimal design damaged identically across several species implies common ancestory of all primates. That's the whole point. Again and again such molecular evidence meets the expectations of common descent, and were common descent not true there would be no reason for it to.

[Diamond]

This earlier optimal design damaged identically across several species implies common ancestory of all primates. That's the whole point.

Not necessarily. It could imply common design rather than common ancestory.

Cordially,