Let me give a counter example. Suppose that at some point, some big old alien space ark showed up and dumped a complete ecosystem into Earth's oceans. We have no evidence that such a thing has happened, of course, other than strangenesses like the sudden appearance of shelly invertebrates: trilobites, brachiopods, mollusks, and so on being rather bizarre. But the point is this -- the current evidence of genetic similarity just means that everthing alive today is related. It doesn't mean that it all derived from common ancestors. All life today could have derived from different ancestors that were also related.
To clarify, if you say that since there is a lack of unrelated genetic code, therefore common ancestry - seems a non sequiter to me. I don't think you can prove common ancestry by the absence of a negative.
If your point is merely that the presence of unrelated genetic codes would refute common descent I would agree. But the absence of unrelated genetic codes does nothing to prove common descent.
Yes. Remember this was one of the examples I gave in response to your post#1729:
Regarding the claim that "all life on earth is descended from one (or very few) common ancestor(s)" -- how is that falsifiable?
If life on Earth did not show identical characteristics at the molecular level, it would falsify common descent and therefore fulfill your request.
I was just pointing out how in fact all remarkably similar the DNA is of all life is from bacteria to humans. I never set out to prove common descent, just to show the prodigious amount of evidence for it.