So, what I would like to see addressed here is, first of all, how strong the indications are that current life has indeed exactly ONE common ancestor.
Imo, if abiogenesis is real (highly doubtful from a science perspective imo), then we can say biology evolves from chemistry (so to speak), but if that is the case, then embedded within the rules of chemistry is the ability to evolve biological life forms. Simple enough, but let's take the next step in logic.
Then, we can say there is a commonality within chemistry that is what gives rise to life.
So the fact all or most life forms share similarities, such as DNA, is not necessarily due to originating from a single biological life form, but can be due to the simple fact of sharing common origins in chemistry. In other words, if chemistry is predisposed to give rise to biology, then there is no reason not to expect these principles to keep working, again and again, and the fact there are wide similarities within all species is not evidence at all for common descent.
Of course, the other commonality that can explain similarity is that there is a Creator (sort of like how one can spot a picasso as oppossed to a Renior).