Even if this wasn't the case there still need not be a single founding species to all present life.
Common Ancestory itself isn't banked on all species having one single ancestor. Even if we find out that there is no universal common ancestor, that doesn't cancel out the common ancestories that are known.
The idea of a universal common ancestor is an extrapolation of the data. It could be more complex than one single common ancestor, but at this time there is no evidence to suggest this and so I think it is a matter of defaulting for the simpler explaination rather than a more complex one.
Agreed. There may be other mechanisms like endosymbiosis that confound universal common descent.
Common Ancestory itself isn't banked on all species having one single ancestor.
I think the theory of universal common descent makes exactly that claim. I thought this is what we were talking about.
Even if we find out that there is no universal common ancestor, that doesn't cancel out the common ancestories that are known.
Agreed. Given the evidence, it is perverse to doubt the common ancestry of humans and chimps as one example. It is when you start going really far back and there isn't much evidence that you get into trouble - is there really a common ancestor for eukaryotes and prokaryotes or is the relationship more complex?