If genetic material is swapped freely between species, there need not be a single founding species to all present life.
I think [uncertainty about biogenesis] is a seperate question from whether common descent is a fact.
It bears on bearing on the question directly. Suppose that life arose twice or more independently and that these several populations could swap genetic material. Now one can conceive of a tracing the ancestry of all present species and never finding a single common ancestor.
Noone can be certain that every species alive today is due to common descent.
I thought it wasn't a question of certainty but rather the perversity of doubting, that this is a difference between absolute and scientific fact.
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.