All evidence at hand is consistent with the descent of all multicellular organisms from a common population. But evolution would still happen if this were not the case.
What you said:
That is the point. One cannot say it is a continuum - a “tree” - in the face of the possibility of multiple episodes of abiogenesis or biogenesis.
Here's how your logic seems to work: "John died with two bullets in his head and no other ecivence of illness or injury; there are many ways that people can die; therefore, because there are many ways to die, we cannot say John died from bullet wounds."