Stipulate the widespread presence of precursor organic compounds -- call it "panfertility". Assume the presence of planetary systems whereupon those compounds are altered and recombined by local conditions to begin self-organization, then self-reproduction: i.e., life.

Each of those systems can be thought of the same way as 'bubble universes' in cosmology, where initial conditions constrain all that evolves afterward. The chance of any two being biochemically similar enough that evolved life from one could survive in another is analogous to matter from one universe being able to persist physically in another with different constants.

Consider the scenario discussed in the paper with which 'SocII' opened this thread.

From our discussion above, it is clear that exchanges of bacteria between planets in different solar systems are only possible during the birth cluster stage of the systems in question. As the number of life-carrying bodies received by the Earth may have been in thousands, so also other planets in other stellar systems may have received their life from other members of our original star cluster, or even from a single source, the Earth. Thus the limited form of lithopanspermia inside a star cluster is possible, while the stronger version of life spreading through the whole Galaxy from a single source could not happen via mechanisms described in this work. But life-carrying bodies originating from our solar system may have found their way to our original neighbours, and that all conditions being optimal, life seeded by our system could have spread to many other solar systems.

[Full text of the paper here; emphasis added]

That is, the paper only discusses the transport mechanism, and ultimately limits its probable upper bound without defining the lower.

My question: If life from one system transferred to another at a sufficiently 'primitive' stage of evolution -- at a sufficiently early time in the history of both systems -- to survive and evolve over some X * 109 years in its new setting, how can we now know? In other words, when is similar biochemistry evidence of common origin, and when is it only evidence of the commonality of potential?