Notwithstanding that Fermi's Paradox is limited to star-faring sentient life, not life in general, and thus irrelevant to the topic of the paper cited by SocratesII...

The "formal" statement in the above post -- Humans have been free from alien competition to evolve into the dominant species on Earth... Ergo interstellar travel is impossible -- assumes too much, and makes a leap of lack of faith, if you'll grant the phrase.

First, it assumes that competition from within the human ecosystem -- i.e., Earth -- has never been, and cannot ever be, sufficient to curtail human evolution. Why? Because we're here to say so. Thus, it is as circular in its reasoning as the anthropic principle in cosmology. It completely ignores the coincidental, non-biological component of survival (such as impacts, nearby supernovae, harsh climatic epochs, and other extinction events). Worse, it ignores the unrepeatable luck of survival of all the precursor species from which humans are descended.

Second... See my "analogy" post below. The conclusion of the paper is clear enough that extra-terrestrial life (a) must be primitive enough to survive interstellar travel encased in rock formed and ejected during an impact, and (b) must have originated in relative proximity to its destination to minimize travel time, in order to arrive viable. The paper does not speak directly to ensuing biological competition once the alien life has arrived, but it implies that both origin and destination are still young enough to be just emerging from their common 'stellar birthplace' -- therefore, that neither planet can be said to have an ecosystem per se at the time of transfer.

I contend that a hypothetical panspermia event occurring any time after either the planet of origin or destination has evolved complex life cannot be successful because both life systems have evolved beyond their ability to interact biochemically... but let us assume for a moment that I'm wrong, and a bacterial population from another planet arrives here encased in a meteorite. It must then be released, somehow, from its entombment into conditions matching its prior habitat, and immediately compete with an entire planet's ecology, beginning with that already occupying the niche it needs to survive in.

It's possible to infer from the "formal" statement quoted above that alien-ness itself grants some competitive advantage. Frankly, this strikes me as baseless fear. Rather than the plot of Crichton's The Andromeda Strain -- during which, by the way, the alien microbe rapidly evolves into a form harmless to Earth life -- the far more likely outcome will resemble Wells' The War of the Worlds, in which billions of years of native evolution defeat the invading species without even breaking a sweat.

We simply cannot know if 'panspermia' has occurred successfully on Earth. If it did, it can only have happened early enough to become an ingredient of terrestrial life (the scenario discussed in the paper), leaving no trace of alien origin.

As to the Fermi Paradox... The fact that humans have not been competed against for 'dominance' of Earth cannot be cited to prove that interstellar travel is impossible. It can only be used to enforce the argument that interstellar travelers have not arrived yet.