The case against "geospermia" here is vastly overstated: there's been a lot of research over the past decade or two establishing very plausible pathways for terrestrial abiogensis. If you're interested, read through some work coming out of Jack Szostak's lab (there's a recent review article here). I'm not as familiar with the literature on prebiotic chemistry as I am with the literature on protocell formation, but I know we've found amino acids on meteorites, and it wouldn't be surprising if they and perhaps some other molecules which are important to life were introduced to earth through meteorites rather than natural syntheses.
But in terms of cell formation, the null hypothesis should probably be that it occured on Earth. Panspermia isn't ridiculous per se, but conditions on Earth appear to have been much more suitable for cell formation than those of the surrounding neighborhood, and sufficiently suitable that terrestrial abiogensis isn't implausible in the least. When it comes to ways in which there could be wild-animal suffering on a galactic scale, I think the possibility of humans spreading life through space colonization is far more concerning.
Also, Zubrin writes:
Furthermore, it needs to be understood that the conceit that life originated on Earth is quite extraordinary. There are over 400 billion of stars in our galaxy, with multiple planets orbiting many of them. There are 51 billion hectares on Earth. The probability that life first originated on Earth, rather than another world, is thus comparable to the probability that the first human on our planet was born on any particular 0.1 hectare lot chosen at random, for example my backyard. It really requires evidence, not merely an excuse for lack of evidence, to be supported.
This is poor reasoning. A better metaphor would be that we're looking at a universe with no water except for a small pond somewhere, and wondering where the fish that currently live in that pond evolved. If water is so rare, why shouldn't we be confused that the pond exists in the first place? Anthropic principle (but be careful with this). Disclaimer: Picking this out because I thought it was the most interesting part in the piece, not because I went looking for bad metaphors.
As a meta-note, I was a little suspicious of this piece based on some bad signaling (the bio indicates potential bias, tables are made through screenshots, the article looks like it wants to be in a journal but is hosted on a private blog). I don't like judging things based on potentially spurious signals, but this might have nevertheless biased me a bit and I'm updating slightly in the direction of those signals being valuable.
Apparently I can add body text to a link post, instead of having to give a TL;DR as a comment? Excellent. Here are some excerpts from the essay:
Let us consider our own Sun as a case in point. At 1 AU, the sun attracts objects to it with a gravitational acceleration of 0.006 m/s2. It also repels objects from it via its light pressure with a force of 0.000009 N/m2. If these two forces are equal, and object will feel no attraction from the Sun, and fly out of the solar system in a straight line with the Earth’s velocity of 30 km/s. (Both of these forces change with the inverse square of their distance from the Sun, so if they are equal at 1 AU, they remain equal at any distance.)
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So, depending on assumed conditions, the diameter of the objects could range from 1 to 10 microns, and be readily projectible across interstellar distances using no other mechanism than the pressure of the star’s light. This is precisely the size range of many typical bacteria.
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So the question is, what kind of information is really worth broadcasting – that is distributed as widely as possible to people who we don’t know and are not likely to hear back from? If human experience is any guide, the answer is propaganda.
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The key to propaganda is in the root of the word itself: propagate. Through propaganda we seek to propagate ourselves across both space and time. This can be in spirit, as in the cases described above, or in the flesh, through physical reproduction.
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It is a striking fact that, despite several centuries of microbial research by thousands of competent investigators, no free-living organisms have been found on Earth that are simpler than bacteria. This is truly remarkable because, as simple as bacteria may be compared to more complex organisms, they are certainly not simple in any absolute sense, incorporating as they do, among other things, the entire elegant double-helix scripted language of DNA. Believing that bacteria were the first life forms to emerge from chemistry is like believing that the iPhone was the first human-invented machine. This is incredible. Just as the development of the iPhone had to be preceded by the development of computers, radio, telephones, electricity, glassware, metallurgy, and written and spoken language, to name just a few necessary technological predecessors, so the creation of the first bacterium had to be preceded by the evolution of a raft of preceding biological technologies. But we see no evidence of any such history. We still see devices all around us that use one or more of the iPhone’s ancestor technologies – telephones, light bulbs, batteries, glass windows, and steel knives, for example – but we see no pre-bacteria organisms. This observation has led many investigators, dating back to Arrhenius (Arrhenius 1908) over a century ago, to postulate that life on Earth is an immigrant phenomenon. According to this “panspermia” hypothesis, bacteria did not originate on Earth, but came here from space, after which they gave rise via generally understood evolutionary processes to all other life forms.
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Indeed, it is hard to escape the conclusion that natural transmission has been going on for at least 3.6 billion years, if from no other original source than the Earth. If the average time between close stellar encounters is 20 million years, with number of microbe inhabited system doubling each time, we could expect 2180 solar systems in our galaxy to have been Earth-progeny-microbe-invaded by now, which is to say all of them, many times over. (This being the case, the probability that the Earth was actually the first of these billions of microbe-inhabited worlds would be vanishingly small.)
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It seems to me that the most portentous form of communication that intelligent extraterrestrials could undertake would be to try to propagate themselves by sending out genetic information to influence this chaotic process in their own direction. Genomes can contain dormant plans for complex traits, as evidenced by recent work in which scientists activated what had been considered junk DNA in chickens to produce long-lost dinosaur features, like teeth.
There's also some discussion of horizontal gene transfer and memetics being mutually beneficial to all, but I think that ignores coordination problems. Contacting aliens may be quite dangerous if it exposes us to maximally viral memes.
This also has huge implications for wild animal suffering.
Full essay here: https://www.centauri-dreams.org/?p=39000