But, look at it from a different perspective. I have read Christian DeDuve (Vital Dust, also Life Evolving) and some other writers on how organic chemists believe life might have originated and, well, let's just say the mechanism envisioned is a bit vague. On Earth, it seems to have occurred quite quickly. Very shortly after the crust cooled enough for oceans to form and the era of hourly impacts of bolides and other debris from the Solar System had quieted enough for stable environments to exist, life emerged. Something on the order of ~4 billion years ago, when the Earth had only existed in any form for about 500 million years. (Interestingly, Mars might have had a more benign environment a little earlier, which invites speculation that life could have originated there, and been transported here on ejecta thrown up by impacts to the Martian surface. Neat idea, but the failure so far to find evidence of life on Mars makes such a scenario highly speculative at best).
So, OK, life may emerge whenever the conditions are right, and relatively quickly. Or, Earth could have been lucky. Maybe even fantastically, phenomenally, once-in-a-thousand-galaxies lucky. We can't know without at least a second example of the phenomenon under discussion, i.e., life.
Or, it has to be acknowledged, life might emerge occasionally, given a soupcon of luck, and favorable conditions, and a fair amount of time. In which case, panspermia might well come in to play.
As I've argued before, the universe is much the same everywhere, and, as well, it has been more or less the same as it is now for something like a third to half of the time it's existed. The early universe was very different; we can actually see those early Galaxies, 10+ billion light years distant (and old; same thing). But galaxies of the universe of the time the Sun formed (4½ b.y. ago) (also visible, at that number of light years distance) look much like those of the current time. A lot of disk population stars not unlike the Sun, some already billions of years old, and no doubt many with planets, some not so different from the Earth. So, given the assumption that life is likely to originate given favorable conditions, life probably originated in many, many, many places much earlier than it did on Earth. But let's speculate that life actually doesn't form all that easily, and that, say, only 1% of the time, all by itself, from nonliving matter, even given favorable conditions, will it emerge. Still, 1% of untold billions of stars is a lot of stars, and life must have originated many, many times even within ordinary middling sized sprial galaxies like the Milky Way.
So here we have, the first "generation" of life-bearing worlds, existing before the Solar System even formed. What if a few times, or even only once, life evolved to the status of a highly technologically advanced civilization, that looked out at the somewhat younger universe that existed then, and said, Yea, behold, there are worlds in their millions and billions yet to form! Let us make it our mission to seed these worlds with life of our own kind, so that life will abide, and not perish in the universe!
We, puny humans, are almost at a technological level where we could build autonomous interstellar craft to travel the spaceways, enter into suitable target systems, and deposit canisters containing living organisms into environments where it stood some chance to survive, and subsequently to evolve. We may well even do this in the future ourselves.
So, then, who is to say that this did not happen in the distant past? It is easy to see, from the logic I’ve tried to follow above, that such an origin of life on Earth is plausible, and, given some quite possible parameters for the likelihood of origin of life in any given environment, might just possibly be more likely than the more usual, single unique origin of life from non-living matter in the earliest days of our world’s existence.
Perhaps someday, not too far in the future, information which will enable us to understand this issue better will be emerging.
When I wrote this several years ago on the whole topic of the Fermi Paradox I commented on the possible role of Von Neumann (self -replicating) machines in cosmic biological evolution. It still seems to me that there are implications of the mere fact that there doesn't seem to be any particular reason such technology could or would not be developed, for the whole issue being discussed here.
Returning to the idea that at the approximate time of the formation of the Solar System (4½ billion years ago), the universe was similar enough to its present state that there were already older, relatively metal rich disk population stars in existence that could have already had time and resources necessary for the evolution of complex life on the surfaces of planets (statistically). The proportion of such stars where this actually happens (one of the factors in the Drake Equation) is essentially unknowable at this point, but there are hints already from the Spitzer Space Telescope data and other exoplanetary search data that planetary systems are highly variable, and the presence of cozy Earth analogs may be somewhat rare, as opposed to very common. That's about all you can really say about that issue right now; the recent discovery of two planets in orbit around a dwarf star not radically different from the Sun (Kepler 62), both of which could have liquid water on their surface, is at least some confirmation that even if not necessarily very common, such planets exist in large numbers. So it's reasonable to posit that there was already a fairly large number of such planets in the Galaxy, including some which had already existed for several billion years, at the time of the formation of the Solar System.
Remember, too, that what was likely in our Galaxy was also likely in a virtually uncountable number of other similarly situated spiral galaxies in the Universe, even though that is not saying that it was likely in all galaxies, since galaxies are not all the same; the point being that we are talking about universal principles here, and even if life turns out to be really quite rare, there is an unbelievably huge sample size if you're talking about the whole universe.
So, could some civilization have arisen before life emerged on Earth, that, as I speculate above, sets as its goal, the propagation of life for its own sake? I see no reason why that could not have happened. But saying that begs a question. Would they have been likely to have developed Von Neumann technology to do it?
The answer to that question is vastly important for whether the panspermia hypothesis should be considered plausible. Because, if something is possible, in a large enough sample size, it's going to happen. As far as I'm aware, there is no reason intelligent beings could not build devices that act as "arks" for living organisms, probably only basic single cell forms, and which then use some form of interstellar propulsion (a difficult, time consuming but not impossible process, particularly suitable for long lived robots, which is what we're contemplating). These machines would scan the heavens for suitable stars where candidate worlds for "seeding" with life might exist, and go there. On arrival, they would deposit their "seeds," then proceed to make some number greater than one copies of themselves, replicating their payload of living organisms as well, and set out, again, this time to multiple new targets. Even allowing for the probability that some kind of error would creep in and cause such systems, no matter how well thought out their self-repair modalities, to eventually fail, it is easy to see that, Sorcerer's Apprentice style, such technology could eventually deposit life in all the suitable star systems in some appreciable chunk of a galaxy, or even possibly all of it. The numbers have been run by others, but suffice it to say that if a Von Neumann propagation was successful through a few hundred iterations, say, and that it could manage an average travel time between stars (including the time needed to replicate itself and start out again) that was measurable in centuries rather than millennia or longer, every star in a given galaxy could be visited in much less time than the age of the galaxy itself. On the order of a million or two years, in galaxies that are (were) several billion years old.
So what's the problem? Life could have originated, somewhere in the Galaxy, from nonliving matter (which after all, without panspermia we hypothesize for every living world); evolved to complexity and the emergence of intelligence (hey, it happened here, unless...); then started such a process, just because. How likely is this? Who can say, but it can't be ruled out. The point is, it's possible that all life in the galaxy at the approximate time of the apparent emergence of life on Earth could have come from such a process, and have had a common origin. (Or a small number of similar origins, it makes little difference).
But this raises another issue. What about since then? The Earth has existed a long, long time. During that time, no civilized creatures, as far as we can tell, have visited here; certainly none has come and stayed. If panspermia-by-Von Neumann process could have occurred 4½ billion years ago, it could have occurred again and again since then. It seems contrived to me to posit that any such device, on encountering worlds where life was already present, would just move on without interfering. Sure, that's a possible program for such machines, but would it always be, like some law of nature, and would it always work? Seems unlikely. So, could life, or variations of already existing life, have been injected into the Earth's biosphere from external sources more than once? Could biological evolution actually be a cosmic story, rather than a planetary one?
Again, as far as I know, there's nothing impossible about such a scenario. But it really, really does beg the Fermi question, all over again. Because you have to ask, even if it were to turn out that intelligent beings are often biophilic, i.e., they see value in propagating life for its own sake, wouldn't at least some of them be territorial, expansionistic, explorational, or otherwise interested in visiting the worlds of the Galaxy themselves, in the flesh? So why, if technological civilizations were, in our scenario, involved in propagating life, have the actual intelligent beings themselves not shown up here, and everywhere, in all that time? It's the Fermi paradox all over again, and all the usual objections apply. (Follow the link above for my thoughts on what that all means). Also, the likelihood that life has been "injected" into the biosphere of Earth multiple times has to be discounted. The generally accepted inference from available data is that all life on Earth shares a single origin and has one common ancestor. Not impossible, but inconsistent with observation, and so requiring extraordinary evidence.
The unspoken possibility here is that panspermia might occur without intelligence. Life might just ride on ejecta and stray comets, and spread through space that way. The problem with that, again, at a rough estimate, is that space is just too vast, such chance depositions probably so rare, that, while occasional transference of life from one star system to another probably can't be ruled out and may well have happened from time to time and place to place in the vast universe, it seems unlikely to have occurred in a general way or on a large scale. The main reason is the same factor as plays a big role in all discussions of the emergence of life and intelligence in the universe: on average, stars are really, really far away from each other. (Alpha Centauri is 40 trillion kilometers, and that's the nearest system). In other words, it takes a bit of consciously directed, deliberate effort, for stuff, like packages of micro-organisms, to get from one star system to another.
So, what, then?
We don't know. Seems to me panspermia is conceivable, but there are objections to it based on probable consequences that don't seem to have occurred. The default hypothesis, I suppose, needs to remain the null one. Which is to say, seems most likely life had a unique origin from nonliving matter on this planet, and most likely on others, in this Galaxy and vast numbers of others in the universe. But, already, it must be said that it looks like this series of events (origin and subsequent evolution of intelligent life) is not necessarily particularly common.
From all this thought, which, I hope, is based on consideration of real information, I draw the conclusion that the evolution of intelligent, technological living beings, and/or another of the Drake Equation factors, the average lifetime of such civilizations, seems likely to be sufficiently improbable that these factors serve as choke points, making the continuous or frequent-over-geological-time presence in this Galaxy (or any galaxy) of starfaring "peoples" unlikely. Still, the possibility that life has been artificially propagated, and that even life on Earth may have originated from such a process, cannot be ruled out, although to advocate that this may have happened requires a bit of special pleading, given the likely corollary events that do not seem to have occurred. And from these, I think another inference can reasonably be drawn:
The next (and, it has to be said, quite possibly first ever) biopropagative starfaring race to emerge in the Galaxy is likely to be our own.