Panspermia, the idea that life on earth may have been accidentally seeded here from somewhere else in the cosmos, is a convenient rescue strategy for materialist explanations of life’s origins. The well known problem with the approach is obvious: it only pushes off the enigma of how the originating, colonizing life itself originated. A Scientific American column by Columbia University astrobiologist Caleb A. Scharf suggests another problem with panspermia that I hadn’t thought of.
In two words, it’s natural selection. You could imagine extraterrestrials as purposefully dropping that first specimen of sulphur-eating bacteria (or whatever) here with high hopes of its developing into you and me three and half billion year later. (That was the premise of this summer’s interesting Ridley Scott science fiction movie Prometheus, with its nod to intelligent design.) Barring the involvement of super-intelligent ETs, the most likely scenario being discussed is that life came here on a comet or an asteroid.
As Scharf writes, such exchanges of material are common in our solar system and presumably elsewhere. Darwinian natural selection adds the further reminder that whatever life form that could successfully hitch a ride and survive the trip would have to be very hardy indeed:

Although it involves a complex web of factors, it seems likely that life driven by cosmic dispersal will probably end up being completely dominated by the super-hardy, spore-forming, radiation resistant, chemical-eating, and long-lived but prolific type of critters. There may be no advantage to a particularly diverse gene pool. Billions of years of galactic transferral will have whittled it down to only the most indelicate and non-fussy microbes — super efficient, super persistent, and ubiquitous — the galactic top dogs.

Where’s the problem? It is that if Dr. Scharf is right — and on its own terms the arguments seems entirely valid — we should expect to find these tough “critters” scattered all around the solar system, wherever we turn our instruments. These little guys are so tough they should be able to survive anywhere, and comets or asteroids should have brought them there by now.

There should be stuff on the Moon, Mars, Europa, Ganymede, Titan, Enceladus, even minor planets and cometary nuclei. Every icy nook and cranny in our solar system should be a veritable paradise for these ultra-tough lifeforms, honed by natural selection to make the most of appalling conditions. So if galactic panspermia exists why haven’t we noticed it yet?

Yes, a dilemma! With a view to saving the bacon for a scenario that would exclude intelligent design, Scharf offers some prospective solutions:

There are all sorts of plausible reasons. The simplest is that we’ve not yet managed to look very hard in all these places. It’s also possible that we’ve just not put two and two together while studying the properties of terrestrial extremophilic organisms. But suppose we keep looking hard and find nothing — this would argue strongly against the possibility of galactic panspermia at all. And this would be interesting, because it would also serve to place a limit to the true extremes of life, a physical and chemical boundary condition. Perhaps the root cause turns out to be gravitational dynamics (interstellar transfer may be horrendously inefficient), or just the environmental limits of bio-chemistry and the molecular machines at the core of it all. In either case a null result might actually tell us something vitally important about the phenomena of life, and our own cosmic significance.

The other interpretation of such a “null result,” hinted at perhaps in the concluding gesture to “cosmic signficance,” would of course be that life is not scattered all around our neighborhood in space because life cannot spontaneously generate itself from inorganic material but rather originated here on earth by purposeful design.
Image: Titan and Saturn from NASA’s Cassini spacecraft, NASA/JPL-Caltech/SSI.