The notion of panspermia - the transferral of viable organisms between planets, and even between star systems, seems to be getting a bit more attention these days. One only has to open this previous week's copy of TIME magazine and there it is, via a very nice piece by Jeffrey Kluger on 'Aliens Among Us'.

There is no doubt that planetary surface material is continually being shipped around between rocky planets and moons in our solar system. Ejected by high energy asteroid or comet impacts, chunks of stuff follow a range of orbital trajectories that result in both eventual return to their origins or transferral to the surfaces of other worlds. Increasing evidence suggests that a variety of (typically microbial) organisms could be carried along, surviving both the extremes of pressure and acceleration, as well as exposure to thousands to millions of years of interplanetary space. They need not do this in stasis, tucked well inside the interstices of rock and ice it's not inconceivable that microbes could be passengers in the natural equivalent of the generation ships of science fiction.

It means that there is a real possibility for life to both cross-infect, and even to be 'seeded' from planet or moon to planet or moon. And I've written about this before, in the context of life on Mars (see 'We Are the Aliens').

Enthusiasts for panspermia go further, and have been known to invoke these mechanisms for galaxy-wide dispersal of life - taking one rare occurrence of life and spreading it across the stars. In some ways the motivation for proposing this kind of cosmic panspermia is a little dated. It comes from a time when we felt that the origin of life of on Earth was such a mystery, and such an unlikely event, that it was convenient to outsource it. Although this didn't actually solve the real question of life's origins, it meant that a specific origin 'event' could be extremely rare among the 200 billion stars of the Milky Way yet life would still show up in other places.

These days I think our discoveries about the remarkable abundance and diversity of so-called pre-biotic chemistry (the stuff that represents all the underlying building blocks of bio-chemistry) in every nook and cranny of our solar system, and even in the proto-stellar nebula of other stars and the wilds of interstellar space - swings the pendulum back to Earth. Nature seems very adept at making all the pieces for life, apparently raising the odds of local bio-genesis.

But this doesn't mean that interstellar, galactic panspermia isn't still relevant. It might be happening. And this gets me to the paradox of the title. There is a factor about large-scale panspermia that to my knowledge is rarely considered, and that is natural selection. You and I, or fluffy bunnies and daffodils are all unlikely candidates for interplanetary or interstellar transferral. The sequence of events involved in panspermia will weed out all but the toughest or most serendipitously suited organisms. So, let's suppose that galactic panspermia has really been going on for the past ten billion years or so - what do we end up with?

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.

Now, we might argue that there are organisms on Earth that could fit the bill, and could represent the most direct descendents of these ancient interlopers. Slow-living, low-metabolism chemoautotrophs abound, and our knowledge about these proverbial bottom-feeders is still very limited.

But the problem, and the potential paradox, is that if evolved galactic panspermia is real it'll be capable of living just about everywhere. 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?

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.

[This post has been adapted and expanded from a post in the Life, Unbounded archives]