Could living things on Earth be descended, in part, from alien life?
I've been pondering the whole concept of panspermia and whether it is possible to conclude with reasonable certainty that all life on Earth is of a single origin.
Perhaps my kind reader will remember ~15 years ago, when scientists rather prematurely announced the discovery of Martian fossil microorganisms in a tektite from Mars. (Which led to the speculation that if life could transfer from the Martian orbit to Earth, the more likely direction 4 billion years ago was from Mars to Earth, so maybe Earth life descended from Martian life?) You'll notice, though, that there was hardly any mention of the possibility that Earth life today is a mix of life having separate planetary origins. And this is because there is a scientific paradigm, so far pretty much unshaken, that the genetic code of Earth, and some of its expressions (such as the specific forms of pigment molecules, like chlorophyll and cytochrome), are in some sense arbitrary; i.e., equally likely biochemical processes could have achieved the same or roughly equivalent functionalities, for example, using different base combinations for DNA, or even if DNA exactly in the form it is found on Earth is universally optimal, surely some of the early-evolved and nearly universal proteins found in every living cell on Earth are not; they are simply contingently evolved. In other words, they could just as well have been different—not radically different, but perceptibly different. But they're not. And the simplest, most likely explanation for everything sharing something that's just the way it is by happenstance is that they all share a common origin. All the biochemical markers, then, point to a single common ancestor.
When it was discovered some years back that what had always been referred to as bacteria were actually two entirely separately evolved lines of similar-morphology organisms, with some differences in biochemistry and genetic markers indicating extremely ancient divergence (the Archaea and the Eubacteria), there was a flurry of discussion in the pop sci rags about the possibility that one or the other of them originated elsewhere, and that we do in fact live in a multiple-origin biosphere.
But from what I understand, those who really understand the molecular chemistry involved (which of course I don't at all), quickly squelched that controversy by demonstrating to any reasonable person's satisfaction that the Archaea and the Eubacteria clearly had many contingent elements in common and were all but unquestionably the descendants of a common ancestor. All the serious discussions of the origin of life question seem to agree that all the evidence so far points to the conclusion that all terrestrial life shares a common ancestor; best guess for time frame is more than 3 but less than 4 billion years b.p. There are some outliers (only among the microbes), which are probably remote isolates of rather early populations that struck out in a different direction from most other life, but in a large set of parameters, every cell shares a significant set of common markers.
So, what does this say about panspermia, or the possibility, often touted in science fiction, that in the long history of life on Earth (at least 12 Galactic revolutions of the Sun (=~250 m.y.)), perhaps there had been visitors... whether sentient or incidental (riding on interstellar flotsam), that penetrated the biosphere of Earth.
What can a rational person actually infer about that?
It seems obvious to me, first, that it cannot be categorically ruled out. A meteorite that entered the Solar system from another star system (unlikely, but not spectacularly so), could have intersected Earth's orbit and survived reentry to crash onto the surface, somewhere in all that time. If it had spores or dormant organisms, they could have survived. This, indeed, could conceivably have been the source of the first life on Earth, although that would require a remarkable coincidence and so falls in the category of special pleading insofar as it should be thought of as a theory.
It's also possible that this could have happened and whatever was on the rock just didn't survive. That eventuality is not particularly interesting: it can't be ruled out, but has no real consequences and so leads nowhere.
It's conceivable, although less and less likely as research into the biochemistry of life gets more and more complete, that this happened, and alternate origin life is here, just not yet discovered.
And then there's the possibility that alien intelligent beings traveled here at some time in that vast expanse of time (after all, from the point of view of the likelihood of time enough for intelligent life to evolve, the universe has been more or less as it is now for much if not all of that time). Perhaps these visitors, finding a world already teeming with life, prudently decided to quarantine themselves so as not to contaminate our world. This too, is intriguing, but unprovable, at least for now, and not supported by any evidence, Nazca megaglyphs and whatnot notwithstanding. (The whole popular meme of "ancient astronauts" is a house of cards of pure silliness; if you don't agree, I'll leave it to you to investigate). Anyway, if alien visitation ever did happen, just by the laws of chance, it was probably many millions of years ago.
But what did not happen, I think can reasonably be inferred from the terrestrial biological evidence, is that alien visitors arrived here, walked around in the open air, allowed our planet to interact with the organisms of their native biosphere, and then left. If that had happened, to a pretty high probability I'm pretty sure, there would have been contamination, and some of that alien life would have likely survived here. It might have overwhelmed our biosphere, or, possibly, simply co-existed, evolving perhaps into forms that filled niches to which its particular predispositions fitted it. But there is simply no evidence of this; and from what I understand, if it had happened, there would be. We could tell if a type of fungus or a type of bacteria were descended from a different origin of life than ours.
I suppose you could say, well, maybe life here originated there, but that just takes us back to panspermia, and it seems to me that just defers the origin question anyway. Somewhere, somehow, life had to have originated, and clearly there just hasn't been enough time for life to have originated somewhere else and filled the universe, or even the Galaxy, through panspermia... unless directed by super beings with nearly divine powers... in which case, why in Their name, would they be so secretive? It just doesn't add up, in my mind.
So this brings me to several inferential conclusions:
Life on Earth originated here, and has never mixed with alien life. To a fair certainty.
Intelligent aliens have never visited Earth, or, if they have, they quarantined themselves and left no biological traces.
Since, if spacefaring intelligent civilizations were extremely common, No. 2 above would seem rather unlikely, they are, and have throughout the history of life on Earth, been rare. (This inference requires some background on the evolution of intelligence and the likelihood of space travel, etc., which I've discussed before on the Gyromantic Informicon; what I am referring to as unlikely is that if there had been a lot of visits, they wouldn't all have been successfully quarantined).
I think these are the only reasonable conclusions from the facts.
Any contrarian comments would be most welcome.
UPDATE:
In defense of my dismissal above of the idea of "ancient astronauts," I add the following:
First, although it is conceivable that alien spacecraft could actually enter the atmosphere without contaminating the Earth with alien life (had such contamination occurred, it would almost certainly have left detectable traces)... it does not appear to me to be likely that any significant intervention in human affairs could have occurred without actual contact with extraterrestrial living things. Which, I reiterate, would almost certainly have left detectable biochemical traces. As Sagan says, the absence of evidence is not the evidence of absence, but the burden of proof is on those making the claim. I submit that since, despite substantial ongoing biochemical research for over a century, no hint of extraterrestrial biochemistry in Earth's biosphere has ever been demonstrated, it is reasonable, using Occam's Razor, to state that to a reasonable certainty there has been no mixing of extraterrestrial life with Earth life at any time.
The next point is that the first point implies, at the very least, that extraterrestrial visits to Earth, which would have at least some probability of introducing such contamination, throughout the history of life on this planet, must have been quite rare, if they occurred at all. Something which has some non-zero probability becomes a near-certainty, given enough time; and if the contamination had ever occurred, it would still be detectable, in all likelihood; there's little reason to just assume that the extraterrestrial life would have died out, every time. So there are three things: (1) a posited nonzero chance of visits, (2) a deduced nonzero chance of contamination, and (3) a deduced relatively high chance that if contamination had occurred, the extraterrestrial life would have survived in some form and would remain detectable and would have been detected by now. Even if you estimate high for (1), low for (2), and minimize (3), either by saying contamination would likely not have occurred or that it wouldn't necessarily have been detected, if you give any reasonable values to these three factors, a large number of visits necessarily implies contamination, and that just does not appear to be the case from all available evidence.
The final point is just how statistically unlikely it is that, in the vast span of Earth's history, one or more of the necessarily small number of visits (given the first two points) that may have occurred would have occurred recently. Let's stipulate that the likelihood of visitation in any given ten million year period throughout the history of life on Earth has to be treated as approximately equal, let's say x% chance in any given ten million year slot; you can say whether x is vanishingly small or substantial, it doesn't matter for this point. Now, why do I say this? Because the universe as a whole is approximately 13 billion years old, and all galaxies are roughly the same age. There was an early period, during which most stars were mostly hydrogen and helium, but the roughly "current epoch," in which stellar evolution has led to abundant metals and the possibility of planetary environments conducive to life, is at minimum several billion years in duration, so the chance of a civilization achieving technology comparable or superior to ours at any time during the history of life on Earth has to be considered to be relatively constant over time. I don't see how this can really be argued against.
The chance of visitation in 3 billion years is then 300x, which if x is high enough, could approach certainty if you assign any substantial value to x, but that chance is spread equally over the entire time. Since there is no actual evidence that this has ever happened, the chance that it likely happened anytime in human history has to be deemed to be exceedingly small. In other words, let's say there were 10 visits to Earth by aliens in 3 billion years, at random. Just statistically, the chance that even one of them occurred in the last 20,000 years is ~0.007%. Not very realistic odds.
I rest my case.
A FINAL COMMENT
16.Aug.11
• Just in case anyone has read this and wondered why anyone would care about any of this, since it's all so removed from our everyday concerns, apart from the pure intellectual curiosity of it, which I believe is essential to the survival of the human race, my answer is this: we have to conclude, as the least unlikely eventuality, that we are effectively on our own as a species. There will likely be no deus ex machina from older and wiser predecessor extraterrestrials, and, as I went into in great detail earlier on the Gyromantic, we are not at any time in the foreseeable future going to have an economically viable alternative habitat to our own Earth. So we better the hell preserve and protect her and make sure she remains habitable. That is probably the essential lesson of our time.
