Scientists are deeply divided about the likelihood of the existence of advanced extraterrestrial civilizations – our peers and betters – in the cosmos. They are much less divided over the likelihood of finding simple unicellular or microbial life which they pretty much agree should be relatively commonplace. The assumption there is that origin of life (pre-biotic) chemistry is pretty easy and straightforward, even though we haven’t yet created life in a test tube. An alternative scenario is that while an origin of life event maybe rare, microbial life can easily spread throughout interstellar space, ultimately in some cases finding suitable abodes to be fruitful and multiply on. That concept is known as Panspermia.
Life on Earth arose very quickly after the planet’s formation. There are two competing theories to explain this. The first is that it’s relatively easy for life to arise naturally, on-site, in short time frames once suitable environmental conditions present themselves. The other explanation, Panspermia, suggests that unicellular life forms present in outer space (in meteors, comets, cosmic dust, etc.) constantly seed planetary environments, and if those environments are environmentally friendly, life flourishes and evolves. Here I suggest that Panspermia is the more probable of origin of life scenarios. It not only allows for a rapid origin of life event on Earth, but for lots of life throughout our own galaxy (perhaps beyond). The evolutionary road to intelligence and technology is another matter. Here I’m just concerned with life – full stop.
When it comes to explaining the quick-smart origin of terrestrial life, I am quite partial to the Panspermia hypothesis. So, my take on the extraordinary early genesis of life on Earth is something supportive of an idea that we were ‘seeded’ by primitive microorganisms, viruses, or spores that pervade outer space. Put another way, the on-site purely terrestrial origin of life is such an improbable event (IMHO) that it’s unreasonable to expect it to have happened very rapidly (there needs to be lots, and lots, and lots of Goldilocks factors that come together just-so). I suggest that the origin of life, from scratch, on-site, is not very easy. If a non-intelligent Mother Nature could create life that easily, from scratch, on-site, in relatively short time frames, then you’d think that hundreds, if not thousands of highly intellectual scientists (biochemists, organic chemists, biophysicists, etc.) would have created a proto-cell in a test tube by now. They haven’t. So, it’s hard. Now the unstated assumption here is that there’s pretty much just one pathway to life (as we know it) – terrestrial life – and like a 5000 piece jigsaw puzzle, but with 50,000 potential pieces, of which 45,000 are false leads or dead ends, it’s difficult. If however there are 50,000 pathways, or ways of assembling that 5000 piece jigsaw puzzle, any one of which terminates in life, then that’s a horse of another color. Even if all 50,000 pathways are hard, the probability has increased remarkably in favor of one of those 50,000 pathways occurring.
Of course the Panspermia hypothesis just puts off the ultimate origin(s) of life event(s) to another elsewhere and else-when, albeit a vastly, many orders of magnitude more vastly, elsewhere and else-when.
As time goes on, the probability of a complex (and the origin of life is hard and complex) or improbable event increases. For example, if you wait long enough, it’s close to certainty that you’ll be dealt a royal flush, or if you shuffle a randomly organized deck of cards long enough, you’ll get an organized arrangement of suits and runs within suits the same as existed as when the deck was new and sealed in its box. But, if that royal flush happened first deal, or the newly organized deck of cards emerged first shuffle, you’d be very surprised and probably suspect some other force at work – sleight of hand hanky-panky magic most likely being afoot. Well, Panspermia is the sleight of hand magic trick in this case.
My simplistic reasoning is to examine the odds of independent individual biogenesis on suitable terrestrial planets vis-à-vis seeding those same terrestrial planets via Panspermia. I would think that it’s more likely to have origin of life success if you plant an existing ‘seed’ in a suitable environment, relative to having independent long cause and effect chain of events leading to a biogenesis. In other words, is it more likely to have one (or a very few) origin of life events that ultimately seed lots of planets, or hundreds to thousands of totally independent origins of life?
Let’s suppose that there’s, say, 1000 relatively new Earth-like planetary environments in our galaxy’s habitable zone that have formed in, say, the past million years or so. What odds these abodes will have life on them in say roughly the same time frame as our own Earth did?
If there are only independent on-site origins of life events that each rely of a chain of involved or complicated Goldilocks required cause and effect events that are necessary for that proto-cell to come into being, then perhaps (guesstimate) only, say, 300 of these planets will become fruitful. It certainly wouldn’t be near to 100%. Now I have to guesstimate as a search of the literature for on-site origin of life probabilities is a worthless exercise. Everyone else’s guesstimates are all over the map! So I guess my guesstimate is as valid as any other! Why 300 out of a 1000? Well, I’m taking the approximately middle ground between the two extremes of near zero percent probability, and near 100% certainty, then erring down toward a more conservative side of that.
Now if Panspermia is operating, (I assume that the Panspermic spore cloud has spread equally throughout the galactic habitable zone over billions of years), and viable seeds from space are sown on our 1000 planets, then one would expect a higher percentage, say 800 planets (guesstimate) to become fruitful. It still wouldn’t be 100% - there are just too many random factors at work – but getting closer to 100%.
Of course if there’s a cosmic Johnny Appleseed* around, then your percentage could easily rise to allow say 990 planets out of 1000 to bear fruit.
What’s a cosmic Johnny Appleseed you ask? Well, here I rely on the thoughts of Francis Crick**. While not the only scientist to ever propose this, he throws a real monkey-wrench into the works in that he has proposed that extraterrestrial intelligence(s) has deliberately seeded suitable planets – “Directed Panspermia” – including us presumably If Crick is right, that really throws calculations for the probability of how prevalent life is in the universe totally out of whack!
The Appleseed idea is akin to the idea that if there is no Mother Nature and you have barren ground then you’d better be prepared to wait a LONG time for something to happen. If there is a Mother Nature, she can seed barren ground in her higgledy-piggledy fashion, and maybe relatively little happens – a few tuffs of grass here and there. But if man (Appleseed) seeds that same ground, you can be assured to a higher degree of confidence that results (a lush lawn) are more likely as not ensured.
It’s a concept hinted at in “Star Trek” to explain why there are so many hominoid races at roughly the exact same level of technology, out of all reasonable probability. [Of course we know the reason why – Captain Kirk meets and squares off against a typical 6 cm trilobite isn’t very riveting TV, and even a T-Rex isn’t a match for a phaser!]
It also reminds me that as a child I tried to write a very amateurish short story about this family of aliens that had a picnic on Earth several billions of years ago. They left their rubbish behind, which of course was ultimately the cause of our biogenesis event! Now that’s such an obvious plot scenario that it has been independently invented dozens (if not hundreds) of times over. Regardless of how many times it’s been thought of, the idea is entirely possible. There’s no violation of any of the laws of physics anywhere in the scenario. No violations remind me that I recall reading various variations of the following theme in quantum physics – ‘anything not forbidden is compulsory’, or, ‘anything that can happen, will happen’ (at least in the Many Worlds scenario of quantum physics).
The Appleseed concept isn’t quite as strange as it might first appear. Haven’t we deposited microorganisms on the Moon; maybe even on Mars (God forbid) if there’s been any slipups in the sterilization of any of the probes that have landed or crashed on the Martian surface. Given that sterilization, aren’t we worried about our biological contamination of pristine environmental friendly abodes of exobiological significance; didn’t we deliberately plunge the main Galileo spacecraft into Jupiter’s atmosphere rather than risk any possibility of it impacting and contaminating Jupiter’s moon Europa? (That might not have been foolproof as parts of the Jovian atmosphere could in itself be environmentally friendly!) Then too, even Carl Sagan once suggested seeding the atmosphere of Venus with photosynthetic microorganisms with a long term view to terra-forming that planet. If it ends up that the number of advanced civilizations in our galaxy is one – terrestrial humanity, it might be the case that we seed the cosmos with life, boldly spreading our germs where no germs have gone before!
It might however have been difficult to have had a cosmic Appleseed 4.5 billion years ago (i.e. – seeding our Earth) in that that might not have been quite enough time for the universe to have evolved a technological civilization capable of doing any seeding – the universe only being some nine billion years old thereabouts and keeping in mind that it takes a while for the cosmos to create the sorts of atoms that are necessary for life. However, that’s no longer the case – a cosmic Appleseed(s) could well exist in the here and now.
Why become a cosmic Appleseed? If you were an extraterrestrial civilization who thought that your planet was the proverbial IT as far as life in the cosmos goes, then you might want to seed suitable sites in the cosmos and spread your collective planet’s genes around. Spreading your genes ain’t much use, but your planet’s microbes, from which you may have evolved, are a suitable substitute. Your planet’s life spreads throughout the cosmos aided by your civilization; it survives, to evolve another day.
Anyway, Appleseed or no Appleseed, I suggest that if you take the concept of just plain Panspermia into account, you can have both a very early genesis of life on Earth, and lots of life within the cosmos. Actually, I’ll propose John’s ‘law’ – the longer it takes for an origin of life event to occur, the more probable that it’s an on-site event. The shorter the time frame, the higher the probability that it’s a Panspermic event. So, if life first arose on Earth say 2 to 3 billion years after formation, then I’d say it was an on-site and purely native event. Alas, since it happened within 500 million years of that formation, I have to cast my vote for a Panspermic seeding from outer space.
Further, Panspermia easily explains a re-origin of life or a re-re-origin of life event on an early Earth following any heavy bombardment wipeout(s). That is to say, at the time of the formation of our solar system and Planet Earth, there was a heck of a lot of rubble floating around the neighborhood: rubble way bigger than the asteroid which KO-ed the dinosaurs 65 million years ago. Visible signs of that are visible today on our Moon as large craters, craters which nearly saturate some parts of the lunar surface. Such large chunks of rubble would have impacted our young Earth again and again and again before ultimately most of the rubble was consumed by the various planetary bodies. Any origin of life event would have gone for naught if a hunk of rubble blasted it away. But, given Panspermia, you’d get an immediate re-seeding, and again, and again, until finally relative peace reigned and life took hold for good.
Finally, there’s one other variation to the Panspermia theme – Bullet Panspermia. The idea here is the exchange of materials between various planetary objects in our solar system. Large impacts (asteroids, comets, etc.) on abodes can knock out small rock-sized chunks of material which can later impact in turn onto other abodes. This works especially well for low gravity, less dense atmosphere abodes. For example, we have lunar and Martian meteorites on Earth. No doubt some terrestrial materials have made the return journey. No doubt Martian material has probably made it to Venus and to the Jovian satellites. The Jovian moons have probably exchanged materials among themselves and some has probably made it to Mars and Earth itself. And while terrestrial or lunar or Martian, etc. rocks can probably make it to Jupiter and Saturn, the reverse isn’t likely. Now, what if some of those rocks happen to have microbial hitchhikers?
Then too, some of those Bullet Panspermic rocks just might escape our solar system entirely. A tiny fraction might eventually drift into neighboring solar systems, and perhaps a fraction of those will seed suitable planetary environments in those systems. Of course the reverse also applies!
Conclusions: The entire business of the origin of life and how common life is (or is not) in the universe is complicated. When it comes to life in the universe, its origin(s) are probably not either/or scenarios. There’s no doubt there’s been at least one on-site origin of life, and probably many. There’s no doubt (in my mind) that Panspermia contributes, to a greater or lesser degree, to populating the cosmos with life, however simple. Finally, there could be a cosmic Johnny Appleseed or two around as well.
*Johnny Appleseed (born John Chapman) was a pioneering American nurseryman who planted lots of apple trees in Ohio , Illinois and Indiana
**Yes, THAT Francis Crick, of DNA/Nobel Prize fame.
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