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The idea that a tiny organism could hitchhike aboard a mote of space dust and cross vast stretches of space and time until it landed and took up residence on the early Earth does seem a bit implausible. More likely any such organisms would have been long dead by the time they reached Earth. But… might those long dead alien carcasses still have provided the genomic template that kick started life on Earth? Welcome to necropanspermia.
Panspermia, the theory that life originated somewhere else in the universe and was then transported to Earth requires some consideration of where that somewhere else might be. As far as the solar system is concerned – the most likely candidate site for the spontaneous formation of a water-solvent carbon-based replicator is… well, Earth. And, since all the planets are of a similar age, the only obvious reason to appeal to the notion that life must have spontaneously formed somewhere else, is if a much longer time span than was available in the early solar system is required.
Opinions vary, but Earth may have offered a reasonably stable and watery environment from about 4.3 billion years until 3.8 billion years ago – which is about when the first evidence of life becomes apparent in the fossil record. This represents a good half billion years for some kind of primitive chemical replicator to evolve into a self-contained microorganism capable of metabolic energy production and capable of building another self-contained microorganism.
Half a billion years sounds like a generous amount of time – although with only one example to go by, who knows what a generous amount of time really is. Wesson (below) argues that it is not enough time – referring to other researchers who calculate that random molecular interactions over half a billion years would only produce about 194 bits of information – while a typical virus genome carries 120,000 bits – and an E. coli bacterial genome carries about 6 million bits.
A counter argument to this is that any level of replication in a environment with limited raw materials favors those entities that are most efficient at replication – and continues to do so generation after generation – which means it very quickly ceases to be an environment of random molecular interactions.
The mechanism through which a dead alien genome usefully became the information template for further organic replication on Earth is not described in detail and the case for necropanspermia is not immediately compelling.
The theory still requires that the early Earth was ideally primed and ripe for seeding – with a gently warmed cocktail of organic compounds, shaken-but-not-stirred, beneath a protective atmosphere and a magnetosphere. Under these circumstances, the establishment of a primeval replicator through a fortuitous conjunction of organic compounds remains quite plausible. It is not clear that we need to appeal to the arrival of a dead interstellar virus to kick start the world as we know it.
Further reading: Wesson, P. Panspermia, past and present: Astrophysical and Biophysical Conditions for the Dissemination of Life in Space.
So being invaded by alien zombies might be possible after all.
Better stock up on ammo and gas for the chainsaw…..for close encounters…..just to be sure. 😀
The one problem with panspermia ideas is that you still must have life originating somewhere and by some mechanism. The only cosmology where panspermia can avoid the origin issue is the steady state universe, which avoids the problem of cosmic origins. We clearly do not live in that sort of universe
LC
Good point, LC. I think of that every time someone mentions panspermia.
Panspermia changes the statistics of an improbable event occurring. Instead of the seeds of life being a very improbable event here on Earth, it becomes an improbable event that need only happen once amongst millions, perhaps billions of exoworlds, many that might only hold the Goldilocks conditions for life briefly.
Once that improbability happens, necropanspermia would allow it to be epidemic across the cosmos and kick start more complex life here. The origination problem still remains but the opportunity for it happen, if only once, is greatly increased.
As a theory it makes a nice clean prediction of what should be hiding in Cabeus’s permanently shadowed moon ice.
I think the idea of Panspermia is primarily proposed as a means of transmission. Of course, life would need to originate somewhere and by some means.
What makes this idea interesting for me is the possibility that life – assuming it exists elsewhere – could potentially share some genetic similarities with life here. This may be on a grand galactic scale or merely a process that’s only feasible for smaller pockets of space.
Of course, all of this is merely speculation without some data to work with. A breakthrough in engineering life would help establish the truth behind the current hypothesis. As for Panspermia, the detailed obseration and sampling of lifeforms on a multitude of planets may be required to test this.
We might have to wait quite sometime )=
A notoriously unsolved problem is, that the available definitions of life list certain features — homeostasis, growth, reproduction, etc. — which may not, or may not all, be intrinsic for life. Because of this we are not really able to discriminate between, well, “real” life and entities which just happen to have the features listed in our — I would say: more or less ad hoc — definitions.
We only know reliably, that we human beings are “real” life — by definition –, and that our biological relatives — based on a vast amount of scientific theories, including the theory of evolution, and observational data, including fossils — are “real” life. It’s questionable, whether templates from space would count as our biological relatives.
Speculating … A variation of necropanspermia would be, that the material from space was not animate itself (or has been animate once before), but was some kind of catalyst, increasing the probability for certain processes, but not being a template or a building block. Well, but in the end, everything on earth came from space, e.g. water, including that in all living cells, came from asteroids, as far as I know.
Having said this, I’m not convinced, that there is enough evidence for panspermia as a serious theory.
The argument there has to be a panspermia because the probabilities are too small is a fallacy. It is similar to what is often said by winners of the lottery. Many will testify this is evidence of some divine hand, or some supernatural process. Of course it is nothing of the sort. The system is guaranteed to produce winners, and when looked at as a total sample space there is nothing surprising about his. In fact there are probabilities for those who win twice. Much the same is at work here. Biological systems are not physically impossible, nor is it impossible for them to emerge given a sufficient number of planets with a range of physical and chemical conditions. In a universe that is sufficiently large, or infinite for practical purposes, then anything which is not physically impossible is then ultimately mandatory.
The Urey-Miller experiment is maybe a first order indication of the chemistry in the Earth’s early atmosphere. Even if this is somewhat oblique it is interesting how organic compounds can form under rough conditions. Throw an electric arc through a gaseous mix of N_2, NH_3, CH_4, CO_2 etc, and you get amino acids and UV radiation give nucleotides. Of course that is a long ways from life, but it is an interesting start. Some organic compounds have been found in nebula for that matter. So let us assume the Earth 3.6 billion years ago had these compounds being formed in this atmosphere that would be fairly toxic to us now. The next requirement is that these elementary compounds enter into longer chains. That again is possible, such as nucleotides will form up 3’-5’ bonds with UV radiation. Amino acids can bond with acetyl groups which then can facilitate di-peptide bonds. In fact ribosomes link up acetylated amino acids into polypeptide residues. So again it is possible for these compounds to emerge in the “soup.” The tough part comes with trying to understand the early mechanisms for self-replication. This was some process which required an energetic path of some form. The adenosine must have interacted with some mechanism which converted ADP to ATP, so then the ATP phosporylated polypeptides which initiate molecular pathways. So in looking at mitochondria the Krebs cycle produces a proton pumped across a membrane, which generates oxidative phosphorylation of ATP. So I conjecture the process involved a geochemical mechanism across a crystalline membrane or the action of electrons on a mineral or crystalline surface.
The other bit which had to be accomplished is some process by which DNA, or maybe more likely RNA codes were converted into polypeptide sequences. This is really hard to understand, and is probably one of the biggest mysteries in the whole problem. Yet RNA binds onto various peptides in complicated way. Ribosomes are essentially protein-RNA complexes, and most mRNA is actually sn-RNA which bind onto polypeptides in a manner which switch on and off their activity. This process is involved with methyl-transferase that methylates DNA and turns it off, or other methyl-transferases which de-methylates DNA to turn it on. Most so called junk DNA turns out to be of this form, where it expresses snRNA that elicits a switching on and off of gene expression. Given that we are speculating some here, in this soup there might have been a huge array of peptide-RNA complexes, where some of them interacted with RNA to promote peptide bonds. Somewhere early on the precursor for the 60s unit of the ribosome was selected for.
So the pre-biotic chemistry most likely involved something of this sort, probably around hot vents where these is some thermal energy flow and a measure of chemical activity. So if there were “pools” with this sort of activity going on there might have then been something of a bio-chemical fitness selection process. So there was a form of molecular evolution which began to set in. This selection process would have given preference to molecular systems or complexes which had some sub-Markovian content. This is similar to biological evolution, but in more of a molecular information theory setting. This is some general form of the evolutionary theory, where evolution theory really applies best to eukaryotic organisms that have sex and generations. Applying Darwinian evolution in its classic form is problematic with prokaryotes. There is frankly a more general setting for evolutionary theory waiting in the wings, which might do to Darwin what Einstein did to Newton.
The molecular selection process would have favored systems which captured energy the most efficiently and for systems which could control this process within their own information content. If pre-biotic chemistry involved some geo-chemical or geo-physical processes selection might have favored pathways and webs of pathways which could function without interacting with geological mechanisms. Then by these proximal or putative ideas of these mechanisms biological systems eventually emerged as self contained cells or the most primitive prokaryotes, which then successfully colonized Earth within a half billion years.
Is this an argument against panspermia? No, but I have never been convinced of arguments for its necessity. Once population I stars emerged with heavier elements or “metallicity” then the chemical elements for life existed. The solar system emerged about 4.5 billion years ago, around the time the universe became vacuum or dark energy dominated. There is a 9.2 billion year time period before then. Based on galaxies we observe around prior to 4-5 billion light years out there were heavy atomic elements and the like, so terrestrial planet probably existed 8 billion years ago. So there may have been biologically active planets, where then maybe a large asteroid gouged out rock with cells on them which then after a considerable time period crash landed on some other planet and took up shop there. In fact in the universe where anything not impossible is mandatory this then must occur in some places in the universe. Panspermia is then not impossible, and maybe Earth is the beneficiary of this sort of thing. However, I have never been convinced that this is somehow a more plausible mechanism for life on Earth than its emergence here to begin with.
LC
Thank you! It is an interesting hypothesis on pathways that may not have been taken here, or even rarely elsewhere, but nevertheless can’t be rejected out of hand.
I’m not generally interested in transpermia for the reason LC mention, it doesn’t predict abiogenesis, but it can putatively have influence somewhere. And now I can gush on a topic that I can relate to! 😀
Concerning the time frame and what one example shows, I have to postpone a thorough discussion, if ever. But even if one accepts that it is somehow too short, the general information argument doesn’t impress.
[I’ll have to leave out detailing Wesson’s model for now, the paper is long!]
Evolution is a process that most generally need and can be defined as such; “Evolution is a process that results in heritable changes in a population spread over many generations.”
It is the genome that distinguish this process of darwinian evolution from other forms of evolution. And as Dawkins describes on the web the genome learns about the environment by bayesian learning (post selection), trial-and-error and trial-and-reward on the “hypotheses” that the preceding generation has made through producing alleles.
[I’ll have to leave out most references for now, this comment is long! (O.o) Ask if needed, and mind that I can misremember and/or misstate.]
If we look at an ancestral state without genome, we still expect variation and selection to work, but we need to revise the process definition. Heavily modifying a suggestion of Sleep, (2010 doi: 10.1101/cshperspect.a002527), one can putatively revise to “Evolution is a process that results in changes in a population of self-organized and reproducing consortia of complicated chemical compounds over time.”
Note that there are no replicators involved, but reproducing consortia. The information on what works in a sufficiently stable environment that is today concentrated in the genome was distributed over the consortium.
It is a therefore bit of an oxymoron to claim that a random assembled replicator contains information gained. Unless one observes that this fortuitous molecule was at least selected by its current environment, but really all the preceding ones. As already Darwin noted and this posts mentions, this process is always observed to proceed in small steps because it is its nature.
The consortium makes this small step advance eminently feasible, the “random” assembled replicator less so. (Note that I would call even Szostak’s semipermeable membranes + activated nucleotides + (say) heat vent chemical production & PCR reproduction, a consortium in this sense.)
Hot from the presses! I don’t think it has been used in a concerted attack on the ALH84001 carbonate deposits. (Paywall.)
But this discovery of a simple atmospheric particulate (!) origin of such on Earth and its putative application on Mars (photolysis ozone and carbon dioxide) makes for it. Indeed they note the same oxygen anomalies from Earth/lab environments in ALH84001.
D’oh! I should have added that even if the carbonates in question most likely comes from the usual water environment deposits, the interesting thing is that as in necrotranspermia the research introduces new pathways. But in this case a putatively major one for funny mineral & chemical assembles.
In my extremely simplistic, uneducated thought process….doesn’t a plant seed die when it falls off the plant, yet, when planted and watered, grows into a new plant? Has anyone bothered to find out whether a typical plant seed, such as corn or beans, when exposed to vacuum and space radiation, then returned to the earth, will grow like it would have otherwise? If that has not been tried, it certainly would be an interesting experiment for the shuttle or space station to try.
@ LC:
Except that it doesn’t seem to be so. If you combine whole-genome data and a transition analysis on characters (here, genes themselves) you capture a fully resolved phylogeny, despite confounds from long branch attraction and horizontal gene transfer.
Actually there is a paper from last year that marginally resolves the sisterhood of eukaryotes and archaeabacteria, as in this current “mega-matrix” tree, purely by whole-genome analysis.
As for extending evolution theory (which is constantly added mechanisms to), it is in practice a definitional issue. Most people believe for good reasons mentioned in the post that the same mechanisms apply early on. (Or similar, if you out-define them by keeping heredity by the genome in the definition.) Some call the remainder “chemical evolution”.
@ Witnessnbr1:
It isn’t simplistic a thought at all, and very relevant. I believe such experiments have been done, and they should be complemented with bacterial spores which are similar to some seeds in that they have evolved for preservation.
As for “die” and “animate” of Duncan Ivry, I think it is begging for problems to use “the NASA definition for life” (i.e. replication & metabolism) too myopically, it is a tool to recognize life easily but not all cases of life. A single individual doesn’t constitute a species. (But the potential for it, among asexual species.)
If evolution is the process of life, life isn’t only a property of the whole population, but one can well separate metabolism and heredity.
Seeds who metabolize somewhat, bacterial spores that likely doesn’t until they become bootstrapped by individuals randomly awakening and then communicating “good environment” hormonally with the sleepers, viruses that doesn’t but can supply deficient bacteria with genes for needed metabolism (Mimi & Mama viruses), and so on and so forth are still “making a living” as far as evolution goes, in the right environment. (And evolution is all about the right environment.)
If you don’t out-define them by appealing to biochemistry in the definitional frame that a process definition sets up, mutating Neumann replicators and evolutionary software may well constitute “life” in the evolutionary sense. And in the later case the “metabolism” is decidedly separable to the hardware substrate.
[Since we usually appeal to “universality” in processes, I would propose to lay off the restrictions. But that is me.]
I don’t like panspermia.
Not a big fan of the panspermia theory actually. As LC said, life still has to evolve somewhere. Earth seems like just as good a place as any, and really the best place we know of.
So for panspermia to work, life has to evolve on a favorable planet, somehow survive a cataclysmic impact (enough to blast the rock into space), travel for thousands, if not millions, maybe even billions of years exposed to vacuum, all kinds of radiation and deep cold, then survive atmospheric re-entry at meteoric speeds, get blasted again (on impact), and then flourish. That’s one hell of a bug.
Or it could have evolved on a favorable planet….and stayed there.
Concerning time frame:
Very true, but today one can say more. [And again I will lay off refs until asked.]
One data point is from the well known Jack Hills zircons that show that a liquid water reservoir in some form existed @ ~ 4.4 Ga. (Even if perhaps pressurized by many atmospheres of CO2 and heated to hundreds of degC.) They also include diamonds that show the ubiquitous carbon isotope signal of metabolism @ 4.25 Ga.
Now Fisher-Tropsch processes, which should be generic in hydrothermal vents (say) and hydrothermal vents generic in the Archaean, is claimed to be able to produce the same signal.
Another data point comes from the recent found faux-amphibolite in Nuvvuagittuq, an igneous or meta-igneous rock derived from material @ 4.28 Ga (at least). It has undergone several thermal events @ 4.0, 3.8 and 3.66 Ga, but it follows that somewhere between 4.28 – 3.8 Ga:
1) There were reservoirs of liquid water. (Wet produced minerals.)
2) Archaean plate tectonics may have existed in the Hadean. (Both wet and dry produced minerals.)
3) LHB impactors didn’t seriously affect the environment. (No oxygen anomalies.)
4) Photosynthesis existed. (Preoxygen atmosphere sulfur photosynthesis drove a biological sulfur cycle, as witnessed by several isotope ratios.)
While the ambiguities doesn’t allow us to test that life existed @ ~ 4.25 Ga as in the parsimonous hypothesis, this means that it isn’t a safe assumption to claim that life didn’t exist then!
So life may have existed a mere 250 My after the Moon creating impact @ ~ 4.5 Ga, a reasonable amount of time. (About the same time it took from the first multicellular body plans to the first land living terapods.)
About the same general question was asked on the astrobiology course I follow. I believe my response is applicable, not as a personal theory but as an attempt of answer.
I tire, so FWIW here is the relevant part of the answer in full, excepting links to refs:
“Many texts describe how biogenesis could be a repeated process of attempts over time and locales. In the simplest model this belongs to the family of Poisson processes. Possible caveats is 1) it isn’t stationary after success (since existing life interrupts it) 2) it can succeed numerous times, in which case it is a Lévy process I believe. However for the purpose of this discussion it suffices.
Such processes stacks most of their their distribution’s probability mass in the beginning. The pdf for the exponential distribution of a first attempt:
http://en.wikipedia.org/wiki/Exponential_distribution .
It is consistent with simplest possible stochastic process fitting the situation that we see a relatively short time. That was all I wanted to say at that time. Any clues why this is wrong is appreciated (but then you may also benefit from reading on).
—————
This time I was prompted to consider deeper:
– We can test the model:
In a normed distribution (observation time t = 1) for a one-sided 3 sigma test we want to have a set of distributions with at least 0.99 of the cdf: F(t,lambda) = 1 – exp(-t*lambda >= 0.99 -> t*lambda > 4.6. t = 1 -> lambda ~ 5, so waiting time T ~ 0.2. With actual time t* ~ 5 Gy, actual wait time T* ~ 1 Gy, which is observed.
Thus a hypothesis that we see a stationary stochastic process with rate at least lambda = 5 passes a 3 sigma test on the available data.
This can fail internally or by putting a more predictive hypothesis (since this is the simplest in its class). For the former, the caveats above first: 1) is not a fail. 2) is complicated, but 2.1) if it looks like a compound Poisson the model still applies and 2.2) post-event the phylogenetics looks like 1 success (a universal tree).
“What if” the observation had been closer to observation time t? _Such_ test wouldn’t have been possible to do with such confidence. That doesn’t affect _this_ test that is to be scrutinized for its failure, not its success.
– How is this “indicative of easy biogenesis”?
In principle a stationary process means stationary mechanism in a stationary environment. This isn’t what happened at the start, since volatiles were collected, temperatures and pressures going down and tectonics started. But since it *looks* stationary it means it was close to stationary in some stochastic sense, even if it was frustrated in actuality. (For frustrated biogenesis, see Lineweaver et al papers.) In this sense the process was robust and the environment stable enough.
Further smaller wait time, which could be even smaller if tectonics had allowed observation, means that there were many parallel attempts. Larger wait time considered above means deterministically either fewer attempts over long time for some reason or fewer successful for some other. Both is indicative of deterministic difficulty.
– Is there a more predictive hypothesis? What would such look like, say for comparison’s sake if possible as stochastic process?
Monod was suggested. I can envision the quoted material as a dynamic process; he seems to say that there was dotted i’s and crossed t’s so in some small volume of phase space biogenesis happened, and all (or enough) of a vast space was explored by the process and its environment before that volume was encountered. However I can’t see how that translates to a stochastic process, except that at the surface it looks like the exact opposite of “stationary”! Which doesn’t seem likely if given the above to compete with.
Due diligence would be to read Monod’s texts. But I’m not tempted, the quote was AFAIU from a book “Chance and Necessity: An Essay on the Natural Philosophy of Modern Biology”, he was an “esteemed writer on the philosophy of science” and “In this book, Monod adopted the term teleonomic to permit recognition of purpose in biology without appealing to a final cause” which is philosophy of theology (!).
“He was also a proponent of the view that life on earth arose by freak chemical accident and was unlikely to be duplicated even in the vast universe. “Man at last knows he is alone in the unfeeling immensity of the universe, out of which he has emerged only by chance,” he wrote in 1971. He used this bleak assessment as a springboard to argue for atheism and the absurdity and pointlessness of existence. Monod believed we are merely chemical extras in a majestic but impersonal cosmic drama—an irrelevant, unintended sideshow.” [Wikipedia material all.]
That sounds like pure philandering philosophy from a biologist, sorry. Yeah, I don’t like philosophy either…
Are there more models that could compete? I know of Koonin’s “Biological Big Bang (BBB) model […] proposed for the major transitions in life’s evolution”, but that is more or less rewritten Monod, IIRC.”
Note that:
a) I’m a terribly difficult student! 🙁
b) Stochastic process modeling directly contradicts the difficulty of the “dynamical” hypotheses of Wesson/Monod (small abiogenesis phase volume) respectively Koonin/Crowell (… and densely visited phase space). 🙂
At least for the data at hand ~ 1 Gy as suggested by earlier fossil data is a sufficiently generous amount of time. And for shorter times, say the ~ 0.25 Gy suggested by modern fossil data, stochastic processes only tells us that the attempt rate or success rate goes up.
There is no real way of comparing what we see with deterministic difficulty. One can only say with the one data point we have, uncertain that it is, that it was sufficiently generous in comparison. Well, duh! Or in more descriptive words, abiogenesis is easy!
Necropanspermia is the ONLY answer to life in the universe, dear carbon copies…
life is NOT limited to this BLUE MARBLE… wouldn’t it be an awful waste of space;-)?
I am not as pessimistic as Monod. In fact prokarotic life or analogues to that might be rather common. A complex bio-planet like Earth is probably fairly rare, and intelligent life even more so. The point is that the origin of life was probably due to some highly unique set of events and condition, but these are not zero probability and as such with a unvierse sufficiently large are probably repeated. For all we know Mars may have originated life unique form Earth’s.
LC
LC, I’m frankly stunned by your response, which doesn’t come to grips with what I wrote. I suspect I expected more.
First I have already noted that Koonin (IIRC) and you aren’t as pessimistic as Monod, since you appeal to a densely visited phase space of possibilities (due to a large universe).
Then I show how a tested model rejects the very idea “that the origin of life was probably due to some highly unique set of events and condition”! The data at hand tells us, overwhelmingly what I can see, that abiogenesis under our conditions was easy. That there were no “unique set of events and/or conditions”. Moreover, conditions which in the model was assumed stable but wasn’t really so had a large window by way of environmental drift.
You can’t shoot down a theory by polishing your own favorite. Which again, I already noted! That favorite is not as pessimistic as Monod, but it is far too pessimistic when compared to a testable hypothesis. And I dunno how Monod/Wesson & Koonin/Crowell are testable in the first place, which is perhaps the graver problem. I asked for a competitive model above, and I already noted that there doesn’t seem to be one!
So, sorry, but I don’t see how repeating a hypothesis makes it more believable or begin to respond to the analysis I made.
What we can agree on, since stochastic modeling tells me this, is that the advent of human equivalent intelligence (whatever that means) was likely hard. Just run the numbers on the above model, and that is what pops out. (Mind that we now are unable to test and reject the possibility that this only *looks* hard, what I can see.)
This is of course what most biologists likes to tell us, since evolution is contingent:
“For Conway Morris, this convergence shows that evolution follows inevitable paths. But that’s a curious argument to use for the “inevitability” of humans which, after all, arose only once. If humanoid intelligence (and, by extension, our ability to apprehend and worship a god) was so inevitable, why did it evolve only a single time?”
Indeed.
[To stave off the inevitable (?) response, observing OOL is different since it only had to happen once and depending on circumstances were unlikely to be observed to repeat. Evolution is ongoing, OOL no longer. Something already Darwin noted and which again I have covered already in laying out the model.]
Oops, I should have added that I was very *glad* for the response. Thank you!
At least it ought to tell me that stochastic modeling isn’t a fully crackpot idea under the circumstances. Personally I believe it may contribute to understanding.
Among other things I think one can see that there is something fishy about the idea that there were “some highly unique set of events and condition”. It is said to be a stochastic process hypothesis (well, usually interpreted as stochastic anyway) but is sure doesn’t look like one!? As they like to say, “what is up with that?”.
@ Witnessnbr1
doesn’t a plant seed die when it falls off the plant, yet, when planted and watered, grows into a new plant?
Interesting point. I’m going with no, it’s in a state of dormancy – since a seed will germinate under the right conditions – but only up to a certain period of time before its internal infrastructure begins to degrade, at which point I would call it dead.
In space, the cold and vacuum conditions won’t be a seed’s biggest problems – the radiation and cosmic rays will probably be what kills it.
The LIFE experiment aboard Fobos-Grunt will carry seeds from Arabidopsis thaliana on a 34 month round trip (something similar was tried back in the Apollo era)
http://en.wikipedia.org/wiki/Living_Interplanetary_Flight_Experiment#Specimens
” Well, but in the end, everything on earth came from space, e.g. water, including that in all living cells, came from asteroids, as far as I know.” — Duncan Ivry
Volcanos expel large amounts of water vapor and this is now thought to be the most likely source of ALL of the water on the surface. This has replaced the cometary-water notion after an isotopic concentration analysis, as explained somewhat more than a decade ago in the “Sky & Telescope” magazine (2/99, p. 35): “Comet water has twice the amount of deuterium [the hydrogen isotope] than terrestrial water does (…). Our oceans are not cometary debris.”
Torbjorn Larsson OM: What I wrote yesterday was a bit brief and cryptic as I was getting tired and about to go to sleep. I was more or less agreeing with you.
When it comes to more complex life things are of course less probable. Prokaryotic life, or life on that level of complexity, may be comparatively common in the universe. What might be less common is the occurrence of complex cells such as eukaryotes. The process by which prokaryotes entered into commensurable assemblies and ultimately defined whole cells is probably far less common. There was then the invention of meiosis, which lead to conjugation and eventually sex. Then we go up mount improbable further to multicellular organisms, and the three main branches (out of over 20) of eukaryotes that went multicellular defined animals, plants and fungis.
Up to that level was the Cambrian explosion. Of course the rest is evolutionary history. Stephen J. Gould maintained there is no goal or direction to evolution. In one sense I agree with him, though the trend does seem to be towards greater complexity. What might manifest itself as complexity on some other planet could of course be vastly different from what we have on Earth. From the estimate I did a number of years ago and presented in my book, there might be a few thousand planets in solar systems and gravitationally favorable conditions similar to Earth. There might be in this galaxy a few thousand planets rich in prokaryotic life and out of those maybe a 10-100 have complex life of some form. These planets might be called bio-planets, where biology is a major driver of physical and chemical conditions on the planet. This is in contrast to Mars, where even if there is life there it really is on the margins and clinging to existence. So if we extrapolate further it probably means that what might be called intelligent life is very rare, maybe occurring at any time on the cosmological Hubble frame in one out of a thousand or a million galaxies.
LC
Torbjorn makes a fairly convincing argument that in relative terms, life arose very quickly after the lunar creation event. If this is indeed the case, then I think this increases the weight behind abiogenisis theory and diminishes the plausibility that panspermia played a major role (if any).
I agree with LC that the more complex the ecosystem, the less likely it is statistically probable. However, under favorable or semi-favorable conditions, wouldn’t life seek to increase it’s complexity? Perhaps complex life follows abiogenisis in most cases, I don’t think we should underestimate this possibility. It may be just a calculation of time and the favorably of conditions.
The grand impact theory for the origin of the moon seems to still hold weight. Presumably Earth and a body about the mass of Mars were in some orbital resonance and they collided. Doubtless that was a huge event, and Torbjorn is right that within 500 million years life did emerge. Even though conditions were doubtless tough during the Hadian period, life did manage to get going around that time. I read sometime back that regular hyper-impacts probably evaporated the oceans and these then precipitated back down.
The panspermia idea is still plausible of course. I question though how prevalent it is. An asteroid impact might impact Earth and knock pieces of material into interstellar space. Of course that would be a small fraction of the material scattered, for it would have a higher velocity after impact than the incoming asteroid. Then we might ponder what the probability any piece would travel the hundred or thousands of light years and land on just the right planet to “plant its seed.” I might imagine this happens here and there, now and then, in the cosmos. However, I can’t see it as a mechanism for spreading life far and wide.
LC