Just as SETI is redefining its parameters, astrobiology has been going through a shift that widens our notion of habitable zones. Not so long ago, the concept seemed simple. Take a Sun-like star and figure out at what distance a planet could maintain liquid water on its surface. Assume, in other words, that the life you’re looking for is more or less like what’s found on Earth, and therefore needs the same conditions to persist. Now we’re finding remote venues like Enceladus that remind us liquid water can turn up in unusual places, and we’ve parachuted a probe onto a world, Titan, where it’s not inconceivable that exotic forms of life can develop.
Throw in the possibility that objects as distant as the Kuiper Belt may contain subsurface liquids and what used to be a constrained habitable zone seems to be vast indeed. And perhaps we’ve already found another living planet, as astrobiologist Dirk Schulze-Makuch tells Lee Billings in a recent interview. Along with David Darling, Schulze-Makuch is the author of the new book We Are Not Alone (Oneworld Publications, 2010), which fires an unexpected shot across the bow with its subtitle: Why We Have Already Found Extraterrestrial Life.
Image: Viking 2 image of Mars’ Utopian Plain. Credit: NASA.
The location of this ‘find,’ of course, is Mars, where the authors argue that the Viking landers probably did discover life there in the 1970s, despite all the subsequent analysis that seemed to rule it out. As Schulze-Makuch tells Billings:
In some ways the timing was bad for Viking. A lot of progress was made after its life-detection experiments were already on or on their way to Mars: The discovery of all the ecosystems at undersea hydrothermal vents, and the extremophile research of the early 1980s really changed how we think about life and its limitations. The Viking researchers thought life on Mars would be heterotrophic, feeding off abundant organic compounds distributed everywhere all over the Martian surface. That picture was wrong, and studies of extremophiles on Earth have made us think differently about Mars. Some people say Viking tried to do too much, too early, and as a result of its ambiguous results, nothing has happened with Martian life-detection experiments ever since.
Viking’s three life-detection experiments produced results that could not have been more frustrating. The Labeled Release Experiment produced a positive result, the Gas Exchange Experiment a negative one, and the Pyrolytic Release Experiment proved ambiguous. That left it up to Viking’s Gas Chromatograph Mass Spectrometer, says Schulze-Makuch, and its subsequent failure to detect organic matter led scientists to conclude Mars was barren of life.
But the latter result should raise the eyebrows:
…the results of the GC-MS were always somewhat odd. This is because we know from the Martian meteorites that there are organics on Mars. Also, it’s been shown that the same instrument could not detect organics in the Dry Valleys of Antarctica or from hydrothermal soil, places on Earth where we know that a small but significant population of microbes makes a living. So the question is, why did the GC-MS not detect the organics present on Mars? Was the concentration too low? Were they in a form that was not detectable? Or, were they all oxidized to carbon dioxide before they could be measured as organics?
Schulze-Makuch opts for the last option and fits it into a hypothesis he and Joop Houtkooper have developed, that Martian organisms use hydrogen peroxide and water as intracellular fluids. Heat, as produced by the GC-MS experiment, would cause the hydrogen peroxide to become unstable and would have oxidized organic compounds, producing carbon dioxide, which is just what GC-MS detected. Life on a dry desert world might well adapt hydrogen peroxide and water for these purposes, and that would also imply that Viking’s Gas Exchange and Pyrolytic Release experiments used too much water, overwhelming Martian microbes.
Mars, so tantalizingly close to our older notions of a habitable zone, is the place we can explore most readily with our robotic instrumentation, but the real action may be much further out in the Solar System. That’s because the Earth is close enough to both Mars and Venus to have exchanged materials with each in the past, leading to the possibility of contamination. To find an independent start to life, what some are calling a ‘second genesis,’ Titan, Europa, Ganymede and Enceladus may be better venues to explore. Of Titan, Schulze-Makuch says:
Titan is a lower priority than Mars, since it is much, much harder to get there, but for finding life that is almost certainly of independent origin, Titan should be the top priority. If we understand organic chemistry correctly and the reactions behind it, it seems reasonable to think there should be life there. Even if we don’t find life there, we can still see how far organic chemistry can evolve in its prebiotic phase. Titan is a natural laboratory for that.
As our astrobiological ideas have widened, we’ve learned not only that we have numerous worlds to explore for life on nearby planets and moons, but that our own planet may contain forms of life we have barely begun to catalog. What we find will invariably lead to speculation about what direction life may have taken on planets around other stars. The big questions will persist: Is life as tenacious elsewhere as it proves to be on Earth? And even if so, is its origin more constrained than the conditions that allow it to spread? Getting past our experience of life on Earth demands preparing ourselves for unusual answers, and it’s not inconceivable that life may exist from the upper clouds of Venus to the remote edges of the Solar System.
Great timing! I first wrote about Schulze-Makuch’s theory in January of 2007, when it first appeared as a scientific monograph. But just last week I enlarged on it: You Only Find What You’re Looking For.
If Mars bacteria exist and use hydrogen peroxide as part of their intracellular complex, they will need to have a chain of enzymes to handle it as gingerly as sappers have handled bombs (terrestrial organisms do, and we only have to deal with minute quantities of the beastie. If you think oxygen is reactive…)
In my view, the biggest piece of evidence against extraterrestrial life is the absence of recognizable global changes that life would cause on a planet. We know life has profoundly changed our own planet, with the most notable probably being the removal of carbon from the atmosphere and the production of free molecular oxygen.
It should be expected that extraterrestrial life would have similar effects. Otherwise, you would have to suppose that Earth is special in providing conditions for an extensive, global biosphere, while in other places life would remain a niche phenomenon with no global effects whatsoever. This remains possible, but is not very satisfying.
Perhaps life without photosynthesis is a lot stealthier than ours. Still, it seems unlikely to me that, for example, the oceans of Enceladus could be teeming with geochemically powered life without any observable chemical signatures of it on the outside, especially now that we have actually sampled the geysers.
Lastly, I find it pathetic to have to go back 40 years to the Viking landers for the best evidence of life, when Mars exploration has progressed in leaps and bounds since then, spectacularly. This may be the fault of mission designers, or it may point to a weakness in Schulze-Makuch’s case, you be the judge.
As I understand, the motivation for hydrogen peroxide are its antifreeze properties and hygroscopy. Neither of these are unique to hydrogen peroxide, nor does it seem to be particularly good at either. As Athena points out, its destructive nature as an agressive oxidant would make it extremely problematic for any type of biochemistry, especially in the high concentrations that appear to be envisioned.
Unless there is another rationale that I missed, the idea strikes me as fanciful, to say the least.
And since I am in a critical mood, here is something he said in the interview that bothers me:
He seems to be implicating that we would not be able to distinguish between indigenous and imported organisms if they were similar. Nothing would be further from the truth. If they are truly similar, we can sequence them and fit them snugly into our tree of life, and determine quite accurately when they made the journey, on space probes or meteorites.
If they are not so similar, than they could, of course, not be contaminants from Earth. You would not need any further proof, such as:
Handedness is a very naive property to look for as proof of extraterrestrial origin. Any number of much smaller, but more substantial differences from the universal machinery of life on Earth would give this away, very conclusively.
On the other hand, handedness in simple organic molecules would be an excellent indicator of the existence of life, if the organisms themselves are elusive. I wonder if it has been sufficiently considered as a “litmus test”.
Thanks very much for including the photograph of the Mars Viking lander. The rocky scene shown looks almost as if it could have come from Grand Canyon National Park or other arid and rocky locations on Earth. Although obvious, by analogy and intuitive schemata based on probabilities, one is given a deep sense of hope that our universe, even our local neck of the Milky Way is suffuse with biospheres, even if many of them are populated only by microbial life forms. This gives us hope that we can terraform such planets and/or terraform planetary moons of extra-solar origin into additional homes for humanity.
Although I do not at all encourage taking over planets all ready inhabited by ETI civilizations, in fact, I could never participate in such disrespect for ETI persons, I am not adverse to terraforming as many extrasolar worlds that we can lay our hands on provided they are not already claimed by ETI civilizations.
Once again, I keep looking at the Viking photo. It is way cool!
Really what we need to do is stop trying to guess the chemistry of alien organisms and send some decently-powerful microscopes. Easier said than done though…
I find it curious that Schulze-Makuch thinks Titan is the highest priority for exploring for possible independent life. I would think that Europa or Enceladus, which both have much more moderate (subsurface) temperature ranges and (subsurface) liquid water, would be more likely to harbour life than frigid Titan. I suppose that Titan is somewhat easier to explore (at least we have experience landing there), but searching in the easiest rather than most likely spots is the same principle that leads drunks to search for their lost car keys under lampposts. It seems to me that, in terms of picking targets for exobiological exploration, the final calculation needs to include both the ease of exploration and the likelihood of success (otherwise we’d send biological probes to the Moon).
For astrobiological purposes & exploration Titan has the advantages of aerobraking, open-sky liquid aggregations, and a photoreactive atmosphere producing high-energy chemicals – i.e. easy to land on and potential for metabolism. One problem is that Titan shared in the common ‘cloud’ of impact debris that Mars, Earth & Mercury have traded back and forth. Material could reach Titan from any of the inner planets and so any life there might not be a wholly independent origin from life on Earth or Mars.
Europa, and maybe Enceladus, is inside the ‘death zone’ of its primary for impactor ferrying of materials – anything would be sterilised by the speed of the impact, thanks to how deep in Jupiter’s gravity well Europa happens to be. An early Europa might’ve had a deep, extended atmosphere, but burn-up was inevitable rather than aerobraking. Any life on Europa will be from a truly independent origin. Even cometary panspermia is unlikely to have delivered viable biological material to a moon so deep in a gravity well.
I think Eniac is right. Earth life leaves quite a large signature in the form of the free Oxygen atmosphere. We do not see anything comparable in the rest of the solar system. If there is life in other solar system bodies, the total mass of it must be relatively small, compared to the Earth’s biomass, which means that the rate of evolution of that life would be quite slow. The smaller the total biomass the smaller the concurrent events that drive that evolution. The rate of evolution would be slower which, in turn, means that any such organisms would be very primitive.
The other issue is temperature. I know we have gotten into it before, but I still maintain that places like Titan are simply too cold to support anything like cellular life. Some of you have said that there are chemical reactions that occur at decent rates at such low temperature and no doubt this is true. However, I do not think that there is the broad range of such reactions that would be necessary to make up an equivalent to Earth biochemisty. There is a lot of chemistry in biological processes. I doubt that a whole suite of such reactions are available in the cryogenic environments of Titan and other places.
I went through a phase where I read all of the recent astrobiology books. One point that stood out from all of them is that life requires energy. Lots and lots of energy (the energy density of Mammalian metabolism is 10 times greater than that of stellar fusion!) Colder environments, even Mars, simply lacks the quantities of energy that propels biochemistry. Microbial life in the clouds of Venus is more believable than anywhere in the outer solar system and even including Mars based on this consideration alone.
I do think Mars and other places should be investigated more thoroughly. Not just for life but for general understanding of the characteristics and processes that make up these places. However, we should not be disappointed to find the solar system, other than the Earth, a barren place. In fact, I fully expect us to find such to be the case.
Yes, yes, a thousand times yes. This question is not asked nearly often enough. Far too many exobiological speculations seem to assume that wherever life can exist, it stands a good chance of originating. We still know far too little about exactly how life on Earth got started.
Good article, and really interesting comments from everyone as well. I thought of a few ‘what if?’s while reading what everyone has written.
Adam wrote about how Jupiter’s gravity well causes the the speed of any impactor to sterilise any life on it. But is this really true for any impactor? I’ve read this before about whether panspermia from Earth could have reached Europa. Such Earth rocks would be quite small. But what about collisions from a large comet and microbes riding inside? A comet of a few kilometers across would smash through many kilometers of ice, perhaps reaching subsurface water below. I wonder whether everything necessarily would be sterilised (I know it’s likely it will, but haven’t seen big comet impacts discussed before).
Eniac and Kurt9 talked about the absence of anything that could indicate life elsewhere in the solar system. How about the methane or Mars which is being somehow replenished?
I agree with duffer’s point that it may be (much) more difficult for life to get started than it is for it survive once it’s going. Maybe cross-contamination from some relatively hospitable planet is a more common way for life to arrive than independent development?
If I understand the article right, no instruments to directly search for life were included on Mars probes after Viking. But methane (a possible biosignature) has been detected in Mars’ atmosphere (I think both from Mars orbit and by Earth-based telescopes) so the possibility of life there remains open.
I recently heard an interview with a planetary scientist who said that one reason that areas like Titan, Europa, and Enceladus are considered outside the habitable zone (defined as orbits where liquid water can exist on a planet or moon surface) even though they may have subsurface liquid water is that life on those bodies would be difficult or impossible to detect remotely. An underground ecosystem would probably not have a biosignature we could find other than by landing on the surface and drilling.
I don’t buy this. First of all, life does not require mammals. Plants have many orders of magnitude lower power densities. Second, as I have said before, the amount of light available for photosynthesis has no bearing on the energy density of animals, because animals use stored chemical energy (On Earth, in the form of oxygen). The density of energy that can be stored chemically does not depend on light levels, nor on temperature. Low light levels reduce the rate at which energy can be replenished by photosynthesis, but that only limits the number of animals, not their metabolism.
As you have said, it is possible that low temperatures restrict the palette of metabolic reactions to such an extent that none are available with Earth-like power density, but I don’t quite see why that is clear or even likely. Even if it was, animals could be warm-blooded and operate at substantially higher temperatures than ambient.
Photosynthesis is needed to replenish the stored chemical energy. On Earth, it works at light levels 1000 times lower than normal, judging from the ocean depths at which photosynthetic bacteria can still operate, so the Saturnian moons should have sufficient light to power a biosphere, including animals with metabolisms every bit as energetic as our own. Depending on the extent to which animal populations are limited by oxygen production (on Earth they almost certainly aren’t), there might just be fewer of them.
However, because of the other things we have both said, it is unlikely that these things, possible or not, have actually happened, because we are not seeing any signs of photosynthesis or oxygen, and carbon is present in all the atmospheres except ours, unclaimed by life.
I agree with duffer. The assumption of an “easy” origin of life is clearly the weakest link in the chain of reasoning for its ubiquity.
I think we know a lot more about that particular origin than is currently appreciated – all sorts of breakthroughs in our understanding of abiogenesis are being reported. For example, this week a news piece from “New Scientist” discusses the affinity of the different biologically important amino acids for different combinations (codons) of basic RNA nucleotides. Seems the ‘genetic code’ really is based on just plain macromolecular chemistry – not some arbitary fixation at random, but something intrinsic to the molecules themselves. So many so-called “mysteries” of the process keep becoming resolved & understood, that it’s hard to not think there will be a full understanding ‘soon’.
Abiogenesis can never be “understood” the way other natural phenomena can. There can be no observation, there are way too few records left to be anything more than tantalizing, and the process is far too complex to be modelled from fundamental physical laws. All we can have is a deepening understanding of how it “could” have happened, but not how it did happen, and particularly not how likely it was to happen.
Unless we can reproduce it in the laboratory, which could be possible if the likelihood was very high. I don’t think so, though.
I disagree with pessimistic views of exobiology. The general trend is that life exists in more forms, more places, and more conditions than previously assumed. Here on Earth there are extremophiles that can live (or even thrive) in environments such as antarctic ice, hot springs and geysers, vents on the ocean floor with no sunlight, toxic waste dumps, and radioactive sites.
It may be true that the origin of life is an important question, and its requirements may restrict life from growing where it could survive. However, I still dont see this restricting life to earthlike planets. Titan may or may not have life, but it would not be surprising if there are simple forms of life there.
Id guess that the majority of exobiology we will find will be microscopic organisms, lichens, and so on; with more complex creatures being more rare.
Whether we detect “life” depends on our definition of “life”. I have said this before on this blog: My definition (r) of ‘life’ is that it is a system exploits a loophole in the 2nd law of thermodynamics (that entropy always increases in a closed system): “Life” is a system that accelerates the “flow” of entropy towards increasing randomness and harvests a small part of the difference between ‘natural’ flow and ‘accelerated’ flow to build a local patch of DECREASING entropy (the second law is still met). Essentially, it’s an eddy in the flow of entropy. This means that “life” is a stable sytem that is far from equilibrium. The nature of the elements involved, the fluids involved (if any) are irrelevant. IF correct, this view implies that locations in which “life” occurs will decay faster than those in which it doesn’t. Given that increasing entropy = increasing randomness = greater heat emission than in lifeless regions, regions that bear life will be warmer than those that don’t. (Measuring this difference would be very difficult, because it would require comparing temperatures of systems that are matched in every other way but the presence of life). But to my point (and here’s a huge [il]logical leap): we may have already observed extraterrestrial life: Jupiter’s Great Red Spot: It maintains itself by feeding off the counter-rotating bands in Jupiter’s atmosphere, and is essentially isolated from them: “Though winds around the edge of the spot peak at about 120 m/s (432 km/h), currents inside it seem stagnant, with little inflow or outflow” (Wikipedia). I’m either a visionary or an idiot. (And according to numerous observations, the latter is far more possible.)
This thread is all over the place… that’s what I get for not checking the site yesterday.
Regarding codons and amino acids… not really sure what the article is talking about, I need to go look at it. We have had a pretty good understanding about translation, the ribosome, tRNA, and codons for some time now. One of the more important accomplishments was recently recognized with a lovely prize.
Adam: The insights into the genetic code you cite, while interesting, really do not bear on abiogenesis, because the genetic code arose long after life existed, speculatively (but unprovably) as a means for RNA based life to acquire a greater range of chemical diversity. Similarly, DNA is speculated to have evolved as a means to stabilize an organism’s genome. I don’t think we even have a good idea which came first, DNA or peptides. There are theories that say RNA was not the first constituent of life, either. Frustratingly, such theories are extremely hard to prove or falsify. RNA may be the earliest constituent that still exists, but that does not mean it was the first. If there were others, all evidence of them disappeared the moment they went extinct. Barring time travel, there is no way to examine it.
bigdan201: The discovery of Earthly life in more and more extreme environments actually decreases the chance of it spontaneously arising easily, since we now know more and more places in the solar system where life could have arisen, but didn’t. Of course this argument collapses if we do find ET life, but for now it holds.
djlactin: Your definition of life is too broad. There are plenty of “stable systems far from equilibrium” that are not life by anyone’s reasonable definition. The temperature difference you mention is useless, it is small and completely masked by other effects, e.g. the reduction of the greenhouse effect from the fixation of CO2.
It wouldn’t be surprising if a “genesis” of life had greater resource demands than survival and continuation. But how much more so? That question remains. It may be that life must start in a warm, brightly lit primordial soup, or that the conditions are much more flexible. I still lean towards the idea that there is flexibility and variety in the formation of life, but I can’t be certain. Either way, there is simply not enough data at this point to come to a conclusion… after all, we haven’t ruled out exobiology in our own solar system.
Methane gobbling gas bag creatures from GJ 436b, or just another hole in our understanding of exoplanets?
Let’s use “hypothesis” instead of “theory”…
RNA World Hypothesis is a better term. I am convinced by the evidence that remains to this day (all around us), but still we aren’t there yet and it is very difficult to justify that we “know” for a certainty what happened billions of years ago.
And with regards to what (if anything) preceded RNA, I would recommend not ever using “theory”.
With regards to peptides vs DNA, I would put forth that RNA –> RNA/Peptides —> RNA/Peptides/DNA
As a genetic component, DNA is only better than RNA because of stability (forget any other differences for the time being), and only if that becomes an issue because the genetic material falls apart before any sort of replication can occur.
However, when considering RNA as a catalytic molecule vs proteins (peptides), the difference here is very very great. Proteins are much better catalysts, allowing quicker metabolism and a much wider array of (potential) functions. If I were an all-RNA organism and you offered to give me either some peptide capabilities or some DNA, I would take the peptide capabilities in a second. Later I would want some DNA to make my genome less susceptible to degradation and attack from ribonucleases (be they protein-based or otherwise), but up front I would want the catalytic potential that comes with utilizing the amino acids in my environment.
bigdan201 said: “I disagree with pessimistic views of exobiology. The general trend is that life exists in more forms, more places, and more conditions than previously assumed…. ”
this is very much the manner that i think about life – it’s likely that more extreme forms of life are yet to be found here on earth.
as for a genesis of life, my opinion is that it’s intuitively obvious that life’s origin is significantly more constrained than the conditions that allow it to spread. it’s a really important point to consider when we think about ETs. i like the high quality discussions on this blog :)
I think the existence of Methane on Mars is more confirmation of Thomas Gold’s abiogenic origin of methane (and of natural gas on Earth) than it is of life on Mars.
Zen: Good point about hypothesis vs. theory. I tend to be lax with words.
I also agree with your reasoning that peptides are more useful than DNA. However, you could argue that a) DNA is a much smaller step from RNA, chemically, and b) RNA might not be stable enough to support the longer genome necessary to encode proteins. Both of these would tend to put DNA first. Anyway, my point was that we don’t know, and probably never will, which exemplifies the frustrating inaccessibility of abiogenesis to any sort of rigorous scientific exploration.
This is one field that would REALLY blossom if ET life was discovered…
That is right, but we should not forget that in addition to the right environment (what you call resource demand), a large amount of sheer luck might also be required. How much luck? That question remains.
The chance for life to arise on a perfectly suited planet is much larger than that for an identically suited petri dish, because the greater surface area provides more opportunity. Why not continue this argument into the cosmos? The chance for life to arise in a galaxy is larger than that on a planet, because the large number of planets provides greater opportunity. What if the chance for life to arise in a galaxy were pegged at, say 50%? This would be plenty to make our own existence plausible, but it would mean that less than one out of N perfectly suited planets have life, where N is the (presumably large) number of such planets in the galaxy.
It would mean that Earth is the only planet in this galaxy with life, more likely than not.
Eniac: You said “There are plenty of “stable systems far from equilibrium” that are not life by anyone’s reasonable definition.”
Please give me some examples. They may help me refine my “idea”.
(Maybe my idea IS too broad, but maybe current definitions are too narrow.)
“The temperature difference you mention is useless, it is small and completely masked by other effects, e.g. the reduction of the greenhouse effect from the fixation of CO2.”
a) I didn’t say the signal would be great (it’s a part that I left out, because my post was getting too long and pompous).
b) you mention CO2 fixation and greenhouse gases, which implies life forms that are like those on earth (CHON-based; photosynthesis). I made no such assumption. In fact, my point was completely the reverse.
Although one can think of different possibilities for how life might push forward and adapt to survive, what evidence exists (in the form of artifacts that we can see today) strongly suggests that DNA is more of an after-thought. For example, making proteins, the codon/anti-codon template is all based on RNA interactions. You don’t need DNA at all for the integration of proteins. There exist RNA viruses (which are not necessarily artifacts) that demonstrate that DNA is not essential.
Regarding RNA–> DNA… this is not near as simple as someone might think. One needs some enzymatic activity that allows for incorporation (or mis-incorporation) of DNA in place of RNA nucleotides (assuming that is the constituent component) and one needs chain elongation to form the polymer. However, for the addition of peptides or amino acids, you don’t need enzymatic activity, you just need RNA-Protein (or amino acid) interactions, which happens on a regular basis. Eventually these interactions can become covalent bonds or even formed via catalysis (if either are required).
My point simply being that although anything is possible some possibilities are not very likely while others are quite likely.
djlactin: You pointed one out yourself, the Great Red Spot. Convection cells are a similar and popular example of stable structures driven by entropy flow. If those were considered life, life would indeed be everywhere.
Zen: You are making a very good case for proteins first, so maybe this is one of those things that we can conclude from the available evidence. It still does not shed much light on how RNA based life originated.
Yes I agree. I think there is some interesting work out there, but we don’t know everything…
there was an article last year that detailed the possibility that phosphorus may have played a KEY role…. I think it was in nature or science. It was a pretty significant piece of work, taking ~10 years to finish. Essentially, they demonstrated that using phosphorus or phosphate (forget which) as a key component one could generate….. hmmmm…
Reference #22 VERY SIGNIFICANT.
This work from Sutherland’s lab is huge. It is not THE final word, but it is a huge step in the right direction. There were several comments/brief summaries about the work. I would recommend reading one of those first if you are not interested in all the details. Anyways, lots of interesting stuff… especially when one realizes we don’t know everything about what chemical reactions are possible under what conditions…. yet.
kurt9: What has oxygen have to do with it ?
Earths atmosphere was almost void of oxygen for a long time after life has developed. Sure huge multicellular organisms may not exist outside earth (in this solar system) but Europa and the clouds of Venus could be full of life without any chance for us to see it with our current methods.
One question with life on Titan is whether life could begin in the subsurface ocean, where temperatures are warmer and reaction rates would therefore be less sluggish, and then subsequently colonise the surface (possibly being transported there via solid state convection in the ice layer or more directly through cryovolcanism).
I wonder how adequately we humans can know which chemical reactions are or are not likely to have occured upon these moons or planet.
Let’s remember, when thinking about life arising on a planetary body – Europa, for example – we are considering a ‘laboratory’ with a volume of 15 trillion cubic kilometres, consisting many different layers and enviroments, which has been ‘running’ for four billion years or so to.
Do we have enough data and insight of our own to know all the reactions that are likely or unlikely to have occured over these vast scales? I think not.
The excellent discussion here has shown one thing again, I think: “life” is a really hard problem. Over and above that, I think, “life” as a problem is hard to a much greater degree than the usual hard problems like, say, quantum physics, or unifying the current theory of gravitation and quantum physics.
Because of the above I expect a first sound scientific solution of the problem of life arriving in the far future and being of a kind rather different from what we could imagine today. And I expect something very, very interesting.
Nowadays the understanding of life is (at least mostly) descriptive, as far as I can see. This makes it difficult to construct falsifiable theories, like in physics, which can be used to make reliable predictions (this is only my impression, and I’m no biologist; I see some promising but rather special approaches; if somebody knows more …). And, as far as I’m concerned, concepts like “self-organization” and “autopoiesis” are not really helpful.
Martin: Any extensive biosphere is bound to have a profound impact on the environment, because it is the nature of life to rearrange the naturally occurring chemicals into biomass and “waste-products”. In our case, one of the waste-products is oxygen, and because it does not naturally occur at all, it is an easy telltale sign of something “fishy” going on on Earth.
Even if you acknowledge that the CO2+H2O+energy => biomass + O2 formula is not the only basis of life possible, the general principle remains the same. It is possible that a waste-product could be hidden if it is already naturally abundant, as might be the case with ammonia and hydrogen sulfide, which are byproducts of some chemotrophic metabolisms found on Earth.
However, the other profound change we see on Earth is the disappearance of CO2 from the atmosphere, caused by the same above reaction. Any biomass, no matter how alien, will be limited in growth by one of its inputs. By definition, the biomass will grow to its limits, leading to strong depletion of the limiting component (CO2 in the photosynthesis case) in the environment, far from its expected natural abundance. Since we have actually analyzed the chemistry of geysers (AFAIK), we would likely have observed and probably recognized any such anomaly.
That said, the absence of proof is obviously not proof of absence, and more research is certainly needed. What we should be looking for is the absence of a particular chemical that is found in most otherwise similar places in the solar system, and which has the potential to be the limiting source of a critical structural component of complex biochemistry (like CO2 as a source of carbon). There are not that many naturally occurring atmospheric compounds, so the candidate list is not too long.
Here are some alternatives, without judging their plausibility:
CH4 + H2O + energy => biomass + H2 (for carbon/oxygen biochemistry)
NH4 + H2S + energy => biomass + H2 (for sulphur/nitrogen biochemistry)
SiO2 + H2O + energy => biomass + O2 (for silicon/oxygen biochemistry)
The last has the potential for the greatest biomass, since its constituents are so extraordinarily abundant.
I like the PAH world hypothesis:
PAH’s are common in the universe and this theory represents a simple, easy way for them to have formed the first RNA molecules.
What’s Oxygen got to do with it?
Oxygen (O2) is a reactive compound with lots of energetic reactions. I maintain my point that life needs energy, lots and lots of energy.
I suggested a while ago that we might search for life by looking for atmospheric deviations from what we would expect in the absence of life, rather than by making assumptions about what life might do.
It was quickly pointed out to me that (besides the problems of observing extraterrestrial atmospheres in adequate detail) a more fundamental problem was establishing a baseline of what atmospheres of planets with no life would look like at various stages of their development. If life is ubiquitous there might be no such baseline, or at least it would be very difficult to establish.
As you say, more research is certainly needed.
I am not fluent enough in chemistry (specifically pre-biotic) to assess how possible the PAH hypothesis is. I would not rule it out based upon what I have seen, but I am definitely more in the direction that something like threose nucleic acid may have preceded RNA.
However, Sutherland’s lab’s recent work hints that you may not need a molecule before RNA…
I should stress that having a scaffold that allows a chain of nucleic acids to form is an important question. Naturally-occurring clays or other similar substances are likely candidates for how one might get chains of nucleic acids without already existing enzymatic activity.
–The clay would act as an area where RNA would essentially come together at high concentrations, which could facilitate chain formation. Depending on temperature, you could then have the complementary strand formed and it would dissociate (higher temp), which would allow another complementary strand to form. The first complementary strand could then serve as a template for additional propagation of the original template. But, this is simply logic at work. Szostak’s lab has some cool work on protocells.
So… if we take some clay and add in temperature variations and some of the pre-biotic chemistry that we are starting to know more and more about… it’s not too difficult to see how you can get the first bits of life going. BUT, that does not mean that RNA came first… simply that it is not completely impossible. Sutherland’s work lends some support to RNA first, but time will tell. Here’s Szostak’s commentary on that work: