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Alien Civilisations: Two Competing Models

by Stephen Ashworth

Being a jazz buff (the 1950s and early 1960s are my era of choice) I naturally note that frequent Centauri Dreams commenter and contributor Stephen Ashworth is a tenor sax man who regularly plays in venues near and around Oxford in the UK. Stephen is also, of course, an insightful writer on matters touching our future in space, not only through his work in the Journal of the British Interplanetary Society but also in his Astronautical Evolution site, which bills itself as studying “The social and political basis for the optimistic, progressive, astronautical society of the present and future.” In this essay, Stephen looks at ways of viewing extraterrestrial intelligence that pose different models for the emergence and spread of life in the universe. NOTE: If you’d like to comment, be aware that today is a travel day for me, so comment moderation will be sporadic, but I’ll catch things up tonight.

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Speculations about the existence of extraterrestrial civilisations analogous to our own fall naturally into two broad camps, which we may for convenience describe as the Steady State model versus the Big Bang model (not to be confused with the cosmological theories of the same names). There is also a Hybrid model which combines the two in true Hegelian fashion (thesis → antithesis → synthesis).

The Steady State model

This is the basis of the famous Drake equation. Drake assumed that for a long time in the past, and for a long time to come, civilisations have been and will continue to be coming into existence, persisting for a while and then vanishing again. The question which interested him was whether the rate of appearance of civilisations capable of interstellar radio communication, and their average longevity, were large enough to make it statistically likely that we would be able to establish contact with a nearby alien society before either they or we became extinct.

Drake regarded civilisations as entirely sedentary or static phenomena. Thus the locations at which they might be found today are always the same as those at which they originally evolved from their biological forebears, and thus closely resemble our planet Earth. They must be found orbiting closely Sun-like stars in circular orbits, thus in the so-called “habitable zone” (planets outside the zone where surface liquid water is possible presumably being habitable only by creatures who were not capable of radio astronomy, or of changing the atmospheric chemistry if there was no atmosphere, and therefore undetectable by astronomical means).

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Diagram 1: The Steady State model.

Diagram 1 shows schematically how many civilisations exist at any one time in the Galaxy on the Steady State model. For simplicity it is assumed that each star has either one civilisation, or none. The total number of stars continues to increase slowly as long-lived dwarf stars are added to the population. The number of civilisations rises slightly faster, as longer-lived planets come into play. We are now at point A on the time axis.

The number of stars occupied at any one time is a small fraction of the total (the diagram exaggerates the fraction for clarity). For example, if we share the Galaxy with a million other civilisations at the present time, as optimists may hope, then only 0.00001 of the stars are currently occupied.

All such civilisations arise independently of one another. Collapsed civilisations are not replaced on their planet of origin, but are replaced by other civilisations arising elsewhere. Civilisations are randomly scattered throughout the Galaxy, though Gonzalez, Brownlee and Ward have presented arguments as to why the centre and the outlying galactic regions may be less hospitable than a ring partway out from the centre, where indeed our own Solar System is found.

Civilisations remain completely dependent upon their planet of origin, and the distance between nearest-neighbour planets of that type (perhaps tens of light-years) forbids interstellar settlement.

The Big Bang model

In his book Contact with Alien Civilisations, Michael Michaud reviews the ideas of a number of people who have gone beyond the Drake equation by taking account of the possibility of interstellar colonisation, including Freeman Dyson and Seth Shostak. A similar view has been taken by Ian Crawford, whose article in Scientific American a few years ago discussed the prospect of a dynamic civilisation colonising the entire Galaxy by star-hopping.

Using technologies readily conceivable today (such as nuclear fusion rocket propulsion), a wave of colonies from an expanding civilisation in our Galaxy might take 1,000 years to make each 5 light-year jump (say, travelling for 500 years at 1% of light speed, then taking another 500 years to build up sufficient infrastructure at the target system to make the next jump). Since the Galaxy is about 100,000 light-years across, such a civilisation could spread daughter civilisations to every suitable star and planetary system in the Galaxy within 20 million years.

This, however, is only 0.2% of the age of the Galaxy. The introduction of faster ship speeds makes absolutely no difference: even without warp drive or FTL travel, on a cosmological timescale such a transition from civilisation nowhere to civilisation everywhere is, as Crawford demonstrated, essentially instantaneous.

For a long time, then, the Galaxy is completely devoid of intelligent life. But then one civilisation appears, and spreads throughout the Galaxy in a burst of expansion which we are calling the Big Bang. Thereafter, the locations at which intelligent, technological life is found are almost all colonies, and that life is ubiquitous and permanent.

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Diagram 2: the Big Bang model.

Diagram 2 shows schematically how many civilisations exist at any one time in the Galaxy on the Big Bang model. If humanity is alone, then we are at point A. But there is a possibility (though a small one) that another civilisation in our Galaxy is, say, only a million years more advanced than we are, and has not yet colonised our part of the Galaxy, in which case we are at point B.

In contrast with the Steady State model, in which stars are occupied randomly, galactic real estate is occupied by colonies in an expanding bubble of space centered on the first civilisation’s planet of origin. Two or more such expanding bubbles may appear, but only if two or more independent civilisations make the breakthrough to space colonisation within about 20 million years of one another – unlikely to occur within any one galaxy. Once the Big Bang is complete the number of stars occupied at any one time is a large fraction of the total, including virtually all main-sequence stars, thus certainly over 0.9 of the total.

Collapsed civilisations are likely to be recolonised from other colonies. In fact, it is possible for every single civilisation to collapse (just as every individual in a population dies), but so long as each civilisation despatches on average more than one colony during its lifetime, the galactic population of civilisations continues to grow.

The original civilisation quickly leaves its planet of origin and adopts a new, space-colony mode of living which allows its offshoots to prosper at all stable stars with orbiting planetary or asteroidal material. This both reduces the distances of interstellar journeys for such a species, and pre-adapts them to living conditions on multi-generational voyages. But all civilisations which evolve after the Big Bang (unless they appear almost simultaneously with it, around point B on the diagram) grow up in an environment dominated by their local colony of the original civilisation.

Their access to space transport would presumably be analogous to the access which a non-developed tribal people on present-day Earth may or may not have to the developed world’s existing motor, rail, shipping and air networks.

The Hybrid model

It is possible to combine these two contrasting models in a single Hybrid model if some emerging technological civilisations get as far as radio astronomy but do not achieve interplanetary and hence interstellar space colonisation.

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Diagram 3: the Hybrid model.

Diagram 3 shows schematically how many civilisations exist at any one time in the Galaxy on the Hybrid model. If our own civilisation collapses before we establish viable extraterrestrial colonies, then we are at point A; if we do succeed in spreading into space, then we are at point B.

In either case, we are unlikely to find interstellar conversation partners. The level of development at which we have radio astronomy but not space colonisation is not in itself a long-term sustainable level, I would argue, but rather an unstable intermediate stage. Having got as far as radio astronomy, a civilisation will either complete the transition to a space-based civilisation within a few centuries, or completely collapse.

This means that the longevity of a society which tried to stabilise itself at that level would be very small, certainly less than 1000 years; the number of such civilisations present at any one time would be correspondingly small; and the distance over which any messages would have to be exchanged correspondingly large, making successful communication correspondingly unlikely.

(If there were as many as 1000 civilisations at any one time, i.e. N = L in Drake’s equation after all the other factors have roughly cancelled each other out, and 1011 stars in the Galaxy, then for an average interstellar spacing of 5 light-years the average spacing between active civilisations would be 2000 light-years, and the waiting time between sending a question and receiving a reply would be greater than the lifetimes of both the transmitting and receiving civilisations.)

The question of longevity

Given modern fears about nuclear war, peak oil, environmental degradation, social degeneration, technological disaster, climate change and terrorism, spiced with a strong dose of post-colonial guilt and self-loathing, for many people it goes against the grain to think of an industrial civilisation like ours as something which might become a permanent feature of the universe.

What is the lesson from the past evolution of life? Firstly, it must be acknowledged that industrial mankind is as different from our pre-industrial forebears as they were from single-celled Precambrian organisms. Unless one is to maintain that science and technology are somehow unnatural, an aberration from the God-given natural order, then the facts must be acknowledged: a new type of life has emerged with capabilities never before seen. They include the abilities to access other heavenly bodies, and to digest the raw materials found there, neither of which was possible before, except in the marginal case of small numbers of bacteria being randomly exchanged between Earth and Mars.

The pattern from evolution is that each level of biology has given rise to a higher level founded on it: thus prokaryotic cells produced eukaryotic cells produced multicellular life produced technological life – in my own terminology: microbiota → gaiabiota → technobiota.

As each new level of complexity appears, the previous level also persists in symbiosis with it. Furthermore, whereas even bacterial life could not originally have hoped to outlive the death of the Earth (and of Mars) when the Sun approaches its red giant phase, providing that our civilisation fulfils its potential then those less complex organisms will, along with ourselves, continue to live and prosper long after the death of the Sun.

The pattern therefore suggests not only that our technological civilisation will produce some kind of successor at a higher level of complexity, but also that it will not die out once it has become properly established.

Clearly our society is still going through a period of rapid transition, and cannot possibly be regarded as well established yet. It is still experiencing technological and social revolutions, it has not yet reached its final form, and it is still a monoculture. Only once it has technologically matured and begun to diversify at a variety of different Solar System locations, and even more so at a variety of nearby stars, will it be possible to say that civilisation has finally arrived.

Once it has arrived in full flower, the more dynamic branches of it will certainly spread, because regardless of the precise impulse at work that is what life has always done.

Consider the question: where can one find bacteria on Earth? The answer is: nowhere, for an unknown period of time early in Earth’s history. Then there is a Big Bang, a relatively brief explosion of bacterial life, and thereafter the answer is: everywhere.

Our industrial society has yet to experience the equivalent of that Big Bacterial Bang, or of the Cambrian explosion of 550 million years ago when a plethora of new and diverse multicellular forms emerged and went their own ways. That will require our descendants to expand on an interplanetary and ultimately an interstellar scale. But when they do, or when some other civilisation does if we don’t make it, and if life develops in the future as it has in the past, then civilisation will certainly become a ubiquitous and universal feature of the Galaxy for as far into the future as it is possible for us to glimpse.

Answering Fermi’s question

I discussed this at length on the I4IS blog last December.

In brief: the question is why civilisation did not arrive before now, with a starting point elsewhere than on Earth, given that the stelliferous universe with earthlike planets is about three times older than the Solar System.

The reason why people have such a problem with this, and refer to it as a paradox, is because they are wedded to the traditional view since Darwin that life evolved from chemistry on Earth, whether in a “warm little pond”, a piece of damp clay or a hydrothermal vent. If that was the case, then since it evolved within about 300 million years after the end of the late heavy bombardment, it should have done so on many other planets, and billions of years earlier.

But Robert Zubrin makes the point that there is a huge complexity gap between the simplest bacterium known to science and the most complex molecule that can be synthesised by shaking up raw materials in a test-tube. Some proto-bacterial form of life must have preceded life as we know it. But there is no evidence of proto-bacterial life on Earth.

This to my mind is strong evidence that, contrary to the generally accepted view, life does not evolve from non-life on Earth-like planets. The obvious alternative scenario involves it first emerging in a microgravity environment in something like a comet nucleus, and doing so only extremely rarely. This decouples its initial emergence from its subsequent evolution to multicellular forms, allows a period perhaps 100 times longer for that initial jump in complexity to occur, explains why proto-bacteria have never been found on Earth, and furthermore adds in the requirement for a low-probability space transfer before evolution towards multi-cellular forms can begin, pushing the Big Bang of technobiotic life to the right on the diagram.

But not too far to the right. For all that the accepted age of the universe of 13.7 billion years seems to us to be unimaginably ancient, on its own terms the universe is still young. Judged by the lifetimes of its longest-lived stars, the red dwarfs, the universe will continue to contain stars and planets as we know them for a period on the order of tens of trillions of years to come, though the brighter stars will fade long before then. If the universe was a human being, it would still only be about a month old.

Another factor may play a part. Carl Sagan has described how the modern alien encounter/alien abduction mania perpetuates the phenomenon of encounters with angels and demons and with the Virgin Mary in earlier centuries. Could the flood of speculations about alien civilisations – where are they? are they hostile or friendly? – be the modern equivalent of the search for God? Do people still yearn to submit to an Overlord (the name given to the aliens in Arthur C. Clarke’s Childhood’s End), whether beneficent, or intent on our punishment?

Until we find any evidence of alien intelligence, the most parsimonious explanation is that there isn’t any where we have looked, that there is no invisible dragon in my garage (Sagan’s image). So we must look further, before it will be possible to use observations to rule out either of the models described here.

References

Ian Crawford, “Where Are They?”, Scientific American, July 2000, p.28-33.

Guillermo Gonzalez, Donald Brownlee and Peter D. Ward, “Refuges for Life in a Hostile Universe”, Scientific American, October 2001, p.52-59.

Michael A.G. Michaud, Contact with Alien Civilisations (Copernicus, 2007).

Carl Sagan, The Demon-Haunted World: Science as a Candle in the Dark (Headline, 1997); Contact (Century Hutchinson, 1986).

Robert Zubrin, “Interstellar Panspermia Reconsidered”, JBIS, vol.54, no.7/8 (July/August 2001), p.262-269.

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Comments on this entry are closed.

  • Wojciech J September 18, 2013, 8:16

    This has been discussed before and I would like to repeat a stance I have taken earlier:there is no reason to assume that advanced civilizations would continue to colonize galaxy in endless wave. As we advance as civilization, our expansion is halting and we tend to concentrate our population. Another point is that you don’t need planets and other solar systems to get living space, once you know how to endure hundreds of years in a space ship, you have already technology to build artificial habitats allowing you to house a huge amount of population without far more complicated and time consuming terraforming. And of course there is a question if we will remain in our biological form as well, if that will change, our need to reproduce or occupy vast space might be subject to change as well.

  • David Cummings September 18, 2013, 9:06

    ” there is a huge complexity gap between the simplest bacterium known to science and the most complex molecule that can be synthesised by shaking up raw materials in a test-tube”

    It’s probably also true that there is a huge complexity gap between the evolution of biological life and the emergence of a technology-capable intelligent life form.

    There are many events in the evolution of humanity that were pure happenstance. If the dinosaurs hadn’t been wiped out… If East African geologically-induced climate changes hadn’t shifted the forest coverage back and forth (encouraging up-on-two-legs)…

    Just as the start of life itself is probably extremely dependent on initial conditions (and is probably relatively rare) so the emergence of intelligence is dependent on lucky events that are also rare.

    Put those two rarities together and you have the Big Bang picture of the spread of civilization. And we humans, here on little ol’ Earth, are quite possibly the progenitors of that Big Bang in the Milky Way Galaxy.

  • Tulse September 18, 2013, 10:05

    “contrary to the generally accepted view, life does not evolve from non-life on Earth-like planets. The obvious alternative scenario involves it first emerging in a microgravity environment in something like a comet nucleus”

    I realize that Ashworth isn’t laying out this argument in much detail, and there may be more explicit and convincing reasoning behind it. That said, the early Earth contained uncountable microenvironments, with the rich availability of complex molecules, liquid water, ready elements, multiple potential sources of energy, reasonably steady temperature, and in a vast variety of specific conditions. Surely this makes it a better candidate for generating life than a random space snowball. (And I am not at all clear why microgravity would be important in this situation.)

    I am always troubled when I see the somewhat popular view that life originated extra-terrestrially, as at best it seems non-scientific, and at worst provides aid to the most pseudoscientific and religious garbage that pervades society. It is just a few short steps from “life did not evolve on earth” to “life did not evolve”, and invoking issues of complexity falls right into the hands of the “Intelligent Design” crowd. There is no compelling reason to suggest a non-earth origin for life, and every reason to avoid giving the most virulently anti-scientific groups further ammunition.

  • M Mahin September 18, 2013, 10:18

    I think the fallacy in this type of analysis is the assumption that an alien civilization would be interested in colonizing all available planets. You have to ask: what are the benefits of establishing an extraterrestrial colony? The Spaniards colonized the New World because they were interested in converting the natives to Christianity, getting them to mine and ship gold back to Spain, and getting them to send back crops back to Spain. An alien civilization might have no such benefits in establishing a colony around another star. They might have no interest in spreading some religion, and the colony may be too far for colonial exports to be practical. So why bother establishing the colony? An alien civilization might be interested in exploring much of the galaxy, but might see no benefits in doing much colonizing. We should not assume that large-scale colonization will occur, when the benefits of it may be slim.

  • Jack William Bell September 18, 2013, 10:25

    There are other possibilities as well. Sessile civilizations that are capable of Intersteller travel and choose not to. Technical Singularities where civilizations turn inwards to virtual realities or are simply replaced by their machines.

    Then there is another possibility: http://www.jackwilliambell.com/?p=476

  • Ron S September 18, 2013, 11:49

    I suggest you remove any correlation with stars. Once a civilization arises and becomes interstellar they will not then behave in the manner of primitives and only set up shop by huddling around other stars. They are unlikely to remain dependent on stars, for anything.

  • Rob Flores September 18, 2013, 11:58

    Sound reasoning, from Mr Ashworth. There are alternative views of why interstellar colonization by ETIs are not apparent in a galaxy teeming with planets theorically capable giving rise to life is possible. But most of these fail because all of them require that ETI behave in predictable and uniform
    behaviour. I think there is room for two types of civs co-existing with
    ours.
    One race of beings that originated in other places in the cosmos and after much seeking have found that only a few score of ETIs have risen from the pond scum in the sum of searching much our universe. Such a race is curious but owing to prior experience, it prefers not to be exposed. Needless to say a civilisation of several billion years of age for example, actively hiding could never be detected by us. For what purpose would they observe and not interfeer? Maybe we are like a soap opera to them, they are tired of simulations where photons die. They want the drama of nature, including us.
    The other type coexisting Civs have been mentioned by other authors, the slow developing ETI where due to a limited energy gradient on their home planet, complete thoughts take decades, reproduction is every millenia and this Civ has just aquired Radio astronomy. In such a case
    this civ would take several Eons to colonize the Galaxy.

  • Thomas Hair September 18, 2013, 12:06

    Wow! From my perspective having published a couple of articles on the subject I find Stephen’s analysis of this subject spot on…goodluck and have fun in Houston Paul.

  • Alex Tolley September 18, 2013, 13:52

    Good exposition. I liked the longer version on Interstellarindex even more. However, I question the hypothesis of the [rare] cometary origin of life.

    Firstly we haven’t found proto bacteria on earth because when they existed, they were effectively out competed and eliminated. They also didn’t leave fossil traces to find, which is much the case with their successor microbial life. Conversely, we also don’t find components like lipid vesicles either, even those they are relatively easy to create in the lab. I personally find the argument that the huge gap between chemistry and life rather akin to teh creationist argument that you don’t find Boeing 707s randomly constructed by natural forces in a junk yard. We just don’t know what level of complexity is required before natural selection algorithms come in to play.

    Secondly, isn’t it rather convenient that a comet may have evolved life and that it was it just luck that Earth was seeded with this life just at the moment it was possible to support it? Isn’t it more likely that if such a scenario were possible, that many comets today would have this proto-life within them? This suggests an obvious test of this hypothesis would be to look for such evidence in dead comets that populate the inner solar system. Perhaps asteroid probes should bore below the surface dust to test this. However I think that the cold environment of a comet is unsuitable for creating life.

    We humans have the illusion that we are the “top of life’s pyramid” and that evolution leads to us. That we even evolved is possibly very path dependent, with many chance events like mass extinctions paving the way for us. It is possible that other organisms could have evolved to technological civilization under different events, but it may also be extremely rare. Again, with a sample of one, and no way to rerun the experiment, we just don’t know.

    We really need some data. I’m hoping that the search for bio signatures provides this in the next couple of decades or so.

  • RonSmith September 18, 2013, 16:04

    Excellent article, but I don’t think the Steady State/ Big Bang dichotomy is all inclusive. I am aware of several speculations regarding the Femi Paradox that do not seem, to me at least, to fall into either description. These mainly involve the suitabilty of the MW galaxy as a whole for life.
    One such speculation is that the MW galaxy has not been quiescent to life until the last 4 or so billion years. This might be explained by the time needed for the metalicity of the galaxy to reach a needed threshold. Other explanations might also apply.
    Another speculation is that the MW galaxy is periodically not able to support life. One possible mechanism for this would be if Sagittarius A* (the super giant black hole at the center of the galaxy) became ‘active’ periodically and the resulting radioactivity is sufficient to sterilize the galaxy. Other mechanisms might also exist.
    This articles concepts are none the less a valuable addition to Fermi Paradox discussions.

  • Stan Clark September 18, 2013, 18:25

    If you accept the Big Bang model. there is always the possibility that we are one of the colonies.
    Which would make for even more interesting speculation…;-)

  • Keith Cooper September 18, 2013, 18:35

    Hi Stephen, that’s an excellent write up. I’m sitting on the fence about a terrestrial genesis or whether life came from space – certainly the latter is a possibility and all kinds of chemical reactions and building blocks essential for life have been discovered in the interstellar medium.

    I would challenge you though on one assumption that is inherent in both your Steady State and Big Bang models, which is the assumption that technology goes hand-in-hand with intelligence. Of course it depends on your definition of intelligence, but I see no reason why life cannot be intelligent but not technological. Intelligent life may be common, excelling in areas other than technology, but technological life could be rare. Ocean dwelling species like dolphins may have no need for technology, lacking the ability or the limbs to create it, and models of planetary formation have shown that terrestrial planets could form with much more water than Earth possesses. The answer to the Fermi Question could be that while complex, intelligent life may well be frequently found around stars, we’re the only ones with that genetic quirk that has allowed us to build radio telescopes and space ships.

  • Swage September 18, 2013, 18:40

    According to the paper “A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around kepler m-dwarfs” the probability of an Earth-like planet (0.5 to 1.4 Earth radii planet orbiting within the habitable zone of the star) orbiting an M-dwarf star is 48%, therefore the mean distance between Earth-like planets is 6.4 ly and the probability of another Earth-like planet within 10 ly is estimated at 94%.

    So thats the situation we are looking at currently. I would add that our data is from indirect observations and small (Earth-like planets) are hard to detect currently. There may be a significant portion under the radar and i expect those probabilitys to increase as our observational equipment improves.

    I am totally with you about an off-world origin for our biology, not only because there is no evidence of any neccesary proto-cell biology (one may argue that the fossil record is missing that piece, because the Earth was still in the planet forming process, which of course also raises the question if environmental conditions were actually suitable for a emerging replication process), not only because (judging by the phylogenic tree’s increasing evolution rate) there was not enough time, but also because its odd that the random emergence of life happened early in Earth’s history (i’d say as soon as conditions permited it). You should expect a random process to kick in at some point in the planetary history and not right from the beginning. Thats extremely odd and those are three very good reasons to seriously question Earth’s position as the cradle of life. Thats… geocentrism, and totally unfounded. There is not one piece of evidence to support such a claim and probably because it simply did not happen.

    If life had its origin in microgravity… i am not sure. Its at least as possible as having a planetary origin. We know for sure comets synthesize pretty much all of the known components and planets do not (gets “delivered”).

    The problem with the models above is in my opinion that they focus too much on technological civilizations (“sentient life”).

    If we assume, and after the ISS space exposure experiments we have every reason to do so, that viable microorganisms from Earth roam space for 3.6 billion years (there is the question how long those organisms can stay viable, of course but there are papers suggesting tens to hundreds of millions of years, for example from sediment scraped from the bottom of the ocean) and if we consider the evolutionary route specifically aiming for higher complexity (“sentients”), i think we get another interesting picture.

    There are extinction events, certainly. The most inevitable is simply when the star of the home system dies, but there are other variants wich do not neccesarily destroy the habitability. So, on a planetary scale, civilization lifetime is limited. There are setbacks and new habitable zones emerge all the time, so there is mixing going on.

    So, by factoring in extinction events and the biological distribution… if we estimate a 6500 years per lightyear for microbe “terraformers”, thats somewhat above 40k years to make the 6.4 ly hop to the next Earth-like planet (by a conservative estimate) naturally.

    And those 40k years is all the second planet would lag behind. What is that on the evolutionary scale? Of course… thats plain linear, wich isn’t reality.

    Given our own fossil record… 5 mass extinctions? I think the odds are pretty good there are advanced civilizations within the next 10 lightyears.

  • Rob Henry September 18, 2013, 18:50

    I agree with so much here it seems wrong to hold you to task, but I just love speculation too much to let the following go. You wrote “The obvious alternative scenario involves [life] first emerging in a microgravity environment in something like a comet nucleus, and doing so only extremely rarely”

    It is great to see some understanding that there can be no long epic of gradual build-up of prebiotic soup on Earth – that would have to fight the laws of entropy for millions of years. On the other hand comets start far from equilibrium (eg loads of carbon monoxide), they just need to be unfrozen for the hundreds of thousands of years presumed necessary but HOW. By T Tauri phase stars? By Al-26? Do you have some other mechanism in mind, such as current short period comets near perihelion?

    And yes, those creatures that currently occupy every niche where simplicity and reproductive pace of freeliving organisms are winning strategies are, in every case, overwhelmingly complex, but to me that is only one of three independent lines of evidence that lead to that same conclusion: abiogenesis is rare. The other two are

    2. Von Neumann showed that non-trivial replication is so complex, that it strains the intellect of our best and brightest, even to design it even in a universe specially made to facilitate it.

    3. Charles Darwin’s theory is all about how new biological traits only ever arise by modifying existing traits by very small steps, every one of which has to provide advantage. No longer can biology rely on the magic forces of organics where life giving properties just pop into existence. From Darwin’s groundbreaking theory we can surmise that that all life descended from one organism, and testing has proved it so. A triumph for evolution, but a killer blow to the “just add water” school for the origin of life.

  • Brett Bellmore September 18, 2013, 18:59

    A minor quibble on the expansion model: Initially, short hops make sense, but as the sphere of colonization becomes large enough, the desire to arrive at destinations nobody has as yet colonized will probably drive longer and faster colonization trips, in an effort by colonies not on the surface of the sphere to reach virgin territory.

    I’m assuming here that there’s a high premium accorded reaching systems not yet colonized, so that you can immediately exploit the best niches.

    Still, this doesn’t really effect the end outcome, which is that the galaxy ‘flips’ from uninhabited to fully colonized in what is, on a galactic scale, a very short time.

  • Geoffrey Hillend September 18, 2013, 22:20

    Carl Sagan answered the Fermi paradox with the absence of evidence does not mean evidence is absent. The same applies to the question: “but there is no evidence of proto-bacterial life on Earth.” Do we really need any? It’s not that life is to fragile to survive inside a comet, atmospheric re-entry, etc., but could it originate without light, ample liquid water etc, a question that has yet to be proven.

    Get a Florence flask put in the elements found the atmosphere of the young primitive Earth such as hydrogen, hydrogen sulfide, methane, ammonia, water. Scientists put electrodes inside the flask and made an electric arc and over time got “a rich variety of complex organic molecules which are the building blocks of life,” a “brown pigment” builds up on the walls of the flash. Cosmos, Sagan, p. 38. The elements necessary for the survival of life are most abundant on the surface of the Earth so that is where the highest chances are for it to originate.

    I like the idea of an advanced galactic society which we are not technologically advanced enough to detect which would explain the Fermi paradox if we think outside the box of today’s technology. Beyond UFO’s, Bennett. As far as the ages of stars is concerned, our Sun is quite young in our galaxy. We could be the new kids on the block. Radio telescopes are a technology which may be limited to a civilization close to our level of advancement.

  • Heath Rezabek September 18, 2013, 23:17

    Stephen – Your article gives much to think on to those (like myself) who still see Fermi’s question as a paradox. If I had to phrase it, I’d phrase the first part of the paradox as: “Complex and technological life, given the time scales involved, ought to be ubiquitous.” And the second part would be, “It is clearly not ubiquitous.” Therein lies the paradox.

    But, you present several interesting possibilities. I did have one question, centering on Diagram 2: “Diagram 2 shows schematically how many civilisations exist at any one time in the Galaxy on the Big Bang model. If humanity is alone, then we are at point A. But there is a possibility (though a small one) that another civilisation in our Galaxy is, say, only a million years more advanced than we are, and has not yet colonised our part of the Galaxy, in which case we are at point B.”

    Which option (A or B) do you find to be more likely than the other, and by what margin?

    @Brett – Your note reminds me of the frontier-waves argument put forth in ‘Burning the Cosmic Commons’ by Robin Hanson. http://hanson.gmu.edu/filluniv.pdf

  • Eniac September 18, 2013, 23:59

    Excellent post, Stephen, spot on.

    I join the minor quibblers in thinking that comets do not really make a good environment for life to evolve, and there seems to be no good reason for supposing that. The absence of probacteria is easily explained by them having been outcompeted to extinction. Gravity plays no role in (bio-)chemical processes on the microscale, but temperature certainly does. Stable temperature and the sheer size of the potentially life supporting environments (the oceans, for example, or the crust) are strong pointers towards Earth as the origin of life. Particularly if abiogenesis is a freak occurrence, as we both agree it most likely is.

    Lastly, to counterpoint the optimistic leaning of your exposition, let me observe that in the second figure, you might have added a third point (say, Z), in the time period before A. That would be us in the case that we do not make it, after all.

  • Eniac September 19, 2013, 0:17

    Astronist:

    If that was the case, then since it evolved within about 300 million years after the end of the late heavy bombardment, it should have done so on many other planets, and billions of years earlier.

    Two more quibbles:

    1) the evidence on those 300 million years is shaky to the extreme. In my view you have to go forward as much as 2 B years until the fossil and/or genetic evidence of life on Earth becomes convincing.

    2) If abiogenesis is extremely rare, there is no expectation about how early or late it would occur on that freak planet where it does. In fact, if you figure in that it takes a few billion years to evolve intelligent life, and our planet might only be good for a few billion more, you would actually expect an early beginning, given the known outcome that we exist.

  • Eniac September 19, 2013, 0:27

    Just as the start of life itself is probably extremely dependent on initial conditions (and is probably relatively rare) so the emergence of intelligence is dependent on lucky events that are also rare.

    I would agree that there is a lot of happenstance here, but not that the “lucky events” are rare. Evolution provides an endless supply of chances for such events. There were many hominid species similar to ours waiting to take our place and ascend to world domination in our stead, had we failed. And had they all failed, other, more distantly related species would have come after them, in endless succession, each with at least the same chance as us. There would have been another billion years left of this constant assault, plenty of time to assure eventual success.

    Abiognesis is special in that there was no evolution to help bring it about.

  • NS September 19, 2013, 2:40

    To Tulse, there is probably no idea (scientific or otherwise) that can’t be misused or abused for some bad purpose. Should Darwin have foreseen what various eugenicists and scientific racists would do with his theories, and kept silent? So little is known about the possible origins of life that at least speculating about the possibility of it arriving from space is not illegitimate. We should let the discussions continue unfettered.

  • Daniel Högberg September 19, 2013, 3:09

    Jack William Bell: Have you been reading Alastair Reynolds or did he copy you? ;)

  • Ronald September 19, 2013, 4:22

    Fascinating topic, article and discussion thread, I will delve into this in more detail later.
    Just a quick related news item now:
    UEA scientists reveal Earth’s habitable lifetime and investigate potential for alien life:
    http://www.uea.ac.uk/mac/comm/media/press/2013/September/habitable-life-on-earth
    And:
    http://www.sciencedaily.com/releases/2013/09/130918211434.htm

    I could not get a link to the original publication yet.

  • Astronist September 19, 2013, 5:10

    Thanks, everyone, for commenting.

    Wojciech J and Ron S: the reason life remains dependent upon stars is that it needs matter, for construction and for replacement of lost volatiles, and energy. If you postulate a purely interstellar lifeform, where does it get these from?

    Tulse: I certainly don’t agree that an extraterrestrial origin for life is non-scientific, or that it supports an intelligent design hypothesis! Alex Tolley suggests an observational test, and a hypothesis which has an observational test is a scientific one. The point I am making is not that life does not evolve on Earth: clearly it does evolve on Earth, from simple to complex forms. But the question is how this process of evolution got started. In his recent book “The Eerie Silence”, Paul Davies emphasises that we don’t actually have any idea of when, where or how life first evolved from non-life. So for all we know it might have been in space.

  • Astronist September 19, 2013, 5:23

    Keith Cooper: thanks! Yes, clearly there are two forms of intelligence. I was referring to technological intelligence, but it is clear that a non-technological form of intelligence is also possible, as exhibited by monkeys, cetaceans and the larger species of octopus on Earth.

    But while non-technological intelligence is, so far as I know, an evolutionary dead end, intelligence in the hands of a land-dwelling species capable of manipulating its environment is capable of self-amplification. Even the simple capability to make written records amplifies our intelligence by providing an accumulating database of precise knowledge. The capacity to make tools, and particularly the capacity to use tools to make better tools, launches an industrial revolution quite different from any evolutionary development seen before.

    So I would suggest that technological intelligence is a radical new departure, on a level with the development of multicellularity, and that what we see in nature is a number of exploratory stabs in this direction, including dolphin-style intelligence, city-building (ants, termites, bees and wasps), tool-using and even tool-making, but that these developments only reached critical mass, so to speak, in our own species. And certainly, yes, we can expect to find similar explorations of biological design space (Daniel Dennett’s useful concept) on Earth-analogue planets.

    Stephen

  • Astronist September 19, 2013, 5:40

    M Mahin: yes, you’ve certainly found a weak point in the Big Bang model. I think an interplanetary-scale civilisation will expand to nearby stars because it can do so, and because as it enriches itself it will need new challenges and new frontiers, and also because I’m looking for broad patterns over the entire history of life. But you are quite right to question this optimistic assumption.

    Do you also provide your own answer, by allowing that an alien civilisation might well be interested in exploration? Unless a “magical” propulsion system (in Clarke’s sense) is affordably available, any biological explorers who venture to another planetary system will need to set up a colony there in which to live while they explore, and if they or their descendants want to return to their star of origin they will need to construct a huge industrial infrastructure in order to generate the fuel for the return journey. But this would not apply to robotic explorers, of course.

    If civilisations in general do not colonise nearby stars, then clearly we are back to the Steady State picture, but a version of that in which successful civilisations have interplanetary scope, giving them sufficient diversity to persist for astronomical periods of time, which is the picture Drake would have preferred.

    Stephen

  • Ron S September 19, 2013, 9:54

    Stephen: “Wojciech J and Ron S: the reason life remains dependent upon stars is that it needs matter, for construction and for replacement of lost volatiles, and energy. If you postulate a purely interstellar lifeform, where does it get these from?”

    How much and for what purpose? Further, why do you seem to assume that a more advanced civilization will not have the ability to produce energy where and when it’s needed, and to marshal and re-purpose matter for most of their needs?

    I suspect you have other unspoken assumptions about advanced civilizations that lead you to make these additional assumptions. For myself, I have difficulty envisioning such a civilization living in the manner of ancient desert nomads, traveling from oasis to oasis for sustenance.

  • Paul W September 19, 2013, 10:47

    So many intelligent possibilities, all in search of a shred of evidence.

    The search and the wait continues.

  • Peter Chapin September 19, 2013, 16:51

    Keith Cooper:

    “I see no reason why life cannot be intelligent but not technological. Intelligent life may be common, excelling in areas other than technology, but technological life could be rare. Ocean dwelling species like dolphins may have no need for technology, lacking the ability or the limbs to create it, and models of planetary formation have shown that terrestrial planets could form with much more water than Earth possesses.”

    I have also thought about the notion of intelligent, non-technical life. Perhaps many ETIs are uninterested in or incapable of creating technological artifacts and yet sing epic poetry to each other as a way of recording and transmitting cultural knowledge.

    Still, it’s hard to image intelligence not turning to technology eventually. Anyone who has lost a loved one due to disease will ask, “What caused this illness and how could it have been prevented?” Won’t all intelligent creatures want to better their lives and use their intelligence to shape the world around them? It seems like a small jump on the galactic time scale from there to starships.

  • Brett Bellmore September 19, 2013, 17:15

    ” Further, why do you seem to assume that a more advanced civilization will not have the ability to produce energy where and when it’s needed, and to marshal and re-purpose matter for most of their needs?”

    We can only reason on what we know of physics, while being aware we don’t know everything. But I’d reason thus: Even if you *can* convert granite into soup, you don’t do it if it’s raining soup. Stars are radiating usable energy whether you use it or not, so why not use it?

  • David C. Morrow September 19, 2013, 18:54

    There’s no reason to assume that all civilizations inevitably collapse. Many human civilizations have but that doesn’t mean they all will and when thinking of alien civilizations we shouldn’t attribute human properties to them.

  • Jack William Bell September 19, 2013, 20:22

    Daniel Högberg: Although I have read some Alastair Reynolds, I’m not sure which book you refer to, so I probably haven’t read it. Most likely we just thought of the same thing. It happens.

    I once asked William Gibson if he had read Vernor Vinge’s ‘True Names’ before he wrote ‘Neuromancer’. He basically told me what I said above, but a lot more vehemently. I think he was tired of answering that question.

  • Daniel September 19, 2013, 21:36

    @Paul W – :)

  • Rob Henry September 20, 2013, 1:57

    I like the approach of Ron S, though, to me he gives only half the possibility. Karl Popper points out that solutions to problems tend to lie at one end or other of the phase space of possibility. ETI will end up following the matter OR the energy. In the first they will be on every rogue planet and round every star, in the second confined to O and B stars, perhaps with a sprinkling of real losers round the brightest A’s.

  • Astronist September 20, 2013, 5:49

    Paul W: the search continues. Precisely!

    Heath Rezabek: thanks for your question. Clearly point A is more probable, statistically speaking, assuming that our observation that industrialised life has not yet reached our Solar System is correct, and assuming that the estimate of the brevity of spread of that life throughout the Galaxy in comparison with the age of the Galaxy to date is also correct. But this proves nothing in our particular case, so we must go on looking. Even if intellectual argument seems convincing, it still needs to be supported with observational data.

    Jack William Bell: thanks for your suggested other possibilities. If civilisations are generally static, however, for whatever reason, then that leads to either the Steady State or Hybrid models. As for your third possibility, that clearly belongs to the Big Bang model, as the aliens you postulate must already be living extremely close to us (unless they have magical transport technologies).

    Stephen

  • Astronist September 20, 2013, 6:13

    Ron S: yes, as you say, I do assume that advanced civilisations will not have the ability to produce energy where and when needed, that they will be as bound by basic laws of physics as we are — though that does not alter the contrast between Steady State and Big Bang models of technological life under discussion here.

    I think there is a difference in viewpoint between those who expect technological progress to continue exponential growth, until powers are attained which seem to us downright magical or godlike, versus those who see technological capabilities as having natural limits, and us already experiencing a deceleration of progress as those limits are approached. One view leads to FTL or spacewarp travel and hyperintelligent machines, the other to slow worldships and clunky robots who have a hard time getting common sense into their circuits. Singularitarians, versus plateaunians; gung-ho optimists versus skeptics. (You often find skeptical plateaunian arguments on Athena Andreadis’s Astrogator’s Logs blog: she has a broader concept of what it is to be human than many futurists.)

    Clearly, throughout my essay above I’m speaking from a plateaunian point of view. No matter how far in the future, industrial beings must still respect the law of conservation of mass-energy, therefore remain dependent upon stars, or artificial fusion plants, or energy drawn from a giant planet’s magnetic field. (Black hole energy? Well, maybe!)

    I wrote about these issues, no doubt at excessive length and insufficient clarity, 6 months ago:
    http://www.astronist.demon.co.uk/astro-ev/ae090.html

    Stephen

  • Swage September 20, 2013, 6:56

    @Eniac

    For all we know water (oceans) poses a problem for pathways leading to RNA. Terran conditions are so problematic for the current pathway models that it has already being suggested life had to have started on Mars first.

    Another problem is an arithmetic one: It took life an estimated 2.6 billion years from single cells to multicellular organisms. Then things happened rather quickly and continued to pick up pace. Thus one may argue that evolutionary novelties were harder to come by the further down the phylogenetic tree you look. Now, the step from pre-biology to cells should have taken a very long time by that estimate, considerably longer than singlecelled to multicellular. The problem here is that Earth is only 4.54 billion years old. By that estimate (one that Francis Crick immediately recognized) there was not enough time for an abiotic process to rise to cellular organisms on Earth, period.

    Of course… we don’t know for sure what this abiotic process was. So its hard to put temporal constrains on it securely. Maybe its an unexpectedly fast process. But there is nothing to support that line of thinking. All the pathways we are considering so far require time that is simply not aviable.

  • Colin Weaver September 20, 2013, 8:54

    I am in emphatic agreement with Paul W above. He has distilled the issue to its very essence.

    When profound claims are made in the complete absence of data or even a testable hypothesis, then I for one choose to remain sceptical and await more evidence. To do otherwise is to make the transition from science to dogma or even religion.

  • David Cummings September 20, 2013, 10:27

    Ronald, an additional 1.7 billion years. That’s 1 billion years longer than I thought. I read somewhere recently that in 700 million years the oceans would boil away because of temperature changes on the sun.

    But it looks like wherever I read that is wrong. This updated calculation is probably more accurate.

    Anyway, long before that time (hopefully on a scale of thousands of years) we should have billions of people living in habitats in various orbits in our solar system.

  • David Cummings September 20, 2013, 10:33

    “need to construct a huge industrial infrastructure in order to generate the fuel for the return journey. But this would not apply to robotic explorers, of course.”

    Stephen, you mention an important point, the difference between robot explorers (in the sense of robot-only missions, with no life-forms along) and “manned” missions (generic “manned” including ETIs).

    In my opinion, the most sensible template for expansion of living space for lifeforms is:
    1) send the robots first, to build the infrastucture
    2) send the life-forms second

    This template applies to the Earth’s moon, Mars, other locations in our solar system (including asteroids and habitats) and most likely applies to ETIs elsewhere in the galaxy.

    The fact that we have not fully embraced this template and are not now building a robotic base on the moon (preferably a telescope on the south pole) and instead are engaging in reality show fantasies of sending humans to Mars proves to me (in my humble opinion) that mankind is not yet taking this subject seriously.

    Sure, the speculation is rampant and loads and loads of fun. But until we commit to building serious infrastructure in advance of our own arrival at various locations, then we are just toying with the idea and not really being serious. In my humble opinion.

  • Alex Tolley September 20, 2013, 11:28

    @Eniac, Swage

    <

    Abiognesis is special in that there was no evolution to help bring it about.

    We really do not know what level of complexity evolution can apply its algorithms. There is a tendency to see evolution as tinkering with a engineered system, whereas life processes, even simple chemical processes, are self-organizing. IOW, you can get order for “free”. This may collapse the time requirement for abiogenesis. Stuart Kauffman’s The Origins of Order is a good place to start.
    I’m also skeptical of the claim that conditions on Earth were unsuitable for some aspect of abiogenesis. I think it suggests a failure of imagination, rather than a statement of fact. It seems to me that we get new phenomena reported very frequently that force us to concede that novel conditions generating these phenomena exist that we had not entertained prior to discovery.

    The hypothesis that seems difficult to test is “visiting astronauts causing panspermia” (Tom Gold?). On this blog we have talked about seeding suitable worlds with life to “terraform” them. We may be capable of actually starting out such a program ourselves within an eyeblink of cosmic time, leaving the intelligence that evolves on those worlds to wonder at their origins. However, if we had evidence that widely separated planets had exactly the same fundamental biology as we do, I might be very suspicious that this was due to chance and not agency.

  • Astronist September 20, 2013, 12:37

    Heath Rezabeck: more on Fermi’s question. The reason why I am so against calling it a paradox is that the two sides of the paradox are so unequal. The idea is: we are certain UFOs are not alien spacecraft, we are similarly certain that alien civilisations exist, therefore: paradox!

    But not so. Yes, we are certain that we have no sign of alien industrial activity at the moment. Tomorrow someone might prove that a pulsar is transmitting prime numbers, or Ellie Arroway/Jill Tarter might get that message from Vega, or real saucers might park themselves over the world’s major cities, but today we see no sign of this. In contrast, the affected certainty that alien civilisations exist is false; their existence is at present pure speculation. It’s, well, the universe is SO big and SO old… but that proves nothing. In a universe of finite age, SOMEONE has to be first!

    If there was a late abiogenesis of life from non-life, and it happened in space, then Fermi’s question simply goes away. Paul Davies in his book The Eerie Silence emphasises this fact: we know nothing of the where, when and how of the origin of life.

    So until we get that signal from Vega, the most parsimonious explanation of the existing facts is that life itself is of recent, space origin and thus Earth had an equal start with all similar worlds in the race to produce the first radio astronomers, the first astronauts.

    But Geoffrey Hillend above says: “The elements necessary for the survival of life are most abundant on the surface of the Earth so that is where the highest chances are for it to originate.” Yes, agreed! So why has life not originated multiple times on Earth? Why is there not an independent abiogenesis on Earth every few hundred million years? Or is there only one possible molecular structure that life can use, so that independently originated organisms appear to have a common ancestry?

    In what I have written above, the idea of the abiogenesis of life in comets seems to be the weakest link. So when we explore Mars, Europa and Enceladus for subsurface life, we should give equal attention to comets. Can we rule in or rule out abiogenesis in comets through in-situ examination?

    Thanks for the discussion.

    Stephen (Ashworth, of course)

  • Rob Flores September 20, 2013, 14:21

    Intriguing Tool “discovered” (didn’t Anathem laugh at such discoveries) to
    calculate quantum behaviour.

    https://www.simonsfoundation.org/quanta/20130917-a-jewel-at-the-heart-of-quantum-physics/

    I bring this up because there is a point of view out there that we are the first intelligence to arrive in the universe due to the “observer” problem in quantum mechanics. I think this new approach to looking at he universe invalidates that view philosophically at least, experimentally? too soon to tell.

    It also points to something some us here probably
    suspect. That being a part of the phenomenon of the universe makes it very difficult to find its “solution”

  • Mark September 20, 2013, 16:22

    This article presumes that interstellar travel will be accomplished by traversing spacetime in the conventional sense of moving from a spatial origin to a spatial destination over a rather lengthy time interval.

    It’s not beyond our capability to imagine an advanced civilization who discovers a new aspect of science somewhat beyond our own understanding which allows for some type of inter-dimensional travel. For example instantaneously winking into the nth dimension and simultaneously winking back out of that dimension somewhere very far removed in space-time. So that “spreading” throughout the galaxy makes no sense because they can be literally anywhere in the entire universe instantaneously. Such a civilization would appear to us to be not merely as Clarke’s magicians, but as gods. And we’d have just as much cognizance of their existence as the fire ants in my backyard do of my existence (unless I inadvertently step into their nest). So if I’m careful the fire ants will never know I’m there, and if these advanced travellers are careful, we’ll never be able to spot them in a crowd.

    I think of the ancient tribes who crossed the land bridge from Asia to North America. They took tens of thousands of years to spread by gradually moving to new hunting grounds until the entire continent was settled. What would they think of a man in a space capsule circling the globe every 88.5 minutes, able to drop out of orbit and parachute to a landing just about anywhere on that continent? And these spacefaring men would be biologically / evolutionarily indistinguishable from the members of the ancient tribes. The only difference is the level of science/technology available at their disposal.

  • Swage September 20, 2013, 16:51

    I politely disagree about the amount of data and the respective results of subsequent testing when it comes to survivability of microorganisms. Kepler planetary data is promising. In ten years we could be able to analyze the atmospheres of the respective candidates. The most critical issue remaining is funding. I also expect major changes to the cosmological model along with that. Reionization remains elusive, the antiproton curve shows no predicted match and the age of the universe has just been pushed back a couple of million years.

  • Eniac September 20, 2013, 23:24

    Swage:

    For all we know water (oceans) poses a problem for pathways leading to RNA. Terran conditions are so problematic for the current pathway models that it has already being suggested life had to have started on Mars first.

    I have not heard this argument before, and is seems plain wrong to me. How precisely are conditions on Mars better than on Earth?

    Now, the step from pre-biology to cells should have taken a very long time by that estimate, considerably longer than singlecelled to multicellular. The problem here is that Earth is only 4.54 billion years old.

    This is an interesting observation. Note, though, that prebiotic conditions are and were present on billions of planets. If only one of those developed life, the total time available is actually a billion times the age of the Earth, since the chance of it happening is proportional to the number of opportunities. It does not matter if the opportunities are in parallel or in series. What we are lacking in time, we can make up in space.

    That, in my opinion, is the reason the universe is so immense. If it weren’t, we wouldn’t be here.

  • Brett Bellmore September 21, 2013, 7:44

    ” Yes, agreed! So why has life not originated multiple times on Earth?”

    The usual explanation is that as soon as life appeared, it ATE all of the almost-life. Or incorporated it into itself, the distinction being kind of academic at that low level of development. Barring extreme barriers to the spread of life, a planet only gets to originate it once, and then becomes unsuitable for it to originate on, because life can’t originate again where there’s already life around to eat the precursors.

  • Alex Tolley September 21, 2013, 10:47

    @Astronist

    Why is there not an independent abiogenesis on Earth every few hundred million years?

    Very simply, once [carbon] life starts on a lifeless world, there is no further possibility of another carbon biology to appear due to competition from the established, more advanced, life form. Now we can create de novo>/i> new synthetic life in the lab, but it must be fully formed and capable to existing either in competition with existing life, or unrecognizable to it.

    Interestingly, the idea of different biologies being formed in space and seeding the Earth is somewhat falsified because we don’t see it on Earth. [Davies might be correct that this shadow life has not been looked for or recognized yet, but I am skeptical about this]. So far we haven’t even seen similar biology with very different DNA codes. Although it is possible that as we sequence all organisms, we miss them as we assume that the code is fixed for translation as we don’t generally test the amino acid composition of proteins. The evidence against this hypothesis is that the DNA sequences we do see have fairly clear lineages and therefore the protein composition and structure must be similar to be functional.

    Therefore even if life evolved to some extent in space, after the first seeding of Earth occurred, further seeding is likely blocked. This, or course, doesn’t rule out life starting in space.

  • Alex Tolley September 21, 2013, 16:21

    . Oops. Closing that tag