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Exobiology / Astrobiology

June 28, 2011

It was noted in the first post of the series on biological ‘dark matter’ that the discovery of truly independent life beyond this planet would be of explosive significance. Even just one proven case of an independently-evolved life would have huge implications for the probability and frequency of life emerging in the universe. But if such life indeed exists, it is currently undefined and therefore relegated in the ‘dark’ category of biological knowledge. In previous posts, we’ve looked at the very interesting proposal that very distinct ‘weird life’ might exist on Earth as a ‘shadow biosphere’, but also noted that it is unlikely that such life could have escaped all influence of conventional life – or, that is to say, that such terrestrial ‘Life 2.0’ could lead a truly orthogonal existence. On the other hand, completely alien extraterrestrial living systems might have radically different ‘design’ features, and (if satisfying the criterion of true independence) would obviously be free of any interaction with known ‘Life 1.0’. Although it could be argued that the study of extra-terrestrial life is science without a subject matter, there are many areas of biology which impinge on the possibility of alien life, and this field has been termed exobiology, or (more commonly in recent times) astrobiology. The analysis of any truly alien life, however simple, would be immensely instructive to modern molecular and cellular biology. This post will deal with the much-grappled question of the likelihood of alien life and its implications. But to start with, we need to revisit a very basic question….

The Meaning of Life

In the very first post of this blog, I quoted a definition of life by the noted crystallographer and biologist J. D. Bernal. To paraphrase it, this description linked life to the electronic properties of matter, the inherent propensities of atoms of many elements to form chemical bonds from which great complexity can arise. At one level, nice, but it still doesn’t really explain what the phenomenon of life is all about. Many others have tried, but no single definition has risen to the forefront, and it remains a deceptively difficult issue to comprehensively pin down despite all modern advances in molecular biological sciences.

Attempting to penetrate to the most basic level, one might think of a living systems as complex organized physical patterns which can replicate themselves by taking in energy and building blocks from the environment. This broad description could even encompass highly speculative science-fiction life based on organized energy patterns or unconventional matter. Often, though, it has been noted that this kind of definition may technically not exclude phenomena such as crystal growth, which are never regarded as exemplary of life. A missing ingredient is the capacity of living systems to undergo Darwinian evolution, and this has been listed as a keynote factor in a number of life-definitions. Indeed, the acquisition of evolvability by complex molecular systems can be cited as a boundary point for separating non-living matter from life.

If we leave aside science fiction conjectures, we are restricted to ordinary matter and chemical bonding as only known source of complex living systems. And complexity is impossible to avoid in such circumstances. Life requires a great many chemical reactions, which must be accelerated by complex macromolecular catalysts known as enzymes. These are not necessarily proteins, as RNA molecules can accomplish much, albeit with less efficiency in most circumstances. It certainly could be the case that other quite distinct macromolecules could act as effective catalysts in alien biosystems, and perhaps the numbers of such alternatives is theoretically very large. What can be predicted, though, is that any of these life-systems would be based upon the construction of complex molecules from simpler molecular alphabets, a limited set of building blocks whose joining together in a vast number of potential combinatorial patterns allows the generation of functional diversity. Any ‘non-alphabetic’ hypothetical alternative soon founders from the inherent difficulties of constructing a complex system where each functional tool is a chemically distinct entity.

Life Is Where You Find It

A great deal has been written about the chances of finding life elsewhere in the universe, but time and again, the arguments all suffer from a basic lack of knowledge. If we could define a detailed mechanism whereby life arose on this planet, it would greatly facilitate extrapolating this event to other planetary conditions. At least, the range of planetary states under which life (at least the life with which we are familiar) could arise could be much more cogently specified.

But at present, we cannot even be certain that life originated on Earth itself. It is increasingly apparent that living cells were present here relatively soon (geologically speaking) after the Earth’s formation during the early phases of the solar system. This presents something of a problem if we assume that early molecular evolution would have required immense time periods, since conditions of the very early Earth (intense heat and high levels of meteoric bombardment) hardly seem ideal for such development. Perhaps ‘islands of stability’ existed on the early Earth where molecular progression could safely occur, or perhaps conditions were (for some reason) less severe than generally modeled. But a logical alternative is to propose that Earth was ‘seeded’ with life from elsewhere, the position of the advocates of ‘panspermia’. While interstellar panspermia is very problematic, a much more feasible form of it proposes that during the early solar system, life actually arose on Mars, and was shuttled to Earth by means of meteoric ejecta.

Be that as it may, it is clear that the emergence of life and the emergence of intelligent life are highly likely to be uncoupled phenomena. In other words, there is no reason to suppose that emergent life elsewhere in the universe will necessarily progress towards intelligence and sentience. It took a very long time in the total history of the terrestrial biosphere for humans to emerge, and this may have been contingent on specific evolutionary and environmental circumstances. At least, it’s clear that a great deal of complex animal life preceded the development of the primate line which led to humans, without showing any tendency towards the evolution of sentience.

So intelligence may be a ‘probability bottleneck’ in an evolving biosphere, but most likely not the only one. It has been suggested that prokaryotic life (bacteria and archaea) may have a relatively good chance of arising on any ‘wet and rocky’ planet, but the evolution of complex multicellular life may depend on truly improbable circumstances. But getting from the A of nonliving matter to the B of even the simplest cell is nevertheless no small move at all. Suggesting that all you need for life to emerge is liquid water and minerals is riding roughshod over a very large body of enigmas and uncertainties. In other words, it is dubious indeed as to how much confidence can be placed in blithe statements as to the ‘inevitability’ of life arising on Earth.

But even if life was, in fact, bound to happen owing to the circumstances existing soon after this planet’s formation, this does not immediately help us understand the frequency of life occurring elsewhere in the universe. What if the Earth was anything but an average ‘wet and rocky’ world, but very, very special indeed? It has become recognized that some geophysical features of the Earth have been important or even essential for either the origin of life, or at least its productive evolution and diversification. These include our exceptionally large moon (stabilization of the Earth’s axial tilt), the Earth’s very significant magnetic field (minimization of ionizing radiation), and plate tectonics (stabilization of oceanic salinity levels and the generation of climatic effects by actions such as mountain building). Certainly no other planet in our solar system is anything like this. Beyond determining mass, size and insolation, identifying all these other ‘special’ properties in astronomically remote extra-solar planets (‘exoplanets’) is a very tall order – so we are still left with a sample size of one.

The distinct possibility that the Earth is a ‘special conditions package’ can also be used as a counter-argument to an implication of the above-mentioned early-onset of abiogenesis on this planet. Aside from the possibility of extra-terrestrial ‘seeding’, an early start to life has been interpreted as evidence that, given the opportunity, life will rapidly spring up, and therefore should be common throughout the universe. Unfortunately, this optimistic view founders once again on ignorance – while life may indeed emerge rapidly given early Earth-like conditions, we cannot yet be sure whether certain special features of the primordial Earth were essential for this to occur, and are exceedingly rare.

Through Anthropic Eyes

Apart from the probable rarity of the ‘package’ of Earth-like conditions throughout the cosmos, it has long been noted that the whole universe often seems as though it was ‘tailored’ to allow the phenomenon of life to emerge. (This could also be taken as an implicit message of the above-mentioned Bernal definition of life as a higher-level outcome of the inherent properties of atomic electron states). Tiny changes in many fundamental physical constants would prevent the existence of matter in its current state, along with the associated chemistry enabling life. Thus, while the physical properties of matter in this universe are such that they are compatible with the development of complex biology (which obviously includes us), it is easy to imagine a host of universes where this is not the case. This apparent ‘life-friendliness’ of the universe has been termed the ‘goldilocks’ effect, after the ‘just right’ conditions which Goldilocks eventually found in the home of the three bears.

Both the apparent ‘special’ nature of the Earth and the universe itself can be viewed through the lens of the anthropic principle. Although this has been put forward in a number of different forms, it can be considered in general as a kind of selection process. Within our observable universe itself, we as conscious observers (and all our non-sentient evolutionary precursors) can only arise where the conditions are favorable for this to occur. So, however demanding these conditions may be, we must necessarily ‘select’ for those circumstances to be present on the planet on which we find ourselves. By this reasoning, even if the Earth is rare to the point of absolute cosmic uniqueness, here we will be, and here we will be placed as observers.

But what about the universe as a whole, and the ‘goldilocks’ effect? The ‘weak’ form of the anthropic principle simply holds that the values of the physical constants must be as they are in order to allow life and intelligent observers to arise at least once during the lifetime of the universe. This has the same logic as for the anthropic ‘selection’ for the Earth itself, but it falls short of being a satisfying explanation unless it is expanded to include the multiverse concept. Here our observable universe is but one of a vast array of alternatives, where any variation on physical laws is possible. By the same token, conscious observers are ‘selected’ only in the universes which permit the phenomenon of consciousness to arise in the first place.

Despite its logic, many people have felt uneasy with anthropic arguments, in that they smack of tautology. (‘We can only be here if it is possible to be here….’). But there are some ways to approach such questions by physical investigation, at least at the planetary level, if not hypothetical alternative universes. Take the alleged ‘rareness’ of the Earth, for example. The more that evidence supports the interpretation that the Earth is indeed endowed with improbable characteristics for a planet of its general type, the more favored is the stance that such conditions have been anthropically selected for, in order to permit our evolutionary appearance. For example, are there features of the Earth’s orbital cycles which are in any way unexpected?  Some such characteristics might be important for planetary stabilization and minimization of constant and drastic climate change which would otherwise pose limitations on the development of complex life. If truly comprehensive models for the origin of life can be derived, clearly the corresponding environmental needs can be related to the likelihood of the frequent occurrence of life in general planetary systems.

Clearly, ongoing astronomic studies of exoplanetary systems are of tremendous value for helping to resolve the debate over the uniqueness (or not) of the Earth. Though no firm conclusions can yet be reached, there is no evidence to suggest that our planet is ‘ordinary’. Some people object to this stance in that it seems to run counter to the long-standing retreat, ever since Galileo and Copernicus, from the view that humans have a special place in the universe. (The Earth revolves around the sun, and not vice versa; the sun is a commonplace star in a unremarkable position within a galaxy of hundreds of millions of stars; our galaxy is undistinguished among millions of galaxies; we are biochemically just another eukaryotic, multicellular animal; we are but one of many types of primates….). But none of these issues exclude the possibility that the Earth possessed a unique (or at least very rare) juxtaposition of physico-chemical features which enabled life to arise and flourish. Giving rise, eventually to us, conscious observers of the universe. Perhaps we are not the only ones to face the universe and wonder, but as yet we know of no others.

From Wild Speculations to Respectable Extrapolations

From its beginnings, alien life at all levels has been a favorite theme of science-fiction. Some stories in this genre falling within the ‘hard’ (more factually scientifically-oriented) camp have offered a little background about the nature of the alien biology. For example, a previous post has noted the theme of  ‘mirror life’, and its fictional treatment on more than one occasion. Many authors (some with scientific backgrounds) have proposed that alien life might be based on silicon rather than carbon, but this seems unlikely from the relatively poor ability of silicon to form complex concatentated bonds. (Recall the comments above regarding the inherent need of any biosystems for some form of molecular alphabets). Most writers of even the hardest science fiction will avoid too much of any such biochemical speculation, for fear of the technical issues getting in the way of the story-line. But such ‘hard’ musings soon reach a dead-end in any case, through limitations of basic knowledge.

Yet such limitations need not apply forever. As synthetic biology teams up with systems biology, increasingly complex ‘alternative biologies’ will become accessible to artificial analyses. (The previous post was devoted to the topic of ‘experimental dark biology’, where advanced synthetic approaches will allow a gamut of novel biosystems to be generated and evaluated). These approaches are certainly applicable to projections of detailed hypothetical alien biosystems. Prior to actual lab studies, much work will be done in preliminary modeling. As an example, consider an interesting proposal for the exoplanetary existence of an alternative type of photosynthesis where free chlorine (or other chlorine compounds) is generated instead of oxygen.  With a rational model of this postulated chlorinic fixation of carbon dioxide, specific predictions can be made as to what planetary ‘signatures’ to search for as evidence that such a biologically-based phenomenon might be taking place. These kinds of studies are thus far from idle indulgences, since they can actively assist in future searches for the signatures of life in the broadest possible terms.

From ranging through multiverses, to this universe, to biopoly(verse):

Of life, is the universe bare?

Save for the Earth, a case so rare?

Or is the stance anthropic

Just a little myopic?

We thus search for others out there.

References & Details

(In order of citation)

‘…..based on organized energy patterns or unconventional matter…’    As an example of a radically different form of life, consider Dragon’s Egg by Robert Forward (Del Rey Impact 2000; first published 1980) with its depiction of sentient life on the surface of a neutron star.

‘……Darwinian evolution…..a keynote factor in a number of life-definitions.’     See Cleland & Chyba (2002) for a discussion of life definitions, including the Darwinian version.

‘…….based upon the construction of complex molecules from simpler molecular alphabets…… Any ‘non-alphabetic’ hypothetical alternative soon founders…..’    This theme is described in more detail in Searching for Molecular Solutions Chapter 10.

A great deal has been written about the chances of finding life elsewhere in the universe…..’    In this post, the main interest is the occurrence of extraterrestrial life in any form, let alone complex life and intelligence. (See the below for reference to the Nick Lane paper considering probabilities of finding simple vs. alien complex biologies). But obviously many people are primarily interested in intelligent alien life, and this is the motivating force behind the SETI project, noted briefly in a previous post.

‘…..proposes that during the early solar system, life actually arose on Mars, and was shuttled to Earth by means of meteoric ejecta.’    Paul Davies (2002) has promoted this idea and a good case can be made for it, since it circumvents the challenges of explaining the origin of life on the seemingly very inhospitable early Earth.

‘…..It has been suggested that prokaryotic life (bacteria and archaea) may have a relatively good chance of arising ….. but the evolution of complex multicellular life may depend on truly improbable circumstances.’    See an on-line paper by Nick Lane in the Journal of Cosmology.

What if the Earth was anything but an average ‘wet and rocky’ world, but very, very special…’     This is the contention of the interesting book Rare Earth: Why Complex Life is Uncommon in the Universe, by Peter Ward and Donald Brownlee (Springer 2003).

‘….an early start to life has been interpreted as evidence that, given the opportunity, life will rapidly spring up…’    As an example of this, see Lineweaver & Davis 2002. Here they conclude that although the early-start effect is compatible with a significant frequency for life in the universe (although intelligence may be far rarer). However, the ‘rare Earth unknown factors’ issue remains a real potential confounding factor.

‘……..the ‘goldilocks’ effect…..’  This has been discussed in detail by Paul Davies in his book The Goldlocks Enigma. (Allen Lane 2006).

‘…….the anthropic principle…..’ Brandon Carter is credited with naming this principle in 1973, as considered in detail in The Anthropic Cosmological Principle, by John D. Barrow & Frank J. Tipler, Oxford U. Press, 1986 .

‘….the multiverse concept….’ Some interesting aspects of this idea, including the question of definitions, is discussed by Vaas (2010).

‘……..are there features of the Earth’s orbital cycles which are in any way unexpected?  ….. might be important for planetary stabilization and minimization of constant and drastic climate change…’  For example, Waltham (2011) has considered climate change deriving from the Earth’s Milankovitch cycles (long term cycles in certain parameters of the Earth’s orbit which affect its exposure to solar energy), and found that the frequency of Earth’s Milankovitch cycles is lower than predicted, possibly contributing to longer term climate stabilization. In turn, this can be framed as a case for anthropic selection. Note, though, that obviously this is not to deny that the Earth has experienced drastic changes in planetary climate over the eons, ranging from ‘Snowball Earth’ in the Pre-Cambrian to a globally ice-free state in the Jurassic. From the point of view of evolutionary development, it is interesting to consider that a ‘goldilocks’ effect of sorts may also exist for planetary stability and a high degree of diversification of complex life. That is to say, neither too much planetary variability nor too little are ideal, but rather some level in between (whether or not it is ‘just right’). From this point of view, an unchangeing environment over vast periods of time is sub-optimal as a driver of the evolution of complexity.

‘… interesting proposal for the exoplanetary existence of an alternative type of photosynthesis….’    See Haas 2010.

Next post: Two weeks from now.

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