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The Limits of Organic Life in Planetary Systems 1 Introduction 1.1 THE SEARCH FOR LIFE IN THE COSMOS The National Aeronautics and Space Administration (NASA) has long given high priority to missions that ask whether extraterrestrial life might exist in the solar system and beyond. That priority reflects public interest, which was enhanced in the mid-1990s when fragments of Mars delivered to Earth as meteorites were shown to contain small structures reminiscent of microbial life. The proper interpretation of those structures remains controversial, but it is certain that nothing would alter our view of humanity and our position in the cosmos more than the discovery of alien life. Nothing would contribute more to NASA’s goal of exploring the cosmos, or to inspiring and educating the next generation of students in the hard sciences and engineering, than a search for alien life. Nothing would be more unfortunate than to expend considerable resources in the search for alien life and then not recognize it if it is encountered. The search for life in the cosmos begins with our understanding of life on Earth. This understanding has grown enormously over the past century. It is now clear that although terran life is conveniently categorized into millions of species, studies of the molecular structure of the biosphere show that all organisms that have been examined have a common ancestry. There is no reason to believe, or even to suspect, that life arose on Earth more than once, or that it had biomolecular structures that differed greatly from those shared by the terran life that we know. Our only example of life has been quite successful in dominating the planet. Earth itself presents a variety of environments, some extreme by human standards. One lesson learned from studies of terran biochemistry and its environmental range on Earth is that the life we know requires liquid water. Wherever a source of energy is found on Earth with liquid water, life of the standard variety is present. That observation has already helped to guide NASA missions through the directive to “follow the water” in searching for life in the solar system. Environments where liquid water might be or might have been present are high on the list of locales planned for NASA missions. Excitement runs high when sites are found where the geology indicates with near certainty the past presence of liquid water in substantial amounts. As pragmatic as the strategy is, scientists and laypeople alike have asked whether it might be parochial, or “terracentric.” As Carl Sagan noted, it is not surprising that carbon-based organisms breathing oxygen and composed of 60 percent water would conclude that life must be based on carbon and water and metabolize free oxygen.1 The depth and breadth of our knowledge of terran chemistry tempts us to focus on carbon because terran life is based on carbon, and organic chemistry as we know it emerged from 19th-century natural-product chemistry
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The Limits of Organic Life in Planetary Systems based on the isolation of compounds from nature. If terran life had provided silicon-based molecules, then our knowledge of silicon-based chemistry would now be advanced. The natural tendency toward terracentricity requires that we make an effort to broaden our ideas of where life is possible and what forms it might take. Furthermore, basic principles of chemistry warn us against terracentricity. It is easy to conceive of chemical reactions that might support life involving noncarbon compounds, occurring in solvents other than water, or involving oxidation-reduction reactions without dioxygen. Furthermore, there are reactions that are not redox. For example, life could get energy from NaOH + HCl; the reaction goes fast abiotically, but an organism could send tendrils into the acid and the base and live off the gradient. An organism could get energy from supersaturated solution. It could get relative humidity from evaporating water. It is easy to conceive of alien life in environments quite different from the surface of a rocky planet. The public has become aware of those ideas through science fiction and nonfiction, such as Peter Ward’s Life as We Do Not Know It.2 The public and the scientific community have become interested in authoritative perspectives on the possibility of life in environments in the solar system very much different from the ones that support life on Earth and life supported by “weird” chemistry in exotic solvents and exploiting exotic metabolisms. To NASA those ideas would help to guide missions throughout the solar system and permit them to recognize alien life if it is encountered, however it is structured. Given the inevitability of human missions to Mars and other locales potentially inhabited by alien life, an understanding of the scope of life will improve researchers’ chance to study such life before a human presence contaminates it or, through ignorance or inaction, destroys it. In broadest outline, this report shows that the committee found no compelling reason for life being limited to water as a solvent, even if it is constrained to use carbon as the scaffolding element for most of its biomolecules. In water, varied molecular structures are conceivable that could (in principle) support life, but it would be sufficiently different from life on Earth that it would be overlooked by unsophisticated life-detection tools. Evidence suggests that Darwinian processes require water, or a solvent like water, if they are supported by organic biopolymers (such as DNA). Furthermore, although macromolecules using silicon are known, there are few suggestions as to how they might have emerged spontaneously to support a biosphere. 1.2 DEFINING THE SCOPE OF THE PROBLEM For generations the definition of life has eluded scientists and philosophers. (Many have come to recognize that the concept of “definition” itself is difficult to define.3) We can, however, list characteristics of the one example of life that we know—life on Earth: It is chemical in essence; terran living systems contain molecular species that undergo chemical transformations (metabolism) under the direction of molecules (enzyme catalysts) whose structures are inherited, and heritable information is itself carried by molecules. To have directed chemical transformations, terran living systems exploit a thermodynamic disequilibrium. The biomolecules that terran life uses to support metabolism, build structures, manage energy, and transfer information take advantage of the covalent bonding properties of carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur and the ability of heteroatoms, primarily oxygen and nitrogen, to modulate the reactivity of hydrocarbons. Terran biomolecules interact with water to be soluble (or not) or to react (or not) in a way that confers fitness on a host organism. The biomolecules found in terran life appear to have molecular structures that create properties specifically suited to the demands imposed by water. Living systems that have emerged on Earth have done so by a process of random variation in the structure of inherited biomolecules, on which was superimposed natural selection to achieve fitness. These are the central elements of the Darwinian paradigm. Various published definitions of life understandably incorporate those features, given that we are the life form defining it. Indeed, because the chemical structures of terran biomolecular systems all appear to have arisen through Darwinian processes, it is hardly surprising that some of the more thoughtful definitions of life hold that it is a “chemical system capable of Darwinian evolution.”4
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The Limits of Organic Life in Planetary Systems 1.3 IS EVOLUTION AN ESSENTIAL FEATURE OF LIFE? Many of the definitions of life include phrases like undergoes Darwinian evolution. The implication is that phenotypic changes and adaptation are necessary to exploit unstable environmental conditions and to function optimally in the environment. Evolutionary changes have even been suggested for the hypothesized “clay crystal life” of Cairns-Smith,5 referring to randomly occurring errors in crystal structure during crystal growth as analogous to mutations. Would a self-replicating chemical system capable of chemical transformations in the environment be considered life? If self-replicating chemical compounds are not life, replication by itself is not sufficient as a defining characteristic of life. Likewise, the ability to undergo Darwinian evolution, a process that results in heritable changes in a population, is also not sufficient to define life if we consider minerals that are capable of reproducing errors in their crystal structure to be equivalent to evolution. Although that property of clays may have been vital in the origin of life and particularly in the prebiotic synthesis of organic macromolecules and as catalysts for metabolic reactions, can the perpetuation of “mistakes” in crystal structure result in the selection of a “more fit” crystal structure? It is important to emphasize that evolution is not simply reproducing mutations (mistakes in clays), but also selecting variants that are functionally more fit. The canonical characteristics of life are an inherent capacity to adapt to changing environmental conditions and to interact with other living organisms (and, at least on Earth, also with viruses).6 Natural selection is the key to evolution and the main reason that Darwinian evolution persists as a characteristic of many definitions of life. The only alternative to evolution for producing diversity would be to have environmental conditions that continuously create different life forms or similar life forms with random and frequent “mistakes” in the synthesis of chemical templates used for replication or metabolism. Such mistakes would be equivalent to mutations and could lead to traits that gave some selective advantage in an existing community or in exploiting new habitats. That random process could lead to life forms that undergo a form of evolution without a master information macromolecule, such as DNA or RNA. It is difficult to imagine such life forms as able to “evolve” into complex structures unless other mechanisms, such as symbiosis or cell-cell fusion, are available. Evolution is the key mechanism of heritable changes in a population. However, although mutation and natural selection are important processes, they are not the only mechanisms for acquiring new genes. It is understood that lateral gene transfer is one of the most important and one of the earliest mechanisms for creating diversity and possibly for building genomes with the requisite information to result in free-living cells.7 Lateral gene transfer is also one of the mechanisms to align genes from different sources into complex functional activities, such as magnetotaxis and dissimilatory sulfate reduction.8 It is possible that this mechanism was important in the evolution of metabolic and biosynthetic pathways and other physiological traits that may have evolved only once even though they are present in a wide variety of organisms. Coevolution of two or more species is also a hallmark of evolution manifested in many ways, from insect-plant interactions to the involvement of hundreds of species of bacteria in the nutrition of ruminant animals. Organisms and the environment also coevolve, depending on the dominant characteristics of the environment and the availability of carbon and energy sources. If the ability to undergo Darwinian evolution is a canonical trait of life no matter how different a life form is from Earth life, are there properties of evolving extraterrestrial organisms that would be detectable as positive signs of life? Evolution provides organisms the opportunity to exploit new and changing environments, and one piece of evidence for the cosmic ubiquity of evolution is that on Earth life occupies all available habitats and even creates new ones as a consequence of metabolism. Another hallmark of evolution is the ability of organisms to coevolve with other organisms and to form permanent and obligatory associations. It is highly probable that an inevitable consequence of evolution is the elimination of radically different biochemical lineages of life that may have formed during the earliest period of the evolution of life. Extant Earth life is the result of either selection of the most fit lineage or homogenization of some or all of the different lineages into a common ancestral community that developed into the current three major lineages (domains). All have a common biochemistry based on presumably the most “fit” molecular information strategies and energy-yielding pathways among a potpourri of possibilities. Thus, one of the apparent generalizations that can be drawn from extant Earth life, and the explanation for the development of a “unity of biochemistry” in all organisms, is that lateral gene transfer is an ancient and efficient mechanism for rapidly creating diversity and complexity. Lateral gene transfer is also an efficient mechanism for
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The Limits of Organic Life in Planetary Systems selecting the genes that are most “fit” for specific proteins and transferring them into diverse groups of organisms. The results are the addition of genes and the replacement of less-fit genes that have similar functions. Natural selection based solely on mutation is probably not an adequate mechanism for evolving complexity. More important, lateral gene transfer and endosymbiosis are probably the most obvious mechanisms for creating complex genomes that could lead to free-living cells and complex cellular communities in the short geological interval between life’s origin and the establishment of autotrophic CO2 fixation about 3.8 billion years ago and microbial sulfate reduction 3.47 billion years ago on the basis of isotope data.9 An important implication of the existence of viruses or virus-like entities during the early evolution of cellular organisms is that their genomes may have been the source of most genetic innovations because of their rapid replication, high rates of mutation due to replication errors, and gene insertions from diverse host cells.10 Is evolution an essential feature of life? Cells are more than the information encoded in their genomes; they are part of a highly integrated biological and geochemical system in whose creation and maintenance they have participated. The unity of biochemistry among all Earth’s organisms emphasizes the ability of organisms to interact with other organisms to form coevolving communities, to acquire and transmit new genes, to use old genes in new ways, to exploit new habitats, and, most important, to evolve mechanisms to help to control their own evolution. Those characteristics would probably be present in extraterrestrial life even if it had a separate origin and a unified biochemistry different from that of Earth life. 1.4 BRIEF CONSIDERATIONS OF POSSIBLE LIFE FORMS OUTSIDE THE SCOPE OF THIS REPORT As discussed in the literature,11 chemical models of non-Earth-centric life reveal much about what the scientific community considers possible, particularly regarding ways in which systems organize matter and energy to generate life. Thus, truly “weird” life might utilize an element other than carbon for its scaffolding. Less weird, but still alien to human biological experience, would be a life form that does not exploit thermodynamic disequilibria that are largely chemical. Weirder would be a life form that does not exploit water as its liquid milieu. Still weirder would be a life form that exists in the solid or gas phase.12 In a different direction, yet also outside the scope of life that most communities think possible, would be a life form that lacks a history of Darwinian evolution. Some features of terran life are almost certainly universal, however. In particular, the requirement for thermodynamic disequilibrium is so deeply rooted in our understanding of physics and chemistry that it is not disputable as a requirement for life. Other criteria are not absolute. Terran biology contains clear examples of the use of nonchemical energy; photosynthesis is the best known, although energy from light is soon converted to chemical energy. Silicon, in some environments, can conceivably support the scaffolding of large molecules. This report explicitly considers nonaqueous environments. Even Darwinian evolution is presumably not an absolute. For example, depending on how human civilization applies gene therapy, our particular form of life could be able to evolve via Lamarckian,a as opposed to Darwinian, processes. Humankind will be able to perceive and solve problems in human biology without needing to select among random events, thus sparing the species the need to remove unavoidable genetic defects through the death of individuals. That will make the human biosphere no less living, even to those who make Darwinian evolution central in their concept of life. Likewise, we can easily conceive of robots that are self-reproducing or computer-based processes that grow and replicate.13 Here, information transfer is not based on a specific molecular replication but on a replication involving information on a matrix. Whether such entities will be called life remains to be seen. What is clear is that the scientific community does not believe that Lamarckian, robotic, or informational “life” could have arisen spontaneously from inanimate matter. At the very least, its matrix would have to be constructed initially by a chemical, Darwinian life form arising from processes similar to those seen on Earth. Again, it is not clear whether those views are constrained by our inability to conceive broadly from what we know or whether they reflect true constraints on the processes by which life might emerge in natural history. a Lamarck recognized a similar principle of evolution referred to as “inheritance of acquired characters,” stating that variations in characteristics seen in organisms were acquired in response to the environment.
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The Limits of Organic Life in Planetary Systems Those thoughts introduce a subsidiary theme of this report. It is conceivable that chemistry, structure, or environments able to support life were not suited for the initiation of life. For example, Earth can support life today, but prevailing views hold that life could not have originated in an atmosphere that is as oxidizing as Earth’s today. If that is true, the surface of Earth would be an environment that is habitable but not able to give rise to life. 1.5 STRATEGIES TO MITIGATE ANTHROPOCENTRICITY We have only one example of biomolecular structures that solve problems posed by requirements for life, and the human mind finds it difficult to create ideas truly different from what it already knows. It is thus difficult for us to imagine how life might look in planetary environments very different from what we find on Earth. Recognizing that difficulty, the committee chose to embrace it. The committee exploited a strategy that began with characterization of the terran life that humankind has known well, first because of its macroscopic visibility and then through microscopic observation that began in earnest 4 centuries ago. This, of course, is like life that is associated with humankind. As the next step in the strategic process, the committee assembled a set of observations about life that is considered exotic when compared with human-like life. Exploration of Earth has taken researchers to environments that human-like organisms find extreme, to the highest temperatures at which liquid water is possible, to the lowest temperatures at which water is liquid, to the depths of the ocean where pressures are high, to extremes of acidity and alkalinity, to places where the energy flux is too high for human-like life to survive, to locales where thermodynamic disequilibria are too scarce to support human-like life, and to locations where the chemical environment is toxic to human-like life. The committee then asked, Can we identify environments on Earth where Darwinian processes that exploit human-like biochemistry cannot exploit available thermodynamic disequilibria? The answer is an only slightly qualified no. It appears that wherever the thermodynamic minimum for life is met on Earth and water is found, life is found. Furthermore, the life that is found appears to be descendant from an ancestral life form that also served as the ancestor of humankind (perhaps we would not necessarily have recognized it if its ancestry were otherwise) and exploits fundamentally human-like biochemistry. The committee then reviewed evidence of abiotic processes that manipulate organic material in a planetary environment. It asked whether the molecules that we see in contemporary terran life might be understood as the inevitable consequences of abiotic reactivity. Although signatures of such predecessor reactivity can be adumbrated within contemporary biochemistry, they are generally faint.14 Some 4 billion years of biological evolution have attached a strong Darwinian signature to whatever went before; hypotheses regarding evidence of our inanimate ancestry within modern biostructures are the subject of intense dispute. If life originated first on Earth, it was long ago when conditions on the surface of this planet were very different from what they are today. We do not know what those conditions were, and we may never know. Furthermore, the organisms around today are all highly evolved descendants of the first life forms and probably contributed long ago to the demise of their less fit, more primitive competitors. The historical slate has been wiped clean both geologically and biologically. Finally, because life forms replicate, singular events can have enormous impacts on future developments. Life does not have to be a probable outcome of spontaneous physicochemical processes, although it may well be. Arguments based on probability are not as powerful in this sphere as they usually are in the physical sciences. The committee surveyed the inventory of environments in the solar system and asked which non-Earth ones might be suited to life of the terran type. Such locales are few, unless there are laws not now understood that could govern the early stages of the self-organization of biochemical structures and processes that could lead inevitably to evolving life forms.15 Subsurface Mars and the putative sub-ice oceans of the Galilean satellites are the only locales in the solar system (other than Earth itself) that are clearly compatible with terran biochemistry. The committee’s survey made clear, however, that most locales in the solar system are at thermodynamic disequilibrium—an absolute requirement for chemical life. Furthermore, many locales that have thermodynamic disequilibrium also have solvents in liquid form and environments where the covalent bonds between carbon and other lighter elements are stable. Those are weaker requirements for life, but the three together would appear, perhaps simplistically, to be sufficient for life. The committee asked whether it could conceive of biochemistry adapted to those exotic environments, much as human-like biochemistry is adapted to terran environments. Few detailed hypotheses are available; the committee reviewed what is known, or might be speculated, and considered research directions that might expand or constrain understanding about the possibility of life in such exotic environments.
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The Limits of Organic Life in Planetary Systems Finally, the committee considered more exotic solutions to problems that must be solved to create the emergent properties that we agree characterize life. It considered a hierarchy of “weirdness”: Is the linear dimensionality of biological molecules essential? Or can a monomer collection or two-dimensional molecules support Darwinian evolution? Must a standard liquid of some kind serve as the matrix for life? Can a supercritical fluid serve as well? Can life exist in the gas phase? In solid bodies, including ice? Must the information content of a living system be held in a polymer? If so, must it be a standard biopolymer? Or can the information to support life be placed in a mineral form or in a matrix that is not molecularly related to Darwinian processes? Are Darwinian processes and their inherent struggle to the death essential for living systems? Can altruistic processes that do not require death and extinctions and their associated molecular structures support the development of complex life? 1.6 REFERENCES 1. Sagan, C. 1973. Extraterrestrial life. Pp. 42-67 in Communication with Extraterrestrial Intelligence CETI (C. Sagan, ed.). MIT Press, Cambridge, Mass. 2. Ward, P. 2005. Life as We Do Not Know It. Viking, New York. 3. Cleland, C.E. 2001. Historical science, experimental science, and the scientific method. Geology 29:987-990. 4. Joyce, G.F., Young, R., Chang, S., Clark, B., Deamer, D., DeVincenzi, D., Ferris, J., Irvine, W., Kasting, J., Kerridge, J., Klein, H., Knoll, A., and Walker, J.1994. In Origins of Life: The Central Concepts (D.W. Deamer and G.R. Fleischaker, eds). Jones and Bartlett, Boston, Mass. 5. Cairns-Smith, A.G. 1982. Genetic Takeover and the Mineral Origins of Life. Cambridge University Press, Cambridge, U.K. 6. See Brown, J.R., 2003, Ancient horizontal gene transfer, Nature Rev. Genetics 4:121-132; Martin, W., Rotte, C., Hoffmeister, M., Theissen, U., Gelius-Dietrich, G., Ahr, S., and Henze, K., 2003, Early cell evolution, eukaryotes, anoxia, sulfide, oxygen, fungi first (?), and a tree of genomes revisited, IUBMB Life 55:193-204; Ochman, H., Lawrence, J.G., and Groisman, E.S., 2000, Lateral gene transfer and the nature of bacterial innovation, Nature 405:299-304; and Woese, C.R., 2002, On the evolution of cells, Proc. Natl. Acad. Sci. U.S.A. 99:8742-8747. 7. Martin, W., Rotte, C., Hoffmeister, M., Theissen, U., Gelius-Dietrich, G., Ahr, S., and Henze, K. 2003. Early cell evolution, eukaryotes, anoxia, sulfide, oxygen, fungi first (?), and a tree of genomes revisited. IUBMB Life 55:193-204. 8. See Grünberg, K., Wawer, C., Tebo, B.M., and Schüler, D., 2001, A large gene cluster encoding several magnetosome proteins is conserved in different species of magnetotactic bacteria, Appl. Environ. 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Claverie, J.M., 2006, Viruses take center stage in cellular evolution, Genome Biol. 7:110; Forterre, P., 2006, The origin of viruses and their possible roles in major evolutionary transitions, Virus Res. 117:5-16; Forterre, P., 2006, Three RNA cells for ribosomal lineages and three DNA viruses to replicate their genomes: A hypothesis for the origin of cellular domain, Proc. Natl. Acad. Sci. U.S.A. 103:3669-3674; and Koonin, E.V., and Martin, W., 2005, On the origin of genomes and cells within inorganic compartments, Trends Genetics 21:647-654. 11. Benner, S.A., Ricardo, A., and Carrigan, M.A. 2004. Is there a common chemical model for life in the universe? Curr. Opinion Chem. Biol. 8:672-689. 12. Allamandola, L.J., and Hudgins, D.M. 2003. From interstellar polycyclic aromatic hydrocarbons and ice to astrobiology. In Proceedings of the NATO ASI, Solid State Astrochemistry (V. Pirronello and J. Krelowski, eds.). Kluwer, Dordrecht. 13. Adami, C., and Wilke, C.O., 2004, Experiments in digital life, Artificial Life 10:117-122; Rosing, M.T., 1999, 13C-depleted carbon microparticles in >3700-Ma sea-floor sedimentary rocks from West Greenland, Science 283:674-676; Shen, Y., Buick, R., and Canfield, D.E., 2001, Isotopic evidence for microbial sulphate reduction in the early Archaean era, Nature 410:77-81; and Shidlowski, M.A., 1988, A 3800-million-year isotopic record of life from carbon in sedimentary rocks, Nature 333:313-318. 14. Benner, S.A., Ellington, A.D., and Tauer, A. 1989. Modern metabolism as a palimpsest of the RNA world. Proc. Natl. Acad. Sci. U.S.A. 86:7054-7058. 15. Kauffman, S.A. 1995. At Home in the Universe: The Search for Laws of Self-organization and Complexity. Oxford University Press, New York.
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