quarta-feira, 13 de junho de 2012

orgânica vs. inorgânica

Mi SciELO Servicios personalizados Servicios Personalizados Articulo Articulo en PDF Articulo en XML Referencias del artículo Como citar este artículo Traducción automática Enviar articulo por email Indicadores Citado por SciELO Accesos Links relacionados Citado por Google Similares en SciELO Similares en Google Bookmark Share on deliciousShare on googleShare on twitterShare on diggShare on citeulikeShare on connotea|More Sharing ServicesOtrosInternational Microbiology versión impresa ISSN 1139-6709 INT. MICROBIOL. v.8 n.1 Madrid mar. 2005 REVIEW ARTICLE -------------------------------------------------------------------------------- Juli Peretó Cavanilles Institute for Biodiversity and Evolutionary Biology, and Department of Biochemistry and Molecular Biology, University of Valencia, Spain Controversies on the origin of life Summary. Different viewpoints, many with deep philosophical and historical roots, have shaped the scientific study of the origin of life. Some of these argue that primeval life was based on simple anaerobic microorganisms able to use a wide inventory of abiotic organic materials (i.e. a heterotrophic origin), whereas others invoke a more sophisticated organization, one that thrived on simple inorganic molecules (i.e. an autotrophic origin). While many scientists assume that life started as a self-replicative molecule, the first gene, a primitive self-catalytic metabolic network has also been proposed as a starting point. Even the emergence of the cell itself is a contentious issue: did boundaries and compartments appear early or late during life's origin? Starting with a recent definition of life, based on concepts of autonomy and open-ended evolution, it is proposed here that, firstly, organic molecules self-organized in a primordial metabolism located inside protocells. The flow of matter and energy across those early molecular systems allowed the generation of more ordered states, forming the cradle of the first genetic records. Thus, the origin of life was a process initiated within ecologically interconnected autonomous compartments that evolved into cells with hereditary and true Darwinian evolutionary capabilities. In other words, the individual existence of life preceded its historical-collective dimension. [Int Microbiol 2005; 8(1):23-31] Key words: autonomy · life definition · open-ended evolution · protocells Received 4 February 2005 Accepted 18 February 2005 Address for correspondence: Institut Cavanilles de Biodiversitat i Biologia Evolutiva Poligon de la Coma, s/n E-46980 Paterna, Spain Tel. +34-963543666. Fax +34-963543670 E-mail: juli.pereto@uv.es -------------------------------------------------------------------------------- Controversias sobre el origen de la vida Resumen. El estudio científico del origen de la vida se ha moldeado desde diferentes perspectivas con profundas raíces filosóficas e históricas. Ciertas hipótesis suponen una vida primordial basada en microorganismos anaeróbicos simples que usaban materiales orgánicos abióticos (origen heterotrófico), mientras que otras postulan que se inició con organizaciones más complejas que se nutrían de moléculas inorgánicas simples (origen autotrófico). Muchos autores postulan que la vida empezó con una molécula autorreplicativa, el primer gen. Sin embargo, otros toman como punto de partida unas redes metabólicas autocatalíticas primitivas. Por último, la aparición de la célula misma es controvertida: ¿membranas y compartimentos tuvieron una incorporación temprana o tardía? Partiendo de una definición reciente de vida, basada en los conceptos de autonomía y evolución abierta, proponemos que, en un principio, las moléculas orgánicas se autoorganizaron en un metabolismo primordial localizado dentro de protocélulas. El flujo de materia y energía a través de dichos sistemas moleculares permitió la generación de estados más ordenados, o sea, la cuna de los primeros registros genéticos. De este modo la vida se inició con compartimentos autónomos interconectados ecológicamente que se transformaron en células dotadas de verdaderas capacidades hereditarias y evolutivas en sentido darwiniano. En otras palabras, la existencia individual de la vida precedió a su dimensión histórico-colectiva. [Int Microbiol 2005; 8(1):23-31] Palabras clave: autonomía · definición de vida · evolución abierta · protocélulas -------------------------------------------------------------------------------- Controvérsias sobre a origem da vida Resumo. O estudo científico da origem da vida foi moldado a partir de diferentes perspectivas sob profundas raízes filosóficas e históricas. Certas hipóteses pressupõem uma vida primordial baseada nos microrganismos anaeróbicos simples que usavam material orgânico abiótico (origem dos heterotróficos), enquanto que outras postulam que aquela se iniciou com organizações mais complexas que se nutriam de moléculas inorgânicas simples (origem dos autotróficos). Muitos autores argumentam que a vida começou com uma molécula auto-replicativa, o primeiro gene. Outros tomam como ponto de partida umas redes metabólicas auto-catalíticas primitivas. Por último, até mesmo a aparição da célula é controvertida: membranas e compartimentos tiveram uma incorporação prematura ou tardia? Partindo de uma definição recente de vida, baseada nos conceitos da evolução aberta, propomos que, no princípio, as moléculas orgânicas se auto- organizaram em um metabolismo primordial localizado dentro de protocélulas. O fluxo de matéria e energia através de tais sistemas moleculares permitiu a geração de estados mais ordenados, ou seja, o berço dos primeiros registros genéticos. Deste modo, a vida se iniciou com compartimentos autônomos interconectados ecologicamente que se transformaram em células dotadas de capacidades hereditárias e evolutivas no sentido darwiniano. Em outras palavras, a existência individual da vida precedeu a sua dimensão histórico-coletiva. [Int Microbiol 2005; 8(1):23-31] Palavras chave: autonomía · definição de vida · evolução aberta · protocélulas -------------------------------------------------------------------------------- This article is dedicated to the memory of three illuminating scientists recently deceased: Ramon Margalef (1919-2004), Joan Oró (1923-2004) and Alfred Giner-Sorolla (1919-2005). Introduction Life is matter with two biographies: (i) ontogenetic, synchronic, developmental, individual existence, based on the autopoietic (i.e. self-building) properties of cells and their ecological (i.e. biogeochemical) consequences, and (ii) the historical-collective, diachronic, populational, evolutionary process. Metabolism and genetics are the two central pillars that sustain life. Every living being uses external sources of both energy and matter to fight against disorder, equilibrium, and death. Membranes and molecular machines located on the frontier between the inside and the outside control and manage flows of energy and matter for the benefit of the cell itself. In current life forms, those activities are dependent on digital genetic records. Change, acquisition, and loss of fragments of the informative molecules are the basic mechanisms of evolution, i.e. the differential capacity for adaptation, survival, and reproduction. The historical persistence of genetic records depends absolutely on the metabolic and ecological abilities of organisms. A fundamental question is thus how the metabolic and genetic functions, which in terrestrial life forms are so intricately intertwined, came to be an integral part of the history of this planet. Which came first, the autopoietic or the hereditary properties of living matter? What is life? Although defining life is a difficult task, we recently proposed a definition that tries to join in a compact sentence both biographical aspects: "a living being is any autonomous system with open-ended evolutionary capacities" [34]. The term "autonomy" refers to the relationship between the living being and its environment and their mutual modifications, as well as the capacity of a living being to use energy and matter to synthesize its own components, i.e. to self-construct, thereby building an identity separated from the ambient. In the simplest case, a living being is a cell (pioneering work on the minimal cell concept can be found in [25]). The term "open-ended evolution" refers to the capability of a living being to explore new functions and relationships with the ambient (including other living beings) and to adapt to diverse situations in an almost unlimited way. The range of physical conditions compatible with-using the words of Gold [13]-planetary infection by microorganisms is astounding: Wherever there is a useful energy source (for living beings, that means visible-light photons, certain inorganic chemical reactions, or organic matter) and liquid water, some form of biological activity can be found. At the molecular level, biodiversity shows a no less striking biochemical unity: the same basic (cellular) organization, the recurrent use of DNA as hereditary material, the universal genetic code, and variations on the same bioenergetic mechanisms (i.e. chemical and chemiosmotic energy currencies). The most parsimonious explanation of those well-documented observations is that all current terrestrial life forms have the same late ancestor, the universal cenancestor-an idea that was already conjectured upon by Charles Darwin, based on much less evidence [4]. The origin of life How can we approach the primordial steps of life evolution? In the "top-down" strategy, all known organisms are compared in order to reconstruct the genetic and metabolic makeup of the universal cenancestor. Thus, in the universal tree of life based on ribosomal structure, this essentially means comparing the two prokaryotic domains, Bacteria and Archaea, since the Eucarya domain is considered to be a chimera derived from prokaryotic partners. Molecular cladistic methods are among the most powerful tools available for making such comparisons, but the possibilities for going even further back in this type of retrospective study and venturing some careful guesses on stages preceding the universal cenancestor are limited. In the opposite direction, the "bottom-up" strategy starts with cosmochemical, planetological, geological, and any other useful source of evidence to reconstruct the ambient, chemical inventory and the processes involved in the origin of life. However, it is also possible to abstractly consider the primordial geochemical conditions and then examine the theoretical principles that could apply to the oldest processes of matter organization. Mathematical models and computer simulations can help to define some general properties of the systems under consideration. Needless to say, many steps, stages, and transitions from chemical to biochemical evolution are, and will be for a long time, hot topics of scientific discussion. Ever since the historical contributions by Aleksandr I. Oparin, in the 1920s, the intellectual challenge of the origin of life enigma has unfolded based on the assumption that life originated on Earth through physicochemical processes that can be supposed, comprehended, and simulated; that is, there were neither miracles nor spontaneous generations. Thus, the scientific merits of Oparin's proposal are not in its details, which are always subject to change with the refinement of scientific knowledge, but in the possibility to experimentally test the plausibility of historical hypotheses and, even, to chemically implement very simple artificial life forms [18]. For example, in 1953, the Urey-Miller experiment [24] inaugurated the prebiotic chemistry research program [3]. Since then, many different reactions-notably, the synthesis of most amino acids, nucleic acid bases (as in the remarkable synthesis of adenine from hydrogen cyanide by Oró [28]), lipids, and other simple biomolecules-have been proposed to explain the abiotic origin of organic matter present in interstellar space [9], meteorites [9,20], comets [9,21], and the early Archean Earth (reviewed in [27]). Although some major abiotic syntheses are easier said than done, including the synthesis of very labile substances, such as sugars (e.g. ribose), or more sophisticated molecular arrangements, such as nucleotides (the abiotic combination of phosphate, [deoxy-] ribose and a base, in the correct positions, is far from trivial and can be well-considered as prebiotically inaccessible), the participation of mineral catalysts (e.g. clays as polymerization promoters [10] and borate as a sugar stabilizer [33]) facilitates some of the more difficult syntheses. Nonetheless, it is not only necessary to have a plausible scheme of abiotic synthesis for a biomolecule but also to explain how it would accumulate (i.e. present over enough time to be used) in the primitive planet [2]. Following Oparin's scenario, prebiotic organic molecules, either terrestrial or extraterrestrial in origin, built up in the oceans, thus promoting the organization of matter into primitive systems able to anaerobically thrive on environmental organics. In this scenario, life would have started with very simple anoxygenic and heterotrophic primordial cells. Difficulties regarding prebiotic chemical reactions in the atmosphere (since we still lack any robust evidence of its exact chemical composition and physical conditions) and uncertainties as to the accumulation of extraterrestrial organics have shifted the focus in search of other locations for primeval living processes. That is the source of interest in submarine hydrothermal vents as chemical cradles of life. In fact, some recent simulations indicate that organic matter can originate in those extreme ambients [11]. The proposal by Günter Wächtershäuser [41] of a thermophilic and autotrophic origin of life strengthened the attention focused on submarine chimneys (see Chap. 4 in [29]). This author suggested that the anoxic synthesis of pyrite was the energy and electron source for carbon dioxide fixation and the genesis of every organic component of nascent living matter. In other words, Wächtershäuser substitutes a primitive autotrophic and bidimensional metabolism on the pyrite surface for the prebiotic soup [23]. This is an acellular, genetically non-instructed metabolism, one that is favored by high temperatures and uses reduced sulfur gases that abound in submarine hydrothermal sources. There is experimental support for some of the chemical postulates but evidence is still lacking for many aspects of an autotrophic origin of life, including the ancestral character of the Arnon cycle [26,30]. A primordial chemolithoautotrophic metabolism based on the net synthesis of acetyl-CoA (Wood pathway) has been proposed as a link between a hydrothermal geochemistry and the most ancient biochemistry [35]. The entire diversity of processes and scenarios-the abiotic generation of chemical complexity in the primitive Earth, both endogenous (atmosphere, sea surface, submarine chimneys) and exogenous (meteorites, comets, interstellar dust particles), the roles of minerals (catalysis and stability) and of different energy sources (electromagnetic, chemical)-leads us, as Antonio Lazcano [19] has proposed, to an eclectic vision of the origin-of-life problem, in which the heterogeneity of mechanisms and conditions is essential to assembling a coherent historical narrative. Confronted standpoints There are several controversies regarding the origin of life, each with deep historical and philosophical roots (for a general discussion, see the book by Fry cited in Table 1, and references therein). In this sense, the scientific discussion of life's emergence does not differ from other intellectual conflicts in biology. Figure 1 summarizes the main subjects under debate. Heterotrophic vs. autotrophic origins. Either life started as a simple system, taking advantage of a chemically complex environment, as suggested, among others, by Oparin and Haldane, or the early emergence of self-sustained systems was based on utilization of the simplest molecules available, as postulated by Wächtershäuser. If we consider that a heterotrophic metabolic chart is simpler than an autotrophic one (such a postulate still needs to be proved), then the first option is the more parsimonious, as admitted by most experts. Replicators or metabolism first. This is a rather old debate, confronting supporters of an early origin of genes (even before their chemical nature was established, e.g. Herman J. Muller's ideas in the 1920s) with those advocating the emergence of proteins first (e.g. Leonard Troland in the 1910s) (see Farley's book cited in Table 1, and [18]). Many authors (including Manfred Eigen and Richard Dawkins) believe that the origin of life is equal to the origin of the first replicators, i.e. the first self-copying molecules. Theoretical and experimental insights (e.g. Leslie Orgel's classic assays on enzyme-free template replication; for a review, see [27]) and the discovery of catalytic RNA (Tom Cech's and Sydney Altman's description of the first ribozymes, in the early 1980s) brought about renewed interest in the idea of a spontaneous and early emergence of genetic polymers. This is one of the fundamental postulates of the RNA-world hypothesis, the alleged solution to the genetic version of the classical "chicken-and-egg" dilemma [8,42]. Nevertheless, other scientists take a different position: how would the origin of order (e.g. polymers synthesized from energetically and chirally pure monomers) have been possible without a plausible and efficient way for funneling energy through the system? Several authors, including Christian de Duve, David Deamer, Tibor Gánti, Harold Morowitz, Stuart Kauffman, Robert Shapiro, and Wächtershäuser, have argued for the appearance of primitive self-catalytic metabolic networks as a first step towards the synthesis of replicators. Those self-organized prebiotic processes would have provided a scaffold for the emergence of genetics [36,37]. However, supporters of the replicator-first approach have refuted the metabolism-first proposals [1,32]. Early or late cellularization. Whether compartment formation was an early or late phenomenon during life's origins is a debate closely related to the above-mentioned controversy on the early origin of metabolism vs. genetics. Many advocates of an early emergence of replicators consider the cell as a mere physical compartment for segregating polymers of differential replicative abilities, albeit some authors have adopted intermediate positions [6]. However, ample biochemical evidence has shown that the cell is not a simple physical space enclosed by a semipermeable membrane. Instead, the inside is chemically and physically different from the outside medium because of the functionally active presence of phospholipid membranes and their molecular (protein) machines, i.e. the active players in bioenergetic transduction and control of the connection between inside and outside. In order to build its own components, living matter must be able to access outside resources through its biomembranes. The essence of bioenergetics lies in the asymmetric disposition of molecular machines on the cell membrane and the corresponding streams of chemical messengers allowing efficient coupling between primary energy sources (visible light, chemicals) and metabolic networks. Some proteins (electron transport chains, ATPases) synthesize bioenergetic currencies (either chemiosmotic or chemical) whereas others use those currencies to execute a wide range of cellular tasks: motility, biosynthesis of complex architectures, maintenance of identity against environmental fluctuations, heredity, etc. We certainly adhere to Franklin M. Harold's [15,16] incapacity to imagine any other way (not to mention acellular systems) to make and maintain complex structures exhibiting complex behaviors using solar energy or exergonic chemical reactions and simple building blocks. Thus, life would have been cellular ab initio (pp. 65-76 in [29]), although many controversial issues remain, including the chemical nature of membranes (e.g. see the theoretical models discussed in [31]) and the kind of primitive molecular transducers of primary energy sources and membrane transporters that would have been necessary to prevent an osmotic crisis [7] prior to the existence of intricate protein machines. This contentious subject has even impregnated "bottom-up" strategies for the artificial (chemical) synthesis of life: those scientists interested either in the design of self-replicative ribozymes [17] or in the chemical implementation of autopoiesis through self-organized and self-reproductive lipid vesicles [14,22]. Other researchers have tried to find common ground in chemical systems, such as vesicles containing both self-replicative ribozymes and ribozymes involved in the synthesis of the vesicle's amphipathic components [40]. Our understanding of the nature and origin of life will advance by closing with educated guesses the gaps between those approaches, both theoretical and experimental, and the "top-down" strategies aimed at imagining the simplest life forms [39]. A conjectured path for the transition from inert to living matter Eörs Szathmáry [12] proposed distinguishing between units of life (entities showing self-maintenance) and units of evolution (entities with replicable, mutable, and heritable instructions). Some typical examples of living beings (e.g. free-living bacteria) are both units of life and units of evolution, and are thus located at the intersection of the two categories. Nonetheless, borderline situations can also be imagined: units of life unable to evolve (sterile animals) or units of evolution strictly dependent on units of life (obligate parasites, such as viruses and viroids) (Fig. 2A). We have proposed a genealogical view of life (see Fig. 1 in [34]), one that can be combined with Szathmáry's scheme but which renames the units of life as autonomous units. In this case, the first step during the self-organization of matter in the primitive Earth would have been the emergence of units of autonomy, lacking the capacity to evolve in the Darwinian sense. In other words, chemical evolution would have produced primitive self-maintaining chemical systems with rudimentary mechanisms of energy transduction but without digital (replicative and mutable) records. We call such "infrabiological" systems (Szathmáry's sensu [39]), involving a primeval association between metabolism and boundary, protocells (Fig. 2B). Once the genetic records were acquired and non-instructed metabolism was taken over by a genetically instructed one, an evolutionary stage was initiated [6] (Fig. 2B). A primordial version of these systems could have been a one-polymer-dependent protocell, as in a cellular RNA world. It must be remarked that most proponents of this one-polymer scenario refer to a hypothetical acellular RNA world, which, in our opinion, is very implausible from a prebiotic perspective. The invention of RNA-catalyzed protein synthesis and the fixation of a code introduced a second key takeover: from a ribozyme-catalyzed primitive metabolism to dynamic control of metabolic networks by proteins. This major evolutionary transition allowed the phenotype (organization)-genotype (structure) decoupling so idiosyncratic to terrestrial life. A final, important takeover was the substitution of DNA for RNA as genetic material, which marked the beginning of the open-ended evolutionary process [34], and the universal cenancestor cells as one of its consequences. Eventually, some autonomous units with open-ended evolutionary capacities completely lost their autonomy (i.e. their metabolic abilities totally vanished) and became obligate dependents on other autonomous units (e.g. viruses and viroids; Fig. 2B). Figure 3 presents a more detailed outline of processes involved in the origin of life following our proposal from prebiotic chemistry to protocells to cells. If we assume, like Simpson, that "the origin of life was necessarily the beginning of organic evolution" [38], then all the stages before the open-ended evolution phase should be considered as protobiological. In summary, during individual life there is permanent communication with the environment, whereas during evolutionary history there is some (unidirectional) communication between past and present populations. As proposed here, during the origin of life the organization and mechanisms allowing a chemical system to be materially and energetically (ecologically) connected with the ambient, and equipped with the ability to self-construct, emerged first. This was before the appearance of complex chemical structures that provided the system with a temporal connection throughout successive generations. Paraphrasing Ramon Margalef, the ecological networks were, also at the very beginning, the cradle of evolution. Future trends into the origin of life research will need a more fluid interaction between those proposing, "the origin of life was the origin of true heredity; we might even say the origin of the first gene" [5], and those claiming "no biopoietic scheme deserves to be taken seriously unless it provides both an explicit and sustainable energy source, and plausible means to couple the energy to the flow of matter through the emerging system" (p. 244 in [16]). Acknowledgements. My interest in the problem of the origin of life has always been motivated by conversations with different people, especially Antonio Lazcano, Álvaro Moreno, and Andrés Moya, who kindly read and commented on an early version of this manuscript. Thanks are given to the organizers of the symposium "Banquete. Comunicación en evolución" (Madrid, January 19-21, 2005), where part of this paper was presented. This work was funded by MECD, Spain (BMC2003-06957). References 1. Anet FAL (2004) The place of metabolism in the origin of life. Curr Op Chem Biol 8:654-659 [ Links ] 2. Bada JL, Lazcano A (2002) The origin of life: some like it hot, but not the first biomolecules. Science 296:1982-1983 [ Links ] 3. Bada JL, Lazcano A (2003) Prebiotic soup-revisiting the Miller experiment. Science 300:745-746 [ Links ] 4. Darwin C (1859) On the origin of species. John Murray, London, p 484 [ Links ] 5. Dawkins R (2004) The ancestor's tale. A pilgrimage to the dawn of life. 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Annu Rev Genet 36:125-151 [ Links ] © 2012 Sociedad Española de Microbiología Editorial Office International Microbiology Jordi Ferrán, 14, ent. E-08028 Barcelona, Spain Telf. +34-934482373 Fax: +34-933341079 int.microbiol@telefonica.net copyright autor do texto

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