Causation and the Origin of Life. Metabolism or Replication First?

Department of Chemistry, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel.
Origins of Life and Evolution of Biospheres (Impact Factor: 1.11). 07/2004; 34(3):307-21. DOI: 10.1023/B:ORIG.0000016446.51012.bc
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The conceptual gulf that separates the 'metabolism first' and 'replication first' mechanisms for the emergence of life continues to cloud the origin of life debate. In the present paper we analyze this aspect of the origin of life problem and offer arguments in favor of the 'replication first' school. Utilizing Wicken's two-tier approach to causation we argue that a causal connection between replication and metabolism can only be demonstrated if replication would have preceded metabolism. In conjunction with existing empirical evidence and theoretical reasoning, our analysis concludes that there is no substantive evidence for a 'metabolism first' mechanism for life's emergence, while a coherent case can be made for the 'replication first' group of mechanisms. The analysis reaffirms our conviction that life is an extreme expression of kinetic control, and that the emergence of metabolic pathways can be understood by considering life as a manifestation of 'replicative chemistry'.

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    • "Whatever their chemical identities were, those small organic molecules must have reacted with each other to produce macromolecules which later formed macromolecular complexes or communities by hypothesized self-assembly processes or coexistence mechanisms of different kinds (Chen and Walde, 2010; Cleaves et al., 2012; Deamer and Weber, 2010; Ehrenfreund and Cami, 2010; Ferris, 2006; Garay, 2011; Johnson et al., 2008; Miller, 1953; Miyakawa et al., 2002; Orgel, 2004; Powner et al., 2009; Rushdi and Simoneit, 2001). The mechanisms of self-assembly and macromolecular community formation are often theoretically problematic, either because the assumptions of the underlying (toy) models are too schematic or because they are physically or chemically unrealistic (Morowitz et al., 2000; Pross, 2004; Szathmáry, 2006; Segré et al., 2001). The actual chemical and evolutionary details of the many different scenarios are usually implicit, so it is often difficult to see how the envisioned macromolecular complex or community could be a self-sustaining and self-regulated unit of life or of evolution (Gánti, 1987; Rasmussen et al., 2009). "
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    ABSTRACT: Metabolically Coupled Replicator Systems (MCRS) are a family of models implementing a simple, physico-chemically and ecologically feasible scenario for the first steps of chemical evolution towards life. Evolution in an abiotically produced RNA-population sets in as soon as any one of the RNA molecules become autocatalytic by engaging in template directed self-replication from activated monomers, and starts increasing exponentially. Competition for the finite external supply of monomers ignites selection favouring RNA molecules with catalytic activity helping self-replication by any possible means. One way of providing such autocatalytic help is to become a replicase ribozyme. An additional way is through increasing monomer supply by contributing to monomer synthesis from external resources, i.e., by evolving metabolic enzyme activity. Retroevolution may build up an increasingly autotrophic, cooperating community of metabolic ribozymes running an increasingly complicated and ever more efficient metabolism. Maintaining such a cooperating community of metabolic replicators raises two serious ecological problems: one is keeping the system coexistent in spite of the different replicabilities of the cooperating replicators; the other is constraining parasitism, i.e., keeping "cheaters" in check. Surface-bound MCRS provide an automatic solution to both problems: coexistence and parasite resistance are the consequences of assuming the local nature of metabolic interactions. In this review we present an overview of results published in previous articles, showing that these effects are, indeed, robust in different MCRS implementations, by considering different environmental setups and realistic chemical details in a few different models. We argue that the MCRS model framework naturally offers a suitable starting point for the future modelling of membrane evolution and extending the theory to cover the emergence of the first protocell in a self-consistent manner. The coevolution of metabolic, genetic and membrane functions is hypothesized to follow the progressive sequestration scenario, the conceptual blueprint for the earliest steps of protocell evolution. Copyright © 2015 Elsevier Ltd. All rights reserved.
    Journal of Theoretical Biology 06/2015; 381. DOI:10.1016/j.jtbi.2015.06.002 · 2.12 Impact Factor
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    • "For example, one might require that every reaction of an autopoietic system be spontaneous, but autopoiesis does not specify the thermodynamic conditions which allow the complete system to function as an autonomous machine, nor does it define its energetic relation with the environment. The specification of the directive forces may be essential to the understanding of the nature of life (Pross 2004). However, in spite of this shortcoming, it appears that autopoiesis (under D1 and D2) is the only simple property actually available which allows a satisfactory definition of living being, satisfying the majority of the theoretical and practical criteria of a good definition. "
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    ABSTRACT: The concept of autopoiesis was proposed 40 years ago as a definition of a living being, with the aim of providing a unifying concept for biology. The concept has also been extended to the theory of knowledge and to different areas of the social and behavioral sciences. Given some ambiguities of the original definitions of autopoiesis, the concept has been criticized and has been interpreted in diverse and even contradictory ways, which has prevented its integration into the biological sciences where it originated. Here I present a critical review and conceptual analysis of the definition of autopoiesis, and propose a new definition that is more precise, clear, and concise than the original ones. I argue that the difficulty in understanding the term lies in its refined conceptual subtlety and not, as has been claimed by some authors, because it is a vacuous, trivial or very complex concept. I also relate the concept of autopoiesis to the concepts of closed systems, boundaries, homeostasis, self-reproduction, causal circularity, organization and multicellularity. I show that under my proposed definition the concept of a molecular autopoietic system is a good demarcation criterion of a living being, allowing its general integration into the biological sciences and enhancing its interdisciplinary use.
    Origins of Life 10/2012; 42(6). DOI:10.1007/s11084-012-9297-y · 1.11 Impact Factor
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    • "In view of the conceptual clarification above, an inspection of the terms used in the debate (Shapiro 2000; Pross 2004; Anet 2004) reveals that so-called replication actually corresponds to ''direction-favoring autocatalysis, based on a template-directing function,'' which is characterized by direction-favoring catalysis, while so-called metabolism actually corresponds to ''speed-favoring autocatalysis and/or self-metabolism,'' primarily characterized by speed-favoring catalysis. Let us review the possible ways leading to the emergence of a ''proto-cell,'' with both the features of ''replication'' and ''metabolism'' (Fig. 1). "
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    ABSTRACT: Self-replication, an important concept abstracted from reproduction, the key feature of life, remains vague in definition and lacking in clear interpretation in terms of its chemical mechanism. Mentioned frequently in discussions concerning the essence of life and its origin, the vague concept has caused a lot of uncertain statements, confusable references, and malposed debates, and has seriously held back efforts in this field. In this article, we try to improve the situation by a conceptual analysis in a more fundamental and clearer background. Self-replication in the substantial world could not mean anything but that "an entity favors the production of its own." The major chemical mechanism for such favoring is catalysis, which can be classified into speed- and direction-favoring types (the template-directing function is actually a type of direction-favoring catalysis). Molecular self-replication could be based on autocatalysis or self-metabolism; the self-replication of a complex entity could be based on autocatalytic and/or self-metabolic sets, and should involve a mechanism of self-division. This conceptual clarification sheds light on the dim areas concerning the essence of life and its origin.
    Theory in Biosciences 12/2010; 130(2):119-25. DOI:10.1007/s12064-010-0117-5 · 1.23 Impact Factor
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