In this article, we would like to discuss some aspects of a theoretical framework for Artificial Life, focusing on the problem of an explicit definition of living systems useful for an effective artificial construction of them. The limits of a descriptive approach will be critically discussed, and a constructive (synthetic) approach will be proposed on the basis of the autopoietic theory of Maturana and Varela.

Key words: self-organization, autopoiesis, biological autonomy, artificial life, constructivism

This article addresses the issue of defining the universal properties of living systems through an organizational approach, according to which the distinctive properties of life lie in the functional organization which correlates its physicochemical components in living systems, and not in these components taken separately. Drawing on arguments grounded in this approach, this article identifies autonomy, with a set of related organizational properties, as universal properties of life, and includes cognition within this set.

Key words: Universal Biology, Organization ǂ Structure, Biological Autonomy, Cognition.

The chronology of encoding amino acids in the genetic code, described by the self-referential model, prompted a search for the supporting biosynthesis pathways since the list of abiotic amino acids was not in close coherence with it. The prediction from the chronology was adequately satisfied with the identification of the Glycine-Serine Cycle of assimilation of C1-units. The start of encoding from C1-derived amino acids sits nicely at the fuzzy borders between methylotrophy and autotrophy. It is not possible to envisage the construction of metabolism from simpler nutrients. These indications support the notion that protein synthesis would be at the crux of the metabolic sink, and that metabolism is sink-driven. Relevance: In spite of the many unknowns in the area of the origin of life, it seems that the self-referential model offers a starting point for experimental verification of the formation of genetic codes. In the metabolic aspect, there is no possibility of getting simpler with respect to nutrients in the routes for metabolic contruction.

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. Relevance: The article deals with autopoiesis and related concepts.

Key words: Autopoiesis, Definition of life, Individual identity, Physical proximity, Self-reproduction, Circular causality

Life is a complex phenomenon that not only requires individual self-producing and self-sustaining systems but also a historical-collective organization of those individual systems, which brings about characteristic evolutionary dynamics. On these lines, we propose to define universally living beings as autonomous systems with open-ended evolution capacities, and weclaim that all such systems must have a semi-permeable active boundary (membrane), an energy transduction apparatus (set of energy currencies) and, at least, two types of functionally interdependent macromolecular components (catalysts and records). The latter is required to articulate a `phenotype-genotype’ decoupling that leads to a scenario where the global network ofautonomous systems allows for an open-ended increase in the complexity of the individual agents. Thus, the basic-individual organization of biological systems depends critically on being instructed by patterns (informational records) whose generationand reliable transmission cannot be explained but take into account the complete historical network of relationships amongthose systems. We conclude that a proper definition of life should consider both levels, individual and collective: livingsystems cannot be fully constituted without being part of theevolutionary process of a whole ecosystem. Finally, we alsodiscuss a few practical implications of the definition fordifferent programs of research.

Key words: artificial life, astrobiology, autonomous agents, definition of life, generalization of biology, genotype-phenotype decoupling, open-ended evolution, origin of life, origins of (genetic) information