A Universal Definition of Life: Autonomy and Open-Ended Evolution

Abstract

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 we claim 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 of autonomous 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 generation and reliable transmission cannot be explained but take into account the complete historical network of relationships among those systems. We conclude that a proper definition of life should consider both levels, individual and collective: living systems cannot be fully constituted without being part of the evolutionary process of a whole ecosystem. Finally, we also discuss a few practical implications of the definition for different programs of research.

Lifeness signatures and the roots of the tree of life

“…Any physico-chemical system is said to be alive, as per a given definition, only if it possesses all the properties mentioned in this definition. Modelbased definitions, on the other hand, define living systems not on the basis of a list of necessary and sufficient properties, but on the basis of a model that describes the very functioning of such living systems (see for instance Maturana and Varela 1973;Gánti [1971Gánti [ ] 2003Ruiz-Mirazo et al 2004). For instance, Gánti's model consists in three functionally dependent cross-catalytic subsystems -a metabolic network, a template information system, and a membrane that encloses everything -whose correct coupling ensures the growth and subsequent replication of the chemoton.…”

“…For instance, some argue that viruses should not be included within the circle of living systems in so far as they lack metabolic activity (e.g. Luisi 1998;Ruiz-Mirazo et al 2004); others argue the contrary, all the more now that it has been discovered that viruses form 'viral factories' that seem to possess the required properties of some of the list-based definitions of life (e.g. Raoult and Forterre 2008).…”

“…Kauffman 1993); others would reject this claim on the grounds that such autocatalytic networks might not be capable of evolution (e.g. Luisi 1998;Ruiz-Mirazo et al 2004). In a similar fashion, one could question whether minimal protoliving systems as those proposed by Szostak, Bartel and Luisi (2001) or Libchaber and his team (Noireaux et al 2005), might more properly qualify as alive than, for instance, those pursued by Rasmussen and colleagues (Rasmussen et al 2003).…”

On What It is to Fly Can Tell Us Something About What It is to Live

“…Such properties typically include growth, reproduction, self-repair, energy-harnessing capability, matter-harnessing capability, variation capability, information capability and so forth (see for instance: Oparin 1961, Bernal 1967, Monod 1970, Crick 1981, Mayr 1982, de Duve 1991, Farmer and Belin 1992, Koshland 2002, Morange 2003. On the other hand, one also finds 'model-based definitions' that define living systems on the basis of a model that describes the very functioning of such living systems (see for instance: Maturana and Varela 1973, Ganti [1971] 2003, Ruiz-Mirazo, Pereto and Moreno 2004. In such cases, the properties of living systems are only secondary in the sense that they are nothing but a consequence of the functioning of the model.…”

“…There is also much debate about the existence of physical, chemical or biological counterexamples that seem to defy both the sufficiency and necessity of any of these definitions (e.g. Luisi 1998, Ruiz-Mirazo, Pereto andMoreno 2004). Many discussions focus on finding 'the' right definition of life.…”