Comment on “Evolutionary dynamics of RNA-like replicator systems: A bioinformatic approach to the origin of life” by Nobuto Takeuchi and Paulien Hogeweg

Abstract

As emphasized, e.g., by Dyson [1] one can envisage life processes from two points of view: the metabolic processes, in which a cascade of free-energy dissipating physico-chemical reactions maintain the living system out of thermodynamical equilibrium, and the information process, whose eventual effect is to allow the living form to produce offspring similar to itself – and to incorporate information about the environment by the mechanism of natural selection. Thus explaining the origin of life poses a two-pronged question: the origin of metabolism, and the origin of biological information. “Either life began only once, with the functions of replication and metabolism already present in rudimentary form and linked together from the beginning, or life began twice, with two separate kinds of creatures, one kind capable of metabolism without exact replication and the other kind capable of replication without metabolism” [1]. However, while “metabolism without exact replication” is already a form of information processing, “replication without metabolism” is highly unlikely to take place, since replication itself cannot take place except far from equilibrium, and to keep a system out of equilibrium some form of metabolic process should take place. I think that this dilemma is ill-placed: the essential difference lies between a mainly analogical and a mainly digital form of information processing. In the present life forms, information is essentially processed in a digital form represented by the nucleotide sequences in the form’s DNA. But one can imagine a kind of prebiotic system similar to Oparin’s coacervates [2] or to Luisi’s liposomes [3], which are able to (inaccurately) reproduce, carrying information to their offspring via their composition, i.e., in analogic language. Takeuchi and Hogeweg [4] start by assuming the existence of a digital reproduction mechanism, in the form of a collection of self-replicating RNA molecules. However, the system is not able to stabilize its information-processing properties without exploiting some form of localization or compartmentation, allowing for the stable coexistence of template and actuator molecules. The need for spatial structure and some form of compartmentation mechanism to avoid the dire consequences of the error threshold was emphasized long ago by Michod [5]. Then the “analogic” information is inscribed in the relative local fraction of template and actuator molecules. Takeuchi and Hogeweg elegantly show that spiral reaction waves can provide some of the necessary localization mechanisms, even in the absence of strict compartmentation. The appearance of active compartmentation adds of course stability to the mechanism. (See, e.g., the mechanisms for the maintenance of producer–nonproducer coexistence in a synthetic biological system reported in [6].) They show moreover that the transition from an RNA-based system to a dual DNAand RNA-based