GARDobes: Primordial cell nano-precursors with organic catalysis, compositional genome and capacity to evolve

The Graded Autocatalysis Replication Domain (GARD) model described here depicts an early primordial scenario, prior to the emergence of biopolymers, such as RNA or proteins. The model describes, with the help of statistical chemistry computer simulations, a collection of organic molecular species capable of rudimentary selection and evolution. The GARD model provides a rigorous kinetic analysis of simple sets of chemicals that manifest mutual catalysis. It is shown that catalytic closure can sustain self-replication up to a critical dilution rate, related to the extent of mutual catalysis. The capacity for self-replication in a mutually catalytic set is shown to be a graded property, quantitated by a critical parameter λci. GARD could be a simple model for a primordial scenario, in which replication and catalysis are performed by the same set of molecules. GARDobes are proposed to be entities that embody a GARD system, endowed with a non-DNA "compositional genome", and are presumed to have replicated slowly and imperfectly through mutually catalytic networks. Therefore, they are not bound by the standard cellular size constraints: GARDobes may be as small as a few nanometers, with 20-50 nanometers being rather large and elaborate. Active GARDobes, if ever found on earth or on other planets, would be distinguished by a highly biased organic chemistry, i.e. having only a small subset of the possible molecules of any given class. Their fossils might still bear the hallmarks of such a bias, with narrow spectra of molecules such as Polycyclic Aromatic Hydrocarbons or even with enantiomeric excesses.