No abstract

The continuity of abiotically formed bilayer membranes with similar structures in contemporary cellular life, and the requirement for microenvironments in which large and small molecules could be compartmentalized, support the idea that amphiphilic boundary structures contributed to the emergence of life. As an extension of this notion, we propose here a 'Lipid World' scenario as an early evolutionary step in the emergence of cellular life on Earth. This concept combines the potential chemical activities of lipids and other amphiphiles, with their capacity to undergo spontaneous self-organization into supramolecular structures such as micelles and bilayers. In particular, the documented chemical rate enhancements within lipid assemblies suggest that energy-dependent synthetic reactions could lead to the growth and increased abundance of certain amphiphilic assemblies. We further propose that selective processes might act on such assemblies, as suggested by our computer simulations of mutual catalysis among amphiphiles. As demonstrated also by other researchers, such mutual catalysis within random molecular assemblies could have led to a primordial homeostatic system displaying rudimentary life-like properties. Taken together, these concepts provide a theoretical framework, and suggest experimental tests for a Lipid World model for the origin of life.

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 λ ci . 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.