Coenzyme world model of the origin of life

Abstract

The origin of life means the emergence of heritable and evolvable self-reproduction. However the mechanisms of primordial heredity were different from those in contemporary cells. Here I argue that primordial life had no nucleic acids; instead heritable signs were represented by isolated catalytically active self-reproducing molecules, similar to extant coenzymes, which presumably colonized surfaces of oil droplets in water. The model further assumes that coenzyme-like molecules (CLMs) changed surface properties of oil droplets (e.g., by oxidizing terminal carbons), and in this way created and sustained favorable conditions for their own self-reproduction. Such niche-dependent self-reproduction is a necessary condition for cooperation between different kinds of CLMs because they have to coexist in the same oil droplets and either succeed or perish together. Additional kinds of hereditary molecules were acquired via coalescence of oil droplets carrying different kinds of CLMs or via modification of already existing CLMs. Eventually, polymerization of CLMs became controlled by other polymers used as templates; and this kind of template-based synthesis eventually resulted in the emergence of RNA-like replicons. Apparently, oil droplets transformed into the outer membrane of cells via engulfing water, stabilization of the surface, and osmoregulation. In result, the metabolism was internalized allowing cells to accumulate free-floating resources (e.g., animoacids, ATP), which was a necessary condition for the development of protein synthesis. Thus, life originated from simple but already functional molecules, and its gradual evolution towards higher complexity was driven by cooperation and natural selection.

Keywords: Coenzyme world; Lipid world; Molecular cooperation; Molecular heredity; Origin of life.

Figure 1

Oil (hydrocarbon) droplets in water as a potential substrate for coenzyme-like molecules. (A) Emulsion of oil/petroleum in water, from http://petrowiki.org/Oil_emulsions. (B) Scenario of coenzyme self-reproduction on oil droplets: a coenzyme molecule makes the surface hydrophilic via oxidation of hydrocarbons; this change facilitates synthesis of coenzymes from precursors on the surface. Hydrophilic oil droplets easily divide and may coalesce with new oil droplets (i.e., capture new oil resource).

Figure 2

Emergence of new coding molecules. (A) Coalescence of oil droplets carrying different self-reproducing coenzymes. (B) Phosphorylation of molecule a, which is stably produced within the chemical community on the surface of oil droplets, results in the emergence of a new coding molecule a_P if this reaction is autocatalytic. (C) Polymerization of molecules b catalyzed by self-reproducing molecules a. (D) Simple repetitive polymers facilitate polymerization of identical polymers aligned on the surface of oil droplets. (E) Template-based universal synthesis (i.e., replication) of coding polymers with any aperiodic sequence of monomers.

Figure 3

The origin of bilayer membranes and transition from surface metabolism to intra-cellular metabolism.