An optimal degree of physical and chemical heterogeneity for the origin of life?

Abstract

The accumulation of pure, concentrated chemical building blocks, from which the essential components of protocells could be assembled, has long been viewed as a necessary, but extremely difficult step on the pathway to the origin of life. However, recent experiments have shown that moderately increasing the complexity of a set of chemical inputs can in some cases lead to a dramatic simplification of the resulting reaction products. Similarly, model protocell membranes composed of certain mixtures of amphiphilic molecules have superior physical properties than membranes composed of single amphiphiles. Moreover, membrane self-assembly under simple and natural conditions gives rise to heterogeneous mixtures of large multi-lamellar vesicles, which are predisposed to a robust pathway of growth and division that simpler and more homogeneous small unilamellar vesicles cannot undergo. Might a similar relaxation of the constraints on building block purity and homogeneity actually facilitate the difficult process of nucleic acid replication? Several arguments suggest that mixtures of monomers and short oligonucleotides may enable the chemical copying of polynucleotides of sufficient length and sequence complexity to allow for the emergence of the first nucleic acid catalysts. The question of the origin of life may become less daunting once the constraints of overly well-defined laboratory experiments are appropriately relaxed.