Abstract
Mineral surfaces are often proposed as the sites of critical processes in the emergence of life. Clay minerals in particular are thought to play significant roles in the origin of life including polymerizing, concentrating, organizing, and protecting biopolymers. In these scenarios, the impact of minerals on biopolymer folding is expected to influence evolutionary processes. These processes include both the initial emergence of functional structures in the presence of the mineral and the subsequent transition away from the mineral-associated niche. The initial evolution of function depends upon the number and distribution of sequences capable of functioning in the presence of the mineral, and the transition to new environments depends upon the overlap between sequences that evolve on the mineral surface and sequences that can perform the same functions in the mineral's absence. To examine these processes, we evolved self-cleaving ribozymes in vitro in the presence or absence of Na-saturated montmorillonite clay mineral particles. Starting from a shared population of random sequences, RNA populations were evolved in parallel, along separate evolutionary trajectories. Comparative sequence analysis and activity assays show that the impact of this clay mineral on functional structure selection was minimal; it neither prevented common structures from emerging, nor did it promote the emergence of new structures. This suggests that montmorillonite does not improve RNA's ability to evolve functional structures; however, it also suggests that RNAs that do evolve in contact with montmorillonite retain the same structures in mineral-free environments, potentially facilitating an evolutionary transition away from a mineral-associated niche.
Highlights
Interactions between minerals and organic molecules likely played a role in the emergence of life on the early Earth and perhaps even play(ed) a role in the emergence of life on other planets
Clay minerals are among those predicted to be present in prebiotic environments, including the early Earth, where they have been proposed to facilitate the transition from abiotic chemistry to biology
We observe that the presence of montmorillonite did not significantly alter the number or identity of RNAs that can adopt functional structures during in vitro evolution
Summary
Interactions between minerals and organic molecules likely played a role in the emergence of life on the early Earth and perhaps even play(ed) a role in the emergence of life on other planets. While direct evidence of clay minerals on the early Earth has been lost due to geological cycling, the presence of 3.5 billion year old clay minerals on Mars (Bristow and Milliken 2011) supports their predicted presence on the early Earth and other potentially prebiotic environments With their small particle size and typically flattened plate-like crystallites, even small proportions of clay minerals in rocks and sediments provide the majority of mineral surface area available for reactions with organic compounds (Ransom et al 1998). Contemporary biology, and evidence of an even greater role tion experiments conducted either in the presence or absence for RNA in early life At least two biologically derived, functional RNA structures (hammerhead and hairpin ribozymes) remain catalytically active in the presence of montmorillonite clay
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