Origins of life

There are many unanswered questions regarding how the biomolecules and biomechanical processes that define life came to be. A collection of Articles in this issue show how intermediates in RNA synthesis might have formed and how the initiation and evolution of RNA replication might have occurred.

Although ribose aminooxazoline has been shown to be a potential intermediate in prebiotic pyrimidine ribonucleotide synthesis, a route by which this could occur has remained elusive. Now, a remarkably efficient photoanomerization reaction has been investigated by theory and experiment. The new route affords enantiomerically pure ribonucleotides when the starting material is enantioenriched.

Chemical reconstitution of the triose glycolysis pathway is controlled by α-phosphorylation and provides a generational link between prebiotic ribonucleotide synthesis, triose glycolysis and serine metabolism. Now, research suggests that unification of nucleotide synthesis and triose metabolism may have been a fundamentally important step towards the origins of life.

An unanswered question in the RNA world scenario is how sequence information could be transferred during replication of duplex RNA. Without the aid of sophisticated enzymes, strand reannealing occurs more quickly than template-directed synthesis. Now, a plausible prebiotic solution to this problem is presented, in which a viscous solvent enables information transfer from a gene-length double-stranded template.

Lysine-rich peptides from the ribosomal core and derived homolysine decapeptides of either L-, D- or mixed chirality have now been shown to enhance RNA polymerase ribozyme activity at low magnesium concentrations, accelerate ribozyme evolution and enable templated RNA synthesis within membranous protocells.

Intracellular bodies called liquid organelles are rich in nucleic acids and proteins, and are thought to occur by liquid–liquid phase coexistence. Now, enzymatic control over the phosphorylation state of a simple cationic peptide, thereby altering its electrostatic interaction with RNA, has been shown to drive formation and dissolution of droplets that mimic these intracellular liquid bodies.

Forged by evolution, the natural enzymatic pathways to aldose carbohydrates are complex. Now, a biocatalytic stereoselective one-pot assembly of these carbohydrates from formaldehyde and glycolaldehyde using engineered D-fructose-6-phosphate aldolase (FSA) variants has been developed that circumvents this complexity.

A minimal cell — one that has all the minimum requirements for life — is still a complex entity comprising informational, compartment-forming and metabolic subsystems. Here it is shown that, contrary to previous assumptions, a common prebiotically plausible chemistry can give rise to building blocks for all the subsystems.

How complex nucleic acids originally formed, despite dilution and degradation reactions, is not clear. Thermal gradients in rock pores have now been shown to be capable of trapping and thermo-cycling genetic polymers during replication. In this system long oligonucleotide strands are seen to outcompete short strands — a prerequisite for the evolution of replicating systems towards increasing complexity.

One theory for the abiogenesis of RNA involves ligation of shorter oligomers that are observed after dry-state condensation of mononucleotides. Here, the chemo- and regioselective acetylation of (oligo)nucleotides in water under prebiotically plausible conditions is described. This remarkable selectivity permits the rapid template-directed ligation of oligomers to favour extant 3′,5′-linkages.

An RNA aptamer and a ribozyme are both observed to retain a surprising degree of activity despite backbone heterogeneity caused by the presence of non-natural 2′–5′ phosphodiester linkages. These results suggest that absolute regioselectivity of non-enzymatic replication may not have been required for the emergence of RNA as the first biopolymer.

RNA compartmentalization is essential for cellular functions and may have played a pivotal role in the emergence of life. However, the consequences of compartmentalization on RNA catalysis have been largely unexplored. Here, partitioning of catalytic RNA in a two-phase aqueous polymer solution increased local RNA concentration, enhancing ribozyme kinetics.

A demonstration of simple sugar synthesis from single carbon feedstocks would provide significant support for the involvement of RNA in the origin of life. Here, hydrogen cyanide is shown to feed a cyanocuprate photoredox cycle that ultimately provides both the starting material and the reducing power necessary for a Killiani–Fischer-type sugar synthesis.

A drawback of recently reported prebiotic routes to RNA is a requirement for enantioenriched reactants. Here, the presence of a slightly enantioenriched amino acid in the reaction mixture is shown to drive the formation of enantiopure RNA precursors. This provides a plausible scenario in which single-handed biological molecules were formed prior to the emergence of self-replicating informational polymers.