A feature of many of the chemical systems plausibly involved in the origins of terrestrial life is that they are complex and messy—producing a wide range of compounds via a wide range of mechanisms. However, the fundamental behaviour of such systems is currently not well understood; we do not have the tools to make statistical predictions about such complex chemical networks. This is, in part, due to a lack of quantitative data from which such a theory could be built; specifically, functional measurements of messy chemical systems. Here, we propose that the pantheon of experimental approaches to the origins of life should be expanded to include the study of ‘functional measurements’—the direct study of bulk properties of chemical systems and their interactions with other compounds, the formation of structures and other behaviours, even in cases where the precise composition and mechanisms are unknown. This article is part of the themed issue ‘Reconceptualizing the origins of life’.

A variety of organic chemicals were likely available on prebiotic Earth. These derived from diverse processes including atmospheric and geochemical synthesis and extraterrestrial input, and were delivered to environments including oceans, lakes, and subaerial hot springs. Prebiotic chemistry generates both molecules used by modern organisms, such as proteinaceous amino acids, as well as many molecule types not used in biochemistry. As prebiotic chemical diversity was likely high, and the core of biochemistry uses a rather small set of common building blocks, the majority of prebiotically available organic compounds may not have been those used in modern biochemistry. Chemical evolution was unlikely to have been able to discriminate which molecules would eventually be used in biology, and instead, interactions among compounds were governed simply by abundance and chemical reactivity. Previous work has shown that likely prebiotically available α-hydroxy acids can combinatorially polymerize into polyesters that self-assemble to create new phases which are able to compartmentalize other molecule types. The unexpectedly rich complexity of hydroxy acid chemistry and the likely enormous structural diversity of prebiotic organic chemistry suggests chemical evolution could have been heavily influenced by molecules not used in contemporary biochemistry, and that there is a considerable amount of prebiotic chemistry which remains unexplored.

In extant biology, biopolymers perform multiple crucial functions. The biopolymers are synthesized by enzyme-controlled biosystems that would not have been available at the earliest stages of chemical evolution and consist of correctly sequenced and/or linked monomers. Some of the abiotic “messy” polymers approximate some functions of biopolymers. Condensation polymers are an attractive search target for abiotic functional polymers since principal polymers of life are produced by condensation and since condensation allows for the accurate construction of high polymers. Herein the formation of hyperbranched polyesters that have been previously used in the construction of enzyme-like catalytic complexes is explored. The experimental setup compares between the branched polyesters prepared under mild continuous heating and the wet-dry cycling associated with environmental conditions, such as dew formation or tidal activities. The results reveal that periodic wetting during which partial hydrolysis of the polyester occurs, helps to control the chain growth and delays the gel transition, a mechanism contributing to the tar formation. Moreover, the NMR and mass spec analyses indicate that continuously dried samples contain higher quantities of crosslinked and macrocyclic products, whereas cycled systems are enriched in branched structures. Ostensibly, environmental conditions have the ability to exert a rudimentary pressure to selectively enrich the polyesterification products in polymers of different structures and properties. At the early stages of chemical evolution, in the absence of biological machinery, this example of environmental control could have been for selectivity in chemical systems. As expected in marginally controlled systems, the identification of each component of the heterogeneous system has proved challenging, but it is not crucial for drawing the conclusions.

Enzymes are biopolymeric complexes that catalyse biochemical reactions and shape metabolic pathways. Enzymes usually work with small molecule cofactors that actively participate in reaction mechanisms and complex, usually globular, polymeric structures capable of specific substrate binding, encapsulation and orientation. Moreover, the globular structures of enzymes possess cavities with modulated microenvironments, facilitating the progression of reaction(s). The globular structure is ensured by long folded protein or RNA strands. Synthesis of such elaborate complexes has proven difficult under prebiotically plausible conditions. We explore here that catalysis may have been performed by alternative polymeric structures, namely hyperbranched polymers. Hyperbranched polymers are relatively complex structures that can be synthesized under prebiotically plausible conditions; their globular structure is ensured by virtue of their architecture rather than folding. In this study, we probe the ability of tertiary amine-bearing hyperbranched polyesters to form hydrophobic pockets as a reaction-promoting medium for the Kemp elimination reaction. Our results show that polyesters formed upon reaction between glycerol, triethanolamine and organic acid containing hydrophobic groups, i.e. adipic and methylsuccinic acid, are capable of increasing the rate of Kemp elimination by a factor of up to 3 over monomeric triethanolamine.This article is part of the themed issue ‘Reconceptualizing the origins of life’.

Speciation, an evolutionary process by which new species form, is ultimately responsible for the incredible biodiversity that we observe on Earth every day. Such biodiversity is one of the critical features which contributes to the survivability of biospheres and modern life. While speciation and biodiversity have been amply studied in organismic evolution and modern life, it has not yet been applied to a great extent to understanding the evolutionary dynamics of primitive life. In particular, one unanswered question is at what point in the history of life did speciation as a phenomenon emerge in the first place. Here, we discuss the mechanisms by which speciation could have occurred before the origins of life in the context of chemical evolution. Specifically, we discuss that primitive compartments formed before the emergence of the last universal common ancestor (LUCA) could have provided a mechanism by which primitive chemical systems underwent speciation. In particular, we introduce a variety of primitive compartment structures, and associated functions, that may have plausibly been present on early Earth, followed by examples of both discriminate and indiscriminate speciation affected by primitive modes of compartmentalization. Finally, we discuss modern technologies, in particular, droplet microfluidics, that can be applied to studying speciation phenomena in the laboratory over short timescales. We hope that this discussion highlights the current areas of need in further studies on primitive speciation phenomena while simultaneously proposing directions as important areas of study to the origins of life.

Enzymes are biological catalysts that are comprised of small-molecule, metal, or cluster catalysts augmented by biopolymeric scaffolds. It is conceivable that early in chemical evolution, ancestral enzymes opted for simpler, easier to assemble scaffolds. Herein, we describe such possible protoenzymes: hyperbranched polymer-scaffolded metal-sulfide nanocrystals. Hyperbranched polyethyleneimine (HyPEI) and glycerol citrate polymer-supported ZnS nanocrystals (NCs) are formed in a simple process. Transmission electron microscopy (TEM) analyses of HyPEI-supported NCs reveal spherical particles with an average size of 10 nm that undergo only a modest aggregation over a 14-day incubation. The polymer-supported ZnS NCs are shown to possess a high photocatalytic activity in an eosin B photodegradation assay, making them an attractive model for the study of the origin of life under the “Zn world” theory dominated by a photocatalytic proto-metabolic redox reaction network. The catalyst, however, could be easily adapted to apply broadly to different protoenzymatic systems.