Abstract:
Life is hypothesized to have originated in primordial or prebiotic soup, which is thought to have been a heterogenous mixture of various chemicals present on early Earth. However, even the simplest unicellular forms of life observed today are far too complex to have formed spontaneously in such a mixture. As a result, researchers investigating the origin of life widely accept that simpler structures, known as protocells, likely preceded modern cells. A protocell, in this context, is defined as a collection of genetic material enclosed within a lipid bilayer. The genetic material in these early protocells is thought to have been RNA, which is capable of both storing the genetic information and catalyzing chemical reactions. This dual functionality is critical because modern enzymes, responsible for the catalysis of biochemical reactions, including replication of the genetic polymer, likely did not exist at the time of origin of life. Thus, RNA replication would have occurred without the aid of enzymes, in the prebiotic environment. Given the lack of specialized enzymatic machinery that ensures the accuracy of replication in modern cells, nonenzymatic RNA replication may have been difficult in the chemically complex environment on the early Earth.
To explore how this chemical complexity might have impacted nonenzymatic RNA replication, we investigated the influence of a mixture of four ‘spent’ nucleotides (5’-nucleoside monophosphates or 5’-NMPs) on RNA replication rates. Our experiments showed that the presence of these nucleotides significantly slowed the replication process. Notably, the results suggest that the extent of inhibition was related to the strength of base-pairing interactions between the spent nucleotides and the template base. The greatest reduction in reaction rate was observed when the spent nucleotide was the Watson-Crick complement of the templating base.
These findings highlight how the heterogeneous composition of the prebiotic soup could have hindered RNA replication during the proposed RNA World stage of early life. They also suggest that compartmentalization may have been essential for the emergence of life. To investigate this, we examined a mixed single chain amphiphile-based liquid-liquid phase-separated (LLPS) system as model protocell. These compositionally diverse coacervates remained stable across a wider pH range than those formed from a single amphiphile. Furthermore, they withstood various plausible prebiotic selection pressures, such as variable temperatures and salt concentrations. Moreover, the coacervates were capable of encapsulating RNA and supporting nonenzymatic RNA replication. Interestingly, RNA encapsulation was enhanced when cationic amino acids were added as co-solutes. Collectively, our results suggest that the chemical diversity of the primordial environment not only influenced RNA replication but may have played a vital role in the formation of stable, functional protocells essential to the origin of life.
