| RNA was indeed the initial replicant, i.e. the first molecule that contained an arbitrary sequence of blocks and which could be used as a template to generate copies at itself. However, such a replicant could appear only after a life form with metabolism already existed. For replicating RNA, there must exist a complex system that extracts energy from the environment and uses that energy to synthesize nucleotides like ATP and then it uses additional energy to polymerize the nucleotides into RNA. RNA itself or any other molecule capable of replication could not have had any role in that living system, because before the existence of replication, any molecule of RNA that would have appeared accidentally would have disappeared eventually, without descendants. Therefore the first molecule of RNA that has survived must have been self-replicating and it could not have other functions. The first self-replicating RNA molecules have diverted resources from a pre-existing living being, by consuming nucleotides like ATP, which must have already been used long before the appearance of RNA, for implementing condensation reactions. In other worrds, the first RNA molecule, i.e. the first nucleic acid molecule was a virus. Some of the present viruses might have had their origin as detached parts from some nuclei of cellular beings, but it is likely that most viruses descend from the primordial viruses and they have never been cellular life forms. The cellular life forms must have appeared by symbiosis between a virus and a life form without nucleic acids. It is a frequently believed myth that life requires a memory molecule, like a nucleic acid. This is a mistake perpetuated by people unfamiliar with engineering. It is perfectly possible to have a chemical system that growths and replicates itself, without containing any molecule able to store arbitrary information, like a nucleic acid. The difference between such a chemical system and the cellular life forms of today has the same nature as the difference between a hard-wired processor and a microprogrammed processor, i.e. the nucleic acids play the role of the microprogram memory that controls the execution units of the processor, allowing the implementation of an arbitrary behavior by changing the sequence of microinstructions, while the hard-wired processor has a fixed behavior, which can be changed only by a redesign of its structure. Information-storage molecules like the nucleic acids are without doubt necessary for the evolution of complex living beings, because they allow the random generation of a huge number of variants that can explore the solution space, from which survival will select optimized variants. The nucleic acids have brought to living beings the same kind of flexibility that programmable embedded computers have brought to various appliances, whose properties can now be changed by a software update, instead of a costly recall and hardware redesign. In a "hard-wired" living being, evolution must be extremely improbable, because any change in some of its component molecules is likely to break the cycle of self-replication, leading to death without descendants. In a self-replicating chemical system, there must be a long chain of chemical reactions, each using as input reactants the products of the previous reaction, while the first reaction in the chain must use the products of the last reaction, closing the cycle. It is very likely that in such a self-replicating chemical system, peptides, i.e. relatively short chains of amino-acids, had a very important role in providing a scaffold that organized the chain of reactions. Even today, most if not all living beings still produce so-called non-ribosomal peptides, which, unlike the proteins, are not produced by templates of RNA. Unlike with the mechanism of protein synthesis by ribosomes, for now the mechanisms that establish the sequence of amino acids in non-ribosomal peptides are very poorly understood. It is likely that at least some of the mechanisms of synthesis of non-ribosomal peptides are a remnant of the synthesis mechanisms used before the appearance of RNA. |
But RNA alone is sufficient to template off itself and create new copies. It can fold into catalytic forms akin to folded proteins. It can even spontaneously generate into chains from the constituent monomers under certain assumed early earth conditions (1). There is a lot of literature behind this formation under various conditions. A lot of guestimates on what these conditions might translate to experimentally, but the general trend is that this seems to be possible under early earth conditions.
1. https://pubs.acs.org/doi/full/10.1021/acscentsci.5c00488
What is left is determining how the consitutent nucleotides might have formed. People have ideas on this though. This review is a bit old now but on topic at least: https://pmc.ncbi.nlm.nih.gov/articles/PMC6316623/#sec4-life-...