Earth, a number of other non-peptide-based plausible types of counter-ion could have also existed, one of which may have been in the form of nucleotides or nucleic acids. There has been much speculation about the existence of nucleotides on early Earth, especially through investigation of the RNA World theory of the origin of life (also plausible would be a pre-RNA or an RNA-DNA World), and researchers have uncovered many ways by which such nucleotides could have been synthesized, including by spark discharge, on meteorites, or as a result of syntheses in cyanosulfidic environments. AMP, ADP, and ATP have all been observed to form in mineral pores, suggesting that nucleotide mono-, di-, and tri-phosphates could have plausibly existed. Once formed, nucleotide monomers, in particular RNA monomers, could then potentially have polymerized through a variety of prebiotic mechanisms such as through dehydration synthesis, clay mineral catalysis, non-enzymatic template-directed polymerization via activated nucleotide monomers, or even lipid-assisted polymerization. It is possible that the early RNA molecules eventually could self-replicate, similar to modern-day viroids, which are small, circular RNA molecules that infect plants and replicate autonomously, not using their own proteins but rather the proteins of the host for replication. Similarly, the discovery of self-replicating RNA molecules, or replicators, that can evolve in the absence of proteins further lends to the possibility that RNA-based replicators could have given rise to the first viruses. Virus-like RNA could have facilitated the transfer of genetic material between RNA, helping to select for more efficient replicators, contributing to the continued evolution of organisms, and culminating in the DNA world. For example, viral-like particles consisting of RNA molecules surrounded by a protein coat, similar to modern viruses, could have arisen spontaneously in the prebiotic environment and could have acted as intermediates in the evolution of more complex life forms; some systems using in vitro peptides and proteins (although not prebiotically plausible) have been demonstrated. The ability of virallike particles to transfer genetic material between RNA molecules or compartments encapsulating RNA could have also facilitated the emergence of more complex replicators and helped to select for more efficient self-replicating RNA molecules. However, the role of viruses in the evolution of life is still not universally accepted. For example, it is unknown how viruses could have emerged in the absence of host cells (which are required for viral replication), which could not have existed on early Earth, although protocells could have existed and acted as primitive hosts. Additionally, it is unknown how viruses could have evolved into more complex structures, for example eventually giving rise to cells. The evolution of cells requires the development of complex cellular machinery, such as membrane-bound organelles and the cytoskeleton, which is simply not observed in viruses. However, while the exact nature of their involvement in the origin of life is not yet clear, there is evidence to suggest that viruses may have played a role in the evolution of primitive RNA and the development of complex genomes. Nevertheless, while there are still many unanswered questions regarding the plausibility of nucleic acids on early Earth, such as degradation or sustained replication, it is still very likely that nucleic acids emerged and were abundant at some point during the progression of early life.