The Origins of the RNA World

Abstract

SUMMARYThe general notion of an "RNA World" is that, in the early development of life on the Earth, genetic continuity was assured by the replication of RNA and genetically encoded proteins were not involved as catalysts. There is now strong evidence indicating that an RNA World did indeed exist before DNA-and protein-based life. However, arguments regarding whether life on Earth began with RNA are more tenuous. It might be imagined that all of the components of RNA were available in some prebiotic pool, and that these components assembled into replicating, evolving polynucleotides without the prior existence of any evolved macromolecules. A thorough consideration of this "RNA-first" view of the origin of life must reconcile concerns regarding the intractable mixtures that are obtained in experiments designed to simulate the chemistry of the primitive Earth. Perhaps these concerns will eventually be resolved, and recent experimental findings provide some reason for optimism. However, the problem of the origin of the RNA World is far from being solved, and it is fruitful to consider the alternative possibility that RNA was preceded by some other replicating, evolving molecule, just as DNA and proteins were preceded by RNA.

Prebiotic selection and assembly of proteinogenic amino acids and natural nucleotides from complex mixtures

“…1a). [2][3][4][5][6][22][23][24] However, the plausibility of this proposed prebiotic synthesis of pentose aminooxazoline (1) has been questioned because it is contingent upon the strictly controlled sequential delivery of pure glycolaldehyde (2a) to cyanamide (3) to yield 2-aminooxazole (4), followed by pure glyceraldehyde (2b) to 2-aminooxazole (4) to yield the desired product (Fig. 1a).…”

“…Accordingly, it has been recognised that "the chief obstacle to understanding the origin of RNA-based life is identifying a plausible mechanism for overcoming the clutter wrought by prebiotic chemistry". [4][5][6][7] For example, the most-efficient and specific proposed prebiotic pathway to the pyrimidine ribonucleotides requires synthesis of the key intermediate pentose aminooxazoline (1) (Fig. 1a).…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] Proteins are built from a highly restricted set of about 20 amino acids according to a universal triplet code of four ribonucleotides. Therefore, it is essential to learn how this specific small constellation of molecules became irrevocably linked at the advent of life.…”

Viruses in Biology

“…Furthermore, the universally conserved ribosome responsible for protein synthesis was shown to be a ribozyme. These discoveries led to the development of the "RNA world" as a powerful model of early life evolution (for a review, see Orgel 2004;Robertson and Joyce 2010). RNA replaced viruses in origin-of-life thinking: nucleocentric views on the origin of life were no longer virocentric.…”

“…Consequently, this definition of life, if applied strictly, does not accommodate viruses either. A self-replicating ribozyme would be alive (Robertson and Joyce 2010), but a virus would not because it cannot self-replicate. Yet, because viruses evolve, many biologists consider them alive.…”