The RNA world hypothesis: the worst theory of the early evolution of life (except for all the others)a

Abstract

The problems associated with the RNA world hypothesis are well known. In the following I discuss some of these difficulties, some of the alternative hypotheses that have been proposed, and some of the problems with these alternative models. From a biosynthetic – as well as, arguably, evolutionary – perspective, DNA is a modified RNA, and so the chicken-and-egg dilemma of “which came first?” boils down to a choice between RNA and protein. This is not just a question of cause and effect, but also one of statistical likelihood, as the chance of two such different types of macromolecule arising simultaneously would appear unlikely. The RNA world hypothesis is an example of a ‘top down’ (or should it be ‘present back’?) approach to early evolution: how can we simplify modern biological systems to give a plausible evolutionary pathway that preserves continuity of function? The discovery that RNA possesses catalytic ability provides a potential solution: a single macromolecule could have originally carried out both replication and catalysis. RNA – which constitutes the genome of RNA viruses, and catalyzes peptide synthesis on the ribosome – could have been both the chicken and the egg! However, the following objections have been raised to the RNA world hypothesis: (i) RNA is too complex a molecule to have arisen prebiotically; (ii) RNA is inherently unstable; (iii) catalysis is a relatively rare property of long RNA sequences only; and (iv) the catalytic repertoire of RNA is too limited. I will offer some possible responses to these objections in the light of work by our and other labs. Finally, I will critically discuss an alternative theory to the RNA world hypothesis known as ‘proteins first’, which holds that proteins either preceded RNA in evolution, or – at the very least – that proteins and RNA coevolved. I will argue that, while theoretically possible, such a hypothesis is probably unprovable, and that the RNA world hypothesis, although far from perfect or complete, is the best we currently have to help understand the backstory to contemporary biology.ReviewersThis article was reviewed by Eugene Koonin, Anthony Poole and Michael Yarus (nominated by Laura Landweber).

Улази Таксона У Нове Еколошке Нише- Адаптивне Радијације

“…Схватања слиједа развоја генетичке информације немају поуздану аргументацију, али откриће саморепродукујућег приона (Prusiner, 1982), поново може да подржи став да су бјеланчевине биле носиоци првих насљедних основа, и да су (обрнутим процесима у односу на савремену транслацију) довеле до РНК-свијета (Gilbert, 1986, Bernhardt, 2012, а затим до ДНК-фазе у постојању Bios-а (као јединственог биотичког система, истовремено и таксона и синтаксона). Унутар РНК-свијета очекивати је да постоје предачко-потомачки затворени системи, који одговарају врсти као основном таксону.…”

The life story of hydrogen peroxide II: a periodic pH and thermochemical drive for the RNA world

“…The RNA world hypothesis for the origin of life is well supported [5], although there are several major questions with it that have been regarded by its opponents as fatal flaws and by its proponents as problems that will be resolved by modelling and experiment. One such question concerns the necessity for thermal cycling, in the non-or proto-cellular milieu, to separate double-stranded RNA into single-stranded templates for replication, and the erstwhile lack of any realistic mechanism for it was acknowledged as a frustrating roadblock for advancement of the hypothesis [6][7][8].…”

A ribonucleopeptide world at the origin of life

“…Accordingly, other ribonucleoproteins (RNPs) such as the spliceosome and the RNase P could have faced the same evolutionary fate. In general, most ribozymes have limited catalytic resources and do not present any evidence of significant complexity (Lilley, 2003;Bernhardt, 2012 , 2016). Activated amino acids and peptides might have interacted with RNA molecules and possibly acted as co-factors for RNA replication and RNA-based catalytic reactions; thereby enhancing RNA stability, replication fitness and ribozyme catalysis.…”

“…This idea led to the formulation of the term "RNA world", and became the almost universally accepted hypothesis regarding the early stages of life and evolution on Earth (Joyce, 2002;Orgel, 2003Orgel, , 2004. However, several problems associated with the RNA world hypothesis emerged (Bernhardt, 2012;Blain & Szostak, 2014). The first disagreement came with the argument that the RNA polymer is too complex to have arisen from abiotic synthesis.…”