a, tRNA structure showing selected ribose and nucleobase modifications. The 3′-amino acid-acylated adenosine is located at the CCA 3′ end in contemporary tRNAs. 5-Methylaminomethyl uridine, mnm5U, is found in the wobble position 34. The amino acid-modified carbamoyl adenosine, (m6)aa6A (aa, amino acid), is present at position 37 in certain tRNAs. b, General RNA–peptide synthesis cycle based on mnm5U and m6aa6A. The structures of oligonucleotides are simplified and only terminal nucleobases are drawn.
It doesn't seem to have dawned on creationists who forlornly claim, despite the daily torrent of evidence to the contrary, that the Theory of Evolution is a theory in crisis. Far from the TOE being a 'theory in crisis', the reality is that it's the childish notion of creationism that's in crisis and which is being increasingly rejected even by religious people as a valid explanation for how life diverged. It's only by creative misrepresentation of the data, or by dismissing it out of hand, that creationist frauds are still able to pull the wool over the eyes of their credulous dupes, to maintain their cult’s superstitions.
One of their mainstays as an 'argument' which they invariably fall back on when confronted with unarguable evidence for evolution by natural selection and other mechanisms, is that none of the mechanisms explain the origin of 'life' itself (abiogenesis, life from non-life'), and without that, evolution would have nothing to work on. It is, of course, irrelevant to the mechanisms by which life diversified once replicators did get going, but, as usual with childish notions, the relevance of using the false dichotomy fallacy is lost on creationists who lack the critical thinking skills to spot the trick, and few if any of them could define the term 'life' in a scientifically meaningful way, anyway, so assume it means some magic ingredient that separates organic' molecules from inorganic ones. It is one of the last remaining gaps which they insist must contain their god and their belief is that, of all the gaps closed by science, this will be the one where a god is found. Their ever-shrinking little god has degenerated to the size of this one remaining gap. The magician that magicked 'life' from non-life and made them 'speshul'.
There are, of course, many possible alternative explanations for how a simple replicator could have got going and, indeed, there doesn't even need to be a single explanation since two or more replicators could have got together early on. If the result had given them an advantage over remaining separate, then this symbiosis would have been almost inevitable.
One of the more popular hypotheses amongst biologists is the 'RNA world hypothesis' that says, because RNA is still the functional unit of biological activity and it is simpler than DNA, it probably came first and DNA followed on as an evolved improvement, acting as RNA's data store and so became the functional organ of inheritance of genetic data.
This is the idea currently being expanded by a team at the Ludwig-Maximilians-Universität München, Germany. Their work has just been published, open access in Nature. They believe they have solved one of the outstanding problems with the theory - how RNA began catalysing the production of chains of amino acids to form peptides and short proteins without the peptides it codes for - a classic chicken and egg conundrum.
Like all chicken and egg conundrums, if this scenario is correct, it turns out that neither of them came first; they both emerged simultaneously and part of a developing complex out of simple pre-existing precursors.
As the Ludwig-Maximilians-Universität news release explains:
Investigating the question as to how life could emerge long ago on the early Earth is one of the most fascinating challenges for science. Which conditions must have prevailed for the basic building blocks of more complex life to form? One of the main answers is based upon the so-called RNA world idea, which molecular biology pioneer Walter Gilbert formulated in 1986. The hypothesis holds that nucleotides – the basic building blocks of the nucleic acids A, C, G, and U – emerged out of the primordial soup, and that short RNA molecules then formed out of the nucleotides. These so-called oligonucleotides were already capable of encoding small amounts of genetic information.
As such single-stranded RNA molecules could also combine into double strands, however, this gave rise to the theoretical possibility that the molecules could replicate themselves – i.e. reproduce. Only two nucleotides fit together in each case, meaning that one strand is the exact counterpart of another and thus forms the template for another strand.
In the course of evolution, this replication could have improved and at some stage yielded more complex life. Genetic material, as we understand it today, is made up of double strands of DNA, a slightly modified, durable form of macromolecule composed of nucleotides.The RNA world idea has the big advantage that it sketches out a pathway whereby complex biomolecules such as nucleic acids with optimized catalytic and, at the same time, information-coding properties can emerge.
Professor Thomas Carell, project leader
Department of Chemistry
Ludwig-Maximilians-Universität (LMU) München, Germany
However, the hypothesis is not without its issues. For example, RNS is a very fragile molecule, especially when it gets longer. Furthermore, it is not clear how the linking of RNA molecules with the world of proteins could have come about, for which the genetic material, as we know, supplies the blueprints. As laid out in a new paper published in Nature, Carell’s working group has discovered a way in which this linking could have occurred.
To understand, we must take another, closer look at RNA. In itself, RNA is a complicated macromolecule. In addition to the four canonical bases A, C, G, and U, which encode genetic information, it also contains non-canonical bases, some of which have very unusual structures. These non-information-coding nucleotides are very important for the functioning of RNA molecules. We currently have knowledge of more than 120 such modified RNA nucleosides, which nature incorporates into RNA molecules. It is highly probable that they are relicts of the former RNA world.
The Carell group has now discovered that these non-canonical nucleosides are the key ingredient, as it were, that allows the RNA world to link up with the world of proteins. Some of these molecular fossils can, when located in RNA, “adorn” themselves with individual amino acids or even small chains of them (peptides), according to Carell. This results in small chimeric RNA-peptide structures when amino acids or peptides happen to be present in a solution simultaneously alongside the RNA. In such structures, the amino acids and peptides linked to the RNA then even react with each other to form ever larger and more complex peptides.In this way, we created RNA-peptide particles in the lab that could encode genetic information and even formed lengthening peptides.
Professor Carell
It’s possible that there never was a pure RNA world, but that RNA and peptides co-existed from the beginning in a common molecule.That was a very surprising discovery. It’s possible that there never was a pure RNA world, but that RNA and peptides co-existed from the beginning in a common molecule.
RNA developed slowly into a constantly improving amino acid linking catalyst. The RNA-peptide world thus solves the chicken-and-egg problem. The new idea creates a foundation upon which the origin of life gradually becomes explicable.
Professor Carell
The ancient fossil nucleosides are therefore somewhat akin to nuclei in RNA, forming a core upon which long peptide chains can grow. On some strands of RNA, the peptides were even growing at several points. As such, we should expand the concept of an RNA world to that of an RNA-peptide world. The peptides and the RNA mutually supported each other in their evolution, the new idea proposes.
According to the new theory, a decisive element at the beginning was the presence of RNA molecules that could adorn themselves with amino acids and peptides and so join them into larger peptide structures. This relationship between RNA and peptides or proteins has remained to this day. The most important RNA catalyst is the ribosome, which still links amino acids into long peptide chains today. One of the most complicated RNA machines, it is responsible in every cell for translating genetic information into functional proteins.
More details and diagrammatic representations of this plausible scenario are given in the team's open access paper published in Nature:
AbstractFor poor creationists, it sometime must seem like the whole of science is refuting them - which is does of course because, unlike creationism, science has a solid foundation in real-world facts which were unknown to the fearful, primitive authors of their favourite fairy tale from the infancy of our species.
The RNA world concept1 is one of the most fundamental pillars of the origin of life theory2,3,4. It predicts that life evolved from increasingly complex self-replicating RNA molecules1,2,4. The question of how this RNA world then advanced to the next stage, in which proteins became the catalysts of life and RNA reduced its function predominantly to information storage, is one of the most mysterious chicken-and-egg conundrums in evolution3,4,5. Here we show that non-canonical RNA bases, which are found today in transfer and ribosomal RNAs6,7, and which are considered to be relics of the RNA world8,9,10,11,12, are able to establish peptide synthesis directly on RNA. The discovered chemistry creates complex peptide-decorated RNA chimeric molecules, which suggests the early existence of an RNA–peptide world13 from which ribosomal peptide synthesis14 may have emerged15,16. The ability to grow peptides on RNA with the help of non-canonical vestige nucleosides offers the possibility of an early co-evolution of covalently connected RNAs and peptides13,17,18, which then could have dissociated at a higher level of sophistication to create the dualistic nucleic acid–protein world that is the hallmark of all life on Earth.
Müller, Felix; Escobar, Luis; Xu, Felix; Węgrzyn, Ewa; Nainytė, Milda; Amatov, Tynchtyk; Chan, Chun‐Yin; Pichler, Alexander; Carell, Thomas
A prebiotically plausible scenario of an RNA–peptide world
Nature 605, 279–284 (2022). https://doi.org/10.1038/s41586-022-04676-3
Copyright: © 2022 The authors. Published by Springer Nature Ltd.
Open access
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
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