The proposed RNA World is a linchpin concept in origin-of-life (OOL) research. RNA seems to conveniently solve the chicken-or-egg dilemma faced by scientists who observe double-stranded DNA coding for the very same protein-based enzymes responsible for its own replication, transcription, and translation processes. In contrast, single-stranded RNA seems to multi-task well, both replicating and providing enzymatic functions. Furthermore, different RNAs mediate critical steps in the process of protein synthesis from DNA; their pivotal intermediary roles assumed to be embedded traces of a past RNA World. Yet the RNA World does not fare well in bottom-up chemical evolution models, especially when carefully managed proof-of-principle experiments in pristine laboratory conditions are compared to the environmental conditions likely present on the early Earth.
The RNA World is a necessary destination on the broader naturalistic pathway called chemical evolution. Upon entry to this phenomenological place of great OOL promise, some assembly will be required. Herein lies many problems. First, water breaks down nuclide polymers, “casting doubt on any soupy version of the RNA world – [e]ven the synthesis of the four bases required as building blocks is not without serious problems.”1 The two known synthetic pathways to produce cytosine are unlikely to have existed in sufficient quantities on the early Earth. Competing chemical interactions would be problematic and the half-life of the desired product insufficiently brief.2 Spark-discharge experiments and meteorites (assumed analogues of the primordial landscape) have even failed to produce any cytosine.3
Next, consider production of the five-carbon sugar ribose required for the RNA molecule’s backbone. The only known mechanism to assemble long-chain sugars is the formose reaction, which produces over forty different sugars along with many more unintended products when contaminants are present. Ribose yield is low and its instability high. Rapid breakdown is inevitable as evidenced by the striking lack of sugars in meteorite samples.4
Finally, phosphates play a crucial role in RNA (and DNA) backbone structure as well as for adenosine tri-phosphate molecules that can generously liberate energy for necessary chemical reactions to take place. Proposed chemical routes to produce polyphosphates encounter typical OOL problems when taking the primordial environmental conditions into consideration: inadequate concentrations of reactants, low product yields, chemical interference, and/or rapid degradation. In addition, although is Earth’s crust in the most phosphorus-rich material in the universe, it bears an elemental abundance of only 1,000 parts per million. Ross and Rana note that “[w]ithout life molecules (already assembled and operating), no known natural process can harvest the amounts of phosphorus necessary for life from the environment.”5
Synthesis of RNA’s homopolymer backbone is contingent on precisely controlled laboratory conditions and a high level of researcher involvement,6 and the characteristic right-handed chirality of RNA sugars only occurs in living systems. Outside a cell, enantiomers almost always tend towards racemic mixtures in relatively short order.7 The purported evolutionary capabilities of RNA also depend on a high level of intervention,8 which better supports intelligent design than chemical evolution by naturalistic processes. Despite their many successes in the laboratory, even the researchers have yet to make truly, self-replicating RNA molecule.9
Given the seemingly intractable problems of the RNA World, as well as dissatisfaction with metabolism-first OOL scenarios, researchers now look back further to a Pre-RNA World to set the stage. Achiral peptide nucleic acid (PNA)10 or threose nucleic acid (TNA) alternatives appear to solve some problems but concurrently add layers of complexity to an already daunting challenge.
“It may be claimed, without too much exaggeration, that the problem of the origin of life is the problem of the origin of the RNA World.” — Leslie E. Orgel11
1 Paul Davies, The Fifth Miracle: The Search of the Origin and Meaning of Life (New York: Simon & Schuster Paperbacks, 2000), 134.
2 Fazale Rana and Hugh Ross, Origins of Life: Biblical and Evolutionary Models Face Off (Covina, CA: Reasons to Believe, 2014), 114.
4 Ibid, 115-6.
5 Ibid, 96.
6 Ibid, 119-120.
8 Fazale Rana and Hugh Ross, Origins of Life: Biblical and Evolutionary Models Face Off (Covina, CA: Reasons to Believe, 2014), 121.
9 Ibid, 121.
10 Ibid, 135.
11 Leslie E. Orgel, “Prebiotic Chemistry and the Origin of the RNA World,” Critical Reviews in Biochemistry and Molecular Biology 39 (March-April 2004): 99-123.
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