Abstract
Spark-tube experiments and analysis of meteorite contents have led to the widespread notion that abiotic organic molecules were the first life components. However, there is a contradiction between the abundance of simple molecules, such as the amino acids glycine and alanine, observed in these studies, and the minimal functional complexity that even the least sophisticated living system should require. I will argue that although simple abiotic molecules must have primed proto-metabolic pathways, only Darwinian evolving systems could have generated life. This condition may have been initially fulfilled by both replicating RNAs and autocatalytic reaction chains, such as the reductive citric acid cycle. The interactions between nucleotides and biotic amino acids, which conferred new functionalities to the former, also resulted in the progressive stereochemical recognition of the latter by cognate anticodons. At this point only large enough amino acids would be recognized by the primordial RNA adaptors and could polymerize forming the first peptides. The gene duplication of RNA adaptors was a crucial event. By removing one of the anticodons from the acceptor stem the new RNA adaptor liberated itself from the stereochemical constraint and could be acylated by smaller amino acids. The emergence of messenger RNA and codon capture followed.
Highlights
Propositions [1], followed by the original Miller-Urey [2] and later spark-tube experiments [3,4], along with meteorite content analyses [5,6], have been interpreted as evidence for a heterotrophic origin of life
Johnson and Wang [14], who have analyzed amino acid side chain-ribonucleotide interactions in the ribosome, concluded that the genetic code may have developed in two stages: a first stage when prebiotically available amino acids formed peptides using a code that was not influenced by anticodon-amino acid interactions and a second stage when these interactions became essential and the code expanded
Concept it is assumed that the first abiotic amino acid synthetic pathways were later replaced by proto-biosynthetic ones [27], an obvious requirement for life to become independent of highly contingent external factors
Summary
Propositions [1], followed by the original Miller-Urey [2] and later spark-tube experiments [3,4], along with meteorite content analyses [5,6], have been interpreted as evidence for a heterotrophic origin of life. As we will see below, views concerning the nature of an initial genetic coding or its absence are highly variable, it is generally agreed that the extensive involvement of RNA in contemporary protein synthesis strongly suggests it played a fundamental role in the very early stages of peptide formation. Consistent with this view RNAs are thought to have appeared very early on Earth both as self-replicators and catalysts [8]. These RNAs should have been capable of undergoing Darwinian evolution catalyzing many relevant reactions in a “RNA World” scenario [10]
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