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rhinocerosThree-digit code or even amino acids in general are not prerequirements for abiogenesis. RNA world without any proteins is feasible and has been confirmed in laboratory experiments where autocatalytic RNA sets formed spontaneously from short strands of RNA. That's abiogenesis for you..
Forgive me, but I do not think that those two realities address the exception issue, to your original astute observation. I am not sure I am aware of a theoretical pre-biotic primitive form of expressive life which did not base its provenance on the possibility of a coacervate; and a coacervate forms from proteins and non aromatic hydrocarbons. RNA is life yes, but we are hinging our context of discussion on the expressive-definition of codons, not simply the existence of carbon-prime spline based nucleotide replication.
RNA is single stranded, short sequenced, less robust, and utilizes uracil in place of thymine - nonetheless, RNA still employs a three digit codon and the same start and stop parameters. If it did not then transcriptase and tRNA could not perform their expressive function. RNA has indeed been constructed in absence of proteins and has been confirmed in laboratory experiments - however, this does not excuse the issue and we still run into the same difficulty as DNA: the issue of first-demand proteins being frame-locked into a 3 codon definition which is parametrized by the other 2 codon defined pre-biotic amino acids. In other words, all the pre-biotic protein assignments bear their archaic definitions.
Our first expressive replicating life, whether archaea or eubacter, employing a simple cytoplasmic genophore still employ a 3 digit codon, despite an ancient phylogeny and pre-chromosomal structure. So in theory here we have in abiogenesis, the collision between a replicating carbon-prime RNA structure and a cohesive coacervate in protein and hydrocarbon.
And in this theoretical abiogenesis, two of our top 3 pre-biotic amino acids assigned immediately to a 3 digit discriminated and framed dependency. The lesser proteins assigned of course to a 2 digit discrimination, as you astutely point out. It is possible then that natural selection simply never differentiated the final digit in those cases, because the third digit did not provide an evolutionary advantage, except in the case of aspartic and glutamic acids; not that the final digit did not exist.
This is still a question. (Not one which disproves evolution by any means - it does not, and that is not my point - I am simply pointing out that the logic has some violations which force us to a plural set of possibilities).