Genetic Code Evolution

The quotation came from this article, "Is a Bigger Genetic Code Better? Get Ready to Find Out | Quanta Magazine". It gave a very clear explanation on how cells work.

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Take any organism on earth, and its DNA and RNA have four nucleotide bases, or letters (usually abbreviated as A, T, C and G in DNA; in RNA, another base, U, takes the place of T). Those letters constitute an alphabet that ultimately spells out how to make proteins. But for that to happen, the cell first has to read and translate that alphabet, using a set of rules - the genetic code - to decipher its meaning.

Basically, the cell's protein-making machinery reads a sequence of DNA as a sentence composed entirely of three-letter words called codons. Codons name amino acids to add sequentially to a protein. With four nucleotide bases at the cell's disposal, 64 codons are possible: One to six codons specify each of the 20 natural amino acids most commonly used, and three tell the cell to stop building the protein.
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Is there any proof that genetic code with 20 amino acids evolved from simpler codes? The quote form this article "The Early Evolution of the Genetic Code: Cell" said that now they may be able to answer that question.

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In the absence of evidence, many of the most interesting questions about the genetic code have fallen into a twilight zone of speculation and controversy. Although it is generally accepted that the modern code evolved from a simpler form, there has been no consensus about when the initial code evolved or what it was like, how and when particular amino acids were added, how and when the modern tRNA/synthetase system arose, or the processes by which the code could have expanded. Now, detailed study of the components of the translation apparatus is at last making these questions tractable.

Three general approaches have recently yielded surprising intimations about how the genetic code evolved. The first is to appeal to general principles at a primary level, in this case the chemistry of nucleic acids and amino acids, to infer how a translation system might be constrained. The second is to alter parts of the translation apparatus in vitro in ways that might reflect earlier states, showing what changes are possible. The third is to examine the phylogeny of particular components, revealing how they have changed since the LUCA (or, in the case of paralogous genes, even before the LUCA), and to extrapolate backward from the principles thus revealed. Here we show how key applications of these approaches begin to provide a general framework for understanding the origin and development of the code.
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