It is widely agreed that the standard genetic code must have been preceded by a simpler code that encoded fewer amino acids. How this simpler code could have expanded into the standard genetic code is not well understood because most changes to the code are costly. Taking inspiration from the recently synthesized six-letter code, we propose a novel hypothesis: the initial genetic code consisted of only two letters, G and C, and then expanded the number of available codons via the introduction of an additional pair of letters, A and U. Various lines of evidence, including the relative prebiotic abundance of the earliest assigned amino acids, the balance of their hydrophobicity, and the higher GC content in genome coding regions, indicate that the original two nucleotides were indeed G and C. This process of code expansion probably started with the third base, continued with the second base, and ended up as the standard genetic code when the second pair of letters was introduced into the first base. The proposed process is consistent with the available empirical evidence, and it uniquely avoids the problem of costly code changes by positing instead that the code expanded its capacity via the creation of new codons with extra letters.

中文翻译：

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标准遗传密码可以从两个字母的GC密码演变而来，而不会造成信息丢失或昂贵的重新分配。

公认的是，标准遗传密码之前必须是编码较少氨基酸的较简单密码。由于该代码的大多数更改都是昂贵的，因此尚未很好地理解此简单代码如何扩展为标准遗传代码。借鉴最近合成的六字母代码的启发，我们提出了一个新的假设：最初的遗传密码仅由两个字母G和C组成，然后通过引入另外一对字母A来扩展可用密码子的数量。包括最早分配的氨基酸的相对益生元丰度，它们的疏水性平衡以及基因组编码区中较高的GC含量在内的各种证据表明，最初的两个核苷酸确实是G和C。代码扩展的过程可能从第三个基数开始，然后从第二个基数继续，直到将第二对字母引入第一个基数后，最终成为标准的遗传密码。所提出的过程与现有的经验证据是一致的，并且其独特之处在于通过放置而不是代码通过创建带有额外字母的新密码子来扩展其容量来避免代价高昂的代码更改问题。