The chemical evolution leading to cellular life on earth almost four billion years ago likely passed through a stage where RNA alone performed all of the functions of the modern macromolecules RNA, DNA and protein. However the so-called RNA world was itself too complex to evolve directly from organic molecules found on the prebiotic earth. More likely, the RNA world emerged from and was supported by a primitive sort of metabolism fueled by the bonds in sulfur-containing compounds called thioesters.

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