“The most ancient (protein) molecules were involved in the interconversion of nucleotides. But they were not synthesizing them,” Caetano-Anollés said. “We see that all the enzymes that were involved in purine synthesis, for example, were very recent. Since these first proteins benefited the formation of building blocks for the primitive RNA world, it makes a lot of sense that we’ve found this origin encased in nucleotide metabolism.”