In an early chemical environment, chiral and non-chiral molecules are thought to have populated the earth (left, top), including sugars (that could be "left" or "right"). Ultimately, D-sugars (dextro or "right") were incorporated into RNA that, in turn, selectively incorporated L-amino acids (levo or "left") in the critical aminoacylation reaction. In the experiments of Tamura and Schimmel, an amino acid was joined to an RNA minihelix that mimics a portion of the modern RNA system. The aminoacylation reaction is an essential first step for making proteins.