Figure 1.

A proposal for the origin of tRNA through the ligation of a hairpin duplex catalyzed by an ancestral self-splicing group I-type intron based on proposals by Di Giulio [41], and Dick and Schamel [49]. In this depiction, the intron is shown as originating from a 3′ extension of one of the precursor hairpins formed by a transcriptional runoff error. aa indicates the amino acid binding site, but is not meant to imply that an amino acid was necessarily attached here during the intron ligation events.