A model is proposed for the early evolution of the coding mechanism. A primordial RNA embodies the functions of today's nucleic acids in a single molecule. The molecule is generated by successive rounds of self-priming and-templating. After proximity is assured by enclosure in a cell, the functions can be partitioned among more efficient specializel molecules. The prediction of sequence homologies in later forms prompted a search for matches between t- and r-RNAs. These are described. Their distributions offer clues to their origins. The existance of overlapping homologies indicates an approach to the reconstruction of an ancestral molecule.

Many tRNAs of E. coli and yeast contain stretches whose base sequences are similar to those found in their respective rRNAs. The matches are too frequent and extensive to be attributed to coincidence. They are distributed without discernible pattern along and among the RNAs and between the two species. They occur in loops as well as in stems, among both conserved and non-conserved regions. Their distributions suggest that they reflect common ancestral origins rather than common functions, and that they represent true homologies.