Figure 7.

Schematic model of how RNAs perform various ribosomal functions and/or encoding various ribosome-related proteins may have become ligated to form the first genome. The visual formalisms are the same as in Figure 6 and illustrate the idea that “genes” that may have existed as overlapping sequences in a variety of reading frames within a self-replicating set or rRNAs may have been separated out into a single reading frame for better functional control. This schema does not identify whether the original primitive genome was constituted of RNA or DNA (though RNA seems likely if a ribosomal origin is correct) but given a common origin for RNA and DNA polymerases (see text) the existence of either one could have provided the basis for the other. It is also important to remember that SINEs and other insertion-facilitating RNAs most likely evolved from tRNAs or that they have common ancestors (see text), so that molecular mechanisms of genome growth would have been present from the outset in a ribosome-first scenario.