Figure 1
An ancient origin for reverse transcriptase. An early origin of a cellular reverse transcriptase is posited by the RNA world hypothesis (Darnell and Doolittle 1986) (left). The widespread existence of RT genes in prokaryotes, in eukaryotes, and their great molecular diversity (represented by the size of the boxes at the tip of each branch) also suggests an early origin for the RT gene. Tree diagram adapted from Eickbush (1994).
