Timing

• To get some idea of the amount of time passed, a phylogenetic tree of the T7-like RNA polymerase genes was constructed.
• This tree contains a node at a known fixed point in time, based on the passage of T7-like RNA polymerase genes into eucaryotes during the mitochondrial endosymbiosis (~1.1 Bya). (Cermakian et al., 1997)
• Updated T7-RNA polymerase tree with cyanophage P60 added and saturation correction:

• The history of the host species is shown in purple.
• There is no evidence for radically unequal rates of divergence.
• The origin of cyanobacteria is dated by the oxygenation of the atmosphere.
• At least some host differences appear too great for the virus to jump.
• I.e., some host species split times may be usable to measure the divergence time between different viruses.
• The relationship between P60 and other T7 structural proteins is much closer than is the relationship of VpV262 and SIO1 to T7.  Hence VpV262 and SIO1 are billions of years diverged from T7.  So although some viral segments may undergo frequent rearrangment and substitution, other segments are among the most conservative arrangements of genes known in the universe.