The fidelity of reverse transcription in retroviruses provides a potential model for modulation of rates of evolution. In normal reverse transcription, the enzyme falls off its template at hairpin loops and other points of three dimensional contour change. This results in an increased rate of transcription errors at these fall-off and reattachment sites compared with stretches of template in linear flat arrays.

Reverse transcriptase is bound to its RNA template at a site that incorporates divalent metal cations. Chelation buffers of trivalent cations of similar cationic radius to magnesium used as an in vitro medium resulted in reverse transcription with the substitute ions. However, the rate of production of large transcripts was greatly accelerated and the production of smaller fragments was reduced. This yielded a much higher rate of cDNA production and an increased fidelity. This experiment therefore demonstrates that changes in cationic environment can alter the fidelity of transcription.

The Cambrian explosion in which most of the extant animal phyla appear relatively suddenly has been difficult to explain because there has been no obvious mechanism whereby a variety of separate lineages could simultaneously undergo accelerated rates of mutation for a limited period of time. The Cambrian transition involved a major environmental change that has been shown to have greatly altered the cationic composition of the ancient sea. We hypothesize that the mechanism we have revealed may be a reasonable model for how an environmental shift that alters the cationic composition of ocean water could trigger an increased rate of mutation in multiple separate lineages.