Our musings about the timing and selection pressures on the evolution of CO2 signal transduction pathways and mechanisms of CCMs are unlikely to progress further until we have identified and characterized at the molecular genetic level more of the key components involved in these two processes. Towards this aim we are in the process of using infrared thermal imaging to identify Arabidopsis mutants that display aberrant stomatal behaviour in response to elevated concentrations of CO2. In addition to these molecular data we also require better proxies for past atmospheric CO2 concentrations, especially in the Precambrian.