It has therefore been suggested that the appearance of land life on a planet has to await the build-up of a sufficient layer of protective ozone. Thus, it is important to understand the origin and evolution of oxygen in planetary atmospheres. Two main phases can be identified in the rise of oxygen in planetary atmospheres. Phase One, which on Earth lasted about 2.4 billion years, involves the release of oxygen from the photochemical dissociation of water vapor. It is important to note that the duration of this first phase is determined by the intensity of the radiation from the star at wavelengths between 100 and 200 nanometers, since only this radiation has the right energy to dissociate water vapor.
Different stars have different intensities at ultraviolet wavelengths.
In particular, more massive stars are hotter and emit more ultraviolet radiation than less massive ones. In fact, I have shown that, in normal stars, the time needed for intelligence life to appear indeed increases significantly when the stellar lifetime increases. As explained above, the existence of such a relation implies that the likelihood of finding the two time-scales to be nearly equal is very high. Once again, please note that the detailed evolution of the atmosphere is surely more complicated than a simple dependence on the intensity of ultraviolet radiation. In particular, it may be that different phases in its evolution have different dependencies on the central star's properties.