The ancient Sun and biogenesis

In this paper evidence is presented that there is a large drop in the surface activity of G-dwarf stars at an age of about 109 years. There are at least five indications that this was also the case for the sun, itself a G-type star. It is suggested that at a critical period in the past the then accentuated UV radiation may have generated HCN in the atmosphere, which, as a result of rain, would fall on the Earth's surface, leading in turn to biogenesis. Before the transition period of approx. 109 years the solar X-ray flux would have been high and the HCN photo-dissociated. (author)