Multiple lines of evidence point to one or more moderately nearby supernovae, with the strongest signal at ∼2.6 Ma. We build on previous work to argue for the likelihood of cosmic ray ionization of the atmosphere and electron cascades leading to more frequent lightning and therefore an increase in nitrate deposition and wildfires. The potential exists for a large increase in the prehuman nitrate flux onto the surface, which has previously been argued to lead to CO2 drawdown and cooling of the climate. Evidence for increased wildfires exists in an increase in soot and carbon deposits over the relevant period. The wildfires would have contributed to the transition from forest to savanna in northeast Africa, long argued to have been a factor in the evolution of hominin bipedalism.
Adrian L. Melott and Brian C. Thomas, "From Cosmic Explosions to Terrestrial Fires?", The Journal of Geology 2019 127:4, 475-481