[muthas]

Based on how we humans use melanin, I would believe that this is a case of having these environmental-stress-reduction "tools" basically already in play (to deal with UV, free radicals, background radiation, etc.), then capitalizing on their functionality when a new source of stress (ionizing radiation) enters their world.

Not to suggest that the melanin with these particular fungi wasn't a wee bit unique to make it better at dealing with converting ionizing radiation usefully, or that natural selection hasn't taken place to improve the existing melanin for even better utilization of this abundant, high-energy source! I just don't think that the fungi created this from scratch after the exposure began.