IIRC genes related to DNA replication are highly conserved; thus our DNA repair mechanism was probably evolutionarily "optimized" for conditions a long time ago

IIRC genes related to DNA replication are highly conserved; thus our DNA repair mechanism was probably evolutionarily "optimized" for conditions a long time ago. Have background radiation levels remained constant on timescales of hundreds of millions of years? I don't know, but I wouldn't be surprised if it varied significantly considering that cosmic radiation is a significant source and the strength of the Earth's magnetic field fluctuates significantly.