There is already one milligram of atmosphere over the cross sectional area of a cell nucleus, so it doesn't seem like it could change much for cosmic rays. All

There is already one milligram of atmosphere over the cross sectional area of a cell nucleus, so it doesn't seem like it could change much for cosmic rays. All DNA in a human cell is about 6 picograms. That's 166 million times more mass of atmosphere. The elements may be less effective etc. but it still seems like any junk dna shielding would be negligible. Maybe cosmic rays aren't the main source of ionizing radiation throughout the evolutionary period though.