I have previously mentioned that our DNA repair mechanisms appear to pick and choose where and when they make repairs.

According to a recent paper:

"Natural selection... acts on the very mechanisms that copy and repair DNA. It makes perfect sense that these mechanisms would be subject to selection pressures: genomes that managed to steer their mutations, even a little, would have pulled way ahead of others that were throwing darts in the dark."

"Certain types of DNA sequence seem to invite mutation and others seem to repel it. One type of mutation-attracting sequence, found in bacteria as well as in us, is "slippery" DNA. Slippery sequences comprise a series of DNA's chemical bases or "letters" repeated over and over again. "

"The mutation rate of slippery DNA can be 1000 times higher than elsewhere in the genome, so in that sense it is not random."

So... when we get a massive influx of cosmic rays, our bodies pick and choose which mutations to repair, and which to experiment with. When we have regions of the DNA that are 1000 times more likely to mutate, then the number of different mutations become less. Taken to an extreme, this means few enough mutations for a new species to arrive at the time of a cosmic ray bombardment!