LisaRose and others,
There are DNA mutations due several factors one of it is UV light and other is natural radioactive decay. UV light on earth was never constant, some period it was much stronger causing mutation of DNA. Ozone layer also did not exist 600 millions year ago and prior, and there were several periods when it was destroyed due catastrophic even (supernova explosion or asteroid impact). UV does alter DNA which causes to species mutated. Natural decay does it as well. We are surrouned by it. The article from Curtis University states:
While radioactivity occurs naturally in our bodies as well as in every living organism across the planet, it was never before thought to affect our DNA in such a direct way. Using high-performance computers, the research team from Curtin and Los Alamos National Laboratory were able to show radioactivity could alter molecular structures which encode genetic information, creating new molecules that do not belong to the four-letter alphabet of DNA.
Of course there is someone who claims that DNA did not change nor there is mechanism to it. UV light, and alpha and beta decay are obviously irrelevant to that person.