i doubt DNA has anything to do with lifespan. Blind luck was the only thing giving any semblance of a long lifespan in those days when hygene was non existant and a common cold was a killer. Lifespan was the same, there are referances to individuals living to their eighties. But wether or not you reached the point where you'd die of age was not a good chance.
as to the theories of nemesis...
if we can assume that dna did NOT affect changes in death rates, then it must not have changed. if so, the the previously mentionioned "genetic purity" could not have changed either.
On the matter of the vapor canopy, where exactly did it go? Nemesis says it helped hyper-oxygenate the earth, but there could not have been anything there that wasn't previously, and if so, then it has to still be there now. "no matter or energy can be created or destroyed", barring nuclear physics. On the same topic, a hyper-oxygenated earth would not do one whit against a virus or bacteria, or lack of hygene. The only thing the oxygen would do is poision all the plants in the world.
I believe in God, I just call it Nature