Ever heard of the second law of thermodynamics? Everything is moving to an increasing state of disorder. Things like harmful mutations, ingesting toxic substances, etc can have effects on the DNA...

So how long ago do DNA-plus-creationists suspect Adam and Eve lived? I believe the bible-literalists' figure, from adding up all the begats, is about 6,000 years. It seems unlikely that enough chance mutations to explain the colossal variance in the human genotype, could happen in so few generations.

Incidentally, this has nothing to do with the second law of thermodynamics.

Peter