Scientific consensus is that we now recognise and diagnose autism better than ever. Previously children that struggled in school would be labelled as troubled or slow or any number of other things. The thing about autism is that like many other things it is a spectrum, and thus previously many people with mild autism would have just cruised through and been thought of as odd or antisocial. Often when really questioned, people like your parents can think of a few people like this from their school days that might now fit the definition of autism spectrum disorder.

Also, it’s worth noting that human DNA does not and cannot degrade in any manner you suggest and that kind of reasoning has unscientific and innapropriate connotations that might associate you with very disagreeable groups.