I think there's a mix-up of the physical and the spritual here. Man has a lower state of a connection with God ... out of the Garden. It doesn't mean man's "state" can't go to a higher level in other, less important, areas. Language has advanced, technology has advanced, why couldn't physical complexity/adaptation advance? Heck, even religion can move to a higher state for man, as it did a couple thousand years ago. I don't see how that conflicts with a fall from grace.
It's merely man trying to be fruitful and multiplying ...
The fall brought about both sin and death according to scripture. It was physical as well as spiritual.
DNA is complex stuff and complex things break down over time, that's just entropy (the second law of themodynamics). Adam, Noah and others back then lived very long lives because they were not subject to the decay that has piled up over generations in our DNA. Physically we have decended, not ascended.