Whatever it was I believe it can be likened to what happen to Eve and childbirth. Apparently prior to the fall Eve would have had children with little or no pain....after the fall God changed Eve physically....from what to what??? I don't know. Was it a change to the birth canal? Was it a change to Eve's hips? We don't know.
But, the bible tells us in Gen 3:16 there was some sort of change.
16 To the woman he said,
“I will surely multiply your pain in childbearing;
in pain you shall bring forth children.
This change...genetic...would now be part of Eves female progeny.
Prior to the fall Adam had no sin nature, or in other words a proclivity to sin. What exactly changed in Adams make-up....I don't know.
Was it a DNA genetic change? Was it a spiritual change? Was it some form of cognitive change? Was it a change in instinct? Perhaps someone here can shed some light on the subject and provide us with an answer.
Whatever happened doesn't change the fact humans now have a sin nature and need Christ Jesus.