They spiritual powers of evil quite likely achieved this evil purpose by altering human DNA. There is no doubt that human DNA has changed over time, but these changes are usually attributed to natural causes. I presume that some of the changes were made after humans were expelled from the garden and thousands and thousands of evil spirits were available to join together in attacking them.
The changes they made to the brains of the first couple would be passed on to all their descendants causing a change in human character and personality. This was too good an opportunity for the powers of evil to go by, as once the numbers of humans on earth increased, they would be spread much thinner, and less effective.
In the New Testament, the “old man” that causes us to sin is described as “the flesh” (Rom 6:19).
That which is born of the flesh is flesh (John 3:6).
The ones according to the flesh are inclined to the things of the flesh;
The inclination of the flesh is death,
The inclination of the flesh is hostile to God
Those who are in the flesh cannot please God
if you live according to the flesh you will die (Rom 8:5-8,13).
While we were in the flesh, the passions of sin… were at work in our members to bear fruit to death (Rom 7:5).
I know that nothing good dwells in me, that is, in my flesh (Rom 718).If we think of “the flesh” as the human body or brain, which we inherited from Adam, and which was damaged when he was attacked by the powers of evil, it would explain why the person who has not been saved by Jesus, struggles to do what is right.
Paul did not understand DNA or the working of the human brain, but he recognized that something in our body (flesh) that we have inherited from Adam has twisted our behaviour towards evil. Like the rest of the people of his time, he assumed that the driver of human behaviour was the heart, but we now know that the mind and the will are located in the brain. Therefore, thinking of the “the flesh” as a corrupted brain might be the best way to understand what Paul was attempting to explain to the Romans.