According to Genesis 1:26, God created man in His image and in His likeness. At creation, Adam and Eve were free from sin. But then Satan appeared in the garden. Eve was deceived and disobeyed God. Thus God cursed both the man and the woman and cast them out of the garden.
Men did not look at the chaos caused by the first act of disobedience and cease from sin. Instead, men grew terribly wicked. Cain killed his own brother. Immorality was embraced and men pursued temporal inventions over their Creator.
At the fall, something changed within man. The evidence of this is present throughout Genesis 4, but in Genesis 5:3, it is proved:
When Adam had lived one hundred and thirty years, he became the father of a son in his own likeness, according to his image, and named him Seth. (Genesis 5:3)What had changed within man? Adam and Eve were created in the image of the perfect God, but Seth was made in the image and likeness of fallen Adam.
From this, it is plainly evident that Adam came into existence in a different condition then those men born after the fall. Adam was created directly in the image of the perfect God, but every man since the fall has been conceived in the image of sinful man.
Natural Death - Genesis 5:4-5
The Genealogy Introduced - Genesis 5:1-2
The Fall: Merciful Curses - The Christian Worldview
Why Is Sin Unavoidable? (Part 1) - The Christian Worldview
Summary of Genesis 4