In the beginning God created two people who were perfect in all ways. They were Adam and Eve, made in the image of God. Then they rebelled against the living God by disobeying the one commandment he had given them—eating fruit from the tree of knowledge of good and evil. I don’t think the fruit was poisonous, since there was nothing in the creation that was, but this was a test of their obedience to the Creator. For without a choice Adam and Eve would have been just ‘robots’.
As a result of their action they were cursed, as was the whole creation (Romans 8). One result of the Curse (the Fall) was degeneration in humans. This would have included errors in the genetic code, not corrected for when copies were made every time cells are replaced in our bodies. (Before the Curse God sustaining power would have corrected any thermodynamically induced copying errors.) As a result the offspring of Adam and Eve, of which there were many, would have in many ways been less than perfect, physically and mentally. They then passed these inferior traits down to all their offspring, which includes all the human race, to us.