The answer is that the fall and our human sin nature are difficult to reconcile theologically with Darwinian evolution, or the first Adam being one of many hominids who descended from previous common ancestors. If there was no historical first Adam and no historical fall, there is no need for redemption, no need for a “second Adam,” and no need for Christ’s death and resurrection.
Adam and Eve were not created with a sin nature. They were created with free will, which gave them the opportunity to make real choices with real consequences. When they chose to disobey God’s one command to them, they became sinners by choice and by nature. And all of their human descendants are now sinners, both by nature and by choice. That means that our relationship with a holy and righteous God is broken.
But because of God’s great love, he provided a rescue plan by sending His Son, Jesus Christ, as a perfect sacrifice for our sins. Any person who accepts Jesus’ gift of salvation and forgiveness then has Jesus’ righteousness imputed to them, and their relationship with God restored. That means that, in the words of the old hymn, “God sees my Savior, and then he sees me, in the beloved, accepted and free.”
This whole biblical passage, although a bit long, is important, as it explains the consequences of the sin of the first Adam, and the contrast and result of the gift of second Adam, who is Jesus Christ our Savior.
Romans 12: 5-19 NIV–
Death Through Adam, Life Through Christ (NIV)
12 Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all people, because all sinned—
13 To be sure, sin was in the world before the law was given, but sin is not charged against anyone’s account where there is no law. 14 Nevertheless, death reigned from the time of Adam to the time of Moses, even over those who did not sin by breaking a command, as did Adam, who is a pattern of the one to come.
15 But the gift is not like the trespass. For if the many died by the trespass of the one man, how much more did God’s grace and the gift that came by the grace of the one man, Jesus Christ, overflow to the many! 16 Nor can the gift of God be compared with the result of one man’s sin: The judgment followed one sin and brought condemnation, but the gift followed many trespasses and brought justification. 17 For if, by the trespass of the one man, death reigned through that one man, how much more will those who receive God’s abundant provision of grace and of the gift of righteousness reign in life through the one man, Jesus Christ!
18 Consequently, just as one trespass resulted in condemnation for all people, so also one righteous act resulted in justification and life for all people. 19 For just as through the disobedience of the one man the many were made sinners, so also through the obedience of the one man the many will be made righteous.