Whether you hold to a young or old Earth view, it is important to realize that through one man sin entered the world. That man was Adam. An excerpt from Romans 5 (NASB):
12 Therefore, just as through one man sin entered into the world, and death through sin, and so death spread to all men, because all sinned—
14 Nevertheless death reigned from Adam until Moses, even over those who had not sinned in the likeness of the offense of Adam, who is a type of Him who was to come.
15 But 1the free gift is not like the transgression. For if by the transgression of the one the many died, much more did the grace of God and the gift by the grace of the one Man, Jesus Christ, abound to the many.
16 The gift is not like that which came through the one who sinned; for on the one hand the judgment arose from one transgression resulting in condemnation, but on the other hand the free gift arose from many transgressions resulting in justification.
17 For if by the transgression of the one, death reigned through the one, much more those who receive the abundance of grace and of the gift of righteousness will reign in life through the One, Jesus Christ.
18 So then as through one transgression there resulted condemnation to all men, even so through one act of righteousness there resulted justification of life to all men.
19 For as through the one man’s disobedience the many were made sinners, even so through the obedience of the One the many will be made righteous.
20 The Law came in so that the transgression would increase; but where sin increased, grace abounded all the more,
21 so that, as sin reigned in death, even so bgrace would reign through righteousness to eternal life through Jesus Christ our Lord.
It's clear that sin entered the world through one man. That sin is then inherited by the rest of humanity. It is critical in the understanding of the work of Christ on the Cross, and the necessity of His death. If sin did not enter the world the way Scripture indicates, then man did not need a savior. Without a requirement for a savior, there is no need for God's only Son to perish.
While we were not there for the creation, and we don't know scientifically what method God used to bring into existence all that is, nor do we know the details of every event that has occurred throughout history, we do know that there is a starting point, and that the first man was Adam, and through him sin entered the world.
Thank God that He provided recourse to the sin under which we have been born!