Fall of man
The Fall of man is the Judeo-Christian theological concept that mankind and the universe existed in a perfect and ideal state prior to a 'fall' triggered by the actions of Adam. In this state, death and suffering did not exist - Adam and Eve were, for practical purposes, immortal - as were all animals. Predation did not occur - animals that are now carnivorous existed solely on a diet of plants.
The one flaw of this perfect world was Satan, who was able to trick Eve into consuming a fruit she had been forbidden by God from eating. She then gave the fruit to Adam, who ate it even though he knew he was doing wrong. 
The immediate effect of this was to introduce Sin into the universe. No longer perfect, but tainted by sin, the perfect universe rapidly decayed with the introduction of death, predation, disease, pain, and all other consequences of sin.
By man came death... in Adam all die. (1 Corinthians 15:21-22)
By one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned. (Romans 5:12)
The creation waits in eager expectation for the sons of God to be revealed. For the creation was subjected to frustration, not by its own choice, but by the will of the one who subjected it, in hope that the creation itself will be liberated from its bondage to decay and brought into the glorious freedom of the children of God. We know that the whole creation has been groaning as in the pains of childbirth right up to the present time. Not only so, but we ourselves, who have the firstfruits of the Spirit, groan inwardly as we wait eagerly for our adoption as sons, the redemption of our bodies. (Romans 8:19-23)
Since God's original creation was "very good," we can safely state that deleterious genetic mutations only came about after the Fall. These mutations gradually built up over time, resulting in gradually decreasing human lifespans  as recorded in Genesis 5.
Young-earth creationists have determined the fall accounts for various apparent contradictions with scientific dating mechanisms which assume an approximately uniform rate of change. Genetic dating, for example, would vastly exaggerate time spans prior to the Fall as in the perfect sinless state genetic mutation would be minimal.
We know that the law is spiritual; but I am unspiritual, sold as a slave to sin. I do not understand what I do. For what I want to do I do not do, but what I hate I do. And if I do what I do not want to do, I agree that the law is good. 17As it is, it is no longer I myself who do it, but it is sin living in me. I know that nothing good lives in me, that is, in my sinful nature. For I have the desire to do what is good, but I cannot carry it out. For what I do is not the good I want to do; no, the evil I do not want to do—this I keep on doing. Now if I do what I do not want to do, it is no longer I who do it, but it is sin living in me that does it... What a wretched man I am! Who will rescue me from this body of death? Thanks be to God—through Jesus Christ our Lord! Romans 7:15-25
Given sufficient time, this accumulation of sin will become so great as to require the reconstruction of the earth, with the second coming of Christ.
- ↑ 1 Timothy 2:14: "And Adam was not deceived, but the woman being deceived was in the transgression."
- ↑ http://www.answersingenesis.org/articles/2008/06/04/did-people-live-over-900-years
- ↑ Genesis 5:2, Hosea 6:7