As I mentioned earlier, Isaac de la Peyrére was a Catholic priest who published his understanding of the scriptures in the seventeenth century, that people just like us lived long before Adam and Eve. I find his writings especially interesting because he reasoned his entire case from New Testament Scripture.
He argued eloquently in "Men Before Adam" that a literal interpretation of Romans 5:12-14 clearly indicated the world was populated before Adam. The key was verse 13: "For before the law was given sin was in the world: but sin is not imputed when there is no law." Peyrére reasoned that the "law" there referred to by Paul was the "law" given to Adam shortly after his creation, and if there was "sin in the world" before that "law" was given to Adam then there must then also have been people to commit that sin.
Peyrére wrote: "... it must be held that sin was in the world before Adam and until Adam: but that sin was not imputed before Adam; Therefore other men were to be allowed before Adam who had indeed sinned, but without imputation; because before the law sins were not imputed."
Peyrére argued that although men and sin were in the world before Adam, the manner of sin was in the form of offenses against nature, violations of "natural law," and all died a natural death. It was not until God imposed moral law, with Adam the first to be subject to it, that men were capable of "legal sin," trespasses against God's law. Beginning with Adam's Fall, human beings die both a natural death and a "legal" or spiritual death.
Today most Bible commentaries avoid any implications that all of humanity did not commence with Adam by saying that the "law" referred to in Romans 5:12 was the law of Moses. But if Mosaic law, and not Adamic law, was intended by Romans 5:13, it would mean that sin was not charged before Moses, and that imputation of sin was through the law of Moses, but that it somehow applied retroactively to Adam and his
descendants. Peyrére railed against this understanding of Romans. Peyrére argued that since the law transgressed was the law given to Adam of Genesis, the sin spoken of in Romans was sin perpetrated by those who co-existed and pre-existed Adam. Sin was not imputed to those forerunners, however, until Adam disobeyed God's law.
Peyrére wrote: "Before the Law of God, or till that Law of God was violated by Adam, sin and death were in the world, yet had gained no power over it : they had got no lawful possession, they had got no absolute power. The reason is, because before that time there was no Law given by God."
Clearly, Peyrére argued, sin was imputed from Adam to Moses. What brought the flood? Was the flood not judgment for sin? Or for that matter, what about the destruction of Sodom and Gomorrah? And if the "law" referred to in Romans 5:13 was Adamic law, the sin that we are there told "was in the world" before that law was given must have been sin committed by people who lived before Adam and Eve. People from whom Cain found his wife.
http://en.wikipedia.org/wiki/Isaac_La_Peyr%C3%A8re