You have to remember, there was no McDonalds, Taco Bell or cigarettes at this time.
I believe Genesis 1 and 2 are sequential (see recent posts for details) and that Adam and Eve were directly created by God, kind of a crazy notion because that is what it says in Genesis 2:7. God could have made Adam to live for any length of time. I feel that this was intentional is that longevity would be a genetic competitive advantage over man created in Genesis 1.
Also not that God limits the lifespan of man to 120 years in Gen. 6:3 Then the LORD said, “My Spirit shall not strive with man forever, because he also is flesh; nevertheless his days shall be one hundred and twenty years.”
How old would he have to have lived for you to believe it? We are just arguing about a number.