It is both puzzling and profound to us today how the ancients attained to such an extraordinary age. 700 years? 900 years? I have a theory to propose with regard to this. One possibility is that Adam and Eve may have eaten from the tree of life. The text does not tell us they didn’t – and remember, they were permitted to eat of it until they were banished from the garden.
We may make the assumption that the fruit of the tree of life was made such that simply eating of it once would be enough to afford eternal life. But by its very nature, the concept of a tree, which continually produces its fruit, would seem to imply otherwise – that you would need to keep eating to continue preserving your life. In the book of Revelation, the tree of life we are told produces twelve kinds of fruit in each month (Rev. 22:2). To this end, it is quite possible that Adam and Eve ate of this fruit one or more times, and doing so may have given such health to their bodies as to give them many years of life. We do know that their offspring did not have access to this life-sustaining fruit, however we do not know of its effect on Adam and Eve’s bodies – perhaps the benefit was a type that was passed on through birth to their children several generations. This may explain the gradual lessening of years until the time of Moses, where the pinnacle of 120 years is established which was spoken of just before the flood.
Josephus, in his “Antiquities of the Jews” proposes the idea that this God-given long lifespan afforded the ancients the ability to learn and master those things which would become the foundation for all humanity. We do not know how much direct help was given to Adam and his relatives (via God or angels) with respect to things such as agriculture, construction, hunting, forging implements of various materials, and basic astronomy for the measuring of time, seasons and even travel. Josephus particularly notes that it would be quite difficult for one to learn astronomy without many years to work with, as you cannot hurry the stars and comets.
Another theory supporting the long lifespans is that before the flood, it was presumed there was some form of ‘canopy’ over the earth, which may have enabled foods to be much more nutritious and life-giving than the are today. During the flood, when the “floodgates of the sky were opened” (Gen. 7:11) this may have accounted for the atmospheric and/or climatic changes that would them adversely affect either the vegetation itself, or the lifespans due to the change in our bodily exposure to the sun’s rays.
Some have speculated that the life spans of the ancients were not in fact that lengthy, but rather were likely measured in other ways. Perhaps they considered every new moon a year for example. I reject this notion as the listing of descendants in Genesis 5 shows a gradual lessening of the lifespan, which continues throughout the entire book of Genesis. It is unlikely that a difference in how a year is measured could account for this gradual decrease.