There are some other theories abut how Adam lived to 900. One theory is that God created man and it was good. The human DNA was uncorrupted at that time, but over time, small mutations (which are pretty much ALWAYS negative and bad) accumulated in humans which reduced the lifespan.
Another theory is that there was a canopy of water/ice over the earth that shielded people from cosmic rays and deleterious effects.
I lean towards the first one, but am not dogmatic on the issue.
WHY CHOOSE One of the "theories?" It seems likely that there is a combination of influences.
It is evident that there was a STEEP drop off after the Flood, which makes me think that conditions in the world after the flood were less conducive to long life.
The longest recorded age was Methuselah who died in the year that the Flood came. I think that he died and then the flood came.
Point is though, that there was no lessening of life spans in the years leading up to the Flood.
There are TWO factors then that the relate to the Flood being the watershed event:
Geo-physical/Environmental Factors:
What was going on in the earth immediately following the Flood? Catastrophic events continued. The surface of the earth shows DOZENS of impact craters which would have occurred in the few hundred years immediately following the Flood. Also there was continuing tectonic activity with major mountain building and movement of plates until finally, sometime after Babel there was a wider separation of the continents as to make migration more difficult. It had to be easier immediately following the Flood, and it had to continue for scores of years to enable the dispersion of the the wide spectrum of animal kinds to the Western Hemisphere and to Australia. But eventually the distances made continued dispersion unfeasible. Dispersion would only take place as a result of botanical rafts after a certain point of rapid continental separation. The point with respect to longevity is this: The living conditions on the the earth were barely survivable: quakes, meteor-strikes, tectonic activity, volcanism, clouds of ash, ice age.
Another environmental factor may be an increase of harmful solar radiation due to a change in the Earth's atmosphere, or even due to a reduction of the force of the Magnetic field which serves as a radiation shield.
A second factor is the genetic "bottleneck" that occurred when the human population was reduced to just THREE reproducing couples. There is no suggestion from Scripture that Noah had additional children, so that it why the number of producing couples should probably be placed at three. Now the longevity begins to drop off. Although even at the END of the book of Genesis people are still living well beyond 100 years. Physical deficiencies tended to be reproduced due to the problem of close genetic ties in the families of the earth after the Flood. The Holy Spirit has clearly declared that ONLY 8 people survived the Flood, and it seems that of these 8, only six were reproducing, and the three males were brothers. The longevity gene was somehow recessive perhaps.
Combine these two factors with the multiplied effects of sin and lifespans have now shortened to about 70 or 80 years as a rule.
