Genesis 1:7 New International Version

"So God made the vault and separated the water under the vault from the water above it. And it was so."

The waters above, some Creation Scientists say, were a thick cloud layer which, prior to the 40 days of rain and flooding, was a shield against the UV rays of the sun.

This protection from the typical DNA disrepair which results from UV light allowed not only long lives but in those creatures which have malleable, more cartilaginous skeleta like reptiles allowed for enormous form - dinosaurs predate the flood by this estimate though are not necessarily concurrent with humans. 

Consider that before the flood, according to our best extant record, people lived 1000 years.  Before the lands and trees were washed away there were many good and safe things to eat.

After the waters fell from heaven the Lord allowed poor Noah, bereft of all produce, to eat meat. Thus was cancer likely introduced to the earth.  UV light directly causes DNA mutation in all creatures exposed & thusly tainted meat enters our gene pool.

Challenge - how to revert to this 1000-year lifespan?  Ostensibly, Avoid the sun.  Don't eat meat.  But our gene pool is very dilute to be sure with transposable cancer vectors.  How can they be identified and removed?