The Bible tells us that the Earth was a very different place back in Noah's time and there were several factors which allowed people to live extremely long lifespans.
Back before the flood, it did not even rain in that time:
Genesis 2:5 – “Neither wild plants nor grains were growing on the earth, for the LORD God had not yet sent rain to water the earth and there were no people to cultivate the soil. (6) Instead, springs came up from the ground and watered all the land.”
However, In Genesis 1:6 we are told that God did place vast amounts of water above the sky, however this water could not have been in the form of regular clouds – as it did not rain:
Genesis 1:6 – “And God said, ‘let there be an expanse in the midst of the waters, and let it separate the waters from the waters.’ (7) And God made the expanse and separated the waters that were under the expanse from the waters that were above the expanse. And it was so."
The NIV translates 2 Peter 3:5 in this way – “They deliberately forget that God made the heavens by the word of his command, and he brought the earth out from the water and surrounded it with water.”
This trend of no rain happening was still occurring in Noah's time, because when Noah and his family exit the Ark God sais:
Genesis 9:13 – “I shall set my bow in the clouds and it shall be for a sign of a covenant between me and the earth.”
Before that time there would have been no rainbows, because large scale precipitation was not happening and therefore no rain or clouds to cause rainbows in the sky.
Also we are told in Hebrews 11:7 - "But by faith, when Noah was warned about things that had never been seen, he built an Ark to save his family."
Things like rain and storms would not have been seen before in Noah’s time.
Genesis 7:11 – “In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened.”
A floodgate or a window is something which typically holds air or water in or out of somewhere. These windows ‘opened’ unleashing the vast reservoirs of water that were suspended above the earth’s atmosphere; which were a major source of the water that caused the flood.
Genesis 7:12 – “And rain fell on the earth forty days and forty nights.”
This is the first mention of rain and it fell in such quantities that it took 40 whole days and nights’ non-stop for it to cease falling.
This layer of water above the sky mentioned in Genesis 1:6 could very well have been suspended in space just outside the earth's atmosphere, where the water would be frozen as ice crystals. The planet’s gravity would be holding it in place, similar to Saturn's rings which are also composed mainly of ice crystals. But this layer would have been a shroud that completely surrounded the earth.
Ice is of course translucent and so the suns light would be able to easily pass through, but at the same time this layer would be filtering much of the harmful radiation coming from space. The light and heat at ground level would probably have been less intense back then, but also the vapour canopy would stop heat escaping into the atmosphere making the planets temperatures far more moderate and less extreme than what they are today.
This water canopy would also condense the air beneath it, increasing the air pressure at ground level. This meant lots more oxygen per square inch for an organism to take in at one time. This is called a hyperbaric condition. It promotes rapid growth and large size as well as rapid healing and cell regeneration.
In October 2006 Science Daily publicized a study led by Arizona State University staff entitled “Giant Insects Might Reign If Only There Was More Oxygen in the Air.”
“Recently, USGS scientists have used a gas QMS to determine the oxygen level of ancient samples of Earth's atmosphere from a most unlikely place - amber.
…Minute bubbles of ancient air trapped by successive flows of tree resin during the life of the tree are preserved in the amber. Analyses of the gases in these bubbles show that the earth s atmosphere once contained nearly 35 percent oxygen compared to present levels of 21 percent.”
This combined with the fact that harmful radiation from space which mutates cells was virtually non-existent; animals, plants, people and other organisms could live up to ten times longer than what is possible today!