Why did people in Genesis live such long lives?

While there is no specific biblical reason for this mystery, there’s a scientific theory that seems to reasonably answer this question. When God sent the flood upon the earth in the account of Noah, it describes the “fountains of the great deep” opening. This opening is likely in the "waters above" mentioned in the creation.

There is a scientific theory that this was a barrier of water surrounding the world. This barrier kept harmful radiation from entering the air, and gave the earth a much higher atmospheric pressure over what we have today. This is similar to the atmosphere in hyperbaric chambers that are used today in hospitals to promote healing. This barrier gave humans (and all creatures) a lifespan much longer than we see today. Not only do we read about the long lifespans of people in the Bible, but fossils have been found that indicate a much longer life years ago: a dragonfly with 3 foot wingspan, cockroaches 18 inches long, 6 foot long centipedes, grasshoppers 2 feet long, cattails 60 feet tall, beavers 8 feet long, and sharks 80 feet long (to contrast, the shark in the "Jaws" movie was 25 feet long).

When that barrier was broken, we can see a gradual decline in the length of life for humans in Scripture, and in the fossil record of life on earth. While this is just a theory, it seems to be supported in Scripture and in science.

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Genesis 1:6-7 — Then God said, "Let there be an expanse in the midst of the waters, and let it separate the waters from the waters."
God made the (Or {firmament}) expanse, and separated the waters which were below the expanse from the waters which were above the expanse; and it was so.

Genesis 5:10 — Then Enosh lived eight hundred and fifteen years after he became the father of Kenan, and he had {other} sons and daughters.

Genesis 5:16 — Then Mahalalel lived eight hundred and thirty years after he became the father of Jared, and he had {other} sons and daughters.

Genesis 7:11 — In the six hundredth year of Noah's life, in the second month, on the seventeenth day of the month, on the same day all the fountains of the great deep burst open, and the floodgates of the sky were opened.