Thank you for the question, “Are the lifespans in scripture neither factual nor metaphorical?
If one does not accept that the lifespans in Genesis as factual or metaphorical, then one as implying by that question that it is a fable. It’s not just that the lifespans of humans that is in question, but the persons and even the Genesis account.
I would like for one to consider from the Biblical perspective of the longevity of humans:
A. If the Genesis account is accurate, there was a body of water protecting the earth’s atmosphere from the sun’s ray. Thus the human body was subject to less decay.
It is common knowledge that the sun emits Ultraviolet A (UVA) and Ultraviolet B (UVB) radiation (in addition to other radiation spectra). These rays create free radicals throughout your skin, damaging DNA, cellular structures, proteins, and connective tissue such as collagen and elastin. If the sun rays were limited by the body of water, then the decay on human body would be slower.
B. If the Genesis account is accurate, the earth had a tropical like weather condition.
The weather pattern prior to the Flood was different than today as there were no hurricanes, tornadoes, snow or even rain. The earth was tropical in weather which would imply the condition of the earth’s atmosphere. Adam and Eve didn’t need clothing to keep warm.
Biologists has found palm trees and tropical plants in Antarctica as well as the Sahara Desert was once a grassland or tropical weather. The earth’s climate was tropical being more conducive for productivity and inhabiting the earth.
C. If the Genesis account is accurate, the rate of decay and or human pollution was much slower than it is today.
After the sin of Adam and Eve, the ground was cursed. The productivity of the earth was slowed with mutations beginning. The beginning of mutation of plants affected humans, animals, and insects as death spread on earth. As humans killed animals for food and clothing, other wild animals and insects feed on the remaining caucus. Germs and diseases began to form. In general, disease was foreign to the human body. It was introduced into the human body, not a natural process of it. The Tree of Life may have been a fruit that would have reverse any decay or disease on humans.
D. The longevity of animals.
If the Genesis record is correct with a perfect environment, there is the possibility that the lifespan of animals and humans would be longer. The Greenland Shark has lived to 400 years and the Bowhead Whale with an average lifespan of 210 years. With all the ocean waters polluted and animals are still able to live that long, is it possible that they could have lived longer then than now since there were limited ocean pollution?
E. The search for the aging DNA.
It would seem that if the earth’s atmosphere prevented harmful sun rays or extreme weather patterns, the natural food supply of humans was organic and healthy, and the body mostly absent of any decay or diseases, wouldn’t human live longer lives?
I came across this article, ““For the first time we are seeing a molecular marker of aging (telomere length), a metabolic marker of aging (fasting insulin levels) and a cardiovascular marker of aging (blood pressure and blood vessel stiffness) all tracking in the same direction in that these individuals were generally protected from age-related changes,” said Vaughan. A paper describing this discovery was recently published in Science Advances.
As research continues into the DNA/RNA of the human body, researchers may find the the causing of aging that leads to death. Is it possible to live beyond 200 years old? It seems impossible now, but who knows what science will discover or will be able to repair so that man can live longer. For now God has limited human life to 120 years. Whether researchers may prolong life beyond 120 seems likely to me. That is a personal opinion with transplants and gene repairs.
SUMMARY: Humans want to live longer and healthier lives whether it is 50 or 500 years old. It may not be as fanciful as one thinks as humans wants to restore the earth to a cleaner environment.
For further perspectives:
https://www.quora.com/Are-the-lifespans-in-scripture-neither-factual-nor-metaphorical