Why don’t we live as long as Methuselah?
And all the days of Methuselah were nine hundred sixty and nine years: and he died.Genesis 5:27
"Probably the most striking feature of Genesis 5 is that our ancestors lived about 10 times longer than people do today; the longest recorded is Methuselah, living for 969 years. This should be believed because God’s Word says it, and He was an eye-witness. Modern science is now shedding light on possible mechanisms.
Noah’s lifespan wasn’t shortened despite spending the last third of his life in the alleged ruined environment. The decrease started only with his descendants.
Indeed a genetic basis for longevity has been demonstrated in animals, e.g. fruit flies, such that genes for longevity can be lost from a population. Recent research on the accumulation of mutations in the human genome has provided further support for the idea.
One of the problems with old-earth belief is that humans add over 30 new mutations every generation. The overwhelming majority of these are not eliminated by natural selection. This should cause an exponential decay in fitness.
So if humans had been around for as long as evolutionists claim, we should have become extinct from the huge mutational load. That we have not become extinct is strong evidence that humans have not been here for longer than a few thousand years.
Recent advanced computer simulations vindicate this proposal, showing that an exponential decay of lifespans fits well with accumulating mutations after the catastrophic population bottleneck at the Flood.
This can be seen from comparing the decay curve produced by the computer simulation with the recorded lifespans from Noah to the present day. As shown, genome decay after a population bottleneck explains the general trend of lifespan decay after the Flood.
But what about Shem, born before the bottleneck, but he lived only ⅔ as long as most of his ancestors? (The lifespans of his brothers, Ham and Japheth, are not recorded.)
--There is of course the ever-present possibility in any individual of a non-aging-related cause of death such as disease or accident.
--But there is also a plausible genetic explanation: he was born when his father was 502, i.e. over half-way through his lifespan. His ancestors were much younger when they fathered their named sons. It has long been known that children born to aged mothers have a higher risk of developing non-hereditary genetic disorders such as Down’s Syndrome, and it is plausible that Mrs. Noah was about the same age as Noah.
But even if she were much younger, more recent research points to aged fathers as a major source of genetic disorders. This should not be surprising since men keep producing sperm throughout their lives from the division of stem cells (some 840 divisions by age 50).
In theory, the risks of spontaneous mutation increase with each round of sperm cell division, so the sperm of older men are more likely to carry mutations.
So it is not surprising that Shem, while very fit by today’s standards, would have been considerably less fit than his parents, and carried extra heritable mutations. So Shem and all his descendants had much lower lifespans than the pre-Flood patriarchs."
CMI
