Coded into human DNA is a limit on the maximum lifespan due to telomere shortening, these end caps protect the chromosomes, but as the cells reproduce they shorten !!
Telomeres and Human Lifespan
a) Telomeres shorten as we get older causing aging in our cells.
b) We inherit telomeres from our parents, but no matter the length of our telomeres at birth, everyone's shorter as they age.
c) Shorter telomeres have a negative effect on our health.
d) Telomere shortening is the main cause of age-related break down of our cells.
e) When telomeres get too short, our cells can no longer reproduce which causes our tissues to degenerate and eventually die.
f) Some cells like those found in the skin, hair and immune system are most affected by telomere shortening because they reproduce more often. ref:Sahin E, DePinho RA. Linking functional decline of telomeres, mitochondria and stem cells during ageing. Nature. 2010;464:520–528
This encoded short lifespan ( a maximum of 125 years by some experts' analysis ) dictates that in the Biblical genealogies of Genesis chapters 5 and 11 each of the long lifespans listed are those of the patriarch's total dynasty.