The way it was explained to me when I was studying the Bible with Jehovah's Witnesses was this (keep in mind they believe in the original sin):
Adam and Eve were created perfect. When they sinned against God, they lost that perfection, which included living forever on earth. As each generation came along, their lifespans became shorter and shorter, which is why if you read the genealogy in Genesis, you see each generation living for a shorter period of time than the one before. The Bible does concur that Moses lived 950 years in Genesis 9:29.
Granted, I don't believe in the original sin anymore...
i also don't believe in the original sin
so what if their life generation becoming shorter is due to the decrease in their understanding to the absolute truth..?
as generation drift away from the correct connection (salat) to Allah SWT by means of spiritual and physical understanding of the absolute truth
they severe their harmony with nature and themselves, so they have less guard (libaas) against pathology be it physical or spiritual
the more awareness of the absolute truth, the more chances that we implement(tend our choices/longing/love toward) them correctly
the better libaas against those pahtology, and the better life expactancy