Aging to death after likely sexually reproducing had long ago most usefully evolved in eukaryotes. This had become advantageously highly important for producing needed inheriting young. This had allowed more frequent descendants to thus sometimes evolve quicker, to thus gradually remain a surviving species.
Once developed beyond an embryo, eukaryotes had in their replicating somatic (body) cells like fibroblasts, (but not germ cells nor stem cells), a chromosome DNA ends shortening characteristic. This restricted such cells to ultimately be able to produce only a limited number of successive healthy cell generations as adults aged. Yet, some aging was needed to occasionally acquire rare random accidental cell chromosome errors in a few reproductive cells, contributing gene alterations, a rare few being beneficial for advancing most important slow gradual natural evolution.
Evolution had long ago developed this important nature, most critical for having evolution continue vs. competing species, thanks to produced new young generations. Such a helpful evolved nature forced limited ages, then death for excessively aging adults, by having chromosome DNA ends shortened each time somatic cells multiplied, due to not fully duplicating the previous length and number of telomere ends there to protect cell chromosomes. Thus the final generations of such somatic cells in adults aging too much would end up also losing ends of good DNA from their repeatedly shortened chromosomes, including ends of proper gene regulating DNA sections, such ends required to be able to keep cells and an adult living healthy.
This became something like an ever more destructive cell ticking time bomb. After too many ticks, the time bomb could fatally ignite. More cells reproduced during increasing body age would result in ever more shortened chromosome flaws, importantly causing death if some adult in that species was living too long.
But cells becoming cancerous often unfortunately switch on a gene that then expresses the chromosome-ends protective enzyme telomerase, which is able to keep them producing chromosome-ends protected bad new expanding cancerous cell generations, (instead of future cancer cell generations loosing ever more from their chromosome ends to most helpfully render them ultimately unable to forever keep harmfully multiplying in forever expanding bad cancers in one's body).
Our required good most importantly proper species reproductive cells had their chromosomes telomere ends protected by telomerase vitally active in them, adding a proper fresh number of protective telomere ends on new reproductive cell generations of their chromosomes, for starting our needed new reproduced descendants, thus resetting to zero in them such a cell ticking time bomb, to let them then live long enough to mature and produce their own future new quite healthy fresh young, to thus keep the species surviving and continuing, (while also slowly sometimes gradually most helpfully competitively evolving).
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