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Each time the DNA replicates itself in cell division, it loses a little bit off the end of each telomere. When the telomeres become too short to form the knot during cell replication, the cell commits suicide. I'm not sure why, and I'm not sure if anybody is. But at any rate, in human beings this usually happens after 50-55 cell replications. And since cells generally replicate themselves approximately every two years, that's why practically no one lives past 120, and not past much that ever.
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The best theory I've heard why telomers exist is an anti-cancer mechanism.
Lets say a cell goes nuts, and starts replicating every minute.
At the same time, your immune system kills 25% of the cancer cells every minute.
So, every 2 minutes, every cancer cell has replicated twice, and there are twice as many.
In 1 hour, there will be a million copies of that cancer cell. But, it will have replicated 60 times. Oh my, it just hit the telomere limit! Bye bye tumor.
Human beings are, biologically, just carriers for our gamites (germ-line cells, sperm production cells and eggs). The rest of the body is intended to shut down rather than live forever, because reproduction is the important part. The telomeres help the body make it to reproduction, and clean up the waste humans that live past reproductive age.
We have gotten to a point beyond this. Evolution applies more to our culture and our minds and thoughts than it does to our biology. A human being who cannot have children still can influence the culture minds and thoughts of other humans, they are no longer a waste product.
A friend of mine is fond of the fact that over 50% of the humans who have ever lived are still alive. Gives him a decent chance at immortality, if that ratio keeps up. =)