I remember an interesting treatise on immortality, a concept that has been around as long as there has been people.
The ancient Greeks had some interesting stories about goddesses making handsome men immortal. The first just did so, but could not inhibit his aging, so he eventually withered to the point of becoming cricket-like. The second goddess wanted a different man to retain his beauty, so she turned him into a statue.
More recent stories go deeply into things like the decision making process, and how mortality makes it easy. An immortal needs an alternative means to force decisions, or they will become inert, never doing anything.
The campy, science fiction movie Zardoz illustrated that with immortality, people reach the limits of their capabilities relatively quickly, which they cannot evolve beyond. But evolution effectively ends the immortality of an individual, because they do not remain the same.
Even with an ordinary life, so much information is recorded by the brain that a few times people have been seen going into a “mental erase” mode, eliminating a vast number of unimportant memories to make room for more.
Then there is the immortal jellyfish, a real thing. Turritopsis dohrnii is a species of small jellyfish which is found in the Mediterranean Sea and in the waters of Japan. It is unique in that it exhibits a certain form of “immortality”: it is the only known case of an animal capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary stage.
http://en.wikipedia.org/wiki/Turritopsis_dohrnii
And do not think that this jellyfish can only do this because it is a simple organism. Some jellyfish have over 200 chromosome pairs, so they are more complicated than humans with a mere 46 pairs.
Oddly enough, though science is putting a lot of research into extending adult life, this jellyfish technique may be far more practical, that is, for people to revert to a far more flexible stage, perhaps just a few years old, using a stem cell ‘flood’ to rebuild themselves as young children.
However, it is very unlikely that either knowledge or personality would be retained in the process. So the body would be “somewhat” immortal, but the self would be forfeit in the process. It will anyway, with mortality.
Obsolescence is built into our DNA, with the telomere chains which shorten with each mitosis of the cells. When the telomere reaches the end of the chain,, the cells die off with fewer and fewer healthy new cells to replace them and maintain the vitality of the body. Death follows.