Some might claim that like the secret of flying is missing the ground, the secret to immortality is simply not dying.
While this may seem like an over simplistic way of looking at the subject, it also happens to be true.
From Wikipedia: http://en.wikipedia.org/wiki/Turritopsis_nutricula "Turritopsis nutricula is a hydrozoan with a life cycle in which it reverts to the polyp stage after becoming sexually mature. It is the only known case of a metazoan capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary stage. It does this through the cell development process of transdifferentiation. Theoretically, this cycle can repeat indefinitely, rendering it biologically immortal until its nerve center is removed from the rest of the body." "Jellyfish usually die after propagating; however, the Turritopsis nutricula has developed the ability to return to a polyp state."
"The ability to reverse the life cycle is probably unique in the animal kingdom, and allows the jellyfish to bypass death, rendering the Turritopsis nutricula biologically immortal."
A hydra apparently, is another kind of cnidarian that is claimed to be immortal. I remember the hydra - I had to make a model of one out of plasticine for biology class once. This model took hours to make, and was based on a cross-section diagram in my biology text book. I remember it was about half a foot high, blue, and made funny marks on the class room walls when thrown by one of my regular class bullies. It looked pretty close to the drawing and aside from the blue color, must've been pretty life-like (scared the bejesus out of the cat) and was more successful than the previous semester's model, which was a miniature volcano for geography class. That one had a paraffin lamp in its base and filled the classroom with smoke when one of the plastic dinosaurs I added for dramatic effect caught fire, along with the bits of grass I had stuck in the plasticine river bank. All that aside, while I enjoyed biology and geography, I much preferred history - and certainly not just because I wasn't required to build any models as projects.
This concept fascinates me. Imagine an animal being born, growing up and maturing, having a good shag and then reverting back to its youthful state. Imagine a dog like that, for example. It would probably alternate between puppy and adulthood - or perhaps just stay youthful? They say such things only occur in "lower animals", so who knows how a "higher animal" with this ability would change in appearance? I could only imagine a daschund alternating between puppy stage and adulthood - with an action that would remind me of a black furry concertina and probably making a noise like a catalini as it did so. The advantages of having such a dog would be easily apparent (for one thing, they would last longer). The question is, would you have to re-house train them every time they revert to puppy-hood? Ah well, that is a topic for another day.
Moving on, perhaps the cnidrian (anybody seen my tonsils?) is the higher life form? I mean, it can live forever, while we just decay and die while it probably wouldn't be bothered much to notice. And since it is possible that this continued life cycle of theirs could theoretically be sustained virtually forever, doesn't this mean there could be creatures out there - even captured examples, living in your fish tank - that were around before Darwin started naming things people dug up in their back yards?
What would such a human be like for example? Let's say some genius figures out the necessary genetic modifications to allow people to do this. What significance would this have? I suppose people would have to start stocking a variety of different sizes of clothing in their closets to begin with. And I suppose there would be periods where you would have to stay off work, and call in "young" again - and forgo driving because you can't reach the pedals this month. Hmm. Could be rather inconvenient. On the other hand, imagine the agony of having to go through puberty every other month, the acne, the squeaky voice, the constantly varying bra sizes... Could this be a little vision of a perpetual hell of its own?
Come to think of it, as a transsexual woman who was once physically male I have gone through puberty twice in my life already - from opposite directions - and believe me, twice was enough for me. No thanks, if I were to be biologically immortal I would have to be certain that along with gray hair and wrinkles and saggy anything, I was done with puberty for good as well, and that my body would remain intact as is, and that everything would remain status quo - at least hormonally, and as far as my clothing sizes go.
While such a creature may be immortal, and the lure of immortality would be powerful indeed - I doubt anybody would find spending eternity as a hydrozoan an attractive prospect. But if I were a hydrozoan - or a cnidiran, then I might have enough time to learn how to pronounce "cnidrian" without blowing my tonsils out through my sinuses.
The older I get, the more I seem to appreciate the life I have - and the more grateful I am that I only have one and that it's quite enough for me. It would seem immortality, like everything, has a price of its own. The question we need to ask ourselves is, is it worth it? In the end, everything dies, without exception - and since that is the single constant in the universe, I find it curiously comforting.