NeuralClone, on Mar 29 2010, 10:15 PM, said:
gsmonks, on Mar 27 2010, 03:12 PM, said:
Not so fast! Single-celled organisms which divide are also immortal.
That isn't the same thing at all. That's a form of reproduction. Single cell organisms still have a life span.
Well, in many ways, unicellular organism *can* be equally immortal. The 'limit' to life span is generally "senescence" (aging), which is actually something that is added in more complex organisms, and not found in more basic life forms.
The real distinction you may be thinking of is that when a single celled organism reproduces by fission, the original "organism" arguably ceases to exist. However single celled organisms don't *have* to reproduce, and many can live indefinitely, especially at low metabolic rates. Viable bacteria, etc. (not just spores) have been found in newly opened pyramids, and reproduction takes more metabolic feedstock and energy than <cue BeeGees> (ah ah ah ah) Staying Alive
Furthermore, just because an organism is unicellular doesn't mean it has to reproduce by fissioning. Many yeasts, for example, can reproduce by budding, in which there is a clear distinction between the daughter cell and the parent cell.
Though single-celled, yeasts are not bacteria, or even prokaryotes. They are eukaryotes, like us. In many ways, they are quite advanced. In fact, the evolutionary divergence between Schizosaccharomyces pombe (a fissioning yeast discovered in African Millet beer) and Saccharomyces cerevisiae (the main European brewing and baking yeast, which reproduces primarily by budding) has been said to be comparable to the divergence between either of those species and Man. Certainly they diverged from each other before there was any (known) life on land.
I mention this because many yeasts do have quite "advanced" cellular mechanisms, and some seem to experience a form of senescence [I don't know which ones, so I hesitate to single out any specific yeast as "immortal"]
Bacteria, on the other hand, have DNA replication and repair that is about 1000x more error prone than eukaryotes, so while a single bacterium *might* live for milennia or indefinitely, it would be quite different genetically than it originally was (and might die from accumulated genetic damage). Is it still the same "individual" that it once was? That's a question for deities far above my pay grade. I get confused enough by simple things like trees: Two different branches on the same 100-year old oak may have more genetic differences (and differ more from the trunk) than two separate saplings from two different acorns from the same branch. Imagine if your left and right hands were more genetically different from each other and your feet than you were from your brother.
The article alludes to this distinction, and it seems to draw the conclusion that because the jelly fish can revert to a more juvenile morphology it somehow overcomes the wear and tear effects (like genetic mutation) that every living organism experiences. I see no evidence that this is true, and indeed it would require a very radical (and fundamental) reworking of basic biochemistry and cell biology to do this. It's hard to see how one species/genus/whatever could develop such a successful mechanism, without throwing off evolutionary sibling branches with similar ability (and survive -- after all mutation is essential to evolutionary adaptation in the long term)
Based on what the article says, I could only conclude that the organism had an unusual life cycle, cycling between forms that are typically called juvenile and adult in other organisms -- not that it was immortal. Think of "adulthood" as something that we are merely *accustomed* of thinking of as fixed and permanent -- like gender. There are many known organisms where individual routinely change gender, often many times in a life, including "large" animals [where "large" is defined as "Hey! look at that X across the street"-sized, a size standard that the great majority of animals fail to meet]
This is really more an issue of not "maturing" permanently (a phenomenon commonly documented in spouses). To actually see if they age, you'd have to watch them for a long time, and I should note that Turritopsis nutricula has only been known for 153 years, and was not well studied (it was recently discovered to be several distinct species).
In point of fact, the reversion to the polyp state has only been seen to be triggered in a lab, never in the wild, and I have underwear that is older than the oldest known individual Turritopsis, whether in the wild or in a lab.
Dark Jedi, on Mar 30 2010, 07:53 AM, said:
Immortal? Cool! Does that mean they can't be destroyed?
Nah, in the real world they don't live very long at all.
BTW: T. nutriculae are much smaller than the photo suggests. 5mm [the size of the smallest watch battery made] is the upper end of their size range. You could fit over a dozen of even those 'big boys' on a dime
...even a Canadian dime
--Orpheus "Canadian dimes are larger than US dimes, and my oldest underwear are rarely found in the wild or in labs"