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As I understand it, single-celled organisms that reproduce via mitosis essentially live forever. When a cell divides, one cannot say that either one of the new cells "is" the parent and the other the child, neither is older than the other, and we might say that the original cell is still alive as long as either of the cells resulting from the division is. With humans the pictures is very different, of course: we have no trouble telling parents and children apart and no one lives longer than about 120-something years.

When in evolutionary history did organisms stop living forever? Was it with the advent of multicellular life with differentiated cells? Sexual reproduction?

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  • $\begingroup$ This is a similar question where the asker is confused about the difference between aging in yeast and in multicellular organisms. The comments back and forth got out of hand. $\endgroup$ – James Jul 4 '16 at 8:29
  • $\begingroup$ Whilst both a single-cellular and a multicellular both appear to age, this is only a rhetorical similarity. You are defining single-cellular organisms as immortal because the lineage doesn't die; this process occurs for the cells in multi-cellular organisms (we are all directly related to the ancestral cell -the lineage of cells is immortal) but the organism dies. I'm voting to close this question because it's unclear what you're asking given that there has never been an instance where an organism was immortal that mortal creatures evolved from (except that one damn jellyfish). $\endgroup$ – James Jul 4 '16 at 8:38
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    $\begingroup$ For the record, this is a good question in the sense that it highlights a common and easy to misunderstanding use of terminology since the media band around "immortal cells" a lot. However the SE Q&A isn't a good forum for ironing out the misunderstanding without a very specific question. $\endgroup$ – James Jul 4 '16 at 8:40
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    $\begingroup$ I think both @Roland and James are telling me that when a cell divides, we should consider the two resulting cells to have age zero rather than age equal to that of the original cell. Under this definition the question is, at best, poorly phrased. I suppose one way we might think about the daughter cells' age is by considering whether they inherit any age-related wear and tear from their parent. $\endgroup$ – kuzzooroo Jul 4 '16 at 17:14
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(Not an evolutionary biologist, but I couldn't resist :)

I don't think your premise is correct. In (symmetric) cell division, we should consider that the old cell "disappears" and two new cells appear; neither of the two daughter cells is the parent cell, they are both descendants of it (hence the term "daughter cells"). The parent cell is gone. Therefore, at least for symmetric cell division, the lifetime of a cell equals the length of a cell cycle. So cells are not immortal --- only the cell lineage is.

Asymmetric cell division (as in stem cell division, not budding) is more tricky. Here it might seem as if there really is an immortal cell (the "stem cell") generating "mortal" offspring. But I think the situation is really the same as in symmetric cell division: most of the cell cycle mechanisms are identical, and again two new daughter cells are generated, and the parent cells disappears. The asymmetric part is that only one of the two daughter cells are "permitted" to continue the line. Again, the cell line is immortal, the cell is not.

But this is still true in higher organisms: the germline is an immortal line, while the somatic cells are not. So the key difference between higher organisms that have a clear life span and unicellular organisms is the appearance of cells that do not generate offspring, but instead support the cells that do.

I'm not sure there is a sharp transition from single cells to the germline/somatic cell situation. Intermediate cases are found in all sorts of single-celled organisms where certain cells "decide" that it's better to support their (genetically identical) neighbor in some way to rather than divide themselves; these are a form of "somatic" cells. All sorts of community behavior in bacteria and other microorganisms could be viewed in this way. During evolution this behavior has gradually got encoded into the DNA itself, with more and more sophisticated "support systems". Higher animals are really just the latest version of this, where the entire body is really just a support system to ensure survival of the germline.

I would suggest reading Richard Dawkin's books on this subject, especially The selfish gene. They are well written and quite accessible.

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  • $\begingroup$ I too am not a specialist, but The Selfish Gene is pretty old fashioned and outdated often being referred to as one of the "blunders of biology". Eo Wilson who was amongst the developers of that theory and many others are outspoken on the issue of the "the selfish gene" mistake. $\endgroup$ – James Jul 4 '16 at 8:11
  • $\begingroup$ @James Say what? Who refers to The Selfish Gene as a blunder? Creationists? $\endgroup$ – Alex - Stop it SE Jul 4 '16 at 10:07
  • $\begingroup$ @Alex I thought this was old news no? There is a large movement of evolutionary biologists, and I'm sure even Dawkins has different views based on new observations. ...also, for different reasons, creationists! From io9 article "To be clear, biologists like West-Eberhard and others are not saying genes and their selfishness aren't important. But they are merely one part of a much larger and more complicated mechanism, which involves inputs from the environment which affect how genes express themselves." $\endgroup$ – James Jul 4 '16 at 10:10
  • $\begingroup$ ... From the mouth of the horse: twitter.com/RichardDawkins/status/… I have read the book cover to cover and there are indeed holes and examples that contradict the model. Paper from the journal of physiology expands on this more than I can! $\endgroup$ – James Jul 4 '16 at 10:18
  • $\begingroup$ @James That a model does not explain every single part of reality does not at all invalidate it. I've read the book cover to cover as well, and it's a fantastic book for anyone wanting to learn the core concepts of evolution, even disregarding the selfish gene theory. If you want to call it outdated, a blunder and a mistake, be aware of many bs-filters engaging. $\endgroup$ – Alex - Stop it SE Jul 4 '16 at 10:58

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