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It's probably just misconstrued pop science, but I thought a read an article recently that said there's no known limit on how long humans can live. I could have sworn though that there were a few automatic processes that took place though, like that the chromosomes all shorten in length every time they're copied (is there any limit to that? Also, why does that matter?), the retinas in the eyes harden, the metabolism slows down, the heart muscles wears out, etc. So, how could it be true?

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closed as primarily opinion-based by David, kmm, another 'Homo sapien', James, AliceD Jul 5 '17 at 21:45

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ I'm not sure this is a very informative question without the original offending article. Specifically I think the key is known limit. We here of people living to increasingly long lengths these days, and with regenerative medicine, this could go on for some time. We don't, and probably can't, know a definitive maximum age currently - even if one exists. $\endgroup$ – Joe Healey Jun 30 '17 at 8:53
  • $\begingroup$ The quote is something akin to "ageing is plastic, and not a natural law". However, without the source material, it's hard to know how to answer this question. This video on ageing from one of my lecturers at undergrad might interest you. $\endgroup$ – James Jul 4 '17 at 9:36
  • $\begingroup$ Mm, no, it doesn't matter what the source material is here. It's a typical scenario, so if someone actually is accredited they will have the knowledge to speak to the phenomena either way, the same way a physicists doesn't need reference material if someone asks "are energy and mass really equivalent?" $\endgroup$ – RayOfHope Jul 4 '17 at 16:06
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You're probably reading about the recently-published responses to a publication that argued there is a limit to human lifespan.

The original article is Evidence for a limit to human lifespan, and in the June 29 issue of Nature there are five responses to it:

Each of these responses has, in turn, a reply from the original authors.

The arguments turn on fairly intricate details of statistical analysis and database interpretation, and I think it's fair to say that outside experts remain unconvinced either way -- neither the original article, nor any of the five responses, nor any of the five responses to the responses, presents a slam-dunk case for or against a limit to human lifespan.

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  • $\begingroup$ "It's therefore pretty pointless to discuss mechanisms how lifespan may or may not be limited, since it's entirely unclear if lifespan is limited and that's not likely to change in the near future." - You might reword this to make clear you are talking about the context of this particular question. Understanding whether lifespan is limited from a research standpoint will almost certainly involve understanding the mechanisms first. $\endgroup$ – Bryan Krause Jun 30 '17 at 18:31
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    $\begingroup$ I've removed that paragraph until I think of a better way to phrase it $\endgroup$ – iayork Jun 30 '17 at 18:35
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There will be no "limit" to human lifespan, if for humans the "survival function" $l(x)$- the probability, for all subsequent ages $x$, that a newborn will still be alive at age $x$ - never hits $0$, but only asymptotes towards it. (The survival function is estimated by the fraction $l_x = \frac{N_x}{N_0}$, of a group of newborns, who in a cohort study are seen to still be alive at age $x$. For finite cohorts the fraction surviving of course will eventually hit $0$, when the last member dies, even if the idealized survival function does not.)

This is the case for "Gompertzian" survival functions, where $l(x)$ decreases super-exponentially, rapidly approaching but never hitting $0$.

(Gompertzian survival functions are more usually described in terms of their age-specific, instantaneous relative rate of decrease [the survival function's slope at age $x$, divided by its height at age $x$, times -1], which is also called the hazard rate or force of mortality $\mu_x = - \frac{{N'}_x}{N_x}$. For Gompertzian survival functions, the force of mortality grows exponentially with age.)

Human survival functions are roughly Gompertzian throughout most of adult life, although the force of mortality's growth may decelerate in very late life.

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  • $\begingroup$ This type of logic doesn't apply to this problem. I agree with the responses in @iayork's answer that the original paper has some flaws, but you can't just say "because this model applies at ages we have observed, therefore this is true." $\endgroup$ – Bryan Krause Jun 30 '17 at 22:18

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