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The lifespan of an individual has great bearing on the future of the Species.

Species with smaller lifespan of individuals would have greater chance of evolving and adapting to the circumstances. On the other hand the shorter lifespan could lead to loss of the individual experiences and wisdom accumulated over the small lifespan to be able to propagated no next generations.

Species with longer lifespan of individuals on the other hand tend to be genetically rigid; lesser chance of mutation-adaptation, but more scope for inter-generation experience-knowledge transfer.

A similar question on Minimum lifespan to be able to be a viable civilization also explores some aspect of my query. But I wanted to have an opinion on what contributes more in the overall development of the Species in the long run? Evolutionary features or the Tools made by the experience-knowledge?

If you're given the task to adjust-calibrate the lifespan of Humans on Earth, what number of years/months/days would you deem appropriate? Given the mental and physical capabilities, i.e. Learning speeds, human strength etc. remain constant.

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    $\begingroup$ Creatures that reproduce quickly have more opportunities to evolve, but their total lifespan doesn't make any difference. A species that reaches sexual maturity after 1 year and then dies after 1 more year will evolve at the same rate as another species that reaches sexual maturity after 1 year and then dies after 100 more years. Lifespan and time to maturity tend to correlate, but they don't have to. $\endgroup$
    – Nuclear Hoagie
    Sep 17, 2020 at 16:24
  • $\begingroup$ I guess by giving more reproductive rate we're ignoring the population pressure on the ecosystem. But that's out of the scope of the question. $\endgroup$
    – Deepak Chaudhary
    Sep 17, 2020 at 16:40

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These two goals aren't mutually exclusive. More learning and persisting knowledge works great with long lifespans. Evolution acts faster when children are born quickly. You can have a new generation every 20 years and still have lifespans approaching a century.

The optimal solution IMHO is to maximise lifespan, and encourage teen pregnancy. Great grandparents, when they're starting to slow down at either work or academia, take on raising their great grandchildren, so their grandchildren can get their PHD and do their big research project.

Natural selection favours those who can most efficiently create children who go on to create more children. Nowhere in there does it require those to die. They can try again to produce favourable offspring, they can teach their descendants things, or they can go do literally any other thing - they've already put the wheels of evolution in motion.

But if they were mutually exclusive

You need a human to:

  • Develop and learn basic fundamentals (walk, read, write, count, minimum science, etc).
  • Reproduce.
  • Learn all knowledge in a speciality.
  • Perform research within that field.
  • Go through rounds of peer review improving their research.
  • And then repeat.

School and early learning could be tightened a lot, but we need to learn to socialise to, reproduce, research, and teach, so I wouldn't want to tighten this below 16 years.

Reproducing, I define as getting offspring ready to survive on their own, which is another 16 years, but that can be done in parallel to the other things part time, and could be shared responsibility with others (teachers, partner(s), babysitters), so I'm adding only 4 years for that.

Learning all you can about a speciality I'd class as getting a PHD - so 7 years.

Doing a research project, getting it reviewed, and released, could be anywhere for 6 months to 10 years. Lets call it 10 years. You may get 1 big project done, you may get 20 little ones done. I'm also guessing a bit high because your first work is rarely your best, and once you're acquainted with a subject it makes sense to spend some time as the expert (rather than do one project, and then let the speciality stagnate for another 20 years until another expert comes).

Sum them all up and that's 16 + 4 + 7 +10. 37 years.

I reckon that's a ballpark answer. It improves the pool of human knowledge slightly with each generation, which is a little above the goal of your question, but this will make up for occasional losses of knowledge if things go wrong when the threshold is quite tight.

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    $\begingroup$ I had similar calculations, and came up with 24(reproduce)+18(raise, contribute to learning) = 42 years. But your calculations are more precise. $\endgroup$
    – Deepak Chaudhary
    Sep 17, 2020 at 16:37
  • $\begingroup$ Is knowledge limited to academia? What about practical skills? Art? Literature? Are you assuming that the absolute majority of people reach their peak in their 30s? $\endgroup$
    – Otkin
    Sep 18, 2020 at 5:48
  • $\begingroup$ @otkin no, but I feel academia is a good proxy because theres a period of learning, a period of creating, and a period of review to improve for next time. That applies to art and literature and anything else. I can get also get a PhD in literature or art and contribute something in my 10 years to that field. $\endgroup$
    – Ash
    Sep 18, 2020 at 6:44
  • $\begingroup$ On average, painters reach the peak (create their most valuable work) at 42, modern-day physicists produce their best work at 48. Experimental thinkers achieve creativity peak at mid-50s after decades of accumulating data and experience. Moreover, there are qualitative differences in knowledge created and collected by conceptual and experimental thinkers. If 37 were the age limit, humanity would not be where we are now. $\endgroup$
    – Otkin
    Sep 18, 2020 at 7:34
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I think that modern humans live sufficiently long. Perhaps, they could live to 100 on average (but maybe I just like round numbers).

If you want to introduce more mutations into the human population you should advocate for later births. If both parents are not in their prime the chances of mutation are increased. Although, as all random changes to the genome, these mutations are not guaranteed to be beneficial.

On a more serious note, humans by their very existence prove that they reached the equilibrium. We are almost at the point where we are no longer subject to evolutionary pressures. Our ability to collect, preserve, and pass the knowledge onto the next generations allows us to adapt better and faster to much more rapid changes in the environment than genetic mutations would ever allow complex organisms.

Intelligence and knowledge are much more effective when it comes to adaptation. The more intelligent your species, the longer they can accumulate knowledge, and the higher density of information they can pass on their offsprings, the less genetic mutations matter.

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  • $\begingroup$ A supplementary question: In what direction you believe the current equilibrium would shift, longer lifespan or shorter one? $\endgroup$
    – Deepak Chaudhary
    Sep 18, 2020 at 7:38
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    $\begingroup$ @DeepakChaudhary Longer if we transcend our own biology. We are too afraid of death. $\endgroup$
    – Otkin
    Sep 18, 2020 at 16:26

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