Human Life Span & Evolution

Yes, improved sanitation and health care technology have resulted in the extension of human lifespan, but what are the evolutionary consequences?First, human lifespan is not, per se, subject to selection. What may be subject to selection is reproductive lifespan, as that can directly affect fitness. Any increase in the former could be merely a secondary consequence of selection acting on the latter.In animals, lifespan appears tightly linked to reproductive strategies. Lifespan is often negatively correlated with reproductive effort or positively correlated with onset of reproduction. For example, in the extreme case of semelparous organisms, reproduction results in death immediately.So we can ask the following questions:1. Are we reproducing less? (that should, in theory, help us live longer). The answer is largely 'yes' in developed countries and 'no' in undeveloped countries, so such a correlation does appear consistent, but does not necessarily imply any causation.2. Are we reproducing later in life? (in theory, this can also lead to longer lifespans under situations where more resources must be garnered before initiating reproduction in porder for it to be successful). Again, the answer is generally 'yes' in developed countries, and 'no' in underdeveloped countries, so the corrlation between reproductive strategy and longevity holds again.So we consider that the developed countries have most of the technology and sanitation to extend not only human life, but human reproductive lifespan. This has lead to consequences quite consistent with evolutionary theory - delayed onset of reproduction (possible because of reduced risk of dying prior to having any offspring) and reduced numbers of offspring (possible because the chances of survival of each are greatly improved).
These trends are not evident in the developing world because lifespan is still substantially shorter there and consequently there is still strong selection for early reproduction and higher fecundity in those environments.The conclusion? If we could improve health care, sanitation and human longevity in the developing countries, the effect would be to reduce their population explosion problem because we would be relaxing selection for early onset of reproduction and high individual fecundity.

Yes, I have pondered this phenomenon.
I would speculate that 'potential reproductive lifespan' has probably been decoupled from 'actualized reproductive lifespan' in humans, especially in the developed world where contraception is readily available.
In other words, the availability of contraception, both male and female-based, may effectively counter what would otherwise be a trend toward earlier onset of reproduction associated with earlier onset of menarche. Also we have cultural factors - teenage pregancy is obviously heavily stigmatized in most modern (although not in many primitive) societies. I would not expect so. If anything, it could facilitate it.
In other words, humans in developed countries can now voluntarily choose to delay onset of reproduction beyond the limits otherwise determined by biology. The question is why they would choose to do so (i.e. why such behavior should be selected). So this sort of approaches your other question: The behavior. And although human behavior has a degree of heritability, it is also strongly affected by cultural selection independently of genetics. Quantity of surviving offspring is far too simple a definition of fitness for humans, regardless of its value in application to plants and other animals. Humans are probably more selected to maximize offspring 'quality' and this can be socially determined. The structure of human society redefines fitness for its own species. I suspect there are strong social and cultural benefits to individuals of delayed reproduction and reduced fecundity. The reduced financial burden is an obvious one. Many people delay having kids until they feel financially secure, and this typically comes later in life than reproductive maturity. (I waited until I was over forty - and I still didn't feel financially secure enough )These same 'socially-influenced reproductive strategies' are not functionally advantageous in generally impoverished countries, so we don't observe the same consequences for human demography.Although I am sure this explanation can be improved on, I think it is basically consistent with the evidence at hand. But feel free to tear it apart...

It is relevant because longevity is a very important life history characteristic and there is every reason to believe it would be molded by natural selection. Apart from the fact that all evidence points to early humans living shorter lives rather than longer, it is perhaps more instructive to consider why selection should be currently acting to increase human lifespan when this increase in lifesapn is in no way correlated with reproductive success.Any ideas ? RazD ?

Yes, I didn’t mean to imply that had to be necessarily the case.
I was just musing about what might select for longer life and what the consequences might be. Obviously, there are human interventions that extend life, but then human activities/technologies/social norms etc. are now all important selective forces in our adaptive topography. I don’t think kids are growing slower or maturing later (if anything, maturing faster) but I do believe that as we are required to invest more and more in each child (in western society anyway) this will cause selection to favor having fewer of them. This is ”r’ versus ”K’ selection in life history. Longer life is associated with ”K’ selection, K being the carrying capacity of the environemtn for that species. K-selected organism take longer to mature, have low fecundity and live a long time. In contrast, r-selected individuals are selected to maximize their intrinsic rate of increase ”r’, which results in them maturing quickly, producing large numbers of offspring with little investment in any one, and living a short life. There have been many criticisms of this conceptualization (primarily there are no common units of measurement to quantify) but it is still a useful descriptive contrast. There is always a cost to parental care because the parent is csacrificing energy and resources that could otherwise be used to produce additional offspring. But your other suggestion is interesting. In strict Darwinian evolution, there is no selective advantage for increasing longevity beyond reproductive age, and yet it is possible that if parents contribute to their offspring’s reproductive success late into life, there would be advantages for increasing parental longevity beyond reproductive age. Think about how many grandparents are important providers of child care for their kids. Edited by EZscience, : No reason given.

I don't think that is likely to happen, but in the case of a highly anomalous result like that, it would not be imediately accepted. It would have to be repeated and confirmed through independent testing by others, preferably using alternative methodologies.