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Author Topic:   Human Life Span & Evolution
Omnivorous
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Posts: 3978
From: Adirondackia
Joined: 07-21-2005
Member Rating: 7.3


Message 9 of 71 (316395)
05-30-2006 7:50 PM
Reply to: Message 3 by ramoss
05-30-2006 10:11 AM


Yes, the average human lifespan has been increasing for the last bunch of years, basically due to the increase of better health care. However, even in roman times, there were a number of individuals from the high classes of soceity that had the better environment to live, and therefore lived into their 80's and 90's.. so the potential has not increased.
Ramoss, I think then as now the primary improvement pushing longer life spans was simply a matter of sanitation and clean water supply. Some very basic principles of human waste disposal and safe water distribution allow the opportunity to reach a genetically-limited age--even today millions die from the diseases that are rampant without rational sanitation practices. Dysentery ravages infants and children in the less developed world.
Invictus, while there have always been some children born to older parents, the percentage is small. Selection doesn't get much opportunity to operate on genes for longevity because prime reproductive years are youthful years. Also, the probability of defective genes increases in aging germ cell lines, e.g., Downs' syndrome among the infants of older mothers, and, recently, a correlation established between having an older father and being at risk for schizophrenia.
The diseases of advancing age--atherosclerosis, cancer, dementia, etc.--have not been strongly selected against because they have little or no impact on reproductive fitness.
Some theorists have argued that elders who survive longer provide an advantaqe to their group via culture/memes to explain why we live past our prime reproductive years. The onset of menopause rather than child-bearing unto death is sometimes cited as an example.
It occurs to me that the near universal tendency for old dominant males to obtain young mates might play some part in conserving longer life spans, but I think I'll say no more about that.
Edited by Omnivorous, : Typo. I'm old, I can't help it.

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Omnivorous
Member
Posts: 3978
From: Adirondackia
Joined: 07-21-2005
Member Rating: 7.3


Message 20 of 71 (316863)
06-01-2006 2:23 PM
Reply to: Message 18 by EZscience
06-01-2006 12:30 PM


Effects of nutrition and contraception?
As usual, EZ, thank you for complicating my understanding.
I wonder what you think about two other factors: the earlier onset of menarche in developed nations (usually attributed to improved nutrition, I believe) and the widespread use of contraceptives
Would not the earlier onset of menarche in the developed regions off-set the effects of otherwise delayed reproduction? We certainly see earlier reproduction in our own, "internal" less (economically) developed areas than we do in more affluent communities...I think.
Doesn't the use of contraceptives to delay reprodution blur the selection effect, since the phenomenon is driven by chemical means rather than genetic ones? If the later-breeding orgamism is selected for, yet is breeding later without any correlation to genetic endowment, what is being selected?
I'm all questions here, and I suspect I'm not seeing the matter clearly.

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 Message 21 by EZscience, posted 06-01-2006 3:36 PM Omnivorous has replied

  
Omnivorous
Member
Posts: 3978
From: Adirondackia
Joined: 07-21-2005
Member Rating: 7.3


Message 22 of 71 (316878)
06-01-2006 4:36 PM
Reply to: Message 21 by EZscience
06-01-2006 3:36 PM


Re: Effects of nutrition and contraception?
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...
Not me, EZ. I appreciate the enlightening comments. I am particularly pleased to see:
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:
omnivorous writes:
If the later-breeding orgamism is selected for, yet is breeding later without any correlation to genetic endowment, what is being selected?
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.
I often feel exasperated when I read analyses that treat humans like pea plants, as though behavioral elements were neither inheritable nor mediated by culture. I find your outline both fascinating and satisfying. Thanks again.

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