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Author Topic:   Human Life Span & Evolution
RAZD
Member (Idle past 520 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


Message 61 of 71 (319273)
06-08-2006 8:30 PM
Reply to: Message 57 by EZscience
06-06-2006 10:04 PM


selection currently acting to increase human lifespan?
it is perhaps more instructive to consider why selection should be currently acting to increase human lifespan

Is it? or is the lifespan increases purely 'environmental' in that it is our increased knowledge of medicine and health that allows people to reach their potential more often?

Average lifespan in developing countries is not all that much different, so when adverse conditions are present it reduces the likelyhood of surviving to old age.

There could be selection for better protection of {children\youth} in populations due to having more able-bodied adults around that would select for the children of those who live {longer+healthy} lives, but this may also select for populations that have fewer children so that they are easier to protect with the resources you have available.

And it may select for populations where children take longer to reach maturity. Some estimates put humans at reaching self-sufficiency at age 10 or so -- compare this to other ape ages or even other mammals -- and that isn't even reaching sexual maturity yet (13?).

Selecting for fewer kids and slower growing kids could select for increased age in adults to compensate for the loss in offspring productivity, as those who were shorter lived would not live long enough to protect the last batches adequately, putting more strain on other members of the group (or increased abandonement).

Isn't there some general relationship between the time it takes for young to reach maturity in a species and the length of life of the species (at least for those where the young are cared for by at least one adult)? Consider elephants for instance.

In animals where there is no 'penalty' for taking care of the kids (say alligators and turtles?) increased longevity of the adult would be selected for by reproductive advantage, yes?


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This message is a reply to:
 Message 57 by EZscience, posted 06-06-2006 10:04 PM EZscience has responded

Replies to this message:
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EZscience
Member (Idle past 4269 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 62 of 71 (319533)
06-09-2006 11:56 AM
Reply to: Message 61 by RAZD
06-08-2006 8:30 PM


Increasing human lifespan more a consequence of selection targeting other traits
RazD writes:

Is it? or is the lifespan increases purely 'environmental' in that it is our increased knowledge of medicine and health that allows people to reach their potential more often?

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.

RazD writes:

Selecting for fewer kids and slower growing kids could select for increased age in adults to compensate for the loss in offspring productivity

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.

RazD writes:

Isn't there some general relationship between the time it takes for young to reach maturity in a species and the length of life of the species (at least for those where the young are cared for by at least one adult)? Consider elephants for instance.

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.

RazD writes:

In animals where there is no 'penalty' for taking care of the kids (say alligators and turtles?) increased longevity of the adult would be selected for by reproductive advantage, yes?

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.


This message is a reply to:
 Message 61 by RAZD, posted 06-08-2006 8:30 PM RAZD has responded

Replies to this message:
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RAZD
Member (Idle past 520 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


Message 63 of 71 (319682)
06-09-2006 7:57 PM
Reply to: Message 62 by EZscience
06-09-2006 11:56 AM


Re: Increasing human lifespan more a consequence of selection targeting other traits
I don’t think kids are growing slower or maturing later ...

I was thinking more in terms of on the way to modern human, say up to 200,000 years ago when Homo sap got his\her first legs.

Looks like sexual maturity is reached about the same time for chimps, bonobos and humans:
National Geographic Article on Gombe (click)
San Diego Zoo Article (click)
Animal Info - Bonobo (click)
12 to 13 years.

When we compare old ages of chimps, bonobos and humans we see 40 to 50 years for chimps and bonobos, comparable to hominid ancestors, but twice that for modern humans?

This is ‘r’ versus ‘K’ selection ... There have been many criticisms of this conceptualization

I was thinking more in terms of subsets than the whole range - comparing all species that cared for young for instance - to see if more can be teased from the data.

... 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.

I ran across this article:
YES Magazine Article - Respecting Elders, Becoming Elders (click)
But haven't read it enough to evaluate how much valid how much opinion. But read the "First Grandmother" section eh?


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Jon
Inactive Member


Message 64 of 71 (320725)
06-12-2006 8:21 AM
Reply to: Message 60 by RAZD
06-08-2006 8:11 PM


Re: Age at Death estimates
I'm just courious. If an early human was dated to have lived 800+ years by modern dating methods, would that date be scientifically accepted, or would it be thrown out as being incorrect. Would the archaeologists discard it because it's unreasonable compared to the majority of ages?

This message is a reply to:
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EZscience
Member (Idle past 4269 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 65 of 71 (320779)
06-12-2006 11:46 AM
Reply to: Message 64 by Jon
06-12-2006 8:21 AM


Re: Age at Death estimates
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.

This message is a reply to:
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RAZD
Member (Idle past 520 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


Message 66 of 71 (320938)
06-12-2006 8:06 PM
Reply to: Message 64 by Jon
06-12-2006 8:21 AM


Re: Age at Death estimates
Dating methods wouldn't bracket the age of a fossil with anything like a valid number, if this is what you are meaning -- we can't date when an organism was born, and only 14C dating method relies on the decay of 14C to measure how long ago the organic sample stopped taking in atmospheric carbon (to date when the sample died). Dating the layer under the fossil could easily provide a false "life" of thousands of years - heck we can walk on rocks that are over 3 billion years old eh?

The methods we have to judging age at death are fairly limited and are based on our medical knowledge of human and animal changes with age.

In order to arrive at a theoretical age of 800 (normal?) years for a fossil there would have to be some rather extraordinary aspects -- all teeth there, but worn down to a nub, severe arthritis, the loss of all cartilege in joints and the subsequent wear on the bones, etcetera.

The fossil would be more likely regarded as deformed and diseased than old.

This is based on applying the tests for age on the fossil to actually test out to 800 years old, rather than taking 800 years and seeing how they would not test to the right ages.

Does that answer your question?


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This message is a reply to:
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Replies to this message:
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Jon
Inactive Member


Message 67 of 71 (321948)
06-15-2006 2:57 PM
Reply to: Message 66 by RAZD
06-12-2006 8:06 PM


Re: Age at Death estimates
The fossil would be more likely regarded as deformed and diseased than old.

So even if the individual had lived an 800-year life, we may not be willing to admit it?

In order to arrive at a theoretical age of 800 (normal?) years for a fossil there would have to be some rather extraordinary aspects -- all teeth there, but worn down to a nub, severe arthritis, the loss of all cartilege in joints and the subsequent wear on the bones, etcetera.

So if the individual had been an extrememely healthy 800-year old man, we wouldn't see these signs. Would we then date the sample as being very young?

This is based on applying the tests for age on the fossil to actually test out to 800 years old, rather than taking 800 years and seeing how they would not test to the right ages.

So then the answer is "yes?" If we did date an individual to be 800 years old, we would look for other explanations instead of accepting the 800-year old number.

Trék


This message is a reply to:
 Message 66 by RAZD, posted 06-12-2006 8:06 PM RAZD has responded

Replies to this message:
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watzimagiga
Inactive Member


Message 68 of 71 (333569)
07-19-2006 11:36 PM
Reply to: Message 67 by Jon
06-15-2006 2:57 PM


Re: Age at Death estimates
Yes, this was one of my points earlier in the discussion. Its a good one, and is yet to really have a good answer.

Matt


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RAZD
Member (Idle past 520 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


Message 69 of 71 (333818)
07-20-2006 8:14 PM
Reply to: Message 67 by Jon
06-15-2006 2:57 PM


Re: Age at Death estimates
So if the individual had been an extrememely healthy 800-year old man, we wouldn't see these signs. Would we then date the sample as being very young?

"Extremely healthy" would allow a person to live to 90 instead of 40 or 50 years, but it would not decrease the wear and tear on teeth and joints, that would still be progressive.

So then the answer is "yes?" If we did date an individual to be 800 years old, we would look for other explanations instead of accepting the 800-year old number.

No, this is taking the tests we have for age and then extrapolating what the results would be for an 800 year old specimen, independent of health. We would then look at that specimen and see what could be concluded -- and as I said we would see "some rather extraordinary aspects -- all teeth there, but worn down to a nub, severe arthritis, the loss of all cartilage in joints and the subsequent wear on the bones, etcetera."

What we would likely conclude about age at death is that it would be indeterminate due to the {excessive damaged and crippled} ("EDC") condition of the specimen.

The age dating methods use cross-references between age and wear that we have accumulated (including references to mummies and other ancient bodies where the age is known). We also have correlation checks with juvenile fossils that show wear at these rates, while the bones are still not fully formed compared to adults in the same populations. These latter fossils rule out cultures with excessive wear pattern due to culture specific behavior (say eating sand) because the increased rate shows up in the young population.

This means that to posit an over 10-fold increase in age from what is know that you need to have some mechanism to reduce wear and tear well below normal levels.

You need to assume a supernatural explanation for how they age without showing it in order to show evidence for a supernaturally aged fossil. This is begging the question.

Now I don't really see the use for speculations like this, because they are all "what if" stories, and the "what ifs" can go on for whatever.

If there were a population that had a high incidence of what appeared to be "EDC" fossils -- all having the same characteristics of excessively worn teeth and bones and the like, then there might be cause for {investigating\theorizing\speculating} why and how this came to be.

There isn't.

If that population were also mixed with others of intermediate ages that showed a clear progression from normal young age to the "EDC" fossils with normal patterns for the very young and sufficient intermediates between them to fill the demographics necessary, showing intermediate stages between normal adult wear and the "EDC" fossils, then you might have evidence to consider an old age.

There isn't.

If there were a number of isolated fossils that showed "EDC" characteristics you might have cause to speculate on an old age.

There isn't.

The worst we have are fossils that cannot be dated because there is not enough there to provide the data. This of course proves that an old age is possible ... :rolleyes: (if that is what you NEED to prove to yourself at all costs so you can be comfortable with some belief or other anyway).

Also see {Bone Maturation}
http://en.wikipedia.org/wiki/Bone_age
http://www.isi.uu.nl/Research/Gallery/Hand/
for fairly accessible articles on some of the changes in the skeleton structure with age.

As long as all of the markers that we have for age development occur in normal patterns you cannot then assume some fantastic age for those fossils.

Now consider Histomorphometric age assessment of the Boxgrove 1 tibial diaphysis:

The bone fragment from Boxgrove 1 presents an unusual situation for age assessment, because the tibia is the only skeletal element recovered for this individual. As a result, the individual’s age-at-death cannot be assessed by macroscopic methods, and it must be evaluated through histological means.

The histological methods developed for estimating age-at-death are bone specific. Several age estimation methods for use on the tibia have been proposed (Kerley & Ubelaker, 1978; Thompson, 1979; Thompson & Galvin, 1983; Uytterschaut, 1985) with Kerley & Ubelaker’s method (1978) being most widely used by biological anthropologists.

Because subsequent application of the original tibial regression formula (Thompson, 1979) found it to overestimate age, a new formula was proposed by Thompson & Galvin (1983). The new formula utilized only the number of whole secondary osteons per mm2 (n.On), which proved to be the best predictor of age in individuals less than 55 years.

Microscopic examination of the fragment of bone from the tibia of Boxgrove reveals substantial histomorphological preservation and virtually all fields are readable. Distinct histomorphological features, such as osteons and osteon fragments, are sufficiently identifiable and quantifiable to permit an histomorphometric age assessment of these remains (Figure 2). Application of the Thompson & Galvin (1983) predicting formula yields an estimate of the age-atdeath of 39·5 years with a range of 31·0–48·0 years for the Boxgrove tibia. The earlier Thompson (1979) formula provides a much higher age (62·6 years+/-8·9 years) (Table 2).

For comparison, histological ages from the Shanidar 2 early Late Pleistocene Neandertal tibia are also provided (Table 2). Macroscopic age indicators for Shanidar 2, in particular the degree of attrition of its complete dentition compared to those of otherwise aged late archaic human specimens from Shanidar and other sites, indicate a young adult age, most likely during the third decade of life (Trinkaus, 1983). Our histomorphometric age estimate using the Thompson & Galvin (1983) formula yields a slightly older age but the probable range encompasses the macroscopic age estimate for this individual. This correspondence provides confidence that the age estimate for Boxgrove 1 in the fourth or early fifth decade of life is appropriate. If anything, the older histomorphometric age, as opposed to dental attrition age-at-death, for Shanidar 2, may suggest that the histomorphometric age for Boxgrove 1 is slightly overestimating its actual age-at-death.

Regardless of the resolution of these issues regarding the calibration of age indicators among pre-Late Pleistocene archaic Homo populations, it is apparent that the known Middle Pleistocene hominid fossil record has an underrepresentation of older individuals, with the current record dominated by young adult and immature remains. Although a variety of taphonomic, behavioral and paleontological processes may contribute to this age bias in the sample, a combination of demographic stress and local population demographic instability may have been a major contributor to the observed pattern (see discussion in Trinkaus, 1995).

At the same time, the present study has shown that histomorphometric methods provide a means to estimate the age-at-death of fragmentary paleontological material such as the Boxgrove 1 tibia. The application of these methods to large samples of Middle Pleistocene archaic Homo where other aging methods can be employed (e.g., the Atapuerca-SH sample) is therefore highly desirable and would help both to refine the level of consistency of these histomorphometric methods with other approaches and expand our paleobiological assessments of these archaic hominids.

Notice four things:

(1) They can measure age up to 55 years by this method -- not just juvenile development.

(2) They also correlate this with samples with teeth wear to show that they have the same age (they didn't have to, and in fact there is a slight difference).

(3) There are other methods for estimating age-at-death.

(4) This paper is discussing the accuracy of the estimates, the correlations that validate the age estimates and how to make those estimates better.

See Dynamic bone remodeling in later Pleistocene fossil hominids. for another early hominid age-at-death study (abstract only).

See also {Dinosaur Fossil Bone Leads to Gender, Age Determinations } and {X-ray microanalysis of fossil dinosaur bone: age differences in the calcium and phosphorus content of Gallimimus bullatus bones. } for some other information related to estimating age-at-death.

Enjoy.


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Aegist
Member (Idle past 2815 days)
Posts: 23
From: Sydney NSW Australia
Joined: 08-21-2006


Message 70 of 71 (342589)
08-22-2006 11:57 PM


Surprisingly I don't think this thread has really touched on the theories of the Evolution of ageing too much, so I thought I would just quickly point out a few things worth reading.

I wrote a paper myself on this about 3 years ago which is a basic introduction to the concept of the evolutionary context of Ageing (http://shanegreenup.blogspot.com/2006/08/why-do-we-age.html). What this paper can probably used for more than anything else for interested parties is the references I have used. particularly worth reading is this paper: shortened link which although it is quite dated now (1957) it still strikes a key theory in the evolution of Ageing and is (for me at least) such a good read.

This doesn't pertain to evolution of human ageing exactly, but the overall concept of evolution of ageing should be no different to humans as it is to all other organisms.

A few other thoughts I have had as I have read through this thread are: 1. Humans are not really living longer today than at any other time, we simply have a higher statistical age brought about by not dying early. Our theoretical maximum life span seems to be static.
2. http://www.imminst.com is the place to look at if you want to learn more about lifespan because there are a lot of people out there who think science will be able to extend our lifespans artificially in the not too distant future. There is therefore a lot of good material on that site about human lifespans.

Shane

Edited by AdminJar, : No reason given.


----------------------------------
http://shanegreenup.blogspot.com
www.sportsarbitrageguide.com

Replies to this message:
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RAZD
Member (Idle past 520 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


Message 71 of 71 (342848)
08-23-2006 7:33 PM
Reply to: Message 70 by Aegist
08-22-2006 11:57 PM


cool
Thanks Shane,

I notice that the second link can be shortened to
http://www.telomere.org/Downloads/Williams_searchable.pdf

I also noticed recent reference to the "hobbit" fossil as a 73 year old woman that lived 18,000 years ago
http://dsc.discovery.com/news/2006/08/21/hobbit_hum.html

And have seen other references to old people in ancient sites and that indicate that such age is not outside the possibilities for those people.

Thanks.


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