Re: selection criteria for obesity

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Date: Fri, 24 Dec 2004 04:09:50 GMT

"Pauline M Ross" <pmross@ross-software.co.uk> wrote in message
news:masls0dd3tce519hchimt3i55dqgiq46q7@4ax.com...
> On Thu, 23 Dec 2004 04:46:37 GMT, "Charles" <lmno@mindspring.com>
> wrote:
>
>>>[Pauline]The fat Venuses I have seen are *clearly* obese,
>>[Charles]do you have any ideas about the selection criteria for obesity?
>>why such a
>>phenomena would continue & apparently even be desirable, in our hss line?
>
> Well, I'm not sure that obesity has been selected for, as such, or
> simply a degree of fat. You said yourself that currently obesity is an
> 'epidemic' and that implies a degree of maladjustment (ie an illness
> rather than a normal state of affairs). Generally speaking, all humans
> have a relatively high level of fat, but not all humans are obese.
>
> As to what is 'desirable' - I think anything which is part and parcel
> of being a normal, healthy individual is likely to be desirable, and
> in a more or less monogamous species, with no alpha-type criteria, the
> range of desirable features is likely to be quite broad. In a
> population where obesity was the norm for healthy individuals (think
> of seals!), I think obesity would be desirable, but not in modern
> humans, where obesity is still a minority state, and is generally
> regarded as being unhealthy.

agreed. I very much enjoyed reading thru your post, thru your notes. with
a little bit of tweaking and beefing-up, you might have something
publishable here.
    meanwhile, I would add two more possible "causes" of obesity and, as
your notes address, SC fat. one is just some unknown, un-imagined,
not-yet-discovered reason that will be clear as day when it *is* found out
(and then you can make a zillion from selling a diet book) and the other is
, .... sexual selection! that is actually what i was trying to achieve
herein. and only got one very strong "no" vote for the possiblity of sexual
selection as the "reason" we became and remain prone to overweight.
however, i would say that sexual selection would fall into the "possible but
not probable" category.
    thank you and everyone for your input.
warmest
charles
>
> As far as the fat Venuses are concerned, we do not know whether they
> represented the norm or an occasional extreme. It may be that they
> were created to celebrate a particularly fruitful period which
> resulted in some individuals achieving a very rare level of obesity,
> who knows?
>
> I made some notes a while back on the possible reasons for human
> subcutaneous fat, which I will dig out and add to the bottom of this
> email (it's rather long).
>
>>>[Pauline] And your point that the large human brain is evidence
>>> for continuous abundance is also a good one.
>>[Charles]thanks! I have fought that fight in the past and got no-where,
>>or have been
>>looked at as some sort of pariah. If we are generalists, then I don't see
>>any particular reason why we would have to starve....
>
> Exactly, and the evidence is that our ancestors were nowhere near
> starvation. Most Homo fossils are tall and clearly healthy, with few
> signs of nutritional deficiences (think of the Nariokatome Boy). It is
> only with early agriculture that such problems become commonplace.
>
>> ... and yes, it occured to me
>>that if we are fat 40 kya, why shouldn't we also have been fat at the
>>beginning of our species... 170 +/- kya. ? And one thing that might make
>>us "fat" is an increase in domesticated, cooked vegetable matter... such
>>as
>>a yam. and if we got yams, why don't we also have the "seed" for
>>agriculture. (pun intended). Maybe hss has always been scratching around
>>in the earth.
>
> It has been proposed that root vegetables were the driving force
> behind the evolution of modern humans - we learned to cook them, they
> were plentiful and fairly nutritious, and needed less jaw-power to
> eat. It may even have been roots which triggered the start of
> agriculture. Personally, I always found it the easiest thing in the
> world to bury a few potatoes in spring and then dig up lots more of
> them a few months later.
>
> Finally, some thoughts on human subcutaneous fat, and the possible
> reasons for it:
>
> It is generally accepted that humans have a lot of subcutaneous fat:
> "thicker than that found in other primates and represents significant
> energy reserves" [JL]; "Only cetaceans, pinnipeds and a few species of
> carnivores and rodents normally have as much fat as 'typical' humans";
> "The average fatness of humans is much greater than that of monkeys"
> [CP1]; "More than half the 31 captive monkeys that we examined were
> less than 5% fat, thinner than most laborotory rodents, although all
> of them had continuous access to food and little opportunity to
> exercise. ... The minimum fatness recorded for teenage girl athletes
> is 7%, and for men 5%. Thus most human beings are not only much fatter
> than most wild and captive mammals, but women and girls are
> consistently fatter than men and boys." [CP2]
> However, hard data is difficult to find. Human body fat levels can
> vary in males from 5% in some athletes, to 25% in older men, with
> averages around 10-20%. Females can vary from less than 10% in some
> athletes, to over 30% in older women, averaging 15-25% [Web]. Obese
> individuals can have much higher values. There seems to be no reliable
> data for hunter/gatherer groups, or for wild apes, although captive
> gorillas have body fat levels within human ranges [ZM]. It is not
> impossible, therefore, that adult males may have fat levels comparable
> to wild apes.
> Adult females and infants have more body fat than males and older
> children. Females whose fat levels drop below a certain point stop
> menstruating, and are therefore infertile. Human infants start to lay
> down fat some two months before birth, and are born with more fat than
> any other mammal infant (15%), although the harp seal (which has an
> unusually short lactation period) and the guinea-pig (which provides
> very un-nutritious milk) have more than 10% [CK].
> There are very few theories on the reasons for general human fat, but
> the most common are: insulation after loss of fur; energy store to
> protect against periodic food shortages; insulation, buoyancy and
> streamlining in an aquatic environment. For infant fat, the standard
> theory is that it assists the fast-growing brain by protecting against
> interruptions in the food supply.
> a) Insulation after loss of fur: this says that we lost our fur (for
> some reason) and needed some compensating insulation. This makes no
> sense at all: in terrestrial environments, fur is the optimum method
> of temperature regulation under any conditions, and even if it had
> already been lost, the most effective means of insulation would be to
> regrow it (unless the reasons for losing it were overwhelming).
> b) Energy store to protect against periodic food shortages: this
> proposes that hominids/humans were at some stage faced with an erratic
> food supply, and laid down fat in the good times as protection. This
> has some merit, as fat does indeed act as an energy store, and humans
> (like many other species) can accumulate a lot of fat if overfed, but
> no other species uses such a method, except for the regular,
> predictable seasonal (winter) shortages in temperate climates (in
> tropical climates, migration is the favoured option), and humans are
> such omnivores that it is difficult to conceive of such severe
> shortages that would trigger this response. Also, since female
> fertility is linked to fat levels, clearly our ancestors always had
> enough food to produce the minimum level of fat required, and this is
> supported by the fossil evidence (Homo specimens were generally tall
> and well-built, with few signs of the nutritional deficiencies which
> are evident in post-agricultural populations). So this theory is
> possible, especially for infant and maternal fat, but not very
> plausible.
> c) Insulation, buoyancy and streamlining in an aquatic environment
> (the AAT idea): this is the most common reason for fat, and is almost
> universal in mammals which spend a lot of time in water. It is also
> true that, among human athletes, swimmers have more body fat than
> runners (in males 12% vs 7%, in females 20% vs 15%) despite having
> comparable training regimes and energy budgets [KJ]. Early hominids
> (Australopithecus and Paranthropus) are strongly associated with
> well-watered and wet habitats, although Homo species are also found in
> drier, more open environments [KR]. This idea also explains the
> greater fat in infants, who would benefit from greater buoyancy and
> insulation until they can swim competently. However, it would have
> needed a lot of time immersed in water (not just shallow wading) to
> produce so much fat, which cannot be verified, and modern humans
> (particularly infants and females) generally retain their fat, despite
> spending very little time in water. It also doesn't account for adult
> females having more fat than males. So this is plausible, but has some
> problems to resolve.
> d) Protects the growing brain against interruptions in the food supply
> (infant fat): the human brain is uniquely large and energy-hungry, and
> this theory proposes that infant fat is designed to protect the brain
> from any energy deficiency. The period of infant fat (from before
> birth to approximately 4 years) tallies with the period of fastest
> brain growth, and there is a huge volume of research showing that
> short-term and/or severe malnutrition produce significant deficits in
> broad measures (such as IQ and school tests) and also subtle
> biochemical effects on the brain itself. However, there appears to be
> no reliable evidence that these effects are permanent if the
> underlying malnutrition is relieved before the age of approximately 4,
> or that infant fat (or maternal fat) in any way protects against these
> effects. It is likely, however, that some infant fat protects very
> well against short-term disruptions resulting from infectious or other
> childhood diseases [CK], but this is more commonplace in modern humans
> living in settled, densely-populated villages, towns and cities.
> Summary: it is unlikely that any one scenario can adequately explain
> human subcutaneous fat. The most plausible solution is that all humans
> acquired fat at some stage in their evolutionary history because of an
> aquatic lifestyle. In modern humans, males and older children no
> longer require much (or any?) fat, but adult females have retained it
> because it has become a fertility signal, and infants have retained it
> because it protects against short-term illness.
>
> References:
>
> [JL] Professor John Langdon
> Biol345 Human anatomy course lecture notes, 2000
> [CK] Christopher Kuzawa
> 'Adipose Tissue in Human Infancy and Childhood: An Evolutionary
> Perspective', Yearbook of Physical Anthropology, 41: 177-209, 1998
> [CP1] Caroline Pond
> 'Adipose Tissue in Human Evolution', in 'The Aquatic Ape: Fact or
> Fiction', 1991
> [CP2] Caroline Pond
> Fat and Figures. New Scientist Vol: ? (1987) Pages:62-66
> [KJ] K T Jang et al
> 'Energy balance in competitive swimmers and runners', Journal of
> Swimming Research, 3, 19-23, 1987
> [KR] Kaye Reed
> 'Early hominid evolution and ecological change through the African
> Plio-Pleistocene', Journal of Human Evolution, 32, 289-322, 1997
> [Web]
> Web sites, various sites, more than one site per reference
> [ZM] Adrienne L. Zihlman, Robin K. McFarland
> 'Body mass in lowland gorillas: A quantitative analysis', Am J Phys
> Anthropol 113:61-78, 2000
>
> --
> Pauline Ross

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