Re: selection criteria for obesity

From: Pauline M Ross (pmross_at_ross-software.co.uk)
Date: 12/23/04 

Date: Thu, 23 Dec 2004 16:58:22 +0000

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.

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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