I am in agreement with both Clark and Andrew's previous posts
that human dentition is adapted for a generalized diet composed of both
plant and animal foods and that human populations show amazing
variability in their plant to animal subsistence ratios.   However, it
is important to recognize that hominids have evolved important metabolic
and biochemical adaptations which are indicative of an increasing
physiological dependence upon animal based foods.    Further,
comprehensive compilations of hunter gatherer subsistence strategies
indicate that whenever it is ecologically possible, humans will almost
always consume more animal food than plant food.
        We have recently compiled the plant:animal subsistence ratios
data in the Ethnographic Atlas (1) for all  world wide181 hunter
gatherer populations which have been studied either historically or by
contemporary anthropologists.    The analysis shows that in the majority
(61.3%) of world wide hunter-gatherers, gathered plant food represents
35 % or less than the total foods utilized.   Only 2.2% of the world's
hunter gatherers derive 66% or more of their total foods from plants;
further not a single hunter gather population derives 86 % or more of
its total calories from plant foods.
The most frequently occurring (mode) plant:animal subsistence ratio for
world wide hunter gatherers is (16-25% plant): (66-75% animal), and the
median value is (26-35% plant): (56-65% animal).   These values
corroborate 5 careful modern studies of hunter gatherers showing a mean
energy intake from animal food sources to be 59% (2).
        Pongids, because thier diet is largely plant based, must
maintain large and metabolically active guts to process the fibrous
plant foods which compose over 93% or greater of their dietary intake.
In contrast, the human gut is much smaller and less metabolically active
than the ape gut.   Presumably this adaptation (reduction in gut size
and metabolic activity) evolved in humans because the inclusion of
nutrient dense, animal based foods by our early hominid ancestors
allowed the selective pressure for a large metabolically active gut to
be relaxed (2,3).
        In addition to the smaller gut that humans maintain relative to
apes, there are other metabolic and biochemical clues which point to
increased utilization of animal food by humans over our evolutionary
history.  By evaluating the metabolic and biochemical dietary
adaptations of cats (obligate carnivores) and those in humans
(omnivores), it becomes apparent that evolution has shaped both hominid
and feline metabolic machinery towards a diet in which animal food was
predominant.
        Obligate carnivores, such as cats, must obtain all of their
nutrients from the flesh of other animals and have therefore evolved
certain  biochemical adaptations which are indicative of their total
dietary dependence upon animal based foods.   Most of these biochemical
adaptations involve either the loss (or reduced activity) of certain
enzymes  required for the synthesis of essential nutrients.   These
adaptations generally occurred because the selection pressure to
maintain these metabolic pathways was relaxed as cats gradually
increased the amount of animal food in their diet as they evolutionarily
progressed from omnivory into obligate carnivory.
        Vitamin B12 is an essential nutrient for both herbivorous and
carnivorous mammals.   Because B12 is not found in higher plants,
herbivorous mammals must soley rely upon absorption of B12 from bacteria
which synthesize it in their gut.   Cats can neither synthesize  B12,
nor absorb it from their gut, consequently they have become wholly
dependent upon animal flesh as their source for this essential nutrient.
Humans, like cats are unable to synthesize or absorb vitamin B12 and are
also reliant upon animal based sources of this essential vitamin, since
it does not occur in a biologically active form in any of the plant
foods which humans normally eat.    The absence of the ability of humans
to either synthesize or absorb bacterially produced B12 is indicative of
the long evolutionary history of animal based foods in our diet.
        Taurine is an amino acid which is not found in any plant based
food (4) and which is an essential nutrient in all mammalian cells.
Herbivores are able to synthesize taurine from precursor amino acids
derived from plants, whereas cats have completely lost the ability to
synthesize taurine (5).    Since all animal based foods (except cow
milk) are rich sources of taurine (4), cats have been able to relax the
selective pressure required for taurine synthesis because they obtain
all of this nutrient that they need from their exclusive meat based
diet.   Humans, unlike cats, still maintain the ability to synthesize
taurine in the liver from precursor substances, however this ability is
quite limited and inefficient when compared to herbivores.   Vegan
vegetarians following diets devoid of animal products display unusally
low levels of both plasma and urinary taurine (6)  - levels which are
indicative of the poor ability of humans to synthesize taurine.
Similar to cats, this inability to efficiently synthesize taurine has
come about because the selective pressure to produce this amino acid has
been gradually reduced due to human kind's long reliance upon animal
food, a food which is quite  high in taurine.
        Plant based foods contain 18 carbon fatty acids of both the
omega 3 and omega 6 families, but are virtually devoid of the 20 and 22
carbon fatty acids which are required for the normal functioning of  all
mammalian cells, whether the mammal is a herbivore or carnivore.
Herbivores have evolved hepatic enzymes (desaturases and elongases)
which allow these precursor  plant based 18 carbon fatty acids to be
chain elongated and desaturated to their 20 and 22 carbon products.
Cats have extremely low levels of the enzymes required to make 20 and 22
carbon fatty acids (7).   Again, the selection pressure to synthesize 20
and 22 carbon lipids has been almost entirely removed because cats
obtain sufficient quantities of these long chain fatty acids by eating
animal tissues which are rich sources of these lipids.   Similarly,
humans also have inefficient elongase and desaturase enzymes (7).
Again, this metabolic change has occurred largely because the need to
desaturate and chain elongate 18 carbon plant fatty acids to their 20
and 22 carbon products has been reduced because humans, like cats, have
obtained their 20 and 22 carbon lipids directly by eating other animal
tissues.
        All animals, whether herbivore or carnivore require vitamin A.
Vitamin A is not found in any plant based food; consequently, herbivores
must synthesize it in the liver from beta carotene consumed from plant
based foods.    Cats have lost the ability to synthesize vitamin A from
beta carotene (8), and must obtain all of their vitamin A from the
organs (liver, kidney) of their prey.   Again, cats have lost the
ability to synthesize vitamin A because the selective pressure (need) to
provide adaptive energy for the synthesis of proteins needed to catalyze
the production of vitamin A were reduced as cats progressively increased
the amount of animal foods in their diets.   Recently, it has been shown
that humans also have limited capactity to synthesize vitamin A from
beta carotene (9), presumably because humans, like cats, have consumed
vitamin A rich animal food sources for eons and are in a transitional
state from omnivory to obligate carnivory.
        These metabolic and biochemical adaptations in humans in
response to increasingly meat based diets as well as the anthropological
evidence provided by both contemporary and historical studies of hunter
gather diets provide strong evidence for the central role of meat and
animal tissues in the human diet.   Although it is true that human
populations can survive under broad plant:animal subsitence ratio's, the
consensus evidence supports the notion that whenever it was ecologically
possible, animal calories would have always represented the majority of
the total daily energy intake.

                                Cordially,


                                Loren Cordain, Ph.D.
                                Dept of ESS
                                Colorado State University
                                Fort Collins, CO 80523
                                (970) 491-7436



                                REFERENCES

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1967;6:109-236.
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the influence of brain and body size on diet and metabolism. Am J Hum
Biol 1994;6:77-88.
3.      Aiello LC, Wheeler P.  The expensive tissue hypothesis. Current
Anthropology 1995;36:199-221.
4.      Laidlow SA et al.  The taurine conte of common foodstuffs. J
Parenteral Enteral Nutr 1990;14:183-88.
5.      Knopf K et al.  Taurine: an essential nutrient for the cat. J
Nutr 1978;108:773-778.
6.      Laidlow SA.  Plasma and urine levels in vegans.  Am J Clin Nutr
1988;47:660-3.
7.      Salem N et al.  Arachidonate and docosahexaenoate biosynthesis
in various species and compartments in vivo. World Rev Nutr Diet
1994;75:114-19.
8.      MacDonald ML et al.  Nutrition of the domestic cat, a mammalian
carnivore. Ann Rev Nutr 1984;4:521-62.
9.      de Pee S, West CE et al.  Lack of improvement in vitamin A
status with increased consumption of dark leafy green vegetables. Lancet
1995;346:75-81.