Humans are Omnivores
Adapted from
a talk by John McArdle, Ph.D.
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There are a number of
popular myths about vegetarianism that have no scientific basis in fact. One of
these myths is that man is naturally a vegetarian because our bodies resemble
plant eaters, not carnivores. In fact we are omnivores, capable of either
eating meat or plant foods. The following addresses the unscientific theory of
man being only a plant eater.
Confusion between Taxonomy and Diet
Much of the
misinformation on the issue of man's being a natural vegetarian arises from
confusion between taxonomic (in biology, the procedure of classifying organisms
in established categories) and dietary characteristics.
Members of the
mammalian Order Carnivora may or may not be exclusive
meat eaters. Those which eat only meat are carnivores. Dietary adaptations are
not limited by a simple dichotomy between herbivores (strict vegetarians) and
carnivores (strict meat-eaters), but include frugivores
(predominantly fruit), gramnivores (nuts, seeds,
etc.), folivores (leaves), insectivores
(carnivore-insects and small vertebrates), etc. Is is
also important to remember that the relation between the form (anatomy/physiology)
and function (behavior) is not always one to one. Individual anatomical
structures can serve one or more functions and similar functions can be served
by several forms.
The key category in the
discussion of human diet is omnivores, which are defined as generalized
feeders, with neither carnivore nor herbivore specializations
for acquiring or processing food, and who are capable of consuming and
do consume both animal protein and vegetation. They are basically
*opportunistic* feeders (survive by eating what is available) with more
generalized anatomical and physiological traits, especially the dentition
(teeth). All the available evidence indicates that the natural human diet is
omnivorous and would include meat. We are not, however, required to consume
animal protein. We have a choice.
There are very few frugivores amongst the mammals in general, and primates in
particular. The only apes that are predominantly fruit eaters (gibbons and siamangs) are atypical for apes in many behavioral and
ecological respects and eat substantial amounts of vegetation. Orangutans are
similar, with no observations in the wild of eating meat.
Gorillas are more typically
vegetarian, with less emphasis on fruit. Several years ago a very elegant study
was done on the relationship between body size and diet in primates (and some
other mammal groups). The only primates on the list with pure diets were the
very small species (which are entirely insectivorous) and the largest (which
specialize in vegetarian diet). However, the spectrum of
dietary preferences reflect the daily food intake needs of each body
size and the relative availability of food resources in a tropical forest. Our
closest relatives among the apes are the chimpanzees (i.e., anatomically, behaviorally,
genetically, and evolutionarily), who frequently kill and eat other mammals
(including other primates).
Evidence of Humans as Omnivores
As far back as it can be traced,
clearly the archeological record indicates an omnivorous diet for humans that
included meat. Our ancestry is among the hunter/gatherers from the beginning.
Once domestication of food sources began, it included both animals and plants.
Relative number and
distribution of cell types, as well as structural specializations, are more
important than overall length of the intestine to determining a typical diet.
Dogs are typical carnivores, but their intestinal characteristics have more in
common with omnivores. Wolves eat quite a lot of plant material.
Nearly all plant eaters
have fermenting vats (enlarged chambers where foods sits and microbes attack
it). Ruminants like cattle and deer have forward sacs derived from remodeled esophagus
and stomach. Horses, rhinos, and colobine monkeys
have posterior, hindgut sacs. Humans have no such specializations.
Although evidence on the
structure and function of human hands and jaws, behavior, and evolutionary
history also either support an omnivorous diet or fail to support strict
vegetarianism, the best evidence comes from our teeth.
The short canines in humans
are a functional consequence of the enlarged cranium and associated reduction
of the size of the jaws. In primates, canines function as both defense weapons
and visual threat devices. Interestingly, the primates with the largest canines
(gorillas and gelada baboons) both have basically vegetarian diets. In
archeological sites, broken human molars are most often confused with broken
premolars and molars of pigs, a classic omnivore. On the other hand, some
herbivores have well-developed incisors that are often mistaken for those of
human teeth when found in archeological excavations.
These indicate we could
be omnivores. Saliva and urine data vary, depending on diet, not taxonomic
group.
Intestinal absorption is
a surface area, not linear problem. Dogs (which are carnivores) have intestinal
specializations more characteristic of omnivores than carnivores such as cats.
The relative number of crypts and cell types is a better indication of diet
than simple length. We are intermediate between the two groups.
Humans are classic examples
of omnivores in all relevant anatomical traits. There is no basis in anatomy or
physiology for the assumption that humans are pre-adapted to the vegetarian
diet. For that reason, the best arguments in support of a meat-free diet remain
ecological, ethical, and health concerns.
[Dr. McArdle
is a vegetarian and currently Scientific Advisor to The American
Anti-Vivisection Society. He is an anatomist and a primatologist.]
The following information is
taken from The New York Times,
Quoted from an editorial by
William Clifford Roberts, M.d., Editor-in-Chief of the
American Journal of Cardiology:
"When
we kill animals to eat them, they end up killing us because their flesh, which
contains cholesterol and saturated fat, was never intended for human beings,
who are natural herbivores."
Quoted from "WHAT DID
OUR ANCESTORS EAT?" in Nutrition Reviews, by
"These
people of Upper Pleistocene, and later those of the mesolithic,
were our immediate ancestors, no longer hunters exclusively and with
whole-grain products and a variable amount of roots, fruits, leafy vegetables
and nuts in their diet. We must grant them a mixed diet, with animal fat
providing a smaller proportion of their food energy than was probably true for
the Neanderthals."
This article was originally
published in the May/June 1991 edition of the Vegetarian Journal,
published by:
The Vegetarian
Resource Group
(410) 366-VEGE
Questions?