Humans are most often described as "omnivores". This
classification is based on the "observation" that humans generally
eat a wide variety of plant and animal foods. However, culture,
custom and training are confounding variables when looking at human
dietary practices. Thus, "observation" is not the best technique
to use when trying to identify the most "natural" diet for humans.
While most humans are clearly "behavioral" omnivores, the question
still remains as to whether humans are anatomically suited for a
diet that includes animal as well as plant foods.
A better and more objective technique is to look at human anatomy
and physiology. Mammals are anatomically and physiologically
adapted to procure and consume particular kinds of diets. (It is
common practice when examining fossils of extinct mammals to
examine anatomical features to deduce the animal's probable diet.)
Therefore, we can look at mammalian carnivores, herbivores (plant-eaters)
and omnivores to see which anatomical and physiological
features are associated with each kind of diet. Then we can look
at human anatomy and physiology to see in which group we belong.
The teeth of a carnivore are discretely spaced so as not to trap
stringy debris. The incisors are short, pointed and prong-like and
are used for grasping and shredding. The canines are greatly
elongated and dagger-like for stabbing, tearing and killing prey.
The molars (carnassials) are flattened and triangular with jagged
edges such that they function like serrated-edged blades. Because
of the hinge-type joint, when a carnivore closes its jaw, the cheek
teeth come together in a back-to-front fashion giving a smooth
cutting motion like the blades on a pair of shears.
The saliva of carnivorous animals does not contain digestive
enzymes. When eating, a mammalian carnivore gorges itself rapidly
and does not chew its food. Since proteolytic (protein-digesting)
enzymes cannot be liberated in the mouth due to the danger of
autodigestion (damaging the oral cavity), carnivores do not need to
mix their food with saliva; they simply bite off huge chunks of
meat and swallow them whole.
According to evolutionary theory, the anatomical features
consistent with an herbivorous diet represent a more recently
derived condition than that of the carnivore. Herbivorous mammals
have well-developed facial musculature, fleshy lips, a relatively
small opening into the oral cavity and a thickened, muscular
tongue. The lips aid in the movement of food into the mouth and,
along with the facial (cheek) musculature and tongue, assist in the
chewing of food. In herbivores, the jaw joint has moved to
position above the plane of the teeth. Although this type of joint
is less stable than the hinge-type joint of the carnivore, it is
much more mobile and allows the complex jaw motions needed when
chewing plant foods. Additionally, this type of jaw joint allows
the upper and lower cheek teeth to come together along the length
of the jaw more or less at once when the mouth is closed in order
to form grinding platforms. (This type of joint is so important to
a plant-eating animal, that it is believed to have evolved at least
15 different times in various plant-eating mammalian species.) The
angle of the mandible has expanded to provide a broad area of
attachment for the well-developed masseter and pterygoid muscles
(these are the major muscles of chewing in plant-eating animals).
The temporalis muscle is small and of minor importance. The
masseter and pterygoid muscles hold the mandible in a sling-like
arrangement and swing the jaw from side-to-side. Accordingly, the
lower jaw of plant-eating mammals has a pronounced sideways motion
when eating. This lateral movement is necessary for the grinding
motion of chewing.
The dentition of herbivores is quite varied depending on the kind
of vegetation a particular species is adapted to eat. Although
these animals differ in the types and numbers of teeth they posses,
the various kinds of teeth when present, share common structural
features. The incisors are broad, flattened and spade-like.
Canines may be small as in horses, prominent as in hippos, pigs and
some primates (these are thought to be used for defense) or absent
altogether. The molars, in general, are squared and flattened on
top to provide a grinding surface. The molars cannot vertically
slide past one another in a shearing/slicing motion, but they do
horizontally slide across one another to crush and grind. The
surface features of the molars vary depending on the type of plant
material the animal eats. The teeth of herbivorous animals are
closely grouped so that the incisors form an efficient
cropping/biting mechanism, and the upper and lower molars form
extended platforms for crushing and grinding. The "walled-in" oral
cavity has a lot of potential space that is realized during eating.
These animals carefully and methodically chew their food, pushing
the food back and forth into the grinding teeth with the tongue and
cheek muscles. This thorough process is necessary to mechanically
disrupt plant cell walls in order to release the digestible
intracellular contents and ensure thorough mixing of this material
with their saliva. This is important because the saliva of plant-eating
mammals often contains carbohydrate-digesting enzymes which
begin breaking down food molecules while the food is still in the
mouth.
Because of the relative difficulty with which various kinds of
plant foods are broken down (due to large amounts of indigestible
fibers), herbivores have significantly longer and in some cases,
far more elaborate guts than carnivores. Herbivorous animals that
consume plants containing a high proportion of cellulose must
"ferment" (digest by bacterial enzyme action) their food to obtain
the nutrient value. They are classified as either "ruminants"
(foregut fermenters) or hindgut fermenters. The ruminants are the
plant-eating animals with the celebrated multiple-chambered
stomachs. Herbivorous animals that eat a diet of relatively soft
vegetation do not need a multiple-chambered stomach. They
typically have a simple stomach, and a long small intestine. These
animals ferment the difficult-to-digest fibrous portions of their
diets in their hindguts (colons). Many of these herbivores
increase the sophistication and efficiency of their GI tracts by
including carbohydrate-digesting enzymes in their saliva. A
multiple-stomach fermentation process in an animal which consumed
a diet of soft, pulpy vegetation would be energetically wasteful.
Nutrients and calories would be consumed by the fermenting bacteria
and protozoa before reaching the small intestine for absorption.
The small intestine of plant-eating animals tends to be very long
(greater than 10 times body length) to allow adequate time and
space for absorption of the nutrients.
In herbivorous animals, the large intestine tends to be a highly
specialized organ involved in water and electrolyte absorption,
vitamin production and absorption, and/or fermentation of fibrous
plant materials. The colons of herbivores are usually wider than
their small intestine and are relatively long. In some plant-eating
mammals, the colon has a pouched appearance due to the
arrangement of the muscle fibers in the intestinal wall.
Additionally, in some herbivores the cecum (the first section of
the colon) is quite large and serves as the primary or accessory
fermentation site.
This is exactly the situation we find in the Bear, Raccoon and
certain members of the Canine families. (This discussion will be
limited to bears because they are, in general, representative of
the anatomical omnivores.) Bears are classified as carnivores but
are classic anatomical omnivores. Although they eat some animal
foods, bears are primarily herbivorous with 70-80% of their diet
comprised of plant foods. (The one exception is the Polar bear
which lives in the frozen, vegetation poor arctic and feeds
primarily on seal blubber.) Bears cannot digest fibrous vegetation
well, and therefore, are highly selective feeders. Their diet is
dominated by primarily succulent lent herbage, tubers and berries.
Many scientists believe the reason bears hibernate is because their
chief food (succulent vegetation) not available in the cold
northern winters. (Interestingly, Polar bears hibernate during the
summer months when seals are unavailable.)
In general, bears exhibit anatomical features consistent with a
carnivorous diet. The jaw joint of bears is in the same plane as
the molar teeth. The temporalis muscle is massive, and the angle
of the mandible is small corresponding to the limited role the
pterygoid and masseter muscles play in operating the jaw. The
small intestine is short ( less than five times body length) like
that of the pure carnivores, and the colon is simple, smooth and
short. The most prominent adaptation to an herbivorous diet in
bears (and other "anatomical" omnivores) is the modification of
their dentition. Bears retain the peg-like incisors, large canines
and shearing premolars of a carnivore; but the molars have become
squared with rounded cusps for crushing and grinding. Bears have
not, however, adopted the flattened, blunt nails seen in most
herbivores and retain the elongated, pointed claws of a carnivore.
An animal which captures, kills and eats prey must have the
physical equipment which makes predation practical and efficient.
Since bears include significant amounts of meat in their diet, they
must retain the anatomical features that permit them to capture and
kill prey animals. Hence, bears have a jaw structure, musculature
and dentition which enable them to develop and apply the forces
necessary to kill and dismember prey even though the majority of
their diet is comprised of plant foods. Although an herbivore-style
jaw joint (above the plane of the teeth) is a far more
efficient joint for crushing and grinding vegetation and
would potentially allow bears to exploit a wider range of plant
foods in their diet, it is a much weaker joint than the hinge-style
carnivore joint. The herbivore-style jaw joint is relatively
easily dislocated and would not hold up well under the stresses of
subduing struggling prey and/or crushing bones (nor would it allow
the wide gape carnivores need). In the wild, an animal with a
dislocated jaw would either soon starve to death or be eaten by
something else and would, therefore, be selected against. A given
species cannot adopt the weaker but more mobile and efficient
herbivore-style joint until it has committed to an essentially
plant-food diet test it risk jaw dislocation, death and ultimately,
extinction.
Human teeth are also similar to those found in other herbivores
with the exception of the canines (the canines of some of the apes
are elongated and are thought to be used for display and/or
defense). Our teeth are rather large and usually abut against one
another. The incisors are flat and spade-like, useful for peeling,
snipping and biting relatively soft materials. The canines are
neither serrated nor conical, but are flattened, blunt and small
and function Like incisors. The premolars and molars are squarish,
flattened and nodular, and used for crushing, grinding and pulping
noncoarse foods.
Human saliva contains the carbohydrate-digesting enzyme, salivary
amylase. This enzyme is responsible for the majority of starch
digestion. The esophagus is narrow and suited to small, soft balls
of thoroughly chewed food. Eating quickly, attempting to swallow
a large amount of food or swallowing fibrous and/or poorly chewed
food (meat is the most frequent culprit) often results in choking
in humans.
Man's stomach is single-chambered, but only moderately acidic.
(Clinically, a person presenting with a gastric pH less than 4-5
when there is food in the stomach is cause for concern.) The
stomach volume represents about 21-27% of the total volume of the
human GI tract. The stomach serves as a mixing and storage
chamber, mixing and liquefying ingested foodstuffs and regulating
their entry into the small intestine. The human small intestine is
long, averaging from 10 to 11 times the body length. (Our small
intestine averages 22 to 30 feet in length. Human body size is
measured from the top of the head to end of the spine and averages
between two to three feet in length in normal-sized individuals.)
The human colon demonstrates the pouched structure peculiar to
herbivores. The distensible large intestine is larger in cross-section
than the small intestine, and is relatively long. Man's
colon is responsible for water and electrolyte absorption and
vitamin production and absorption. There is also extensive
bacterial fermentation of fibrous plant materials, with the
production and absorption of significant amounts of food energy
(volatile short-chain fatty acids) depending upon the fiber content
of the diet. The extent to which the fermentation and absorption
of metabolites takes place in the human colon has only recently
begun to be investigated.
In conclusion, we see that human beings have the gastrointestinal
tract structure of a "committed" herbivore. Humankind does not
show the mixed structural features one expects and finds in
anatomical omnivores such as bears and raccoons. Thus, from
comparing the gastrointestinal tract of humans to that of carnivores,
herbivores and omnivores we must conclude that humankind's
GI tract is designed for a purely plant-food diet.
CARNIVORE: Reduced to allow wide mouth gape
Jaw Type
CARNIVORE: Angle not expanded
Jaw Joint Location
CARNIVORE: On same plane as molar teeth
Jaw Motion
CARNIVORE: Shearing; minimal side-to-side motion
Major Jaw Muscles
CARNIVORE: Temporalis
Mouth Opening vs. Head Size
CARNIVORE: Large
Teeth: Incisors
CARNIVORE: Short and pointed
Teeth: Canines
CARNIVORE: Long, sharp and curved
Teeth: Molars
CARNIVORE: Sharp, jagged and blade shaped
Chewing
CARNIVORE: None; swallows food whole
Saliva
CARNIVORE: No digestive enzymes
Stomach Type
CARNIVORE: Simple
Stomach Acidity
CARNIVORE: Less than or equal to pH 1 with food in stomach
Stomach Capacity
CARNIVORE: 60% to 70% of total volume of digestive tract
Length of Small Intestine
CARNIVORE: 3 to 6 times body length
Colon
CARNIVORE: Simple, short and smooth
Liver
CARNIVORE: Can detoxify vitamin A
Kidney
CARNIVORE: Extremely concentrated urine
Nails
CARNIVORE: Sharp clawsOral Cavity
Carnivores have a wide mouth opening in relation to their head
size. This confers obvious advantages in developing the forces
used in seizing, killing and dismembering prey. Facial musculature
is reduced since these muscles would hinder a wide gape, and play
no part in the animal's preparation of food for swallowing. In all
mammalian carnivores, the jaw joint is a simple hinge joint lying
in the same plane as the teeth. This type of joint is extremely
stable and acts as the pivot point for the "lever arms" formed by
the upper and lower jaws. The primary muscle used for operating
the jaw in carnivores is the temporalis muscle. This muscle is so
massive in carnivores that it accounts for most of the bulk of the
sides of the head (when you pet a dog, you are petting its
temporalis muscles). The "angle" of the mandible (lower jaw) in
carnivores is small. This is because the muscles (masseter and
pterygoids) that attach there are of minor importance in these
animals. The lower jaw of carnivores cannot move forward, and has
very limited side-to-side motion. When the jaw of a carnivore
closes, the blade-shaped cheek molars slide past each other to give
a slicing motion that is very effective for shearing meat off bone.Stomach and Small Intestine
Striking differences between carnivores and herbivores are seen in
these organs. Carnivores have a capacious simple (single-chambered)
stomach. The stomach volume of a carnivore represents
60-70% of the total capacity of the digestive system. Because meat
is relatively easily digested, their small intestines (where
absorption of food molecules takes place) are short -- about three
to five or six times the body length. Since these animals average
a kill only about once a week, a large stomach volume is
advantageous because it allows the animals to quickly gorge
themselves when eating, taking in as much meat as possible at one
time which can then be digested later while resting. Additionally,
the ability of the carnivore stomach to secrete hydrochloric acid
is exceptional. Carnivores are able to keep their gastric pH down
around 1-2 even with food present. This is necessary to facilitate
protein breakdown and to kill the abundant dangerous bacteria often
found in decaying flesh foods.Colon
The large intestine (colon) of carnivores is simple and very short,
as its only purposes are to absorb salt and water. It is
approximately the same diameter as the small intestine and,
consequently, has a limited capacity to function as a reservoir.
The colon is short and non-pouched. The muscle is distributed
throughout the wall, giving the colon a smooth cylindrical
appearance. Although a bacterial population is present in the colon
of carnivores, its activities are essentially putrefactive.What About Omnivores?
One would expect an omnivore to show anatomical features which
equip it to eat both animal and plant foods. According to
evolutionary theory, carnivore gut structure is more primitive than
herbivorous adaptations. Thus, an omnivore might be expected
to be a carnivore which shows some gastrointestinal tract
adaptations to an herbivorous diet.What About Me?
The human gastrointestinal tract features the anatomical
modifications consistent with an herbivorous diet. Humans have
muscular lips and a small opening into the oral cavity. Many of
the so-called "muscles of expression" are actually the muscles used
in chewing. The muscular and agile tongue essential for eating,
has adapted to use in speech and other things. The mandibular
joint is flattened by a cartilaginous plate and is located well
above the plane of the teeth. The temporalis muscle is reduced.
The characteristic "square jaw" of adult males reflects the
expanded angular process of the mandible and the enlarged
masseter/pterygoid muscle group. The human mandible can move
forward to engage the incisors, and side-to-side to crush and
grind.Summary
Facial Muscles
HERBIVORE: Well-developed
OMNIVORE: Reduced
HUMAN: Well-developed
HERBIVORE: Expanded angle
OMNIVORE: Angle not expanded
HUMAN: Expanded angle
HERBIVORE: Above the plane of the molars
OMNIVORE: On same plane as molar teeth
HUMAN: Above the plane of the molars
HERBIVORE: No shear; good side-to-side, front-to-back
OMNIVORE: Shearing; minimal side-to-side
HUMAN: No shear; good side-to-side, front-to-back
HERBIVORE: Masseter and pterygoids
OMNIVORE: Temporalis
HUMAN: Masseter and pterygoids
HERBIVORE: Small
OMNIVORE: Large
HUMAN: Small
HERBIVORE: Broad, flattened and spade shaped
OMNIVORE: Short and pointed
HUMAN: Broad, flattened and spade shaped
HERBIVORE: Dull and short or long (for defense), or none
OMNIVORE: Long, sharp and curved
HUMAN: Short and blunted
HERBIVORE: Flattened with cusps vs complex surface
OMNIVORE: Sharp blades and/or flattened
HUMAN: Flattened with nodular cusps
HERBIVORE: Extensive chewing necessary
OMNIVORE: Swallows food whole and/or simple crushing
HUMAN: Extensive chewing necessary
HERBIVORE: Carbohydrate digesting enzymes
OMNIVORE: No digestive enzymes
HUMAN: Carbohydrate digesting enzymes
HERBIVORE: Simple or multiple chambers
OMNIVORE: Simple
HUMAN: Simple
HERBIVORE: pH 4 to 5 with food in stomach
OMNIVORE: Less than or equal to pH 1 with food in stomach
HUMAN: pH 4 to 5 with food in stomach
HERBIVORE: Less than 30% of total volume of digestive tract
OMNIVORE: 60% to 70% of total volume of digestive tract
HUMAN: 21% to 27% of total volume of digestive tract
HERBIVORE: 10 to more than 12 times body length
OMNIVORE: 4 to 6 times body length
HUMAN: 10 to 11 times body length
HERBIVORE: Long, complex; may be sacculated
OMNIVORE: Simple, short and smooth
HUMAN: Long, sacculated
HERBIVORE: Cannot detoxify vitamin A
OMNIVORE: Can detoxify vitamin A
HUMAN: Cannot detoxify vitamin A
HERBIVORE: Moderately concentrated urine
OMNIVORE: Extremely concentrated urine
HUMAN: Moderately concentrated urine
HERBIVORE: Flattened nails or blunt hooves
OMNIVORE: Sharp claws
HUMAN: Flattened nails