The Comparative Anatomy of Eating
By: Dr. Milton R. Mills, M.D. November 2009
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.
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
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.
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
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.
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.
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
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.
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.
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
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
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.
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
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
(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
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
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
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
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 or risk jaw dislocation, death
and ultimately, extinction.
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. 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
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
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-
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
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
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
Carnivore Angle not expanded
Herbivore Expanded angle
Omnivore Angle not expanded
Human Expanded angle
Jaw Joint Location
Carnivore On same plane as molar teeth
Herbivore Above the plane of the molars
Omnivore On same plane as molar teeth
Human Above the plane of the molars
Carnivore Shearing; minimal side-to-side motion
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
Major Jaw Muscles
Herbivore Masseter and pterygoids
Human Masseter and pterygoids
Mouth Opening vs. Head Size
Carnivore Short and pointed
Herbivore Broad, flattened and spade shaped
Omnivore Short and pointed
Human Broad, flattened and spade shaped
Carnivore Long, sharp and curved
Herbivore Dull and short or long (for defense), or none
Omnivore Long, sharp and curved
Human Short and blunted
Carnivore Sharp, jagged and blade shaped
Herbivore Flattened with cusps vs complex surface
Omnivore Sharp blades and/or flattened
Human Flattened with nodular cusps
Carnivore None; swallows food whole
Herbivore Extensive chewing necessary
Omnivore Swallows food whole and/or simple crushing
Human Extensive chewing necessary
Carnivore No digestive enzymes
Herbivore Carbohydrate digesting enzymes
Omnivore No digestive enzymes
Human Carbohydrate digesting enzymes
Herbivore Simple or multiple chambers
Carnivore Less than or equal to pH 1 with food in stomach
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
Carnivore 60% to 70% of total volume of digestive tract
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
Length of Small Intestine
Carnivore 3 to 6 times body length
Herbivore 10 to more than 12 times body length
Omnivore 4 to 6 times body length
Human 10 to 11 times body length
Carnivore Simple, short and smooth
Herbivore Long, complex; may be sacculated
Omnivore Simple, short and smooth
Human Long, sacculated
Carnivore Can detoxify vitamin A
Herbivore Cannot detoxify vitamin A
Omnivore Can detoxify vitamin A
Human Cannot detoxify vitamin A
Carnivore Extremely concentrated urine
Herbivore Moderately concentrated urine
Omnivore Extremely concentrated urine
Human Moderately concentrated urine
Carnivore Sharp claws
Herbivore Flattened nails or blunt hooves
Omnivore Sharp claws
Human Flattened nails
According to biologists and anthropologists who study our anatomy
and our evolutionary history, humans are herbivores who are not
well suited to eating meat.
Unlike natural carnivores, we are physically and psychologically
unable to rip animals limb from limb and eat and digest their raw
flesh. Even cooked meat is likely to cause human beings, but not
natural carnivores, to suffer from food poisoning, heart disease,
and other ailments.
People who pride themselves on being part of the human hunter
tradition should take a second look at the story of human evolution.
Prehistoric evidence indicates that humans developed hunting skills
relatively recently and that most of our short, meat-eating past was
spent scavenging and eating almost anything in order to survive;
even then, meat was a tiny part of our caloric intake.
Humans lack both the physical characteristics of carnivores and
the instinct that drives them to kill animals and devour their raw
carcasses. Ask yourself: When you see dead animals on the side
of the road, are you tempted to stop for a snack? Does the sight of
a dead bird make you salivate? Do you daydream about killing cows
with your bare hands and eating them raw? If you answered "no"
to all of these questions, congratulations—you're a normal human
herbivore—like it or not. Humans were simply not designed to
By: Dr. Milton R. Mills, M.D.
Article: The Comparative Anatomy of Eating