Comparative Anatomy of Eating- Were We Meant to Be Vegetarian?

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Misty L. Trepke

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The Comparative Anatomy of Eating

 

Summary:

 

" 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. "

 

The Comparative Anatomy of Eating

 

by Milton R. Mills, M.D.

 

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.

 

Oral 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.

 

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.

 

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 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.

 

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.

 

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.

 

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

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.

 

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

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.

 

Summary

 

Facial Muscles

 

CARNIVORE: Reduced to allow wide mouth gape

HERBIVORE: Well-developed

OMNIVORE: Reduced

HUMAN: Well-developed

 

Jaw Type

 

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

 

Jaw Motion

 

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

 

CARNIVORE: Temporalis

HERBIVORE: Masseter and pterygoids

OMNIVORE: Temporalis

HUMAN: Masseter and pterygoids

 

Mouth Opening vs. Head Size

 

CARNIVORE: Large HERBIVORE: Small OMNIVORE: Large HUMAN:

Small

 

Teeth: Incisors

 

CARNIVORE: Short and pointed

HERBIVORE: Broad, flattened and spade shaped

OMNIVORE: Short and pointed

HUMAN: Broad, flattened and spade shaped

 

Teeth: Canines

 

CARNIVORE: Long, sharp and curved

HERBIVORE: Dull and short or long (for defense), or none

OMNIVORE: Long, sharp and curved

HUMAN: Short and blunted

 

Teeth: Molars

 

CARNIVORE: Sharp, jagged and blade shaped

HERBIVORE: Flattened with cusps vs complex surface

OMNIVORE: Sharp blades and/or flattened

HUMAN: Flattened with nodular cusps

 

Chewing

 

CARNIVORE: None; swallows food whole

HERBIVORE: Extensive chewing necessary

OMNIVORE: Swallows food whole and/or simple crushing

HUMAN: Extensive chewing necessary

 

Saliva

 

CARNIVORE: No digestive enzymes

HERBIVORE: Carbohydrate digesting enzymes

OMNIVORE: No digestive enzymes

HUMAN: Carbohydrate digesting enzymes

 

Stomach Type

 

CARNIVORE: Simple

HERBIVORE: Simple or multiple chambers

OMNIVORE: Simple

HUMAN: Simple

 

Stomach Acidity

 

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

 

Stomach Capacity

 

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

 

Colon

 

CARNIVORE: Simple, short and smooth

HERBIVORE: Long, complex; may be sacculated

OMNIVORE: Simple, short and smooth

HUMAN: Long, sacculated

 

Liver

 

CARNIVORE: Can detoxify vitamin A

HERBIVORE: Cannot detoxify vitamin A

OMNIVORE: Can detoxify vitamin A

HUMAN: Cannot detoxify vitamin A

 

Kidney

 

CARNIVORE: Extremely concentrated urine

HERBIVORE: Moderately concentrated urine

OMNIVORE: Extremely concentrated urine

HUMAN: Moderately concentrated urine

 

Nails

 

CARNIVORE: Sharp claws

HERBIVORE: Flattened nails or blunt hooves

OMNIVORE: Sharp claws

HUMAN: Flattened nails