Comparison of the Human and Animal

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Comparison of the anatomy of gastrointestinal tract

People are most often classified as "omnivores". This classification was made upon observing that people include a wide range of plant and animal food in their diet. However, culture, tradition and education also have a major impact on the human diet, making observation not the best technique to discover the most natural diet for humans. Although most people behave as omnivores, the question is whether human anatomy is adjusted to a diet that includes plant and animal food. A much better and more objective technique that can give an answer to this question is to look at human anatomy and physiology. Mammals are anatomically and physiologically adapted to obtain and consume certain types of food (the common practice when examining fossils of extinct mammals is to study anatomical features to determine the likeliest nutritional habits). By studying the anatomy and physiology of mammals, carnivores, herbivores and omnivores, we can determine anatomical and physiological features connected with a particular type of food. If we compare human anatomy and physiology to those of other animals, we can see to which group we belong.

Oral cavity

Carnivores have a wide mouth opening in comparison to the size of their head. This has obvious advantages for the development of hunting power, killing and dismembering of prey. The facial muscles are weakened, otherwise they would hinder opening the mouth, and they play no part in the preparation of food for swallowing. The jaw of all mammalian carnivores is a simple hinge joint in line with the teeth. This kind of connection is very stable and acts as a central point of the lever arms consisting of the upper and lower jaw. The main muscle that moves carnivores' jaw is the temporalis muscle. This muscle is so massive that it takes up most of the head's circumference.

When you pet a dog, you are petting its temporalis muscles. The angle of the upper jaw in carnivores is small because the muscles that are attached to it (masseter and pterygoids) are not of great importance for these animals. The lower jaw in carnivores cannot move forward, and side to side movement is limited. When the jaw closes, the blade-shaped molars glide toward each other, creating an ideal mechanism for tearing flesh from bone. Carnivores' teeth are separated so that tough pieces of meat would not get stuck between them. The incisors are small and are used to grasp and shred prey. The canines are quite long and dagger-like. The animal uses them to wound, tear and kill prey. The molars are flattened and triangular with jagged edges so that they can function like serrated blades. Due to the ability of the jaw to unfold, the molars of the upper and lower jaws are connected from behind, just like scissors.

The saliva of carnivores does not contain digestive enzymes. When a carnivorous mammal greedily swallows the food, it does it so fast that it does not even chew. Since the enzymes for protein digestion cannot be released inside the mouth because of the danger of destroying the oral cavity (auto-digestion), carnivores do not need to mix their food with saliva. They simply bite off huge chunks of meat and swallow them whole, without crushing.

According to the evolution theory, the anatomical features that are connected with the plant-based diet were developed later than those of carnivores. Herbivorous mammals have well-developed facial muscles, thick fleshy lips, a relatively small mouth opening and a thickened, muscular tongue. The lips help to bring food into the mouth and are involved in chewing, along with the muscles of the face and tongue. In herbivores, the jaw joint has moved to a position above the teeth. Even though such structure is less stable than the carnivore joint, it is much more mobile and allows the complex jaw motions needed when chewing plant foods. In addition, such joint enables the upper and lower molars to overlap along the entire jaw during the closing of the mouth. This creates a platform for grinding food (this kind of jaw joint is so important for animals that feed on plants that it has evolved over 15 times in different species of herbivorous mammals). The temporalis muscle is small and of minor importance, while the masseter and pterygoids hold the upper jaw like a swing which they move from one side to another. Accordingly, the lower jaw has emphasized lateral movement.

This lateral movement of the jaw is necessary to grind food while chewing. The arrangement of the teeth greatly varies from species to species, depending on the kind of vegetation they consume.

Although these animals differ by the type and number of teeth, their teeth share common structural features. The incisors are wide, flattened and rhombus. Canines may be small as in the horse, prominent as in hippos, pigs and some primates (presumably they are used for defense) or completely absent. In general, the molars are square and flattened on top to provide a grinding surface. The molars do not glide side by side vertically in a slicing motion, but glide across one another horizontally to crush and grind. The surface of the molars varies, depending on the type of plants they feed on.

The teeth of herbivorous animals are closely grouped so that the incisors form a mechanism for biting and cropping, and the upper and lower molars form extended platforms for crushing and grinding food.

The "walled-in" oral cavity has the ability to greatly expand during eating. Herbivores carefully and methodically chew their food by pushing it with their tongue and facial muscles back and forth between their teeth for grinding or milling. This thorough process is necessary to mechanically disrupt plant cells in order to release the digestible intracellular content and mix it with saliva. Mixing food with saliva is very important because the saliva of omnivorous and herbivorous mammals contains carbohydrate-digesting enzymes that break down food molecules while the food is still in their mouth.

Stomach and small intestine

Striking differences between carnivores and herbivores are seen in these organs. The stomach volume of a carnivore represents 60-70% of the total capacity of the digestive system. The small intestine (where the absorption of food molecules takes place) in carnivores is short—about 3-5 times the body length. Considering that these animals kill once a week, a large stomach is an advantage because it allows them to overfeed, stuffing themselves as much as possible. That food is digested later on while the animal is resting. In addition, the carnivore's stomach has an exceptional ability to secrete hydrochloric acid. Carnivores are able to keep the pH in the stomach at the level of 1-2, even if the food is still in the stomach. This is necessary to facilitate protein breakdown and destroy dangerous bacteria often found in decaying flesh.

Due to the difficulty of digesting certain types of plant food (due to large amounts of indigestible fibers), herbivores have significantly longer and, in some cases, far more elaborate guts than carnivores. Herbivores that eat plants rich in cellulose have to ferment food (digest by bacterial enzyme action) to obtain the nutrient value. Herbivores are divided into 2 groups: ruminants (who ferment food in the front part of the intestine) and those who ferment food in the hindgut.

The ruminants are the herbivores with the famous multi-chambered stomach. Herbivores that eat soft vegetation do not need a multi-chambered stomach, so they have a simple stomach and a long small intestine. These animals ferment difficult-to-digest fibrous portions in the colon. Many of these herbivores improve the effectiveness of their digestive tract with the help of carbohydrate-digesting enzymes in their saliva. The fermentation process in a multiple-stomach would be energetically wasteful in animals that feed on soft, pulpy vegetation. The bacteria and protozoa involved in fermentation would consume nutrients and calories before they reach the small intestine for absorption into blood. The small intestine of herbivores is quite long (over 10 times body length) in order to provide sufficient time and space for the absorption of nutrients.


The colon of carnivores is very short and simple because its only function is to absorb water and salt. The diameter of the colon is approximately the same as the small intestine and it is therefore a reservoir of limited capacity. The colon is short and is not separated into pouches. The muscle is distributed throughout the colon, giving it a smooth, cylindrical shape. Even though bacteria are present in the colon, their activities are essentially connected with decay.

Herbivores' colon is a highly specialized organ involved in water and electrolyte absorption, vitamin production and absorption and/or fermentation of fibrous plant materials. The colon is usually wider than the small intestine and relatively long. In some herbivorous mammals, colon looks pouched due to the way that the muscle fibers are positioned in the intestinal wall. In addition, the first part of the colon in some herbivores is quite large and serves as the primary or additional place for fermentation.

What about omnivores?

One would expect an omnivore to show anatomical features of animals that eat meat and those who eat plant foods. According to the evolution theory, the carnivore gut structure is more primitive than that of herbivores. It could be assumed that omnivores are carnivores with some adaptations to the herbivorous diet. That is the case with bears, raccoons and some other specific species that are considered part of the Canine families (this discussion will be limited to bears since bears are, in general, representative of omnivores).

Bears are classified as carnivores, but according to their anatomy, they are omnivores. Although the bear diet includes meat, their nutrition is primarily plant-based (78-80% of their diet consists of plant foods, except for the polar bear, who lives in the Arctic, which is poor in vegetation, and feeds primarily on seals). Bears cannot digest fibrous vegetation and are therefore very picky.

Their diet is dominated by primarily succulent herbage, tubers and berries. Many scientists believe that the reason bears hibernate is that their chief food (succulent vegetation) is absent during the cold northern winters (it is interesting to note that Polar bears hibernate during the summer months, when seals are unavailable).

In general, bears exhibit anatomical features of carnivores. Their jaw joint is level with the molars. The temporalis muscle is massive and the angle of the mandible is small, which is in accordance with the limited role of the pterygoid and masseter muscles in operating the jaw. The small intestine is short (less than five times body length), as in the case of the pure carnivores, and the colon is simple, smooth and short. Bears' and other omnivores' most significant adjustment to plant food is the modification in their teeth arrangement. Bears retain the peg-shaped incisors, large canines and shearing premolars, while the molars have become squared with rounded cusps for crushing and grinding.

Bears do not have the flattened and blunt nails seen in most herbivores, but retain the elongated, pointed claws of a carnivore. An animal that hunts, kills and eats other animals must have the physical equipment to make it happen and make it easier. As the bear diet includes meat, they must maintain anatomical characteristics that allow them to hunt and kill animals. Therefore, they have the jaws, muscles and teeth arrangement that enable the development and use of the power necessary for killing and dismembering animals, even though their diet is mainly herbivorous. The jaw joint of herbivores (above the level of the teeth) is much more effective for breaking and grinding vegetation and would potentially provide bears with a greater selection of plant food. However, such joint is much weaker than the carnivores'.

Herbivores' jaw joint would be likely to dislocate and would not be able to withstand harsh blows from the animal fighting for its life in the mouth of a predator, nor the pressure when breaking bones. Besides, such a joint does not provide a mouth opening large enough to catch and swallow prey. In the wild, an animal with a sprained jaw would starve to death or would be eaten by other animals.

We can conclude that a species like bears would be doomed to have a jaw like herbivores, because such an agile and efficient, but weaker jaw joint would require a shift toward an essentially vegetarian diet. It would expose these animals to the risk of dislocating the jaw, death, and finally extinction.




Based on a chart from A.D. Andrews' book, "Food Fit for Men" (Chicago: American Hygiene Society, 1970)

What about us?

The human digestive tract shows the anatomical changes in accordance with the plant-based diet. Humans have muscular lips and a small mouth opening. Many of the muscles that form facial expressions are actually involved in the chewing process. The muscular and agile tongue that we use while eating has developed other functions; for example, it is involved in speech. The jaw joint is flattened by a cartilaginous plate and is located quite above the teeth level. The temporalis muscle is weakened.

The characteristic "square jaw" in males reflects the expanded angular lower jaw and enlarged chewing muscles. The human jaw can be moved forward to engage the incisors, as also from side to side, allowing food crushing and grinding.

Human teeth are also similar to those of herbivores, except canines (the fangs in some apes are elongated and serve as a warning sign or for defense). Our teeth are generally rather large and compressed together. The incisors are flat and spade-shaped. They are useful for peeling, cutting and biting relatively soft materials. The canines are neither serrated nor conical (they are small, flattened, and function like incisors). The premolars and molars are somewhat rectangular, flattened and knobby, and are used for crushing, grinding, and the conversion of food into mush.

Human saliva contains the enzyme for the decomposition of carbohydrates, the so-called salivary amylase. This enzyme is responsible for the digestion of starch. The esophagus is small and suitable for small, soft balls of well-masticated food. If you eat quickly, swallow large amounts of food or fibrous and/or poorly chewed food (such as meat), you can suffocate.

The human stomach has only one chamber and is slightly acidic (clinically speaking, when a person has the pH of a full stomach less than 4-5, there is cause for concern). The stomach volume occupies 21-27% of the total volume of the digestive system of humans. The stomach is used for storing, mixing and liquefying food and regulating its entry into the small intestine. The human small intestine is long, around 10-11 times the body length (our small intestine is 6-9 meters long). The human body is measured from the top of the head to the end of the spine and the average is around 60-90 centimeters (in humans of average height).

The human colon is pouched, which is inherent to herbivores. The stretchable colon in cross section is much larger than the small intestine and is quite long. The human colon is responsible for the absorption of water and electrolytes and production and absorption of vitamins. Fermentation of fibrous plant material with the production and absorption of significant amounts of energy (volatile short-chain fatty acids) also occurs there. We have only recently started learning about the extent of the fermentation and absorption of metabolites in the human colon.

In conclusion, we can say that the structure of the human digestive tract is the same as in herbivores. Humankind does not show mixed features in the structure of digestive tract, as would be expected in omnivores such as bears and raccoons.

Therefore, comparing the human digestive system and those found in carnivores, herbivores and omnivores, we can conclude that the human digestive tract is designed to digest only plant foods.

Now, imagine that there are no slaughterhouses, knives and butchers, and that you found yourself face to face with a cow that you want to eat. You have only your natural weapons—your hands and teeth—and you need to kill the cow to get food. Would you be able to do it? Jump on her and bite her neck? Do you think that you would do better with a rabbit, which is significantly smaller than a cow? Even if you broke his neck, would you tear it apart with your teeth?!?

"People are just not designed for eating meat. You put a baby in a crib with an apple and a rabbit. If it eats the rabbit and plays with the apple, I'll buy you a new car!" Harvey Diamond (writer)

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