Humans are herbivores, Comparative Anatomy

How do we determine what our natural diet is?

Here’s a look at some of the supporting evidence and how we arrive at the conclusion that humans are herbivores and that a diet of primarily fruits and greens is optimal.

The study of comparative anatomy clearly shows that humans are a type of specialized herbivore called a frugivore.

Comparative anatomy is the primary pillar of the modern taxonomy and classification of animals.  This branch of biology looks at the parts of the animal and puts them into groups according to similarities and differences.  The terminology of carnivore, omnivore, and herbivore come from this discipline because the anatomy and physiology of an animal are all geared toward how it acquires food to sustain itself.  

Carl Linnaeus, the man who is responsible for the modern taxonomy system classified humans as frugivores.  He wrote: “To say that humans have the anatomical structure of an omnivore is an egregious inaccurate statement. Man’s structure, internal and external compared with that of the other animals, shows that fruit and succulent vegetables are his natural food.”

Based on all the anatomy and physiology of humans, we are very clearly herbivores.

Why?  Let’s examine the evidence ourselves.  

We will take a quick look at our teeth, our jaws, our intestinal canals, our stomach pH, our hands and feet, our skin, and our livers and kidneys.

Our teeth are similar to other herbivores.   The most common objection here is that humans have canines, but guess what?  So do many other herbivores.  Hippos, baboons, gorillas, and even some types of deer and camels have massive canines or fangs.  Compare human teeth to cats, dogs, bears, lions, etc.  They are much more similar to cows, even though cows are ruminant herbivores and humans are frugivores.  Human teeth are nearly identical to ape and monkey teeth.

“Herbivores (like the cow) have 24 molars, eight jagged incisors in the lower jaw and a horny palate in the upper jaw. Their jaws move vertically, laterally, forward, and backward, enabling the herbivore to tear and grind coarse grasses. Omnivores (like the hog) can have tusk-like canines allowing them to dig up roots. Frugivores (like the chimpanzee) have 32 teeth: sixteen in each jaw including four incisors, two cuspids, four bicuspids, and six molars. The cuspids are adapted for cracking nuts, and the uniform articulation of the teeth enables the frugivore to mash and grind fruits.  On the contrary, carnivores (like the cat family) have markedly developed canines that are long, sharp, cylindrical, pointed, and set apart from the other teeth. Fangs and sharp pointed teeth that penetrate and kill, that rip and tear flesh, are a feature of all true carnivores (except certain birds). The powerful jaws of the carnivore move only vertically, and are ideal for ripping and tearing flesh that is swallowed virtually whole and then acted upon by extremely potent gastric juices.” (Source:

Human intestinal canals are also sacculated.  This means they have grooves which is a feature that only herbivores have.  The grooves are designed to slow down the transportation of digesting plant matter which is helped along by fiber to get all the nutrition and juices out.  

Omnivores and carnivores have smooth digestive tracts because the waste is much harsher from the flesh foods, and after eating and digesting it they are designed to eliminate the waste much more quickly.  

Carnivores and omnivores also have differences in their livers and kidneys to help with this waste management.  They are also capable of digesting whole bones.  If a human eats bone fragments, their stomach acid isn’t strong enough to break it down and digest it.  Omnivores and carnivores can break down whole bones with no issue because they are designed to do so.

“That brings us to stomach form and size: In the carnivore the stomach is a small, round sack designed to dissolve flesh quickly and then pass it on for removal. In plant eaters (particularly ruminants) stomachs are complicated adjoining sacks with ring-like convolutions. The frugivore stomach (including in humans) is oblong and is characterized by folds called rugae which serve to retain food for relatively long periods.

Organ sizes of various species also markedly vary. The liver and kidneys in the carnivore are much larger than in vegetarian animals. A lion’s kidney is twice the size of a bulls, and not much smaller than the elephants. This allows the lion to handle large amounts of protein and nitrogenous waste products contained in its natural flesh diet. The carnivores huge liver secretes larger amounts of bile into the small intestine than does the herbivores liver. There is a direct relation between the quantity of meat eaten and the amount of bile secreted. Meat-eating therefore, places a strain on the small liver of humans which impairs the organ’s function over a long period of time.  When you place humans on a diet for which they are NOT naturally adapted, this places unnatural stress on the organs of elimination. Humans have never adapted to the carnivorous diet that is high in animal products. The human liver is smaller than the carnivores and as a result, we cannot detoxify the poisonous products inherent within animal foods such as uric acid (discussed below). Our kidneys are also smaller and become diseased from overwork caused by a diet high in animal protein.”

The chief difference between the natural human diet and many modern diets is the inclusion of irritants, cooked foods, and excess protein.  How do we know that humans are designed to thrive on less protein?  Let’s look at human mother’s milk compared to other species and some commentary and analysis by T.C. Fry.

A table and excerpt from T.C. Fry on protein and mother’s milk

The first number is the average number of days a newborn takes to double its weight.  The second number is the average percentage of protein present in the mother’s milk of that species.

SpeciesAvg # of Days for a newborn to double weight % protein present in mother’s milk
man180 days 1.6% protein
horse60 days2.0 % protein
calf47 days3.5% protein
kid19 days4.3% protein
pig18 days5.9% protein
lamb10 days6.5% protein
dog8 days7.1% protein
cat7 days9.5% protein

“The highest need for protein in the diet occurs for most animals during the above periods when the newborn is doubling its birth weight.  It is important that we realize the protein content in mother’s milk, the optimum food nature has provided for rapid growth of the young, is far below the usual foods that are recommended because of their protein content (such as meat, nuts, legumes, grains, etc.).  Protein is indeed important for growth, but we might well question the alleged necessity for concentrated, high-protein foods.

The second role of protein is in the repair of tissues of replacement of worn-out cells.  After an organism reaches its full growth (usually between 18 and 22 years for humans), protein is needed only to supply the loss incidental to tissue waste.  Cell degeneration and waste occur primarily because of toxicity in the body.  If we adopt a lifestyle and diet that introduces a minimal amount of toxins into the body, then tissue waste will decrease significantly.  As a result, actual protein needs will also diminish.  After an individual reaches adulthood, the only protein needs are for the repair and replacement of tissues that have deteriorated, due largely to body toxicity.

Protein is not used directly as fuel for the body or for muscular activity.  In muscular work, excretion of nitrogen as a result of protein usage increases only very slightly.  Instead, it is the excretion of carbonic acid and absorption of oxygen that increase.  These changes indicate that an expenditure of energy is derived mainly from non-nitrogenous foods (such as carbohydrates and fats) and not from protein.  It is true that the body can use protein to generate fuel for physical activity, but it does so by breaking the protein down into a carbohydrate form.  Protein is used as fuel only when there is either an excess of proteins or a lack of carbohydrates.  When this occurs, the body splits off the nitrogenous matter from the protein molecule and uses the remaining carbon contents to produce fuel.  This process not only involves a net loss of energy, but it also places an unnecessary strain on the liver, kidneys and other organs to eliminate the unusable nitrogenous wastes.

It is for this reason that the popular high-protein, low-carbohydrate diets in weight loss and also why they are dangerous.  Since the body has to expend so much energy in converting the excess protein into the needed carbohydrates for fuel, a net loss occurs in the body and the dieter loses weight.  At the same time, he also places a heavy burden on his kidneys to eliminate all the uric acid generated by this protein breakdown and simultaneously overworks an already exhausted liver.

If more physical activity is anticipated, it is only necessary to increase the carbohydrate intake of the diet.  Proteins are very poor in fuel-efficiency and do not aid directly or efficiently in muscular activity.” – T.C. Fry, Life Science Course

Disease causation

Now how do we know that less ideal foods are causing disease issues?  Let’s go into a presentation of the seven stages of disease and a discussion of how disease is formed:

We can see disease build slowly going through each one of these stages if we continue our bad habits.  We can also see disease melt away if we stop our bad habits and resume our natural diet.  I understand it’s a tough pill to swallow, but the gist is that eating less ideal foods causes far too much burden to be placed on our body’s waste management system – i.e., the lymphatic system.  As the lymphatic system gets overwhelmed and dehydrated, symptoms worsen and compound over time, and our bodies will do their best to respond with cleaning events that we typically recognize as colds or flu.

Then we tend to compound the problem by medicating the symptoms, be it with herbs, drugs, or even homeopathic remedies.  Ultimately treating symptoms just burdens the body further and interferes with the body’s natural cleaning processes.  Treating symptoms, at best, essentially just kicks the can down the road.  You may be more comfortable in the short term, but you’re not addressing the underlying cause that’s creating the problem.  That’s why we generally just recommend rest, drinking plenty of water, and eating as close to ideal simple meals as you can muster.  Of course, no one is perfect and it’s up to you how much you want to embrace this type of diet and lifestyle.  It’s just our passion to get this information out there so people can make better-informed decisions on how to care for their health.