In my book, Humans are Herbivores: A Scientific Case for Veganism, we explored the medical data illustrating the many health risks of consuming animal products. Now, we will evaluate the anatomical evidence supporting the conclusion that human beings are categorically herbivorous animals. The question of our taxonomical classification is really one of evolutionary biology. Fortunately, scientists have identified obvious physical distinctions among mammals that are dead giveaways as to whether or not a particular animal is naturally designed to eat meat, plants and meat, or just plants.

As Milton R. Mills, M.D. points out in The Comparative Anatomy of Eating,

“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.”

40 Anatomical Features that Classify Humans as Herbivores

There are several types of features that help biologists determine the feeding behaviors of animals. For instance, digestive abilities, “hunting or gathering” abilities, reproductive habits, locomotive abilities, and circadian rhythms are all factors taken into consideration. To illustrate this better, here are two simple examples. Stomach acidity differs greatly between herbivores, omnivores, and carnivores, which would demonstrate an animal’s evolutionary adaptation to digest simple carbohydrates such as fruit or complex proteins and fats such as meat. Additionally, whether or not an animal has claws will likely determine if it uses them for hunting or not.

It is important that a conclusion is not reached after reviewing one or two data points. Rather, we must make our conclusion by assessing the totality of all available data. To conclude humans are anything other than herbivores is to ignore the following striking pieces of evidence.

1. Carnivores have facial muscles that are reduced to allow a wide mouth gap to swallow large chunks of meat or entire animals whole. Omnivores’ facial muscles are also reduces. However, herbivores and humans have in common well-developed facial muscles for chewing plant matter.
2. Both carnivores and omnivores have a jaw angle that is acute. Both herbivores and humans have an expanded jaw angle.
3. The location of the jaw joint in carnivores and omnivores is on the same plane as their molar teeth. The location of the jaw joint in herbivores and humans is above the plan of the molars.
4. The jaw in carnivores and omnivores is designed to shear and has minimal side-to-side motion. The jaw in herbivores and humans is very dexterous and moves side-to-side and front-to-back.
5. The major jaw muscle in carnivores and omnivores is the temporalis. The major jaw muscles in herbivores and humans are the masseter and pterygoids.
6. The size comparison between mouth opening and head size is very exaggerated in carnivores and omnivores. However, in herbivores and humans the mouth opening to head size is quite small.
7. In carnivores and omnivores, the incisor teeth are short and pointed. In herbivores and humans, the incisor teeth are broad, flattened, and spade-shaped.
8. In carnivores and omnivores, the canine teeth are long, sharp, and curved. In herbivores, the canine teeth are dull and usually short, although sometimes they are long for defense. Other times, herbivores have no canines at all. In humans, they are short and blunted. It is worth pointing out that just because we refer to our own teeth as canines, does not mean they have anything in common with actual canines in carnivores and this name has more to do with their location on the jaw.
9. In carnivores and omnivores, their molar teeth are sharp, jagged, and blade-shaped. In herbivores, they are flattened with cusps or have a complex surface. In humans, they are flattened with nodular cusps.
10. Carnivores do not chew their food, they swallow it whole. Omnivores swallow food whole or perform simple crushing before swallowing. Herbivores and humans require extensive chewing before swallowing food.
11. The saliva in carnivores and omnivores contains no digestive enzymes whatsoever. The saliva in herbivores and humans contains carbohydrate digesting enzymes.
12. Carnivores and omnivores have what is called a “simple” stomach. Herbivores can either have simple stomachs or stomachs with multiple chambers. In this case, humans have simple stomachs. However, the pH of carnivore and omnivore stomachs is less than or equal to 1. In herbivores and humans, the stomach pH is between 4 and 5.
13. The stomach capacity of carnivores and omnivores is roughly 60% to 70% of the total volume of the digestive tract. In herbivores, it is less than 30% of the total volume of the digest tract. In humans and frugivores, or animals that only eat fruit, it is 21% to 27% the total volume of the digestive tract.
14. Carnivores have a liver that is proportionally 50% larger than others. Omnivores have a liver that is proportionally larger than herbivores. Herbivores have a liver that is proportionally larger than frugivores. Frugivores and humans have the smallest livers.
15. In carnivores and omnivores, the length of the small intestine is 3 to 6 times the body length, measured from neck to anus. In herbivores, the length of the small intestine can measure 10 to 12 times the body length, and sometimes more. In humans, the small intestines are 10 to 11 times the body length.
16. Carnivores and omnivores have colons that are simple, short, and smooth. Herbivores have colons that are long, complex, and may be sacculated. Humans have colons that are long and sacculated.
17. Carnivores and omnivores can detoxify preformed vitamin A from food with their liver. Herbivores and humans cannot and require pro-vitamin A carotenoids.
18. Carnivores and omnivores have kidneys that produce extremely concentrated urine. Herbivores and humans produce moderately concentrated urine.
19. Carnivores and omnivores have bile flow that is comparatively moderate to heavy. Herbivores and humans have bile flow that is comparatively weak.
20. The kidneys of carnivores and omnivores produce urate oxidase, or uricase. The kidneys of herbivores and humans do not.
21. The colons of carnivores and omnivores are alkaline. The colons of herbivores and humans are acidic.
22. For carnivores and omnivores, peristalsis does not require fiber to stimulate. For herbivores and humans, it does.
23. Carnivores and omnivores can metabolize large amounts of cholesterol efficiently. Herbivores and humans can only metabolize phytosterols efficiently.
24. Carnivores require approximately 2 to 4 hours to digest a meal. Omnivores require approximately 6 to 10 hours to digest a meal. Herbivores require approximately 24 to 48 hours to digest a meal. Frugivores and humans require approximately 12 to 18 hours to digest a meal.
25. Carnivores and omnivores cannot convert short chain fatty acids into long chain fatty acids. Herbivores and humans can.
26. Carnivores and omnivores have sharp claws. Herbivores have flattened nails or blunt hooves. Humans have flattened nails.
27. Carnivores and omnivores have zonary-shaped placentas. Herbivores and humans have discoid-shaped placentas.
28. Carnivores and omnivores cool themselves by panting and only have sweat glands in their paws if they have paws. Herbivores and humans have sweat glands all over their bodies.
29. Carnivores and omnivores are 100% covered in hair. Herbivores and humans have pores with extensive hair covering their bodies.
30. Carnivores and omnivores have multiple teats for nursing litters of offspring. Some herbivores also have multiple teats. Frugivores and humans have dual breasts for nursing one to two offspring.
31. Carnivores, omnivores, and herbivores walk on all fours. Humans and frugivores walk upright or at least have the ability to do so.
32. Carnivores, omnivores, and some herbivores produce vitamin C endogenously. Frugivores, some herbivores, and humans must consume vitamin C through their diet.
33. Carnivores require taurine in their diet which is found in most animal tissues such as muscle, viscera, and brain but is not found in plants. Humans and most omnivores and herbivores synthesize taurine endogenously.⁹⁰
34. Carnivores and omnivores do not have prehensile arms, hands, feet, or tails. Herbivores and humans do.
35. The brains of carnivores and omnivores are fueled by fats and proteins. The brains of herbivores and humans are fueled by glycogen.
36. Carnivores and omnivores do not have full-color vision. Herbivores and humans have full-color vision.
37. Humans and herbivores sleep approximately 8 hours per 24 hour cycle, whereas carnivores and omnivores spend approximately 18 to 20 hours sleeping per 24 hour cycle.
38. Carnivores and omnivores drink by lapping their tongue. Herbivores and humans drink by sipping with their upper lip.
39. Carnivores are generally adapted for short sprints to catch their prey. Herbivores are generally adapted for endurance to outlast and outrun their predators. Humans are adapted for endurance (with the assistance of the aforementioned sweat glands).
40. Male carnivores do not have seminal vesicles as part of their reproductive anatomy. Male herbivores and male humans do have seminal vesicles. Erectile dysfunction is more often seen in men with elevated cholesterol levels and high levels of LDL “bad” cholesterol.

Moreover, our closest primate relatives eat an almost exclusively herbivorous or frugivorous diet. Also, ironically, the number one cause of choking deaths in humans is from eating meat according to a 2007 study by Dolkas et al.

The Evolution of Binocular Vision

A false assertion made my meat proponents is that humans have binocular vision which proves we are designed to hunt. In 1974, Matt Cartmill proposed the Visual Predation Hypothesis which essentially states that prey species typically have eyes on the sides of their heads to look out for predators while predators have evolved binocular vision to stalk their prey.

If we examine our pre-hominid ancestors starting with Dryomomys some 55 million years ago, which was essentially a tree-dwelling shrew, which did not have forward facing eyes but was arguably our first ancestral creature to switch from eating insects to eating fruit. Around the same time period, Carpolestes, similar to today’s wooly possum, also had eyes that were not forward facing. Nonetheless, it had teeth that were highly specialized for eating flowers, seeds, and fruit.

Then came Notharctus at 45 million years ago, similar to a modern day lemur, whose diet consisted primarily of fruit and leaves based on the fossil remains we have of its teeth. This was our first primate-like ancestor and it just so happened to have evolved binocular vision – not for hunting, but for navigating tree branches easier in the forest canopy as it leaped long distances high up in the air. This is known as the Arboreal Locomotion Hypothesis.

There are a few exceptions to this hypothesis, such as squirrels which live in trees but have eyes on the sides of their heads. However, some have pointed out that the only exceptions to this rule are smaller mammals, concluding that there is a larger selective pressure for larger animals who stand to risk greater injury if they fall from the treetops.

Some biologists posit that binocular vision was also imperative for early primates to begin manipulating plant foods, such as twisting and plucking fruit from branches or peeling it. According to a 2004 paper by R. A. Barton entitled Binocularity and brain evolution in primates,

“Fine-grained  stereopsis  is  likely  to  be  critical  for  the  visually guided,  delicate  manipulation  of  plant  foods,  which  has  been proposed  as  a  key  adaptation  of  ancestral  primates.”

Approximately 30 million years ago, Aegyptopithecus emerged, similar to a modern day howler monkey, which had eyes even closer together on the front of its face. Both Aegyptopithecus and the howler monkey are considered herbivores and predominantly eat fruit.

What we see is a complete inversion of Cartmill’s Visual Predation Hypothesis. In fact, during the period of time where our early ancestors evolved binocular vision, they actually adapted from a diet consisting mostly of insects to one consisting mostly of fruit. In the same time frame, our ancestors evolved from having claws to having fingernails and they lost their overlapping predatory molars used for shearing flesh and snapping bones (or insect carcasses) and developed in-line molars for grinding plant matter. Additionally, our vision and smell dulled, both of which are highly refined for predators.

In 2008, Changizi and Shimojo published a report called “X-ray vision” and the evolution of forward facing eyes, which is perhaps the most compelling hypothesis to date.⁹⁷ In their own words it states,

“…the degree of binocular convergence is selected to maximize how much the mammal can see in its environment. Mammals in non-cluttered environments can see the most around them with panoramic, laterally directed eyes. Mammals in cluttered environments, however, can see best when their eyes face forward, for binocularity has the power of “seeing through” clutter out in the world. Evidence across mammals closely fits the predictions of this “X-ray” hypothesis.”

In fact, the scientists actually found a direct correlation between the relative degree of how leafy an animal’s environment is and the distance between its eyes. This is especially true for predators in the ocean such as sharks, dolphins, and octopi which have eyes on the sides of their heads but have no problems finding and killing their prey. Moreover, many large herbivorous mammals have eyes on the front of their heads including koalas, tree kangaroos, and sloths.

Diets of Early Hominids and Brain Development

Meat proponents like to attribute humans’ rapid brain development with eating meat. Of all the arguments put forward by meat proponents, this one is perhaps the silliest. There are tens if not hundreds of thousands of species that have consumed meat for much longer than humans and our early hominid ancestors. In fact, many species have eaten an exclusively carnivorous diet for millions of years longer than us. Why aren’t sharks, crocodiles, or tigers the smartest species on the planet?

In 2014, Melin and associates published a review in the Journal of Human Evolution claiming that the change of seasons and the quest for elusive bugs spurred tool use and problem-solving skills among primates. Although they may have been on the “hunt” for insects, this likely comprised less than five percent of their overall diet, the rest consisting of fruit, leaves, seeds, and flowers.

Another hypothesis was postulated by Harvard biologist Richard Wrangham that stated the sudden and dramatic availability to calories and carbohydrates through the use of fire to cook food afforded early hominids the ability to more adequately meet their nutritional needs and provide much more fuel for their brains. Fire happens to be a tool unique to humans and has played a major role in our diet for quite some time.

According to Smithsonian Magazine,

“[Wragham’s colleague Rachel] Carmody explains that only a fraction of the calories in raw starch and protein are absorbed by the body directly via the small intestine. The remainder passes into the large bowel, where it is broken down by that organ’s ravenous population of microbes, which consume the lion’s share for themselves. Cooked food, by contrast, is mostly digested by the time it enters the colon; for the same amount of calories ingested, the body gets roughly 30 percent more energy from cooked oat, wheat or potato starch as compared to raw, and as much as 78 percent from the protein in an egg.”

A California study published in the Journal of Nutrition in 1999 concluded that,

“Anthropoids, including all great apes, take most of their diet from plants, and there is general consensus that humans come from a strongly herbivorous ancestry. Though gut proportions differ, overall gut anatomy and the pattern of digestive kinetics of extant apes and humans are very similar. Analysis of tropical forest leaves and fruits routinely consumed by wild primates shows that many of these foods are good sources of hexoses, cellulose, hemicellulose, pectic substances, vitamin C, minerals, essential fatty acids, and protein. In general, relative to body weight, the average wild monkey or ape appears to take in far higher levels of many essential nutrients each day than the average American and such nutrients (as well as other substances) are being consumed together in their natural chemical matrix. The recommendation that Americans consume more fresh fruits and vegetables in greater variety appears well supported by data on the diets of free-ranging monkeys and apes.”

Atherosclerosis Only Affects Herbivores

In Chapter One of Humans are Herbivores, we looked at the causal relationship between dietary cholesterol and coronary heart disease, multiple types of cancer, and Alzheimer’s. Now, let’s examine the various ways scientists know that cholesterol causes atherosclerosis, a disease that scientists are only able to create in herbivores.

At the 39^th Annual Williamsburg Conference on Heart Disease held in 2012, medical doctors Mina Benjamin and William Roberts pointed out that while there are 10 risk factors for atherosclerosis, or narrowing of the arteries, 9 of these factors are contributory at most but do not cause the disease in and of themselves. The only risk factor known to cause atherosclerosis is dietary cholesterol.

After spending 50 years researching coronary heart disease, William Roberts published his four key findings proving that this disease manifests from the consumption of cholesterol.

“Atherosclerosis is easily produced experimentally in herbivores (monkeys, rabbits) by giving them diets containing large quantities of cholesterol (egg yolks) or saturated fat (animal fat). Indeed, atherosclerosis is one of the easiest diseases to produce experimentally, but the recipient must be an herbivore. It is not possible to produce atherosclerosis in carnivores (tigers, lions, dogs, etc.). In contrast, it is not possible to produce atherosclerosis simply by raising a rabbit’s blood pressure or blowing cigarette smoke in its face for an entire lifetime.

Atherosclerotic plaques contain cholesterol.

Societies with high average cholesterol levels have higher event rates (heart attacks, etc.) than societies with much lower average cholesterol levels.

 When serum cholesterol levels (especially the low-density lipoprotein cholesterol [LDL-C] level) are lowered (most readily, of course, by statin drugs), atherosclerotic events fall accordingly and the lower the level, the fewer the events (“less is more”). Although most humans consider themselves carnivores or at least omnivores, basically we humans have characteristics of herbivores.”

Individuals who have a cholesterol score between 50mg and 70mg per dl do not develop atherosclerosis whereas a cholesterol score above 75mg/dl causes the progression of atherosclerosis and this relationship is linear, as in the higher the cholesterol score the faster the atherosclerotic progression.

Incidence of Chronic Diseases Unseen in Predominantly Vegetarian Societies

In a 2007 article published in the Baylor University Medical Center Proceedings, William Roberts, M.D. also pointed out the following.

“Some extremely common conditions in the Western world are relatively uncommon in purely or predominantly vegetarian and fruit-eating societies. These include 1) severe atherosclerosis and its devastating consequences (heart attacks, brain attacks, etc.); 2) systemic hypertension: in societies that eat minuscule amounts of salt, the systemic arterial blood pressure is usually about 90/60 mm Hg, a level near what it is at birth but a level in the Western world often associated with shock; 3) stroke; 4) obesity; 5) diabetes mellitus; 6) some common cancers (colon, breast, prostate gland); 7) constipation, cholecystitis, gallstones, appendicitis, diverticulosis, hemorrhoids, inguinal hernia, varicose veins; 8) renal stones; 9) osteoporosis and osteoarthritis; 10) salmonellosis and trichinosis; and 11) cataracts and macular degeneration.”

A 2014 meta-analysis conducted by Lap Tai Le and Joan Sabaté published in the Journal of Nutrients reviewed thirteen articles involving hundreds of thousands of participants in total. They concluded,

“In summary, vegetarians have consistently shown to have lower risks for cardiometabolic outcomes and some cancers across all three prospec tive cohorts of Adventists. Beyond meatless diets, further avoidance of eggs and dairy products may offer a mild additional benefit. Compared to lacto-ovo-vegetarian diets, vegan diets seem to provide some added protection against obesity, hypertension, type-2 diabetes; and cardiovascular mortality. In general, the protective effects of vegetarian diets are stronger in men than in women.”

In 2013, Tantamango-Bartley and colleagues performed an analysis on the prevalence of cancer between vegetarian and non-vegetarian groups to assess animal products as dietary risk factors. They concluded that vegetarian diets offer protection against multiple forms of cancer. In their own words,

“We examined the association between dietary patterns (non-vegetarians, lacto, pesco, vegan, and semi-vegetarian) and the overall cancer incidence among 69,120 participants of the Adventist Health Study-2. Cancer cases were identified by matching to cancer registries. Cox proportional hazard regression analysis was conducted to estimate hazard ratios, with “attained age” as the time variable.”

Red Meat and Mortality

A 28-year study conducted by Pan et al. which ran from 1980 to 2008 examined 121,342 participants to determine the health risk, if any, that red meat consumption poses to humans.¹¹⁰ In total, they documented 23,926 deaths and 2.96 million person-years of aggregate data. Suffice to say, this study has attained the gold-standard of medical evaluation.

“We prospectively followed 37698 men from the Health Professionals Follow-up Study (1986-2008) and 83644 women from the Nurses’ Health Study (1980-2008), who were free of cardiovascular disease (CVD) and cancer at baseline. Diet was assessed by validated food-frequency questionnaires and updated every four years.”

The study’s authors concluded that red meat consumption is associated with an increased risk of cardiovascular disease mortality, cancer mortality, and all cause mortality. Additionally, they found that substitution of other healthy protein sources for red meat is associated with a lower mortality risk.

The exact numbers that the team estimated for the risk factors were quite alarming. At just one serving increase per day of unprocessed red meat, total lifetime mortality rose by 113%. For cardiovascular disease mortality, one serving increase of red meat raised the hazard ratio by 118% for men and 121% for women. For cancer, the hazard ratio was raised by 110% for men and 116% for women.

Substituting for just one serving of red meat per day with fish, poultry, nuts, legumes, low-fat dairy, or whole grains resulted in a 7% to 19% reduction in mortality risk. The authors also estimated that 9.3% of male deaths and 7.6% of female deaths could have been prevented if all individuals consumed less than half a serving per day of red meat.

If you are compelled to try out a plant-based diet but you are worried that it won’t meet your body’s nutritional needs, Chapter Four of my book demonstrates that this fear is supported by the scientific community. Check out my book, Humans are Herbivores: A Scientific Case for Veganism, on Amazon.