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In reply to: Humans are natural omnivores; we digest meat, have canine teeth, and have front-facing eyes

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Those objecting to veganism often bring up one or more in a series of related complaints: that a vegan diet is not natural, that humans are omnivores and can digest meat, or that canine teeth and front-facing eyes are indications we are predators and not prey. These protests are adequately dismissed with the first point below, which explains why they are not pertinent to the validity of veganism and therefore cannot diminish the case for veganism. Although no further exploration of these claims is necessary once their lack of pertinence is demonstrated, we expound on these claims in case you're interested. It turns out that even if the objections were pertinent, they'd be nevertheless weak.

Talking Points

The case for veganism does not depend on humans being natural herbivores or having specific physical traits.

Vegan diets are beyond sufficient for human health. Even if humans were natural omnivores and our teeth and eye locations supported that assertion, the science is clear that a strictly herbivorous vegan diet is not only adequate but also beneficial to our health. This is confirmed by Harvard Medical School, Mayo Clinic, Cleveland Clinic, Kaiser Permanente, NewYork-Presbyterian, the Academy of Nutrition and Dietetics operating in the United States, the Dietitians of Canada, the British Dietetic Association, the Dietitians Association of Australia, and others.[1]

These prominent organizations and others could only have made statements declaring the adequacy and salubriousness of a vegan diet if science supported such statements. Cleveland Clinic even explicitly states, "There really are no disadvantages to a herbivorous diet"[2]

The case for veganism has nothing to do with this issue. Simply put, the case for veganism is that it's ethically wrong to cause unnecessary harm to animals. Because it's not necessary to eat animal products for nutrition, any claims that we are natural herbivores are rendered meaningless in this context.

The evidence is strong that we lean toward being herbivorous.

The fact that humans are behavioral omnivores and are able to get nutrition from both plants and animals says nothing about what is natural or optimum.

Our anatomy and physiology suggest that we are more herbivorous than omnivorous. A number of people have observed that anatomical and physiological traits of humans closely match herbivores'. Dr. Mills's The Comparative Anatomy of Eating shows we more closely match herbivores in eighteen traits, as summarized below.

The Comparative Anatomy of Eating (Dr. Milton Mills)
Trait Human Herbivore Omnivore Carnivore
Intestines: Small 10–11 times body length 10–12+ times body length 4–6 times body length 3–6 times body length

Intestines: Colon

Long, sacculated Long, complex; may be sacculate Simple, short, and smooth Simple, short, and smooth
Teeth: Incisors Broad, flat, and spade shaped Broad, flat, and spade shaped Short and pointed Short and pointed
Teeth: Canines Short and blunted Dull and short or long (for defense) or none Long, sharp, and curved Long, sharp, and curved
Teeth: Molars Flat with nodular cusps Flat with cusps vs. complex surface Sharp blades or flattened Sharp, jagged, and blade shaped
Saliva Carbohydrate-digesting enzymes Carbohydrate-digesting enzymes No digestive enzymes No digestive enzymes
Stomach: Type Simple Simple or with multiple chambers Simple Simple
Stomach: Acidity with Food pH 4–5 pH 4–5 ≤ pH 1 ≤ pH 1
Stomach: Capacity 21% to 27% of total volume of digestive tract < 30% of total volume of digestive tract 60% to 70% of total volume of digestive tract 60% to 70% of total volume of digestive tract
Chewing Extensive chewing necessary Extensive chewing necessary Swallows food whole or simple crushing None; swallows food whole
Nails Flat nails Flat nails or blunt hooves Sharp claws Sharp claws
Jaw: Type Expanded angle Expanded angle Angle not expanded Angle not expanded
Jaw: Joint Location Above the plane of the molars Above the plane of the molars On the same plane as molar teeth On the same plane as molar teeth
Jaw: Motion No shearing; good side-to-side, front-to-back motion No shearing; good side-to-side, front-to-back motion Shearing; minimal side-to-side motion Shearing; minimal side-to-side motion
Major Muscles Masseter and pterygoids Masseter and pterygoids Temporalis Temporalis
Mouth Opening vs. Head Size Small Small Large Large
Facial Muscles Well developed Well developed Reduced Reduced to allow wide mouth gape
Liver Cannot detoxify vitamin A Cannot detoxify vitamin A Can detoxify vitamin A Can detoxify vitamin A
Kidney Moderately concentrated urine Moderately concentrated urine Extremely concentrated urine Extremely concentrated urine
Source: Mills, M.D., Milton. “The Comparative Anatomy of Eating | Digestion | Gastrointestinal Tract.” Scribd. Accessed February 15, 2019.

Used by...

Percy Bysshe Shelley was a poet, not a scientist, but it's interesting to note that he wrote an entire book, A Vindication of Natural Diet, published in 1884, that drew on comparative anatomy to argue that humans were best suited to a vegetable diet.[3] This predates Dr. Milton Mills's work by over 100 years.

Evolution and anthropology may support the contention that we are more herbivorous. Biologist Rob Dunn declares in Scientific American that "human ancestors were nearly all vegetarians." In making that assertion, and in questioning the validity of paleo claims, he deems it important to look at the diets of our ancestors at the time our guts were evolving. He states that for primates, a group to which humans belong, plants "were our paleo diet for most of the last thirty million years during which our bodies, and our guts in particular, were evolving. In other words, there is very little evidence that our guts are terribly special and the job of a generalist primate gut is primarily to eat pieces of plants."[4]

Dr. Colin Barras, a paleontologist and science writer, believes that "archaeologists tend to emphasise the role of meat in ancient human diets, largely because the butchered bones of wild animals are so likely to be preserved at dig sites. Edible plants may have been overlooked simply because their remains don’t survive so well."[5]

Our inability to kill and eat animals and process meat without sophisticated tools is telling. Omnivores and carnivores who eat animals have the athletic prowess and anatomical features necessary to not only catch and kill their prey but also to tear and rip apart the carcass and process it for eating. Humans lack these features and must use sophisticated tools, such as spears and knives, to accomplish these tasks.

The adverse effects of eating animal products suggest that we are more herbivorous. Supporting the contention that our evolution and physiology are herbivorous is the overwhelming scientific evidence that eating animal products contributes to all manner of health problems, including increased risk for obesity, high blood pressure, diabetes, heart disease, and cancer.[6] [7] [8] [9]

The notion of a natural diet is problematic.

The concept of a natural diet might make some sense in the context of gatherers and hunters. But since the invention of agriculture, with its selective breeding of both plant and animal species, the label loses its meaning.

Also, to make the claim that humans are natural omnivores, one needs to define what is meant by "natural" in this context. If by "natural" you are referring to the ability to obtain nutrients, then humans are omnivores, as we can digest both plants and meat. But, as shown earlier, that still cannot negate the case for veganism.

If you mean it's natural because it's nutritionally the best diet for humans, then you are on shaky ground. There's an increasingly large body of research, as mentioned and cited above, supporting the contention that the closer we are to a varied herbivorous diet, the greater our general health and the lower our risk for a multitude of chronic diseases.

Finally, the claim that humans are natural omnivores can be thought of as an example of the naturalistic fallacy. That is to say, being natural doesn't make something ethically or nutritionally sound.

Canine teeth are not indicators of dietary requirements.

As elaborated on earlier, the argument for veganism does not depend on humans having any specific physical traits. But the part of the objection that pertains to canine teeth is discussed here only because it is frequently voiced.

Hippopotamuses, gorillas, camels, and saber-toothed deer all have sizable canines, and all are herbivorous. Herbivores use canine teeth in various ways. Sizable canines in herbivores are often for defense. The relatively short, blunted canines in humans can assist in biting into hard, crunchy plants (such as apples) and ripping vegetable matter, preparing the food for grinding by the other teeth. One thing seems obvious—human canines are not adequate to kill prey or tear raw flesh for eating.

Front-facing eyes are not necessarily indicative of predator status.

The claim is made that since many prey animals have eyes on the side of the head and many predator animals have eyes on the front of the head, it follows that humans, who have eyes on the front of the head, are designed to eat copious amounts of meat. The point is made moot, however, not only by the fact that the argument for veganism does not depend on physical traits but also by the fact that our closest relatives in the animal kingdom, the primates, have eyes in the front of the head.

At least three advantages of frontal eyes for primates have been proposed.

Binocular vision is crucial for the manipulation of plant foods. A study titled "Binocularity and brain evolution in primates," published by the National Academy of Sciences, concludes that "fine-grained stereopsis [binocular vision] is likely to be critical for the visually guided, delicate manipulation of plant foods."[10]

The ability to "see through" foliage is advantageous. Theoretical neurobiologist Mark Changizi proposes in the Journal of Theoretical Biology the "X-ray vision" hypothesis. According to Changizi, front-facing eyes gave our ancestors the advantage of being able to "see through" the cluttered foliage in the forest. You can see this effect, he states, by placing a finger in front of your eyes and noting that the finger does not block the view of anything behind it.[11]

Arboreal locomotion requires accurate depth and distance perception. The depth and distance perception afforded by front-facing eyes was useful to our ancestors in jumping from branch to branch and tree to tree. This idea was proposed in 1922 by Edward Collins and has subsequently been expanded and refined.[12]

See Also


  1. In reply to: We need animal products to be healthy
  2. “Understanding Vegetarianism &amp Heart Health.” Cleveland Clinic, December 2013.
  3. Shelley, Percy Bysshe. A Vindication of Natural Diet. Percy Bysshe Shelley. Kindle e-Book, A public domain book. Vegetarian Society, 1883.
  4. Dunn, Rob. “Human Ancestors Were Nearly All Vegetarians.” Scientific American Blog Network, July 22, 2012.
  5. Barras, Colin. “Ancient Leftovers Show the Real Paleo Diet Was a Veggie Feast | New Scientist.” New Scientist, December 5, 2016.
  6. M.D, Michael Greger, and Gene Stone. How Not to Die: Discover the Foods Scientifically Proven to Prevent and Reverse Disease. 1 edition. New York: Flatiron Books, 2015
  7. “The China Study: The Most Comprehensive Study of Nutrition Ever Conducted and the Startling Implications for Diet, Weight Loss and Long-Term Health: Thomas M. Campbell II and T. Colin Campbell: 8580001064130: Amazon.Com: Books.” Accessed January 12, 2018.
  8. Davis, Brenda, and Melina Vesanto. Becoming Vegan: The Complete Reference to Plant-Based Nutrition (Comprehensive Edition). Accessed January 12, 2018.
  9. “PlantBasedResearch | An Online Library of Research Relevant to Plant-Based Nutrition.” Accessed January 12, 2018.
  10. Barton, R. A. “Binocularity and Brain Evolution in Primates.” Proceedings of the National Academy of Sciences of the United States of America 101, no. 27 (July 6, 2004): 10113–15.
  11. Changizi, Mark A., and Shinsuke Shimojo. “‘X-Ray Vision’ and the Evolution of Forward-Facing Eyes.” Journal of Theoretical Biology 254, no. 4 (October 21, 2008): 756–67.
  12. Goldman, Jason G. “Evolution: Why Do Your Eyes Face Forwards?” BBC, October 28, 2014.


This article was originally authored by Greg Fuller and copyedited by Isaac Nickerson. The contents may have been edited since that time by others.