On One Hand

December 21, 2005

Diet and Anatomy: Humans as Natural Vegetarians?

Filed under: Uncategorized — ononehand @ 3:08 am

This article and chart seeks to understand the “natural” diet for humans by looking at human physiology. The chart considers things like teeth, facial structure, and stomach acidity, comparing humans to known herbevores, omnivores, and carnivores. The website I got this from clearly has a pro-vegetarian bias, but the facts in the chart are technically correct so to debunk the argument you’d have to try to come up with other anatomical features not listed.

When the chart describes herbivores, think of things like horses, cows, deer, goats, camels, gorillas, orangutans, lemurs, and most monkeys.

When the chart describes omnivores, think of pigs, bears, some monkeys, and raccoons. (Omnivores are technically quite common: many birds are omnivores, as well as many fish and rodents, but they’re so anotomically different from larger animals that it’s harder to compare.)

When the chart describes carnivores, think especially of cats (including lions, tigers, leapords, and so forth), wolves, crocodiles, sharks, weasels, and meat-eating dinosaurs.

Facial Muscles

Carnivore: Reduced to allow wide mouth gape
Omnivore: Reduced
Herbivore: Well developed
Human: Well developed

Jaw Motion

Carnivore: Shearing; minimal side-to-side motion
Omnivore: Shearing; minimal side-to-side motion (again, think of bears, racoons)
Herbivore: No shear; good side-to-side, front-to-back
Human: No shear; good side-to-side, front-to-back

(“Shear” means the back teeth close down before the front during a bite, creating a scissor-like biting motion. Humans, obviously, chomp vertically; it’s impossible to bite down with your molars without biting your front teeth as well.)

Incisors (Front teeth)

Carnivore: Short and pointed
Omnivore: Short and pointed
Herbivore: Broad, flattened, and spade-shaped
Human: Broad, flattened, and spade-shaped

Canines (pointed teeth just behind incisors)

Carnivore: Long, sharp, and curved inward
Omnivore: Long, sharp, and curved inward
Herbivore: Dull and short (or long for defense: think gorillas, which are herbivores but have sharp canines for fighting) or none
Human: Short and blunted

Molars (back teeth)

Carnivore: Sharp, jagged, and blade-shaped
Omnivore: Sharp blades and/or flattened
Herbivore: Flattened with cusps or complex surface
Human: Flattened with nodular cusps


Carnivore: None; swallows food whole
Omnivore: Swallows food whole and/or simple crushing
Herbivore: Extensive chewing necessary
Human: Extensive chewing necessary


Carnivore: No digestive enzymes in saliva
Omnivore: No digestive enzymes in saliva
Herbivore: Carbohydrate-digesting enzymes in saliva
Human: Carbohydrate-digesting enzymes in saliva (salivary amylase and others)

Stomach Acidity (High acidity is intended to kill bacteria in meat)

Carnivore: Around a pH of 1 with food in stomach
Omnivore: Around a pH of 1 with food in stomach
Herbivore: pH of 4 to 5 with food in stomach
Human: pH of 4 to 5 with food in stomach (which is why meat must be cooked for humans to avoid illnesses, while animals don’t have this problem)

Length of Small Intestine (short intestines needed for carnivores to prevent meat from rotting in gut, while herbivores need long intestines to foster bacteria that help digest tough plant cell walls)

Carnivore: 3 to 6 times body length
Omnivore: 4 to 6 times body length
Herbivore: 10 or more times body length
Human: 10 to 11 times body length


Carnivore: Sharp claws
Omnivore: Sharp claws
Herbivore: Flattened nails or blunt hooves
Human: Flattened nails

2 Milton R. Mills, M.D., “The Comparative Anatomy of Eating”.
3 William C. Roberts, “Facts and Ideas From Anywhere,” Baylor University Medical Center Proceedings, Oct. 1999.

It is notable that, unlike herbevores like horses and cows, humans are not intended to live on coarse plant material like bark and grasses. Humans evolved to seek out “higher” plant material with higher calorie and nutritional content, such as fruits, roots, soft leaves and nuts. Tomatoes and oranges have many, many more calories per pound than grass does. This allowed for the shrinking of the size of the digestive system while increasing the size of the brain. Incidentally, it is the larger brain that allowed humans and higher primates to hunt for the good fruit and roots and avoid the need to eat grass. Many herbivores will occasionally eat meat if dire circumstances press: it is thought that Homo sapiens started eating meat during ice ages, when the proper plant material became extremely scarce. At most carnivorous the early humans were scavengers; they didn’t start hunting and actually eating considerable amounts of fresh meat until more recently, with the recent advent of weapons. It’s notable that human instincts do not follow natural predatory instincts, as carnivorous and omnivorous animals are inclined to kill with their paws and mouths only, have no desire or need to cook their food, and go straight for the nutritious and easily digestable organs, while humans usuall go for the muscles.



  1. This appears to be an extensive critical examination of studies like that:

    The summary:
    Comparative anatomy is a valid tool, but simplistic applications are often fallacious. The basic question of this paper is not whether comparative anatomy and physiology are valid tools (they clearly are, though we will see that applying comparative anatomy and physiology to humans is problematic), but whether the simplistic analyses presented by dietary advocates are legitimate or “scientific” (we will see later that they are not).

    Humans can be regarded as natural omnivores, so long as one uses the common definition of the term: a natural diet that includes significant amounts of both plant and animal foods. A short summary of some of the evidence supporting this follows (the material below was discussed in depth in earlier sections of this paper).

    # The fossil record. Approximately 2.5 million years of human omnivory/faunivory are apparent in the record, with genetic adaptation to that diet the inevitable and inescapable outcome of evolution. The supporting evidence here includes isotope analysis of fossils, providing further evidence of consumption of animal foods.

    # Comparative anatomy of the human gut. The best scientific evidence available to date on gut morphology–analyzed using two different statistical approaches–shows evidence of adaptations for which the best explanation is the practice of faunivory. (Faunivory as an explanation is also supported by optimal foraging theory in hunter-gatherer tribes.) Further, the human gut morphology is not what might be expected for a strict vegetarian/fruit diet.

    # Comparative physiology (metabolism)

    * Intestinal receptors for heme iron. The existence of intestinal receptors for the specific absorption of heme iron is strong evidence of adaptation to animal foods in the diet, as heme iron is found in nutritionally significant amounts only in animal foods (fauna).

    * B-12 an essential nutrient. Similarly, the requirement for vitamin B-12 in human nutrition, and the lack of reliable (year-round) plant sources suggests evolutionary adaptation to animal foods in the human diet.

    * Plant foods are poor sources of EFAs. In general, the EFAs in plant foods are in the “wrong” ratio (with the exception of a very few exotic, expensive oils), and the low synthesis rates of EPA, DHA, and other long-chain fatty acids from plant precursors point to plant foods as an “inferior” source of EFAs. This strongly suggests adaptation to foods that include preformed long-chain fatty acids, i.e., fauna.

    * Taurine synthesis rate. The low rate of taurine synthesis in humans, compared to that in herbivorous animals, suggests human adaptation to food sources of taurine (fauna) in the human diet.

    * Slow conversion of beta-carotene. The sluggish conversion rate of beta-carotene to vitamin A, especially when compared to the conversion rate in herbivorous animals, suggests adaptation to dietary sources of preformed vitamin A (i.e., a diet that includes fauna).

    * Plant foods available in evolution were poor zinc and iron sources. The plant foods available during evolution (fruits, vegetative plant parts, nuts, but no grains or legumes) generally provide low amounts of zinc and iron, two essential minerals. These minerals are provided by grains, but grains are products of agriculture (i.e., were not available during evolution), and contain many antinutrients that inhibit mineral absorption. This suggests that the nutritional requirements for iron and zinc were primarily met via animal foods during human evolution.

    * Bitter taste threshold as a trophic marker. An analysis of the human bitter taste threshold, when compared to the threshold of other mammals, suggests that our sensitivity to the bitter taste is comparable to that of carnivores/omnivores.

    Comment by jdhenchman — December 21, 2005 @ 5:10 pm | Reply

    • Continued

      # There is no such thing as a veg*n gatherer tribe. And there are no records to indicate that any such tribes ever existed; also no evidence of any vegan societies either.

      # The actual diets of all the great apes includes some fauna–animal foods. Even the great apes that are closest to being completely vegetarian, gorillas, deliberately consume insects when available. Chimps and bonobos, our closest relatives, hunt and kill vertebrates and eat occasional meat.

      Humans are classic examples of omnivores in all relevant anatomical traits. There is no basis in anatomy or physiology for the assumption that humans are pre-adapted to the vegetarian diet. For that reason, the best arguments in support of a meat-free diet remain ecological, ethical, and health concerns.

      The evolutionary or hunter-gatherer diet (discussed in earlier sections) consists of a diet of wild plant foods (fruits, nuts, some leaves/stems, starchy tubers–possibly cooked), insects, and the lean meat and organs of wild animals.

      Note that grains, legumes, and/or dairy are generally not available to hunter-gatherers; such foods are provided in significant quantities only via agriculture, and have been a significant part of the human diet for only about 10,000 years or less. The extent of human genetic adaptation to such foods is a controversial point, but the majority view is that the genetic adaptation that has taken place in the last 10,000 years is quite limited. (See the discussions earlier herein regarding hereditary hemochromatosis, and the carnivore connection hypothesis.) Similarly, modern processed foods have been with us for only a few generations, and genetic adaptation in such a short period is highly unlikely.

      Comment by jdhenchman — December 21, 2005 @ 5:10 pm | Reply

      • Re: Continued

        And, finally, this page specifically addresses the comparative anatomy claims, http://www.beyondveg.com/billings-t/comp-anat/comp-anat-6a.shtml:

        A simple, summary answer to the question of how humans can hunt animals and eat meat without the physical adaptations of the lion and tiger is the obvious one implicitly ignored and rationalized by the advocates of simplistic comparative “proofs”: We don’t need sharp teeth, powerful jaws, or claws to capture and butcher animals because we have used (since our inception as a genus ~2.5 million years ago) tools (or technology–stone weapons) for that purpose. Over the eons, evolution itself has adapted our physiologies to the results of this behavior along unique lines, quite regardless of the hue and cry over the “illegitimacy” with which these behaviors/skills are regarded by those extremists promoting the bizarre idea that human dietary behavior should be strictly limited to what we could do “naked, without tools.”

        Technology, driven by our intelligence, supports adaptive behavior that allows us to easily overcome the physical limitations that the comparative “proofs” regard (incorrectly) as being limiting factors. Along similar lines, we don’t need the strong bodies of a lion or tiger because we have something much more powerful: high intelligence, which allowed humans to become the most efficient hunters, and the dominant mammalian species, on the planet.

        Comment by jdhenchman — December 21, 2005 @ 5:16 pm

    • That article is just as bias as the one I cited. If all it’s attempting to do is debunk the argument that human beings are meant to be strictly vegetarian, I can agree to the article. No primate is committed by nature to any diet that is “strict” – they are all fairly adaptable in times of environmental stress. But inasmuch as the article denies that humans are “omnivores” or claims that meat is any large proportion of a natural diet, it is absolutely untrue. It is also of note that insectivores, though technically carnivorous, are not considered to be eating traditional “meat” in the sense that iscects are not consumed or digested the same way that meat is (human teeth are much more adapted to eating insects than they are to eating flesh, in which case sharp canine teeth are useless). You can debunk some forms of humans’ natural “vegetarianism,” but you can’t debunk the “vegetarian diet, which is better described as “mostly vegetarian.”

      Now, to the claims, as you summarized them: I do not deny that humans have eaten an occasional piece of meat for quite a while and have gone through evolutionary adaptations to handle it, such as protease in the stomach. I also don’t deny that some communities ate meat more often. However, your arguments don’t imply that we are designed to eat meat regularly. An excellent example involves something you mentioned: vitamin B-12.

      True, it’s hard to get enough vitamin B-12 from plants. Large herbivores manufacture it in their long intestines with the aid of bacteria, while carnivores get it quickly and easily from meat. The few plants that have B-12 are unreliable sources, as you mentioned (though I’m not sure what you meant by “year-round.” Algaes and seaweeds have B-12, and such foods are obviously rare in arid or inland areas. It seems that the best place to get vitamin B-12 is meat. However, coming back to your mention of the fact that B-12 is not available in plants “year round” – note that the human liver is designed to store enough vitamin B-12 to supply the entire body with all the B-12 it needs for up to twelve years. If the body is designed to keep stores of something for twelve years when those resources would easily be replenished by eating meat, the only explanation can be that human beings are designed to go long periods of time without meat.

      Take an essential vitamin for which the opposite is true: almost all mammals have the capacity to manufacture vitamin C out of sugar, and ALL carnivores that use vitamin C can manufacture it themselves (otherwise they would die). But humans don’t have that ability. Instead, we have to regularly consume this nutrient which is only found in reasonable amounts in plants (especially fruits) and is flushed out of the bloodstream quickly. The body has no way to store vitamin C, and those on high-meat diets are very suceptible to the crippling and often fatal disease of scurvy. In fact, those who consume high-meat diets are also more prone to osteoperosis, heart disease, cancers, and countless other lethal conditions. The same is not true for vegetarian diets. As I said, humans are designed to go a considerable amount of time eating absolutely no meat before they start having problems. (Anemia is no higher in vegetarians than meat-eaters and it’s rare in vegans. As a vegetarian, I’ve talked to doctors worried about getting anemia, and they always say “there are no risks of a vegetarian diet if you eat beans or soy for protein. Your body recycles it’s iron and green vegetables have enough of it. If you go completely vegan, take a suppliment once in a while because after several years you might run out of B-12”).

      Comment by ononehand — December 22, 2005 @ 2:14 am | Reply

    • *but whether the simplistic analyses presented by dietary advocates are legitimate or “scientific” (we will see later that they are not).

      I absolutely agree. Hopefully this helps you understand why I don’t listen to economists talking about Global Warming.

      Comment by ononehand — December 22, 2005 @ 2:34 am | Reply

  2. Before even getting bogged down in the details of the claims, how do we expect to derive normative premises here, if that is the goal? Even supposing that in our evolutionary history we did not eat meat, this does not entail that we should not eat meat. One would need to adopt the premise that “If X is not a pattern present in our evolutionary history (prior to some time t), then we should not do X”. But this is dubious. Taking antibiotics is not in our evolutionary history either, but presumably we should not cease doing so for this reason. In other words, it seems that the “facts” presented here are at best interesting anthropological notes, not moral revelations which equate vegetarianism with The Good.

    Comment by lackingquality — December 22, 2005 @ 11:01 am | Reply

    • Feel free to study that post more closely in case you come up with something I haven’t seen, but I’m pretty sure that there is no place in that document that suggests that human beings “should not” eat meat or that vegetarianism is “good” or led to by “moral revelations.” It looks to me like it’s trying to uncover the natural diet of Homo sapiens in prehistoric times, not suggesting what anyone has eaten or should eat in modern imes. But again, feel free to look it over and let me know.

      If you followed links to the website, where the claim is made that vegetarianism is an ideal diet for several reasons, you will see that this page is only a small part of a much larger argument and it isn’t intended to stand alone. You will see that that particular document is intended to refute the “we should eat meat because it’s natural to” arguent, which fails for the exact reason that you have stated.

      I don’t know if you eat meat or not so I don’t know if this applies to you, but I always find it amusing how defensive non-vegetarians get when you simply state that you are one. They always come up with some quasi-moral rebuttal, such as “you’re a vegetarian? Well, I don’t think animals have souls,” or “you’re a vegetarian? Well, it’s kill or be killed, so if we didn’t eat animals they would eat us,” or, “you’re a vegetarian? You know, that’s soooo unnatural.” It’s like the fact that I choose to not eat meat for my own reasons is some kind of moral attack on them, even though I haven’t said anything about them. It’s hard to even talk about vegetarianism; everyone who eats meat tries so hard to justify what they are doing that they resort to all kinds of “you fucking hippy!” type attacks. Some of them (many became vegetarians later) have later told me that they got defensive because they were conflicted on the matter and my vegetarianism evoked some guilt. For the other ones, I don’t know why they get so defensive/aggressive.

      Comment by ononehand — December 22, 2005 @ 6:25 pm | Reply

      • That is good, I was just clarifying. The word “natural” often comes with a strong normative flavor, so it is important to make sure we distinguish a number of different arguments.

        And yes, that is an amusing phenomenon. I encounter the same thing just stating that I am an atheist, or a moral skeptic, or countless other things that the masses of idiots find discomforting. As should be clear, I was not being defensive about meat eating in general, as I could not really care less either way about this issue, but was only preventing a potential misstep that commonly occurs when a pro-vegetarian bias and historical notes get combined to derive shaky moral guidelines.

        Comment by lackingquality — December 22, 2005 @ 10:01 pm

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