Humans are naturally plant-eaters

according to the best evidence: our bodies

by Michael Bluejay  June 2002  Updated December 2015



A fair look at the evidence shows that humans are optimized for eating mostly or exclusively plant foods, according to the best evidence: our bodies. We're most similar to other plant-eaters, and drastically different from carnivores and true omnivores.1,2,3 Those who insist that humans are omnivores, especially if their argument is based on canine teeth, would do well to look at what the evidence actually shows. We'll cover that below.



I first wrote this article many years ago, but since then Milton Mills, M.D. published an excellent paper which covers the anatomy of eating, so let's skip right to my table-ized summary of his research:

Humans are biologically herbivorous



Carnivores

Omnivores

Herbivores

Humans

Facial muscles

Reduced to allow wide mouth gape

Reduced

Well-developed

Well-developed

Jaw type

Angle not expanded

Angle not expanded

Expanded angle

Expanded angle

Jaw joint location

On same plane as molar teeth

On same plane as molar teeth

Above the plane of the molars

Above the plane of the molars

Jaw motion

Shearing; minimal side-to-side motion

Shearing; minimal side-to-side motion No shear; good side-to-side, front-to-back

No shear; good side-to-side, front-to-back Major jaw muscles

Temporalis

Temporalis

Masseter and ptergoids

Masseter and pterygoids

Mouth opening vs. head size

Large

Large

Small

Small

Teeth: Incisors

Short and pointed

Short and pointed

Broad, flattened and spade-shaped

Broad, flattened and spade-shaped

Teeth: Canines

Long, sharp, and curved

Long, sharp and curved

Dull and short or long (for defense), or none

Short and blunted

Teeth: Molars

Sharp, jagged and blade-shaped

Sharp blades and/or flattened

Flattened with cusps vs. complex surface

Flattened with nodular cusps

Chewing

None; swallows food whole

Swallows food whole and/or simple crushing

Extensive chewing necessary

Extensive chewing necessary

Saliva

No digestive enzymes

No digestive enzymes

Carbohydrate digesting enzymes

Carbohydrate digesting enzymes

Stomach type

Simple

Simple

Simple or multiple chambers

Simple

Stomach acidity with food in stomach

≤ pH 1

≤ pH 1

pH 4-5

pH 4-5

Length of small intestine

3-6 times body length

4-6 times body length

10-12+ times body length

10-11 times body length*

Colon

Simple, short, and smooth

Simple, short, and smooth

Long, complex; may be sacculated

Long, sacculated

Liver

Can detoxify vitamin A

Can detoxify vitamin A

Cannot detoxify vitamin A

Cannot detoxify vitamin A

Kidney

Extremely concentrated urine

Extremely concentrated urine

Moderately concentrated urine

Moderately concentrated urine

Nails

Sharp claws

Sharp claws

Flattened nails or blunt hooves

Flattened nails

From The Comparative Anatomy of Eating, by Milton R. Mills, M.D. | "Body length" measured from neck to anus, as with the other animals



The details are in Mills' paper. The rest of this article covers mostly angles not in that paper.





Summary of this article

The anatomical evidence tells us that we're optimized for eating mostly or exclusively plant foods. The only way to come to another conclusion is to ignore the bulk of the anatomical evidence. (Critics of this article either use inferior evidence, such as disputed assumptions about the prehistoric diet, or they cherry-pick the anatomical evidence while ignoring the bulk of it.)

Plant-eaters eat small amounts of non-plant foods, usually insects (either intentionally or inadvertently). The small non-plant consumption of plant-eating animals doesn't mean that they're "omnivores" in the classical sense, and certainly doesn't justify the idea that humans are adapted to a very mixed diet of plant and non-plant foods.

"Omnivore" doesn't mean 50% plants and 50% animals. Many of my critics consider chimpanzees to be omnivores but 95-99% of the chimp diet is plants, and most of the remainder isn't meat, it's termites. (see below) They gleefully send me picture after picture of primates eating animal foods, as though they've made some sort of point. If humans are omnivores, then the anatomical evidence suggests that we're the same kind: the kind that eats almost exclusively plant foods.



Chimpanzee: Pretty amazing canines for an animal that's as much as 99% vegetarian (and whose main non-veg food isn't meat, it's termites). (Source: Creative Commons )

The animals most similar to us, the other primates, eat an almost exclusively vegan diet.

Saying we're omnivores because we're capable of eating meat is just silly. We're capable of eating cardboard, too. And by the "capable" argument, then cats are omnivores too, since nearly every commercial cat food has plant ingredients. (Check the label.) Nobody would ever make the argument that cats are omnivores based on what they're capable of eating. But they sure make that argument for humans, enthusiastically.



Our so-called "canine teeth" are "canine" in name only. Other plant-eaters (like gorillas, horses, and hippos) have "canines", and chimps, who are almost exclusively vegan, have massive canines compared to ours.



Our early ancestors from at least four million years ago were almost exclusively vegetarian. (source, article #5)



Among animals, plant-eaters have the longest lifespans, and humans are certainly in that category (and yes, this was true even before modern medicine).

(and yes, this was true even before modern medicine).

We sleep about the same amount of time as other herbivores, and less than carnivores and true omnivores.





The most common cause of choking deaths is eating meat. ( source ) Real carnivores and omnivores don't have that problem.





Real carnivores and omnivores don't have that problem. The idea that a switch to meat-eating is what sparked early humans' brain development has no more evidence to support it than the competing theories (such as that it was a switch to cooked foods that did the trick), and certainly doesn't square with what comparative anatomy tells us.







Looking at the evidence fairly



The meat-eating reader already has half a dozen objections to this before s/he's even read the rest of the article, and I will address those objections specifically, but first let me address them generally: It's human nature to want to feel that what we're doing is right, proper, and logical. When we're confronted with something that suggests that our long-held belief might actually be wrong, it's uncomfortable. We can either consider the new evidence fairly, adding to the discomfort about our possible error, or we can reject that premise without truly considering it, which allows us to sidestep any uncomfortable feelings. And we do this by searching our minds for any possible arguments for why the challenge must be wrong, to justify our current beliefs. This practice is so common psychologists have a name for it: cognitive dissonance. While we prefer to believe we act on reason, a formidable amount of psychological research shows the opposite: Often to usually, our actions aren't based on our arguments. Rather, our actions come first and then we come up with the arguments to try to support those actions. If we were truly logical, we'd consider the evidence first and then decide the best course of action. But often we have it in reverse, because it's too difficult to accept that we might have been wrong. Vegan bodybuilders shatter the myth that vegans are skinny and malnourished. (Pictured: Avi Lehyani, anonymous, Ryan Wilson, Robert Cheeke)

This is particularly true when it comes to vegetarianism. It's easy to identify because the anti-vegetarian arguments are usually so extreme, compared to other kinds of discourse. A person who would never normally suggest something so fantastic as the idea that plants can think and feel pain, will suddenly all but lunge for such an argument if they feel a need to justify their meat-eating. That's psychology for you.

So I challenge you: stop trying to figure out ways that I "must" be wrong even before you've bothered to read the rest of this article. Instead, read it, and actually consider it rather than reflexively trying to come out with ways to dismiss it out of hand. You can certainly still disagree after you've considered all the evidencebut not before. (Most of my critics miss that point, emailing me arguments that are already rebutted here in the article, which they didn't bother to actually read.)

Many readers will find it necessary to try to defeat me, at least in their minds, so let's agree that that would mean providing more and better evidence for your position. One does not win the argument by making a single point, as most who've written to me seem to think. The evidence favoring a plant-based diet for humans is clear, convincing, and overwhelming. There is definitely some evidence for the other side, to be sure, but it's simply not nearly as strong. While that ought to be obvious, I mention it because my critics seem to believe that all the evidence I present somehow vanishes into thin air when they present their one lone argument. As though making a single point and ignoring all other evidence to the contrary wins the day.

In graphical form, it goes like this:

Evidence that humans' anatomy favors a plant-based diet

Evidence to the contrary



Lunging at the minority of evidence in the red box doesn't make the contrary position more compelling. The only way to make the contrary position compelling is to present more and better evidence (making the red box bigger), not to pretend that the green box doesn't exist.



The most common counter-arguments



Chimpanzee: Pretty amazing canines for an animal that's as much as 99% vegetarian (and whose main non-veg food isn't meat, it's termites). (Source: Creative Commons )

The truth is our so-called "canine teeth" are canine in name only. Humans' "canine teeth" are unlike the canine teeth of actual canines, which are really long and really pointed. Our teeth are absolutely not like theirs. In fact, other vegetarian animals (like gorillas, horses, and hippos) possess the same so-called "canine" teeth, which are often used for defensive purposes rather than for eating. Check out the chimpanzee picture at right, and consider that chimps' diets are up to 99% vegetarian (and what little non-vegetarian food they eat usually isn't meat, it's termites). And remember that we're more similar to chimps than to any other animal.



John A. McDougall, M.D., has a good take on this:

Our dentition evolved for processing starches, fruits, and vegetables, not tearing and masticating flesh. Our oft-cited "canine" teeth are not at all comparable to the sharp teeth of true carnivores. I lecture to over 10,000 dentists, dental hygienists, and oral specialists every year, and I always ask them to show me the canine teeth in a persons mouth  those that resemble a cats or dogs teeth  I am still waiting to be shown the first example of a sharply pointed canine tooth.



If you have any doubt of the truth of this observation then go look in the mirror right now  you may have learned to call your 4 corner front teeth, canine teeth  but in no way do they resemble the sharp, jagged, blades of a true carnivore  your corner teeth are short, blunted, and flat on top (or slightly rounded at most). Nor do they ever function in the manner of true canine teeth. Have you ever observed someone purposely favoring these teeth while tearing off a piece of steak or chewing it? Nor have I. The lower jaw of a meat-eating animal has very little side-to-side motion  it is fixed to open and close, which adds strength and stability to its powerful bite. Like other plant-eating animals our jaw can move forwards and backwards, and side-to-side, as well as open and close, for biting off pieces of plant matter, and then grinding them into smaller pieces with our flat molars.

The canine argument is important for what it reveals: The people who make it place so much importance on it, insisting that humans having canines immediately wins the whole argument, all by itself, case closed! But when they discover that they were wrong, then suddenly the canine issue really wasn't so important to them after all, and they simply move on to their next misconception, as though their previous argument never happened. That really lays their motivations bare: They were never really interested in evaluating the evidence, they were only interested in being right. But really, if someone thinks that canine teeth are the be-all and end-all of the herbivore vs. omnivore debate, then when they find out that they're wrong about teeth, that ought to tell them something. But does it ever? Nope. If we needed evidence of bias, there you have it.



"Humans have always eaten meat."

No, we haven't, and I'll provide evidence for that shortly. More importantly, early humans, like modern humans, could have simply acted outside of instinct, and made interesting dietary choices contrary to their anatomy. We really have to look at our anatomy to get the best evidence for what we're optimized for eating, not what some humans chose to eat. Otherwise, thousands of years from now anthropologists might conclude that eating McDonald's is natural because humans in the 20th and 21st centuries used to eat a lot of it. Also, of early humans who did eat meat, they might have eaten it as sparingly as modern chimps do.



"We're capable of eating meat, therefore we're omnivores. Case closed."

Okay, fine, then cats are omnivores, too. ("Case closed.") Commercial cat foods, both wet and dry, contain things like rice, corn, and wheat. In fact, some people feed their cats a pure vegan diet with no meat at all. But of course, cats are true carnivores. We don't call them omnivores just because they'll eat things contrary to what nature intended. That would be silly. No one makes that argument for cats. But they make it for humans, enthusiastically. However, they can't have it both ways: Either we don't assume humans are omnivores just because we can eat meat, or we apply the same standard to other animals and conclude that cats are omnivores, too. Which is it?

"Humans are omnivores."

"Omnivore" doesn't mean 50% plants and 50% animals. Many consider chimpanzees to be omnivores but 95-99% of their diet is plants, and most of the rest isn't meat, it's termites. If humans are omnivores, then the anatomical evidence suggests that we're the same kind: the kind that eats almost exclusively plant foods. And if an omnivore is an animal that is capable of eating both plants and animals, and ever does so, then sure, we're omnivores, but then again, so are cats. (See above.) A true omnivore would have a body optimized for eating both plants and animals. With non-humans we can look at what they eat in the wild to figure out their preferred diets, but humans lost our instincts long ago, so we can look only at our anatomy and digestive systems. And that evidence is compelling.

"You're not a credible source."

You don't have to believe me, you can look at the evidence I cite. My critics talk as though I claim this article to be original research, but really, I'm just reporting on what the science says, citing credible sources along the way.



It's funny, my critics think I'm not allowed to speak without credentials, but somehow they don't need any themselves in order to argue the contrary position. For example, when Internet forums discuss whether meat-eating is natural, someone will undoubtedly lunge at the "canines = meat-eater" argument, then someone else will point them to this article where I explain that other plant-eaters have larger canines than ours, then the canine guy will exclaim, "Bluejay isn't a credible source! He has no formal training!" As though the critic has a Ph.D. in biology. And as though my own lack of credentials means than gorillas, horses, and hippos don't have large canine teeth. The critics don't think they need any special training to spout off (wrong) pronouncements about human canines, for example, but anyone who dares disagree with their misinformed assumptions had better have multiple advanced degrees and tenure at an Ivy League university.



What matters is the evidence, and I cite my sources. If the critics have better evidence, rather than pulling pronouncements about canine teeth out of the ether, they'd do well to present it, rather than just dismiss evidence they don't like out of hand.

By the way, doctors like John McDougall and Milton R. Mills (both M.D.'s) believe that human anatomy favors plant foods. I wonder whether the people who send me hate mail about this article and tell me I'm an idiot would feel just as confident in telling these two doctors that the doctors are idiots, too?

"Vitamin B12. End of story."

I'm not joking when I tack on "End of story" to the sample counter-arguments. People actually make them that way, literally. Here again, they think one point invalidates all other evidence. Amazing. The argument here is that since B12 isn't found in plant foods and modern vegans must supplement, a vegan diet can't be natural. Here's what's wrong with that argument: B12 isn't made by animals, it's made by bacteria. (source) It's found in animal foods because they're a hotbed of bacterial activity. It's also found in feces of most species. Historically it was easier for vegans to get B12 because their environment was so dirty. Plants pulled from the ground and not washed scrupulously could have bacterial contamination, and thus B12. (source)

B12 is also found in lakes, before the water is sanitized. (source) Remember that "plant-eaters" aren't exclusively plant-eaters; they eat some small amounts of non-plant foods. For example, of the 1-5% of chimps' diets that aren't plants, most is generally termites, which happen to be loaded with B12. (source) We saw that fecal matter contaminating the environment can provide B12. But not taking any chances, many plant-eating animals actually eat their own feces. Prehistoric humans might have done the same. (Human feces is loaded with B12.) (source) Because the ability to absorb B12 decreases with age, the Food and Nutrition board says that all people over 50 should eat B12-fortified food or take B12 supplements, not just vegans. (source)

So the idea that our bodies are designed to eat large amounts of meat because of a single vitamin made by bacteria isn't very compelling.

In any event, there's no question that modern vegans must take a B12 supplement, recommended at least weekly. (source) No plant food is a reliable source, and most fermented products (like tempeh) which list B12 on the Nutrition Facts actually don't have any, because the FDA mandates the wrong test for B12. (source)

"Other primates eat meat."

Hardly. Various sources (below) say that a chimp's diet is 95-99% plant foods, and the primary non-plant food isn't meat, it's termites. Remember also that primates are intelligent and can make choices outside of instinct, just like humans do, so the tiny amount of meat they might eat could be due to choice, not instinct.



I cover the primate diet in more detail below.

"You're not considering evolution."

How so? However much our species has changed, the end result is that our anatomy still favors a predominantly plant diet.

But haven't humans always eaten a lot of meat?

In a word, no, which we'll discuss in a moment, but first there's something more important: Best evidence is our anatomy, not what our ancestors ate in the recent or distant past. Humans act by idea rather than by instinct, so we certainly can and do make choices that go against what's most "natural". Most other animals are programmed to know what their ideal food is. We are not. For us, it's learned behavior. Or in some cases, guessed behavior. We can make choices about what we should eat even if that's contrary to good health, as millions prove every day when they eat at McDonald's. If our ancestors ate meat, they were simply being human and making choices rather than acting on instinct. Think about it: Do you really believe that cavemen were true experts about nutrition? If so, what other major decisions about your life would you like to put in the hands of a caveman?

And in fact, even other primates learn their eating behavior from others:

Just as human travelers often adopt the local cuisine, wild monkeys learn to eat what those around them are eating, new research finds. A study of wild vervet monkeys (Chlorocebus aethiops) in South Africa provides proof that primates other than humans adopt and conform to cultural behaviors. Given a choice between two foods, infant monkeys ate only the foods that their mothers ate. And young males that ventured to other groups soon switched to the local diet, researchers report online today (April 25) in the journal Science. "Some of the ways of learning that we have thought were distinctly human are more broadly shared across nonhuman primates," said study co-author Andrew Whiten, a cognitive biologist at the University of St. Andrews in the United Kingdom. (Live Science, April 2013)



So the best evidence isn't historical diets, best evidence is our own bodies. If we'd really been eating a lot of meat for a long time, that would be reflected in our anatomy, but it's not. Anyway, let's return to the assumption that our ancestors did eat a lot of meat. I can't think of a better example of a case in which people believe something to be true just because they assume it is. We all grew up thinking that our predecessors were huge meat-eaters, but where did we get that idea? Is it true just because it's part of our collective consciousness? More importantly, what does the evidence say?

John A. McDougall, M.D., perhaps the most knowledgeable expert on the relationship between diet and disease, asserts that our early ancestors from at least four million years ago followed diets almost exclusively of plant foods. (source, article #5) Many other scientists believe that early humans were largely vegetarian. For example, biologist Robert Dunn, writing in Scientific American, says:

The majority of the food consumed by primates todayand every indication is for the last thirty million yearsis vegetable, not animal. Plants are what our apey and even earlier ancestors ate; they 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. (source; see also articles by Grande & Leckie and Derek Wall)

Then there's this research:

Robert W. Sussman, Ph.D., professor of anthropology, [spoke at] the American Association for the Advancement of the Science's Annual Meeting....[E]arly man was not an aggressive killer, Sussman argues. He poses a new theory, based on the fossil record and living primate species, that primates have been prey for millions of years, a fact that greatly influenced the evolution of early man. "Our intelligence, cooperation and many other features we have as modern humans developed from our attempts to out-smart the predator," says Sussman.... The idea of "Man the Hunter" is the generally accepted paradigm of human evolution, says Sussman, "It developed from a basic Judeo-Christian ideology of man being inherently evil, aggressive and a natural killer. In fact, when you really examine the fossil and living non-human primate evidence, that is just not the case." Sussman's research is based on studying the fossil evidence dating back nearly seven million years. "Most theories on Man the Hunter fail to incorporate this key fossil evidence," Sussman says. "We wanted evidence, not just theory. We thoroughly examined literature available on the skulls, bones, footprints and on environmental evidence, both of our hominid ancestors and the predators that coexisted with them." ... But what Sussman and Hart discovered is that Australopithecus afarensis was not dentally pre-adapted to eat meat. "It didn't have the sharp shearing blades necessary to retain and cut such foods," Sussman says. "These early humans simply couldn't eat meat. If they couldn't eat meat, why would they hunt?" It was not possible for early humans to consume a large amount of meat until fire was controlled and cooking was possible. Sussman points out that the first tools didn't appear until two million years ago. And there wasn't good evidence of fire until after 800,000 years ago. Bio-Medicine.org, 2006

Consider also that the Maasai in Kenya, who still eat a diet high in wild hunted meats, have the worst life expectancy in the world. ( Fuhrman )

In any event, the idea that our ancestors might have decided to mimic other animals and eat more meat isn't a particularly compelling argument that it's natural for us to do so. Given that humans act outside of instinct, looking at historical behavior isn't as convincing as looking at anatomyas we'll do in a moment.

Considering the other primates

Our closest animal relatives are, of course, the other primates. They provide clues about our natural diet since our anatomy is so similar to theirs. Very few of them eat any significant amount of animals, and those who do typically mostly stick to things like insects, not cows, pigs, and chickens. Jane Goodall, famous for her extensive study of apes while living with them, found that it was very rare for the primates she saw to eat other animals. Critics lunge all over the fact that Goodall discovered that primates occasionally eat meat. But the key word here is occasionally. If we ate meat is infrequently as the other primates did, our health would certainly be a lot better. Goodall herself apparently wasn't impressed by primates' occasional eating of meat: Jane Goodall herself is a vegetarian.



Chimpanzee: Eats 95-99% plants, and most of the rest is termites (not meat). (Source: Creative Commons )

Which brings up another point: The people who hysterically scream at me that chimps are omnivores, besides ignoring that chimps' meat consumption is so small as to be virtually non-existent, never acknowledge that the non-plant foods chimps eat are not the same things humans eat. Chimps do not eat cattle and chickens. And humans don't eat termites. The idea that the meat-laden American diet can be justified because chimps may eat a whopping 5% of non-plant foods, none of it cattle or chickens, and much of it termites, is rather silly.

Let's do the math to see what 1% of a diet as non-plant foods actually looks like. If all of that 1% were beefwhich for chimps, it is nothow much beef would that be? For an adult human, it's a mere 8 grams a day (about 1/3 of an ounce, or just 0.02 pounds). That's about 1/9th of a medium carrot. Go get a carrot, cut it into nine pieces, and each piece then represents the amount of meat (or termites) you could eat every day to have your diet match that of a chimp. Yes, there you have chimps' overwhelming "omnivorism".

Here's how another writer put it:

"Meat makes up only 1.4% of [chimps'] diet which in any statistical study or analysis would be considered as quantitatively unimportant. In longitudinal studies it has been found that 90% of all kills were by males and as the females rarely hunt they receive a share in return by begging only after she allows him to mate with her.... On rare occasions chimps do eat and kill a baby chimp. So if you follow this argument to its conclusion, humans should kill and eat their babies, meat should only make up 1.4% of the human diet, [and] females should only receive meat by begging for it and allowing the giver to mate with her!" (source)



Consider also that even though primates eat meat sparingly, there again it's likely because they're intelligent and like humans are able to make choices to act outside of instinct. As other writers put it, "While chimpanzees are known to kill, this behaviour is not necessarily dietary but ritualistic." In 2006 the journal Nature published research about how chimpanzees have culturebehaviors copied from peers rather than being genetic. (See "Case Closed: Apes Got Culture".)

Eugene Khutoryansky who does believe that eating meat is natural, still cautions that the implications of chimps' killing should give us pause:

Eating meat is indeed natural in the sense that other animals do it as well. In fact, it is even done on occasion by our closest living relatives, the chimpanzees. However, there are many other things which are also natural. For example, chimpanzee males sometimes rape the females in their tribe. Chimpanzees sometimes engage in organized warfare against other tribes with which they compete for territory. A chimpanzee male, in a moment of rage, sometimes picks up a nearby infant, and crushes his skull against a rock. And chimpanzees do on occasion eat meat, and they do on occasion engage in cannibalism, in spite of the fact that there is a plentiful supply of food from other sources. So eating meat is indeed absolutely natural. However, the fact that it is natural does not imply that it is ethically permissible. If we believed that eating meat was ethically permissible simply because other animals did it as well, then this would imply that there is nothing wrong with rape, cannibalism, or infanticide, all of which routinely occurs throughout the animal kingdom. (source)







What it means to be an omnivore

There is no question that humans are capable of eating meat. But that certainly doesn't mean it's ideal for us. As I mentioned earlier, commercial cat foods typically contain rice, corn, and wheat. But of course, cats are true carnivores. We don't call them omnivores just because they'll eat things contrary to what nature intended. That would be silly. No one makes that argument for cats. But they make it for humans, enthusiastically.

McDougall explains how the ability to digest animal foods didn't hurt our survival as a race, although it takes a toll on our lifespan:

"Undoubtedly, all of these [meat-containing] diets were adequate to support growth and life to an age of successful reproduction. To bear and raise offspring you only need to live for 20 to 30 years, and fortuitously, the average life expectancy for these people was just that. The few populations of hunter-gatherers surviving into the 21st Century are confined to the most remote regions of our planetlike the Arctic and the jungles of South America and Africasome of the most challenging places to manage to survive. Their life expectancy is also limited to 25 to 30 years and infant mortality is 40% to 50%. Hunter-gatherer societies fortunately did survive, but considering their arduous struggle and short lifespan, I would not rank them among successful societies."

The problem with the term "omnivore" is that it's used in different ways. Many folks assert that if a primate ever eats any meat at all, no matter how small or insignificant, then bam!  they're an omnivore. But cats eat copious amounts of rice, corn, and wheat in commercial cat food, and have far more plants in their diet than meat in primates' diets. So why do these people insist that the piddling, insignificant amount of animal foods consumed by primates makes them omnivores, while cats are carnivores no matter how much plant food they eat?

And once they (think) they've shown that primates are omnivores, they then use this "fact" to justify the huge amount of meat that people eat today. This of course is ridiculous.

A more reasonable definition would be that a true omnivore would routinely eat large quantities of both plants and animals. A creature consuming less than 5% of its calories from animals just doesn't seem very omnivorous to me. (This includes other primates, our ancestors, and traditional Okinawans.) But for the record, if you insist that such creatures are omnivores, then I'll agree with you  as long as you agree that humans should also eat less than 5% of our calories from animals, just like the other creatures you're basing our "omnivorism" on. And that cats are omnivores, too.

Humans lack a desire to eat whole animals

True carnivores (and omnivores) salivate about the idea of eating whole prey animals when they see them. Humans do not. We're interested in eating the body parts only because they've been removed from the original animal and processed, and because we grew up eating them, making it seem perfectly normal. It's amazing how much of a disconnect we've been able to learn about the difference between animals and food. As GoVeg puts it:

While carnivores take pleasure in killing animals and eating their raw flesh, any human who killed an animal with his or her bare hands and dug into the raw corpse would be considered deranged. Carnivorous animals are aroused by the scent of blood and the thrill of the chase. Most humans, on the other hand, are revolted by the sight of raw flesh and cannot tolerate hearing the screams of animals being ripped apart and killed. The bloody reality of eating animals is innately repulsive to us, more proof that we were not designed to eat meat. Ask yourself: When you see dead animals on the side of the road, are you tempted to stop for a snack? Does the sight of a dead bird make you salivate? Do you daydream about killing cows with your bare hands and eating them raw? If you answered "no" to all of these questions, congratulations--you're a normal human herbivore--like it or not. Humans were simply not designed to eat meat. Humans lack both the physical characteristics of carnivores and the instinct that drives them to kill animals and devour their raw carcasses.

And here's one of my favorite passages by John A. McDougall, M.D.:

Cats are obligate carnivores (they must live on a diet primarily of meat) and their taste buds reflect this by having abandoned the tongue sensors that respond to sweet-tasting carbohydrates. Dogs are omnivores (they have retained both kinds of taste buds) those enjoying carbohydrates and amino acids. Humans tongues respond pleasurably to sweet (carbohydrates), but have lost the taste for amino acids, placing us undeniably in the category of herbivores (plant eaters). Many of your friends and family are confused, thinking people are omnivores, needing both meat and plants in their diet. We only appear to be omnivorous because we have the ability to "doctor up" meat with salt and sauces (barbecue, sweet and sour, marinara, etc.) sufficiently enough to make it palatable. Prove this for yourself. The next person you meet head-on who claims meat is "tasty," stop him in his tracks and insist that he eat a large plate of plain, unseasoned, boiled beef or boiled chicken in front of you; note their displeasure. Then offer that same meal to the dog or cat and note how eagerly this critter devours the meat. You would be hard-pressed to find a person who did not enjoy a bowl of perfect, ripe bananas, but try to get your cat to eat this sweet food. I have a Rottweiler dog named Bodega who is a true omnivore and enjoys bananas as much as meat. A careful observer notices that an animal's taste buds are no mistake of naturethey clearly define the proper diet that the animal should eat.

Comparing humans to other animals

Human physiology is strikingly similar to that of other plant-eaters, and quite unlike that of carnivores. It is telling that in none of the missives that readers have sent in to argue with me do they ever deny the data in the following table. They simply think that by making some other point (e.g., that humans possess canine teeth) that somehow obliterates the more convincing data in the table. This is the same table presented at the beginning, but it's important enough that it bears repeating.



Humans are biologically herbivores



Carnivores

Omnivores

Herbivores

Humans

Facial muscles

Reduced to allow wide mouth gape

Reduced

Well-developed

Well-developed

Jaw type

Angle not expanded

Angle not expanded

Expanded angle

Expanded angle

Jaw joint location

On same plane as molar teeth

On same plane as molar teeth

Above the plane of the molars

Above the plane of the molars

Jaw motion

Shearing; minimal side-to-side motion

Shearing; minimal side-to-side motion No shear; good side-to-side, front-to-back

No shear; good side-to-side, front-to-back Major jaw muscles

Temporalis

Temporalis

Masseter and ptergoids

Masseter and pterygoids

Mouth opening vs. head size

Large

Large

Small

Small

Teeth: Incisors

Short and pointed

Short and pointed

Broad, flattened and spade-shaped

Broad, flattened and spade-shaped

Teeth: Canines

Long, sharp, and curved

Long, sharp and curved

Dull and short or long (for defense), or none

Short and blunted

Teeth: Molars

Sharp, jagged and blade-shaped

Sharp blades and/or flattened

Flattened with cusps vs. complex surface

Flattened with nodular cusps

Chewing

None; swallows food whole

Swallows food whole and/or simple crushing

Extensive chewing necessary

Extensive chewing necessary

Saliva

No digestive enzymes

No digestive enzymes

Carbohydrate digesting enzymes

Carbohydrate digesting enzymes

Stomach type

Simple

Simple

Simple or multiple chambers

Simple

Stomach acidity with food in stomach

≤ pH 1

≤ pH 1

pH 4-5

pH 4-5

Length of small intestine

3-6 times body length

4-6 times body length

10-12+ times body length

10-11 times body length

Colon

Simple, short, and smooth

Simple, short, and smooth

Long, complex; may be sacculated

Long, sacculated

Liver

Can detoxify vitamin A

Can detoxify vitamin A

Cannot detoxify vitamin A

Cannot detoxify vitamin A

Kidney

Extremely concentrated urine

Extremely concentrated urine

Moderately concentrated urine

Moderately concentrated urine

Nails

Sharp claws

Sharp claws

Flattened nails or blunt hooves

Flattened nails

From The Comparative Anatomy of Eating, by Milton R. Mills, M.D. * "Body length" measured from neck to anus, as with the other animals

As another writer said, "The human body was not designed to catch or eat animals. You have no claws. Your teeth do not rend flesh. Your mouth can not seriously wound nor is it made to really get a good bite into a struggling victim like true carnivores can. You are not fit to run fast to catch prey. Meat-eaters have fast enough reflexes to ambush or overtake a victim. You do not. Try catching a pig or a chicken with your bare hands; see what happens."

Plant-eaters have the longest lifespans

In general, plant-eating creatures have the longest lifespans. Elephants, horses, and chimpanzees are at the top of the list while lions, tigers, and wolves are about half that. Humans' lifespans are even longer than the elephants etc. (even before modern medicine), providing more evidence that we're in the plant-eating camp.



Omnivores

Herbivores (least sleep)



Carnivores (most sleep)

We sleep like herbivores

Carnivores sleep the most, herbivores the least, and omnivores in the middle. Guess which group our own sleep correlates to. Here are some charts from an article in Nature (PDF). They have arbitrarily stuck us (and other primates) in the omnivore group, because that's what everyone assumes we are, but notice that we're at the extreme end of that chart, with nearly every other single omnivore sleeping more than we do. We fit nicely in the herbivore chart, and I added a prominent dot for us in that one so you can see how we fit in at eight hours a night. If we use a figure of 6-7 hours a night (suggested as natural by longevity research), our placement in the chart becomes even more compelling.

"But what about canine teeth and binocular vision?"

It's part of our collective consciousness that we have "canine teeth" and that this "proves" that we're meat eaters. But the truth is that this argument couldn't be weaker.

Canine teeth are canine in name only. Humans' so-called "canine teeth" are unlike the canine teeth of actual canines, which are really long and really pointed. Our teeth are absolutely not like theirs. In fact, other vegetarian animals (like gorillas and horses) possess the same so-called "canine" teeth.

Overall, our teeth resemble those of plant-eaters much more than meat-eaters. For example, we have molar teeth (plant-eaters do, carnivores don't). Try to find a human-type molar inside your cat's mouth. Our teeth can also move side to side to grind, just like the other plant-eaters, and completely unlike the carnivores. Their jaws go only up and down.

My favorite counter to someone trying the canine rationalization on a message board:

"Hey Julia--we evolved with canine teeth? I'd like to see you tackle a steer and tear it apart with those ferocious incisors."

What's funny to me is how the teeth argument is so important to meat proponents when they make their point about canine teeth, and then as soon as they find out that our teeth are much more similar to those of herbivores than of carnivores, and therefore consideration of our teeth suggests that we're designed to be plant eaters -- suddenly what kind of teeth we have is not so important to them after all.

Others have argued that predators have eyes on the front of their heads for binocular vision, while prey animals have eyes on the sides, indicating that we fall into the predator camp. This ignores the fact that the animals that we're most similar to -- the other primates -- have eyes on the front of their heads, and are almost exclusively vegetarian. It's also important to remember what I said at the top of this article: There is certainly evidence on both sides of this debate, but the preponderance of evidence clearly shows that we're suited to eating plants almost exclusively.

Does the unhealthfulness of meat mean that it's not natural?

The medical evidence is overwhelming: The more animal foods we eat, the more heart disease, cancer, diabetes, and other degenerative disease we suffer. This has been exhaustively demonstrated beyond any doubt. Dean Ornish, M.D. showed that heart disease can be reversed, and he did so by feeding his patients a vegetarian diet. John McDougall, M.D. has also written extensively about how the overwhelming scientific evidence shows that animal foods cause disease, and how for 25 years he's helped patients regain their health by eating unprocessed vegan foods instead. The science journals are filled with articles that come to the same conclusion: more plants means better health, more animal foods means more illness. And as mentioned earlier, the Maasai in Kenya, who still eat a diet high in wild hunted meats, have the worst life expectancy in the world. ( Fuhrman )

But is this evidence that meat-eating is unnatural? Maybe so, but maybe not. On the one hand we expect that's what's natural for us to eat should keep us in the best health (and that would discount meat, at least in the amounts it would take to make us true omnivores). But on the other hand, people can certainly live well beyond their reproductive years on a mixed animal-plant diet, which is mostly what evolution cares about. So it's hard to say whether the unhealthfulness of meat is evidence that we're naturally plant-eaters, but then again, we have other, better evidence anyway. The best evidence that we're supposed to eat primarily plants is the obvious one: our anatomy.

Human performance on meat-free diets

Not only do vegetarians and vegans easily build muscle, they often excel as athletes too, winning Olympic gold medals and world championships. In fact, some of the most famous bodybuilders in history were vegetarian. Here's a list of vegan and vegetarian athletes.

Examples of successful vegetarian and vegan athletes

(Note: As of July 2012, I've stopped adding to this list, because I've made my point that there are numerous vegans who among the top athletes in their sports. The number of vegetarian and vegan athletes is growing rapidly now and there's no way I'll be able to keep up with all the new ones. By the way, not all these athletes were/are exclusively veg*n for their entire athletic careers, the point is that they were/are successful as athletes even when veg*n.)



Other Vegan Athletes



Billy Simmonds (bodybuilder) Natural Universe champion



Bill Pearl (bodybuilder) Mr. Universe (3 times), World's Best Built Man, Mr. America, Mr. California, numerous Halls of Fame

Roy Hilligenn (bodybuilder) Mr. South Africa (4x), Mr. America, Olympic lifter

Ricky Williams (football) Miami Dolphins

The research on veg vs. non-veg athletes is fairly sparse, but what does exist has failed to show any clear performance benefit for meat-eaters. (See my separate article, Protein and Strength.)







Summary

Human anatomy is much more similar to herbivores than carnivores.

If humans are omnivores because we're capable of eating meat, then so are cats, since most cats eat plant foods every day. If we're supposed to eat animal foods, the evidence suggests it's a tiny part of our diets, not a large one.



of eating meat, then so are cats, since most cats eat plant foods every day. If we're supposed to eat animal foods, the evidence suggests it's a tiny part of our diets, not a large one. Making one contrary point does not magically invalidate all the other evidence as soon as it's made.



(Google picks the ads, not me.)

More on how human physiology favors a diet of plant foods:

Other objections answered



What fueled humans' brain development?

Some critics have argued that it was early humans' introduction of meat into their diets that sparked the evolutionary shift that gave us exceptional intelligence. (Too bad that meat-eating didn't provide the same benefit to carnivores like wolves and lions, huh? They're still stuck with those smaller brains.)



We'll start with this by turning to the most recent evidence on the subject, a 2011 paper from the science journal Nature, described by one paleontologist thusly:

"The paper is pretty good evidence that meat wasn't essential to our evolution. Meat, it turns out, probably didn't make us smart, after all."

In fact, the exact opposite argument has been made: That it was our plant-based diet that was responsible for our evolutionary spark. For example: "Developing a better memory for the exact location of favoured trees, the shortest routes between them and a timetable for when they would likely be fruit-bearing would definitely favour survival." (Grande & Leckie) The T.A. of my nutrition class, after achieving his doctorate, later wrote a book positing that our evolutionary spark was caused by beginning to cook starches, like potatoes, instead of eating them raw. And here's an excerpt from an article about work by University of California researchers: Man's ability to digest starchy foods like the potato may explain our success on the planet, genetic work suggests. Compared with primates, humans have many more copies of a gene essential for breaking down calorie-rich starches, Nature Genetics reports. And these extra calories may have been crucial for feeding the larger brains of humans, speculate the University of California Santa Cruz authors. Previously, experts had wondered if meat in the diet was the answer. However, Dr Nathaniel Dominy and colleagues argue this is improbable. "Even when you look at modern human hunter-gatherers, meat is a relatively small fraction of their diet. To think that, two to four million years ago, a small-brained, awkwardly bipedal animal could efficiently acquire meat, even by scavenging, just doesn't make a whole lot of sense." Other scientists have suggested that it was the introduction of cooked foods that provided the ability for our brains to advance. Of course, these ideas are just as much conjecture as the idea that it was meat that made us smart. But the existence of these contrary scholarly opinions serve to show that the idea that meat made us smart isn't widely regarded as fact, not by a long shot.

BeyondVeg.com article

Some readers have argued that Tom Billings' enormous article critiquing the comparative anatomy approach debunks this article. I believe they're missing the main points of Billings' work, but they can be forgiven because Billings is far from clear about what he's trying to prove. Billings essentially argues two main points:

The natural human diet is not 100% plants, even if we're geared towards having plant foods make up the overwhelming bulk of our diet. He's arguing against those who suggest that a natural human diet doesn't contain even a trivial amount of non-plant foods. Fruitarian extremists who insist that humans are primarily fruit-eaters haven't proved their point. The thing is, I don't necessarily disagree about either of these things. After all, as I pointed out, chimps are 95-99% vegan, but not 100%, and there's certainly compelling evidence that the natural human diet includes a lot more than fruit.



For the first point, it basically comes down to a question of semantics: I'm comfortable classifying a creature who eats 95-99% plants as essentially a plant-eater. Billings apparently is not. Here's one quote:

Virtually all of the great apes consume some animal food, even if only limited amounts of insects. Humans doing this would not be considered "vegetarian," yet attempts are often made to characterize apes as essentially vegetarian by falsely characterizing such insectivory as insignificant.

So even though he admits that great apes might eat only a "limited amount of insects" in addition to their otherwise vegan diet, he still thinks it's wrong to characterize them as "essentially vegetarian". Right.



Billings has various objections to anatomical comparisons, which range from straw men ("a comparison is not 'proof'") to poor conclusions "chimps are omnivores") to inaccuracies ("comparative 'proofs' ignore current knowledge of ape diets"). To the point:

Mills' "A Comparative Anatomy of Eating" didn't use the word "proof" or "prove" even once. Comparative anatomy is simply excellent evidence, and in science you go with what the best evidence tells you.



When the chimp diet is 95-99% vegan, and when most of the remainder isn't meat, it's termites, chimps hardly seem "omnivorous".



Apes' piddling non-plant consumption doesn't mean that apes are not predominantly plant-eaters.

Billings seems to throw a lot of stuff at the wall, hoping something will stick, because really, he's assumed the unenviable task of trying to convince us that it's wrong to read any meaning into 18 striking biological similarities between humans and plant-eaters, or the 18 differences when we compare ourselves to carnivores. Well, he's made an effort, but despite his verbosity, it's far from compelling. (It might be more so if he reduced the length and instead tried to more clearly explain why his evidence supposedly undermines comparative anatomyor to split his critique of Mills into a separate article rather than lumping it in with that of fruitarian extremists, who are too easy a target. Because he sure cites a lot of sources and introduces lots of facts in isolation of any clear point, but relating them to a refutation seems rather lacking.)



Footnotes

(Note: Most sources are cited, in the text, not here. If I can find the time I'll break them out into more footnotes.)

1 Meat in the Human Diet, John A. McDougall, M.D. The McDougall Newsletter, July 2003

2 The Comparative Anatomy of Eating, Milton R. Mills, M.D.

3 What's Wrong with Eating Meat?, Barbara Parham, 1979