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Some vegans claim that humans are herbivores, not omnivores, and that we are not physiologically designed to eat meat (see here: http://www.peta.org/living/food/natural-human-diet/).

"According to biologists and anthropologists who study our anatomy and our evolutionary history, humans are herbivores who are not well suited to eating meat. Humans lack both the physical characteristics of carnivores and the instinct that drives them to kill animals and devour their raw carcasses. Although many humans choose to eat a wide variety of plant and animal foods, earning us the dubious title of “omnivore,” we’re anatomically herbivorous."

They go onto claim that we don't have tiger claws and teeth for catching and eating meat, that our jaws and teeth are designed for grinding plants not meat, that our stomach acid is too week to break down meat, that our intestinal tract is long like a herbivore for digesting plants and that it takes so long for meat to pass through that it rots and poisons us causing cancer, that we lack a hunting drive, that meat with it's cholesterol and protein is the cause of all disease so obviously its poison that we are not supposed to eat, that we get food poisoning from meat (like that never happens with vegetables...) and that the only reason humans started eating meat was to survive in times when there was a lack of plant foods available but according to our anatomy it's clear we're actually herbivores not designed for eating meat.

Can this be debunked?

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    $\begingroup$ Look at our teeth - our front teeth are the teeth of a carnivore. Compare that to the teeth (or lack thereof) of horses, sheep, cattle, etc. $\endgroup$ – MattDMo Sep 18 '16 at 12:16
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    $\begingroup$ Relevant: quora.com/… $\endgroup$ – Always Confused Sep 18 '16 at 12:44
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    $\begingroup$ "Debunk this nonsense" sounds like a cognitive bias. But that's just my opinion $\endgroup$ – Chimango Chisuwo Dec 26 '16 at 14:54
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    $\begingroup$ Although we are omnivores (as most of the answers below explain), the main problem is the amount of meat that we consume. There is a very good chance that in the beginning of our species history, we didn't have access to so much meat. So, I would say that we need a better balance and we are naturally more herbivores than carnivores. And if you check the diet of people 100 years ago, meat was more the exception, rather than the rule. $\endgroup$ – BioGeo Dec 26 '16 at 17:20
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    $\begingroup$ The fundamental (no pun intended) problem here seems to be the idea that human are "designed". We evolve, and we do that by exploiting new environmental niches. The "eat meat when you can catch it" niche seems to be one that our ancestors moved into before they split off from the chimpanzee lineage. A more recent example of such evolution would be the preservation of lactose tolerance into adulthood among humans whose ancestors were herders. $\endgroup$ – jamesqf Feb 3 '17 at 18:45
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As the answers given already point out, humans are generally considered omnivores that are able to use food sources available to them opportunistically. Apart from that, there actually is not much to debunk in the PETA statement - it is just a slightly one sided view on the history of human diet. Just as one sided as the claims that humans would not have evolved a really big brain without consuming animal products. Especially the canine teeth cannot be seen as evidence for prehistoric hunting, as in great apes the size of canine teeth is ridiculously well explained not only by defensive but mostly by mating behaviour; apart from that, humans do not even have those, and that most likely is caused by the trend towards monogamy which reduces inter-male aggression.

However, coming back to the actual question, in the relative short history (approx. 200-300ky) of our species there is evidence for 'several transitions towards or away from a vegetarian diet in humans' (Nielsen et al., 2017).

And in fact, there even is a human-specific evolutionary derived haplotype (younger than the Human-Neandertal split) that shows signatures of positive selection and is associated with increased efficiency in the synthesis of long chain fatty acid from plant precursor molecules - which would not really be necessary for meat eaters, as these fatty acids are readily available in animal products (Ameur et al., 2012). More specifically, the genes lying on that haplotype belong to the FADS cluster that seemed to have had adaptive value - also for synthesising long chain fatty acids from medium chain plant precursor fatty acids - in African populations even more recently (approx. 85 kya, Mathias et al. 2012).

This finding has been supported by a more recent study by Kothapalli et al. (2016). They investigated a 22bp-indel allele in the FADS2 gene that is associated with both higher blood levels of long chain fatty acids and exhibits high frequency (signature of positive selection) especially in South Asia and Africa where there is long tradition for plant-based diet.


Conclusion:

Modern humans (and this is relevant, as diet varies strongly between species and therefore the species comparison does not need to be very informative) appear to have changed diet frequently but there is evidence for human-specific molecular adaptations towards plant-based diets.

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  • $\begingroup$ Thank you. Well thought through. $\endgroup$ – Lisa Jan 15 at 2:50
  • $\begingroup$ There are several claims in this that could use sourcing, like that eating meat was instrumental in human evolution is one sided, your links only confirm this view. $\endgroup$ – John Jan 17 at 15:26
  • $\begingroup$ But you also show some adaptations AWAY from a plant based diet, so your own evidence does not support your conclusion. Also I would like to see support for your claim that South Asia and Africa have a long tradition of plant based diets, any claim about native African populations a a whole is immediately suspect. $\endgroup$ – John Oct 23 at 15:17
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The simple answer is humans are omnivores – physiologically – because we have the capability to digest both plant and animal matter. Many humans are behavioural omnivores as well, consuming both as part of their diet, although many people can and do live while consuming only plant products.

Most if not all humans can or could digest animal products in some form - a brief search doesn't bring up any counterexamples of people who cannot physiologically do this (in the same way that many people cannot digest milk well in adulthood as they are lactose intolerant). Even if some people do not have this capability, they would be exceptional and broadly speaking you could describe humans as omnivorous, at least physiologically.

The Wikipedia page for omnivore is a good starting point if you want to know more about consumption classifications.

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    $\begingroup$ What about B12 deficiency and anemia risk in vegans, isn't that a clear indication humans are not designed as herbivores? Vegans have to take supplements, which is a modern invention, without supplements and fortified foods vegans risk serious deficiencies which can prove fatal if untreated from a lack of animal products in their diet. We can't get sufficient b12 from plants but herbivores can because their digestive system works differently, right? Can you explain about that please if you can? $\endgroup$ – Big Skeptic Sep 19 '16 at 6:50
  • $\begingroup$ Furthermore, fossil record shows humans have been eating meat in the last milions of years. $\endgroup$ – Pere Sep 19 '16 at 10:12
  • $\begingroup$ @BigSkeptic Vegetarians also drink more milk and lots of milk-products (curdled milk, sweets, butters, curd, etc) (and also used in many worships, sometimes in wasting amount). But milk is actually source of animal-proteins. $\endgroup$ – Always Confused Sep 19 '16 at 12:47
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    $\begingroup$ B12 is only sourced from dirt. Whether you are getting it from a supplement or animal meat - the source of that vitamin was found in the dirt. Humans used to get it from the dirt off fruits, veggies, etc. Now? Every bit of b12 you get comes from supplements. Even the animal meat you consume, the b12 you get from them was supplemented to them $\endgroup$ – Justin Beagley Feb 19 at 8:43
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We are not herbivorous because our digestive system is too short. Humans do not have many of the features required to digest plant matter efficiently. We do not have a multi chamber stomach like ruminants have. We do not have an enlarged cecum like rabbits

In fact, the human gut as a percentage of body mass is half that of a chimpanzee, (10% vs 20%). Also the composition of that gut is different, 50-60% are small intestine in human vs 20% in chimp. The human colon is also reduced: 17-23% in human, vs 52-54% in chimp.

http://www.beyondveg.com/billings-t/comp-anat/comp-anat-6c.shtml

Also gorillas, which are more herbivorous than humans, have cecum (7% of the Gut volume compared to 0% in humans.) http://huntgatherlove.com/content/human-colon-evolution-part-1-comparative-anatomy

If anything, the argument that the human gastrointestinal system is long like a herbivore's is false. It is short and reduced, and does not have the right components to ferment plant matter. However, it is longer than an obligate carnivore.

As a species, human are an ape (a fruit eating animal) that has started to evolve to eat meat. It is an intermediate species so to speak, it still doesn't digest meat well. It has many features of its fruit eating ancestor, but has already taken step to eating meat, while having lost features (like large gut size and big colon) that digest plant matter more efficiently.

Put a child on a vegan diet and the kid might die from B12 deficiency, insufficient calories, and lack of fatty acids.

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    $\begingroup$ Some apes eat meat as well, chimps f.e. $\endgroup$ – RHA Dec 26 '16 at 10:41
  • $\begingroup$ That is true. Estimated that Chimps obtain 6% of its calories from meat. Gorilla are more herbivorous and have larger percentage of their gut devoted to a cecum (7% in Gorilla, 5% in Chimp and N/A in humans). <huntgatherlove.com/content/…> $\endgroup$ – JayCkat Dec 26 '16 at 16:53
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    $\begingroup$ I think when you say humans "digest plant matter efficiently", you are perhaps over-simplifying. Humans seem to digest many fruits, seeds, and vegetables quite well. We just don't have the extra machinery to digest things high in cellulose, as grazers do. So like many other species, including some (like wolves/dogs) that are popularly supposed to be carnivores, we eat both. Though some human groups, e.g. the Inuit, have traditionally lived on an almost exclusively animal diet. $\endgroup$ – jamesqf Dec 26 '16 at 20:50
  • $\begingroup$ I don't think you realize that those added machinery is required. The vegetable and fruits that we eat today are no way similar to the non domesticated versions of the same plants. It is like saying bananas are very easy to eat... only problem being the wild version of bananas are small, gummy sappy, full of large hard seeds.. and really only edible by straining out the seed and cooking the resulting pulp. The plants we are familiar today are the product of thousands of years of domestication. wheat is the product of nearly 12000 yrs of selective breeding. $\endgroup$ – JayCkat Dec 26 '16 at 22:03
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    $\begingroup$ @JayCkat: But there are many fruits, nuts, and vegetables that have not been subjected to that 12K years of selective breeding (and which aren't sold commercially in any large quantity), yet which sustained native populations. Hereabouts there are pine nuts, cattail roots & pollen, camassia bulbs, elderberries, wild blackberries, and much more, all of which I eat on occasion. $\endgroup$ – jamesqf Feb 6 '17 at 20:05
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There are several lines of evidence.

  1. Behavior, we regularly eat meat.

This is the one that matters most, omnivore is a behavioral classification. All humans societies eat meat, a few eat little else or nothing else. The fact that a small subset of well off people in developed countries don't does not change this. Our two closest relatives also eat meat and many of our ancestral species show direct signs of eating meat.

  1. Gall bladders.

One somewhat useful piece of evidence is humans have well developed large gall bladders. Gall bladders function to store bile, a large one functions to be able to break down large quantities of fatty foods eaten all at once, which really only occurs in carnivores, omnivores. (there is also a weird thing ruminants do, but we are not ruminants so it can be ignored)

  1. Dentition

Our dentition is that of an omnivore, Its not specialized for a meat or plant diet. As an example gorilla show signs of specializing in a plant diet. Our detention however does show specialization for eating cooked food. Which we do to both meat and plant products.

  1. Digestion

We havelarger powerful livers able to breakdown toxins better than many related animals, We are also more sensitive to the smell of rot. Some have argued this is a sign of being a scavenger but this is more likely another adaptation to eating cooked food, the toxins we are best a breaking down are those that come from cooking meats and plants.

Humans are also better to digesting iron from animal sources than plant sources, and show adaptations towards this. We also show adaptations for digesting some plant materials better supporting an omnivorous line of adaptations.

Humans lack any of the specialized gut features seen is exclusive herbivores, we have no way to breakdown cellulose nor a structure to house bacteria that could do it for us, that means you could classify us as a frugivore at best if trying to argue for herbivory. Our gut length is what is expected form an omnivore, longer than a carnivore shorter than an herbivore.

  1. B-12

We can't make vitamin B-12. Humans and all our other great ape relatives get b-12 from eating animals.

Now some people argue humans should be classified as cucinivores (a unique classification) as well as omnivores, becasue we are specialized for eating cooked food.

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Humans benefit from a plant based diet. The B12 argument is only valid at face value as domesticated animals that are raised to be slaughtered are given B12 supplements as they would to obtain enough of it through a "natural" diet. B12 is found in bacteria in the soil so washing vegetables and removing the dirt is also removing the B12. Animal products are high in saturated fats which is the main contributor to atherosclerosis and therefore heart disease which kills millions of people around the world each year. True ominovores do not have the ability to develop atherosclerosis. The argument that humans had always eaten animal products renders us complete omnivores is downright wrong as eating something for survival is completely different than eating for optimal health. Our ancestors had no concept of biochemistry or the effects food has on the cellular level of our body. Our ancestors also usually died at rather young ages of nutritional deficiencies.

It is not difficult to obtain protein, fatty acids, or calories for that matter as many nuts, beans, seeds, and legumes are high in unsaturated fats and flax seeds them selves are a better source of Omega 3s than fish. Lentils, dark beans especially are high in iron and, and virtually every food has some sort of minor iron content and vegans have an infinitesimal chance of becoming anemic compared to an omnivorous diet or a vegetarian who consumes dairy. Dairy products are known to hinder iron absorption.

Animal products also raise levels of a hormone called Insulin like Growth Factor 1 or IGF1. IGF1 is known to cause insulin resistance, promote cancer cell growth and release free radicals throughout the body.

The argument of what we are is really a nonsensical argument as it does not matter what the label is as opposed to what kind of diet we can optimally benefit from in the long term.

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    $\begingroup$ Welcome to Biology.SE. Please take a look at what is considered a good answer on this site. Some of your statements might be challenged, so please provide reference to any claims you make. Thanks! $\endgroup$ – AlexDeLarge May 11 '17 at 14:59
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    $\begingroup$ This posting contains lots of wild claims - please add serious references for them. $\endgroup$ – Chris May 11 '17 at 16:54
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    $\begingroup$ B12 deficiency in livestock is due to cobalt deficiency, these animals can manufacture their own B12 ( well the bacteria in their complex gut can), they lack cobalt mostly due to human agricultural methods which prevents animals from wandering and eating a more diverse diet. Humans can't get B12 by this method. Also the nutrient content of modern crops is meaningless for our evolutionary history, since they are highly modified compared to their ancestors, more importantly most of them don't occur where humans evolved so mentioning them is pointless. $\endgroup$ – John Jan 17 at 15:14
  • $\begingroup$ You didn't explain how Cobalt deficiency leads animals to be b12 deficient. You also didn't explain how they'd get more Cobalt if they had a more diverse diet. Humans get b12 the same way animals get b12. Bacteria in dirt. $\endgroup$ – Justin Beagley Feb 19 at 8:57
  • $\begingroup$ @JustinBeagley if you want a full breakdown ask it as a question but a good summation can be found here. agric.wa.gov.au/livestock-biosecurity/…. And you are wrong ruminants GUT bacteria manufacture their B12. $\endgroup$ – John Oct 23 at 15:11
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It is physiologically possible for a human to be an omnivore, carnivore or herbivore. It's not what we are, but what we can be or what we want to be.

Strictly speaking, it seems that humans can not be strict herbivores, because natural plant foods do not contain enough of the essential nutrient vitamin B12 and they are vegan/vegetarian websites that tell you that (Vegan Health, Vegetarian Nutrition Dietetic Practice Group).

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