This article reviews the myriad of different genetic, physiological, morphological and nutritional adaptations in humans to eat meat, including research on vitamin B12, 20 and 22 carbon fatty acid, Taurine, haem absorption, dental anatomy, teeth morphology, meat-adaptive genes and parasite co-evolution. These suggest human ancestors had a high reliance on meat and challenge the idea held by a substantial portion of the general public, and even some academics, that humans are “naturally” herbivores and that our current meat-eating habit is facultative.
Arguments that humans are adapted to live a herbivorous lifestyle are explored. Contrary to the claims, the evidence suggests that humans are omnivores, being well equipped to eat substantial portions of animal tissue and have eaten meat since the dawn of our genus “Homo” millions of years ago.
[For the following review the word “meat” encompasses all animal tissue, i.e. mammals, reptiles, birds, insects, fish, etc.]
For millions of years, since our genus “Homo” originated about 2.5 million years ago, and likely before that time also, our ancestors have dined on large portions of animal matter, this view is supported by fossil evidence and isotopic evidence.    Before the dietary shift to include more meat in our diet, our diet was probably like that of modern chimpanzees, mostly herbivorous containing plant foods including fruits, seeds, nuts, leaves and flowers with minor amounts of meat and insects. 
Today meat is still an important part of our diet, shown by a recent report from the OECD with the FAO has estimated the consumption amounts of the main types of meat eaten. For example, the United States consumed 48.9 kg/capita of poultry, 25.9 kg/capita of beef and veal, 23.1 kg/capita of pork, 0.4 kg/capita of sheep in 2017, and that amount is predicted to increase .
Despite our long evolutionary history and our current desire to eat meat, there is still a substantial portion of the public and even some academics which believe that humans are “naturally” herbivores and that our current meat-eating habit is facultative.
It is necessary to understand our current and ancestral diet, not only for the intrinsic value of knowing about our past, and how our diet may have morphed our evolutionary history, but it could also play a role in the field of evolutionary medicine through the application of modern evolutionary theory to understanding health and disease. For example, there is a well-known diet called the “stone age diet”, or “the paleo diet” which suggests that it is the diet that ideally fits our genetic makeup, the diet is about eating what our bodies are “designed” to eat, that is what our ancestors ate, this diet suggests a relatively large amount of energy should be from animal foods . Regardless of if the diet works well or if it is the “optimal” diet, it has attracted a large following. And on the other side of the coin many vegan or vegetarian groups and individuals claim that humans are naturally herbivores – so a vegetarian diet is healthiest.
The following is a review of the evidence for many adaptations in human beings for eating animal tissue. The proportion of the type of animal matter we are adapted to eating along with what diet would be optimal for human health will not be discussed.
Possible adaptations in humans for meat eating:
1) Vitamin B12
Vitamin B12 (cobalamin) is an important vitamin that plays a key role in human health, deficiencies of which are able to cause megaloblastic anemia, fatigue, weakness, constipation, loss of appetite, and weight loss as the main problems. Additionally, it can cause difficulty in maintaining balance, depression, confusion, dementia, soreness of the mouth or tongue and poor memory, as well as being able to cause irreversible neurological damage.  
The natural source for cobalamin in human is via the consumption of animal products , as there are very few plant sources containing significant amounts of cobalamin, which likely would not have made up a significant portion of the human diet as we evolved.
Herbivores get most of their cobalamin from gut bacteria which synthesize the vitamin.  In humans, however, it is unlikely that gut bacteria are able to act as a significant source of cobalamin, due to the fact that cobalamin produced by gut microbes represents a miniscule amount (about 2%) of the total corrinoid content in feces and that it is produced in the colon, which is below the small intestine where cobalamin can be absorbed.  
This evidence suggests an adaptation to meat eating, in order to get the required amount of B-12. This conclusion gains even more support when studying people on vegetarian and omnivorous diets, which finds that there are lower levels of B12 in vegetarian and vegan diets and that vegetarians and vegans are at higher risk of B12 deficiency than meat consumers.   
2) Taurine synthesis–
Another adaptation, our species has a limited ability to synthesize the biologically important amino acid, taurine, which is essential for cardiovascular function, and development and function of skeletal muscle, the retina, and the central nervous system  ; it also has many other benefits, non-essential, effects; as such taurine deficiency can have severe adverse effects. Vegetarian and vegan diets in humans result in lower concentrations of taurine , which is found naturally in meats and fish, but hardly ever in plants . Like felines  the need to internally synthesize taurine may have been evolutionarily reduced in humans because it had been obtained in the diet, which had relaxed the selective pressure formerly requiring the need to synthesize this essential amino acid.
3) 20 and 22 carbon fatty acids-
Like obligate carnivores , humans have an ineffective ability to chain elongate and desaturate 18 carbon fatty acids to form their product 22 and 20 carbon fatty acids. These fatty acids are crucial for the function of the cell membrane and brain tissue among other things , so it is odd that there were reductions in the enzymes desaturase and elongase activity, which are necessary to synthesise, these fatty acids. This indicates 22 and 20 carbon fatty acids must have been obtained via a different method, namely diet. 20 and 22 fatty acids are found mostly in animal food. Although they can be found in plant foods the amount in plant sources are only trace quantities, and it is more likely that animal tissue was the main source of 22 and 20 carbon fatty acids available to our Hominid ancestors.  This indicates that animal foods were increasingly incorporated into our ancestor’s diet instead of them being synthesized from 18 carbon plant fatty acid.
Physiological and morphological adaptations-
4) Dental anatomy
Analysis of teeth from early Homo species have shown that early Homo’s teeth are adapted to eat tough food, meaning they were more adapted to fracture tough, pliant foods. Meat seems to be this key tough-food resource, as one of the other main sources of tough-foods early Homo could have obtained sufficiently to cause this adaptation in the teeth, USOs (carbohydrate-rich underground storage organs of plants), are often fairly brittle compared to animal tissue and they are of limited nutritional value, so likely they would not be a cornerstone resource.  Some recent studies which used dental topography to analyze dental specimens of our ancestor’s show that increased occlusal relief and steeper sloped cusps yield sharper cutting surfaces that would give animal tissues less of an opportunity to stretch and absorb energy, thereby allowing easier consumption . So it is probably increased consumption of animal products may have played a role in the dental adaptations of our genus. Similarly, reviews of dental anatomy of our ancestors which analyze Dental Microwear, Dental Structure, Occlusal Morphology, and Tooth size have shown suggested that food required more preparation from the incisors and greater amount of molar shearing. The available evidence suggests a shift in the diet in early Homo and especially H. erectus with broadening of diet to include some more tough foods such as meat .
However the dental evidence is quite limited as there are only small numbers of samples of fossils from our ancestors, to be more certain in the results larger numbers of fossils would need to be analysed.
5) Gut morphology:
By observing the differences between the gut of carnivores, herbivores, and omnivores and comparing their guts to our gut we can predict what diets we are adapted to.
Carnivores and herbivores have morphological differences in their gut. Carnivores tend to have well-developed stomachs and long small intestines, while herbivores tend to have a chambered stomach with a well-developed caecum and colon. Humans fit neither of these patterns. The human gut has a simple stomach, relatively elongated small intestine and reduced caecum and colon, which suggests a relatively high dependency on meat.  
It should be noted that the gut is quite malleable and it is able to adapt slightly to the current diet, altering the proportions of the gut. This plasticity does not change the fact that gut is adapted to eating at least some meat as gut plasticity is quite limited in scope.
It should also be noted that suggesting that the human gut has adaptations to meat eating does not necessarily mean that humans have a dietary preference to faunivory (animal matter eating). Analysis of whether the human gut specification fits a preferred faunivorous or frugivorous (fruit-eating) diet often yields contradictory results depending on which technique is used, some methods give results that the gut is firmly in the faunivory range and others suggesting the frugivore range.  However, the human gut is probably more firmly implanted into the frugivore range, similar to that of organisms which eat mostly fruit with the inclusion of a large portion of insects and small amounts of vertebrates. 
6) Haem Absorption:
Humans have the ability to digest haem iron due to receptors or transporters , which are specific for the absorption of heme iron. In Western societies, iron derived from heme sources make up about two-thirds of the average person’s total iron stores despite only constituting one-third of the iron that is ingested  Although heme iron is found in plants at very low levels, the amount in plants is not nutritionally significant, and heme iron is sourced almost entirely from animal foods (Which also explains why vegetarians are more prone to lower iron levels and status than people who eat meat).   The presence of these receptors and transporters indicate a physiological adaptation to animal foods in the diet, this point is further compounded by the fact that herbivores are unable to absorb these haem complexes and are reliant on the absorption of ionic iron and that carnivores and other omnivores, such as pigs , are able to absorb this molecule.
Other possible adaptations-
7) Meat-adapted genes:
There are some genes in the human genome that appear to be adaptations to eating meat. Some hypothesize that the increased consumption of animal-sourced food during human evolution, selected for “meat-adaptive genes” to increase resistance to harmful effects of fat, toxins, and pathogens, delaying dysfunctions and diseases of the brain and heart, etc. caused by this increasingly meat-rich diet. And enable a major increase in lifespan, which if true could partly explain the difference in life expectancy between humans and the other great apes.  Meat-adaptive genes enabled the shift from an herbivorous ape diet to the more omnivorous diet of hominids. However, this is just a hypothesis at the moment, and requires further research to be verified.
Another hypothesis is that there might be a genetic basis for food preferences, affecting the choice of meat in the human diet. Indeed twin studies have found that dietary patterns, are partly heritable, for example, the heritability of red meat consumption in a UK sample was 39%  and certain genes have been found to correlate with increased meat consumption. 
8) Coevolution with parasites:
Many parasites and their hosts undergo co-evolution . Taeniidae are a family of parasites spread by the consumption of meat. Taenia saginata (commonly known as “beef tapeworm”) and T. solium (commonly known as “pork tapeworm”) use humans as their definitive host, whereas these parasites are not definitive for the other great apes . This parasite which specifically lives of humans indicates a substantial period of coevolution and meat consumption by humans and their ancestors. It is hypothesized that dietary shifts by humans from herbivory to increased carnivory during the evolution of early Homo caused these parasites to start infecting humans. 
Claims that humans are herbivores.
As stated in the introduction there are still many who believe humans are herbivores, despite overwhelming evidence opposing this. Many of these people are part of animal rights groups or vegan groups, hence it would be advantageous to claim that humans are naturally herbivores, since it would help with the spread of their ideology. For example, the well-known animal rights organization “PETA” holds the position that “According to biologists and anthropologists who study our anatomy and our evolutionary history, humans are herbivores who are not well suited to eating meat.” 
There are also a few academics who hold this position, such as Milton R. Mills, M.D., who wrote about humans being herbivores in his article “The comparative anatomy of eating.”  and William Clifford Roberts, M.D who concludes that humans “are natural herbivores” in his article “We think we are one, we act as if we are one, but we are not one” written 1990, and has cited the paper in more of his work since then.  Seeing as the article by Roberts appeared in the peer-reviewed journal “The American Journal of Cardiology”, whereas Mills’ article was not, I will focus on the evidence presented by Roberts.
Before considering the individual evidence presented by Roberts I must point out that he wrongly assumes that humans must either be natural herbivores or carnivores, and he allows no option that humans could be omnivores. This binary view on diet means that the evidence presented by Roberts is merely a comparison between humans, carnivores, and herbivores. It should also be noted that Roberts presents barely any sources for his claims.
That evidence is evaluated here:
- Firstly, Robert argues that humans can’t be carnivores as they do not have claws or sharp teeth, which they need to hunt and kill prey (there is no reference that it is universal that carnivores require/have sharp teeth and claws), and since they aren’t adapted like carnivores that means they must be herbivores. As stated before diet is not necessarily as binary as either carnivore or herbivore, so you could not just say that “something is not X, therefore it is Y”. Also, humans do not need sharp claws to eat meat, as humans have developed tools which have been used to butcher carcasses  and hunt  for millions of years,. In fact, large claws could be more of a hindrance than a help, as humans use their hands for more than just hunting. Also, as stated above, the human teeth are adapted to eat tough and plant foods like meat. (See Dental anatomy section)
- Next, he goes on to say that the human gut is similar to that of a herbivore. He states that the intestine of a carnivore is short and the intestine of a herbivore is long and that a human intestine is also long. This is a very simplistic examination of the intestines of organisms (and there are no references). Also, as stated above more in-depth analyses show that the human gut is likely like animals which eat mainly frugivore diet with relatively high portions of animal tissue. (See gut morphology section)
- He next states that carnivores lap up water and pant cool down, whereas herbivores sweat and sip water, and that since humans also sip water and sweat so humans are also herbivores. This is a completely irrelevant point, even if we assume the cooling and drinking method for herbivores and carnivores is an almost universal rule (there are no references to back up that claim). The way of cooling and drinking has little to nothing to do with the diet of an organism, so should not be used to support the idea that humans are herbivores.
- Roberts then says that carnivores can synthesize vitamin C, herbivores, however, must get vitamin C by the diet, and since humans are not able to synthesise vitamin C, humans are herbivores. Again, there is no reference.
The inability to synthesise vitamin C is due to mutations in the GLO gene (in most or all cases so far studied)  Contrary to what Roberts states these GLO gene losses and reactivations are unrelated to the diet of the species involved.  So the lack of vitamin C synthesis does not imply that an organism is a herbivore.
- Finally, Roberts argues that carnivores cannot suffer (cholesterol induced) atherosclerosis whereas humans and herbivores can, so humans are herbivores. To support this idea, he cited a study which showed that dogs didn’t suffer atherosclerosis even when cholesterol 200 times that of human’s average level, whereas if you add just 2 grams to a rabbit’s diet for 2 months produce great fatty changes in the arteries. First I would say that, although carnivores are less likely to develop atherosclerosis, carnivores can and do suffer it, for example, diet-induced atherosclerosis can be caused in the domestic cat (a classic carnivore) . Not only that but omnivores have also been shown to develop diet-induced atherosclerosis , such as pigs and mice. Therefore, atherosclerosis formation does not necessarily indicate the diet of an organism. However, this argument was likely the most convincing out of all of them.
Thus, I find the evidence presented by Roberts for humans being herbivores is lacking, whereas there is evidence that humans are adapted to meat eating.
Another often cited piece of evidence that humans are herbivores is that meat consumption is linked to a host of health problems.  Indeed, meat consumption (especially red and processed meat) is often linked to increase your chances of cancers , diabetes, stroke, heart disease  (albeit inconsistently), and obesity/weight gain , among other problems. This however does not necessarily mean that humans are not evolved to eat meat. It is possible that this is a maladaptive effect, in the past it may have been quite beneficial to consume meat, but in modern times where meat is much easier to come by, our naturally evolved inclination to eat meat leads to negative effects healthwise. Also many of the negative effects of meat could be due to effects of cooking,  processing and preservatives, rather than the meat itself.
The idea that humans are herbivores is not without its merits, although humans have been eating meat for millions of years, before that our ancestors ate very little meat. Similarly our closest living relatives, chimpanzees, do not eat very much meat, with the majority of its diet consisting of plant matter. Combining this with the fact that meat seems to have negative health effects then it is not difficult to see why people would think that humans should be natural herbivores.
Human ancestors have consumed relatively large portions of meat, this long period of dieting on meat has caused adaptations to take place in humans, to obtain key nutrients and better consume animal matter.
If you were to take the evidence of adaptations listed above individually it would be possible to reject each of the points, but when taken all together it would be difficult to say that humans have no adaptations to meat eating, in fact, the available evidence suggests that humans are quite well adapted to meat eating.
The evidence outlined above shows clearly that as humans evolved and consumed greater amounts of meats and have developed genetic, physiological, morphological and nutritional adaptations to eating animal tissue.
I also conclude that the evidence cited by some proving that humans are herbivores is lacking, although in theory it could make sense. Thus, animal rights organizations, vegan and vegetarian groups and many scientists are pushing a controversial belief that “humans are naturally herbivores”. I suggest that further evidence should be collected before propagating a certain worldview or lifestyle as it lacks sufficient evidence.