So, Peta had done an article on their website (http://www.peta.org/living/food/natural-human-diet/) proclaiming that humans are naturally herbivores. Now I have no problems against vegetarians/vegans, but I do LOVE me some evolutionary biology, so I decided to go through the article and evaluate their claims that humans are naturally herbivores and not omnivores [See my previous blog, [here] where I examine the myriad of adaptations in humans to eat meat].
So, let’s get started, and see what evidence they present to support their claim.
[For this article I will use the word meat to describe all animal tissue, whether from mammals, birds, insects, etc.]
The Claim: “Carnivores swallow their food whole, relying on their extremely acidic stomach juices to break down flesh and kill the dangerous bacteria in meat that would otherwise sicken or kill them. Our stomach acids are much weaker in comparison because strong acids aren’t needed to digest pre-chewed fruits and vegetables.”
First, something which Peta does multiple times in this article is compare human physiology to that of carnivores and point out differences between us and them, this, of course, is problematic, as humans are hardly ever described as (obligate) carnivores , but omnivores (Note: there is no clear line that differentiates facultative carnivores from omnivores , but due to the black and white view that Peta displays in their article we can assume that Peta are talking about only obligate carnivores and obligate herbivores). So even if the evidence presented in this article did prove that are not carnivores it would not prove that humans are herbivores, or do not naturally eat meat.
Secondly, analysis the stomach acidity of the 68 species (25 birds and 43 mammals) from seven trophic groups found that generally the stomach pH is: Obligate scavengers (1.3 ± 0.08), facultative scavengers (1.8 ± 0.27), generalist carnivore (2.2 ± 0.44), omnivore (2.9 ± 0.33), specialist carnivore (3.6 ± 0.51), hindgut herbivore (4.1 ± 0.38) and foregut herbivore (6.1 ± 0.31), humans. The pH of the human stomach acid was 1.5 ± 0.44. As you can see contrary to Peta’s claim human stomach acid is a lot weaker than that of carnivores, is incorrect.  To give you a more specific example let’s look at the pH of a well-known carnivore, cats, which have the average stomach pH of 2.5 ± 0.07 
Humans, (uniquely among the primates considered) surprisingly, appeared to have stomach pH values more akin to those of carrion feeders [the high acidity levels normally associated with scavengers] than to those of most carnivores and omnivores. They then go on to suggest that carrion feeding was more important in human evolution or alternatively it is due to protection against pathogens, independent of diet. So, although interesting, the stomach pH of humans does not support the idea that humans are herbivores, in fact, it could suggest quite the opposite. 
As a side note, the idea that humans were scavengers is not a new one, in the scientific community there are two main hypotheses about human ancestry and meat eating, which are that humans were mainly scavengers or that they were mainly hunters . Personally, I’m more of a fan of the idea that humans were hunters and got most of their animal flesh by hunting it, although this stomach pH evidence could be explained by the scavenger hypothesis.
Teeth, Jaws, and Nails-
The Claim: “Humans have short, soft fingernails and pathetically small “canine” teeth. In contrast, carnivores all have sharp claws and large canine teeth that are capable of tearing flesh.”
Again, humans aren’t carnivores, but I digress.
Okay, so the argument is that humans do not have some of the adaptations which other animals do to rip and tear meat and kill prey. Well the obvious answer is that humans don’t need those things as we can make tools to cut up animal tissue, in fact, the evidence suggests that humans have been using tools to butcher carcasses for at least 2.5 million years at the dawn of our genus “Homo”, which is when the major shift in our diet occurred to increased carnivory  . There is also some evidence of tool-assisted hunting by humans about 2 million years ago  as well as the fact that chimpanzees have been known to use tools for hunting (as well as hunting without tools). 
So clearly, humans do not need claws or carnivore-like teeth to be effective predators, in fact sharp, long claws could arguably have been more of a detriment than a help, as they could interfere with some of the functions and adaptations of the current (and frankly more useful) adaptations, e.g. grabbing.
But let’s look more in depth, analysis of teeth from early Homo has shown that early Homo’s teeth are adapted to eat tough food, meaning early Homo was more adapted to fracture tough, pliant foods (and the further down the human lineage you go the more these adaptations are exaggerated). Meat seems to be most likely be this key tough-food resource which caused the teeth to evolve in this way, as one of the other main sources of tough foods early Homo could have obtained, USOs (carbohydrate-rich underground storage organs of plants) are often fairly brittle compared to animal tissue and they are of limited nutritional value, so probably would not be a cornerstone resource, as well as the fact that we know during early Homo there was a shift to increased carnivory (either way whether the adaptation is due to USO’s or meat our teeth are adapted to eat tough, elastic foods, such as meat). So again, contrary to Peta’s claims human teeth do not show that we are herbivores.  
The Claim: “Carnivores have short intestinal tracts and colons that allow meat to pass through the animal relatively quickly, before it can rot and cause illness. Humans’ intestinal tracts are much longer than those of carnivores of comparable size.”
Carnivores tend to have well-developed stomachs and long small intestines; herbivores tend to have a chambered stomach with well-developed caecum and colon. Humans fit neither of these patterns with its simple stomach, relatively elongated small intestine and reduced caecum and colon. The human gut has a simple stomach, relatively elongated small intestine and reduced caecum and colon. Which suggests a relatively high dependency on meat.  
Looking merely at length and complexity is not a good way of analysis, however.
More complex analysis of whether the human gut specification fits a preferred faunivorous (animal matter eating) 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 it is a gradient. But I would say, and I’m sure that Peta would agree, that the human gut is probably more firmly implanted into the frugivore range, like that of organisms which eat mostly fruit with the inclusion of a large portion of insects and small amounts of vertebrates. 
The Claim: “Humans also lack the instinct that drives carnivores to kill animals and devour their raw carcasses. 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 ate the raw corpse would be considered deranged.”
This statement is based on opinion rather than on a scientific basis. It is probably true that most humans in civilised societies, where meat can be gotten easily and whenever wanted that people don’t want to kill, even if those individuals enjoy eating meat. In many modern hunter-gatherer societies, however, they relish hunting, and that these societies eat a very large portion of animal food because meat is so nutritious.  Here is one example I found of a tribe hunting and fishing, I just did a quick YouTube search and instantly found this example: (https://www.youtube.com/watch?v=tpdpffv9dH8)
When it comes to a “natural” diet you won’t be able to get closer to humans surviving in nature than hunter-gatherer tribes like these.
As for why cooked meat is preferred? Well again this is subjective, many people do eat raw meats such as sashimi, some types of sushi and steak tartare and may like it more than when it is cooked. However, there are multiple reasons for the preference of cooked meat. Evolutionarily speaking it makes sense that cooked food (not just meat) would be preferred over raw food, as it kills harmful microbes and parasites as well as making the food easier to digest. Also, when meat is cooked a whole host of reactions occur, for example, Maillard reactions, which produce a wide range of aromas and flavours which make it more desirable. 
The Peta article doesn’t stop there, it goes on to talk about health, but honestly, I cannot be bothered to go through all of it. So, I will stop here. I may do another blog post where I analyse those points as well, but don’t count on it.
To end though I would like to say that there is just because humans are omnivores does not mean that human must or even should eat meat. In a way I admire vegans and vegetarians for their choice, whether it is for health, moral or environmental reasons, the reason I chose to respond to this article is because I just like analysing claims with a critical eye.
 “Mammals: Carnivores. Duane E. Ullrey. Encyclopedia of Animal Science”. and “Mammals: Omnivores. Duane E. Ullrey. Encyclopedia of Animal Science.”
 Beasley, D., Koltz, A., Lambert, J., Fierer, N. and Dunn, R. (2015). The Evolution of Stomach Acidity and Its Relevance to the Human Microbiome. PLOS ONE, 10(7), p.e0134116. (http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0134116)
 Nutrient requirements of cats and dogs. (2006). 1st ed. Washington, D.C.: National Academies, p.8. (https://books.google.co.uk/books?id=aqeCwxbRWvsC&printsec=frontcover&dq=Nutrient+Requirements+of+Dogs+and+Cats&hl=en&sa=X&ved=0ahUKEwjWjPn8hdzSAhVoL8AKHdIkD_IQ6AEIHDAA#v=onepage&q&f=false)
 de Heinzelin J, Clark JD, White T, Hart W, Renne P, WoldeGabriel G, Beyene Y, Vrba E. Environment and behavior of 2.5-million-yearold Bouri hominids. Science. 1999 Apr 23;284(5414):625-9
 Semaw, Sileshi (2000). “The World’s Oldest Stone Artefacts from Gona, Ethiopia: Their Implications for Understanding Stone Technology and Patterns of Human Evolution Between 2·6–1·5 Million Years Ago”. Journal of Archaeological Science. 27: 1197–1214
 Domínguez-Rodrigo, M. (2002). Hunting and Scavenging by Early Humans: The State of the Debate. Journal of World Prehistory, 16(1), 1-54.
 Wong, Kate (18 March 2014). “How Hunting Made Us Human”. Scientific American. 310 (4). Retrieved 26 March 2014.
 Pruetz, J., Bertolani, P., Ontl, K., Lindshield, S., Shelley, M. and Wessling, E. (2015). New evidence on the tool-assisted hunting exhibited by chimpanzees (Pan troglodytes verus) in a savannah habitat at Fongoli, Senegal. Royal Society Open Science, 2(4), pp.140507-140507.
 McKie, Robin (23 September 2012) Humans hunted for meat 2 million years ago. Guardian. Retrieved 17/03/2017.
 Peter S. Ungar. (2012) Dental Evidence for the Reconstruction of Diet in African Early Homo. Current anthropology. 53(6), pp. 318-329
 Peter Ungar. (2004) Dental topography and diets of Australopithecus afarensis and early Homo. Journal of Human Evolution 46 (5), pp. 605–622
 Milton K. (1986) Primate diets and gut morphology: implications for hominid evolution. In: HarrisM, RossEB, eds. Food and Evolution: Toward a Theory of Human Food Habits. Philadelphia, PA: Temple University Press; pp. 93–116.
 Martin R. (1992) The life of primates. In: JonesS, MartinR, PilbeamD, eds. The Cambridge Encyclopaedia of Human Evolution. Cambridge: University Press; pp. 39–97.
 Sussman, R.W., (1987). Species-specific dietary patterns in primates and human dietary adaptations. In: Kinzey, W.G. (Ed.), The Evolution of Human Behavior: Primate Models. SUNY Press: Albany, NY, pp. 131–179.
 Ungar, P. (2007). Evolution of the Human Diet: The Known, the Unknown, and the Unknowable. 1st ed. Oxford: Oxford University Press, pp.316-318.
 L. Cordain, J.B. Miller, S.B. Eaton, N. Mann, S.H. Holt, J.D. Speth Plant-animal subsistence ratios and macronutrient energy estimations in worldwide hunter-gatherer diets Am. J. Clin. Nutr., 71 (2000), pp. 682–692
 Shahidi, F. (1998). Flavor of meat, meat products, and seafoods. 1st ed. London: Blackie Academic & Professional, p.145+.