The proposition is that eating meat is not only natural for humans; it is often also presented as a key to our evolutionary success. By becoming omnivorous, human beings were able to survive on an additional energy source (other than plants). In fact, it is widely accepted that ancient humans were hunters and gathers--surviving on plant energy, spiked occasionally with animal meat. Thus, some conclude, eating meat is a natural part of our diet.
I would suggest that this argument fails on two significant levels. The primary flaw is the extent in which the human body has evolved to eat meat. The second flaw is that, if indeed if it is natural for human beings to eat meat, it is an inconsistent argument when juxtaposed with the other foods that we eat.
Comparative anatomy, as wonderfully presented by Milton R. Mills, M.D., suggests that human beings are much more related to herbivores than they are to carnivores. It is a convincing journey throughout the human digestive system that consistently reveals our relationship to animals that exclusively eat plants--even more so than common omnivores.
To offer just a few examples, consider the act of chewing. Carnivores do not chew their food (they will break it apart so that it can be swallowed, but do not chew their food). Ever say to your dog, "chew your food, you're going to choke"? Herbivores, meanwhile chew their food extensively. Omnivores typically swallow their food whole or engage in simple crushing. Human beings, particularly if they are to access maximum nutritional benefits, are required to chew their food extensively.
The length of the small intestine respective to an animal's body size is revealing as to the type of diet consumed. Carnivores usually have a small intestine 3-6 times their body length; herbivores have a small intestine 10-12 times their body length. Omnivores are about 4-6 times their body length. Human beings, it turns out, have a small intestine approximately 10-11 times their body length.
The stomach acidity of carnivores and omnivores is less than or equal to a pH of 1 (with food in the stomach). For Herbivores and human beings, the stomach acidity has a pH of 4-5.
And this goes on and on. Space doesn't permit discussion on each digestive trait, but in each case--from everything like jaw type, teeth, saliva, stomach size, liver, kidney to nails and colon--human beings resemble the herbivore more than carnivores and even typical omnivores. Rare is any trait without exception in nature, but taken together--it is clear that human beings, though they can eat meat, were designed to eat a plant-based diet.
In analyzing the omnivore diet, there is evidence that less meat is healthier. It is not a secret that human beings who eat a large amount of meat and animal protein have a higher risk of cardiovascular disease and cancer. Our bodies have simply not evolved quickly enough, within the context of the omnivorous or carnivorous diets, to process large quantities of cholesterol and saturated fats.
However, for argument sake, let us suppose that all of that is wrong. Let us argue that because human beings can consume meat, regardless of the amount, it is indeed natural--and not subject to argumentation. Then, I would encourage consistency, and offer examples of unnatural human habits--in comparison to other animals.
Other animals do not drink the milk of other species (except in unusual circumstances) and never past the infant stage. Human adults, who drink milk of cows or goats, are engaging in one of the truly most unnatural behaviors in nature. This unnatural behavior is the reason that a majority of the world's population is unable to digest lactose--it's natural for mammals to lose that ability after weaning. However, we do see some evolutionary movement on this trait (though the cause/extent of that movement is widely debated).
In the consumption of meat, carnivores typically use their large canines to rip apart their prey's body. Carnivores eat the meat right off the bone, and do not take the time to cook the food, or season it to taste. Thus, to be consistent with the argument of our "natural" omnivore tendencies, we ought not to find walking up to a cow and ripping off a piece of its thigh and eating it raw as unusual.
Eggs are a natural food source and many species will eat the concentrated protein and energy sources dedicated to a developing young. However, like our meat . . . in nature, it goes down raw. Snakes and raccoons do not add salt and pepper and eat them scrambled with ham, cheese and onions.
Finally, our "natural" foods are quite extensively altered in modern food processing. Milk is pasteurized and injected with vitamins. It is turned into cheese and sour cream--things you'll never find in nature. Livestock is genetically selected and injected with growth hormones and antibiotics to increase food production.
Human beings have long passed living in the "natural" world. While we can debate the merit of our cultural evolution, and the optimal human diet, I think it is a stretch to justify eating meat as a natural behavior of human beings. Human beings, through comparative anatomy, seem clearly to be designed to eat a diet made up of primarily plants. Furthermore, the argument that eating meat is a natural perspective of the human omnivore is inconsistent when we consider other unnatural part of our diets.
One can't argue it both ways.
We no longer live in the natural world, and, as such, we can no longer give credence to that argument as the justification for eating meat--unless we are willing to revert to a truly "natural" diet. While we can biologically consume meat, it is a choice--not a requirement or some predisposition based on our evolutionary history.