This argument usually takes the form of an appeal to nature, either referencing evolution, our canine teeth or the fact that meat was important in our brain development. The argument holds that since eating animals is natural to us as a species, then it can’t be wrong, since it is in some way what we are “meant” to be doing.
In terms of our evolution, whether or not meat is what we are naturally supposed to be eating is debatable; humans share some qualities with herbivores as well as omnivores. Our facial muscles are well developed, our jaw is expanded and goes side-to-side rather than up-and-down, our incisors are broad and flattened and those much cited canines are short and blunted rather than the sharp and curved canines we see in many carnivores. Our saliva contains carbohydrate digestive enzymes, our stomach has simple chambers, our small intestines is 10-11 times the length of our body, our liver cannot detoxify vitamin A and our nails are flattened. These are all traits typically seen in many herbivores; many of the animals who are the most similar to us subsist mostly on plant matter.
To a certain extent however, eating animals can be argued to be natural on the basis that we have evolved to at least be able to process meat, though recent research by The World Health Organisation suggests that doing so is extremely harmful to our health. Despite any evidence to the contrary, we can still happily accept the designation of humans as omnivores and accept the idea that eating meat is natural, because what comes naturally to us should not in any way inform our ethics. Just because something is natural, does not make it right. An overwhelming body of research tells us that humans can thrive on a plant based diet, with both the American Dietetic Association and the National Health Service telling us that vegan diets are nutritionally adequate and appropriate for all stages of life. Eating meat may well be “natural”, but this fact is ethically irrelevant; most us us can live a healthy life without any animal products and so the decision to consume them is as open to criticism as any other freely chosen behaviour. The idea that we are adapted to do something and therefore should do it is utterly irrational.
Many people also want to point to the fact that animals eat other animals in defence of the idea that eating animals is ethically justified. This ignores the fact that other animals lack the same moral agency which humans do, not to mention the fact that they lack the choice to act otherwise. This is also a clear example of cherry picking. Pointing to one specific behaviour in specific animals and basing our understanding of “natural” around that, while ignoring all incidences to the contrary, is fallacious reasoning. Lions consume animals, but we are certainly no closer to lions than we are to our herbivorous primate ancestors. That meat eating alone should be picked out as justified because some other animals do it and ignoring other behaviours which other animals exhibit, like infanticide, cannibalism and incest, is arbitrary at best. If we were going to truly base our ethics around what other animals do, our world would be a brutal place indeed. When it comes to deciding on what values we base our society and our personal ethics around, we can surely do a little better than imitating the behaviour of predatory animals.
When we point to the existence of canine teeth, the fact that other animals kill animals, or that we can process meat does not in any way support the notion that doing so is ethical. It is beyond doubt that eating animals is within our capacity as humans, but it does not follow that this is the action we should be performing. Murdering other humans to compete for scarce resources and territory is also adapted and well within our capacity, but we now recognise that this behaviour is no longer ethical in the modern world. The same should be stated of the consumption of animal products. We must not forget that we are not discussing hunter gatherers with bows and arrows, but the industrialised slaughter of billions of animals, most of whom will be pumped with drugs and antibiotics, whose flesh will probably be bought pre-packed from a supermarket. This is about as far from “natural” as can be imagined.
We are not hunter gatherers anymore, we live in a technologically advanced and culturally complex society. If what is natural is the basis of our ethics, then surely we should all be condemning computers, cellphones, glasses, medication and surgery, too. We cannot simply pick and choose which parts of our natural behaviour to observe and which parts to utterly discard based solely on which of our primal behaviours we would like to be able to justify. It seems reasonable to conclude that this is an argument which is more about having a convenient approach to ethics than a rational one.