Evolutionarily speaking, our physiological adaptation is frugivorous, as in, our health depends upon a large portion of our diet to consist primarily of fruits, roots, shoots, leaves, seeds, and nuts – much like our primate cousins. We are largely related mostly to bonobos and chimpanzees, and although their behavior shows that some (raw) meat eating occurs, the primary and predominant diet of our closest genetic relatives remains essentially frugivorous, none the less. Despite this physiological reality, our species has devolved into becoming more omnivorous behaviorly. Yet, our behavior is not indicative of what our bodies were adapted to thrive on over millennia.
This may seem like an unpopular notion, as many articles from plenty of people providing their opinions suggest that our brains, or our hands, or teeth indicate our adaptation to omnivore-ism. Even if we find evidence of our ancestors hunting for meat with their arrows, and other Paleolithic tools, the fact that our ancestors behaved this way, or the fact that we continue to behave this way, does not answer whether or not we are truly meant to eat a more omnivorous diet.
It has become apparent from the answers within this thread alone, that some people will argue over physiological adaptations and that comparisons through anatomy are open to conjecture. Be that as it may, there are many factors which can illustrate which diet provides for optimum health and longevity.
What greater example is there than our own bodies? It is through the myriad of diseases that our bodies present with by eating foods not really meant for regular and consistent consumption, that we can more easily identify which foods our bodies truly need to thrive.
It is through our physiological maladies that it becomes clear what kinds of foods we should be avoiding, unless of course you don’t mind suffering from the deleterious health consequences from the consumption of animal products.
The more meat you eat, the more likely your general morbidity for a number of major diseases. Diseases such as atherosclerosis , many forms of cancer, rheumatoid arthritis, Parkinson’s disease, Crohn’s disease, type 2 diabetes, gallbladder stones, and the list could go on, continue to plague mankind due to this omnivorous dietary behavior.
We are more able to ascertain what type of diet is actually the most beneficial for us simply by understanding what hurts us, and what heals us. Multiple studies confirm that the more red and processed meats you eat, the more likely you are to decrease your lifespan with the most common causes of death being cardiovascular disease and cancer. 
There are many scientific articles that point to animal products being contributing factors to these diseases for a myriad of reasons from the bacteriological vectors, to our inability to process the toxic elements within and or the byproducts that are produced from our consumption of flesh, organs, eggs, and mammary fluids.
Meat contains blood heme which is toxic to us and is a very high contributor to the third-highest cancer humans suffer from – colorectal cancer. When you cook meat, it also produces heterocyclic amines, which are a probable cause of cancer as they are mutagenic  There’s also a very high correlation with morbidity for sufferers of prostate cancer when consuming eggs or foul.  Not to mention, the damage which high saturated fats and cholesterol cause to our circulatory and vascular system. In addition, the gut flora which help to digest these products for you give off toxic by-products as well and we are harmed by that. Yet, when you see the beneficial byproducts that you get from gut flora helping to digest fiber in our systems, as well as the more alkaline state our body prefers to exist within, and the deleterious health consequences of remaining in the state of acidosis for a long period of time due to the continual digestion of animal products, it becomes quite obvious that our bodies thrive and are the most healthy the more whole plant-based foods we incorporate into our diets. The more you actually begin to study what foods truly give you the most health benefits, it’s really a no-brainer, what human beings respond the best to, which in turn more clearly illustrates our essentially frugivorous nature.
Popular myths have been prevalent throughout our Modern Day history. In previous centuries, consider the fact most of humanity believed that the earth was the center of the solar system, or that the Earth was flat. People even in this day in age believe the Earth is flat! To this day, a great deal of humankind still buy into the myth of separate races even though modern genetics have proven that popular belief to be false. Our diet is no different. Just because you can eat a lot of different kinds of foods for a long time and seem like you’re doing all right, doesn’t mean that you should be eating everything that’s available in the market. Just because everyone is doing it doesn’t mean that we should be doing it. Oftentimes, people who do buy into the myth of humans being omnivores, inevitably see negative consequences for that behavior, but don’t realize that a lot of the things that they suffer from in the forms of diseases are directly related to the dietary lifestyle’s that they have engaged in for decades. Some diseases are silent killers, and you don’t know to change your diet until it’s too late. Often times, individuals have a difficulty in changing, due to their habits, and surrounding cultures not providing enough plant based options at restaurants, or other social gatherings. We’ve been enculturated to believe in a myth. It takes a lot of courage to deviate from popular opinion, but with enough consideration for our health, and a general consensus within our communities, the shift to a more plant based culture will become inevitable. It’s already happening.