The morphology of mammal's teeth and digestive tract are reflective of evolutionary adaptation to different diets, but of course there are many exemptions to the apparent general patterns. Most obligate carnivores have
razor sharp canines and lack molars, which means they have to tear the meat off and swallow it more or less whole. Human teeth are shaped to be able to both tear at pieces of meat (we have smaller and less developed canines than obligate carnivores) as well, as grinding up various vegetable matter (large molar teeth). This would indicate our dentition is set up for omnivorous diets, but it is also dependent on which part of the world one comes from and the diet of our ancestors from those particular parts.
Inuit people of the Arctic had to survive on more or less 100% carnivorous food, but
more research has indicated they weren't as healthy as some sources would like to claim. The
Masai people of Africa have lived on mostly carnivorous diets and displayed very few signs of heart disease. But of course, the
Masai also spend a lot of their time running and chasing their bloody food around, which may explain why they are still fit and healthy, dhuh.
However, the human dentition is reflective of our tendency for being generalists, which is partially a reason for our success ( if you want to call it that) as a species. We can survive on and adapt to different diets in times of increased environmental pressures. Think Darwin's finches and all the different beak shapes that were a direct result of high competition and the resulting need for specialisation in a confined spatial context. Humans have also cooked food for long enough for our physiology to gradually adapt to it, so the claim that we should only be eating raw food is not entirely correct either, but of course there are benefits to eating some foods raw.
The fact that humans are neither obligate herbivores or carnivores means we can survive for longer on either diet, depending on climate, competition, energy consumption and genetics. Obligate carnivores have short small intestines which allows for more efficient processing of meat. Human small intestines are shorter than obligate herbivores, but longer than obligate carnivores. True carnivores swallow their meat whole because they don't have digestive enzymes in their saliva. Humans have digestive enzymes in their saliva, which aids the breakdown of various foods before it passes through the digestive tract. Contrary to popular belief, meat is easily digestible, which is why carnivores and omnivores have shorter small intestines. Cellulose is difficult to digest, which is reflected in ruminant's need to chew for hours, multiple stomachs and long intestinal tracts. But someobligate carnivores still have to ingest small amounts of vegetable matter to facilitate digestion. This is true for cats who eat grass almost every day to eliminate fur balls and speed up digestion. And carnivores do ingest small amounts of vegetable and grains indirectly as a result of eating birds and mammal's stomach contents.
So yeah, to expend a lot of energy in a really cold climate on a strict vegan diet would most likely not be sustainable for long. But probably fine for sitting inside, playing videogames. Likewise, eating a high-energy, purely meat-based diet in a temperate climate, sitting in front of a box all day would have equally disastrous consequences for long term health....
Of course there is the claim that there are many "modern"diseases that have proliferated as a result of the agricultural revolution and mass production of grain- based, carbohydrate foods; diabetes, heart disease, dental diseases, etc. Coeliac disease is interesting as it is an immunoreaction to gluten, a protein found in many grains. However, it may also be worth considering that proliferation of cancers and so-called modern "lifestyle diseases" are more obvious or even possible because of our longer lifespans. With my own background in palaeopathology and dentistry, I would speculate there would be plenty of disease data unavailable or hidden in the fossil record that we simply cannot access, for obvious reasons. But human mummies of very ancient origins have been found with signs of
atherosclerosis. The idea that certain diseases are thus a result of modern lifestyle is therefore up for debate. Seeing that humans have spread all over the planet since their inception, it would be natural to assume that humans would have adopted to different diets according to the surrounding climate and environment over time, and we would therefore perhaps have developed geographically different tolerances and/or intolerances to certain diets. I therefore think future medical research will be focusing even more on genetics and individual risk, rather than assuming we are all equally set up for the same disease development.
