• To start with, humans get easily sick if they eat raw meat, a sign that neither their stomach nor their immune system are designed for meat. Real carnivores have stronger stomachs that also "eat" the parasites, bacteria and worms of rotting meat.
• Humans cannot digest meat well: mostly they have to cook it. Carnivores don't cook. Humans began eating meat on a large scale after the invention of cooking.
• Humans are the only primates that eat meat (any animal can eat meat in small quantities, but no primate eats meat on a regular basis).
• Humans who eat a lot of meat get heart disease, cancer, diabetes, osteoporosis, and all sorts of degenerative diseases. Carnivore animals who eat a lot of meat live healthy lives.
• When they don't abuse of meat, humans live very long lives by the standards of the animal kingdom.
• Humans need to sleep about the same amount of time as other herbivores, who sleep a lot less than carnivores.
• Human "canine" teeths are shared with horses, not with carnivores. There is no carnivore that has teeth like our canines. Horses have them. Whatever their function, it is not to eat meat.
• Humans are the only carnivore that enjoys putting "sauces" on meat. In fact, very few humans would eat meat cooked with no sauces. What sauces do is to hide the taste of meat.
• If a vegetarian diet were dangerous, half the population of India would be dead or very sick. On the other hand, in the places where the diet is mainly carnivorous, people do get sick and die by the thousands of all sorts of diseases.
• It is fairly easy for a meat-eater to become a vegetarian; it is difficult for a vegetarian to eat meat. This is a sign that the human digestive system has to be trained from childhood to digest meat, otherwise it wouldn't.
• As a world traveler, i noticed that the percentage of vegetarians among independent traveler is way higher than in any of their countries. After enough years, i started realizing that it's just natural selection at work: meat eaters are more likely to get sick when they travel, and therefore are more likely to stop traveling. All it takes is a slightly different way of cooking the meat and humans cannot digest it anymore. Over the long run, this creates a disproportional number of vegetarians among frequent travelers.
• At a personal level, i have always noticed that my friends who eat meat are afflicted with weak health, get easily injured and are more likely to get a flue. Needless to say, they would claim that this has nothing to do with meat, and that they would be sicker and weaker if they stopped eating it.
• Last but not least, most humans are disgusted by dead animals. If you see a dead cat or dog or raccoon in the street, brains and entrails splattered all over the asphalt, you are more likely to vomit than to salivate the way a real carnivore would.

Meat is the primordial fast food. At some point humans realized that they could absorb a lot of nutrients (notably, proteins) by just eating someone else's brain. Humans were cannibals before they were carnivores. The human brain was the first meat item in the menu (it is also one of the easiest to digest for non-carnivore). Then humans started eating also other animals, but to this day they are disgusted at the idea of eating most animals. Humans still don't eat the majority of animals. The reason they started eating animals is simple: it was easier than hunting and eating humans. Why did humans started eating human brains? I think it was one of the many accidental discoveries of human civilization: humans were killers before they were cannibals, and one day they realized that they could eat the brains of their victims, and that the brains were highly nutritional. My feeling is that "hunting" is just an evolution of "fighting". Humans are equipped with an uncanny ability and passion to kill fellow humans. It is an art in itself. Humans were killers from the very beginning. They became cannibals much later, more or less by accident. They became meat eaters and animal hunters even later.

Fish is a different story. Humans can and do eat raw fish with a relatively high chance of not getting sick. And there is evidence that a fish diet is not easily replaced by a vegetarian diet.

I would be happy to list counter-arguments that meat is a natural diet for humans. So far i haven't found one. Many readers send me links to "scientific" studies but either the studies were done by meat companies (...) or they end up proving the opposite. For example, the often repeated story that chimps eat meat too: in fact, chimps "hunt" but it does not seem that the reason they hunt animals is nutritional, and in any case meat represents at most 5% of their diet. There seems to be consensus that chimps (our closest relatives) use meat as status symbol to win sexual favors and establish power structures (so there are also theories that humans started hunting for the same reason, and only later meat became a staple of the human diet). (This article correctly points out that "In the early 1960's...it was thought that chimpanzees were strictly vegetarian." This was not only the opinion of western experts who check out a chimp for a few minutes, but the opinion of all the African natives who had lived with chimps for millennia. How could something so visible have escaped thousands of generations of African observers? is it possible that chimps started eating meat only since the 1960s? is it possible that chimps learned it from their cousins?)

P.S.

• I am aware of Wrangham's theory that meat accounts for 30% of a chimp's diet. However, other studies showed that eating meat is actually detrimental to chimps: chimps don't have a digestive apparatus that can dispose of the high calories of meat in an efficient manner, and therefore the protracted process of chewing the meat is offset by the effort it takes to eat it. The net gain in calories is lower than when they eat the easier to chew and digest plant food. It is still a mystery what makes chimps eat meat (which, by the way, is almost always monkey meat).
• I am aware of Wrangham's theory that Homo Sapiens is biologically adapted to eating cooked meat. But by studying the biosynthesis of DHA, Michael Crawford has concluded that it is unlikely that the human brain would have evolved to become this big if humans relied on meat: the richest source of DHA is the marine food chain. (DHA is a fatty acid that is essential to the development of the cortex)
• I am aware of theories that eating meat is fundamental to sustain the large brain of Homo Sapiens. However, Home Habilis had big teeth and powerful jaws (just like today's chimps) and its successor Homo Erectus lost both, despite the fact that the brain of Homo Erectus was twice the size of the brain of Homo Habilis. So the correlation is just not there.
• Also, Harry Jerrison has noted that brain sizes of both predators and preys increase in size: preys need a larger smarter brain to solve the problem of how to avoid being eaten. So the cause of a larger brain size might be more nutrients *or* the need to be smarter than the predator. In other words, we might have large brains because we were eaten by other animals, not because we ate other animals.
• I am aware of the discovery of August 2010 that hominids may have started eating meat a lot earlier than previously thought. I am waiting to find out more.
• See the pages on cognitive science for more details on the human brain and mind.
• See this article for neurological differences in the brains of vegetarians and meat eaters.