Only in recent history have humans elected specific diets for themselves. Reasons are multiple: health, weight loss, ethical/moral, the environment, financial, social... We are still in a period of experimentation but there's an entire spectrum of choice, ranging from purely meat based to the hilarious breatharians who claim to live on fresh air and sunlight.

[man eating trees, artist: John Sokol, source]

Physiology

teeth comparison source
Our species evolved to fully exploit the food available. We use canines and incisors to tear and chop flesh. Unlike the ruminant's flat molars, our molars have sharp edges to help grind meat and various plant matter into pulp.

Raw meat meal. Pic source
Humans eating raw meat. Notice how even small children can chew through flesh. One even enjoys a nice cup of blood.

Saliva

Human saliva contains enzymes which begin to digest carbohydrates and fats, also adding moisture to make food easier to swallow. Unlike herbivores – we don't chew, partially digest, then re-chew the same mouthful multiple times. We chew for a short time; the food is swallowed.

Stomach and down...

The human stomach, liver and pancreas produce digestive enzymes: pepsin, trypsin, chymotrypsin, lipase and bile. These easily break down meat proteins and fats into essential nutrients. This is why you never see lumps of meat in your poo. As an ex-vegetarian, I can attest to the fact my poos often had colourful nuggets of vegetable matter – sweetcorn, carrot, nuts and other indigestible plants.

rainbow poo, source

Unlike the virtually sterile human stomach, herbivorous stomachs contain bacteria and parasites, which ferment and partially digest plant matter. This micro-community converts cellulose and fibre into short-chain fatty acids which the animal uses for fuel. Effectively, they're living on a high-fat diet.

Our digestive flora live at the hind end of our very short digestive tract. By the time the bacteria in our colon start converting plantstuff into something useful, we're ready to shit it out.

gorilla eating his own shit to get vital nutrients, source

Unless they eat their own shit, vegans suffer from low vitamin B12 because they don't eat animal products. This essential nutrient is abundant in animal foods. Bacteria in our colon can convert plants into B12 but it's just not enough for the human body. This is why vegans absolutely must inject synthetic B12 into their buttocks. Long term use of this has yet to be understood. It's not wise for a healthy person to rely on big pharma to stay alive.

Vegans will tell you we're not omnivores, and say we're not structurally similar to famous omnivores like bears or raccoons. Are bears similar to mice or chimps or chickens or turtles or catfish or rhinos or dolphins or flies? Not really, but these are all omnivorous animals (and there are many more). Just because a human isn't a bear or a raccoon, doesn't mean we're not omnivorous.

bear-dolphin, source

There is zero evidence of any vegan societies in the history of this planet, certainly none that reproduced and flourished. This is because it is impossible for a vegan to maintain health without synthetic chemicals. You will, however, find lots of evidence for omnivorous and even carnivorous human societies.

Don't take my word for it. Do your own research. But please don't get your information from PETA or the Vegetarian Society; they're prone to cherry-picking data and quoting pseudoscience to further their agenda.