By nature the human species is omnivorous. Distinctively human evolution began about six million years ago when the Rift Valley in eastern Africa divided the common ancestor of humans and Chimpanzees. After the split meat became important to the proto human diet, and hunting became important to human evolution. Even during the paleolithic era meat was supplemented with plant foods.
With the beginning of agriculture in the Mid East some ten thousand years ago, grains and beans became important to the human diet. Nevertheless, it took several thousand years of evolution for humans to easily process the higher carbohydrate diet that a largely plant diet provided. Populations that more recently adopted an agricultural diet, such as North American Indians, and Australian Aborigines are more vulnerable to type 2 diabetes.
One can live on an all plant diet if one mixes grains and beans to get complete protein, and if one takes vitamin B 12 supplements. Vitamin B 12 is the one nutrient not found in plant foods.
Yup. According to the Vegan Society, there is no reliable natural source of B12 to be found in a vegan diet. They encourage vegans to eat foods fortified with B12. The need for B12 is what really refutes the argument that human beings were meant to be vegetarian.
You’re right, however, in practice, there’s also no such thing as a natural, reliably-vegan diet - staple foods such as grains and pulses tend to contain insect remains (especially once the presence of the insect remains has been obscured by processing - and it’s exceptionally hard to prevent this).
And insects happen to be quite a good source of vitamin B12.
According to this page, mealworm larvae contain 12.3 micrograms of B12 per kg of insects. Adult human RDA for B12 bottoms out at 0.3 micrograms, so you’d have to eat about 25g (~an ounce) of mealworms to get this - which seems like quite a lot to eat by accident.
However, I think I’m right in saying that the B12 in insects is produced by their gut flora and that their droppings also contain the vitamin - and probably more concentrated than in the insect bodies, so maybe it’s as little as 5 or 10 grams of mealworm poo that you need in your daily loaf of bread.
On reflection, it seems like you might obtain enough of the nutrient to stave off serious malnutrition, but not enough to thrive.
Besides B-12, another nutrient to consider is omega-3 fatty acids, and while they can be obtained from plants (mainly nuts and seeds), they are in a form (ALA) that must be converted by your body into more useful forms (DHA and EPA), which it does very inefficiently, especially in men:
Also, while not necessarily related to a meat/plant diet, the high omega-6 content typical of many grains (the average Western diet has a 6:3 ratio as high as 30:1) has been thought to contribute to heart disease and other disorders:
This also applies to most meat consumed today; grain-fed beef has a much higher omega-6 content than grass-fed beef (and is also much worse environmentally and uses up food crops that could feed many more people):