Very pleasant to eat, but can any white rice be called good nutrition? Isn't it just high calorie carbohydrate? Good as a low cost energy supplier, but poor in nutrients.
No. Grains are high in protein. Although the amino acids are not properly balanced so you need to augment a grain-intensive diet with nuts and seeds, which have a complementary protein profile, or just a little bit of dairy which has all the amino acids.
But more to your point, even
whole grains are not a complete diet for humans--or most mammals for that matter, just the ones with a rich culture of gut flora like cows and elephants and bunny rabbits. They have a few of the vitamins and minerals that are removed by the refining process, but they do not have all the nutrients we need.
Putting together a complete, well-balanced vegetarian diet is not terribly difficult with today's knowledge of biochemistry and today's food technology, but still it's not something a person would stumble onto accidentally.
In fact when farming was first invented twelve thousand years ago, it kicked off a negative trend in human health that was not reversed until the Modern Era. By the era of the Roman Empire, the population density was so great that it was not economically feasible to set aside huge swaths of land for grazing herds very close to the cities. The slow and costly transportation technology of the era, coupled with the primitive food-storage technology, made it unaffordable for the average city-dweller to eat meat very often, and even dairy foods (it's ten times as resource-efficient to raise cattle for milk than for meat) were not a staple for the average Roman.
As a result most people ate a grain-intensive diet. Nobody knew about vitamins and minerals in those days, so all they were getting (even from whole grains) was an adequate intake of protein and calories. The average life expectancy of a commoner
who had already survived the high infant mortality rate fell from the low 50s in the Paleolithic Era to the
low 20s in the Roman Era!
If the Christian occupying armies had not obliterated the Aztec and Inca civilizations, it's not clear what the future would have held in store for them as their population increased and crowding became a problem in their empires. The only grain they were able to cultivate in the New World was corn, which is an absolutely wretched source of nutrition compared to wheat and rice.
