I have since determined that the human digestive tract is so versatile, that we can exist on just about anything.
Actually that's not quite true. All the other Great Ape species are herbivores. They graze on leaves and other plant tissue. Like the ruminants (cattle, hippos, deer, camels, etc.) they have a bacterial culture in their gut that produces the enzymes necessary to break down the cellulose in plant tissue so they can digest it as starch. They augment this diet with insects, frogs, etc., as well as nuts and seeds, which, in addition to the bodies of the bacteria themselves, satisfy their dietary need for protein. (Goats, wildebeest, giraffes, etc., actually don't need the extra protein like gorillas and orangutans do because they have better bacteria, but that's another story.)
As our evolutionary line diverged from the chimpanzees, one of our ancestral species figured out how to knap flint and make blades. They used these blades to scrape the leftover meat off the bones left by the predators. This greatly increased their protein consumption. This allowed their brains to grow much larger (your brain metabolizes more protein than all the rest of your body, IIRC) and they kept getting smarter. Before long they were attaching their blades to sticks and going hunting. Each subsequent species had a slightly larger brain and a slightly larger protein intake. Eventually a species called
Homo sapiens emerged that had (proportional to size) a colossal brain. He developed language, planning and organizational skills, fancy new weapons, and became the first full-time predatory ape.
During this transition (which took several million years) his gut got smaller. He was getting so many calories from the meat he was eating that he no longer needed to eat leaves and no longer needed that large bacterial culture.
This is the reason that you're wrong: we indeed
cannot exist on just about anything. We need massive amounts of protein, we need vitamins that we can no longer synthesize, and although we can digest starch to increase our calorie intake, we cannot eat it raw. Cellulose has to be broken down by heat ("cooked") before we can digest it.
If we're diligent and careful, we can get our protein from plant tissue. The seeds of most plants have quite a bit of protein. This includes nuts, legumes, and grains, in addition to obvious sources like sunflower seeds. But many of these protein sources are wrapped in cellulose. This is why we have to cook our grains and our legumes, although we can eat most nuts and seeds raw.
But there's more. Our bodies are pretty good at taking the amino acids in the protein we eat and converting them into whichever amino acids are needed for today's maintenance and repair. However, there are several amino acids that we need in relatively small quantities, so our bodies never developed the ability to synthesize them. They're abundant in meat, of course. Regardless of your ethical feelings about meat, we are carnivores just like tigers and weasels, and can subsist exclusively on meat. But if we're trying to get by on the protein in plant tissue, we have to be careful. Several of those "essential amino acids" are missing in grains and legumes but plentiful in nuts and seeds, while another set is missing in nuts and seeds but plentiful in grains. So you have to be sure to balance your plant proteins or you'll get some icky protein-deficiency disease like marasmus or kwashiorkor. (Ooh, just typing their names makes me feel icky.)
When you think about, we seem to have an odd doubt about whether the stuff we eat is sufficient in nutrients.
Throughout history, this doubt has been quite reasonable.
At the end of the Paleolithic Era, just before the Agricultural Revolution ushered in the Neolithic, when everybody ate three meals a day of meat, the life expectancy of a human who had survived childhood (infant mortality was around 80%) was 50-55 years. Then civilization was founded. Raising animals for their meat is a very inefficient use of land. When communities started growing to the size that we would call genuine "cities," it took a lot of farmland to feed all those people meat. Remember, there was no transportation technology yet. No wheels, no carts, no oxen, goats or horses to pull them. Everything had to be carried by hand, or on a travois, a triangle of sticks that people put over their shoulders and let one point drag on the ground. Imagine not just growing enough pigs, chickens or cattle to feed all the people in a city, and then imagine
getting the meat to them before it spoiled. Sure, they could dry the meat, but that just made the process even slower and more complicated.
Instead of growing crops to feed the cattle, it was much more efficient to feed the crops to the humans. Bread (or rice, corn, or whatever grain was growing in a particular region) became a staple. Augmented with a little bit of milk or eggs (which are much more efficient use of the land and which also can be kept longer than meat), humans had their full complement of protein.
However, people in the Bronze Age and even the Iron Age knew nothing about vitamins and minerals. People could survive for quite a while on grains with a little dairy, but their bodies began to break down before long.
Bottom line? Starting with the 50-55 year adult life expectancy of the Stone Age, the life expectancy of a surviving adult in the Roman empire was
less than 30 years. Of course the nobility got lots of meat and they lived longer, but the peasants and slaves did not.
So if you think we eat bad now, look how far we've come from the Romans.
If pandas can exist on bamboo, then we ought to do fine on just about any protein source, with a few fruits, grains and vegetables to supplement it.
As for the pandas, they are a species of bear, but they're as different from all other bears as we are different from all other apes. All other bears are carnivores, although most of them are lazy-asses and will scavenge bread and pasta from your trash if its convenient. But just as we changed from herbivores to carnivores, pandas did just the opposite and changed from carnivores to herbivores. That change is much more complicated! Digesting plant tissue is a much more complex process than digesting meat: the very same tissue we ourselves are made of so the molecules just slide right in there. As a result, pandas can digest
only bamboo. They evolved for its particular kind of cellulose. Their gut has exactly the right chemistry to nurture exactly the right bacteria that have the secret skills to break down the specific kind of cellulose in bamboo and turn it into protein and starch.
So don't be envious of the panda. If some horrible pest gets loose and kills all the world's bamboo forests, pandas will become extinct.
In other words, this need for something else, or the fear of a nutritional imbalance, must be some kind of delusion.
I hope you understand now why that is not correct.