[notechback]

Just to mention that staples like grain have indeed much protein, so your sentence actually doesn't make much sense.

The conception of food consisting of vegetable + meat/fish + staple (rice, bread, potato, ..) is an utterly modern one.

All across history in most settled cultures will have lived principally of local plants of some kind, with meat being a rate treat. The exception being fishing villages but even there vegetables/grains will always have been a principal source of calories and nutrients.

[Mirioron]

Yes, and people nowadays are taller, smarter, and live longer. A large part of the Flynn effect is usually attributed to nutrition. Humans can survive with poor nutrition (not getting the right amount of nutrients they need) for a very long time, but it usually has consequences, especially when it happens during childhood.

Another thing to keep in mind is that humans didn't evolve to be farmers. We evolved to be hunter-gatherers. Just because for a slice of our existence people ate one way doesn't mean that that's the diet most suited for us.

By the way, there's a difference between animal proteins and protein in grain. They aren't quite the same composition of amino-acids.