Oats were a staple of some primitive cultures in Scotland, and rye grain was a staple of some primitive cultures in the Swiss Alps. Both cultures were studied by Weston Price and he found their health to be excellent, in fact these and other cultures were the basis of his book and the Weston Price Foundation.

He has hypothesized that these grains were much more nutritious than what we buy in our stores because of the way they were growth, in rich soil, at certain altitudes, etc. and how they were prepared, primarily soaked and fermented, which makes them much more nutritious.

These people also got a lot more exercise, exposure to the elements, and led relatively stress-free lives.

Virtually every culture that we know about (which is to say, those that survived well enough to leave remnants) was based on some grain. Cultivation of grains is what made it possible for many of these cultures to exist in the first place. I suppose you could say that these people were all unhealthy and died at an early age. You could also say the same thing about hunter gatherer societies. We will probably never know.

I'm not defending grains in a modern diet, and I avoid them. But their role in history is unmistakeable.