If you're looking for evidence, that's going to be hard to come by as the damage that grains do take decades to show up, and there are few examples of societies not eating grain nowadays. I wish someone would quickly go out and take blood samples from the few relatively unsullied hunter gather groups out there now so we could get some idea of what kinds of differences there might be between grain eaters and non-grain eaters.

Other than that, we have anthropological evidence of shorter stature, worse bones, more dental issues when humans turned from HG to grain eating. And historical evidence from earlier in the century, such as Taubes wrote about.

I think that, my own body's reaction to grains, plus what we know about evolution is good enough for me to decide that humans aren't currently designed (evolved) to eat grains. We can survive but they'll take their toll eventually, unless you're genetically gifted.

Fruit is another matter. I think we've probably been eating fruit a lot longer than grains. However, not in the quantities we currently do and certainly not concentrated into high fructose doses!