I'm amused by the reasoning that we shouldn't eat grains because early man was a hunter and gatherer.
Those primitive savages didn't have half the life expectancy of a modern human. And, it doesn't necessarily follow that what we had in the past is best for us now.
I also doubt that early man was so much a hunter and a gatherer. Farming makes life so much easier, and so would have been one of man's earliest innovations. Why run around the woods looking for fruit when you can grow an orchard? Likewise, someone would have early on discovered that grain is edible, and grain is easy to grow. That's why it's the cheapest food in the grocery store. And, wild-gathered anything is expensive.
We all die, and even if grain is bad for me, I'd prefer to eat it than not. But, it's not bad. It makes it easier to get the nutrition you need. I saw a study that said people who eat brown rice live longer than people who don't eat any rice (and, both live longer than people who eat white rice).
The key is not abstinence, but moderation.