With regards to eating grains, there is plenty of evidence to support the idea that humans ate limited amounts of grains at times before 10,000 years ago. However, those grains were wild grains and a very, very small part of the human diet. It was the advent of the agricultural revolution that saw humans suddenly move to a diet based on grains.
The advantage of an agricultural-based grain diet was that the sheer number of calories supported larger (though sicklier) populations. The numeric advantage allowed work specialization and military superiority even though it diminished the well-being of individuals. What's good for the group isn't necessarily good for the individuals. In fact, the most grain-based societies (such as ancient Egypt) show some of the shortest lifespans, even among the elite. Cleopatra needed bypass surgery.
For me, a Primal diet ended the horrible acid reflux problems I was facing and allowed me to end a 30-year need for asthma medications. I had tried vegetarian diets previously. They proved less-than-healthy for me. I did stay slim that way, but had lots of other problems including issues with blood sugar stability during exercise.
There is no universal consensus on what is the healthiest diet. If you really want to succeed you're just simply going to have to read as much material on the subject as you can and then test it on yourself. I strongly recommend keeping a food and health journal to help you find cause and effect in your own health.
Life is short: Void the warranty.