Of course pretty much the entire world is now suffering from high and increasing rates of obesity, diabetes, and cancers too. There's quite the correlation there.
Most HG societies ate less than 30% carbs and these are the groups that have no indication of all these chronic diseases that are now on the rise. Here is a link:
Whole Health Source: Composition of the Hunter-Gatherer Diet
"Many groups were almost totally carnivorous, with 46 getting over 85% of their calories from hunted foods. However, not a single group out of 229 was vegetarian or vegan. No group got less than 15% of their calories from hunted foods, and only 2 of 229 groups ate 76-85% of their calories from gathered foods (don't forget, "gathered foods" also includes small animals)."
The evidence is obviously lost to the ages. Now that most of these tribes are extinct or have had their traditional cultures destroyed there is little left to study now a days. It's rather sad. But, we do have some inclination as to what they didn't eat...grains, legumes, or seed oils. They did eat meat, nuts, seeds, berries, fruit, and tubers with some vegetables. The groups that had access got more energy from starch and fruit, but there are many that did not.
Your remark about grain is interesting. Surely you have seen the data damning wheat in grain eating societies...basically in comparison to rice usually. So rice is obviously less toxic than wheat right? So there are levels to which grains are better or worse when your picking your poison...that has been addressed numerous times.