well in the interest of brain-storming. I doubt chimps eat grains, they certainly are not cultivating them. So even if they did eat them, collecting a few grass seed in the fall would not amount to a significant percentage of their diet over all. Even if they gorged on them when they were available.
The thing with grass seeds are that they are pretty darn tough, they have been naturally selected to discourage eating and when they are to pass through animals digestive tract and grow where they were deposited (apparently some farmers use this method to plant new pasture land). These plants stood the test of time because the animals couldn't eat them. I think it is therefore logical to assume that we started out intolerant of grains. But with our big brains we learned how to defeat those defenses of the seed by soaking and fermenting and grinding and nixtamalization to make them edible.
I also believe that our artificial selection of these grains has led to a situation where these defense mechanism have been increased. We have selected the grains that are the most disease resistant, meaning they have the best defenses. These defenses by design are damaging to our gut, the faster the seed pass, the more likely they are to grow. IBS and all these other, sort of sub-clinical ailments that are rampant in our society today, and probably have been for years, we just didn't have a name or TV commercials for them in the past.
you might find this discussion interesting
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