If you think in terms of what people ate during the Paleolithic era, and you do NOT need to be savvy in the sciences to guess, it would have been food that could either have been picked or foraged, and eaten AS IS, or an animal that either ran across the land or swam in the sea.
Both grains and legumes contain anti nutrient properties. While growing from the ground, these very components serve to protect them from pesticides and predators, but when we ingest them, they work against us by adhering to many of the vitamins and minerals in our food, preventing us from properly absorbing them and causing microscopic tearing in the intestines, thus increasing intestinal permeability.
The usual stuff.
UPDATE: For the true Paleoista!