Interesting article about human ancestors eating grass: A Grassy Trend in Human Ancestors' Diets.
The implication is that our ancestors have eaten grasses right along. But the following statement is important. It would come as no surprise that our early ancestors were eating nutrition-packed roots and rhizomes.
The isotope method cannot distinguish what parts of grasses and sedges human ancestors ate -- leaves, stems, seeds and-or underground storage organs such as roots or rhizomes.