There was a large shift in our ancestors diet about 3.5 million years ago to food derived from grasses and/or sedges. This has potentially important implications for what diet we’re adapted to. Unfortunately, the evidence isn’t specific enough to be very useful:
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. The method also can’t determine when human ancestors began getting much of their grass by eating grass-eating insects or meat from grazing animals.