The first recognisable humans evolved 2.5 million years ago. That is 2,500,000 years and our current counting system has only racked up 2,013 of them or 0.08% of the time we’ve been around. These people probably ate a mixture of foods including fruit, nuts, tubers and insects.
It is easy to imagine the different early versions of our ancestors eating progressively larger insects and broadening the range of foodstuffs they tackled.
Hunter-gatherer Homo sapiens seems to have eaten a wide range of plant and animal materials adapting to what was available in their environment. North Africans ate a lot of plant foods because the animals were not fat enough to support energy needs while Eskimos living where plants do not grow fast enough to be a viable food source lived off fat animals, fish, and their own stored body fat.
None of our primate cousins eat grasses and there is little evidence that humans did until at most 15000 years ago and even then grains seem to have been a very limited source of food. In order for the nutrients in grasses and grains to be accessible either the grain must be ground and the wheat separated from the chaff or more than one stomach is needed. Processing the grain requires equipment which was too heavy to be carried around and humans have only one stomach.
Before we ate wheat, it seems as though we ate a wide range of plant and animal material, the exact composition of which depended on where we lived. The advantage of this was that famine was rare as any food shortages could be compensated with by other sources.
Humans did not settle down and start to farm until 10,000 years ago and when they did so, they traded variety for staying put.