There -are- clear accounts of hunter/gathers cultivating grass seeds.
However, it is also fairly clear that these foods were eaten only
seasonally and that it never formed the major portion of the diet. And
furthermore, it was only in limited regions where the stuff was available
at all.

For the vast bulk of human history, cereal grain seeds would have been
completely unavailable to any humans.  Where they were available, they
would have been much smaller, tougher, and harder to cultivate or eat than
the grains grown today.

Their widespread regular cultivation and use as a primary food source
started about 10,000 years ago, and apparently only started due to
population pressure.