This was my thought too -- at least about Paleo diet proponents rejecting grains.

IMHO these early people were smart enough to work out ways to eat things that were available to them. They probably had cooking with heat, boiling in water, grinding with stone, and probably worked out using salted water for preservation, and kept the spoiled food that had random yeast that tasted good and used it for culture.

Rice is a soft grain and can be eaten whole. Grains like wheat are HARD and need grinding to be edible. The hard grains keep well, so there was a big incentive to work out how to use them. A tribe that got it right probably had quite an advantage.

The movie "10,000" works on this idea.