They bent the grass over and hit the heads with a stick to loosen the grains. This would also favor ripening over time and seed dormancy. The key factor in domestication was when humans began to replant the seeds. Eventually, the crop only survives because of humans and vice versa. They are important in all cultures.
Cooking improves edibility.
Most parched or popped originally. They can be mechanically harvested. Selection pressure was exerted by man. The method of harvesting was important. Replanting was a key step.