According to the below article from New Scientist, stone age humans knew how to grind and mill flour to make bread, or something similar.
The article explains how traces of plants on 30 000 year old grindstones found in Italy, Russia and the Czech Republic indicate that we knew how to mill flour at that time.
It indicated that these grindstones had been used like mortar and pestles to grind plants such as cat’s tail and fern root. These are beleived ot have provided an energy hit to early man. It is thought that this may have assisted early man during times of climate change.