Could domestcatable grain be a rather subtle filter for technological civilization?
This seems somewhat unlikely to me, and we might be able to answer it by exploring "grain." It seems to me that there are a handful of non-grain staple crops around the world that suggest that a planet would need to have no controllable vegetation sufficient for humans to sustain themselves on (either directly, or indirectly through feed animals). Even ants got agriculture to work.
Potatoes, sweet potatoes, turnips, taro, tapioca, those weird south american tubers related to Malibar spinach, and the tubers of runner beans immediately come to mind as long term storable calorie crops.
Of note, the consumption of flour has recently been pushed back to at the very least 32,000 years ago, probably much longer, even if field agriculture has not:
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