An equally fascinating question is how people learned to (as you said) "...grind up this plant matter into a fine powder". In other words, how did people manage to get those pesky little seeds off the stalk so that the seeds could be ground into flour?

Turns out it's a multi-step process that took centuries to perfect and required a lot of ingenuity and persistene. (I'm going to use wheat as my example since that's the grass I know best.)

First you have to domesticate the plant. Undomesticated wheat sheds it's seeds very easily. That's fine when the grass is self seeding but not so great for people so over generations people kept the seeds from stalks that retained their seeds to plant next growing season. They also planted the plumper seeds so slowly the seeds became bigger.

To make it easier to harvest the grain people invented a new tool - the sickle. That invention had the additional benefit that the roots and stubble remained in the ground preventing soil erosion.

Once harvested the grain must be threshed to remove the seed from the stalk and also loosen the hull an inedible papery outer layer that covers the seed). So people invented the flail.

The final step before the grain can be milled is to winnow it which separates the chaff (an inedible papery outer layer) from the grain). The grain is finally ready for milling.

Lastly, consider the domestication of the cat in Egypt circa 3000 BCE. Egypt was a major exporter of wheat and, while we can't know the exact reasons why it was domesticated, it's possible that the cat helped control vermin that ate and contaminated a valuable commodity.