I read years ago a quote that said 'It was a brave man who first ate frogs' legs.' I doubt it. He was probably just hungry.
People who are now in the 50s and 60s who are children of European migrants have pointed to a lot of our 'weeds' and explained how important those plants were as food to their parents. I suppose that's how the plants arrived here.
As our 'civilisation' has progressed, we eat from an ever more narrow menu of foods.